Strategy and experimental progress of the EXL-50U spherical torus in support of the EHL-2 project

The EXL-50U is China’s first large spherical torus device with a toroidal field reaching 1 T. The major radius of the EXL-50U ranges from 0.6 m to 0.8 m, with an aspect ratio of 1.4−1.8. The goal of plasma current in the first experimental phase is 500 kA, and in the future second phase, the goal of plasma current is 1 MA. On the EXL-50U project, the ENN fusion team expeditiously accomplished a series of comprehensive tasks including physical and engineering design, main component construction installation, and system commissioning, all within a mere eighteen-month timeframe. In the experiments of 2024, the EXL-50U achieved a 500 kA limiter configuration discharge using ECRH (Electron Cyclotron Resonance Heating) for non-inductive current start-up and a current ramp-up with the synergetic effect of ECRH and central solenoid (CS). Preliminary divertor configuration plasmas were also obtained under 200 kA plasma current. The core ion temperature of 1 keV was achieved with low-power NBI heating, and the energy confinement time of 30 ms was reached with Ohmic heating in the flat-top phase. The current and future experiments of EXL-50U will strongly support the physical design and operational scenarios of EHL-2 in the areas of current drive, high ion temperature exploration, energy transport and confinement, and hydrogen-boron physical characteristics. At the same time, the experience in the design, construction, and commissioning of the engineering, heating, and diagnostics systems on EXL-50U is also very beneficial for enhancing the feasibility of the engineering design for EHL-2.