VUV irradiance measurement of a 2.45 GHz microwave-driven hydrogen discharge

TL;DR

This study measures the VUV emission of a 2.45 GHz microwave hydrogen plasma, revealing photon emission rates, plasma process diagnostics, and reaction rates, providing insights into plasma behavior and energy dissipation.

Contribution

It introduces a simple photodiode-based method for measuring VUV emission and applies it to analyze plasma processes and reaction rates in microwave-driven hydrogen discharges.

Findings

VUV photon emission rate is 10^16-10^17 1/cm^3s.

Approximately 8% of microwave power is dissipated via VUV emission.

Metastable neutral density is about 0.5% of neutral gas density.

Abstract

Absolute values of VUV-emission of a 2.45 GHz microwave-driven hydrogen discharge are reported. The measurements were performed with a robust and straightforward method based on a photodiode and optical filters. It was found that the volumetric photon emission rate in the VUV-range (80-250 nm) is $10^{16}$-$10^{17}$ 1/cm$^3$s, which corresponds to approximately 8% dissipation of injected microwave power by VUV photon emission. The volumetric emission of characteristic emission bands was utilized to diagnostics of molecular plasma processes including volumetric rates of ionization, dissociation and excitation to high vibrational levels and metastable states. The estimated reaction rates imply that each injected molecule experiences several inelastic electron impact collisions. The upper limit for the total density of metastable neutrals ($2S$ atoms and $c^3\Pi_u$ molecules) was estimated to be approximately 0.5% of the neutral gas density.