1
Electric Propulsion-1
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
Electrostatic Propulsion
Ion Engines (Ion Thrusters)
Electric Propulsion-2
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
– ….
2
Electric Propulsion-3
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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)
Electric Propulsion-4
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
Electric Propulsion-5
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
Electric Propulsion-6
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
Hollow Cathode Ionization Chamber
Space Propulsion Analysis and Design,
Humble, Henry and Larson, 1995
4
Electric Propulsion-7
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
• 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
24
Electric Propulsion-8
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
5
Electric Propulsion-9
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
Electric Propulsion-10
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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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Electric Propulsion-11
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
Electrostatic Propulsion
Ion Thruster Analysis
Electric Propulsion-12
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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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Electric Propulsion-13
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
Electric Propulsion-14
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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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Electric Propulsion-15
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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)
.
Electric Propulsion-16
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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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Electric Propulsion-17
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
.
.
Electric Propulsion-18
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
.
. .
10
Electric Propulsion-19
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
.
.
.
.
.
Electric Propulsion-20
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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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Electric Propulsion-21
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
Electric Propulsion-22
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
Additional Slides
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Electric Propulsion-23
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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
Electric Propulsion-24
Copyright © 2003-2006, 2017, 2019 by Jerry M. Seitzman. All rights reserved. AE6450 Rocket Propulsion
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