Swarm parameter measurement in hydrogen, considering secondary photonic electron emission

TL;DR

This study models secondary electron emission in hydrogen discharges, demonstrating UV light as the primary emission source below 200 Td and confirming high photo-cathode efficiency for UV photons above 8 eV.

Contribution

It provides a detailed modeling of secondary electron emission mechanisms in hydrogen discharges, clarifying the role of UV light and validating previous experimental findings.

Findings

Secondary emission is due to UV light below 200 Td

Simulation aligns with observed waveform oscillations

High photo-cathode efficiency for UV photons above 8 eV

Abstract

Discharges in hydrogen at pressures above $1\,$kPa and a reduced electric field of $E/N=100-200\,$Td show a characteristic current oscillation in Pulsed Townsend experiments. This is explained by secondary emission of electrons from the photo-cathode: some hundreds of nano-seconds after the laser-pulse that released the initial $10^4-10^6$ primary electrons, secondary electrons are emitted from the cathode. Mechanisms discussed in literature are UV-emission from neutral molecules, emission by positive ions reaching the cathode, and back-scattering of excited neutrals. For a measurement up to $500\,$Td and pressures up to $2\,$kPa we model different sources, and agree with previous findings that the observed secondary emission is purely due to ultra-violet light below $200\,$Td: the simulation fits with the complicated form of the oscillating waveform. Obtained swarm parameters agree well with the literature. Our findings suggest a very high efficiency of the photo-cathode for UV light of energies above $8\,$eV.