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 USA Laura P. Rose, Michael Keidar Micropropulsion and Nanotechnology Laboratory, George Washington University, Washington, D.C USA Lubos Brieda Particle in Cell Consulting LLC, Falls Church, VA th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, July 2013, San Jose, California

Outline Motivation Background Experimental Method Simulation Method Results & Discussion Conclusions Acknowledgements Questions 2

Motivation The interaction between the plasma and wall is critical in electric propulsion devices –Power Deposition  Performance –Wall Erosion  Lifetime 3

Background Plasma-wall interaction: the plasma sheath Non-neutral region that forms near walls interacting with plasma to equalize fluxes of + and – charge. 4 - Theory for floating wall, collisionless Argon plasma with cold ions

Background 5

Background Research objectives: –Experimentally characterize plasma-wall interactions –Develop predictive and efficient simulation capability –Validate theoretical models 6 Enable designers to take advantage of plasma-wall interaction and not be hindered by it

Background Where to start? 7 In HET’s, decreasing current utilization and electron temperature saturation with high SEE (BN) vs. low SEE (carbon velvet) discharge channel wall Raitses, Y., et al. "Measurements of secondary electron emission effects in the Hall thruster discharge." Physics of Plasmas 13 (2006): Performance limitation due to wall interaction (SEE)

Experimental Method To experiment with sheaths: Plasma cell –Multidipole-type plasma device selected Proven 2 low n e, n i Stability In-vacuum 8 Heated Filaments Cusp shaped field Permanent Magnets Aluminum Frame Create thick-sheath plasma for interrogation 2 Lang, Alan, and Noah Hershkowitz. "Multidipole plasma density." Journal of Applied Physics 49.9 (1978):

Initial study: Measure sheath potential profile over wall material sample Layout: 9 F B M LP EP W Experimental Method FFilaments MPermanent Magnets BMagnetic Field LPLangmuir Probe EPEmissive Probe WWall material sample XMeasurement location Key: 3’ 2’

10 Plasma Cell, on Experimental Method

Simulation Method 11 Simulate sheath and compare to experiment

Results & Discussion Langmuir Probe 12

Results & Discussion Emissive Probe 13 Increasing Emission

Results & Discussion Emissive Probe 14

Results & Discussion Experimental Results, BN (HP) 15 Pressure Electron Density Electron Temperature Sheath Voltage (10 -5 Torr-Ar)(10 14 m -3 )(eV)(V) 10.0 ± ± ± ± ± ± ± ± ± ± ± ± 2.4 Filament Bias Voltage: -87 V

16 Potential difference across the sheath is significantly larger than predicted using theory / measured T e –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) 6.4 ± ± ± ± ± ± 2.4

Experiment vs. Simulation 17 Pressure Electron Density Electron Temperature Sheath Voltage (10 -5 Torr-Ar)(10 14 m -3 )(eV)(V) 10.0 ± ± ± ± ± ± ± ± ± ± ± ± 2.4 Filament Bias Voltage: -87 V Results & Discussion

18 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.

19 Filament Bias Below Ground Experimental Results, Al 2 O 3 Filament Bias Electron Density Electron Temperature Sheath Voltage (V)(10 14 m -3 )(eV)(V) -60 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 2.4 Neutral Pressure (Torr-Ar): 7.5 x Results & Discussion

20 What causes the sheath disappearance? Filament bias voltage increased Primary electron energy increased Energy flux to Al 2 O 3 surface increased Secondary electron emission increased Sheath potential drop decreased Sheath disappearance! Results & Discussion

21 When does the sheath disappearance occur? –For Argon plasma, predicted to occur when wall SEE yield reaches Experimental electron temperatures are too low to elicit this yield, Results & Discussion 3 Viel-Inguimbert, V. "Secondary electron emission of ceramics used in the channel of SPT." IEPC , Toulouse, France but high temperature electrons could. Electron kinetic effects are significant

22 Experiment, BN vs. Al 2 O 3 PressureBias Electron Density Electron Temperature Sheath Voltage (10 -5 Torr-Ar)(V)(10 14 m -3 )(eV)(V) Al 2 O ± ± ± ± 2.0 BN7.5 ± ± ± ± 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 23 Conclusions

Acknowledgements –This work is supported by the Air Force Office of Scientific Research through Grant FA 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

Background SEE Yield Al 2 O 3 = High SEE BN = Med SEE 3 Viel-Inguimbert, V. "Secondary electron emission of ceramics used in the channel of SPT." IEPC , Toulouse, France

28 Plasma Cell Experimental Method