Spoke Instabilities Research Articles

Boundary-induced effect on the spoke-like activity in E × B plasma

The spoke instability in an E × B Penning discharge is shown to be strongly affected by the boundary that is perpendicular to B field lines. The instability is the strongest when bounded by dielectric walls. With a conducting wall, biased to collect electron current from the plasma, the spoke becomes faster, less coherent, and localized closer to the axis. The corresponding anomalous cross-field transport is assessed via simultaneous time-resolved measurements of plasma potential and density. This shows a dominant large-scale E × B anomalous character of the electron cross-field current for dielectric walls reaching 40%–100% of the discharge current, with an effective Hall parameter βeff ∼ 10. The anomalous current is greatly reduced with the conducting boundary (characterized by βeff ∼ 102). These experimental measurements are shown to be qualitatively consistent with the decrease in the E field that triggers the collisionless Simon-Hoh instability.

Rotating spoke instabilities in a wall-less Hall thruster: experiments

Properties of rotating spokes have been investigated in the plasma discharge of the 200 W-class permanent magnet ISCT200 Hall thruster (HT) in wall-less (WL) configuration under various operating conditions. For comparison purposes, the HT was also operated in the standard configuration. High-speed camera imaging combined with sophisticated image processing allowed the observation of the plasma inhomogeneity dynamics and the determination of characteristics such as mode number, sizes, rotation frequency and propagation velocity. The time varying Xe+ ion azimuthal velocity distribution function has been measured in front of the anode of the WL thruster by means of laser-induced fluorescence spectroscopy in photon-counting regime. Experiments show ions move at the thermal speed on average, yet some ions have a negative velocity, i.e. they rotate in the direction opposite to the E × B drift, in agreement with numerical simulations performed with a 3D-3V PIC code.

Rotating spoke instabilities in a wall-less Hall thruster: simulations

The low-frequency rotating plasma instability (spoke) in the ISCT200 thruster operating in the wall-less configuration was simulated with a 3 dimensional PIC MCC code. In the simulations an m = 1 spoke rotating with a velocity of 6.5 km s−1 in the E × B direction was observed. The rotating electron density structure in the spoke is accompanied by a strongly depleted region of the neutral gas, which clearly shows that the spoke instability is of an ionization nature, similar to the axial breathing mode oscillations. In the simulation the electron cross-field transport through the spoke core was caused by diffusion in the high-frequency (4–10 MHz), short-scale (3 mm) electric field oscillations. These short-scale oscillations play a crucial role in the thruster discharge as over 70% of the electron current to the anode originates from the spoke core. The rest of the current originates from the spoke front where the electron cross-field transport toward the anode is due to the E × B drift in the spoke macroscopic azimuthal electric field.

Low frequency azimuthal stability of the ionization region of the Hall thruster discharge. II. Global analysis

The linear stability of the Hall thruster discharge is analysed against axial-azimuthal perturbations in the low frequency range using a time-dependent 2D code of the discharge. This azimuthal stability analysis is spatially global, as opposed to the more common local stability analyses, already afforded previously (D. Escobar and E. Ahedo, Phys. Plasmas 21(4), 043505 (2014)). The study covers both axial and axial-azimuthal oscillations, known as breathing mode and spoke, respectively. The influence on the spoke instability of different operation parameters such as discharge voltage, mass flow, and thruster size is assessed by means of different parametric variations and compared against experimental results. Additionally, simplified models are used to unveil and characterize the mechanisms driving the spoke. The results indicate that the spoke is linked to azimuthal oscillations of the ionization process and to the Bohm condition in the transition to the anode sheath. Finally, results obtained from local and global stability analyses are compared in order to explain the discrepancies between both methods.

Ultrahigh Speed Images of Hall Thruster Azimuthal Instabilities

Visible emission of the azimuthal plasma instabilities were observed within the plume of a low-power Hall thruster utilizing ultrahigh speed imaging. The captured images provide time-dependent and spatial information on the azimuthal instability in the axial-radial plane of the thruster, which have not been well characterized. These images are then compared with observations made based on their probe measurements of azimuthal instabilities termed spoke instabilities. Data from the images show that the spoke instabilities are more complex than originally described and have additional features not detectable with probe measurements in a single plane.

Rotating Spoke Instabilities in Hall Thrusters

High-speed imaging of a Hall thruster plume reveals near-omnipresent rotating regions of elevated light emission, dubbed rotating spokes, in the annular thruster discharge channel. Azimuthal oscillations have long been suggested to induce cross-field electron transport in Hall thrusters, but conclusive experimental identification of such oscillations with probes is often challenging. However, simple processing of high-speed images taken at a few tens of thousands of frames per second clearly reveals long-wavelength rotating spokes at very low frequencies, corresponding to velocities of only a few hundred meters per second.