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

Fundamental Erosion Processes

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)

Two Success Stories as Examples

• 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 1: Barium Transport Processes in Xenon Hollow Cathodes

Example 2: Barium Surface Kinetics in Lithium Plasma Thrusters

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

Seeing a MC Cathode Heat up

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