Non inductive formation of an extremely overdense spherical Tokamak by electron Bernstein wave heating and current drive on LATE

An extremely overdense special Tokamak plasma has been non-inductively formed and maintained by electron Bernstein (EB) wave heating and current drive in the Low Aspect ratio Torus Experiment (LATE) device. The plasma current reaches 12 kA and the line-averaged electron density exceeds 7 times the plasma cut off density by injecting a 2.45 GHz microwave power of 60 kW. Such a highly overdense plasma is obtained when the upper hybrid resonance layer lies to the higher field side of the 2nd harmonic ECR layer, which may realize a good coupling to EB waves at their first propagation band. The effect of the injection polarization on the mode conversion rate to EB waves at the extremely overdense regime has been investigated and an improvement in the plasma current is observed.