The requirements to stabilize microinstabilities with velocity shear in tokamaks are examined for both aspect ratio A = 3 and A = 1.4. A comprehensive linear gyrokinetic code is used to compute growth rates in realistic numerical equilibria. Growth rates for A = 3 and A = 1.4 are generally similar for electron drift modes and ion temperature gradient modes. Velocity shear is stronger at low aspect ratio; however, low A profiles have a stronger microtearing mode which is more difficult to shear stabilize. Nonetheless, low aspect ratio devices are predicted to have good confinement and may ignite at very small size. Profiles are presented which may allow high β, MHD stable operation at A = 1.4 with high confinement. The possibility of using a controlled application of non-turbulent loss processes to control profiles to prevent turbulent transport and to maximize β is suggested.
Linear gyrokinetic stability of a high β non-inductive spherical tokamak