Probe Measurement of Electron Density in Magnetized Plasma

Abstract

Electrostatic probe measurement of electron density in magnetized plasma is discussed. Experiments are made in helium at pressures ranging from 0.3 to 1.6mmHg with 1011-1012/cc of plasma electron density and 0-5000 gauss of magnetic field strength.The influence of magnetic field, pressure and discharge current on the saturation electron current to cylindrical probe is investigated, for which the plasma electron density determined by the following two methods is taken as the true density : one is based on probe characteristic of ion current almost free from the effect of magnetic field, the other is with a microwave cavity devised to become free from the effect of magnetic field. Results of experiment show that the thickness of sink region around the probe perpendicular to the magnetic field is given by Ve/ (ω2p+ω2B) 1/2where Ve is the thermal velocity of electron, and ωP and ωB are respectively the angular frequencies of electron plasma and cyclotron.Use of this region and the theory developed by Dote enables the numerical determination of electron density of magnetized plasma. The electron density calculated by using electron current characteristics of the probe with its axis parallel to the magnetic flux agrees well with the true electron density of magnetized plasma. The electron density measurement with the probe perpendicular to the magnetic line of force is also investigated.