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MODELING OF MICRODISCHARGES FOR USE AS MICROTHRUSTERS Ramesh A. - PowerPoint PPT Presentation

MODELING OF MICRODISCHARGES FOR USE AS MICROTHRUSTERS Ramesh A. Arakoni a) , J. J. Ewing b) and Mark J. Kushner c) a) Dept. Aerospace Engineering University of Illinois b) Ewing Technology Associates c) Dept. Electrical Engineering Iowa State


  1. MODELING OF MICRODISCHARGES FOR USE AS MICROTHRUSTERS Ramesh A. Arakoni a) , J. J. Ewing b) and Mark J. Kushner c) a) Dept. Aerospace Engineering University of Illinois b) Ewing Technology Associates c) Dept. Electrical Engineering Iowa State University mjk@iastate.edu, arakoni@uiuc.edu, jjewingta@comcast.net http://uigelz.ece.iastate.edu 52 nd AVS International Symposium, November 2, 2005. * Work supported by Ewing Technology Associates, NSF, and AFOSR.

  2. AGENDA • Introduction to microdischarge (MD) devices • Description of model • Reactor geometry and parameters • Plasma characteristics • Effect of geometry, and power • Incremental thrust, and effect of power • Concluding Remarks Iowa State University Optical and Discharge Physics AVS2005_RAA_01

  3. MICRODISCHARGE PLASMA SOURCES • Microdischarges are plasma devices which leverage pd scaling to operate dc atmospheric glows 10s –100s µ m in size. • Few 100s V, a few mA • Although similar to PDP cells, MDs are usually dc devices which largely rely on nonequilibrium beam components of the EED. • Electrostatic nonequilibrium results from their small size. Debye lengths and cathode falls are commensurate with size of devices. ( ( ) ) = ε ≈ − µ 1 / 2 L 2 V / qn 10 20 m cathode Fall c 0 I 1 / 2 ⎛ ⎞ T ⎜ ⎟ λ ≈ ≈ µ eV 750 cm 10 m , ⎜ ⎟ − D 3 ⎝ ⎠ n ( cm ) e • Ref: Kurt Becker, GEC 2003 Iowa State University Optical and Discharge Physics AVS2005_RAA_02

  4. APPLICATIONS OF MICRODISCHARGES • MEMS fabrication techniques enable innovative structures for displays and detectors. • MDs can be used as microthrusters in small spacecraft for precise control which are requisites for array of satellites. FLOW THRU MICRO- DISCHARGE HOT GAS TO GAS IN NOZZLE Ewing Technology Associates Ref: http://www.design.caltech.edu/micropropulsion Iowa State University Optical and Discharge Physics AVS2005_RAA_03

  5. DESCRIPTION OF MODEL • To investigate microdischarge sources, nonPDPSIM, a 2- dimensional plasma code was developed with added capabilities for pulsed operation. • Finite volume method in rectilinear or cylindrical unstructured meshes. • Implicit drift-diffusion-advection for charged species • Navier-Stokes for neutral species • Poisson’s equation (volume, surface charge, material conduction) • Secondary electrons by impact, thermionics, photo-emission • Electron energy equation coupled with Boltzmann solution • Monte Carlo simulation for beam electrons. • Circuit, radiation transport and photoionization, surface chemistry models. Iowa State University Optical and Discharge Physics AVS2005_RAA_04

  6. DESCRIPTION OF MODEL: CHARGED PARTICLE, SOURCES • Continuity (sources from electron and heavy particle collisions, surface chemistry, photo-ionization, secondary emission), fluxes by modified Sharfetter-Gummel with advective flow field. � � ∂ N = − ∇ ⋅ φ + i S ∂ i t − ∇ ⋅ ε ∇ Φ = ρ + ρ • Poisson’s Equation for Electric Potential: V S • Photoionization, electric field and secondary emission: � � ′ ⎛ − − ⎞ � � � r r ′ ⎜ ⎟ ′ ⌠ σ 3 N ( r ) N ( r ) exp d r ⎜ ⎟ λ i ij j ⎮ � ⎝ ⎠ = S ( r ) � � ⎮ Pi ′ π − 2 ⎮ 4 r r ⌡ ( ) ( ) ⎛ ⎞ − Φ − ε 1/2 3 E/ q ∑ ⎜ ⎟ = −∇ ⋅ = = γ φ 2 W 0 S j , j AT exp , j ⎜ ⎟ Si E S ij j kT ⎝ ⎠ j S Iowa State University Optical and Discharge Physics AVS2005_RAA_05

  7. ELECTRON ENERGY, TRANSPORT COEFFICIENTS • Bulk electrons: Electron energy equation with coefficients obtained from Boltzmann’s equation solution for EED. ( ) � � � � ∂ ε ⎛ ⎞ n 5 ∑ = ⋅ + σ − κ − ∇ ⋅ εϕ − λ ∇ = φ ⎜ ⎟ 2 e j E E n N T , j q ∂ EM e i i e e ⎝ ⎠ t 2 i • Beam Electrons: Monte Carlo Simulation • Cartesian MCS mesh superimposed on unstructured fluid mesh. Construct Greens functions for interpolation between meshes. Iowa State University Optical and Discharge Physics AVS2005_RAA_06

  8. DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT • Fluid averaged values of mass density, mass momentum and thermal energy density obtained using unsteady, compressible algorithms. ∂ ρ � = −∇ ⋅ ρ + ( v ) ( inlets , pumps ) ∂ t � ( ) ( ) � ∂ ρ ⎛ ⎞ � � v ( ) ∑ ∑ = ∇ ⎜ ⎟ − ∇ ⋅ ρ − ∇ ⋅ τ + − µ N kT v v q N E S m q E ∂ i i i i i i i i i ⎝ ⎠ t i i ( ) ∂ ρ � � ( ) � c T ∑ ∑ = −∇ − κ ∇ + ρ + ∇ ⋅ − ∆ + ⋅ p T v c T P v R H j E ∂ p i f i i i t i i • Individual species are addressed with superimposed diffusive transport. ( ) ⎛ ⎞ ⎛ ⎞ + ∆ � ( ) ( ) N t t ⎜ ⎟ ⎜ ⎟ + ∆ = − ∇ ⋅ − ∇ + + i N t t N t v D N S S ⎜ ⎟ ⎜ ⎟ i i f i T V S ⎝ ⎠ N ⎝ ⎠ T Iowa State University Optical and Discharge Physics AVS2005_RAA_07

  9. GEOMETRY AND MESH • Geometry A • Geometry B • Plasma dia: 150 µ m at inlet, 250 µ m at cathode. • Electrodes 130 µ m thick. • Dielectric gap 1.5 mm. • Geometry B: 1.5 mm dielectric above the cathode. • Fine meshing near electrodes, less refined near exit. • Anode grounded; cathode bias varied based on power deposition (0.25 - 1.0 W). • 10 sccm Ar, 30 Torr at inlet, 10 Torr at exit. Iowa State University Optical and Discharge Physics AVS2005_RAA_08

  10. EXPERIMENT: GEOMETRY • Modeled geometry similar to experimental setup. • Plume characterized by densities of excited states. • Ref: John Slough, J.J. Ewing, AIAA 2005-4074 Iowa State University Optical and Discharge Physics AVS2005_RAA_09

  11. CHARGED SPECIES: GEOMETRY A [Ar + ] (10 11 cm -3 ) [e] (10 11 cm -3 ) Potential (V) E-field (kV/cm) 1 200 0 -250 1 18 • Power deposition occurs in the cathode fall by • 10 sccm Ar, 0.5 W collisions with hot electrons. • Very high electric fields near cathode. Iowa State University Optical and Discharge Physics AVS2005_RAA_10

  12. NEUTRAL FLUID PROPERTIES: GEOMETRY A Ar 4s (10 11 cm -3 ) Ar 4p (10 11 cm -3 ) Gas temp (°K) 100 200 700 1 1 300 • Plume extends downstream, can be used for diagnosis. • • 10 sccm Ar, 30 – 10 Torr Gas heating and consequent expansion is a source of thrust. • 0.5 W. • Ref: John Slough, J.J. Ewing, AIAA 2005-4074 Iowa State University Optical and Discharge Physics AVS2005_RAA_11

  13. VELOCITY INCREASE WITH DISCHARGE Without With • Gas heating and discharge discharge subsequent expansion causes increase in velocity. • Steady state after one or two bursts of flow. • At high plasma density, momentum transfer between charged species and neutrals is also important. V max 130 m/s V max 170 m/s • 10 sccm Ar, 30 Torr at inlet, 10 Torr at exit. • 0.5 Watts. • Power turned on at 0.5 ms. 160 0 Animation Iowa State University Axial velocity (m/s) Optical and Discharge Physics 0 – 0.55 ms AVS2005_RAA_12

  14. POWER DEPOSITION: IONIZATION SOURCES • 0.5 W • 1.0 W 0.5 W 1.0 W Max 5 x 10 20 Max 7.5 x 10 20 Max 1.5 x 10 20 Max 2 x 10 20 Bulk ionization (cm -3 sec -1 ) 100 Beam ionization (cm -3 sec -1 ) 1 • Ionization rates increase with power. • Beam electrons are equally as important as bulk electrons. • 10 sccm Ar, 30 Torr at inlet, 10 Torr at exit. Iowa State University Optical and Discharge Physics AVS2005_RAA_13

  15. POWER DEPOSITION: PLASMA PROPERTIES • 0.75 W • 0.5 W • 1 W • 0.5 W • 0.75 W • 1 W Max 3.5 x 10 13 Max 2.25 x 10 13 Max 2 x 10 13 Max 700 Max 900 Max 980 [e] (cm -3 ) 5 x 10 11 Max Temperature (°K) 300 Max • Hotter gases lead to higher ∆ V and higher thrust production. • Increase in mean free path due to rarefaction may affect power deposited to neutrals. • With increasing [e], increase in production of electronically excited states. • 10 sccm Ar, 30 Torr at inlet, 10 Torr at exit. Iowa State University Optical and Discharge Physics AVS2005_RAA_14

  16. POWER DEPOSITION: FLOW VELOCITY Power off 0.5 W 1.0 W Max 80 Max 160 Max 200 • 10 sccm Ar, 30 Torr at V y compared in the above plane. inlet, 10 Torr at exit. • Power turned on at 0.5 ms. MAX 5 Iowa State University Axial velocity (m/s) Optical and Discharge Physics AVS2005_RAA_15

  17. BASE CASE RESULTS: GEOMETRY B [e] (cm -3 ) Potential (V) Bulk Ionization(cm -3 s -1 ) Gas temp (°K) Max 8 x 10 20 Max 1 x 10 14 1 901 100 100 301 1 0 -320 • Electrons are confined, discharge operates in an unsteady regime. • Ionization pulses travel towards anode. • Power densities are greater than that of Geometry A. • 10 sccm Ar, 30 – 10 Torr Iowa State University • 0.5 W, turned on at 0.5 ms Optical and Discharge Physics AVS2005_RAA_16

  18. VELOCITY INCREASE: GEOMETRY B 0.5 W • Increase in velocity is due to expansion of hot gas. V y compared • Axial-velocity increase Max 140 Max 400 in the above not substantial at exit. plane. • 10 sccm Ar, 30 – 10 Torr • 0.5 W, turned on at 0.5 ms Animation MAX Axial velocity (m/s) Iowa State University 5 Optical and Discharge Physics 0 – 0.65 ms AVS2005_RAA_17

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