First achievement of high-confinement mode in proton-boron plasmas
Dear Editor,
High-confinement mode (H-mode) discharges have been achieved for the first time in proton-boron (p-11B) plasmas on the EXL-50U spherical torus (ST). A working gas mixture consisting of 30% diborane (B2H6) and 70% hydrogen (H2) was employed, along with boron powder injection for real-time boronization, during the discharges. The boron concentration in the plasma is estimated to be approximately 10%. H-mode operation was obtained using neutral beam injection (NBI) with injected power ranging from 0.5 to 2.5 MW at an on-axis magnetic field of 0.8 T. Edge-localized modes (ELMs) with a characteristic frequency of about 100 Hz were observed. A clear edge pedestal structure in the electron density profile was formed during the H-mode phase. These results represent the first demonstration of H-mode operation in p-11B plasmas in magnetic confinement devices.
The H-mode was first discovered in the ASDEX tokamak and was subsequently reproduced in a wide range of conventional tokamaks.1 In STs, H-mode operation has also been achieved, such as MAST,2 MAST Upgrade, and NSTX.3 Owing to its ability to significantly enhance plasma pressure and energy confinement time, H-mode has become the standard operational regime envisioned for future fusion devices, such as ITER.4
Previously, H-mode operation has been achieved exclusively in hydrogen, deuterium, and tritium plasmas, since the D-T reaction possesses the largest fusion cross-section at achievable plasma temperatures. H-mode in helium plasma was first achieved in ASDEX Upgrade5 and then in other devices. In recent years, p-11B fusion has attracted increasing theoretical interest due to its potential for aneutronic operation.6 Experimentally, alpha particles generated from p-11B fusion reactions have been successfully detected in a magnetically confined plasma.7 Building on these theoretical and experimental advances, a roadmap for p-11B fusion development has been proposed by ENN Science and Technology Development Co., Ltd.8 EHL-2, an ST designed for the demonstration of thermal p-11B fusion reactions, has been designed and is currently under construction.9 The device is equipped with a total auxiliary heating power of 31 MW, and H-mode operation has been predicted based on empirical scaling laws derived from hydrogen and its isotope plasmas. However, despite extensive studies of H-mode in hydrogenic and helium plasmas, the compatibility of H-mode operation with p-11B plasmas has not yet been experimentally demonstrated using the existing EXL-50U device.
The experimental setup
EXL-50U is an upgraded version of EXL-50, featuring the installation of central solenoid (CS) coils to enable ohmic heating and assist plasma current ramp-up.10 A photograph and model of EXL-50U are shown in Figures 1A and 1B, respectively. An electron cyclotron resonance heating (ECRH) system with a source power of 0.8 MW has been installed for plasma heating and current drive. The NBI system consists of two injectors: the first beam delivers up to 1 MW at a beam energy of 25–30 keV, while the second beam delivers 1 MW at 25 keV. Both beams are injected in the co-current direction and are primarily used for ion heating.
