Pulsed Langmuir Probe Measurements in a Helium Afterglow Plasma

Abstract

The electron‐temperature history and density decay rate of a 1 Torr pulsed helium afterglow plasma has been studied with electrostatic probes. The plasma was repetitively formed by electrodeless rf break‐down at 21 MHz. A cylindrical double probe was positioned at the center of the plasma and, in addition, two large metal electrodes were mounted on the inner cylindrical walls of the plasma container. The arrangement of electrodes allowed for both symmetric double‐probe measurements and single‐probe measurements. A new pulsed probe technique was developed which makes it possible to limit the plasma depletion by the probe to that of a floating probe. At early times in the afterglow (0≤t≤2 msec) the electron energy distribution was found to be non‐Maxwellian with two energy groups of electrons. At late times in the afterglow the electron energy distribution was found to be Maxwellian with a temperature corresponding to the ambient temperature. In the case of the single‐probe measurements the accuracy of the temperature determination is a new result which indicates that a correct collisional probe model must predict a linear region in the semi‐logarithmic plot of the probe characteristic.