Electron Density Determination in an Are Plasma by Laser Interferometry

In collison dominated plasmas the electron density is the most important parameter. Therefore laser interferometry is very attractive because it allows a direct determination of this quantity without any rigid assumptions such as, e.g., local thermodynamical equilibrium (LTE). For references see, e.g., Griem \ Alpher and White 2 or Kunze 3. In the experiment described in this paper we intended to test how accurate the electron density can be measured in a well-defined plasma by means of an appropriate interferometer. We have chosen a wallstabilized arc and a Michelson interferometer. By switching off the arc current the change of refractivity in the arc axis can be measured, whereas the special arrangement of the interferometer allows simultaneous fringe detection al two different wavelengths. That should be an improvement to the arrangement described in the very extensive paper of Hearn and Konjevic4, who have measured the electron density in the axis of an arc column by means of a "coupled cavity" interferometer, whereas the two fringe measurements could be performed only time sequentially with a loss in accuracy. Baum et al. 5 carried out side-on observations of the arc column with a two wavelength interferometer utilizing the radial symmetry for Abel inversion. Because of the differentially thin homogeneous layers and the errors introduced by the inversion procedure — especially in the vicinity of the axis — the accuracy of the electron density determination is only 10 to 15%. For this reason we observed the arc end-on as described by Hearn and Konjevic. End-on