Stabilizing impact of high gradient of β on microturbulence

It is shown here that microturbulence can be stabilized in the presence of steep temperature and density profiles. Indeed in high β plasmas, pressure profile gradients are associated with high vertical bar β' vertical bar =-∂β/∂ρ, where β=P/(B2/2μ0) and ρ the square root of the toroidal flux normalized to its edge value. It is shown here that high values of vertical bar β' vertical bar have a stabilizing influence on drift modes. This may form the basis for a positive feedback loop in which high core beta values lead to improved confinement, and to further increase in β. A gyrokinetic electromagnetic flux tube code, GS2 [M. Kotschenreuther, G. Rewoldt, and W. M. Tang, Comput. Phys. Commun. 88, 128 (1995)], is used for analyzing the microstability. In high β spherical tokamak plasmas, high vertical bar β' vertical bar rather than low aspect ratio is a source of stabilization. Therefore, the effect of high vertical bar β' vertical bar should be stabilizing in the plasmas of the National Spherical Torus Experiment [Y.-K. Peng, M. G. Bell, R. E. Bell et al., Phys. Plasmas 7, 1681 (2000)]