Cathode Coupling Physics in a Hall Thruster

Introduction

The main components of Hall thrusters consist of a discharge channel with an interior anode, internally or externally mounted cathode, and magnetic circuit. A propellant gas, typically xenon, is fed into the annular discharge channel through the anode, and electrons are emitted from the cathode. The cathode emits electrons to ionize the propellant gas in the discharge channel and to neutralize the ion beam downstream of the thruster. How easily these electrons are transported from the cathode to the anode for the ionization can be described by the cathode coupling voltage which is the potential difference from cathode common to the beam plasma potential.  A lower cathode coupling voltage increases voltage utilization efficiency which affects the total thruster performance.

As the design for Hall thruster becomes more mature and optimized, the major concern that potentially limits Hall thruster for challenging deep-space exploration missions is, now, the failure of hollow cathodes due to erosion of cathode orifice plate and keeper. This is mainly due to the presence of high-energy ions generated from the interaction of electrons and neutrals just outside of the hollow cathode keeper, and striking back to the cathode. The evolution of plasma in the near-plume of hollow cathode is critical in understanding of the source(s) of high-energy ions as well as the transport of electrons from the cathode to the anode.

The ultimate goal of this project is to acquire better understanding of 1). cathode coupling physics in a Hall thruster, and 2). electron transport from the cathode to anode across the magnetic field.  Both numerical and experimental studies are necessary to provide the first-principles description of electron transport and cathode coupling in a Hall thruster.

Background

While the "fluid" assumption for neutrals is valid inside the cathode emitter, it losses the continuum approximation as Knudsen number (Kn) becomes much greater than one for the flow going out the cathode orifice. Knudsen number is defined as the ratio of the mean free path to the characteristic length, and is an important parameter that indicates the validity of continuum approaches.  Generally, the continuum approximation is valid for Kn < 0.01, the continuum assumption becomes not applicable for Kn > 0.1 because intermolecular effects are not significant due to collisionless flow regime, and 0.01 < Kn < 0.1 represents a transition region between the continuum and free molecular regimes.  Typically, the computational approach to simulate the partially-ionized gas in hollow cathodes and Hall thrusters is to use "hybrid" method: 1) solve the fluid conservation laws (i.e., mass, momentum, and energy) for the electrons, and 2) use kinetic, particle approach to track the evolution of the heavy species, i.e., ions and neutrals. 

To account for the long mean free path for collisions, especially those that lead to an ionization, neutral gas is typically simulated using the particle-in-cell (PIC) combined with direct simulation Monte Carlo (DSMC) to account for ionization. Generally, the DSMC method is effective in simulating dilute neutral gas flow, while the PIC method is effective in simulating dilute flows with charges and electric and magnetic field effects, i.e., plasma[1].  A particular DSMC code coupled with PIC, called MONACO-PIC (or MPIC) is used to investigate the hollow cathode near-plume plasma in this project.

Approach

As the first step of this project, a two-dimensional (2-D) axisymmetric orifice cathode (OrCa2D)[2] fluid code developed at the Jet Propulsion Laboratory (JPL) has been used to simulate the internal and near-plume plasma of a hollow cathode. The OrCa2D uses finite volume scheme with first-order upwind and strong implicit, time-split approach. Currently, preliminary study of plasma and neutral flows in hollow cathode is being conducted using the MONACO-PIC code, which will be verified with the simulation result from Ref. [3].  Then, the computational domain of the OrCa2D simulation will be regenerated using the MONACO-PIC to compare the results. 

Expected Outcomes

Expected outcomes of this project are as follows:

  1. A more detailed knowledge of a first-principles description of electron transport and cathode coupling in Hall thruster
  2. An improved numerical model that will accurately capture the phenomena of cathode coupling and the electron transport between the electrodes in a Hall thruster
  3. Useful insight to improve the thruster performance and critically aid in the development of next generation Hall thrusters

Investigators

Maria Choi

Acknowledgments

This research is supported by NASA Space Technology Research Fellowship (NSTRF).

References

  1. Cai, C., Theoretical and numerical studies of plume flows in vacuum chambers, 2005.
  2. Mikellides, I., Katz, I., Goebel, D., and Polk, J., °Hollow cathode theory and experiment. II. A two-dimensional theoretical model of the emitter region," Journal of applied physics, Vol. 98, No. 11, 2005, pp. 113303-113303.
  3. Boyd, I.D., and M.W. Crofton. "Modeling the plasma plume of a hollow cathode." Journal of applied physics 95.7 (2004): 3285-3296.