Electron Swarm Parameters in Helium in a Strong Electric Field

Dependences of kinetic and transport coefficients of electrons in helium on the electric field strength E reduced to the atomic concentration N in the E/N range from 15 to 1000 of Td are calculated using the earlier developed Monte-Carlo code. The calculation results are compared to the published experimental data. It is shown that for the available experiments, significant inaccuracy is inherent in the range of high E/N magnitudes caused by electrons not being in equilibrium with the field in strong fields. It is shown that the diffusion-drift approximation for the space-time evolution of electron concentration in helium is not applicable above E/N approximate to 200 Td. The gas gain G of helium-filled proportional counter is calculated; the computed G magnitudes agree well with the measurement data. The discrepancy between the measured G values and previously published calculated G values is explained.