A high-density and high-confinement tokamak plasma regime for fusion energy.pdf
Nature  |  Vol 629  |  16 May 2024  |  555 Article A high-density and high-confinement  tokamak plasma regime for fusion energy S. Ding1 ✉, A. M. Garofalo1, H. Q. Wang1, D. B. Weisberg1, Z. Y. Li1, X. Jian1, D. Eldon1,  B. S. Victor2, A. Marinoni3,4, Q. M. Hu5, I. S. Carvalho1, T. Odstrčil1, L. Wang6, A. W. Hyatt1,  T. H. Osborne1, X. Z. Gong6, J. P. Qian6, J. Huang6, J. McClenaghan1, C. T. Holcomb2 & J. M. Hanson7 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 energy1,2. To reach the goal of an  economical reactor, most tokamak reactor designs3–10 simultaneously require  reaching a plasma line-averaged density above an empirical limit—the so-called  Greenwald density11—and attaining an energy confinement quality better than the  standard high-confinement mode12,13. 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 scenario14,15. 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. Fusion energy is the ultimate energy source for humanity16. A promis- ing approach is a steady-state fusion reactor using magnetic confine- ment in the tokamak configuration17,18. With a deeper understanding  of tokamak plasma physics and the development of reactor-relevant  technologies, many fusion reactor designs have been proposed3–10.  When the ion temperature is above 13 keV (1.5 × 108 K) in D–T fusion  reactions, the thermonuclear power density19 Pfus = nfuel 2⟨σv⟩E/4 is pro- portional to the fuel density (nfuel) 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 vari- ables 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 density11 (nGr),  in tokamak high-confinement mode (H-mode) plasmas13. The energy  confinement quality, represented by the H-factor20 (for example, H98y2),  is believed to be the highest leverage parameter for fusion capital  cost8. H98y2 is usually required to exceed the standard H-mode level  (H98y2 = 1.0) for good fusion economy. FPP designs3–10 simultaneously  require 1 ≤ Greenwald fraction (fGr) ≤ 1.3 and 1 ≤ H98y2 ≤ 1.65. However,  such a tokamak operating regime is an uncharted area that has never  been verified in experiments. The empirical nGr is a density limit for the pedestal density in an  H-mode plasma21,22. The pedestal is a narrow region of plasma at the  edge with suppressed turbulent transport and a steep pressure gra- dient. When approaching nGr 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, there- fore, required to achieve a line-averaged density above the pedestal  density limit. Possible approaches include relying on the natural peak- ing at low collisionality23 and the potential inward particle pinch24. The  previous DIII-D experiment24 can achieve a transient fGr of about 1.4 with  D2 gas puffing. A large pinch velocity has been measured. H98y2 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 fGr ≈ 1.5 with pellet injection25,26. However, the  H98y2 values in those discharges were less than 1. More examples with  https://doi.org/10.1038/s41586-024-07313-3 Received: 25 July 2023 Accepted: 14 March 2024 Published online: 24 April 2024 Open access  Check for updates 1General Atomics, San Diego, CA, USA. 2Lawrence Livermore National Laboratory, Livermore, CA, USA. 3University of California San Diego, La Jolla, CA, USA. 4Plasma Science and Fusion Center,  Massachusetts Institute of Technology, Cambridge, MA, USA. 5Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA. 6Institute of Plasma Physics, Chinese Academy of  Sciences, Hefei, China. 7Department of Applied Mathematics and Applied Physics, Columbia University, New York, NY, USA. ✉e-mail: dingsiye@fusion.gat.com 556  |  Nature  |  Vol 629  |  16 May 2024 Article H98y2 < 1 at high density are well documented22. As no tokamak experi- ment has yet attained a sustained fGr above 1 and H98y2 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 magnetohydro- dynamic (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 wall27,28. ELM con- trol is an active research area and various approaches have been pro- posed29–33. However, compatibility among small/no ELM solutions, high  density (above nGr) and high confinement quality (H98y2 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 nGr. It exploits an operating regime recently established  in the DIII-D tokamak that allows simultaneous fGr > 1.0, H98y2 ≈ 1.5 and  small ELMs and could support many existing designs for future reac- tors3–10. The approach elevates the plasma density in the core while  keeping the pedestal fraction of the Greenwald density at moderate  levels (for example, fGr,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 (βP) scenario15,34–36.  More information about the high-βP research can be found in Methods.   In experiments, the on-axis fraction of the Greenwald density (fGr,0) can  reach up to 1.7, resulting in a line-averaged fGr of 1.3. ITBs in the density  and temperature profiles also greatly improve the energy confinement  quality (H98y2 up to 1.8), compared to the standard H mode (H98y2 = 1) at  the same engineering and operating parameters. Figure 1 shows a plot of the DIII-D database and illustrates the pro- gress made in extending the plasma operating space towards high  fGr and high H98y2. The 2019 high-βP experiments with impurity injec- tion15 have simultaneously achieved fGr > 1.0 and H98y2 > 1.0. However,  in these experiments, too much impurity injection also increases the  radiative energy loss in the plasma core, limiting H98y2 at high density.  Of the violet diamonds in Fig. 1, some have H98y2 ≤ 1.2 when fGr ≥ 1.15.  However, these results are not good enough for FPP designs. A major  improvement in the 2022 DIII-D high-βP experiment used additional  D2 gas puffing (Fig. 2) instead of impurity injection. This approach  effectively reduces the core radiation and improves H98y2, 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  fGr > 1 and H98y2 ≈ 1.5. For comparison, other scenarios presented run on  DIII-D have not achieved such simultaneous normalized performance  (yellow circles). 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, H98y2  increased along with fGr (Fig. 2a) until the ramping down of the heat- ing power (Extended Data Fig. 1e). This is opposite to the common  observation of reduced energy confinement quality in higher density  H modes22, especially for experiments close to the Greenwald den- sity. The plasma was maintained at fGr > 1.0 and H98y2 > 1.0 for about  2.2 s, which was 2.2 times the current diffusion time (τR) or 24 times  the energy confinement time (τE). 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  βN ≈ 3.5 and βP ≈ 2.9 was achieved at safety factor q95 ≈ 8.5 (Fig. 2b) with  plasma current Ip = 0.73 MA and toroidal magnetic field BT = 1.89 T, and  with mixed co- and counter-Ip neutral beam injection (NBI). Note that  nGr = 6.7 × 1019 m−3 in this discharge, close to the Greenwald density of  the ITER 9 MA steady-state scenario at 7.2 × 1019 m−3. The dedicated D2  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, deute- rium (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 prom- ising 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 neu- tron rate, an indicator of fusion performance, increased substantially  (67% higher, from 0.6 × 1015 to 1.0 × 1015 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 fGr 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 den- sity 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 fGr > 1.0. Attractive FPP designs DIII-D data; 3,600+ discharges 0 0.5 1.0 1.5 2.0 2.5 0 0.5 1.0 1.5 fGr 2019–2022 non-high-EP experiments 2022 high-EP experiments without impurity injection 2019 high-EP experiments with impurity injection H98y2 Fig. 1 | Database of H98y2 and fGr for DIII-D discharges. More than 3,600  discharges are included. Violet diamonds show high-βP experiments  performed in 2019 with impurity injection. Blue squares are the new high-βP  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 fGr = 1.0 and H98y2 = 1.0, respectively. Article 0 2 4 6 DIII-D # 190904 nD (1019 m-3) a 0 2 4 6 8 10 0.00 0.04 0.06 0.02 nC/ne b Safety factor c 0.0 0.2 0.4 0.6 0.8 1.0  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. ψN is  normalized poloidal flux. ψN < 1.0 locates within the private flux region. Color  coding shows the measured Te,div. Dashed lines indicate the actual positions of  Langmuir probes. Article Extended Data Table 1 | Terminology
Title: A high-density and high-confinement tokamak plasma regime for fusion energy ​
Authors: S. Ding, A. M. Garofalo, H. Q. Wang, D. B. Weisberg, Z. Y. Li, X. Jian, D. Eldon, B. ​ S. Victor, A. Marinoni, Q. M. Hu, I. S. Carvalho, T. Odstrčil, L. Wang, A. W. Hyatt, T. H. Osborne, X. ​ Z. Gong, J. P. Qian, J. Huang, J. McClenaghan, C. T. Holcomb, J. M. Hanson ​
Publisher: Nature
Date: 2024-04-24 00:00:00
Abstract: 
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. ​ To reach the goal of an economical reactor, most tokamak reactor designs simultaneously require reaching a plasma line-averaged density above an empirical limit—the so-called Greenwald density—and attaining an energy confinement quality better than the standard high-confinement mode. ​ 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. ​ 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. ​ 