Thermalisation time of electron swarms in xenon for uniform electric fields
Boyle, G. J., Casey, M. J. E., Cocks, D. G., White, R. D., and Carman, R. J. (2019) Thermalisation time of electron swarms in xenon for uniform electric fields. Plasma Sources Science and Technology, 28 (3). 035009.

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Abstract
We have calculated the thermalisation time for an electron swarm in gaseous xenon using a multiterm timedependent Boltzmann equation (BE), for a range of instantaneously applied reduced electric fields 1 Td < E/N < 1000 Td. Starting from a Maxwellian electron energy distribution function (EEDF) at room temperature for a given E/N, the timeevolution of the EEDF and associated electron swarm parameters (drift velocity We , mean energy <epsilon >, ionisation coefficient k(i) , excitation coefficient k(ex)) are followed as they converge to steadystate values. For all values of E/N considered, the individual swarm parameters are found to converge at different rates. For E/N > 5 Td, they converge in order We (fastest), <epsilon >, k(ex), and k(i) (slowest). The time taken for the slowest swarm parameter to converge to an acceptable level (e.g. to within 10% of its steadystate value) is used universally as the benchmark for evaluating the thermalisation time tau(th). This time is found to be strongly dependent on the value of the reduced electric field E/N, dropping by almost 5 orders of magnitude for increasing E/N fields 1 Td < E/N < 1000 Td. As a key outcome from this work, the calculated thermalisation times tau(th).p are expressed as a general formula, as a function of both the reduced electric field E/N and a user defined convergence level between 1% and 20%. We show that ballpark estimates of thermalisation times, based on the inverse of the collision frequency for energy dissipation 1/nu(e)(epsilon) at typical average electron energies, are likely to be unreliable if applied to the heating phase. We also undertake a brief analysis of the cooling phase when the electric field is instantaneously removed from the plasma (i.e. fieldfree) after it evolves to steadystate conditions during the previous heating phase. Finally, we compare calculated thermalisation times with the typical risetimes of the voltage pulse waveforms for several experimental 'nanosecond' pulse excited plasma discharge devices.
Item ID:  57825 

Item Type:  Article (Research  C1) 
ISSN:  13616595 
Keywords:  plasma, electron, thermalisation, xenon, nanosecond discharge, multiterm Boltzmann equation 
Copyright Information:  © 2019 IOP Publishing Ltd 
Date Deposited:  03 Apr 2019 07:36 
FoR Codes:  51 PHYSICAL SCIENCES > 5106 Nuclear and plasma physics > 510602 Plasma physics; fusion plasmas; electrical discharges @ 100% 
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