Single Electrode A.C. Plasma Anemometer in High Speed H2 Jet With Background RF Plasma

Measurements in a high-speed, low pressure hydrogen jet using a cylindrical single- electrode AC-driven glow discharge flow sensor are presented. These results examine the sensitivity and voltage-current characteristics of the plasma sensor with and without the presence of a background RF plasma. These results extend the utility of the plasma sensor to a regime of rarefied, high speed gases. This sensor is being developed to fill a gap in diagnostic capabilities for use in Extreme Ultra-Violet (EUV) lithography systems that use hydrogen as a purge gas within the main vacuum chamber. The plasma sensor utilizes a high frequency (0.05-2 MHz+) AC discharge between two electrodes as the main sensing element. The voltage drop across the discharge correlates to changes in the external flow which can be calibrated for mass-flux (ρU ) or velocity. Recent experiments examine the effects of electrode geometry, AC frequency, and background ionization on the plasma sensor response. The velocity sensitivity was improved by the new electrode geometry with good response even while operating in a background plasma.