Hall Thruster Simulations Research Articles

Data-driven analysis of oscillations in Hall thruster simulations

A data-driven modal analysis of plasma oscillations in a stationary plasma thruster-100-like Hall thruster in the 1–120 kHz range is presented. Data are generated by a two-dimensional (axial-radial) hybrid particle-in-cell/fluid simulation code. While proper orthogonal decomposition is unable to successfully uncouple the different dynamics, higher order dynamic mode decomposition (HODMD) cleanly isolates the breathing and ion transit time (ITT) modes. Indeed, the computed HODMD components can be clustered into two distinct groups, enabling the separate reconstruction of the dynamics of the two oscillation modes. It is also shown that each plasma variable exhibits a different behavior in each cluster. The breathing oscillations have a global or standing-wave character for the plasma density, neutral density and ion current density inside the thruster, and an axially progressive-wave structure for the electron temperature and plasma potential in the near plume. The ITT mode shows a progressive-wave structure for all variables, except for the neutral density whose amplitude is negligible. This work serves as an illustration of the applicability of data-driven techniques like HODMD to the analysis of plasma flows relevant to electric propulsion.

A zero-equation turbulence model for two-dimensional hybrid Hall thruster simulations

We present a model for electron transport across the magnetic field of a Hall thruster and integrate this model into 2-D hybrid particle-in-cell simulations. The model is based on a simple scaling of the turbulent electron energy dissipation rate and the assumption that this dissipation results in Ohmic heating. Implementing the model into 2-D hybrid simulations is straightforward and leverages the existing framework for solving the electron fluid equations. The model recovers the axial variation in the mobility seen in experiments, predicting the generation of a transport barrier which anchors the region of plasma acceleration. The predicted xenon neutral and ion velocities are found to be in good agreement with laser-induced fluorescence measurements.

Implementation of an Entropy Closure Model for 2-D Hybrid Hall Thruster Simulations

An entropy closure formulation for Hall thrusters was implemented in a radial-axial hybrid simulation. We modeled the entropy production and its scaling with effective collision frequency and magnetic field. This entropy source was used in an entropy transport equation resulting in a differential equation for the effective electron mobility. With this formulation, the effective electron mobility is a computed variable. This paper presents the results of 2-D hybrid simulations of a laboratory Hall thruster that incorporate this model and compares these results to simulations using both Bohm and experimental measurements for the mobility. Simulations were also performed using this model for varying discharge voltage and background gas density. The background gas density is found to have a significant effect on the computed discharge current. The entropy closure model is found to better capture the current-voltage characteristics that are measured experimentally than other models used for electron transport.

General Cause of Sheath Instability Identified for Low Collisionality Plasmas in Devices with Secondary Electron Emission

A condition for sheath instability due to secondary electron emission (SEE) is derived for low collisionality plasmas. When the SEE coefficient of the electrons bordering the depleted loss cone in energy space exceeds unity, the sheath potential is unstable to a negative perturbation. This result explains three different instability phenomena observed in Hall thruster simulations including a newly found state with spontaneous ∼20 MHz oscillations. When instabilities occur, the SEE propagating between the walls becomes the dominant contribution to the particle flux, energy loss and axial transport.

Ion Current in Hall Thrusters

To match the observed thrust and discharge current in Hall thrusters, computer simulations have typically assumed anomalous electron transport mechanisms, such as Bohm diffusion, to enhance electron mobility across magnetic field lines. Without enhanced electron transport, the simulations predict a much lower discharge current than observed, and too much of the potential drop is downstream of the channel exit. Rather than search for mechanisms to increase the electron scattering frequency, we seek to identify mechanisms that would increase the fraction of the current carried by ions, thus reducing the required electron current. We describe two mechanisms that enhance the current carried by ions. The first mechanism increases the ion current carried in the channel by simply including the effects of multiply charged ions on the plasma response. The importance of using accurate ionization cross sections and the need to include multiply charged ions even for discharge voltages of 300 V are discussed. The second mechanism is a process we term ldquoion reflux.rdquo In this process, current is carried by ions generated downstream of the channel exit. Portions of these ions impact the center of the thruster and are neutralized by cathode electrons. A large fraction of the resultant neutral atoms are reionized as they pass through the main exhaust beam. These newly born ions then carry additional current through the plume. Since neutrals freely move across magnetic field lines, ion reflux effectively enhances the cross-field electron transport. This second mechanism operates downstream of the exit plane and does not enhance electron transport in the acceleration region. Both of these mechanisms, i.e., multiply charged ions and ion reflux, reduce the need to invoke anomalous electron transport mechanisms in Hall thruster computer simulations. However, to date, no Hall thruster simulation has produced results in agreement with data without assuming some anomalous electron transport.

Shear-Based Model for Electron Transport in Hybrid Hall Thruster Simulations

An electron cross-field transport model based on instantaneous simulated plasma properties is incorporated into a radial-axial hybrid simulation of a Hall plasma thruster. The model is used to capture the reduction of fluctuation-based anomalous transport that is seen experimentally in the region of high axial shear in the electron fluid. Similar transport barriers are observed by the magnetic confinement fusion community due to shear suppression of plasma turbulence through an increase in the decorrelation rate of plasma eddies. The model assumes that the effective Hall parameter can be computed as the sum of the classical term, a near-wall conductivity term, and a fluctuation-based term that includes the effect of shear. A comparison is made between shear-based, experimental, and Bohm-type models for cross-field transport. Although the shear-based model predicts a wider transport barrier than experimentally observed, overall, it better predicts measured plasma properties than the Bohm model, particularly in the case of electron temperature and electric potential. The shear-based transport model also better predicts the breathing-mode oscillations and time-averaged discharge current than both the Bohm and experimental mobility models. The plasma property that is most sensitive to adjustment of the fitting parameters used in the shear-based model is the plasma density. Applications of these fitting parameters in other operating conditions and thruster geometries are examined in order to determine the robustness and portability of the model. Without changing the fitting parameters, the simulation was able to reproduce macroscopic properties, such as thrust and efficiency, of an SPT-100-type thruster within 30% and match qualitative expectations for a bismuth-fueled Hall thruster.