Physics Conclusions of the PI3 Spherical Tokamak Program

Plasma Injector 3 (PI3) is a large scale (2m vessel diameter) spherical tokamak (ST) device in which plasmas are formed by fast coaxial helicity injection (CHI) from a magnetized Marshall gun and stabilized by axial current driven on a central shaft. The Ohmically heated ST plasmas generated by PI3 are suitable as initial target plasmas to be compressed to fusion conditions in a Magnetized Target Fusion compression concept. Best results to date exhibit electron temperatures of 450 eV as measured by Thomson scattering and ion temperatures of 600 eV as estimated through measured neutron yield and density profile. Plasma current is typically 300-500 kA, with closed poloidal flux of 150-250 mWb.  Thermal energy confinement is typically 7-15 ms, while total plasma lifetime is 20-30 ms.  We find evaporative lithium coating (~micron thickness) of the main flux conserver greatly improves plasma lifetime and temperature, with best results within 30 shots after a new layer of coating. Performance of CHI improves with total capacitor bank size, motivating larger banks in future devices.