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X- and O-mode breakdown and startup are compared in a tokamak with a 39 GHz high field side launch. The HE(11) beam is reflected from a mirror movable in both the toroidal and poloidal directions. The importance of the upper hybrid resonance (UHR) layer for power absorption in the cold breakdown plasma is demonstrated for both injected modes. The low first-pass O-mode absorption, resulting in multiple wall reflections and mode mixing, explains power deposition at both the cyclotron and UH resonances, the latter expected only for X-mode polarisation. X-mode launch, however, deposits power only at the UHR, creating a more local deposition profile which is displaced further to the low field side than with O-mode launch. This effect produces an initial startup current channel for X-mode launch located far to the low field side and reduces the current ramp rate, particularly at high ECH power, where electron densities are high. The broad O-mode deposition profile permits a more central startup and a more rapid current rampup over a broader range of experimental conditions. Results of EC heating experiments are also presented.

Additional information

Authors: GOODMAN T ET AL., Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, Lausanne (CH)
Bibliographic Reference: Paper presented: 33rd Annual Meeting, Division of Plasma Physics, Tampa (US), Nov. 4-8, 1991
Availability: Available from (1) as Paper EN 36307 ORA
Record Number: 199111114 / Last updated on: 1994-12-02
Original language: en
Available languages: en