Electron Cyclotron/Upper Hybrid Resonant Preionization in the ISX-B Tokamak.

Preionization experiments have been performed on a tokamak by injecting about 80 kW of microwave power at 35 GHz for up to 15 ms. Microwave absorption occurs at the electron cyclotron and upper hybrid resonance frequencies as predicted by theory. Preionization causes substantial (40%) reductions in loop voltage during the initial phase of the tokamak shot. Flux (volt second) savings with preionization are about 30% in the first 2 ms or about 2% of the total flux expenditure in a tokamak shot. The plasma current starts 20 microsec earlier and rises 1.4 times more rapidly in the preionized case. Electron densities of 5 x 10 to the 12th power/cu cm can be sustained with only a toroidal magnetic field during microwave injection. The bulk electron temperature in the preionized plasma is about 10 eV although there are indications of higher electron temperatures (50 eV) in the upper hybrid resonance layer. During the early stages of ohmic heating the preionization is effective in decreasing the peak of the radiated power. (Author)