optimizing the performance of plasma based microthrusters*

OPTIMIZING THE PERFORMANCE OF PLASMA BASED MICROTHRUSTERS*

Ramesh A. Arakoni,a) J. J. Ewingb) and Mark J. Kushnerc)

a) Dept. Aerospace EngineeringUniversity of Illinois, Urbana, IL

b) Ewing Technology Associates, Bellevue, WAc) 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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Iowa State University

Optical and Discharge Physics

AGENDA

Microdischarge (MD) devices as thrusters

Description of model

Scaling of thrust

Geometrical effects

Conclusions.

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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.

MICRODISCHARGE PLASMA SOURCES

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GAS IN HOT GAS TO NOZZLE

FLOW THRU MICRO-DISCHARGE

Iowa State UniversityOptical and Discharge Physics

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.

Iowa State UniversityOptical and Discharge PhysicsICOPS06_MT_03

300 m hole diameter

Ref: J. Slough, J.J. Ewing, AIAA 2005-4074 Ref: Kimura, Horisawa, AIAA 2001-3791

The force provided by the thruster is calculated by:

where dm/dt is the mass flow rate, Ve is the exit.


CALCULATION OF THRUST

aeee PPAVdtdmF

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Ref: Robert G. Jahn, Phys. of Electric Propulsion, Mc-Graw Hill, 1989.

The incremental thrust obtained due to the discharge is given by:

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.

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.


EFFICIENCY OF THRUSTER

PlasmaWithoutPlasmaWith

VdtdmV

dtdmF

.

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PowerF /



DESCRIPTION OF MODEL

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To investigate microdischarge sources, nonPDPSIM, a 2-dimensional 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.



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:

Secondary electron emission:

ii S

tN

SV

DESCRIPTION OF MODEL: CHARGED PARTICLE, SOURCES

j

jijSj

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ELECTRON ENERGY, TRANSPORT COEFFICIENTS

Bulk electrons: Electron energy equation with coefficients obtained from Boltzmann’s equation solution for EED.

e

ieiie

2EM

e qj,T25NnEEj

tn

Iowa State UniversityOptical and Discharge PhysicsICOPS06_MT_08

Beam Electrons: Monte Carlo Simulation

Cartesian MCS mesh superimposed on unstructured fluid mesh.

Greens functions for interpolation between meshes.



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.

)pumps,inlets()v(t

iiiiiiii

iii EqmSENqvvkTN

tv

i i

iiifipp EjHRvPTcvTtTc

DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT

SV

T

iTifii SS

NttNNDvtNttN

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Plume characterizes densities of excited states.


EXPERIMENTAL GEOMETRY (BY OTHERS)

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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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 Iowa State University


15 SCCM: PLASMA CHARACTERISTICS

-2700 1401.4 1401.4 22.50

Potential (V) [Ar+] 1011 cm-3

Logscale

[e] 1011 cm-3

Logscale

E field (kV/cm)

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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


15 SCCM: NEUTRAL FLUID

2 200 300 6754 400

Ref: John Slough, J.J. Ewing, AIAA 2005-4074

[Ar(4p)] 1011 cm-3

Logscale

[Ar(4s)] 1011 cm-3

Logscale

Gas temp (K)

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Expt. plume



VELOCITY INCREASE WITH DISCHARGEAnimation 0 – 0.6 msPower onCold flow

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.

0 300Axial velocity (m/s)ICOPS06_MT_14

15 sccm Ar, 30 – 10 Torr 0.5 W.



30 sccm, 1 W: AXIAL VELOCITY, THRUST

30 sccm Ar, 30 – 10 Torr 1.0 W

6000

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.

Axial velocity (m/s)ICOPS06_MT_15

Power onCold flow Animation 0 – 0.55 ms



POWER DEPOSITION: PLASMA, GAS HEATING

(°K) Ionization efficiency increases with power due to larger excited

state density At higher temperatures and lower densities decouple power

transfer from ions to neutrals.

Max675 K

Max875 K

300 Max[e] cm-3 (logscale)

1 100

0.5 W 0.75 W0.5 W 0.75 W2.6 x 10131.4 x 1013

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POWER DEPOSITION: FLOW VELOCITY

Increase in flow speed and thrust of 250% predicted with 0.75 W

0 MAX

0.5 WPower off 0.75 W

Max 160 Max 300 Max 400

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Vy in exit plane.



EFFECT OF GEOMETRY: CATHODE THICKNESS

No significant effect of electrode thickness on velocity profile.

Thicker electrode could lead to longer service life.

30 sccm Ar, 30 / 10 Torr

1.0 W

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EFFECT OF GEOMETRY: END CAP

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

PlasmaWithoutPlasmaWith

VdtdmV

dtdmF

.

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OPTIMAL GEOMETRY: DOWNSTREAM PRESSURE

1001 MAX400[e] cm-3 logscale Gas temp (°K)

5 Torr 5 Torr 10 Torr 10 TorrMax1920

Max1440Max 6 x 1014 Max 2.5 x 1014

Lower downstream pressure produces a more confined plasma (a bit counter-intuitive)

Higher power density leads to hotter neutral gas.

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1W, 30 sccm Ar



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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optimizing the performance of plasma based microthrusters*

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