Predictions of H-mode access and edge pedestal instability in the EHL-2 spherical torus

The EHL-2 spherical torus is designed to demonstrate proton-boron (p-¹¹B) fusion within a compact spherical tokamak. Its planned heating system includes a negative ion-based neutral beam injection (N-NBI), two positive ion-based NBI systems (P-NBI), electron cyclotron resonance heating (ECRH), ion cyclotron resonance heating (ICRH), and high harmonic fast wave (HHFW), with a total power output of 31 MW. According to scaling law estimates, the device is capable of achieving H-mode operation. The plasma density, $n_\text{e,min}$, at the minimum L-H power threshold, $P_\text{LH}$, is estimated to be $4.4\times10^{19}$ $\text{m}^{-3}$. The pedestal parameters were calculated using the REPED model. Assuming B as the primary impurity ion, the predicted pedestal width and height are lower compared to the typical case with carbon impurities. The pedestal collisionality for EHL-2 is estimated to range between 0.06 and 0.17, indicating the potential for significant energy loss due to edge localized modes (ELMs). The heat flux on the divertor plate has been calculated using the JOREK code. The peak heat fluxes during ELM bursts are approximately 31.0 MW/m² at the lower inboard target and 39.5 MW/m² at the lower outboard target. A preliminary design of the resonant magnetic perturbation (RMP) coils has been completed to both control type-I ELMs and correct error fields. The system comprises 16 coils arranged into 2$\times$4 pairs. In ELM control mode, a 14/2 component is generated at 1.7 G/kAt, with a current of 4.9 kA required to achieve $\sigma_\text{Chirikov} = 1$ at the resonant surface, where the normalized poloidal magnetic flux is 0.85. In error field (EF) modulation mode, 2/1 and 3/1 components are generated at 3.5 G/kAt and 2.8 G/kAt, respectively.