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

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Presentation on theme: "Plasma-Electrode interactions in high- current-density plasmas Edgar Choueiri (Princeton) & Jay Polk (NASA-JPL) 3."— Presentation transcript:

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 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 Cathode Technologies That Would Be Impacted by This Research

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 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 Small Orifice Cathode Xenon Solution: Plasma is Concentrated Near Orifice Neutral Xenon Density, n e /10 21 (m -3 ) Electron Temperature, T e (eV) Emitter Temperature and Electron Current Density

10 Small Orifice Cathode Xenon Solution: Plasma is Concentrated Near Orifice 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, n e /10 19 (m -3 ) Equipotentials, (V)

11 Momentum Equation for Species i Simplified Form for Ba Ions Equation for Ba Ion Flux Corresponding Equation for Ba Atom Flux Continuity Equations for Atoms and Ions Numerical Model of Barium Transport 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

12 Example 1: Barium Transport Processes in Xenon Hollow Cathodes

13 Example 2: Barium Surface Kinetics in Lithium Plasma Thrusters

14

15 Equilibrium surface coverage of activator supplied from the vapor phase is given by: k a j n j,s = k d j N j 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 = T s The adsorption isotherm is given by: –P/(2  mkT) 1/2 =  j exp(-E d j /kT s )N j min f j 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 T s 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 Planck’s LawEmissivity CameraBeam Splitter Lens

19 MCVP Data MCVP views thruster end-on Cathode tip temperature 15 seconds after start-up: 560 nm532 nm 630 nm600 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


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