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MODELING OF MICRODISCHARGES FOR USE AS MICROTHRUSTERS

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

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MODELING OF MICRODISCHARGES FOR USE AS MICROTHRUSTERS

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  1. MODELING OF MICRODISCHARGES FOR USE AS MICROTHRUSTERS Ramesh A. Arakonia) , J. J. Ewingb) and Mark J. Kushnerc) a) Dept. Aerospace Engineering University of Illinois b) Ewing Technology Associates c) Dept. Electrical Engineering Iowa State University mjk@iastate.edu, arakoni@uiuc.edu http://uigelz.ece.iastate.edu June 20, 2005. * Work supported by Ewing Technology Associates, NSF, and AFOSR. ICOPS2005_RAA_00

  2. AGENDA • Introduction to microdischarge (MD) devices • Description of model • Reactor geometry and parameters • Plasma characteristics for dc case • Incremental thrust, and motivation for pulsing • Effect of power on plasma characteristics • Effect of pulse width, power on incremental thrust • Concluding Remarks Iowa State University Optical and Discharge Physics ICOPS2005_RAA_01

  3. MICRODISCHARGE PLASMA SOURCES • Microdischarges are plasma devices which leverage pd scaling to operate dc atmospheric glows 10s –100s 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. • Ref: Kurt Becker, GEC 2003 Iowa State University Optical and Discharge Physics ICOPS2005_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. Ewing Technology Associates Ref: http://www.design.caltech.edu/micropropulsion Iowa State University Optical and Discharge Physics ICOPS2005_RAA_03

  5. DESCRIPTION OF MODEL • To investigate microdischarge sources, nonPDPSIM, a 2-dimensional DC plasma code was developed with added capabilities for pulsed and rf discharges. • Finite volume method used 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 ICOPS2005_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. • Poisson’s Equation for Electric Potential: • Photoionization, electric field and secondary emission: Iowa State University Optical and Discharge Physics ICOPS2005_RAA_05

  7. ELECTRON ENERGY, TRANSPORT COEFFICIENTS • Bulk electrons: Electron energy equation with coefficients obtained from Boltzmann’s equation solution for EED. • 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 ICOPS2005_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. • Individual species are addressed with superimposed diffusive transport. Iowa State University Optical and Discharge Physics ICOPS2005_RAA_07

  9. GEOMETRY FORMICROTHRUSTER  Fine meshing near the cathode to capture beam electrons accurately. • Anode grounded, cathode biased –300 to –500 V.  50 Torr Ar at inlet, expanded to 10 or 1 Torr at exit.  Flow rates 3 or 6 sccm.  Plasma dia. 150 m  Cathode 100 m  Anode 50 m  Dielectric gap 0.5 mm Iowa State University Optical and Discharge Physics ICOPS2005_RAA_08

  10. PLASMA CHARACTERISTICS OF DC CASE Animation Axial velocity 0 – 1.005 ms  Power deposition occurs in the cathode fall by collisions with energetic electrons.  Gas heating is the primary source of thrust. 250 0 Axial velocity (m/s) -280 500 0 2 50 300 Tgas (°K) E-source (1019 cm-3 s-1) Potential (V) •  6 sccm Ar • -300 V • Power turned on at 1 ms Iowa State University Optical and Discharge Physics ICOPS2005_RAA_09

  11. EFFECT OF PLASMA ON VELOCITY Without discharge With discharge •  6 sccm Ar • -300 V • Power turned on at 1 ms 250 0 Iowa State University Optical and Discharge Physics Axial velocity (m/s) ICOPS2005_RAA_10

  12.  (10-6 g/cc) power off 5 100 3 sccm 6 sccm PLASMA CHARACTERISTICS: CURRENT-VOLTAGE • Discharge operates on the near side of the Paschen curve. • Neutral density increases with higher flow rate leading to higher ionization rates. Iowa State University Optical and Discharge Physics ICOPS2005_RAA_11

  13. INCREMENTAL THRUST: MOTIVATION FOR PULSING  Thrust calculated by:  Increase in thrust is the rate of momentum transfer to the neutrals when the discharge is switched on.  Meaningful incremental thrust occurs when plasma power to is greater than that of the flow  6 sccm Ar -300 V Power turned on at 1 ms  F calculated close to cathode Iowa State University Optical and Discharge Physics ICOPS2005_RAA_12

  14. PULSING: VOLTAGE vs TIME • Fluid initialized by simulating for 1 ms. • Pulses of 300, 400, 500 V applied for 0.5, 1 s. • Pulse width has effect on incremental thrust. Optimize the duration of the pulse for maximum thrust per energy expended. Iowa State University Optical and Discharge Physics ICOPS2005_RAA_13

  15. PLASMA CHARACTERISTICS : PULSED  Plasma reaches steady state quickly whereas the fluid does not. 425 300 -280 2 50 0 Ionization (1019 cm-3 sec-1) Potential (V) Beam electrons (1019 cm-3 sec-1) Tgas (°K)  6 sccm Ar -300 V 1 s pulse Iowa State University Optical and Discharge Physics ICOPS2005_RAA_14

  16. VARIATION OF [e] AND TGAS WITH BIAS VOLTAGE -300 V -400 V -500 V -300 V -400 V -500 V  Absence of electrons near cathode fall due to high fields and gas rarefaction.  Beam electrons are important to sustain the plasma. 25 500 1 5 [e] (1011 cm-3) Logscale Logscale Beam electrons (1020 cm-3 sec-1) • 6 sccm Ar • Values shown at the end of a 1 s pulse Iowa State University Optical and Discharge Physics ICOPS2005_RAA_15

  17. VARIATION OF VAXIAL AND TGAS WITH BIAS VOLTAGE -300 V -400 V -500 V -300 V -400 V -500 V • Power deposition varies non-linearly with applied voltage. • Gas heating leads to stagnation of flow upstream of the power deposition zone. 250 0 420 600 900 Axial velocity (m/s) 300 300 300 Gas temperature (°K) • 6 sccm Ar • Values shown at the end of a 1 s pulse Iowa State University Optical and Discharge Physics ICOPS2005_RAA_16

  18. EFFECT OF PULSE WIDTH ON THRUST 500 ns  Incremental value is dependent on the duration of the pulse.  Pulse widths of 500 ns are too short for the flow to attain equilibrium with the plasma.  The effect of the discharge is felt even after the plasma is turned off due to gas heating. 1 s  6 sccm Ar  -400 V Iowa State University Optical and Discharge Physics ICOPS2005_RAA_17

  19. EFFECT OF POWER ON THRUST  Incremental thrust increases nearly linearly with power.  Higher power with small pulses may lead to higher efficiency.  Heat transfer to walls is small with if the pulse width is much less than conduction time scales.  6 sccm Ar  1 s pulse Iowa State University Optical and Discharge Physics ICOPS2005_RAA_18

  20. CONCLUDING REMARKS  An axially symmetric microdischarge was computationally investigated with potential application to microthrusters.  Higher flow rates resulted in higher power deposition for a fixed voltage due to increase in density of neutral species.  It was observed that pulsing of the discharge might be an efficient in terms of power consumed per thrust produced.  Studies were conducted to find the effect of parameters such as flow rate, power, and pulse width.  Small pulses with larger voltages might be more efficient at generating thrust. Iowa State University Optical and Discharge Physics ICOPS2005_RAA_19

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