Volume 28A, number 5 P H Y SIC S L E T T E R S 16 December 1968 MEASUREMENT OF ARC ELECTRON DENSITIES USING A CO 2 N. KONJEVI~ Institute of Physics, Beograd, P.O. Box 57, Yugoslavia and K. R. HEARNE Department of Electrical Eng. and Electronics, University of Liverpool, England Received 29 October 1968 LASER This letter describes the application of a carbon dioxide laser interferometer, 10.6~, to the measurement of electron density in an arc plasma. Results are given for the axial electron density in argon, current range 30-90 A. Helium-neon laser interferometers which are widely used for the measurement of time depen- dent electron densities have recently been sucess- fully applied to arc plasmas at atmospheric pres- sure [1-3]. In these latter experiments it was necessary to employ a two wavelength method and hence the advantage of using the 10.6 ~ line is that greater sensitivity can be obtained and this per- mits shorter plasmas to be studied. In the present experiments the electron densities obtained using the three wavelengths are compared and this can clearly be used as a basic check on the validity of the method, originally proposed by Alpher and White [4]. The arc chamber used in the present exper- iments has been described previously [1] and con- sisted of a set of water cooled copper discs which surrounded an arc channel of length 10.5 cm, the electrodes were made of densametal. An inter- ferometric arrangement similar to that of Ashby et al. [5] was employed with the chamber located in the external cavity. The steady state electron density was determined by measuring the total change in electron density which occurred follow- ing arc interruption. The arc itself was main- rained by a 460 V d.c. supply and rapid interrup- tion (~ 5g sec) was achieved by diverting the cur- rent through a silicon controlled rectifier situ- ated in parallel with the arc. The d.c. CO2 laser which operated under constant gas flow conditions was 1.3 m long, the radius of the curved mirror being 2 m. Both laser mirrors were made of stain- less steel coated with gold and each had a 2 mm hole in the centre covered by Itran windows. (a) (b) Fig. I. Fringe oscillograms following interruption, arc current =46 A. (a) X= 3.39g(b) X= 10.6/2. Before using this laser with the arc an effort was made to suppress the multimode operation by in- serting a copper ring in the laser plasma tube. Varying both the position and size of the ring, mainly by trial and error, resulted in a satis- 309