K. Makowski

Wall material effects in stationary plasma thrusters. II. Near-wall and in-wall conductivity

Simulations and experimental characterizations of a stationary plasma thruster are compared for four different wall materials to investigate near-wall conductivity (dielectric materials) and in-wall conductivity (conducting materials) in such a discharge. Using a one-dimensional transient fluid model that takes into account a possible electron temperature anisotropy, it is shown that electron-wall backscattering plays a crucial role by maintaining a relatively high electron temperature along the magnetic field lines which in turn drives large electron currents toward the walls. The large differences in discharge current observed experimentally for the dielectric materials are qualitatively recovered, confirming that near-wall conductivity results from the combined effects of secondary electron emission and electron backscattering. A clear correlation is found between the appearance of space charge saturation at the walls and a jump of the discharge current observed in experiments when varying the discharg...

Transit-time instability in Hall thrusters

Longitudinal waves characterized by a phase velocity of the order of the velocity of ions have been recurrently observed in Hall thruster experiments and simulations. The origin of this so-called ion transit-time instability is investigated with a simple one-dimensional fluid model of a Hall thruster discharge in which cold ions are accelerated between two electrodes within a quasineutral plasma. A short-wave asymptotics applied to linearized equations shows that plasma perturbations in such a device consist of quasineutral ion acoustic waves superimposed on a background standing wave generated by discharge current oscillations. Under adequate circumstances and, in particular, at high ionization levels, acoustic waves are amplified as they propagate, inducing strong perturbation of the ion density and velocity. Responding to the subsequent perturbation of the column resistivity, the discharge current generates a standing wave, the reflection of which sustains the generation of acoustic waves at the inlet ...

Spectral analysis of Hall-effect thruster plasma oscillations based on the empirical mode decomposition

Hall-effect thruster plasma oscillations recorded by means of probes located at the channel exit are analyzed using the empirical mode decomposition (EMD) method. This self-adaptive technique permits to decompose a nonstationary signal into a set of intrinsic modes, and acts as a very efficient filter allowing to separate contributions of different underlying physical mechanisms. Applying the Hilbert transform to the whole set of modes allows to identify peculiar events and to assign them a range of instantaneous frequency and power. In addition to 25kHz breathing-type oscillations which are unambiguously identified, the EMD approach confirms the existence of oscillations with instantaneous frequencies in the range of 100–500kHz typical for ion transit-time oscillations. Modeling of high-frequency modes (ν∼10MHz) resulting from EMD of measured wave forms supports the idea that high-frequency plasma oscillations originate from electron-density perturbations propagating azimuthally with the electron drift v...

IS NEAR-WALL CONDUCTIVITY A MISNOMER?

The near-wall conductivity theory outlined by Morozov in 1968 constitutes one of the most prominent outcome of the early investigations of Hall thrusters, which describes the cross-eld electron diffusion mechanism induced by electron-wall collisions. The present work generalizes the near-wall conductivity theory to the case of a non-zero sheath potential at the walls. The general solution is found to differ qualitatively from the no-sheath solution of the classical theory, and puts into question the hypothesis that the so-called near-wall currents are conned to the close vicinity of the walls.

Chaotic waves in Hall thruster plasma

The set of hyperbolic equations of the fluid model describing the acceleration of plasma in a Hall thruster is analyzed. The characteristic feature of the flow is the existence of a trapped characteristic”; i.e. there exists a characteristic line, which never intersects the boundary of the flow region in the thruster. To study the propagation of short wave perturbations, the approach of geometrical optics (like WKB) can be applied. This can be done in a linear as well as in a nonlinear version. The nonlinear version describes the waves of small but finite amplitude. As a result of such an approach one obtains so called transport equation, which are governing the wave amplitude. Due to the existence of trapped characteristics this transport equation appears to have chaotic (turbulent) solutions in both, linear and nonlinear versions.

Search for the Frequency Content of Hall Effect Thruster HF Electrostatic Wave with the Hilbert-Huang Method

Hall Effect Thruster (HET) plasma oscillations are studied. A set of antennas and an electric probe is used to pick‐up the signals. All the detectors are located in the thruster channel exit plane, at its outer circumference, close to the zone of maximum magnetic barrier of SPT100‐ML device. Each non‐stationary signal is expanded into a finite set of intrinsic modes with the use of Empirical Mode Decomposition (EMD) method. Characteristic bands of instantaneous frequency and power are filtered out by means of Hilbert transform. The analysis is applied to signals recorded in different operating conditions of the HET. The HF oscillations in the frequency range of ∼ 1 ÷ 20 MHz are identified as an electrostatic drift wave propagating along the thruster azimuth. In this band the decrease of discharge voltage results in less defined and broadened frequency spectrum when compared to nominal operating conditions.

High Frequency Oscillations In 2‐D Fluid Model Of Hall Effect Thrusters

2‐D linearised plasma dynamics equations for Hall Thruster plasmas are examined with special attention focused on the effect of spatial non‐uniformity of the unperturbed state. Introducing fast and slow (separated) scales of plasma parameter variation, the stability conditions for HF modes are discussed.

Model of Stationary Plasma Thruster with Conducting Walls

Although stationary plasma thrusters typically operate with dielectric walls, several attempts have been made at using floating conducting segments in the channel. In such a configuration, the wall potential cannot be computed locally like in the case of dielectric walls, but results from a global balance of currents over the whole plasma-wall interface. It is shown that for graphite and therefore for most conducting materials, the potential of the conducting section is almost uniform to within a few mV . Numerical results for a fully conducting channel show that the wall potential stabilizes to a value close to the plasma potential in the exit plane, which strongly contrasts with the situation met in the Hall thruster with anode layer (TAL) where the walls are biased at the potential of anode. The large discharge currents measured with graphite walls are well reproduced in simulations which highlight the effect of plasma short-circuiting induced by currents through the walls.