Linear Gyrokinetic Stability Analysis of ST40 Hot Ion Plasmas

The high field compact spherical tokamak ST40 is an important step toward ST-based fusion reactor [1]. Temperature and density profiles, and their uncertainties in recent hot ion ST40 plasmas with central ion temperature exceeding 8.6 keV have been inferred from experimental and modeling constraints in the absence of full profile measurements. Linear gyrokinetic stability analysis has been carried out to identify most unstable micro-instabilities in these hot ion plasmas. In one of these plasmas, it is found that linear growth rates of both ion- and electron-scale ($k_\theta\rho_s \geqslant 0.2$) modes decrease from the edge toward the core of the plasma (i.e. from $\rho=0.8$ to 0.3 where $\rho$ is the square root of normalized toroidal magnetic flux) while the change of linear growth rates at $k_\theta\rho_s < 0.2$ is non-monotonic. In particular, at $\rho=0.3$ no unstable mode was found at $k_\theta\rho_s> 5$, and about an order of magnitude reduction in the maximum linear growth rate in the ion-scale wavenumber range of $ 0.2\leqslant k_\theta\rho_s < 1$ is seen at $\rho=0.3$ compared with the $\rho=0.8$ location. TEM and/or ubiquitous mode are found to be important in this hot ion plasma. \\

\noindent[1] M. Gryaznevich and O. Asunta, Fusion Eng. Des. 123 177 (2017)