Overview of the physics design of the EHL-2 spherical torus

ENN is planning the next generation experimental device EHL-2 with the goal to verify the thermal reaction rates of p-11B fusion, establish spherical torus/tokamak experimental scaling laws at 10's keV ion temperature, and provide a design basis a succeeding experiment aiming to test and achieve p-11B fusion burning plasma. Based on 0-dimential system design and 1.5-dimentional transport modelling analyses, the main target parameters of EHL-2 have been basically determined, including the plasma major radius, R0, of 1.05 m, the aspect ratio, A, of 1.85, the maximum central toroidal magnetic field strength, B0, of 3 T, and the plasma toroidal current, Ip, of 3 MA. The main heating system will be the neutral beam injection at a total power of 17 MW. In addition, 6 MW of electron cyclotron resonance heating will serve as the main means of local current drive and MHD instabilities control. The physics design of EHL-2 is focused on addressing three main operating scenarios, i.e., 1) high ion temperature scenario, 2) high-performance steady-state scenario and 3) high triple product scenario. Each scenario will integrate solutions to different important issues, including equilibrium configuration, heating and current drive, confinement and transport, MHD instability, p-11B fusion reaction, plasma-wall interactions, etc. Beyond that, there are several unique and significant challenges to address, including 1）establish a plasma with extremely high core ion temperature (Ti,0>30keV), and ensure a large ion-to-electron temperature ratio (Ti,0/Te,0 > 2), and a boron concentration of 10-15% at the plasma core; 2）realize the start-up by non-inductive current drive and the rise of MA-level plasma toroidal current. This is because the volt-seconds that the central solenoid of the ST can provide are very limited; 3）achieve divertor heat and particle fluxes control including complete detachment under high P/R (>20MW/m) at relatively low electron densities. This review will introduce the advanced progress in the physics design of EHL-2.