J. Quant. Speetrosc. Radiat. Transfer Vol. 41, No. 2, pp. 147-152, 1989 0022-4073/89 $3.00+0.00 Printed in Great Britain. All rights reserved Copyright © 1989 Pergamon Press plc TEMPERATURE FLUCTUATIONS IN AN ATMOSPHERIC PRESSURE d.c. THERMAL PLASMA J. GRANDY, VIVEK BAKSHI, and R. J. KEARNEY Department of Physics, University of Idaho, Moscow, ID 83843, U.S.A. J. BATDORF INEL EG&G, Inc., P.O. Box 1625, Idaho Falls, ID 83415, U.S.A. (Received 2 March 1988) Almtraet--Measurements are presented which allow determination of temperature fluctuations in the plasma column of a non-transferred d.c. argon plasma. Temperatures, computed from spectral line emission data, show short-term fluctuations of up to 10%. INTRODUCTION A typical d.c. plasma jet with cylindrical symmetry (see Fig. 1) is best described in terms of the radial coordinate r and the axial distance z. The temperature of a plasma as a function of position is an important macroscopic parameter of the system and is well defined under conditions of local thermodynamic equilibrium (LTE). 1,2 Optical measurements are now used almost exclusively to determine temperature and, although there are a variety of optical ways to determine the plasma temperature, the preferred method is to measure a single line emission intensity I(x) as a function of the flame width x at axial coordinate z) '4 The Abel integral equation is then used to transform the intensities I(x) across the width of the flame into radial emissivities e (r). Under conditions of an optically-thin plasma (no absorbtion of radiation within the plasma) and LTE, determination of the absolute line emission coefficient allows a temperature to be calculated. Although various investigatorss,6 have observed short-term fluctuations in plasmas, mainly with high speed movies, and others have examined temperature variations in response to alternating currents in transferred arc systems,7-9 there appears to have been no detailed study of short term (~ 10 #s or less) temperature-fluctuation measurements in non-transferred d.c. systems. ;'::,Z,:½"%',I Cathode ~-/ r/ // / • "//;Y/:'/4~ ~" , Y, " >?'z; N X x x - -% % % \ X ~, )) iiii2, Gas Inlet B C ~---z Fig. 1. Basic configuration of the non-transferred argon plasma torch. Standard operating parameters were 500 A, 22 V and an argon gas flow of 1.2 m3/h. 147