Stabilization of ion-scale electrostatic drift-type modes with decreasing aspect ratio through major or minor radius in tokamaks

In the conventional tokamak with a high aspect ratio (A), turbulent transport is known to be dominated by the ion-scale electrostatic drift-type modes, such as the ion temperature gradient (ITG) or trapped electron mode (TEM). When tokamak type changes from the conventional to the spherical one by decreasing A, these modes are often observed to be stabilized, making turbulent transport then dominated by the electron-scale mode or the electromagnetic ones, such as the kinetic ballooning mode (KBM) or micro-tearing mode. Here, a modeling study is presented on how the ion-scale electrostatic drift-type modes are linearly stabilized when A decreases through major or minor radius in the s–α equilibrium model. Especially, following two mechanisms are newly identified to play an important role in the linear stabilization. One is the enhancement of the threshold temperature gradients for the ITG and TEM. This enhancement occurs clearly when A decreases through the major radius, but is also effectively possible when A is reduced though the minor radius. The other is the increment of the ballooning force parameter α which roughly varies in proportion to 1/A2 when we assume a fixed safety-factor profile. This increment enhances the linear electromagnetic and Shafranov-shift effects, which provide additional stabilization for the ITG and TEM, respectively. With the increment of α, the standard KBM can be excited at a smaller pressure gradient, but it is expected to have the 2nd stability regime access if plasma shape is strong, as typically taken in the low A spherical tokamaks. As shown in the recent simulation work by Kennedy et al (2023 Nucl. Fusion63 126061), however, the hybrid-KBM may be excited in the low A, high beta plasmas, and a brief discussion is given about its possible origin in terms of the present modeling results.