Effectiveness of X- and O-mode ECRH breakdown and startup in TCA
Comparative studies of ECRH startup have been performed to assess the merits of X/O-mode low loop voltage startup with the aim of minimising volt-second consumption and runaway electron content in ECRH started plasmas. The TCA ECRH system includes a 39 GHz gyrotron, designed and built by CRPP and ABB, which operates in the TE(02) mode with a maximum efficiency of 42%. The microwaves are launched from the high field side of TCA in HE(11) in either X- or O-mode, and are reflected from a mirror steerable in both toroidal and poloidal directions. Absorption in the preionisation plasma and the subsequent profile evolution are followed using, in particular, a high-resolution vertical H(alpha) camera and a five-channel density interferometer. Optimisation studies of startup at constant loop voltage are also reported. The rf power level, type of mode launched and the ECRH resonance position significantly affect the current rise rate. Although X- and O-mode show different startup characteristics, much of the power of both modes is found to be absorbed in the upper hybrid resonance region, which places significant absorbed power to the low field side of the cyclotron resonance. Operation regimes producing low-runaway-content plasmas have been found; these depend strongly on use of proper gas injection schemes.
Bibliographic Reference: Paper presented: 18th European Conference on Controlled Fusion and Plasma Physics, Berlin (DE), June 3-7, 1991
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Record Number: 199110438 / Last updated on: 1994-12-02
Original language: en
Available languages: en