Pedestal stability and its variation with decreasing aspect ratio in H-mode plasmas with very large elongation

Abstract As an effort to understand the H-mode property of the spherical tokamak (ST), a modeling study is presented about the pedestal stability in the very large elongation ( κ ) regime and how it then varies when the aspect ratio ( A ) decreases from a high value of the conventional tokamak. It is first shown that, when κ is very large (>2), the peeling–ballooning mode (PBM) eigenvalue spectrum has a complete shift to the n = 1 limit, where n is the toroidal mode number. This shift makes the mode sensitive to the edge safety-factor q ( a ), resulting in an oscillating behavior of the threshold pedestal height ( P ped ) when q ( a ) increases, as expected for the peeling-type mode. In addition, it allows the PBM to couple with the n = 1 external kink mode (EKM) when the normalized beta ( β N ) approaches the no or ideal wall limit. When A decreases through the major or minor radius, these mode characteristics are maintained well, while P ped has a different behavior depending on whether q ( a ) is fixed or varies with A . When q ( a ) is fixed, P ped has a non-negligible increment with decreasing A , mainly due to the increase of plasma current by enhanced toroidicity effect. Meanwhile, when q ( a ) varies with A, P ped has an oscillating behavior as expected for the n = 1 peeling-dominant mode. If plasma beta or β N has a large increment through the toroidal field reduction with decreasing A , a sudden drop of P ped is also shown to be possible by the excitation of the high- n ballooning-branch modes around the pedestal or the coupling to the n = 1 EKM near the no or ideal wall limit. Finally, similar to those observed in some previous analysis works, a discrepancy is found between the present modeling results based on the ideal MHD code and experimental measurements in the contemporary ST devices, and a brief discussion is given about its possible origin and relevance in future ST devices.