Predicted optimization of beta(N) limit for steady-state scenario with double transport barriers in HL-2M tokamak

The equilibria of the HL-2M tokamak, designed for steady-state operation with reversed magnetic shear and double transport barriers, are calculated using TOQ code, and the n = 1 linear ideal magnetohydrodynamic (MHD) stability of the equilibria is investigated using GATO code. To improve the beta (beta(N)) limit, the MHD stability of the equilibria with different heights of internal transport barrier (ITB) and edge transport barrier (ETB) is simulated. The simulation results indicate that the ITB drives low m globalized modes while the ETB drives high m edge localized modes, and thus a moderate barrier height ratio is beneficial to MHD stability. By adjusting the heights of both barriers with different plasma shapes, as well as different locations and widths of ITBs, optimal equilibria with improved MHD stability are achieved, which always have a moderate barrier height ratio unless the ITB is extremely wide or very close to the edge. In ideal-wall conditions, when the ITB is wide enough or very close to the edge, beta(N) limits could exceed six, or even reach seven in some special cases. Increasing elongation and triangularity, broadening the ITB, as well as moving the ITB to a large minor radius, can contribute to the optimization of the beta(N) limit, especially when combined with the stabilizing effect from an ideal wall.