Figure 1. Implementation of flush-mounted Langmuir probes
along the inner and outer channel walls of the 6-kW Hall thruster
. Each wall contained five Langmuir probes at various axial locations, as well as a null probe to characterize line capacitance. (From ref. 3.)
Figure 2. Measured sheath potentials at eight different operating conditions along the walls of the 6-kW Hall thruster
, as a function of measured electron temperature.
Sheath potentials are close to theoretical values given no secondary electron emission (SEE).
The Hobbs and Wesson solution for tungsten (W) and boron nitride (BN) are also given, indicating sheath potential behavior for space-charge-limited SEE; this solution corresponds to complete thermalization of secondary electrons in the bulk plasma. (From ref. 3.)
A major component of any Hall thruster is the channel wall lining adjacent to the discharge plasma.
While primarily used to protect the magnetic circuit from the harsh plasma environment, the channel walls are also a significant factor in Hall thruster performance and lifetime through its interactions with the discharge.
These interactions are governed by the sheath formed along the walls, a non-neutral layer which regulates the plasma current absorbed by the surface.
Thus, properties of the sheath determine the amount of electron energy absorbed by the wall, which in turn affects the electron dynamics within the bulk discharge.
Furthermore, the energy imparted by the sheath to the ions within the discharge determines the impact energy and incident angle of ions upon the surface, thus affecting the amount of material sputtered and consequently the wall erosion rate.
Since the primary failure mechanism in Hall thrusters is the erosion of the discharge channel walls, the sheath and related wall physics heavily influence thruster lifetime.
While significant progress has been made in understanding Hall thruster sheath physics, studies have been primarily based on theory and simulation.
Unfortunately, limitations have been reached in accurately determining the secondary electron emission characteristics as well as the sputtering yield of pertinent wall materials.
To date, simulations have been unable to accurately reproduce the erosion progression measured by experiment.
Thus, an experimentally-based study is necessary to illuminate the strengths and weaknesses of present sheath and erosion models.
Since the sheath thickness within Hall thrusters is typically less than a millimeter, traditional means of interrogating the channel (e.g. high-speed reciprocating probes, LIF) are unable to achieve the required proximity to the walls.
Thus, flush-mounted Langmuir probes are employed in this study along the inner and outer channel walls to experimentally determine pertinent properties such as ion currents, sheath potentials, and electron energy distribution functions (EEDFs).
Figure 3. Measured electron energy distribution function (EEDF) at the inner wall of the 6-kW Hall thruster
, upstream of the acceleration zone.
The corresponding Maxwellian distribution given the measured electron temperature is shown for comparison.
The measured EEDF gives further evidence of the presence of a high-energy electron tail close to the channel wall.
Analysis of the resulting data has shown that the plasma adjacent to the wall contains high-energy electrons, with a near-Maxwellian energy distribution before the plasma is accelerated out of the channel.
Measured sheath potentials on the order of five electron temperatures indicate that the high-energy population of electrons is not being lost to the walls but is in fact being replenished.
This is strong evidence of thermalization of the electron population, including secondary electrons emitted from the discharge channel walls.
The degree of thermalization strongly affects the resulting sheath properties; therefore, this result is significant towards properly understanding and modeling Hall thruster sheath physics.
Future work includes using the resulting data to predict erosion rates and comparing the resulting erosion profiles to those actually observed in the thruster.
This study will help illuminate necessary sputtering yield features to match predicted and observed profiles.
Selected Relevant Publications
Rohit Shastry, Alec Gallimore, Richard Hofer, "Near-Wall Plasma Properties and EEDF Measurements of a 6-kW Hall Thruster,"
AIAA-2009-5356, 45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Denver, CO, 2-5 August 2009.
Rohit Shastry, Alec D. Gallimore, and Richard R. Hofer, "Near-Wall Plasma Characterization of a 6-kW Hall Thruster,"
IEPC-2009-133, 31st International Electric Propulsion Conference, Ann Arbor, MI, USA, September 20-24, 2009.
R. Shastry, A. D. Gallimore, and R. R. Hofer, "Erosion Characterization via Ion Power Deposition Measurements in a 6-kW Hall Thruster,"
57th Joint Army Navy NASA Airforce (JANNAF) Propulsion Meeting, Colorado Springs, CO, May 3-7, 2010.