Transport and Microinstability Properties of ST40 Plasmas

ST40 is a high toroidal field (≤2.1 T) compact spherical tokamak that has performed scans in plasma isotope, plasma current, toroidal field, and collisionality to determine the dependence of global confinement and local transport at toroidal field values higher than those in any other operating spherical tokamak. The scans show a strong increase of ion temperature and confinement with increasing plasma mass. The reduction in ion thermal diffusivity with increasing plasma mass plays a major role in the attainment of 10 keV central ion temperatures in deuterium plasmas with deuterium neutral beam injection. Furthermore, preliminary results indicate confinement scalings with plasma current, toroidal field and collisionality in hot ion mode plasmas differ from those trends observed in H-mode plasmas at both low and higher aspect ratio, with the confinement trends being governed primarily by the electron temperature profile. Microinstabilty analyses indicate ion-scale modes may be important at all locations in driving transport in ST40 high ion temperature plasmas. These modes are sensitive to changes in plasma beta, indicating that these may be Kinetic Ballooning Modes. The gyrokinetic analyses also indicate modes that are sensitive to trapped electron effects. Electrostatic electron temperature gradient modes are calculated to be unstable in the outer region of the plasma. The isotope dependence of microinstabilities will also be presented.