Bochum 2002 – scientific programme

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P: Plasmaphysik

P 23: Poster: Plasmatechnologie, Plasma-Wand-Wechselwirkung, Staubige Plasmen, Plasmadiagnostik

P 23.13: Poster

Wednesday, March 20, 2002, 17:40–18:40, HZO Foyer

Studies of ICRF antenna voltage stand-off — •Vl.V. Bobkov, J.-M. Noterdaeme, F. Wesner, R. Wilhelm, and ASDEX Upgrade Team — Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, D-85748 Garching

The voltage stand-off of ICRF antenna can sometimes limit the power launched to the plasma. We study how to improve this voltage stand-off. A well-diagnosed RF probe for ASDEX Upgrade has been developed as a tool to get experimental knowledge on breakdown phenomena at high voltages with different conditions near the plasma boundary. The exchangeable open end of coaxial line of the probe represents a model of the antenna high voltage region.
Voltages of 60 kV for 200 ms were achieved in a teststand in high vacuum at the probe head. At higher voltages vacuum arcs appear. An ion source with a high aperture beam was used to provide plasma density of 10 ⁹ cm⁻³. The presence of plasma produced by the ion source has no influence on voltage stand-off of the probe unless neutral pressure is increased. Increasing the pressure gives rise to a glow discharge. Measurements for self-sustained glow discharge were made in addition.
Experiments with the probe have started on ASDEX Upgrade. H-mode discharges with the plasma shape fitting well the ICRF antenna contour are used.
For comparative analysis the signals from the ICRF antennas are used. Data from the probe and the antennas indicate multiple reasons for a breakdown leading to the reduced voltage stand-off, but the breakdown often correlates with ELMs on the probe and on the antennas.