Sedina Tsikata

Dispersion relations of electron density fluctuations in a Hall thruster plasma, observed by collective light scattering

Kinetic models and numerical simulations of E×B plasma discharges predict microfluctuations at the scales of the electron cyclotron drift radius and the ion plasma frequency. With the help of a specially designed collective scattering device, the first experimental observations of small-scale electron density fluctuations inside the plasma volume are obtained, and observed in the expected ranges of spatial and time scales. The anisotropy, dispersion relations, form factor, amplitude, and spatial distribution of these electron density fluctuations are described and compared to theoretical expectations.

Hall thruster plasma fluctuations identified as the E×B electron drift instability: Modeling and fitting on experimental data

Microturbulence has been implicated in anomalous transport at the exit of the Hall thruster, and recent simulations have shown the presence of an azimuthal wave which is believed to contribute to the electron axial mobility. In this paper, the 3D dispersion relation of this E×B electron drift instability is numerically solved. The mode is found to resemble an ion acoustic mode for low values of the magnetic field, as long as a non-vanishing component of the wave vector along the magnetic field is considered, and as long as the drift velocity is small compared to the electron thermal velocity. In these conditions, an analytical model of the dispersion relation for the instability is obtained and is shown to adequately describe the mode obtained numerically. This model is then fitted on the experimental dispersion relation obtained from the plasma of a Hall thruster by the collective light scattering diagnostic. The observed frequency-wave vector dependences are found to be similar to the dispersion relation of linear theory, and the fit provides a non-invasive measurement of the electron temperature and density.

Development and experimental characterization of a wall-less Hall thruster

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.

Three-dimensional structure of electron density fluctuations in the Hall thruster plasma: E¯×B¯ mode

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 th...

Collective Thomson scattering for studying plasma instabilities in electric thrusters

Collective (or coherent) Thomson scattering has recently emerged as an important tool for identifying and characterizing certain instabilities in Hall thrusters. Plasma instabilities in electric thrusters are implicated in diverse phenomena, including reduced efficiency, lifetime and anomalous particle transport. This work discusses the main features of the collective scattering diagnostic PRAXIS, and recent applications of the diagnostic to study the nature of microturbulence at different thruster operating regimes. Early measurements show the presence of a small-scale azimuthal instability may be linked with regimes of unstable thruster operation.

Device convolution effects on the collective scattering signal of the E × B mode from Hall thruster experiments: 2D dispersion relation

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.

Hall thruster microturbulence under conditions of modified electron wall emission

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.

Pseudo-3D PIC modeling of drift-induced spatial inhomogeneities in planar magnetron plasmas

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 ...

Azimuthal micro-instability inside a wall-less hall thruster

Summary form only given. In a Hall Thruster (HT), a discharge is ignited inside an annular ceramic chamber between an anode situated at the rear of the device and an electron-emitting hollow cathode downstream of the channel. Xenon gas propellant is injected through the anode plane. A magnetic field is created using internal and external coils to achieve a quasi-radial magnetic field profile. A discharge voltage applied between the anode and the cathode is responsible for the acceleration of the ions and the engine thrust. Nevertheless, a fraction of the energetic ions bombards the ceramic walls, resulting in erosion that reduces the thruster lifetime1.An alternative architecture has recently been tested for a low power HT. In this new configuration, the anode is now a ring very close to the channel exhaust. Magnetic coils have been replaced by permanent magnets. This new HT configuration has been named the Wall-Less HT (WLHT) because ionization and acceleration occur outside the channel2. Because in such a configuration the plasma-wall interactions are strongly reduced, the WLHT may offer the opportunity of an increased lifetime with performance levels similar to those of a conventional HT. We have developed fully Particle-In-Cell (PIC) Monte Carlo Collisions (MCC) model of the WLHT. The two-dimensional PIC MCC model describes the azimuthal and axial directions3. A hybrid parallel programming technique that combines OpenMP and MPI on 16 processors is employed to shorten computation time to a few days. Simulations reveal the presence of a micro-instability that participates to the cross-B field electron transport.

Influence of size scaling and plasma-wall interaction on features of hall thruster microturbulence

Summary form only given. Turbulence at length scales on the order of the electron Debye length has been shown to be a possible contributor to anomalous electron transport in the Hall thruster. Simulations1 and coherent Thomson scattering experiments2 have identified an azimuthal instability of megahertz frequency and millimeter wavelength which is associated with increased electron current.Recent observations show that the instability is present in a range of thrusters, ranging in power from a few hundred watts to a few kilowatts. The mode amplitude, however, appears to depend on features such as the ratio of channel width to thruster diameter. Radial-azimuthal PIC simulations have also recently demonstrated the strong coupling between the instability and secondary electron emission from the walls3. The contribution of secondary electron emission to the electron current is influenced partly by plasma heating induced by the instability. Experimental investigations performed with highand low-emission wall materials reveal that the emission characteristics influence the fluctuation amplitude and localization of the instability. This work discusses recent results from experiments and PIC simulations showing the influence of such size scaling and wall interaction effects on observed microturbulence characteristics.