Transcript Document

OPTIMIZING THE PERFORMANCE OF PLASMA BASED
MICROTHRUSTERS*
Ramesh A. Arakoni,a) J. J. Ewingb) and Mark J. Kushnerc)
a)
Dept. Aerospace Engineering
University of Illinois, Urbana, IL
b) Ewing Technology Associates, Bellevue, WA
c) Dept. Electrical and Computer Engineering
Iowa State University, Ames, IA
[email protected], [email protected], [email protected]
http://uigelz.ece.iastate.edu
ICOPS 2006, June 4 - 8, 2006.
* Work supported by Ewing Technology Associates, NSF and
AFOSR.
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AGENDA
 Microdischarge (MD) devices as thrusters
 Description of model
 Scaling of thrust
 Geometrical effects
 Conclusions.
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MICRODISCHARGE PLASMA SOURCES
 Microdischarges are plasmas that leverage pd scaling to operate
at high pressures (10s-100s Torr) in small reactors (100s m).
 Typically operated as a dc discharge using wall stablization.
 High E/N in the cathode fall generates energetic electrons
producing high ionization.
 High power densities (10s kW/cm3) owing to small volume of
discharge, producing high neutral gas temperatures.
 Increase in gas temperature in flowing gas produces thrust.
FLOW THRU MICRODISCHARGE
GAS IN
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HOT GAS TO
NOZZLE
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MICRODISCHARGES AS MICROTHRUSTERS
 Micro-satellites weighing < few kg or require Ns to mNs of
thrust for station keeping.
 Thrusters based on MD devices can deliver the required thrust
using a only a few Watts of power.
 The MD operates as an efficient heat source for the propellant.
Expansion of the hot gas provides the required thrust.
300 m hole diameter
Ref: J. Slough, J.J. Ewing, AIAA 2005-4074
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Ref: Kimura, Horisawa, AIAA 2001-3791
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CALCULATION OF THRUST
 The force provided by the thruster is calculated by:
dm
F
Ve  Ae Pe  Pa 
dt
where dm/dt is the mass flow rate, Ve is the exit.
Ref: Robert G. Jahn, Phys. of Electric Propulsion, Mc-Graw Hill, 1989.
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EFFICIENCY OF THRUSTER
 The incremental thrust obtained due to the discharge is given
by:
.
 dm 
F   V 
 dt With Plasma
 dm 

V
 dt Without Plasma
 Common metric for efficiency is the thrust per unit power
input to the system. In this case, we look at incremental thrust
per unit power.
  F / Power
 Typical values of the efficiency for electro-thermal and arc
thrusters are about 0.1 – 0.2 N/kW.
 Theoretical limit on efficiency is 2/Ve, where Ve is the exit
velocity.
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DESCRIPTION OF MODEL
 To investigate microdischarge sources, nonPDPSIM, a 2dimensional plasma-hydrodynamics code was used.
 Finite volume method used on cylindrical unstructured
meshes.
 Implicit drift-diffusion-advection for charged species
 Navier-Stokes for neutral species
 Poisson’s equation (volume, surface charge)
 Secondary electrons by ion impact.
 Electron energy equation coupled with Boltzmann solution
 Monte Carlo simulation for beam electrons.
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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 i
     Si
t
 Poisson’s Equation for Electric Potential:
     V  S
 Secondary electron emission:
jS    ij j
j
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ELECTRON ENERGY, TRANSPORT COEFFICIENTS
 Bulk electrons: Electron energy equation with coefficients
obtained from Boltzmann’s equation solution for EED.

ne  
5
 
2
 j  E  EEM  ne  Ni i       Te , j  qe
t
2

i
 Beam Electrons: Monte Carlo
Simulation
 Cartesian MCS mesh
superimposed on unstructured
fluid mesh.
 Greens functions for
interpolation between meshes.
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DESCRIPTION OF MODEL:
NEUTRAL PARTICLE TRANSPORT
 Fluid averaged values of mass density, mass momentum and
thermal energy density obtained using unsteady, compressible
algorithms.


   ( v )  ( inlets, pum ps)
t


 v 



   N i kTi     v v        qi N i Ei  S i mi  i qi E
t
i
 i



 
 c pT 

  T  v c pT   Pi   v f   Ri H i   ji  E
t
i
i
 Individual species are addressed with superimposed diffusive
transport.

 N i t  t   
   SV  S S
N i t  t   N i t      v f  Di NT 

N
T



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EXPERIMENTAL GEOMETRY (BY OTHERS)
 Plume characterizes densities of excited states.
 Ref: John Slough, J.J. Ewing, AIAA 2005-4074
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GEOMETRY OF THE MICROTHRUSTED
 Plasma channel geometry:
 300 m at inlet, 500 m at cathode.
 130 m thick electrodes, 1.5 mm dielectric
gap.
 Anode grounded; cathode bias varied
based on power deposition (a few W).
 30 Torr (4 kPa) Argon at inlet, expanded to
low pressures (5 - 10 Torr) downstream.
 Gradation of meshing with a fine mesh
near the discharge and coarse mesh near
the outlet.
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15 SCCM: PLASMA CHARACTERISTICS
Potential (V) [Ar+] 1011 cm-3 [e] 1011 cm-3
Logscale
0
-270
1.4
140
E field (kV/cm)
Logscale
1.4
140
0
22.5
 Power deposition occurs in the cathode fall by beam electrons and
ion drift.
 Electric fields of > 22 kV/cm in cathode fall.
 15 sccm Ar, 30/10 Torr, 0.5 W
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15 SCCM: NEUTRAL FLUID
[Ar(4s)] 1011 cm-3 [Ar(4p)] 1011 cm-3 Gas temp (K)
Logscale
2
200
Expt. plume
Logscale
4
400
300
675
 Gas heating and consequent expansion is a source of thrust.
 More extended plume in experiment due to supersonic status.
 15 sccm Ar, 30/10 Torr, 0.5 W
 Ref: John Slough, J.J. Ewing, AIAA 2005-4074
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VELOCITY INCREASE WITH DISCHARGE
Cold flow
Power on
Animation
0 – 0.6 ms
 Gas heating and
subsequent expansion
produces increase in
velocity.
 When turning on
discharge, pulsation
initially occurs.
 Incremental thrust: 0.05
mN,
  thrust/power: 0.1 N/kW
Total thrust: 0.12 mN.
 15 sccm Ar, 30 – 10 Torr
 0.5 W.
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0
300
Axial velocity (m/s)
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30 sccm, 1 W: AXIAL VELOCITY, THRUST
Cold flow
Power on
Animation
0 – 0.55 ms
 Increasing power
produces increase Mach
number near 1.
 Incremental thrust: 0.2 mN
 Total thrust of  0.5 mN.
 Thrust per unit power:
0.17 N/kW.
 30 sccm Ar, 30 – 10 Torr
 1.0 W
0
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600
Axial velocity (m/s)
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POWER DEPOSITION: PLASMA, GAS HEATING
0.5 W
0.75 W
Max
875 K
Max
675 K
Max
300
(°K)
0.5 W
0.75 W
1.4 x 1013
2.6 x 1013
100
1
-3
[e] cm (logscale)
 Ionization efficiency increases with power due to larger excited
state density
 At higher temperatures and lower densities decouple power
transfer from ions to neutrals.
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POWER DEPOSITION: FLOW VELOCITY
Power off
0.5 W
0.75 W
Max 160
Max 300
Max 400
Vy in exit
plane.
 Increase in flow speed and
thrust of 250% predicted
with 0.75 W
0
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MAX
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EFFECT OF GEOMETRY: CATHODE THICKNESS
 30 sccm Ar,
30 / 10 Torr
 1.0 W
 No significant effect of
electrode thickness
on velocity profile.
 Thicker electrode
could lead to longer
service life.
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EFFECT OF GEOMETRY: END CAP
.
 dm 
F  
V
 dt With Plasma
 dm 

V
 dt Without Plasma
 Maximum increment in velocity for end cap thickness of 500 m.
 Optimal thickness required to expand (and not cool) the hot gas.
 1W, 30 sccm Ar, 30/10 Torr
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OPTIMAL GEOMETRY: DOWNSTREAM PRESSURE
 5 Torr
Max 6 x 1014
 10 Torr
Max 2.5 x 1014
100
1
-3
[e] cm logscale
 5 Torr
Max
1920
 10 Torr
Max
1440
MAX
400
Gas temp (°K)
 Lower downstream pressure produces a more
confined plasma (a bit counter-intuitive)
 Higher power density leads to hotter neutral
gas.
 1W, 30 sccm Ar
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CONCLUDING REMARKS
 A microdischarge was computationally investigated for
potential use in microthrusters.
 At flow rates of a few 10s sccm and up to 1 W power, 0.1 – 0.5
mN of thrust were achieved.
 Thrust specific power consumption of 0.1-0.2 N/kW is
predicted in-line with other arc discharge thrusters.
 Placement of electrodes is important with respect to
confinement of plasma and possible cooling of gas.
 Slightly embedded electrodes resulted in maximum
incremental thrust for a given flow rate and power.
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