Study of the Plasma-Wall Interface – Measurement and Simulation of Sheath Potential Profiles
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Transcript of Study of the Plasma-Wall Interface – Measurement and Simulation of Sheath Potential Profiles
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
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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