Ratio Of Radial Diffusion Coefficient Research Articles

Time of flight measurements of the ratio of radial diffusion coefficient and mobility for electron swarms in helium, argon, neon and krypton at high E/N: S A J Al-Amin and J Lucas, J Phys D: Appl Phys, 20, 1987, 1590–1595

Time of flight measurements of the ratio of radial diffusion coefficient and mobility for electron swarms in helium, argon, neon and krypton at high E/N: S A J Al-Amin and J Lucas, J Phys D: Appl Phys, 20, 1987, 1590–1595

Electron swarms in mixtures of metal vapour and argon gas

Calculations have been made for the electron swarm parameters in mixtures of argon and metal vapour for E/N varying from 2.82 to 282 Td. Three metal vapours, namely sodium, potassium and caesium, have been investigated for conditions in which the concentrations of metal vapour varied from 0.001% to 20%. The results have shown that for a small percentage of metal vapour both the ionisation coefficient and the drift velocity have higher values than in either pure argon or pure metal vapour. The mean energy and the ratio of radial diffusion coefficient to mobility have both shown monotonic variations at a fixed E/N from the higher values in pure argon to the lower values in pure metal vapour. The radiation emission has also been computed. It is observed that for a range of E/N determined by the percentage of metal vapour, that most of the swarm energy is emitted as metal vapour radiation.

Time of flight measurements of the ratio of radial diffusion coefficient and mobility for electron swarms in helium, argon, neon and krypton at high E/N

A time of flight pulse technique has been used for the measurement of the ratio of radial diffusion coefficient to mobility (Dr/ mu )d for electron swarms in helium, argon, neon and krypton. The experimental range for helium and argon is 56.5<E/N<5650 Td (where E/N is the ratio of electrical field to gas number density), whilst the experimental range for neon and krypton is 1.41<E/N<5650 Td. The values of (Dr/ mu )d increase with increasing E/N to a maximum value and then decrease in value. The values vary from 6.52 to 21.7 V in helium, 6.40 to 16.4 V in argon, 1.23 to 28 V in neon and 5.24 to 13.3 V in krypton. The results in helium and argon show good agreement with the result of Lakshminarasimha et al. (1977) obtained by using a steady-state technique over an experimental range which extended up to E/N=1300 Td. There are no experimental results in neon and krypton for comparison. The present results have been compared with theoretical values computed by various workers using the Boltzmann and Monte Carlo technique. Good agreement can generally be obtained between selected theoretical and the experimental results for the entire E/N region.

The ratio of radial diffusion coefficient to mobility for electrons in hydrogen, nitrogen and carbon monoxide at high E/N

Measurements of the ratio of radial diffusion coefficient to mobility, Dr/ mu , have been obtained for carbon monoxide, nitrogen and hydrogen. A pulse technique has been used covering a wide range of E/N from 28 to 5650 Td. The values of Dr/ mu vary between 0.8 and 17 eV in carbon monoxide, between 1.2 and 16 eV in nitrogen and between 1 and 12 eV in hydrogen. Comparison has been made with results of a Monte Carlo computer simulation. For all the gases, fair agreement has been obtained with the theory except for the highest of range E/N. This discrepancy has been explained by the presence of a non-equilibrium electron energy distribution in the experimental measurements.

Electron swarm parameters in oxygen and methane

Experimental measurements of the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons have been obtained for the range of 25<or=E/N<or=848 Td for oxygen and 0.28<or=E/N<or=848 Td for methane (1 Td=10-17 V cm2). The results obtained from a Monte Carlo simulation have been compared with the present and previously obtained data and a good fit has been achieved for the entire range of E/N. As a result of this investigation, two sets of cross sections are given which accurately simulate electron swarm parameters in oxygen and methane. The simulation also produced results for the ionisation coefficient and the ratio of radial diffusion coefficient to mobility and these values have also been compared with other theoretical and experimental values, where available. The mean energy has also been computed.

Electron swarm parameters in argon and krypton

Experimental measurements of the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons have been obtained for the range of 2.8<or=E/N<or=566 Td for argon and 14<or=E/N<or=849 Td for krypton (1 Td=10-17 V cm2). The results obtained from Monte Carlo simulation have been compared with the present and previously obtained data and a good fit has been achieved for the entire range of E/N. The simulation also produced results for the ionisation coefficient and the ratio of radial diffusion coefficient to mobility and these values have also been compared with other theoretical and experimental values, where available. The mean energy has also been computed and compared with the experimental breakdown voltage. This comparison has shown that equilibrium conditions exist in the region in which the present measurements have been made. As a result of this investigation, two sets of cross-sections are given which accurately simulate electron swarm parameters in argon and krypton respectively.

Electron swarm parameters in helium and neon

Experimental values have been given for both the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons in helium (1.4<or=E/N<or=226 Td) and neon (1.4<or=E/N<or approximately=339 Td). Comparison has been made with a Monte Carlo computer simulation and a good fit has been obtained with the present and previously measured data. The simulation results for ionisation coefficient and ratio of radial diffusion coefficient to mobility have been compared with both experimental and other theoretical values over a wide range of E/N. A comparison between the mean swarm energy and the breakdown voltage has clearly shown that equilibrium conditions exist at the highest E/N (2260 Td for helium and 5600 Td for neon) for which the computations were carried out.

A pulse technique for the measurement of the ratio of diffusion to mobility for electron swarms in gases at high E/N

A pulse technique has been developed for the measurement of the ratio of radial diffusion coefficient to mobility (Dr/ mu ) for electrons in gases. Measurements have been made in carbon dioxide for the experiment range 14<E/N<5650 Td, where E/N is the ratio of electric field to gas number density. The magnitude of Dr/ mu increases with increasing E/N; values vary from 0.85 V to 18.0 V and show good agreement with the results of Lakshminarasimha et al. (1974) obtained by using a steady-state technique for E/N up to 1553 Td. A Monte Carlo simulation of the present experimental range shows that the equilibrium conditions exist after the electrons have travelled a short distance within the gap.

Electron drift velocity and momentum cross-section in mercury, sodium and thallium vapours. II. Theoretical

For pt.I see ibid., vol.11, p.325 (1978). Theoretical values for the electron drift velocity have been calculated in mercury, sodium and thallium for 7*10-18<or=E/N<or=5*10-16 V cm2 using Boltzmann's equation. The momentum cross-section for electron/atom collisions has been obtained as a function of electron energy by using an iterative procedure which produces a good agreement between the calculated and experimental values of the drift velocity. Other electron swarm parameters have been calculated as a function of E/N: namely, electron mean energy, the ratio of radial diffusion coefficient to mobility and the fraction of swarm energy loss to inelastic collisions.

The ratio of radial diffusion coefficient to mobility for electrons in helium, argon, air, methane and nitric oxide

The ratio of the radial diffusion coefficient to mobility Dr/ mu exp has been measured for the first time in five gases at high E/N (ratio of electric field to gas number density). The range of E/N was as follows: helium 3<or=E/N<or=847 Td; argon 8<or=E/N<or=1271 Td; air 14<or=E/N<or=1412 Td; methane 14<or=E/N<or=5650 Td; nitric oxide 56<or=E/N<or=1412 Td. The Townsend ionization coefficient alpha T/N has also been measured in each gas for the same E/N range.

Electron swarm parameters in CO2 laser gas mixtures

Electron swarm parameters for gas mixtures used in CO2 lasers have been measured experimentally for the first time, using a new UHV discharge chamber specially constructed for the purpose. Studies have been made on six laser gas mixtures with He, N2 and CO2 in the ratio 3:1:2, 8:1:1, 3:2:1, 30:7:1, 3:1/4:1, 3:1:1. Measurements of ionization coefficient and the ratio of radial diffusion coefficient to mobility in the range 14.1<or=E/N<or=282 Td are reported. A comparison is made with the theoretical calculations of Lowke et al. (1973); the experimental values are about 50% lower than the theoretical predictions at high E/N.

Electron swarm parameters in nitrogen

Measurements have been made, using recently developed experimental and theoretical techniques, of: the ratio of radial diffusion coefficient to mobility, Dr/μexp; the amplification coefficient, (λL-uL)/p; and Townsend's ionization coefficient, αT/p: for electron swarms in nitrogen for 10 less-than-or-eq, slant E/p less-than-or-eq, slant 450 V cm−1 Torr−1 at 0 °C. The results have been used for the estimation of electron energy distribution and of the mean energy for 30 less-than-or-eq, slant E/p less-than-or-eq, slant 450. It has been shown that at high E/p the distribution becomes strongly anisotropic, the degree of anisotropy being described by a coefficient δ.

The measurement of the ratio of radial diffusion coefficient to mobility and the amplification coefficient for electrons at high E/p

A generalized method is given for the measurement of the amplification coefficient g and the ratio of radial diffusion coefficient to experimental mobility, Der/μexp, for electron swarms moving through uniform electric fields in the presence of secondary electrons. The technique requires the experimental measurement of both the current amplification and the radial distribution of anode current. Measurements are reported for swarms in hydrogen for 30<or=E/p0<or=350 V cm−1 Torr−1 (at 0°C).

The measurement of the ratio of diffusion coefficient to mobility for electrons at low E/p

An accurate method is given for the calculation of the ratio of radial diffusion coefficient to mobility (Der/μe) for electron swarms in the absence of electron ionization. The technique required the measurement of the radial distribution of electron current across the anode, and the analysis is based upon a modified form of the drift and diffusion equation. Experimental values are given for electrons crossing uniform electric fields in hydrogen with 5<or=E/p0<or=30 V cm−1 Torr−1 at 0°C. The analysis of the radial distributions by the Huxley equation is shown to give values of De/μe which are not constant across the anode. The variations, which are as large as ±5% of the mean value, are explained.