Pulsed Langmuir Probe Measurement of Ionization in a High-Pressure Plasma

Measurements with a negatively pulsed probe in an atmospheric-pressure high-density plasma over wide ranges of ionization density (1015-1018/meters3), probe voltage (10-400 volts), and flow velocity (5-40 m/s) have confirmed previous results obtained by Thomas, working at a single voltage and flow velocity in which the current initially falls off with a t-y dependence where y ¿ 0.58 and t = time. The results together with those of Thomas show good agreement with a theory based on a physical model involving space-charge controlled expansion of the sheath towards its final position. It is expected that this technique of probe operation will afford a simple and sometimes unique method of measuring ionization density with excellent spatial resolution, under high-pressure conditions where the plasma velocity relative to the probe is not accurately known (for atmospheric-pressure dc measurements this factor often becomes significant with flow velocities > 0.1-1 m/s).