Plasma Decay Technique to Measure Density in Discharge Tubes

Abstract

The decay technique measures plasma density over a wide range; in particular, below 109 cm−3, where, for discharge tubes of a few liters in volume, Langmuir probes can give ambiguous results. The method is most easily used in systems of simple geometry; for example, spheres and cylinders. The ion current to the discharge-tube wall is sampled by a negatively-biased probe that forms a part of the wall. An average value for the positive charge density in the tube before turnoff is obtained by integrating the total ion current to the probe during decay, multiplying this quantity by the ratio of wall area to probe area, and dividing by the volume of the tube. An advantage of the decay technique is that the electron and the ion velocity distributions need not be known.