Experiments on Minimizing ELM-induced Fast Wave Antenna Breakdown in DIII-D

All present-day fast wave (FW) antennas operating into H-mode plasmas encounter a limit in coupled power set by antenna breakdown. This limit is due to the low antenna/plasma coupling characteristic of H-mode plasmas with a large antenna/plasma distance (“outer gap”), which leads to high antenna rf electric fields that exceed the dielectric strength of the antenna parts immersed in the far scrape-off layer (SOL) plasma. As previously observed by the AUG [1] and DIII-D [2] groups, this problem is aggravated by large edge localized modes (ELMs): the arrival of an ELM at the antenna can substantially reduce the voltage at which breakdown occurs. This issue has been addressed experimentally in three ways at DIII-D. An approach to this problem, suggested by previous work on AUG and DIII-D, is to reconsider the use of a nearly optically-opaque Faraday screen (FS) to reduce the ELM-induced plasma density and ultraviolet light around the antenna elements. A second approach is to stabilize the ELMs with resonant magnetic perturbations (RMP) [3], since it may be that large ELMs are intolerable in ITER and future devices for other reasons in any case. Thirdly, the physics of the antenna breakdown mechanism has been studied at extremely high time resolution (data sampling rates up to 1 GHz). This work may lead to new antenna ar