Physics design of current drive and strategy of heating system for EHL-2 spherical torus

ENN He Long-2 (EHL-2) is the next-generation large mega-Ampere (MA) spherical torus (ST) proposed and funded by the ENN company. The design parameters are: T_i0 > 30 keV, n_\mathrme0\sim\mathrm 1\times10^20\mathrm\ m^-3 , I_p ~ 3 MA, B_t ~ 3 T. One of the biggest challenges of EHL-2 is how to achieve several MA current flat-tops with limited voltage-seconds (Vs) of the center solenoid (CS) coils. In order to minimize the consumption of Vs, a fully non-inductive start-up by electron cyclotron resonance heating (ECRH) will be applied in EHL-2. The ramp-up phase will be accomplished with the synergetic mode between the CS and non-inductive methods. The strategy of non-inductive start-up and ramp-up with synergetic mode has been verified on EXL-50U’s experiments. Based on this strategy, numerical simulations indicate the feasibility of EHL-2 achieving 3 MA plasma current. A high-performance steady-state scenario with I_p ~ 1.5 MA is also designed. In this scenario, the bootstrap current fraction f_BS > 70%, the safety factor q at the magnetic axis q₀ > 2, the minimum safety factor q_min > 1, the poloidal beta β_p > 3 and normalized beta β_N > 2.3. Each design iteration integrates the validation of physical models with the constraints of engineering implementation, gradually optimizing the performance of the heating and current drive (H&CD) systems. Numerical simulation results for general auxiliary H&CD systems such as neutral beam injection (NBI), electron cyclotron (EC) wave, ion cyclotron wave (ICW), and lower hybrid wave (LHW) are presented. These simulation results ensure that the 31 MW H&CD systems comprehensively cover all scenarios while maintaining engineering feasibility.