Analysis of Kelvin-Helmholtz-like instabilities in strongly rotating tokamak plasmas

Toroidal rotation in tokamak plasmas has been shown to stabilize several performance-limiting instabilities. However, for toroidal flows of the order of the ion sound speed a new rotation and rotation shear-driven Kelvin-Helmholtz-like instability can grow [1]. Experimental evidence for this mode may have been observed in previous NSTX data. The experiments indicate that gradients in density co-existing with sonic-ordered flows could play an important role in its drive [2]. In [1], only strong gradients in plasma rotation were considered.

A large aspect ratio magnetohydrodynamic model with sonic ordered flows was therefore used to analyse the driving and damping effects of strong density, temperature, pressure and rotation gradients on the Kelvin-Helmholtz-like instability. Performing an asymptotic expansion for large, sonic toroidal rotation allowed for the parametric dependences of the mode growth to be investigated. These analytic results were additionally compared against a full magnetohydrodynamic code, VENUS-MHD, which was further used to model relevant low magnetic shear NSTX discharges.

[1] C Wahlberg et al 2013 Plasma Phys. Control. Fusion 55 105004

[2] Hao, G.Z., Heidbrink, W.W., Liu, Y.Q., Yang, S.X., Fredrickson, E.D., Podestà, M., & Crocker, N.A. (Oct 2018). Centrifugal Force Driven Low Frequency Modes in Spherical Tokamak (IAEA-CN--258). International Atomic Energy Agency (IAEA)