Electron-impact Ionization Rate Coefficients Research Articles

Maxwellian and non-Maxwellian rate coefficients of the tungsten ions: W46+–W55+

This study focuses on the significance of suprathermal (‘hot’) electrons in the tokamak device. Hot electrons, which follow a non-Maxwellian energy distribution, are high-energy electrons that exert a substantial influence on various processes taking place within the plasma. Our aim was to investigate the influence of non-Maxwellian distribution on the rate coefficients of highly charged tungsten ions. This paper presents Maxwellian and non-Maxwellian electron impact ionization rate coefficients for W46+ to W55+ ions. The cross sections were calculated using the fully relativistic flexible atomic code with level-to-level distorted-wave method. We found that even for a small fraction of hot electrons, the contribution of hot electrons to the rate coefficients is still dominant at low bulk temperature.

Maxwellian rate coefficients for electron-impact ionization of W26+

Excitation-autoionization (EA) and direct ionization (DI) Maxwellian rate coefficients (MRC) are presented for the levels of the ground configuration of the W26+ ion. The study of EA and DI cross sections is performed by applying the Dirac–Fock–Slater method within the level-to-level distorted-wave approximation including radiative damping. Excitations to the high-nl shells (n ≤ 25) are taken into account for the EA process. It is shown that electron-impact ionization from the different levels of the ground configuration produces a significant variation of MRC. Excitations to configurations with energy levels that straddle the ionization threshold play the main role in enhancing MRC for the highest level of the ground configuration. The difference of 40% among the EA MRC is determined for the levels of the ground configuration.

Ionization rate coefficients and induction times in nitrogen at high values of E/N.

Electron-impact ionization rate coefficients in nitrogen at values of E/N, the ratio of the electric field to the neutral density, up to 12 000 Td (1 Td${=10}^{\mathrm{\ensuremath{-}}17}$ V ${\mathrm{cm}}^{2}$), are reported. In addition, we report experimental measurements of the ionization induction time, the time during the early portion of an applied electric field when the electron energy distribution function is transient and the plasma is characterized by nonexponential growth of the electron density. For nitrogen, we show that the induction period is approximately equal to the inverse of the ionization frequency for a large E/N range. Time-dependent Boltzmann calculations of the electron energy distribution function yield instantaneous ionization rates that are in good agreement with both the measured ionization rates and the induction period. The measurements were made in an electrodeless cell contained in an S-band waveguide immersed in a dc magnetic field and subjected to a pulsed rf electric field at cyclotron resonance. We show that our measurements are equivalent to experiments in dc electric fields; the equivalent dc electric field strength being uniquely related to the rf electric field strength. The use of an rf field for these high-E/N measurements circumvents complications that would be introduced by electrode effects. This is the first direct measurement of ionization rates at these extreme values of E/N.

Nonequilibrium ionization of nitrogen: The role of stepwise ionization from metastable states in the presence of superelastic electronic collisions

Electron impact ionization rate coefficients involving N2 metastable electronic states have been calculated in discharge and postdischarge conditions by using the electron energy distribution functions which take into account both the presence of superelastic electronic (SEC) and vibrational (SVC) collisions [J. Appl. Phys. 59, 4004 (1986)]. The results show that stepwise ionization from metastable states can overcome the corresponding rates from the ground state especially in the absence of SVC. Finally, a comparison of the present results with those coming from associative ionization involving metastable electronic and vibrational states is presented and discussed.

Electron impact ionization rate coefficients and cross sections for highly ionized iron

Electron impact ionization cross sections for the ions Fe(XVII)-(XXVI) have been computed in a distorted wave exchange approximation. Analytic fits are provided for the cross section data, as well as for the rate coefficients assuming a Maxwellian electron velocity distribution. For ejection of a 2 p ground state electron, the scaled ionization rate was found to depend linearly on the number of 2 p electrons in the ion.

Experimental ionization-rate coefficients for hydrogenlike, heliumlike, and lithiumlike ions

Electron-impact ionization-rate coefficients are obtained for lithiumlike, heliumlike, and hydrogenlike ions by the plasma spectroscopy method. The plasma is produced in a 140-kJ large-bore theta pinch, and electron density and temperature are determined by 90\ifmmode^\circ\else\textdegree\fi{} Thomson scattering. The basic requirements of the evaluation technique employed are briefly discussed and the results are compared with other measurements and theoretical calculations.

Electron-impact ionization-rate coefficients for lithiumlike nitrogen and oxygen

The electron-impact ionization-rate coefficients for lithiumlike nitrogen and oxygen are obtained from the time evolution of spectral lines emitted by a hot plasma. The plasma is produced in a 37-kJ theta pinch. Laser scattering diagnostics indicate that ${N}_{e}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and $k{T}_{e}\ensuremath{\approx}80$ eV during the time of principal interest. Under these conditions, the ionization-rate coefficient for lithiumlike nitrogen is (7.4\ifmmode\pm\else\textpm\fi{}2.2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$, and that for lithiumlike oxygen is (4.1\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$. The rate coefficients measured here and all other measurements of lithiumlike ionization rate coefficients are compared to calculations of the ground-state ionization-rate coefficient and the total-ionization-rate coefficient.