Optical Emission Spectroscopy-Based Tomography for Compact Low-Pressure Microwave Plasma in a Multicusp

Abstract

Microwave-induced argon and hydrogen plasmas confined in a compact multicusp (MC) are studied in the pressure range of 0.2–2 mtorr and at microwave powers of 216–324 W. Plasma parameters such as electron temperature ( $\text{T}_{\text{e}}$ ) and density ( $\text{N}_{\text{e}}$ ) are evaluated using optical emission spectroscopy (OES) and Langmuir probe (LP) measurements. The estimated $\text{T}_{\text{e}}$ and $\text{N}_{\text{e}}$ from OES measurements lie in the range of 5–13 eV and $0.6 \times 10^{\mathrm {{10}}}$ – $4.0 \times 10^{\mathrm {{10}}}$ cm $^{-\mathrm {{3}}}$ , respectively, for the Ar and H2 plasmas, and are comparable to the values obtained from the LP measurements. The objective of this paper is to develop an emission tomographic measurement system for plasma confined in a compact MC. In general, OES is carried out to determine the wavelength of the most intense emitted light from the plasma, which is then applied to design the tomographic system to finally obtain a 2-D emission profile of light from the whole plasma cross section. Tomographic reconstruction of the plasma emission indicates two bright rings, one at the core and another near the plasma boundary, which are influenced by higher electron density at the core region and higher electron temperature near the electron cyclotron resonance zone.