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Plasma-Electrode interactions in high-
current-density plasmas
Edgar Choueiri (Princeton) &
Jay Polk (NASA-JPL)
3
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
Example: Erosion Processes in a Thoriated-Tungsten Cathode
TemperatureFeedback
DeterminesCathode
Temperature
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?
• 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
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)
• 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, ne/1021 (m-3)
Electron Temperature, Te (eV)
Emitter Temperature and Electron Current Density
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 orificeXenon Plasma Density, ne/1019 (m-3)
Equipotentials, (V)
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
Example 2: Barium Surface Kinetics in Lithium Plasma Thrusters
0.001
0.01
0.1
1
10
100
1000
Current Density (A/cm
2 )
4000350030002500200015001000Temperature (K)
PBa = 100 Pa
10 Pa
1 Pa
Pure W (110) Pure Ba Ba-W (110)
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 = kd
jNj
• 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)N
jminfj
• This approach neglects:– Activator transport through concentration boundary
layer– Electric field effects on ionized activator species
transport in plasma
5
4
3
2
1
Desorption Energy (eV)
3.02.52.01.51.00.50.0Coverage, f
Ba-W (110) Li-W (110)
100
80
60
40
20
0
Pre-exponential Factor (10
12 s
-1)
45004000350030002500200015001000Temperature (K)
Ba-W (110) Li-W (110)
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
0.01
2
468
0.1
2
468
1
2
4
Coverage, f
45004000350030002500200015001000Temperature (K)
P=103 Pa
104 Pa
105 Pa
0.01
2
4
68
0.1
2
4
68
1
2
4
Surface Coverage, f
4000350030002500200015001000Temperature (K)
PBa = 100 Pa
10 Pa
1 Pa
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
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
Camera Beam Splitter Lens
MCVP Data
• MCVP views thruster end-on • Cathode tip temperature 15 seconds after start-up:
560 nm 532 nm
630 nm 600 nm
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