Simulation investigation of MHD instabilities in high-β_N H-mode plasma discharges of HL-2A tokamak

A stationary high normalized beta ( \beta _\mathrmN > 2) H-mode discharge was achieved in the HL-2A tokamak using pure neutral-beam injection (NBI) heating. While transient high performance occurred, \beta _\mathrmN often plateaued near 3. To understand this limitation, magnetohydrodynamic (MHD) instabilities potentially limiting \beta _\mathrmN were simulated in the edge and core regions using the MHD analysis of the resistive spectrum (MARS) and the extended fluid code (ExFC) codes, respectively. MARS edge simulations show that, without wall stabilization, the ideal external kink mode (EKM) with toroidal mode number n = 1 —driven by combined Ohmic and bootstrap currents—becomes strongly unstable as \beta _\mathrmN approaches a critical value of about 3. Meanwhile, ExFC simulations with the H-mode pedestal artificially removed reveal a low n electromagnetic instability in reversed shear plasmas, identified as a kinetic infernal mode (KIM), which also becomes unstable near \beta _\mathrmN ≈ 3. A global ExFC simulation with complete H-mode profiles simultaneously captures both the core KIM and an n = 1 edge global mode (EGM) analogous to the EKM from MARS simulations. This simulation reproduces the observed trend of the \beta _\mathrmN limit. The results indicate that the \beta _\mathrmN plateau near 3 in HL-2A plasmas likely results from the combined destabilization of the core KIM and the edge n = 1 global instability.