X-Mode and O-Mode Electron-Cyclotron Heating Breakdown and Startup in Tca

A comparative study of X- and O-mode high field side launch for electron cyclotron heating (ECH) breakdown and startup of tokamak plasmas has been performed. It is found that X-mode power is not absorbed at the cyclotron resonance but uniquely at the upper hybrid resonance, displaced to the low field side of the cyclotron resonance. O-mode power, however, is absorbed at the cyclotron resonance as well. The displacement of the upper hybrid resonance to the low field side with O-mode launch is significantly smaller than that with X-mode launch because of the lower densities produced by O-mode launch at the same microwave power level. The result is a more central and less localized breakdown with O-mode launch. The breakdown characteristics of X- and O-mode launch are seen to affect the position of the initial plasma current centroid in the poloidal cross-section. There is a strong correlation between the initial current ramp rate and the initial plasma current position which is most likely due to the dependence of the plasma inductance, toroidal electric field and field line connection lengths on the plasma major radius. O-mode launch starts the plasma more centrally than X-mode launch and results in higher current ramp rates. X-mode startup occurs further to the low field side where current ramp rates are observed to be poor.