The transport of electrons through nitrogen gas

Abstract

The photon flux technique has been used to measure the drift velocity W, the radial and longitudinal diffusion coefficients DT and DL and Townsends ionization rate alpha T for electrons in nitrogen. Theoretical support for the use of photons to determine electron transport parameters in the presence of small electron density gradients is also presented.

Cathode Region of a Steady-state Townsend Discharge in Nitrogen

“…The experimental apparatus used in the investigation of the cathode region has been described elsewhere (Wedding et al 1985). The only modification to this apparatus has been an improvement in the design of the planar collimator which detects photons in the plane of the discharge at a given axial position z.…”

“…The present collimator consists of two parallel plates, each 9 cm long and 6 cm wide, separated by precision shims. This design is superior to that of Wedding et al (1985) as deep wells milled into the surface of the plates minimise the possibility of internal photon reflections. The light output from the cathode region of the discharge was thus measured with a spatial resolution at the centre of the drift tube of 0·8 mm (FWHM).…”

“…With the aforementioned pressures and electrode separation the applied gap voltage was sufficiently remote from the breakdown voltage that the contribution from cathode-produced secondary electrons to the overall discharge current was negligible. The discharge current to the anode was used in a feedback circuit to control the thermal emission from the oxide cathode source (Wedding et al 1985). With this arrangement typical discharge currents of the order of 100 nA could be maintained to within ±o ·1%.…”

“…It is the purpose of this paper to consider in more detail the cathode non-equilibrium region of a steady-state Townsend discharge in molecular nitrogen, a region characterised by spatial variations in the electron energy distribution. The Holst-Oosterhuis (1921) luminous layers observed in rare gas discharges (see also Druyvesteyn 1932;Fletcher 1985) provide a striking example of such phenomena. Electrons leaving the cathode gain energy from the field as they move through the drift tube and suffer elastic or inelastic collisions with the gas particles.…”

“…This region, where the normalised energy distribution for all electrons at a given position has reached a steady state, may be defined as a region of equilibrium. In this hydrodynamic regime the energy distribution function depends on both E/N and spatial gradients in the electron concentration (Wedding and Kelly 1989), but is independent of source conditions. In the proximity of the anode and cathode of the discharge, regions of non-equilibrium exist where the hydrodynamic model is not applicable.…”