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

A stationary high normalized beta (β_N > 2) H-mode discharge was achieved in the HL-2A tokamak using pure neutral-beam injection (NBI) heating. While transient high performance occurred, β_N often plateaued near 3. To understand this limitation, magnetohydrodynamic (MHD) instabilities potentially limiting β_N were simulated in the edge and core regions using the MARS (the MHD Analysis of Resistive Spectro) and ExFC (the Extended Fluid Code) 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 β_N 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 β_N ≈ 3. A global ExFC simulation including the full H-mode profiles, and thus both the core KIM and edge EKM, reproduces the observed β_N limit trend. These results suggest that the β_N plateau around 3 in HL 2A plasmas may arise from the combined destabilization of the core KIM and the n = 1 edge EKM.