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P: Plasmaphysik

P 11: Dusty Plasmas and Low Temperature Discharges (Poster Session)

P 11.18: Poster

Thursday, April 5, 2001, 12:30–15:00, AT2

Measuring the RF-Impedance of a Low Current Argon Arc — •Stefan Lichtenberg, Jens Luhmann, and Jürgen Mentel — Ruhr-University Bochum, Fundamentals of Electrical Engineering Group, Universitätsstr. 150, 44780 Bochum, Germany

A low current argon arc is operated at 1.5 - 6 amps in a model-lamp at a pressure of 0.26 MPa. By superposition of an RF-signal with low energy onto the arc-current the complex-impedance of the DC-arc can be determined. A measuring system was developed whose calibration procedure eliminates the impedance of the external circuit. The calibration procedure is derived from network analysis. By variation and extrapolation of arc length the real and the imaginary part of the impedance of the space charge layers in front of the cathodes can be determined. The complex impedance was measured at frequencies from 12.5 - 40 MHz, for different electrode diameters (0.6 - 2 mm) and materials (pure tungsten and doped tungsten). It was found that the impedance is strongly inductive when the ion fraction of the total current to the cathode is high and changes to capacitive values when the thermionic emission becomes dominant. Financial support is supplied by the German Federal Ministry of Education and Research (BMBF, Ref.-No. 13N7763).