Controlling the electron energy distribution function of electron beam generated plasmas with molecular gas concentration: II. Numerical modeling

Abstract

In this work, the second in a series of two, a spatially averaged model of an electron beam generated Ar-N 2 plasma is developed to identify the processes behind the measured influence of trace amounts of N 2 on the development of the electron energy distribution function. The model is based on the numerical solution of the electron Boltzmann equation self-consistently coupled to a set of rate balance equations for electrons, argon and nitrogen species. Like the experiments, the calculations cover only the low-energy portion (<50 eV) of the electron energy distribution, and therefore a source term is added to the Boltzmann equation to represent ionization by the beam. Similarly, terms representing ambipolar diffusion along and across the magnetic field are added to allow for particle loss and electrostatic cooling from the ambipolar electric field. This work focuses on the changes introduced by adding a small admixture of nitrogen to an argon background. The model predictions for the electron energy distribution function, electron density and temperature are in good agreement with the experimentally measured data reported in part I, where it was found that the electron and ion energy distributions can be controlled by adjusting the fraction of nitrogen in the gas composition.

Microplasma thruster powered by X-band microwaves

“…It is noted that the effects of adding molecular H 2 and N 2 gases to monoatomic rare gas Ar plasmas have so far been studied both experimentally and numerically/theoretically (see the Appendix). [107][108][109][110][111][112][113][114][115][116][117][118][119] In the present experiments, we did not measure the T e as well as electron energy distribution function (EEDF), and so we cannot say quantitatively the effects of adding a small amount (<1%) of H 2 and N 2 to Ar on the plasma properties. However, the present optical emission spectroscopy indicated that the addition of <1% H 2 and N 2 to Ar plasmas gave rise to no significant change of the Ar I line intensities in the visible and near-ir region (and thus no significant change of the intensity ratio of the visible Ar I lines to the near-ir ones); this implies that no significant change in the EEDF and/or T e occurred with the H 2 and N 2 addition, since the upper-level excitation energies for visible Ar I lines are >1 eV higher than those of near-ir ones (as mentioned above).…”

“…39 When N 2 is added to Ar, a cooling effect occurs on the plasma electrons, and the electron temperature T e tends to be reduced. [117][118][119] The EEDFs in pure Ar plasmas are nearly Maxwellian below the inelastic collision thresholds with a slight depletion of electrons for energies E e > 11.6 eV [the first excitation threshold for Ar or the excitation energy for Ar*(3p 5 4s 3 P 2 )]; 116 with increasing the fraction of N 2 , a reduction in the EEDF occurs in the range E e ∼ 1-5 eV, owing to some resonant electron-molecule vibrational excitation processes. 112,[116][117][118] In these situations, the electron density n e tends to decrease 116,118 and the gas temperature T g tends to increase 113 with the N 2 fraction.…”

“…[117][118][119] The EEDFs in pure Ar plasmas are nearly Maxwellian below the inelastic collision thresholds with a slight depletion of electrons for energies E e > 11.6 eV [the first excitation threshold for Ar or the excitation energy for Ar*(3p 5 4s 3 P 2 )]; 116 with increasing the fraction of N 2 , a reduction in the EEDF occurs in the range E e ∼ 1-5 eV, owing to some resonant electron-molecule vibrational excitation processes. 112,[116][117][118] In these situations, the electron density n e tends to decrease 116,118 and the gas temperature T g tends to increase 113 with the N 2 fraction. Moreover, the addition of N 2 may alter the plasma properties through a resonant electronic energy transfer between Ar metastable atoms and N 2 molecules [Ar*(3p 5 4s 3 P 0,2 ) + N 2 (X 1 Σ g + ) → Ar(3p 6 1 S 0 ) + N 2 (C 3 Π u )], 120,121 which can lead to an overestimation of the gas temperature derived from the vibronic spectrum of the N 2 2nd positive band assuming T g ≈ T rot .…”

Electron energy probability function modulation with external electron beam in capacitive coupled radio frequency discharges

“…They have demonstrated that the main effect of the external EB was the control of rate constant for ionization, excitation, and momentum transfer. Boris and Petrov et al have written a series of two papers to report the control of the EEPF experimentally and theoretically in EB‐generated plasmas, without any conventionally driving electric field. The ability for control of the EEPF has been demonstrated, by adding small admixture of nitrogen to a background argon gas, at pressure of 25 mTorr.…”

Electron kinetics in capacitively coupled plasmas modulated by electron injection

“…Such a configuration has been proposed and attracted great attention recently. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The groups of Fernsler and Manheimer 1,2 have studied the fundamental physics of the EI generated plasmas. Chen and Eden 3 have realized a microplasma transistor by injecting electrons into the sheath of a plasma with a controllable electron emitter.…”