Microinstability properties of small-aspect-ratio tokamaks

Advanced tokamak configurations can have improved stability properties for high-n microinstabilities such as the toroidal drift mode (trapped-electron-η_i, mode) and the kinetically-calculated magnetohydrodynamic (MHD) ballooning mode. A promising means to achieve this end involves employing tokamak configurations with very small aspect ratio, as in the proposed National Spherical Tokamak Experiment (NSTX) [M. Ono et al., Bull Am. Phys. Soc. 40, 1655 (1995)] or the existing Small Tight Aspect Ratio Tokamak (START) experiment [R. J. Colchin et al., Phys. Fluids B 5, 2481 (1993)]. Kinetic instabilities are analyzed here using a comprehensive toroidal eigenvalue code with realistic equilibria for cases based on NSTX and START along with artificial cases to study parametric sensitivities. It is found that, as the aspect ratio decreases, the amount of "bad" magnetic curvature decreases, causing stabilization of both electrostatic and electromagnetic high-n instabilities at sufficiently small aspect ratio.