X- 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 was performed. It was 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 localised breakdown with O-mode launch. The breakdown characteristics of X- and O-mode launch were seen to affect the position of the initial plasma current centroid in the poloidal cross-section. There was a strong correlation between the initial current ramp rate and the initial plasma current position which was 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 started the plasma more centrally than X-mode launch and resulted in higher current ramp rates. X-mode startup occurred further to the low field side where current ramp rates were observed to be poor.
Bibliographic Reference: Article: Nuclear Fusion, Vol. 32 (1992) No. 5, pp. 757-771
Record Number: 199211031 / Last updated on: 1994-12-02
Original language: en
Available languages: en