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 for subsequent experiments to test and realize the p-11B fusion burning plasma.Based on 0-dimensional（0-D） system design and 1.5-dimensional 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● establish a plasma with extremely high core ion temperature(Ti,0> 30 keV),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;● 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;● achieve divertor heat and particle fluxes control including complete detachment under high P/R（> 20 MW/m） at relatively low electron densities.This overview will introduce the advanced progress in the physics design of EHL-2.