Study of the Plasma-Wall Interface – Measurement and Simulation of

Sheath Potential Profiles Samuel J. Langendorf, Mitchell L.R. Walker

High-Power Electric Propulsion Laboratory, Georgia Institute of Technology, Atlanta, GA 30332 USA

Laura P. Rose, Michael KeidarMicropropulsion and Nanotechnology Laboratory, George

Washington University, Washington, D.C. 20052 USA

Lubos Brieda Particle in Cell Consulting LLC, Falls Church, VA 22046

49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 14 -17 July 2013, San Jose, California

Outline

• Motivation• Background• Experimental Method• Simulation Method• Results & Discussion• Conclusions• Acknowledgements• Questions

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Motivation

• The interaction between the plasma and wall is critical in electric propulsion devices

– Power Deposition Performance– Wall Erosion Lifetime

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Background

• Plasma-wall interaction: the plasma sheathNon-neutral region that forms near walls interacting with plasma to equalize fluxes of + and – charge.

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- Theory for floating wall, collisionless Argon plasma with cold ions

Background

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Background

• Research objectives: – Experimentally characterize plasma-wall

interactions– Develop predictive and efficient simulation

capability– Validate theoretical models

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Enable designers to take advantage of plasma-wall interaction and not be hindered by it

Background

• Where to start?

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In HET’s, decreasing current utilization and electron temperature saturation with high SEE (BN) vs. low SEE (carbon velvet) discharge channel wall.1

1. Raitses, Y., et al. "Measurements of secondary electron emission effects in the Hall thruster discharge." Physics of Plasmas 13 (2006): 014502.

Performance limitation due to wall interaction (SEE)

Experimental Method

• To experiment with sheaths: Plasma cell– Multidipole-type plasma device selected

• Proven2

• low ne, ni

• Stability• In-vacuum

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Heated Filaments

Cusp shaped field

Permanent Magnets

Aluminum Frame

Create thick-sheath plasma for interrogation

2Lang, Alan, and Noah Hershkowitz. "Multidipole plasma density." Journal of Applied Physics 49.9 (1978): 4707-4710.

• Initial study: Measure sheath potential profile over wall material sample

• Layout:

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F

B

M

LP EPW

Experimental Method

F Filaments

M Permanent MagnetsB Magnetic FieldLP Langmuir Probe

EP Emissive ProbeW Wall material sampleX Measurement location

Key:3’

2’

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• Plasma Cell, on

Experimental Method

Simulation Method

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Simulate sheath and compare to experiment

Results & Discussion

• Langmuir Probe

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Results & Discussion

• Emissive Probe

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IncreasingEmission

Results & Discussion

• Emissive Probe

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Results & Discussion

• Experimental Results, BN (HP)

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Pressure Electron Density

Electron Temperature

Sheath Voltage

(10-5 Torr-Ar) (1014 m-3) (eV) (V)

10.0 ± 2.5 4.6 ± 1.1 1.23 ± 0.35 20.5 ± 2.0

7.5 ± 1.88 2.9 ± 0.7 1.66 ± 0.30 39.1 ± 3.5

5.0 ± 1.25 1.8 ± 0.4 2.16 ± 0.25 51.8 ± 2.4

Filament Bias Voltage:

-87 V

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• Potential difference across the sheath is significantly larger than predicted using theory / measured Te

– High-energy electron populations in multidipole plasma devices

Results & Discussion

Electron kinetic effects are significant

Experimental Results, BN (HP)

Sheath Voltage,Theoretical

Sheath Voltage,Experimental

(V) (V)6.4 ± 1.8 20.5 ± 2.08.6 ± 1.6 39.1 ± 3.5

11.2 ± 1.3 51.8 ± 2.4

• Experiment vs. Simulation

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Pressure Electron Density

Electron Temperature

Sheath Voltage

(10-5 Torr-Ar) (1014 m-3) (eV) (V)

10.0 ± 2.5 4.6 ± 1.1 1.23 ± 0.35 20.5 ± 2.0

7.5 ± 1.88 2.9 ± 0.7 1.66 ± 0.30 39.1 ± 3.5

5.0 ± 1.25 1.8 ± 0.4 2.16 ± 0.25 51.8 ± 2.4

Filament Bias Voltage:

-87 V

Results & Discussion

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• Simulated potential profiles agree with measurements within convolved experimental error when a potential drop is specified.

Results & Discussion

Confirmed that electrostatics are driving the sheath structure in this case, not SEE or ion-neutral collisions.

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Filament BiasBelow Ground

• Experimental Results, Al2O3

Filament Bias

Electron Density

Electron Temperature

Sheath Voltage

(V) (1014 m-3) (eV) (V)

-60 ± 0.25 3.5 ± 1.1 1.25 ± 0.35 38.8 ± 2.0

-70 ± 0.25 4.2 ± 1.1 0.95 ± 0.35 39.7 ± 2.0

-90 ± 0.25 3.6 ± 0.7 1.10 ± 0.30 8.5 ± 2.0

-120 ± 0.25 3.0 ± 0.4 1.15 ± 0.25 -2.6 ± 2.4

Neutral Pressure(Torr-Ar): 7.5 x 10-5

Results & Discussion

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• What causes the sheath disappearance?

Filament bias voltage increased

Primary electron energy increased

Energy flux to Al2O3 surface increased

Secondary electron emission increased

Sheath potential drop decreasedSheath disappearance!

Results & Discussion

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• When does the sheath disappearance occur?– For Argon plasma, predicted to occur when wall

SEE yield reaches 0.97.• Experimental electron temperatures are too

low to elicit this yield,

Results & Discussion

3Viel-Inguimbert, V. "Secondary electron emission of ceramics used in the channel of SPT." IEPC-2003-258, Toulouse, France. 2003.

but high temperatureelectrons could.

Electron kinetic effects are significant

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• Experiment, BN vs. Al2O3

Pressure Bias Electron Density

Electron Temperatur

e

Sheath Voltage

(10-5 Torr-Ar) (V) (1014 m-3) (eV) (V)

Al2O3 7.5 ± 1.88 90 3.6 ± 0.7 1.10 ± 0.30 8.5 ± 2.0

BN 7.5 ± 1.88 87 2.9 ± 0.7 1.66 ± 0.30 39.1 ± 3.5

Results & Discussion

– Observed sheaths in agreement with shape predicted by theory and simulation, but larger

• Believed due to incomplete knowledge of EEDF– Experimentally verified that SEE can alter both size and

shape of sheath potential profile and cause sheath disappearance

• Mechanism for increased energy loss to the wall

• Future Work– Improve Langmuir probe measurement to get EEDF– Incorporate measured EEDF into simulation– Measure SEE sheath with increased spatial resolution– Develop simulation of effects of SEE

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Conclusions

• Acknowledgements– This work is supported by the Air Force Office

of Scientific Research through Grant FA9550-11-10160

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Conclusions

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Experimental Method

Axial distance from magnet (in)

Radial distance from magnet (in)

Magnetic Field

Bulk plasma largely field-free

(G)

Gaussmeter 200

180

160

140

120

100

80

60

40

20

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.00.0 0.2 0.4 0.6 0.8 1.0 1.2

Background

SEE Yield

Al2O3 = High SEEBN = Med SEE

3Viel-Inguimbert, V. "Secondary electron emission of ceramics used in the channel of SPT."  IEPC-2003-258, Toulouse, France. 2003.

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• Plasma Cell

Experimental Method