<title>Transverse rf Plasma Discharge Characterization For CO2 Waveguide Lasers</title>

Abstract

AbstractThe transverse RF discharge plasma characteristics of HejN2:CC>2 gas mixtures used in C02 waveguide lasers has been predicted and measured. Using theoretical electron transport coefficients and a classical RF plasma model, predictions of discharge conductance, E/N, and excitation efficiency were obtained. Experimental values of gas conductance and E/N were reduced from measurements of laser voltage and input power. Laser efficiencies of have been attained,IntroductionAlternate forms, of pumping gaseous lasers, other than by normal glow discharges,1*2 have been sought which would eliminate the deliterious effects of such discharges, and yet still be efficient. A unique system of transverse RF electric field discharge3*^ eliminates many of these problems, such as voltage chorona, discharge ignition difficulties, cathode sputtering, and energy ion bombardment. The transverse RF discharge is a cathodeless system which relies largely on the bulk plasma effects of the ionized laser gas to sustain the discharge. The voltages obtained by this technique of transverse discharge are sig­ nificantly lower than encountered in the usual longitudinal excitation (100V versus several kilovolts). Additionally, the RF system allows direct laser pump modulation, and therefore direct amplitude modulation of the laser without an EO modulator.This paper compares experimental measurements of the RF discharge plasma characteristics of He:N2:C02 gas mixtures as they apply to C02 waveguide lasers, against results derived from theoretical electron transport coefficients,and a classic plasma model,Laser RF gas plasma modelThe classic analysis of an RF gas plasma gives the major analogous circuit elements of the laser discharge. The analysis neglects the interaction of the gas with the walls by assuming a boundaryless regime. This is far from the truth when considering wall dimensions on the order of one to two millimeters; however, the trend reflects the dominant macro­ scopic prop-erties of the gas. The gas can then be thought of an an RF circuit element, and suitable power delivery networks can be examined.From plasma physics5* 6 an ionized plasma in the presence of an RF field can be shown to have a steady state current density which varies ascosUt - ) . (1)This result is based on the following assumptions which reflect the general laser situations:1. Ion motion is neglected since the ions are 1800 times heavier than the electrons,2. Operation is in the high pressure, low frequency regime where the molecular collisional frequency is greater than the driving RF frequency,3. Induced moments do not greatly perturb u or e of the gas from their vacuum values.4. RF magnetic effects are neglected because the drift velocities of the particles are relatively small.The above expression can be rearranged as an analogous series RF circuit, wheremvd md R = - and L = - , (2)& fcwN e e2 B fcwN ee2 The most important aspect of the result is the collisional frequency dependence of the resistive term, Rg. Since it is assumed that v is a very large number, Rg will be the dominant impedance term of the resultant analogous circuit for moderate RF frequencies.