Plasma-Electrode interactions in high-current-density plasmas

1. Plasma-Electrode interactions in high-current-density plasmas • Edgar Choueiri (Princeton) • & • Jay Polk (NASA-JPL) • 3

2. Relevance • Why are electrode-plasma interactions important? • Electrodes are often the life-limiting components in high-current-density devices (e.g. electric thrusters) • Plasma-surface interactions drive electrode life

3. Fundamental Erosion Processes

4. Example: Erosion Processes in a Thoriated-Tungsten Cathode Temperature Feedback Determines Cathode Temperature

5. Fundamental Questions that Should be Addressed • Critical fundamental issues for electrodes in contact with plasmas • What are the mechanisms controlling electrode erosion? • What steps are rate-controlling? • How can they be modeled? • How do we maintain a low work function surface? • What are the material transport processes in the near-electrode plasma?

6. Cathode Technologies That Would Be Impacted by This Research • Dispenser Cathodes • (Low work function barium activator material in the cathode) • Lanthanum Hexaboride Cathodes • (Low work function bulk material) • Multi-Channel Hollow Cathodes • (activator material in propellant vapor stream) • Field Emission Cathodes

7. Approaches--Modeling • Model transport processes in plasmas • Oxidizing contaminants responsible for chemical erosion • Low work function activator materials (example 1: barium in xenon dispenser cathodes) • Evaporated bulk cathode materials • Model surface reactions such as oxidation in chemical attack • Surface kinetics (adsorption/desorption) of low work function activators (example 2: barium on tungsten in lithium multi-channel hollow cathodes)

8. Two Success Stories as Examples

9. Small Orifice Cathode Xenon Solution: Plasma is Concentrated Near Orifice Neutral Xenon Density, ne/1021 (m-3) • Results for the small orifice configuration with Jd=13.3 A, m=3.7 sccm • Small orifice leads to high neutral density, drops rapidly near orifice • Electron temperature peaks in the orifice • Electron emission current density is concentrated in the first 4 mm of the insert • Emitter temperature peaks at the orifice Electron Temperature, Te (eV) Emitter Temperature and Electron Current Density

10. Small Orifice Cathode Xenon Solution: Plasma is Concentrated Near Orifice Equipotentials, (V) • The electric field points out of the ionization zone • Large potential drop near the emitter surface • High plasma density with a peak near the orifice Xenon Plasma Density, ne/1019 (m-3)

11. Numerical Model of Barium Transport Corresponding Equation for Ba Atom Flux Momentum Equation for Species i • Other model components: • Collision frequencies based on measured cross sections or Coulomb collisions • Results of xenon discharge model used to specify major species parameters • Xenon plasma parameters treated as constant values in minor species solution Simplified Form for Ba Ions Continuity Equations for Atoms and Ions Equation for Ba Ion Flux

12. Example 1: Barium Transport Processes in Xenon Hollow Cathodes

13. Example 2: Barium Surface Kinetics in Lithium Plasma Thrusters

14. Example 2: Barium Surface Kinetics in Lithium Plasma Thrusters

15. Example 2: Barium Surface Kinetics in Lithium Plasma Thrusters • Equilibrium surface coverage of activator supplied from the vapor phase is given by: • kajn j,s = kdjNj • Assumptions for the coverage model: • Non-activated adsorption • Non-localized adsorption sites • No competing absorbate species • Flux to surface equals thermal flux of vapor at T = Ts • The adsorption isotherm is given by: • P/(2πmkT)1/2 = ωj exp(-Edj/kTs)Njminfj • This approach neglects: • Activator transport through concentration boundary layer • Electric field effects on ionized activator species transport in plasma

16. Adsorption Isotherms Give Required Partial Pressures of Vapor-Phase Activators • The relationship describing a balance between adsorption and desorption can be solved for the equilibrium surface coverage for a given P and Ts • Lithium requires extremely high vapor pressures to maintain a high surface coverage • Barium appears to require very modest partial pressures for reasonable coverage

17. Approaches--Experiments • Measure plasma flow properties inside cathodes • LIF • Line emission spectroscopy • Fast microprobes • Measure transport of minor species through the plasma • LIF • Line emission spectroscopy • Mass spectrometry • Characterize surface reactions and desorption rates • Surface diagnostics (SEM, XPS, EDS, etc.) • Reaction kinetics measurements (time resolved concentrations) measurements) • Measure electrode temperatures • Multi-wavelength pyrometry • Small embedded thermocouples • Fast fiber optic probes

18. Multi-Color Video Pyrometry • Intensity measured at four wavelengths and data fit to appropriate intensity model: • Image split four ways to pass through separate narrow bandwidth optical filters and recorded with a digital camera Emissivity Planck’s Law Camera Beam Splitter Lens

19. MCVP Data • MCVP views thruster end-on • Cathode tip temperature 15 seconds after start-up: 560 nm 532 nm 630 nm 600 nm

20. Seeing a MC Cathode Heat up

21. Conclusions • Plasma-electrode interactions are critical to many high-current-density devices including plasma thrusters • Requires collaboration between plasma physicists and material scientists • Need for more predictive/accurate models • Need for more specialized diagnostics with high accuracy and high temporal and spatial resolution