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 1T. The major radius of the EXL-50U ranges from 0.6m to 0.8m, with an aspect ratio of 1.4 to 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 achieved 1 keV 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.