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International audience; Nonlinear interaction between kinetic instabilities in partially magnetized plasmas in the presence of multiply charged ion streams is investigated using kinetic simulations. It was observed by Hara and Tsikata [Phys. Rev. E 102, 023202 (2020)] that the axial ion–ion two-stream instability due to singly and doubly charged ion streams, coupled with the azimuthal electron cyclotron drift instability (ECDI), enhances cross-field electron transport. In the present study, it is observed that the addition of triply charged ions (as a third ion species) contributes to damping of the excited modes, leading to a reduction in the cross-field electron transport. The net instability-driven electron transport is shown to be a function not only of the azimuthal modes, such as the ECDI, but of the multiple ion species that dictate the development of additional plasma waves. It is found that trapping of the higher ion charge states within the plasma waves results in broadening of the ion velocity distribution functions.

International audience; In recent numerical, theoretical, and experimental papers, the short-scale electron cyclotron drift instability (ECDI) has been studied as a possible contributor to the anomalous electron current observed in Hall thrusters. In this work, features of the instability, in the presence of a zero-electron emission material at the thruster exit plane, are analyzed using coherent Thomson scattering. Limiting the electron emission at the exit plane alters the localization of the accelerating electric field and the expected drift velocity profile, which in turn modifies the amplitude and localization of the ECDI. The resulting changes to the standard thruster operation are expected to favor an increased contribution by the ECDI to electron current. Such an operation is associated with a degradation of thruster performance and stability.

A pseudo-3D modeling approach, based on a particle-in-cell (PIC)-Monte Carlo collisions algorithm, has been developed for the study of large- and short-scale organization of the plasma in a planar magnetron. This extension of conventional PIC modeling permits the observation of spontaneous organization of the magnetron plasma, under the influence of crossed electric and magnetic fields, into the well-known, large-scale regions of enhanced ionization and density known as spokes. The nature of complex three-dimensional electron trajectories around such structures, and non-uniform ionization within them, is revealed. This modeling provides direct numerical evidence for the existence of high-amplitude internal spoke electric fields, proposed in earlier works. A 3D phenomenological model, consistent with numerical results, is proposed. Electron density fluctuations in the megahertz range, with characteristics similar to the electron cyclotron drift instability experimentally identified in a recent Letter, are also found.

Recent experimental studies on nanocrystalline metals have shown that mechanical strength may decrease with decreasing grain size d for d 7 nm) to localized grain boundary migration (d

An alternative Hall thruster architecture that shifts the ionization and acceleration regions outside the plasma chamber is demonstrated. This unconventional design is here termed a “wall-less Hall thruster,” as the bulk of the magnetized discharge is no longer limited by solid boundaries. A 200 W prototype with permanent magnets has been developed and characterized. Experimental results concerning the thruster operation, discharge oscillations, electric field distribution, and ionization zone characteristics are presented and discussed. Our first experiments show that the cross-field discharge can be moved outside the cavity without drastically disturbing the ion production and acceleration mechanisms. This design offers the benefit of reduced plasma-wall interaction and lower wall losses, while also greatly facilitating diagnostic access to the entire discharge ionization and acceleration regions.

International audience; The effect of the collective light scattering diagnostic transfer function is considered in the context of the dispersion relation of the unstable E×B mode previously reported. This transfer function is found to have a contribution to the measured frequencies and mode amplitudes which is more or less significant depending on the measurement wavenumbers and angles. After deconvolution, the experimental data are found to be possibly compatible with the idea that the mode frequency in the jet frame (after subtraction of the Doppler effect due to the plasma motion along the thruster axis) is independent of the orientation of the wave vector in the plane orthogonal to the local magnetic field.

International audience; Collective scattering measurements have been conducted on the plasma of a Hall thruster, in which the electron density fluctuations are fully characterized by the dynamic form factor. The dynamic form factor amplitude distribution has been measured depending on the k-vector spatial and frequency components at different locations. Fluctuations are seen as propagating waves. The largest amplitude mode propagates nearly along the cross-field direction but at a phase velocity that is much smaller than the E×B drift velocity. Refined directional analysis of this largest amplitude mode shows a thin angular emission diagram with a mean direction that is not strictly along the E×B direction but at small angles near it. The deviation is oriented toward the anode in the (E,E×B) plane and toward the exterior of the thruster channel in the (B,E×B) plane. The density fluctuation rate is on the order of 1%. These experimentally determined directional fluctuation characteristics are discussed with regard to the linear kinetic theory model and particle-in-cell simulation results.