7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Physics Basis Development for Spherical Torus p-11B Plasma Fusion

Y.-K.M. Peng1, B. Liu1, Y.J. Shi1, J.Q. Dong1, D. Guo1, W.J. Liu1, A. Ishida1, D. Wu2, S.J. Liu3, Y.C. Liu3, Z.
Lee1, D. Luo1, M.Z. Tan1, X. Xiao1, G.C. Zhao1, B.S. Yuan1, T.T. Sun1, X.M. Song1, X. Gu1, L.L. Dong1, X.C.
Jiang1, M.Y. Wang1, H.S. Xie1, M.S. Liu1 and the ENN Fusion Technology Research Team11Hebei Key Laboratory of Compact Fusion and ENN Science and Technology Development Co., Ltd, Langfang, Hebei
065001 China, 2 Key Laboratory for Laser Plasmas and School of Physics and Astronomy, and Collaborative Innovation
Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240 China, 3 Institute for Fusion Theory andSimulation, School of Physics, Zhejiang University, Hangzhou 310058 Chinae-mail (speaker): pengyuankai@enn.cn

Research toward commercial p-11B fusion
power in a Spherical Torus (ST) [1] takes advantageof the aneutronic reaction to permit a relativelyslender toroidal field coil center-post, and retain highperformance (e.g., plasma ), compact size andmodular design [2]. Compared to the DT tokamak,the challenges of reaching ~100keV plasma
temperatures, sufficient densities for adequate fusion
power, and high energy confinement times must beovercome. A mid-size ST p-B experiment, EXL-50,
was initiated at ENN in October 2018 to explore thephysics of ECRH current initiation, ramp-up, heating
and sustainment without a central solenoid [3].
Progress in physics basis so far included an
improved equilibrium model [4] for p-B plasmas toreproduce the observed plasma conditions [5,6],
which contain populations of un-equilibrated suprathermal (~200keV) electrons of very low densities(~several1016/m3) [7,8]. Under ECRH power, theseplasmas exhibit transitions (Figure) to a quiescentstate. Transition in ST p-B plasmas becomes a viable
research goal aimed at improving ion turbulence andconfinement, given strong auxiliary and/or internalfusion alpha particle heating [9].
These transitions are likely to change the ST pB plasma properties of equilibrium; supra-thermal
particle kinetics; macroscopic stability; neoclassicaltransport; microscopic turbulence and transport
including isotope effects; heating and current drive
via ECRH, LHCD, ICRH, NBI and the fusionalphas; plasma-material interactions of limiter anddivertor configurations involving boron; and charge
separation mechanisms at the plasma edge.
Upcoming research will develop a p-11B physics
basis toward igniting the plasma and allowing thecentralized fusion  heating and auxiliary heating toexceed the local Bremsstrahlung radiation andtransport losses, with margins to reach fusion burn.
The EXL-50U [3,10] is being built to raise in
2024 its operational capabilities to test the p-B
plasma properties at 0.5MA current, several-keV
central temperatures, and ~1020/m3 central densities.
This will provide opportunities for addressing newphysics issues encountered. The EHL-2 [1] will

follow to produce plasmas of 3MA current and 3T
toroidal field at 1.05m major radius, to achieve
substantial p-11B fusion reactions for the first time.
Research aimed at raising plasma performanceand improving the physics basis of these new ST p11
B experiments [1,3,10] will be reviewed, including
new performance analysis based on the open,
thermally un-equilibrated plasma system model [11].
References[1] M.S. Liu and H.S. Xie, ENN's Roadmap for ProtonBoron Fusion Based on Spherical Torus, to be presented
at the FEDC2023, London, UK.
[2] STEP, https://step.ukaea.uk/.
[3] Y. Shi et al., to present at the FEDC2023, London, UK
https://conferences.iaea.org/event/316/contributions/27889/.
[4] A. Ishida et al., 2021 Phys. Plasmas 28 032503
https://doi.org/10.1063/5.0027718.
[5] Y. Shi et al., 2022 Nucl. Fusion 62 086047
https://doi.org/10.1088/1741-4326/ac71b6.
[6] D. Guo et al., 2022 PPCF 64 055009
https://doi.org/10.1088/1361-6587/ac5a08.
[7] D. Choi et al., 2020 PPCF 62 115012
https://doi.org/10.1088/1361-6587/abb0d3.
[8] T. Maekawa et al., 2023 Nucl. Fusion 63 076014
https://doi.org/10.1088/1741-4326/acd2eb.
[9] H. Han et al. 2022 Nature 609, 269–275
https://doi.org/10.1038/s41586-022-05008-1.
[10] Y.M. Yang et al., Engineering Design of the ENN
Spherical Torus EXL-50U, presented at the SOFE2023,Oxford, UK.
[11] Z. Fan et al., 2016 Phys. Plasmas 23 010703
http://dx.doi.org/10.1063/1.4940315.

A self-organized transition from a turbulent state (left,1.18s, 28 + 50GHz) to a quiescent state (right, 1.80s,50GHz) of the EXL-50 ECRH-only p-B plasma underboron powder fueling (Ip~100kA, nel~1.51018/m2)