Electrostatic Propulsion Ion Engines (Ion...

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Electric Propulsion-1

Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion

Electrostatic Propulsion

Ion Engines (Ion Thrusters)



Electrostatic Thrusters

• Thrust provided by “static” electric field in the direction of the acceleration

• Propellant is often an ionized gas

– so often denoted ion thruster

• Typically operate at low pressure (near vacuum)

• Handful of different technologies

– gridded ion thruster or ion engine

• original development at NASA, 1950’s-60’s

– Hall effect thruster (HET) or Hall thruster or ungriddedion thruster

• original development in Russia (Stationary Plasma Thruster, SPT), 1950’s-60’s

– electrospray (or colloidal) thruster

– ….

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Missions and Lifetime• Satellite Station-keeping

– GEO communication satellites (including orbit raising as backup to chemical propulsion system)

– LEO: ESA GOCE

• Satellite orbit raising (and station-keeping)

– Boeing 702HP - XIPS ion engine

– AEHF GEO comm. satellite, BPT-4000 HET

• Lunar orbit

– ESA SMART-1 (2003) employed Hall thruster (PPS-1350-G)

• Deep space exploration

– Deep Space 1 (1998), Dawn (2007) – asteroid belt protoplanets

– JAXA Hayabusa (2014) - asteroid rendezvous and return

• Lifetime

– NASA Evolutionary Xenon Thruster (NEXT), 7 kW ion engine,>43,000 hours (5 years) of continuous operation (ground test)



Components• Ion sources

– usually electronbombardmentplasma

– RF discharge – ion contact:

liquid metal (e.g., Cs)flowing through hot porous tungsten

– field emission: charged droplets/particles

• Accelerator

• Neutralizer– electrons added to make exhaust charge neutral– typically thermionic emitters or hollow cathodes

Space Propulsion Analysis and Design,

Humble, Henry and Larson, 1995

L

Accelerator

Gridded Ion Engine example

3



Ionization by Electron Bombardment

1) produce seed electrons (e-)

– typically from thermionic source

2) accelerate e- to high velocity/energy

– E field or B field approaches

– B field gyrate e-

• increased energy and frequency of

collisions with neutrals

3) electrons collide with neutral gas

molecules and ionize them

– also produces majority of electrons

used in ionization



Hollow Cathode Ionization Chamber

Space Propulsion Analysis and Design,

Humble, Henry and Larson, 1995

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• Electron emission from hot source with low work potential (s)

– filament example shown here

– B field acceleratese-

– high KE e- ionize neutralpropellant flow and createmore e-

Sutton

Thermionic Emitter/B Field Ion. Chamber

kTqee seeTh

kmqJ

2

3

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Accelerator: Gridded Ion Thruster• Acceleration E field provided by electrodes

– propellant passes through many small cells• Grid of multiple electrodes (forms ion optics)

– 2-3 grids typical

Screen Grid

AccelGrid

DecelGrid

• screen and decel grids help prevent high energy ions from impacting accel grid (sputtering)

• decel grid reduces sputtering from backflow of charge-exchange ions

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Hall (Effect) Thruster

• Uses applied magnetic (B) field (external magnets) to create ionization and acceleration in same chamber

• Typically axisymmetric geometry

– apply radial B field

• E field created between internal anode and external cathode

– but strongly influenced by plasma at open end

after Space Travel Aided by Plasma Thrusters: Past, Present and

Future , DeFusco, Craddock, Faler, DSIAC Journal, 2017

Electron Source

Azimuthal example



Hall (Effect) Thruster

C. Mullins, Non-invasive Hall Current

Distribution Measurement System for

Hall Effect Thrusters (2015)

• Ionization

– axial E and radial B generate azimuthal e- acceleration/motion (Hall current)

– high energy e- ionize neutrals

– heavy ions have larger radius of gyration, so less deflection by B

• pick weak enough B

• Acceleration: two interpretations– electrons largely trapped by B,

so negative plasma potential near exit accelerates ions

– Hall effect from electron current, induced E jB

j

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Electrostatic Propulsion

Ion Thruster Analysis



Electrostatics – Recall Definitions

• V – potential or voltage(sometimes ) (Volts)

• E – electric field (V/m, N/C)

• q – charge (C) (qe-=1.60210-19 C)

• Force, 𝐹 = 𝐸𝑞

• Potential Energy

• J – current (A, C/s)

• j – charge current density (A/m2, C/sm2 )

• nq – number density charged part. (1/m3)

• u – velocity of charged particles (m/s)

x

V

qunj q

VqqEdxFdx

V𝐸

𝐸 = −𝛻𝑉 = −𝑑𝑉

𝑑𝑥

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Electrostatic Thruster Performance - ue

• Specific impulse

• Find exhaust velocity from energy

balance per particle

• So maximum (ideal) specific impulse limited

by voltage difference across accelerator

inletexite

VVm

qu 2

s

m

MW

Vaccel )volts(890,13

for singly ionized

Vqmu 2

2

1

ue

o

esp

g

uI

kgAMU 27106605.11

q=1.602210-19 C



Gridded Ion Thruster Performance -

• Thrust 𝜏 = 𝑚𝑢𝑒• Mass flow rate related to current

• Maximum thrust limited by

achievable current density

• For gridded ion engine,

ion current limited

by space-charge

– field from dense

ions creates “shield”

from applied E field

2

23

max

2

9

4

L

V

m

qj accelo

meter

Farado

1210854.8 permittivity

of free space

Child-Langmuir

Law

accelerator electrode spacing

22

23

8

maxm

)volts(10467.5

m

Amps

LMW

Vj accel

for singly ionized

mn q

mjAm enquj

++ +

++ +

+

++

+

++

+

+

++

+

L+ -

.

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Gridded Ion Thruster Performance -

• Maximum thrust

• High requires high V and aspect ratio

– space charge(D/L)max~1

– use many small ion beams toget more thrust

ee uqmjAmu

eu

q

mAj

maxmax V

m

q

q

mA

L

V

m

qo

22

9

42

23

22

max98 LVA

o

A=cross-sectional flow area

for circular cross-section

of diameter, D 22

max92 VLD

o

NewtonsVLD in1018.6 2212

DL

f(q/m)

.



Electrostatic Thruster - Propellant

• Thrust performance

• For fixed Isp

• So choose propellant with high m/q

– heavy molecules• xenon (Xe) good choice (MW=131.3) and

noble gas, so easy to store

• Cs, Hg heavier, but storage issues

– singly ionized ions preferable

– also macro particles (colloidal thrusters)

ospe gIq

mju

q

mj

A

q

m

Am

qj maxmax

q

mj

A

for gridded ion engine

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Electrostatic Thruster Power

• Jet power

– at maximum current for

gridded ion thruster

• Accelerator electrical power

• Power supplied to thruster (not just accel.)

spoebj IgumP 2

1

2

1 2

thjth PP

2

252

9

4

L

V

m

qAP accel

o

j

accelaccelelec VjAVJP

accele Vm

qu 2

2

1

2

23

max,

2

9

4

L

V

q

mAm accelo

b

2

23

max

2

9

4

L

V

m

qj o

jetelec PP ideally

Ion beam flowrate @ exit

th = thruster efficiencynot thermodynamically limited

.

.



Electrostatic Thruster Power - Losses

• Ionization losses

– energy used to create ions (e.g., XeXe+)

– minimum loss given by ionization potential (I) for atom (typically 4-20 eV, electron volts) times current

• Thrust correction

– beam divergence, multiple ionization, sputtering (ion impacts on grid)

• Propellant utilization efficiency

• Neutralization losses

– energy to create e-

qm

mJP ionionlossion

ideal

mmbu

neutneut VJP

.

. .

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Nonideal Performance and Typical Values

• Nonideal performance equations

• Typical values– ion I up to 100-300 eV/ion– 0.8-0.95– u 0.8-0.95– Vneut 10-20Vs

idealspusp II ,

neutaccelionthth VVVJVJP

oidealspueb gImum ,

Vaccel , Isp

th

beam voltage

dominates

low Isp ion engines

are inefficient

accel

neution

u

th

accelb

th

eb

th

j

th

V

VVV

m

qm

Vm

qm

VJ

um

P

P

1

2 2

2

2

.

.

.

.

.



Example: Electron Bomb., Xe+ Gridded Thruster

• Operating conditions

– Vaccel=700 V, L=2.5 mm, 2200 holes (grids) each with D=2.0 mm

– MW(Xe)=131.3, I(Xe)=12.08 eV

• Determine– , – ue, Isp– mprop

– power required including only minimum needed for ionization andneutralization (10 V)

..

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Solution

2212

max1018.6 VLD

gridN /1094.17005.2/21018.6 62212

mNgridNgridstotal

3.4/1094.12200 6

,max

smsmMWVue 3.131700890,13890,13

smue 070,32 sI sp 3270

sgumm eb

41034.1

mmqVmuPPPP neutIeneutionjet 22

kgkgMWm

25

27

1018.21066.1

Cq 1910602.1 for singly ionized molec.

WW 17.29.68

WP 1.71 maximum th=68.9/71.1

= 96.9% for Vion=200eV, th = 77%

assume u=1. .

. .

ion mass



Additional Slides

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Child-Langmuir Law Derivation

• Poisson’s Eqn.

– 1-do

ii

o

q qnV

2

oo

ii

u

jqn

dx

Vd

2

2

xVVq

mj

oo

1

2

+

xo

2

22

2

1

dx

Vd

dx

dV

dx

dV

dx

d

21

2

2

2

2

1VV

dx

d

q

mj

dx

dV

dx

do

o

21

21

2VV

q

mj

dx

dVo

o

VVq

mj

dx

dV

dx

dVo

oo

2

422

2

oE

Integrate

Assume

E(0)=Eo=0



Child-Langmuir Law Derivation

4121

22

VV

q

mj

dx

dVo

o

xq

mjVdVV

o

V

Vo

o

4121

41

22

SOV and Integrate

43

3

4VVo

344121

22

3

x

q

mjVV

o

o

Voltage reduced over applied

voltage due to space charge

2

23

max

2

9

4

x

V

m

qj o

Maximum current density possible

x

E

negl. current

space charge limited

x

V

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