Open AccessLow n electromagnetic modes in spherical tokamaks
Author(s) -
Jugal Chowdhury,
B. F. McMillan
Publication year - 2021
Publication title -
plasma physics and controlled fusion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.328
H-Index - 111
eISSN - 1361-6587
pISSN - 0741-3335
DOI - 10.1088/1361-6587/ac031b
Subject(s) - magnetohydrodynamics , tokamak , physics , plasma , long wavelength limit , wavelength , diamagnetism , atomic physics , ion , computational physics , pressure gradient , spherical tokamak , atmospheric pressure plasma , population , magnetic field , mechanics , optics , nuclear physics , condensed matter physics , quantum mechanics , demography , sociology
The performance of spherical tokamak reactors depends on plasma β , and an upper limit is set by long-wavelength kinetic ballooning modes (KBMs). We examine how these modes become unstable in spherical-tokamak reactor relevant plasmas, which may contain significant fast-ion pressure. In a series of numerically generated equilibria of increasing β , the KBM becomes unstable at sufficiently high plasma β , and for such cases, it is also significantly unstable even in the long-wavelength limit. The β threshold for the KBMs is similar to the ideal Magnetohydrodynamics (MHD) threshold, and in cases without fast ions, their frequencies are as predicted by diamagnetic-drift stabilised MHD. To isolate and explore the KBMs, simulations are performed where the pressure gradient is entirely due to the density profile, or entirely due to the temperature profile; the resulting KBMs have similar properties in the long-wavelength regime. The introduction of energetic ions restricts the KBMs to longer wavelengths, and reduces the β threshold somewhat; for parameter regimes of current-day devices, this is such long wavelength that a global analysis would become necessary. Mode frequencies in plasmas with a significant fast particle population are seen to be controlled by fast particle precession frequencies.