In this paper, the effects of non-Maxwellian distribution of electrons on the characteristics of magnetized plasma sheath with secondary electron emission are investigated by using a magnetic fluid model of one-dimensional velocity and three-dimensional space. The velocity of electrons follows the non-extensive distribution, and the ions are magnetized in a magnetic field with a certain tilt angle relative to the wall. The effects of the non-extensive electron distribution parameter <i>q</i> and the magnetic field strength and angle on the Bohm criterion, the floating wall potential, the secondary electron number density at the sheath edge, the sheath thickness and the ion velocity are studied by establishing the self-consistent equations. When the electron velocity distribution deviates from the Maxwellian distribution, the results show that as the <i>q</i>-parameter increases, the value of the Bohm criterion decreases, the floating wall potential increases, the number of secondary electrons at the sheath increases, the sheath thickness decreases, the number density of ions and electrons decline faster, the number density of ions near the wall is higher, and the velocities of the ions in the three directions are all reduced. In addition, as the magnetic field strength increases, the sheath thickness decreases, and the number density of ions and electrons in the sheath area decrease rapidly; the larger the magnetic field angle, the more significant the influences of the parameter <i>q</i> on the wall potential and the sheath thickness are, while the velocity component of the ion in the <i>x</i>-direction decreases with the increase of the magnetic field angle, but in the case of super-extensive distribution (<i>q</i> < 1), the velocity change near the wall presents an opposite trend, the increase of magnetic field angle causes wall velocity to increase; when it is close to Maxwellian distribution (<i>q</i> → 1), the velocity near the wall does not depend on the change of the magnetic field angle and basically tends to be identical; in the case of sub-extensive distribution (<i>q</i> > 1), the velocity near the wall decreases with the magnetic field angle increasing.
The secondary electron emission and inclined magnetic field are typical features at the channel wall of Hall thruster acceleration region, and the characteristics of the magnetized sheath have significant effect on the radial potential distribution, ion radial acceleration and wall erosion. In this paper, the magnetohydrodynamics model is used to study the characteristics of the magnetized sheath with secondary electron emission in acceleration region of Hall thruster. The electrons are assumed to obey non-extensive distribution, the ions and secondary electrons are magnetized. Based on the Sagdeev potential, the modified Bohm criterion is derived, and the influences of the non-extensive parameters and magnetic field on the acceleration region sheath structure and parameters are discussed. Results show that, with the decrease of the parameter q, the high-energy electron leads to an increase of the potential drop in the sheath, and the sheath thickness expands accordingly, the kinetic energy rises when ions reach the wall, which can aggravate the wall erosion. Increasing the magnetic field inclination angle in the AR of the Hall thruster, the Lorenz force along the x direction acting as a resistance decelerating ions is larger which can reduce the wall erosion, while the strength of magnetic field in the AR has little effect on Bohm criterion and wall potential. The propellant type also has a certain effect on the values of wall potential and secondary electron number density and sheath thickness.
The properties of atmospheric pressure collisional plasma sheath with non-extensively distributed electrons and hypothetical ionization source terms are studied in this work. The Bohm criterion for the magnetized plasma was extended in the presence of an ion-neutral collisional force and ionization source. The effects of electron non-extensive distribution, ionization frequency, ionneutral collision, magnetic field angle and ion temperature on the Bohm criterion of the plasma sheath are numerically analyzed. The fluid equations are solved numerically in the plasma-wall transition region using a modified Bohm criterion as the boundary condition. The plasma sheath properties such as charged particle density, floating sheath potential and thickness are thoroughly investigated under different kinds of ion source terms, the contributions of collisions, and magnetic fields. The results show that the effect of the ion source term on the properties of atmospheric pressure collisional plasma sheath is significant. As the ionization frequency increases, the Mach number of Bohm criterion decreases and the range of possible values narrows. When the ion source is considered, the space charge density increases, the sheath potential drops more rapidly, and the sheath thickness becomes narrower. In addition, ion-neutral collision, magnetic field angle and ion temperature also significantly affect the sheath potential profile and sheath thickness.
The direct and resonant single-photon photoionization of W63+ ions from their ground state 1s 22s 22p 63s 2 S 1/2 and four lowly-excited states 1s 22s 22p 63p 2 P 1/2,3/2 and 1s 22s 22p 63d 2 D 3/2,5/2 has been studied within the framework of the relativistic R-matrix method and the multiconfigurational Dirac–Fock method. Special attention has been paid to obtaining direct and resonant ionization limits and to identifying possible resonant symmetries of W63+ ions. To this end, the photoionization cross-sections have been calculated using the Dirac Atomic R-matrix Code package. Direct and resonant ionization limits were obtained for the ground-state photoionization of W63+ ions, and the resonant peaks identified were found to be associated with the resonances 1s 22s 22p 53lnl′ and 1s 22s2p 63lnl′ (l, l′ = s, p, d) for the fine-structured energy level or configuration at the level of fine structure or configuration. For excited-state photoionization, however, the resonant ionization limits and resonant peaks were hardly distinguishable due to much more densely spaced resonant peaks and a complicated resonant structure, although the respective direct ionization limits are obtained. We expect that this work will remedy the current lack of fundamental studies of the photoionization of W63+ ions and be helpful in the diagnosis and simulation of fusion plasmas in conjunction with the available photoionization data for tungsten ions in other charge states.
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