Investigations on argon and hydrogen plasmas produced by compact ECR plasma source

Abstract

Argon and hydrogen plasmas were produced by a Compact ECR Plasma Source (CEPS) attached coaxially to a large chamber. This paper presents characterization results of the two plasmas using a newly designed Langmuir Probe. Experiments in argon were conducted to benchmark the plasma parameters and to determine the efficacy of the CEPS for thruster applications, recommended by recent results (Ganguli et al 2019 Plasma Sources Sci. Technol. 28 035014), while the hydrogen experiments were undertaken to determine the typical range of plasma parameters for assessing the usefulness of CEPS for different applications. In argon plasma, high densities (≈1012 cm-3), high electron temperatures and plasma potentials are obtained at fairly low pressures ≈0.3-0.5 mTorr. The plasma potential drops substantially (≈65 V) within the CEPS itself, demonstrating its suitability as a plasma thruster. Onthe other hand, plasma densities are lower for hydrogen in front of the CEPS, and the electron temperatures and plasma potentials, higher. The hydrogen experiments have helped establish an important aspect of such plasmas. At low pressures (≈0.5 mTorr) plasma density is relatively low with a single, high temperature electron population; on the other hand, at higher pressures (≈6 mTorr), the plasma has two electron populations, a low temperature, high density population and another low density, high temperature warm population. For hydrogen this transition from the former (with single electron population) to the latter state (with two electron populations) occurs at a pressure, ≈3 mTorr while similar transition for argon occurs at ≈0.3 mTorr. Apart from the observed two states of plasma, each state with its own distinctive properties, another important feature of the hydrogen plasma is that the axial density profiles obey approximately n/B = constant although no such scaling is observed for argon.