Measurements of electron densities and temperatures in low-pressure arcjet plasmas operating under diamond growth conditions

Abstract

Summary form only given. We have measured the spatial variation of plasma parameters in a low-pressure DC arcjet plasma used in the deposition of diamond thin films. A Langmuir probe was inserted into the plasma jet from the side. Various gas mixtures were studied, including pure argon, argon-hydrogen mixtures, and argon-hydrogen-methane mixtures, with and without NO seeded into the gas as an convenient laser-induced fluorescence (LIF) probe species for gas temperature measurements. When hydrogen and methane was added, the electron density increased by about an order of magnitude, but these electrons were cooler, approximately 1.5 eV, with no high energy tail. Adding NO did not change the plasma density or electron temperature significantly, but it did tend to shift the plasma potential the magnitude and sign of the shift depended on the hydrogen concentration and distance downstream from the orifice. We have also used the Saha equation and examined the plasma densities expected if the gas and electrons were in thermal equilibrium. We cannot achieve good agreement between the predicted densities and measured densities, which lends support to our contention that the plasma in this case is not in equilibrium. Most of the plasma appears to be formed in the intense arc region and is subsequently swept downstream to the probe or deposition substrate and does not equilibrate with the heavy gas molecules as they undergo chemical reactions. We present data for the spatial variation of these plasma parameters and compare them to the gas species and temperatures determined from laser-induced fluorescence measurements.