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Nuclear Power in a Clean Energy System

About this report.

With nuclear power facing an uncertain future in many countries, the world risks a steep decline in its use in advanced economies that could result in billions of tonnes of additional carbon emissions. Some countries have opted out of nuclear power in light of concerns about safety and other issues. Many others, however, still see a role for nuclear in their energy transitions but are not doing enough to meet their goals.

The publication of the IEA's first report addressing nuclear power in nearly two decades brings this important topic back into the global energy debate.

Key findings

Nuclear power is the second-largest source of low-carbon electricity today.

Nuclear power is the second-largest source of low-carbon electricity today, with 452 operating reactors providing 2700 TWh of electricity in 2018, or 10% of global electricity supply.

In advanced economies, nuclear has long been the largest source of low-carbon electricity, providing 18% of supply in 2018. Yet nuclear is quickly losing ground. While 11.2 GW of new nuclear capacity was connected to power grids globally in 2018 – the highest total since 1990 – these additions were concentrated in China and Russia.

Global low-carbon power generation by source, 2018

Cumulative co2 emissions avoided by global nuclear power in selected countries, 1971-2018, an aging nuclear fleet.

In the absense of further lifetime extensions and new projects could result in an additional 4 billion tonnes of CO2 emissions, underlining the importance of the nuclear fleet to low-carbon energy transitions around the globe. In emerging and developing economies, particularly China, the nuclear fleet will provide low-carbon electricity for decades to come.

However the nuclear fleet in advanced economies is 35 years old on average and many plants are nearing the end of their designed lifetimes. Given their age, plants are beginning to close, with 25% of existing nuclear capacity in advanced economies expected to be shut down by 2025.

It is considerably cheaper to extend the life of a reactor than build a new plant, and costs of extensions are competitive with other clean energy options, including new solar PV and wind projects. Nevertheless they still represent a substantial capital investment. The estimated cost of extending the operational life of 1 GW of nuclear capacity for at least 10 years ranges from $500 million to just over $1 billion depending on the condition of the site.

However difficult market conditions are a barrier to lifetime extension investments. An extended period of low wholesale electricity prices in most advanced economies has sharply reduced or eliminated margins for many technologies, putting nuclear at risk of shutting down early if additional investments are needed. As such, the feasibility of extensions depends largely on domestic market conditions.

Age profile of nuclear power capacity in selected regions, 2019

United states, levelised cost of electricity in the united states, 2040, european union, levelised cost of electricity in the european union, 2040, levelised cost of electricity in japan, 2040, the nuclear fade case, nuclear capacity operating in selected advanced economies in the nuclear fade case, 2018-2040, wind and solar pv generation by scenario 2019-2040, policy recommendations.

In this context, countries that intend to retain the option of nuclear power should consider the following actions:

• Keep the option open:  Authorise lifetime extensions of existing nuclear plants for as long as safely possible. 
• Value dispatchability:  Design the electricity market in a way that properly values the system services needed to maintain electricity security, including capacity availability and frequency control services. Make sure that the providers of these services, including nuclear power plants, are compensated in a competitive and non-discriminatory manner.
• Value non-market benefits:  Establish a level playing field for nuclear power with other low-carbon energy sources in recognition of its environmental and energy security benefits and remunerate it accordingly.
• Update safety regulations:  Where necessary, update safety regulations in order to ensure the continued safe operation of nuclear plants. Where technically possible, this should include allowing flexible operation of nuclear power plants to supply ancillary services.
• Create a favourable financing framework:  Create risk management and financing frameworks that facilitate the mobilisation of capital for new and existing plants at an acceptable cost taking the risk profile and long time-horizons of nuclear projects into consideration.
• Support new construction:  Ensure that licensing processes do not lead to project delays and cost increases that are not justified by safety requirements.
• Support innovative new reactor designs:  Accelerate innovation in new reactor designs with lower capital costs and shorter lead times and technologies that improve the operating flexibility of nuclear power plants to facilitate the integration of growing wind and solar capacity into the electricity system.
• Maintain human capital:  Protect and develop the human capital and project management capabilities in nuclear engineering.

Executive summary

Nuclear power can play an important role in clean energy transitions.

Nuclear power today makes a significant contribution to electricity generation, providing 10% of global electricity supply in 2018.  In advanced economies 1 , nuclear power accounts for 18% of generation and is the largest low-carbon source of electricity. However, its share of global electricity supply has been declining in recent years. That has been driven by advanced economies, where nuclear fleets are ageing, additions of new capacity have dwindled to a trickle, and some plants built in the 1970s and 1980s have been retired. This has slowed the transition towards a clean electricity system. Despite the impressive growth of solar and wind power, the overall share of clean energy sources in total electricity supply in 2018, at 36%, was the same as it was 20 years earlier because of the decline in nuclear. Halting that slide will be vital to stepping up the pace of the decarbonisation of electricity supply.

A range of technologies, including nuclear power, will be needed for clean energy transitions around the world.  Global energy is increasingly based around electricity. That means the key to making energy systems clean is to turn the electricity sector from the largest producer of CO 2 emissions into a low-carbon source that reduces fossil fuel emissions in areas like transport, heating and industry. While renewables are expected to continue to lead, nuclear power can also play an important part along with fossil fuels using carbon capture, utilisation and storage. Countries envisaging a future role for nuclear account for the bulk of global energy demand and CO 2 emissions. But to achieve a trajectory consistent with sustainability targets – including international climate goals – the expansion of clean electricity would need to be three times faster than at present. It would require 85% of global electricity to come from clean sources by 2040, compared with just 36% today. Along with massive investments in efficiency and renewables, the trajectory would need an 80% increase in global nuclear power production by 2040.

Nuclear power plants contribute to electricity security in multiple ways.  Nuclear plants help to keep power grids stable. To a certain extent, they can adjust their operations to follow demand and supply shifts. As the share of variable renewables like wind and solar photovoltaics (PV) rises, the need for such services will increase. Nuclear plants can help to limit the impacts from seasonal fluctuations in output from renewables and bolster energy security by reducing dependence on imported fuels.

Lifetime extensions of nuclear power plants are crucial to getting the energy transition back on track

Policy and regulatory decisions remain critical to the fate of ageing reactors in advanced economies.  The average age of their nuclear fleets is 35 years. The European Union and the United States have the largest active nuclear fleets (over 100 gigawatts each), and they are also among the oldest: the average reactor is 35 years old in the European Union and 39 years old in the United States. The original design lifetime for operations was 40 years in most cases. Around one quarter of the current nuclear capacity in advanced economies is set to be shut down by 2025 – mainly because of policies to reduce nuclear’s role. The fate of the remaining capacity depends on decisions about lifetime extensions in the coming years. In the United States, for example, some 90 reactors have 60-year operating licenses, yet several have already been retired early and many more are at risk. In Europe, Japan and other advanced economies, extensions of plants’ lifetimes also face uncertain prospects.

Economic factors are also at play.  Lifetime extensions are considerably cheaper than new construction and are generally cost-competitive with other electricity generation technologies, including new wind and solar projects. However, they still need significant investment to replace and refurbish key components that enable plants to continue operating safely. Low wholesale electricity and carbon prices, together with new regulations on the use of water for cooling reactors, are making some plants in the United States financially unviable. In addition, markets and regulatory systems often penalise nuclear power by not pricing in its value as a clean energy source and its contribution to electricity security. As a result, most nuclear power plants in advanced economies are at risk of closing prematurely.

The hurdles to investment in new nuclear projects in advanced economies are daunting

What happens with plans to build new nuclear plants will significantly affect the chances of achieving clean energy transitions.  Preventing premature decommissioning and enabling longer extensions would reduce the need to ramp up renewables. But without new construction, nuclear power can only provide temporary support for the shift to cleaner energy systems. The biggest barrier to new nuclear construction is mobilising investment.  Plans to build new nuclear plants face concerns about competitiveness with other power generation technologies and the very large size of nuclear projects that require billions of dollars in upfront investment. Those doubts are especially strong in countries that have introduced competitive wholesale markets.

A number of challenges specific to the nature of nuclear power technology may prevent investment from going ahead.  The main obstacles relate to the sheer scale of investment and long lead times; the risk of construction problems, delays and cost overruns; and the possibility of future changes in policy or the electricity system itself. There have been long delays in completing advanced reactors that are still being built in Finland, France and the United States. They have turned out to cost far more than originally expected and dampened investor interest in new projects. For example, Korea has a much better record of completing construction of new projects on time and on budget, although the country plans to reduce its reliance on nuclear power.

Without nuclear investment, achieving a sustainable energy system will be much harder

A collapse in investment in existing and new nuclear plants in advanced economies would have implications for emissions, costs and energy security.  In the case where no further investments are made in advanced economies to extend the operating lifetime of existing nuclear power plants or to develop new projects, nuclear power capacity in those countries would decline by around two-thirds by 2040. Under the current policy ambitions of governments, while renewable investment would continue to grow, gas and, to a lesser extent, coal would play significant roles in replacing nuclear. This would further increase the importance of gas for countries’ electricity security. Cumulative CO 2 emissions would rise by 4 billion tonnes by 2040, adding to the already considerable difficulties of reaching emissions targets. Investment needs would increase by almost USD 340 billion as new power generation capacity and supporting grid infrastructure is built to offset retiring nuclear plants.

Achieving the clean energy transition with less nuclear power is possible but would require an extraordinary effort.  Policy makers and regulators would have to find ways to create the conditions to spur the necessary investment in other clean energy technologies. Advanced economies would face a sizeable shortfall of low-carbon electricity. Wind and solar PV would be the main sources called upon to replace nuclear, and their pace of growth would need to accelerate at an unprecedented rate. Over the past 20 years, wind and solar PV capacity has increased by about 580 GW in advanced economies. But in the next 20 years, nearly five times that much would need to be built to offset nuclear’s decline. For wind and solar PV to achieve that growth, various non-market barriers would need to be overcome such as public and social acceptance of the projects themselves and the associated expansion in network infrastructure. Nuclear power, meanwhile, can contribute to easing the technical difficulties of integrating renewables and lowering the cost of transforming the electricity system.

With nuclear power fading away, electricity systems become less flexible.  Options to offset this include new gas-fired power plants, increased storage (such as pumped storage, batteries or chemical technologies like hydrogen) and demand-side actions (in which consumers are encouraged to shift or lower their consumption in real time in response to price signals). Increasing interconnection with neighbouring systems would also provide additional flexibility, but its effectiveness diminishes when all systems in a region have very high shares of wind and solar PV.

Offsetting less nuclear power with more renewables would cost more

Taking nuclear out of the equation results in higher electricity prices for consumers.  A sharp decline in nuclear in advanced economies would mean a substantial increase in investment needs for other forms of power generation and the electricity network. Around USD 1.6 trillion in additional investment would be required in the electricity sector in advanced economies from 2018 to 2040. Despite recent declines in wind and solar costs, adding new renewable capacity requires considerably more capital investment than extending the lifetimes of existing nuclear reactors. The need to extend the transmission grid to connect new plants and upgrade existing lines to handle the extra power output also increases costs. The additional investment required in advanced economies would not be offset by savings in operational costs, as fuel costs for nuclear power are low, and operation and maintenance make up a minor portion of total electricity supply costs. Without widespread lifetime extensions or new projects, electricity supply costs would be close to USD 80 billion higher per year on average for advanced economies as a whole.

Strong policy support is needed to secure investment in existing and new nuclear plants

Countries that have kept the option of using nuclear power need to reform their policies to ensure competition on a level playing field.  They also need to address barriers to investment in lifetime extensions and new capacity. The focus should be on designing electricity markets in a way that values the clean energy and energy security attributes of low-carbon technologies, including nuclear power.

Securing investment in new nuclear plants would require more intrusive policy intervention given the very high cost of projects and unfavourable recent experiences in some countries.  Investment policies need to overcome financing barriers through a combination of long-term contracts, price guarantees and direct state investment.

Interest is rising in advanced nuclear technologies that suit private investment such as small modular reactors (SMRs).  This technology is still at the development stage. There is a case for governments to promote it through funding for research and development, public-private partnerships for venture capital and early deployment grants. Standardisation of reactor designs would be crucial to benefit from economies of scale in the manufacturing of SMRs.

Continued activity in the operation and development of nuclear technology is required to maintain skills and expertise.  The relatively slow pace of nuclear deployment in advanced economies in recent years means there is a risk of losing human capital and technical know-how. Maintaining human skills and industrial expertise should be a priority for countries that aim to continue relying on nuclear power.

The following recommendations are directed at countries that intend to retain the option of nuclear power. The IEA makes no recommendations to countries that have chosen not to use nuclear power in their clean energy transition and respects their choice to do so.

• Keep the option open:  Authorise lifetime extensions of existing nuclear plants for as long as safely possible.
• Value non-market benefits:  Establish a level playing field for nuclear power with other low carbon energy sources in recognition of its environmental and energy security benefits and remunerate it accordingly.
• Create an attractive financing framework:  Set up risk management and financing frameworks that can help mobilise capital for new and existing plants at an acceptable cost, taking the risk profile and long time horizons of nuclear projects into consideration.
• Support new construction:  Ensure that licensing processes do not lead to project delays and cost increases that are not justified by safety requirements. Support standardisation and enable learning-by-doing across the industry.
• Support innovative new reactor designs:  Accelerate innovation in new reactor designs, such as small modular reactors (SMRs), with lower capital costs and shorter lead times and technologies that improve the operating flexibility of nuclear power plants to facilitate the integration of growing wind and solar capacity into the electricity system.

Advanced economies consist of Australia, Canada, Chile, the 28 members of the European Union, Iceland, Israel, Japan, Korea, Mexico, New Zealand, Norway, Switzerland, Turkey and the United States.

Reference 1

Cite report.

IEA (2019), Nuclear Power in a Clean Energy System , IEA, Paris https://www.iea.org/reports/nuclear-power-in-a-clean-energy-system, Licence: CC BY 4.0

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• Open access
• Published: 24 April 2024

A high-density and high-confinement tokamak plasma regime for fusion energy

• S. Ding   ORCID: orcid.org/0000-0002-1930-0439 1 ,
• A. M. Garofalo 1 ,
• H. Q. Wang   ORCID: orcid.org/0000-0003-1920-2799 1 ,
• D. B. Weisberg 1 ,
• Z. Y. Li   ORCID: orcid.org/0000-0003-3932-9244 1 ,
• X. Jian 1 ,
• D. Eldon 1 ,
• B. S. Victor 2 ,
• A. Marinoni   ORCID: orcid.org/0000-0003-1004-5782 3 ,
• Q. M. Hu   ORCID: orcid.org/0000-0002-8877-4988 4 ,
• I. S. Carvalho   ORCID: orcid.org/0000-0002-2458-8377 1 ,
• T. Odstrčil 1 ,
• L. Wang 5 ,
• A. W. Hyatt 1 ,
• T. H. Osborne 1 ,
• X. Z. Gong 5 ,
• J. P. Qian 5 ,
• J. Huang 5 ,
• J. McClenaghan 1 ,
• C. T. Holcomb 2 &
• J. M. Hanson   ORCID: orcid.org/0000-0003-2432-4870 6  

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• Magnetically confined plasmas
• Nuclear fusion and fission

The tokamak approach, utilizing a toroidal magnetic field configuration to confine a hot plasma, is one of the most promising designs for developing reactors that can exploit nuclear fusion to generate electrical energy 1 , 2 . To reach the goal of an economical reactor, most tokamak reactor designs 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 simultaneously require reaching a plasma line-averaged density above an empirical limit—the so-called Greenwald density 11 —and attaining an energy confinement quality better than the standard high-confinement mode 12 , 13 . However, such an operating regime has never been verified in experiments. In addition, a long-standing challenge in the high-confinement mode has been the compatibility between a high-performance core and avoiding large, transient edge perturbations that can cause very high heat loads on the plasma-facing-components in tokamaks. Here we report the demonstration of stable tokamak plasmas with a line-averaged density approximately 20% above the Greenwald density and an energy confinement quality of approximately 50% better than the standard high-confinement mode, which was realized by taking advantage of the enhanced suppression of turbulent transport granted by high density-gradients in the high-poloidal-beta scenario 14 , 15 . Furthermore, our experimental results show an integration of very low edge transient perturbations with the high normalized density and confinement core. The operating regime we report supports some critical requirements in many fusion reactor designs all over the world and opens a potential avenue to an operating point for producing economically attractive fusion energy.

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Fusion energy is the ultimate energy source for humanity 16 . A promising approach is a steady-state fusion reactor using magnetic confinement in the tokamak configuration 17 , 18 . With a deeper understanding of tokamak plasma physics and the development of reactor-relevant technologies, many fusion reactor designs have been proposed 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . When the ion temperature is above 13 keV (1.5 × 10 8  K) in D–T fusion reactions, the thermonuclear power density 19 P fus  =  n fuel 2 ⟨ σv ⟩ E /4 is proportional to the fuel density ( n fuel ) squared, as the change of normalized reaction rate ⟨ σv ⟩ with temperature is relatively small. Here, E is the fusion energy released per reaction. Detailed definitions of all variables mentioned in this paper can be found in Extended Data Table 1 . Therefore, to achieve attractive fusion goals, most of the recent fusion pilot plant (FPP) designs require very high plasma densities, higher than the empirical edge density limit known as the Greenwald density 11 ( n Gr ), in tokamak high-confinement mode (H-mode) plasmas 13 . The energy confinement quality, represented by the H-factor 20 (for example, H 98y2 ), is believed to be the highest leverage parameter for fusion capital cost 8 . H 98y2 is usually required to exceed the standard H-mode level ( H 98y2  = 1.0) for good fusion economy. FPP designs 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 simultaneously require 1 ≤ Greenwald fraction ( f Gr ) ≤ 1.3 and 1 ≤  H 98y2  ≤ 1.65. However, such a tokamak operating regime is an uncharted area that has never been verified in experiments.

The empirical n Gr is a density limit for the pedestal density in an H-mode plasma 21 , 22 . The pedestal is a narrow region of plasma at the edge with suppressed turbulent transport and a steep pressure gradient. When approaching n Gr at the pedestal, various unfavourable phenomena can be observed in experiments. These cause a strong decrease of the confinement quality or even a sudden, complete loss of plasma energy (disruption) 22 . A peaked core density profile is, therefore, required to achieve a line-averaged density above the pedestal density limit. Possible approaches include relying on the natural peaking at low collisionality 23 and the potential inward particle pinch 24 . The previous DIII-D experiment 24 can achieve a transient f Gr of about 1.4 with D 2 gas puffing. A large pinch velocity has been measured. H 98y2 in this case is around 1. ASDEX Upgrade experiments took a different approach by using pellet injection to improve the core fuelling. The experimental results show a transient f Gr  ≈ 1.5 with pellet injection 25 , 26 . However, the H 98y2 values in those discharges were less than 1. More examples with H 98y2  < 1 at high density are well documented 22 . As no tokamak experiment has yet attained a sustained f Gr above 1 and H 98y2 well above 1 (for example, 1.5) at the same time, experimentally verifying the desired operating regime in FPP designs is a great challenge for the magnetic confinement fusion community.

Another challenge with H-mode reactor plasmas is the very high transient heat load produced by quasi-periodic edge magnetohydrodynamic (MHD) instabilities known as type-I edge-localized-modes (ELMs). Without control, ELMs in a reactor can severely damage plasma-facing-components, for example, the first wall 27 , 28 . ELM control is an active research area and various approaches have been proposed 29 , 30 , 31 , 32 , 33 . However, compatibility among small/no ELM solutions, high density (above n Gr ) and high confinement quality ( H 98y2 well above 1, for example, 1.5) has not been demonstrated in experiments.

We report a new experimental approach for achieving a line-averaged density above n Gr . It exploits an operating regime recently established in the DIII-D tokamak that allows simultaneous f Gr  > 1.0, H 98y2  ≈ 1.5 and small ELMs and could support many existing designs for future reactors 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . The approach elevates the plasma density in the core while keeping the pedestal fraction of the Greenwald density at moderate levels (for example, f Gr,ped  ≈ 0.7), thus not violating the empirical density limit. It does so by exploiting self-organized internal transport barriers (ITBs) at large minor radius in the high poloidal-beta ( β P ) scenario 15 , 34 , 35 , 36 . More information about the high- β P research can be found in Methods . In experiments, the on-axis fraction of the Greenwald density ( f Gr,0 ) can reach up to 1.7, resulting in a line-averaged f Gr of 1.3. ITBs in the density and temperature profiles also greatly improve the energy confinement quality ( H 98y2 up to 1.8), compared to the standard H mode ( H 98y2  = 1) at the same engineering and operating parameters.

Figure 1 shows a plot of the DIII-D database and illustrates the progress made in extending the plasma operating space towards high f Gr and high H 98y2 . The 2019 high- β P experiments with impurity injection 15 have simultaneously achieved f Gr  > 1.0 and H 98y2  > 1.0. However, in these experiments, too much impurity injection also increases the radiative energy loss in the plasma core, limiting H 98y2 at high density. Of the violet diamonds in Fig. 1 , some have H 98y2  ≤ 1.2 when f Gr  ≥ 1.15. However, these results are not good enough for FPP designs. A major improvement in the 2022 DIII-D high- β P experiment used additional D 2 gas puffing (Fig. 2 ) instead of impurity injection. This approach effectively reduces the core radiation and improves H 98y2 , as shown in Fig. 1 (blue squares). Thus, this paper reports a clear experimental demonstration of an accessible operating point in an existing tokamak that can meet a few of the FPP requirements, including simultaneous f Gr  > 1 and H 98y2  ≈ 1.5. For comparison, other scenarios presented run on DIII-D have not achieved such simultaneous normalized performance (yellow circles).

More than 3,600 discharges are included. Violet diamonds show high- β P experiments performed in 2019 with impurity injection. Blue squares are the new high- β P experiments performed in 2022 without impurity injection. Yellow circles represent all other experiments performed in 2019–2022. The area shaded in orange indicates the parameter space for attractive FPP designs. Vertical and horizontal dashed lines show f Gr  = 1.0 and H 98y2  = 1.0, respectively.

a , f Gr in blue and H 98y2 in green. b , β N in blue, β P in green and q 95 in violet. c , D 2 gas puffing in feedback control in black and dedicated feedforward D 2 gas puffing in vermillion. d , Peak pedestal electron density gradient in blue and pedestal total pressure in vermillion. e , Separatrix electron density in green and ratio between pedestal electron density and separatrix electron density in violet. f – i , Profiles of electron temperature ( f ), ion temperature ( g ), electron density ( h ) and carbon density ( i ) at the time slices shown in the vertical dashed lines in a . Dots with error bars are measurements. j – l , D α data for the three periods shown in the shaded area in d . a.u., arbitrary units. m – o , Total pressure profiles at the time slices of the vermillion dots in d .

Figure 2 shows detailed data from an example discharge (190904) in 2022. The striking feature in this discharge is the dynamic synergy between energy confinement quality and plasma density. That is, H 98y2 increased along with f Gr (Fig. 2a ) until the ramping down of the heating power (Extended Data Fig. 1e ). This is opposite to the common observation of reduced energy confinement quality in higher density H modes 22 , especially for experiments close to the Greenwald density. The plasma was maintained at f Gr  > 1.0 and H 98y2  > 1.0 for about 2.2 s, which was 2.2 times the current diffusion time ( τ R ) or 24 times the energy confinement time ( τ E ). Thus, the high normalized density and confinement phase was not transient, which is imperative for application in future long-pulse FPPs. A normalized plasma pressure β N  ≈ 3.5 and β P  ≈ 2.9 was achieved at safety factor q 95  ≈ 8.5 (Fig. 2b ) with plasma current I p  = 0.73 MA and toroidal magnetic field B T  = 1.89 T, and with mixed co- and counter- I p neutral beam injection (NBI). Note that n Gr  = 6.7 × 10 19  m −3 in this discharge, close to the Greenwald density of the ITER 9 MA steady-state scenario at 7.2 × 10 19  m −3 . The dedicated D 2 gas puffing time trace is shown in vermillion in Fig. 2c . This approach ensures that there is a sufficient source of particles in the plasma, and it pushes the plasma density to a higher level, regardless of the change in the feedback gas (black line in Fig. 2c ).

Profiles of the temperature and density for electrons, deuterium (main ion) and carbon (main impurity) are shown in Fig. 2f–i and Extended Data Fig. 2a . The evolution of the on-axis densities for electrons, deuterium and carbon is displayed in Extended Data Fig. 1c . One can see that ITBs developed in all density channels. The increased deuterium density in this experiment suggests the promising application of this scenario in future FPPs, as it can attain a higher fuel density to give a higher fusion power. A related piece of experimental evidence is shown in Extended Data Fig. 1d . It is clear that with increased plasma density and energy confinement, the neutron rate, an indicator of fusion performance, increased substantially (67% higher, from 0.6 × 10 15 to 1.0 × 10 15  s −1 ) from 2 to 4.8 s, whereas the injected power (blue line in Extended Data Fig. 1e ) was almost constant. Moreover, a very mild increase of the radiated power was observed in the very-high-density phase of the experiment (Extended Data Fig. 1e ). The core radiated power as a fraction of the injected power increased from 10% to 20% as f Gr increased from 0.7 to 1.1. The edge radiation remained about 25% of the injected power. Note that for either Bremsstrahlung radiation or impurity line emission, the radiated power was roughly proportional to the electron density squared. Therefore, some increase in the radiated power was expected even with the same impurity level, when the plasma density was increased significantly. Regarding the impurity behaviour, one can see a well-developed ITB at large radius in the carbon density profiles (Fig. 2i ). Despite the ITB at large radius, the carbon density inside the ITB did not have a significant central peak, which would usually cause a large amount of core radiation and a reduction of core performance. The ratio between carbon density and electron density stayed around 4–5% during the evolution (Extended Data Fig. 2b ). This is consistent with the well-controlled radiated power in the phase with f Gr  > 1.0.

The evolution of the safety factor profile ( q -profile) is shown in Extended Data Fig. 2c . The figure shows the self-organized q -profile evolution, which reflects the change of the local bootstrap current density associated with the development of a large-radius ITB. The local minimum q ( q min ) in the outer half of the plasma was at ρ  ≈ 0.6 for around 2 τ R . q min in this discharge stayed above 2.

When a density ITB built up over time and was sustained, the total pedestal pressure at ρ  = 0.88 did not change significantly (Fig. 2d ). However, other pedestal parameters and the ELM behaviour changed. At f Gr  < 0.8, typical standard H-mode pressure profiles and typical large type-I ELMs were observed (Fig. 2j,m ). At 0.8 ≤  f Gr  < 1.0, pressure profiles with an ITB and compound ELMs emerged (Fig. 2k,n ). Finally, pressure profiles with a large ITB and small ELMs dominated the f Gr  ≥ 1.0 phase (Fig. 2l,o ). During the evolution, a decreased peak pedestal electron density ( n e,ped ) gradient, increased separatrix electron density ( n e,sep ) and decreased ratio between n e,ped and n e,sep were observed, as shown in Fig. 2d,e . These parameter evolutions are consistent with the favourable conditions needed to access the small-ELM regime discussed in the literature 29 . A more detailed modelling analysis of the pedestal for different ELM behaviours will be discussed later in this paper.

Although addressing the transient heat load is crucial, mitigating the stationary heat load is equally important for an FPP. Divertor detachment is widely considered to be a necessary solution for realizing an acceptable stationary heat load in the operation of future FPPs. Even without detachment-oriented impurity seeding, Extended Data Fig. 3 shows that the electron temperature at the divertor plates ( T e,div ) clearly reduced from over 35 eV (before 1.8 s) to 20–25 eV (1.8–2.8 s) and finally to 10–15 eV (after 2.8 s) in the f Gr  > 1.0 and H 98y2  ≈ 1.5 phase, and there were small ELMs. The lowest T e,div phase is consistent with the existence of an ITB at large radius. Although T e,div  ≤ 15 eV is not yet considered as divertor detachment (usually T e,div  < 10 eV), it already suggests that there would be mitigation of tungsten erosion under the experimental stationary heat load, if a tungsten wall had been used. Note that although the integration of full divertor detachment and high-confinement core has been achieved in previous DIII-D high- β P experiments and reported 15 , 37 , the experimental approach and the operating parameter space were both different. The previous results used impurity seeding and f Gr  ≈ 0.9, which are not sufficient for FPP designs.

Therefore, the analysed typical DIII-D high-β P discharge has demonstrated a sustained, accessible operating point in a present tokamak that integrates high normalized density and confinement quality, small ELMs and reduced divertor electron temperature, thus addressing the key requirements of FPP designs for simultaneous high-performance core and excellent core-edge integration.

To understand the physics that enables high confinement quality at high normalized density, we performed a gyro-fluid transport analysis using the TGLF code 38 on the experimental data from the discharge shown in Fig. 2 . Figure 3a,b shows the dependence of the normalized electron turbulent heat flux Q e / Q GB (where  Q GB is the Gryo-Bohm heat flux) on the fractional contribution of the density gradient to the pressure gradient ( F p  =  T ∇ n / ∇ p ) at mid-minor radius in the plasma. The gyro-fluid modelling indicates that when using either numerical approach to vary F p (constant ∇ T or constant ∇ p ), the decreasing trend of Q e for increasing F p is robust. A similar result was obtained for the ion energy transport. These results reveal an important feature in the high- β P scenario, namely that anomalous turbulent transport, which leads to poor global confinement, can be reduced with a high density gradient, that is, a high density in the core with the pedestal density maintained below n Gr . This is consistent with the experimental observation of synergy between high confinement quality and high density. If F p were increased by 30%, the normalized Q e would decrease by a factor of 2 compared with the prediction at the experimental value, when the normalized pressure gradient α MHD (approximately − q 2 / B T,unit 2 R d p /d r ) was moderate (1.13), as shown in Fig. 3a . However, the reduction of the transport can be 2–3 orders of magnitude stronger when α MHD is high (2.75) in the experimental equilibrium (Fig. 3b ). Note that this finding is also consistent with the previous nonlinear gyro-kinetic theoretical modelling 39 , which found an extreme reduction in the transport coefficient when high α MHD was combined with moderate density gradients. The underlying physics includes 1) the low drive of the ion-temperature-gradient turbulence at high density gradient (that is, there is a low ratio between the density gradient scale length and the ion temperature scale length ( η i )), and 2) less effective coupling between trapped electrons and the trapped-electron-mode turbulence owing to the much narrower turbulence eigenfunction at high α MHD .

a , Moderate α MHD case from the high- β P discharge in Fig. 2 . F p scan with the constant ∇ p approach in blue and with the constant ∇ T approach in vermillion. The experimental (Exp.) value of F p is indicated by the black arrow. b , High α MHD case from the high- β P discharge in Fig. 2 . Same colour coding as in a . c , d , Temperature ( c ) and density ( d ) profiles for the low- q 95 H-mode case analysed in e and f . Dashed lines show the radial location for transport analysis. e , f , Two-dimensional scans of normalized electron turbulent heat flux on F p and local q based on the low- q 95 H-mode data shown in c and d . Full experimental β e ( e ) and half experimental β e ( f ). The experimental data point from the low- q 95 discharge is indicated by a blue star in e .

We also applied the same gyro-fluid transport analysis to a standard H-mode discharge to reveal the requirement for realizing the favourable conditions for low transport at high density. A low- q 95 standard H-mode discharge (DIII-D 187019) with strong D 2 gas puffing and high density was investigated. Compared with the high- β P discharge discussed above, this discharge had the same heating power (9 MW), comparable line-averaged density (5.0–6.5 × 10 19  m −3 ), slightly lower β N (approximately 2.5), but much lower q 95 (4 versus 8.5). This was because of a much higher I p (1.3 versus 0.73 MA). Typical standard H-mode profiles are shown in Fig. 3c,d , which are different from the ITB profiles in Fig. 2 . Figure 3e presents the transport analysis of a two-dimensional scan on F p and local q at ρ  = 0.65. The modelling uses the experimental β e value. As illustrated by the horizontal dashed lines, the figure can be roughly divided into three regimes. At low local q , such as for the standard H-mode experimental data point, the modelling predicts high turbulent transport at high F p . This is consistent with the experimental observation of decreased H 98y2 at high density in this discharge. At medium q , transport is predicted to be almost independent of F p . Finally, low transport at high F p can be found in the high- q regime (top right corner of this figure highlighted by the blue dashed line). This example suggests that a minimum of the local q  ≈ 4.4 is required to access this regime. Note that the analysed high- β P case has local q  ≈ 5.1. However, high local q alone is insufficient to access this regime. Figure 3f indicates the importance of sufficient β e , or the plasma pressure ( β ). Note that β changes accordingly in the modelling when scanning β e . The range of the two-dimensional scan is the same. However, this scan uses half of the experimental β e in the modelling. As one can see, the results are significantly different. For most of the q values in the scan, high turbulent transport at high F p is predicted. The favourable low transport at high F p may still exist but probably requires very high local q , which is less realistic in present tokamak experiments or future machine designs.

In summary, the transport analysis suggests that the standard H-mode could access the favourable low-transport regime at high density, with the following necessary conditions: high local q and high plasma pressure β , which are two key components in the expression of α MHD . Thus, sufficient α MHD is essential for realizing the favourable operating regime. As summarized in the literature 15 , 37 , 40 , α-stabilization is considered as the primary turbulence suppression physics in the high- β P scenario, as it provides a reactor-relevant rotation-independent mechanism for high confinement 40 . On the other hand, given that β P   ∝   β N q 95 , high q 95 and high β N lead to high β P . Therefore, the high- β P scenario is naturally an excellent candidate for pursing this goal.

We performed a pedestal analysis to evaluate the pedestal stability and understand the evolution of the ELM behaviour in the high- β P discharge described in Fig. 2 . The ELITE calculations 41 shown in Fig. 4a predict the stability boundary for peeling–ballooning modes in the pedestal, for each of the three ELM states. In the type-I ELM state, the pedestal lies near the unstable ballooning region. Evolving to the small-ELM state, the experimental point moves along the ballooning boundary towards a lower pedestal pressure gradient and lower pedestal current density. Moving further away from the peeling boundary is consistent with the observation of no giant ELMs in the later phase. Modelling with BOUT++ (refs. 42 , 43 , 44 ) provides details on the instability growth rate in Fig. 4b . The dominant low- n peeling–ballooning mode was identified at n  ≈ 10, which agrees with the ELITE result. The predicted low- n growth rate is smallest for small ELMs. BOUT++ modelling also resolves high- n resistive ballooning modes near the separatrix, when considering the plasma resistivity. It is clear that the high- n separatrix modes are dominant in the small-ELM case in contrast to other results in Fig. 4b . The modelling suggests that the high- n separatrix modes played an essential role in the observation of small ELMs in high- β P plasmas.

a , b , Results for the type-I ELM in blue, the compound ELM in green and the small ELM in violet. a , Pedestal stability versus normalized pedestal current density ( y axis) and normalized pressure gradient at the pedestal peak gradient location ( x axis). j max , j sep and ⟨ j ⟩ are the maximum pedestal current density, the current density at the separatrix and the average current density in the pedestal region, respectively. Stability boundaries are shown as solid lines. Experimental points are indicated as open squares with error bars. b , Linear mode growth rate (normalized by Alfvén frequency, ω A ) at different toroidal mode numbers.

In this paper, we have extended the operating space of a tokamak plasma towards a regime with simultaneous f Gr up to 1.25 and H 98y2  ≈ 1.3–1.8, using the high- β P scenario in DIII-D. The achievement of entering this previously uncharted regime provides essential support to many attractive FPP designs all over the world. The increased deuterium density and neutron rate in the experiment confirm the promising application of this scenario for higher fusion performance in future FPPs. Unlike many previous high-density H-mode experiments, the high- β P scenario uniquely features a synergy between high confinement quality and high density, especially around the Greenwald value. We have also elucidated the important role of α-stabilization in this achievement, showing that the favourable regime of low turbulent transport at high density is predicted and achieved only at relatively high local q and high β , namely for sufficient α MHD at high β P . This successful experiment not only addresses a few of the key requirements on FPP core plasma parameters but also suggests a potential solution for core-edge integration by demonstrating sustained small ELMs together with f Gr  > 1.0 and H 98y2  > 1.0. Realizing the small-ELM regime is understood as a combination of the reduced growth rate of low- n modes and the predominance of the high- n resistive ballooning mode near the separatrix because of the decreased peak density gradient in the pedestal, increased separatrix density and high β P . During the natural small-ELM phase with a high normalized density and confinement, the plasma is close to divertor detachment, which is believed to be the most promising solution for achieving steady-state plasma–wall-interactions in FPPs 37 , 45 . The natural proximity of detachment conditions shows the potential of a fully integrated scenario with high-performance core and cool edge. As the divertor detachment was not optimized in the discussed experiment, doing so will be important work for future experiments. Note that the compatibility of the high- β P scenario with full divertor detachment has been demonstrated 37 . So far, neither a significant central peak in the density profile of the impurity (carbon) nor a significant increase in the core radiated power has been observed when the density is above the Greenwald value. Dedicated impurity transport experiments and modelling work are also under consideration for this operating regime. Fast-particle confinement is important for future FPPs. Experiments on the high- β P scenario in DIII-D usually exhibit classical fast-ion transport. More discussion of previous results is presented in ‘DIII-D high- β P experiments’ ( Methods ).

We fully appreciate that further work is needed to address other critical issues related to FPP compatibility, for example, operating with a metal wall and helium exhaust. On DIII-D, limited experiments with high- β P plasmas operating with a divertor strike point on a (temporary) ring of tungsten tiles have shown promising results, with no significant degradation of core performance. However, to fully address the compatibility with a metal wall, we are collaborating closely with the Experimental Advanced Superconducting Tokamak (EAST) and Korea Superconducting Tokamak Advanced Research programme in the development of high- β P scenarios so that we can exploit their metal wall and long-pulse operation capabilities. Long-pulse operation (over 10 s) will further address the alignment for steady-state q -profiles and pressure profiles with ITB in the high- β P scenario. With regard to the helium exhaust, several review papers give favourable conclusions for high- β P plasmas with ITBs in JT-60U 46 , 47 . The results for JT-60U high- β P ITB plasmas show that the helium density in the core was controlled well and that no helium accumulation was observed, even with helium NBI for the core helium source. Moreover, the results also emphasize the importance of helium exhaust techniques, such as pumping, for controlling the helium content in the core.

Furthermore, there has been recent activity on extending the high- β P scenario towards true long-pulse operation, including modelling work for EAST 48 , ITER 49 , 50 and FPPs under design 10 . Depending on the design philosophy of each group, the high- β P scenario can be applied in a wide range of FPP designs, from large conventional tokamaks 14 to relatively small and compact devices 9 , 10 . One example from CAT-DEMO (Case D) 9 shows a possible design point of an FPP using the high- β P scenario: R  = 4 m, R / a  = 3.1, B T  = 7 T, I p  = 8.1 MA, q 95  = 6.5, f Gr  = 1.3, f Gr,ped  = 1.0, β N  = 3.6, H 98y2  = 1.51, fusion gain Q  = 17.3 and output electric power 200 MWe. The experimental achievement and the increased understanding reported in this paper may open a potential avenue to an operating point for producing economically attractive fusion energy.

DIII-D tokamak

The DIII-D National Fusion Facility 51 is a tokamak research device operated by General Atomic in San Diego, California, for the US Department of Energy. DIII-D is the largest magnetic fusion research facility in the USA. The DIII-D programme is focused on establishing the scientific basis for the optimization of the tokamak approach to fusion energy production, in part through the development of advanced steady-state operating scenarios. The major and minor radii of DIII-D are 1.66 and 0.67 m, respectively. The toroidal field is up to 2.2 T at the magnetic axis, and the plasma current is up to 2 MA. DIII-D has four high-power NBI systems with a total power of up to 20 MW. Unique features of the DIII-D NBI systems include: (1) two horizontally rotatable beamlines, providing a capability for switching the off-magnetic axis neutral beam current drive from the co plasma-current direction to the counter plasma-current direction and (2) two vertically movable beamlines that enable the selection of on- or off-magnetic axis plasma heating and current drive in the experiment. DIII-D has six operational gyrotrons for heating the electron cyclotron and driving the current in the plasma. Each gyrotron is designed for 1 MW continuous-wave operation for several seconds at a central frequency of 110 GHz. Steerable antennas offer flexible heating and current drive methods in the plasma, including mid-plane launchers and top launchers. DIII-D is also developing a helicon system and a lower-hybrid wave system for additional radio-frequency heating and current drive capabilities. The extensive diagnostic set and the sophisticated plasma control system support various plasma experiments on DIII-D.

DIII-D high- β P experiments

Many international tokamaks have investigated high- β P scenarios 34 , 35 , 52 , 53 , 54 , 55 , 56 , 57 ever since these scenarios were first proposed 14 in 1990. High- β P experiments were explored by DIII-D 58 in the 2000s and have been extensively investigated since 2013 by the joint DIII-D/EAST research team 15 . An ITB at large minor radius is the signature of the high- β P scenario in DIII-D. The latest physical understanding for developing such an ITB is through the reduced magnetic shear and sufficient α-stabilization effect 40 , 59 (magnitude of α MHD ). As α MHD   ∝   q 2   ∝   I p −2 , a relatively low plasma current and a broader current-density profile (having a higher q value and reduced magnetic shear at large radius) are beneficial for realizing ITB. DIII-D high- β P experiments usually begin with a lower I p flat top, aiming at q 95  ≈ 10 in the first phase of the discharge. An I p ramp-up or B T ramp-down can be used in the later phase of the discharge to reduce the experimental q 95 to a desired level, for example, 6–8. Edge perturbations, such as ELMs, active gas puffing and impurity injection have experimentally been found to trigger the formation of an ITB by creating a ‘low-(magnetic) shear detour’ at large radius, which gives access to the second stability regime. Experimentally, it has been found that an empirical β P threshold of between 1.7 and 1.9 in DIII-D can sustain a strong ITB at large radius 15 . Regarding the plasma shape and configuration, a double-null configuration and an inverted ITER-similar true single-null shape have been successfully tested in experiments. Despite the high q min (over 2) in experiments, the high- β P scenario exhibits good fast-ion confinement, which is usually found to be classical. The present understanding  2, high-β steady-state scenarios on DIII-D. Phys. Plasmas 22, 055904 (2015)." href="/articles/s41586-024-07313-3#ref-CR60" id="ref-link-section-d110295379e2335">60 , 61 indicates that: (1) The high- β P plasmas have a shorter slowing-down time (because of the high density) and lower ∇ β fast , where β fast is the ratio of volume-averaged fast-ion pressure to the pressure of the toroidal magnetic field, which reduces the drive for Alfvénic modes. (2) The reverse-shear Alfvén eigenmodes are weaker or stable because the negative magnetic shear region is at higher radius, away from the peak of the fast-ion profile. Additionally, independent modelling of a high q min (over 2) ITER 8 MA steady-state scenario (not the high- β P scenario) found that there was negligible fast-ion loss because of a mismatch between the loss boundaries and the locations of the Alfvén eigenmodes 62 . A more detailed description of experimental waveform designs and a comprehensive review of the high- β P scenario development on DIII-D in the last decade can be found in a review paper 15 .

Diagnostics for profiles

A multi-pulsed high-resolution Thomson scattering system 63 , 64 was used to measure the core electron density and temperature in the DIII-D experiments. The ion temperature and the carbon density profiles were measured by a charge-exchange recombination spectroscopy system for C 6+ particles 65 . The measurements were arranged radially on the low-field side.

Statistical analysis

Our statistical analysis used experimental data from DIII-D discharges during 2019–2022. We recorded data pairs of ( f Gr , H 98y2 ) from two time slices for each discharge: (1) the highest H 98y2 and the corresponding f Gr at the same time and (2) the highest f Gr and the corresponding H 98y2 at the same time, unless the two time slices were the same. Several filters were applied: I p  ≥ 0.55 MA, d I p /d t  < 0.5 MA s −1 , P tot  ≥ 5 MW, W  ≥ 500 kJ and (d W /d t )/ P tot  ≤ 0.1. Here, W is the total energy stored by the plasma and P tot is the total heating power. A smoothing window of 40 ms was applied. The minimum and maximum H 98y2 values were truncated at 0.5 and 2.5, respectively. More than 3,600 discharges from 2019–2022 were used in the analysis.

Reconstruction of the kinetic equilibria

A multi-step workflow was used to add constraints on the pressure and plasma-current density in the equilibrium reconstruction to improve the accuracy of the reconstructed equilibrium. The workflow (for one iteration) has three parts: fitting the profile based on the existing equilibrium, calculating the power balance for the total pressure and plasma-current components, and reconstructing the equilibrium with pressure and current-density constraints. Usually, two or three iterations would be sufficient to produce high-quality equilibria for the transport study. When performing power balance calculation, NUBEAM 66 was used for the NBI-driven current and fast-ion pressure calculation, and the Sauter model 67 was used for the bootstrap current calculation. ONETWO 68 was used to create the total pressure by combining the thermal pressure and fast-ion pressure and to provide the total plasma-current density by considering the external driven current, the bootstrap current and the calculated Ohmic current and by solving the poloidal field diffusion. The equilibrium was reconstructed with the EFIT code 69 .

Gyro-fluid transport modelling

The TGLF code 38 was used to calculate the turbulent heat fluxes in the high- β P case and the low- q 95 H-mode case. This modelling used a more recent saturation rule, SAT2 (ref. 70 ). Electromagnetic effects were included. The modelling took an experimental kinetic equilibrium from one time slice and focused on one radial location, for example, ρ  = 0.65. The turbulent heat fluxes were predicted based on the local parameters of the selected time and radial location by taking contributions from several turbulent modes (low k and high k ) into account. Quasi-neutrality was maintained when scanning the local density gradient F p , meaning that the density gradients for all species changed accordingly. When scanning the local q , other quantities that are not independent of q were scanned accordingly. When scanning β e , the entire plasma β was changed accordingly, as T i / T e and n i / n e were fixed in the modelling.

Pedestal modelling

The ELITE code 41 was used to calculate the growth rate of the peeling–ballooning mode instability, which is believed to limit the achievable pedestal height by triggering ELMs when the plasma crosses the instability boundary. ELITE uses reconstructed equilibria with pressure and current constraints (kinetic equilibria) as input. On the basis of the input, a set of equilibria were generated by independently varying the edge pressure and current in a separate ELITE calculation to obtain the peeling–ballooning boundary.

The reduced three-field fluid module under the BOUT++ framework 42 , 43 , 44 was used to simulate the edge modes. The simulation evolved several physics parameters: perturbed pressure, magnetic flux and vorticity. The three-field module included not only basic peeling–ballooning physics but also non-ideal effects, such as ion diamagnetic drift, E  ×  B drift, resistivity and so on. The simulation domain in the normalized poloidal flux was 0.80 <  ψ N  < 1.05 and the grid resolution was n ψ  = 512 and n y  = 64. The kinetic equilibria were used as input. The Spitzer–Härm resistivity η Sp was used by considering realistic plasma kinetic profiles. The hyper-resistivity was taken as a constant value η H  = 10 −16 in the generalized Ohm’s law for current diffusion. The radial electric field ( E r ) profile calculated from the ion momentum balance equation was used in the simulation.

Data availability

Raw data were generated by the DIII-D team. The data that support the findings of this study are available from the corresponding author upon request.

Code availability

The computational codes used in the analysis of this paper are managed by General Atomics. They are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank the DIII-D team and the joint DIII-D/EAST research team for their support in the machine operation and data analysis and modelling. We express our deep gratitude to E. J. Strait of General Atomics for his suggestions on improving the manuscript text. Additionally, we thank M. T. Kotschenreuther of ExoFusion for insightful discussions on transport physics, which contributed significantly to the development of this work. This work is supported by the US Department of Energy (Grant Nos. DE-SC0010685, DE-FC02-04ER54698, DE-AC52-07NA27344, DE-FG02-04ER54761, DE-AC02-09CH11466 and DE-SC0016154). This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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S. Ding, A. M. Garofalo, H. Q. Wang, D. B. Weisberg, Z. Y. Li, X. Jian, D. Eldon, I. S. Carvalho, T. Odstrčil, A. W. Hyatt, T. H. Osborne & J. McClenaghan

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Contributions

S.D. and A.M.G. conceived the experimental idea regarding DIII-D. S.D. led the experimental demonstration and conducted the DIII-D database analysis, equilibrium reconstructions, gyro-fluid transport modelling and writing the manuscript. A.M.G. provided guidance during the writing of the manuscript. H.Q.W. performed the pedestal stability and divertor condition analysis and provided the target low-q 95 discharge for analysis. Z.Y.L. performed the pedestal modelling. S.D., A.M.G., H.Q.W., X.J., L.W., X.Z.G., J.P.Q., J.H., J.M. and C.T.H. participated in designing and executing the experiments. D.B.W., D.E., B.S.V., A.M., Q.M.H., I.S.C., T.O., A.W.H., T.H.O. and J.M.H. participated in executing the experiments. T.O. conducted the impurity analysis. All authors reviewed the manuscript. A.M.G. and C.T.H. polished the manuscript.

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Correspondence to S. Ding .

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Extended data figures and tables

Extended data fig. 1 additional time histories for diii-d # 190904..

(a) Plasma current in blue and toroidal field in green; (b) Line-averaged density in blue and stored energy in green; (c) On-axis electron density in blue, on-axis deuterium density in green and on-axis carbon density in violet; (d) Measured neutron rate; (e) Injected NBI power in blue, measured total radiated power in green and core radiated power in violet.

Extended Data Fig. 2 Additional profiles for DIII-D # 190904.

Deuterium density profiles in (a), ratio between carbon density and electron density in (b) and safety factor profiles (q-profiles) in (c). Different color indicates the time slice shown in Fig. 2a . Additionally, profiles for a pre-ITB time slice (1.89 s) shown in gray dashed line are added.

Extended Data Fig. 3 Spatial and temporal evolution of electron temperature at the divertor plates, measured by Langmuir probes.

ψ N is normalized poloidal flux. ψ N  < 1.0 locates within the private flux region. Color coding shows the measured T e,div . Dashed lines indicate the actual positions of Langmuir probes.

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Ding, S., Garofalo, A.M., Wang, H.Q. et al. A high-density and high-confinement tokamak plasma regime for fusion energy. Nature (2024). https://doi.org/10.1038/s41586-024-07313-3

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Nuclear fusion experiment overcomes two key operating hurdles

Two important barriers to a stable, powerful fusion reaction have been leapt by an experiment in a small tokamak reactor, but we don’t yet know if the technique will work in larger devices

By Matthew Sparkes

24 April 2024

Inside the DIII-D tokamak fusion reactor

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A nuclear fusion reaction has overcome two key barriers to operating in a “sweet spot” needed for optimal power production: boosting the plasma density and keeping that denser plasma contained. The milestone is yet another stepping stone towards fusion power, although a commercial reactor is still probably years away.

One of the main avenues being explored in efforts to achieve fusion power is using tokamak reactors. These have a doughnut-shaped chamber where plasma hotter than the surface of our sun is contained by vast magnets .

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It had been thought that there was a point – known as the Greenwald limit – above which you couldn’t raise the density of the plasma without it escaping the clutches of the magnets, potentially damaging your reactor. But raising density is crucial to increasing output, as experiments have shown that the output of tokamak reactors rises proportionally with the square of the fuel density.

Now, Siye Ding at General Atomics in San Diego, California, and his colleagues have shown that there is a way to raise the plasma density, and proved that it can be stable, by running the DIII-D National Fusion Facility tokamak reactor for 2.2 seconds with an average density that is 20 per cent above the Greenwald limit. While this barrier has been passed before, with less stability and for shorter durations, this experiment crucially also ran with a metric known as H98(y,2) of above 1.

H98(y,2) is a complex blend of measurements and values that shows how well the plasma is contained by the magnets, says Gianluca Sarri at Queen’s University Belfast, with a value of 1.0 or above signifying that plasma is being successfully held in place.

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“You’re now starting to show some sort of stable operation where you can consistently be in the sweet spot,” says Sarri. “This was done in a small machine. If you take these results and extrapolate it to a larger machine… that is expected to put you in a situation where gain and significant power production can be achieved over a significant amount of time.”

The DIII-D experiment relied on a mix of approaches that aren’t themselves new, says Sarri, but together seem to have created a promising approach. The team used higher density in the core of the doughnut shaped plasma, to increase output, while allowing it to dip at the edges nearest the containment vessel to drop below the Greenwald limit, therefore avoiding any plasma escape. They also puffed deuterium gas into the plasma to calm reactions in specific spots.

DIII-D’s plasma chamber has an outside radius of just 1.6 metres, and isn’t yet know whether the same method would work for ITER, the next-generation tokamak under construction in France, which will have a radius of 6.2 metres and is expected to create plasma as soon as 2025.

UK nuclear fusion reactor sets new world record for energy output

“These plasmas are very complicated,” says Sarri. “A small change in conditions leads to a big change in behaviour. And experimentally it has been more like a trial-and-error sort of approach, where you try many different configurations and basically see which one is best. It’s all about forcing the plasma to do something that is completely against its nature, that it really doesn’t want to do.”

Ding says the experiment bodes well for the future of fusion power. “Many reactor designs require simultaneous high confinement and high density. Experimentally, this is the first time it is realised,” he says. “The next step is expensive, and currently research is going in many different directions. My hope is that this paper will help focus the efforts worldwide.”

The work is another step towards a practical fusion power plant, says Sarri, but nobody should expect to see a commercial reactor in the next five, or even 10, years.

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Nature DOI: 10.1038/s41586-024-07313-3

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Nuclear Power in the World Today

• The first commercial nuclear power stations started operation in the 1950s.
• Nuclear energy now provides about 10% of the world's electricity from about 440 power reactors.
• Nuclear provides about one-quarter of the world’s low-carbon electricity.
• Nuclear is the world's second largest source of low-carbon power (26% of the total in 2020). 
• Over 50 countries utilize nuclear energy in about 220 research reactors. In addition to research, these reactors are used for the production of medical and industrial isotopes, as well as for training.

Nuclear technology uses the energy released by splitting the atoms of certain elements. It was first developed in the 1940s, and during the Second World War research initially focused on producing bombs. In the 1950s attention turned to the peaceful use of nuclear fission, controlling it for power generation. For more information, see page on  History of Nuclear Energy .

Civil nuclear power can now boast around 20,000 reactor years of operating experience, and nuclear power plants are operational in 31 countries (plus Taiwan) worldwide. In fact, through regional transmission grids, many more countries depend in part on nuclear-generated power, particularly in Europe.

When the commercial nuclear industry began in the 1960s, there were clear boundaries between the industries of the East and West. Today, the nuclear industry is characterized by international commerce. A reactor under construction in Asia today may have components supplied from South Korea, Canada, Japan, France, Germany, Russia, and other countries. Similarly, uranium from Australia or Namibia may end up in a reactor in the UAE, having been converted in France, enriched in the Netherlands, deconverted in the UK and fabricated in South Korea.

The uses of nuclear technology extend well beyond the provision of low-carbon energy. It helps control the spread of disease, assists doctors in their diagnosis and treatment of patients, and powers our most ambitious missions to explore space. These varied uses position nuclear technologies at the heart of the world's efforts to achieve sustainable development. For more information, see page on Nuclear Energy and Sustainable Development .

In 2022 nuclear plants supplied 2545 TWh of electricity, down from 2653 TWh in 2021.

Figure 1: Nuclear electricity production (source: World Nuclear Association, IAEA PRIS)

Figure 2: World electricity production by source 2020 (source: International Energy Agency)

Thirteen countries in 2022 produced at least one-quarter of their electricity from nuclear. France gets up to around 70% of its electricity from nuclear energy, while Ukraine, Slovakia, Belgium and Hungary get about half from nuclear. Japan was used to relying on nuclear power for more than one-quarter of its electricity and is expected to return to somewhere near that level.

Figure 3: Nuclear generation by country 2021 (source: IAEA PRIS)

Developments in 2024 

Grid connections

Construction starts

Reactor shutdowns

World overview

All parts of the world are involved in nuclear power development, and some examples are outlined below.

For up-to-date data on operable, under construction and planned reactors worldwide, see table of  World Nuclear Power Reactors & Uranium Requirements .

For detailed country-level information, see the  Country Profiles section of World Nuclear Association's Information Library.

North America

Canada  has 19 operable nuclear reactors, with a combined net capacity of 13.6 GWe. In 2022, nuclear generated 13.6% of the country's electricity.

All but one of the country's 19 nuclear reactors are sited in Ontario. The four units at Darlington and units 1-6 at Bruce are undergoing lifetime extension refurbishment.

The programme will extend the operating lifetimes by 30-35 years. Similar refurbishment work enabled Ontario to phase out coal in 2014, achieving one of the cleanest electricity mixes in the world.

Mexico has two operable nuclear reactors, with a combined net capacity of 1.6 GWe. In 2022, nuclear generated 4.5% of the country's electricity.

The USA has 94 operable nuclear reactors, with a combined net capacity of 97.0 GWe. In 2022, nuclear generated 18.2 % of the country's electricity.

Two AP1000 reactors are now in operation at Vogtle. There had been a further two AP1000 reactors under construction at VC Summer, but these were cancelled. One of the reasons for the hiatus in new build in the USA to date has been the extremely successful evolution in maintenance strategies. Over the last 15 years, improved operational performance has increased utilization of US nuclear power plants, with the increased output equivalent to 19 new 1000 MWe reactors being built.

The number of operable reactors has reduced in recent years, from a peak of 104 in 2012. Early closures have been brought on by a combination of factors including cheap natural gas, market liberalization, over-subsidy of renewable sources, and political campaigning.

South America

Argentina has three reactors, with a combined net capacity of 1.6 GWe. In 2022, the country generated 5.4% of its electricity from nuclear. 

Brazil has two reactors, with a combined net capacity of 1.9 GWe. In 2022, nuclear generated 2.5% of the country's electricity.  

West & Central Europe

Belgium  has five operable nuclear reactors, with a combined net capacity of 5.9 GWe. In 2022, nuclear generated 46.4% of the country's electricity.

Finland  has five operable nuclear reactors, with a combined net capacity of 4.4 GWe. In 2022, nuclear generated 35.0% of the country's electricity. Finland's fifth reactor – a 1600 MWe (net) EPR – was connected to the grid in March 2022.

France has 56 operable nuclear reactors, with a combined net capacity of 61.4 GWe. In 2022, nuclear generated 62.5% of the country's electricity.

Government policy, set under a former administration in 2014, aimed to reduce nuclear's share of electricity generation to 50% by 2025. This target was delayed in 2019 to 2035, before being abandoned in 2023.

One reactor is currently under construction in France – a 1750 MWe EPR at Flamanville.

The Netherlands has a single operable nuclear reactor, with a net capacity of 0.5 GWe. In 2022, nuclear generated 3.3% of the country's electricity.

Spain  has seven operable nuclear reactors, with a combined net capacity of 7.1 GWe. In 2022, nuclear generated 20.3% of the country's electricity.

Sweden  has six operable nuclear reactors, with a combined net capacity of 6.9 GWe. In 2022, nuclear generated 29.4% of the country's electricity.

The country is closing down some older reactors, but has invested heavily in operating lifetime extensions and uprates.

Switzerland  has four operable nuclear reactors, with a combined net capacity of 3.0 GWe. In 2022, nuclear generated 36.4% of the country's electricity.

The United Kingdom  has 9 operable nuclear reactors, with a combined net capacity of 5.9 GWe. In 2022, nuclear generated 14.2% of the country's electricity.

A UK government energy paper in mid-2006 endorsed the replacement of the country’s ageing fleet of nuclear reactors with new nuclear build. Construction has commenced on the first of a new-generation of plants.

Central and East Europe, Russia

Armenia  has a single nuclear power reactor with a net capacity of 0.4 GWe. In 2022, nuclear generated 31.0% of the country's electricity. 

Belarus  has two operable nuclear power reactors, with a combined net capacity of 2.2 GWe. Almost all the rest of the country's electricity is produced from natural gas. In 2021, nuclear generated 14.1% of the country's electricity.

Bulgaria  has two operable nuclear reactors, with a combined net capacity of 2.0 GWe. In 2022, nuclear generated 32.6% of the country's electricity.

The Czech Republic  has six operable nuclear reactors, with a combined net capacity of 3.9 GWe. In 2022, nuclear generated 36.7% of the country's electricity.

Hungary  has four operable nuclear reactors, with a combined net capacity of 1.9 GWe. In 2022, nuclear generated 47.0% of the country's electricity.

Romania  has two operable nuclear reactors, with a combined net capacity of 1.3 GWe. In 2022, nuclear generated 19.4% of the country's electricity.

Russia has 36 operable nuclear reactors, with a combined net capacity of 26.8 GWe. In 2022, nuclear generated 19.6% of the country's electricity.

A government decree in 2016 specified construction of 11 nuclear power reactors by 2030, in addition to those already under construction. At the start of 2023, Russia had three reactors under construction, with a combined capacity of 2.7 GWe.

The strength of Russia's nuclear industry is reflected in its dominance of export markets for new reactors. The country's national nuclear industry is currently involved in new reactor projects in Belarus, China, Hungary, India, Iran and Turkey, and to varying degrees as an investor in Algeria, Bangladesh, Bolivia, Indonesia, Jordan, Kazakhstan, Nigeria, South Africa, Tajikistan and Uzbekistan among others.

Slovakia  has four operable nuclear reactors, with a combined net capacity of 1.8 GWe. In 2022, nuclear generated 59.2% of the country's electricity. A further two units are under construction.

Slovenia  has a single operable nuclear reactor with a net capacity of 0.7 GWe. In 2022, Slovenia generated 42.6% its electricity from nuclear.

Ukraine has 15 operable nuclear reactors, with a combined net capacity of 13.1 GWe. In 2022, nuclear generated an estimated 58.7 TWh of electricity.

Turkey commenced construction of its first nuclear power plant in April 2018, with start of operation expected in 2024.

Bangladesh  started construction on the first of two planned Russian VVER-1200 reactors in 2017. Construction on the second started in 2018. It plans to have the first unit in operation by 2024. The country currently produces virtually all of its electricity from fossil fuels.

China has 55 operable nuclear reactors, with a combined net capacity of 53.3 GWe. In 2022, nuclear generated 5.0% of the country's electricity.

The country continues to dominate the market for new nuclear build, with 25 reactors under construction at the end of October 2023. In 2018 China became the first country to commission two new designs – the AP1000 and the EPR. China is marketing the Hualong One for export, a largely indigenous reactor design.

The strong impetus for developing new nuclear power in China comes from the need to improve urban air quality and reduce greenhouse gas emissions.

India  has 23 operable nuclear reactors, with a combined net capacity of 7.4 GWe. In 2022, nuclear generated 3.1% of the country's electricity.

The Indian government is committed to growing its nuclear power capacity as part of its massive infrastructure development programme. The government in 2010 set an ambitious target to have 14.6 GWe nuclear capacity online by 2024. At the end of October 2023 eight reactors were under construction in India, with a combined capacity of 6.7 GWe.

Japan  has 33 operable nuclear reactors, with a combined net capacity of 31.7 GWe. As of October 2023, 11 reactors had been brought back online, with a further 16 in the process of restart approval, following the  Fukushima accident in 2011. In the past, 30% of the country's electricity has come from nuclear; in 2022, the figure was just 6.1%.

South Korea has 26 operable nuclear reactors, with a combined net capacity of 25.8 GWe. In 2022, nuclear generated 6.1% of the country's electricity.

The country has three new reactors under construction domestically and is constructing a four-unit plant in the United Arab Emirates.

Pakistan  has six operable nuclear reactors, with a combined net capacity of 3.3 GWe. In 2022, nuclear generated 16.2% of the country's electricity.

Egypt started construction in July 2022 of the first of four Russian-designed VVER units to be built at the El Dabaa site on the Mediterranean coast. The second unit began construction in November 2022, the third in May 2023, and the fourth in January 2024. All four reactors are expected to be operational by 2030.

South Africa has two operable nuclear reactors, with a combined net capacity of 1.9 GWe, and is the only African country currently producing electricity from nuclear. In 2022, nuclear generated 4.9% of the country's electricity. South Africa remains committed to plans for further capacity, but financing constraints are significant.

Middle East

Iran  has a single operable nuclear reactor with a net capacity of 0.9 GWe. In 2021, nuclear generated 1.7% of the country's electricity. A second Russian-designed VVER-1000 unit is under construction.

The United Arab Emirates  has three operable nuclear reactors with a capacity of 6.8 GWe. A fourth unit is under construction at the same plant (Barakah). In 2022, nuclear generated 6.8% of the country's electricity.

Emerging nuclear energy countries

As outlined above, Bangladesh, Turkey and the United Arab Emirates are all constructing their first nuclear power plants. A number of other countries are moving towards use of nuclear energy for power production. For more information, see page on  Emerging Nuclear Energy Countries .

Improved performance from existing reactors

The performance of nuclear reactors has improved substantially over time. Over the last 40 years the proportion of reactors reaching high capacity factors has increased significantly.

Figure 4: Long-term trends in capacity factors (source: World Nuclear Association, IAEA PRIS)

It is also notable that there is no significant age-related trend in the mean capacity factor for reactors over the last five years.

Figure 5: Mean capacity factor 2018-2022 by age of reactor (source: World Nuclear Association, IAEA PRIS)

Need for new generating capacity

There is a clear need for new generating capacity around the world, both to replace old fossil fuel units, especially coal-fired ones, which emit large amounts of carbon dioxide, and to meet increased demand for electricity in many countries. In 2021, 61% of electricity was generated from the burning of fossil fuels. Despite the strong support for, and growth in, intermittent renewable electricity sources in recent years, the fossil fuel contribution to power generation has not changed significantly in the last 15 years or so (66.5% in 2005).

The OECD International Energy Agency publishes annual scenarios related to energy. In its World Energy Outlook 2023 1 there is an ambitious ‘Net Zero Emissions by 2050 Scenario' (NZE), which "maps out a way to achieve a 1.5°C stablisation in the rise in global average temperatures, alongside universal access to modern energy by 2030." The NZE in WEO 2023 sees nuclear capacity increase to 916 GWe by 2050.

Other nuclear reactors

In addition to commercial nuclear power plants, there are about 220 research reactors operating in over 50 countries, with more under construction. As well as being used for research and training, many of these reactors produce medical and industrial isotopes.

The use of reactors for marine propulsion is mostly confined to the major navies where it has played an important role for five decades, providing power for submarines and large surface vessels. Over 160 ships, mostly submarines, are propelled by some 200 nuclear reactors and over 13,000 reactor years of experience have been gained with marine reactors. Russia and the USA have decommissioned many of their nuclear submarines from the Cold War era.

Russia also operates a fleet of large nuclear-powered icebreakers and has more under construction. It has also connected a floating nuclear power plant with two 32 MWe reactors to the grid in the remote arctic region of Pevek. The reactors are adapted from those powering icebreakers.

For more information see page on The Many Uses of Nuclear Technology . 

Notes & references

References .

1. OECD International Energy Agency, World Energy Outlook 2023  [ Back ] 2. OECD International Energy Agency Statistics [ Back ]

General references

World Nuclear Association, World Nuclear Performance Report 2023

Related information

Scientists Fused Two Methods to Contain Nuclear Plasma

This hybrid approach could finally fuel the drive for clean energy.

• New research suggests two methods could work together to better contain nuclear plasma.
• Nuclear fusion energy is a “holy grail” idea, but we have a long way to go to make it happen.
• No one to date has demonstrated a reactor that’s even close to generating net energy.

Nuclear fusion is one of the holy grails of modern science. The nuclear reaction involves forcing particles together, which releases an enormous amount of energy. It’s the same physics eruption that powers many nuclear weapons. That’s compared to our current ecosystem of nuclear fission reactors, where particles are instead divided. Fission reactions happen at much lower temperatures and require much less shielding or elaborate containment, but they also produce much less energy than proponents say a nuclear fusion power plant could produce.

The problem is—and it’s a big one—successful nuclear fusion for energy is very, very far away. Today’s milestones are often reported without the fact that these facilities take an enormous amount of energy in order to get up to productive speed. Ignoring that high energy cost creates a misleading media climate, like reporting a company’s gross income as its profits. Fusion reactors also can’t run for more than a short time because of plasma containment.

It’s a scientific puzzle to contain a swirling mass that’s as hot as a star, and which could easily melt anything on Earth that it ever touched. These reactors include General Atomic's DIII-D tokamak, operated on behalf of the U.S. Department of Energy, which is the subject of this research study. A tokamak uses an outer shell of powerful magnets to contain a stream of plasma. Physicists have spent decades working on magnetic fields to contain the plasma. Still, the edges are vulnerable to edge-localized modes (ELMs), while the interior of the donut-shaped plasma stream can experience hot spots from the magnetic field itself.

Two leading methods to safely contain plasma are electron cyclotron current drive (ECCD) and resonant magnetic perturbations (RMP). ECCD involves heating electrons in one direction to reduce collisions with electrons going the other direction. This reduces chaotic reactions within the stream in order to keep plasma better confined. RMPs are “ spiraling rippled magnetic fields ” that can be used on the surface to calm edge nodes, with hot spots, or magnetic islands, as a side effect.

Using both methods together, scientists believe they can use RMP to bring edge nodes into line and then ECCD to “heal” the resulting islands. ECCD, which is done using microwaves, was thought to be too potentially damaging to nuclear fusion reactor equipment. Lead author Qiming Hu of PPPL said in a press release that this isn’t the case. Hu specializes in ELM control and is part of another recent publication on modulating magnetic fields to reduce them in Korea’s KSTAR tokamak reactor.

“We’ve shown that it’s doable, and we’ve demonstrated the flexibility of the approach. This might open new avenues for designing future devices,” he said.

Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all. 

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