Low n electromagnetic modes in spherical tokamaks

• Published in: Plasma physics and controlled fusion 2021-07, Vol.63 (7), p.75017
• Main Authors: Chowdhury, J, McMillan, B F
• Format: Article
• Language: English
• Subjects: electromagnetic modes; gyrokinetic simulation; kinetic ballooning mode; microinstability; spherical reactor; tokamak
• ISSN: 0741-3335; 1361-6587
• DOI: 10.1088/1361-6587/ac031b

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.