Electron number density and collision frequency measurements in a microwave surface wave discharge

Abstract

Summary form only given, as follows. We present results of electron number density and collision frequency measurements in an 1.5 m length and 18.6 mm ID argon plasma column produced by a microwave surface wave. The surface wave, propagating in the plasma column, is excited by an electromagnetic launcher built from a rectangular waveguide operating in a TE/sub 101/ mode and at a frequency of 2.45 GHz. This launcher is able to excite a TM/sub 01/ mode in the plasma column. Two different measurement methods were used in order to determine the electron number density as a function of the plasma column axial position: the first uses a moving axial electric probe mounted over the glass tube. The second uses a double Langmuir probe. A mean electron number density of 7/spl times/10/sup 10/ cm/sup -3/ was obtained for an argon pressure of 2 mTorr and a microwave power of 750 W. Two dispersion equations for the wave propagation were also obtained: one where the electron-neutral atoms collision frequency is much lower than the microwave frequency (/spl nu/<</spl omega/) being expressed as a ratio of modified Bessel functions. The other when the electron-neutral atoms collision frequency is of the same magnitude of the microwave frequency (/spl nu//spl sim//spl omega/) being expressed as a ratio of Hankel functions. Using these dispersion equations and the plasma relative complex permittivity /spl epsiv//sub p/, the electron number density is obtained. Also it was noticed that the electron number density is a decreasing linear function of the axial position with a slope proportional to the collision frequency /spl nu//spl sim/10/sup 8/ rad s/sup -1/. The operational characteristics of the electric discharge concerning different glass tubes of internal diameters equal to 18, 12, and, 8 mm, respectively, were also investigated.