Magnetic Shielding in Hall Thrusters:
Breakthrough Space Propulsion Technology for the 21st Century
Richard Hofer
Jet Propulsion Laboratory, California Institute of Technology
Presented at the Michigan Institute for Plasma Science and Engineering (MIPSE) Seminar Series
at the University of Michigan, Ann Arbor, MI
March 20, 2013
National Aeronautics and
Space Administration
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California
Acknowledgements
Jet Propulsion Laboratory
California Institute of Technology
• The research described here is the result of a multi-year investigation of
magnetic shielding in Hall thrusters conducted by the Electric Propulsion group
at JPL.
– Modeling: Ioannis Mikellides, Ira Katz
– Experiments: Dan Goebel, Jay Polk, Ben Jorns
• The research described here was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the National
Aeronautics and Space Administration. Program sponsorship includes:
– JPL R&TD program
– JPL Spontaneous Concept program
– NASA In-Space Propulsion project in the Space Technology Mission Directorate
(STMD)
2
MSL
Jet Propulsion Laboratory
California Institute of Technology
3
Dawn
Jet Propulsion Laboratory
California Institute of Technology
4
Asteroid Retrieval Mission
Jet Propulsion Laboratory
California Institute of Technology
5
Asteroid Retrieval Mission
Diameter
(m)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
Asteroid Mass (kg)
1.9 g/cm3
7,959
15,544
26,861
42,654
63,670
90,655
124,355
165,516
214,885
273,207
341,229
419,697
509,357
610,955
725,237
852,949
994,838
2.8 g/cm3
11,729
22,907
39,584
62,858
93,829
133,596
183,260
243,918
316,673
402,621
502,864
618,501
750,631
900,354
1,068,770
1,256,977
1,466,077
3.8 g/cm3
15,917
31,089
53,721
85,307
127,339
181,309
248,709
331,032
429,770
546,415
682,459
839,394
1,018,714
1,221,909
1,450,473
1,705,898
1,989,675
Jet Propulsion Laboratory
California Institute of Technology
KISS Study
Concept
About the same mass
as the International
Space Station
350
6.5-m dia. 340-t
“Levitated Mass”
at the LA County
Museum of Art
2008 HU4 at 1,000 t
341
300
40-kW SEP
is enabling
IMLEO (t)
250
200
180
150
100
50
18.8
0
Brophy, J. and Oleson, S., "Spacecraft Conceptual Design for Returning Entire nearEarth Asteroids," AIAA-2012-4067, July 2012.
ZBO LH2/O2
N204/MMH
SEP/Xe
Historical Perspective
14.0
Delta-V Beyond Earth Escape (km/s)
12.0
10.0
On-board Propulsion
Launch Vehicle
Electric Propulsion Missions
8.0
6.0
4.0
2.0
0.0
Jet Propulsion Laboratory
California Institute of Technology
Uses for Electric Propulsion
High ΔV space missions
Jet Propulsion Laboratory
California Institute of Technology
Low-disturbance station keeping
High precision spacecraft control
8
Hall Thruster Operation
Jet Propulsion Laboratory
California Institute of Technology
1. Electrons from the cathode are trapped in an azimuthal drift by the applied electric (E)
and magnetic fields (B).
2. Neutral propellant gas is ionized by electron bombardment.
3. Ions are accelerated by the electric field producing thrust.
4. Electrons from the cathode neutralize the ion beam.
9
Conceptual Framework
•
Hall thrusters use an axial electric field
and a radial magnetic field to
accelerate ions and confine electrons.
•
The magnetic field intensity is sufficient
to magnetize electrons while the crossfield configuration induces an
azimuthal electron drift in the ExB
direction.
•
These conditions severely restrict the
axial electron mobility allowing for
efficient ionization of the neutral
propellant and the establishment of
the self-consistent electric field, which
must sharply rise in the region of
maximum magnetic field intensity in
order to maintain current continuity.
•
Due to their much greater mass, ions
are unimpeded by the magnetic field
but are accelerated by the electric field
to produce thrust.
Br
Xe o
Ez
w
Xe +
R e  L  R b
 b  1   e
L
Anode
Ae
Distribution of E & B along the channel
(Kim, JPP 1998)
Magnetic field lines
vE =
EB
B
2

Ez
Br
=
Jet Propulsion Laboratory
California Institute of Technology
Vd
BrL
Fundamental properties of the
Hall thruster discharge
Jet Propulsion Laboratory
California Institute of Technology
• Along field lines electrons stream freely
Isothermality
ϕ
• and the electric field is balanced by the electron pressure
“Thermalized Potential”
Te
• The transverse B and ExB configuration implies a high Hall
parameter
• with the cross-field mobility reduced by ~1/Ω2
ne
11
Experimental evidence demonstrating the
isothermality of lines of force
Jet Propulsion Laboratory
California Institute of Technology
Outer wall
probe locations
Inner wall probe
locations
AIAA-2012-3789
Measured
magnetic
field lines
12
Two big problems in Hall thruster research
have remained unresolved for over 50 years
Jet Propulsion Laboratory
California Institute of Technology
• Cross-field e- mobility limiting performance
– Still don’t understand, but we can measure it and model it in an ad hoc fashion
– Developed design strategies to regulate the electron current and achieve highperformance
• Discharge chamber erosion limiting life
– Magnetic shielding essentially eliminates the primary failure mode
0.71
0.57
0.69
0.56
0.67
0.55
Anode Efficiency
Anode Efficiency
0.58
0.54
0.53
0.52
0.51
0.50
0.49
D-80, 4.0 mg/s
SPT-1, 2.42 mg/s
BHT-1000, 2.76 mg/s
0.65
0.63
0.61
NASA-173Mv1
NASA-173Mv2
0.59
0.57
0.48
1500 1750 2000 2250 2500 2750 3000 3250 3500 3750
Anode Specific Impulse (s)
0.55
1500 1750 2000 2250 2500 2750 3000 3250 3500 3750
Anode Specific Impulse (s)
Advanced magnetic field topologies vastly improved the performance at high-Isp.
Hofer, R. R. and Gallimore, A. D., "High-Specific Impulse Hall Thrusters, Part 1: Influence of
Current Density and Magnetic Field," Journal of Propulsion and Power 22, 4, 721-731 (2006).
13
H6 Hall Thruster
World’s Most Efficient Xenon Hall Thruster
•
•
•
The H6 is a 6 kW Hall thruster designed in
collaboration with AFRL and the University
of Michigan
The thruster was designed to serve as a
high-performance test bed for
fundamental studies of thruster physics
and technology innovations
High-performance is achieved through
advanced magnetics, a centrally-mounted
LaB6 cathode, and a high uniformity gas
distributor
– Throttleable from 2-12 kW, 1000-3000 s,
100-500 mN
– At 6 kW, 300 V (unshielded): 0.41 N
thrust, 1970 s total Isp, 65% total efficiency
– At 6 kW, 800 V (unshielded): 0.27 N thrust,
3170 s total Isp, 70% total efficiency
– Highest total efficiency of a xenon Hall
thruster ever measured.
Jet Propulsion Laboratory
California Institute of Technology
H6
LaB6 Hollow Cathode
14
Performance has improved by ~40%
since 1998
Jet Propulsion Laboratory
California Institute of Technology
0.70
Total Effic ency
0.65
P5
NASA-173Mv1
0.60
NASA-173Mv2
0.55
H6US
0.50
H6MS
0.45
1500 1750 2000 2250 2500 2750 3000 3250 3500 3750
Total SpecificI mpulse (s)
P5 (1998)
Haas, Gulczinski,
Gallimore
NASA-173Mv1 (2001)
Hofer, Peterson,
Gallimore
NASA-173Mv2 (2003)
Hofer, Gallimore
H6 (2006)
Hofer, Brown,
Reid, Gallimore
15
Thruster life has been the major technology
challenge in electric propulsion since 1959
Jet Propulsion Laboratory
California Institute of Technology
• Thruster life is a fundamental constraint on mission performance affecting
– ΔV capability
• NASA’s Dawn spacecraft carries 2 prime + 1 redundant thruster strings in order to meet
the mission requirements.
– Spacecraft dry mass
– Cost
– Reliability
Xenon propellant
450 kg
Specific Impulse
3100 s
Thrust
92 mN
Burn time
41.3 kh
Burn time through wear test
30.4 kh
Life margin
1.5
Allowable burn time per
thruster
20.2 kh
Number of required
thrusters
2
Discharge chamber erosion from high-energy ion
impact is the primary life limiting failure mode in
(unshielded) Hall thrusters
Redeposition
Zone
Erosion
Zone
•
•
Volumetric Wear Rate (Arb)
•
•
AIAA-2005-4243
Jet Propulsion Laboratory
California Institute of Technology
As a fundamental constraint on mission performance,
thruster life has been the major technology challenge in
electric propulsion since 1959.
In Hall thrusters, high-energy ions sputter erode the
ceramic walls of the discharge chamber, eventually
exposing the magnetic circuit and leading to thruster
failure.
The erosion rate decreases with time as the walls recess
causing the angle of the ions with respect to the wall to
become increasingly shallow. However, the erosion never
stops and eventually the walls erode away and expose the
magnetic circuit.
Until recently, thruster lifetime has always been the major
hurdle towards widespread adoption of Hall thrusters on
deep-space missions
Volumetric erosion rate decreases with time as the
ion incidence angle becomes increasingly shallow. In
traditional (unshielded) Hall thrusters, the erosion
rate never decreases enough to avoid failure.
Time (h)
Magnetic Shielding in Hall Thrusters
•
What does it do? It eliminates
channel erosion as a failure mode by
achieving adjacent to channel
surfaces:
–
–
•
•
•
Jet Propulsion Laboratory
California Institute of Technology
Isothermal field lines
Thermalized potential
high plasma potential
low electron temperature
How does it do it? It exploits the
isothermality of magnetic field lines
that extend deep into the
acceleration channel, which
marginalizes the effect of Te×ln(ne) in
the thermalized potential.
Why does it work? It reduces
significantly ALL contributions to
erosion: ion kinetic energy, sheath
energy and particle flux.
Status? Peer-reviewed, physicsbased modeling and laboratory
experiments have demonstrated at
least 100X reductions in erosion rate.
Mikellides, I. G., Katz, I., Hofer, R. R., and Goebel, D. M., "Magnetic Shielding
of Walls from the Unmagnetized Ion Beam in a Hall Thruster," Applied Physics
Letters 102, 2, 023509 (2013).
18
Physics-based design methodology utilized to modify
the H6 in order to achieve magnetic shielding
Jet Propulsion Laboratory
California Institute of Technology
• Design modifications achieved
through virtual prototyping
– JPL’s Hall2De used to
simulate the plasma and
erosion
– Infolytica’s Magnet 7 used to
design magnetic circuit
• Goal was to achieve magnetic
shielding while maintaining
high performance
Mikellides, I. G., Katz, I., Hofer, R. R., and Goebel, D. M., "Design of a
Laboratory Hall Thruster with Magnetically Shielded Channel Walls, Phase
III: Comparison of Theory with Experiment," AIAA-2012-3789, July 2012.
19
Experimental Apparatus
•
•
H6 Hall thruster
Owens Chamber at JPL
–
–
–
•
Jet Propulsion Laboratory
California Institute of Technology
3 m diameter X 10 m long
Graphite lined
P ≤ 1.6x10-5 Torr
Sixteen diagnostics for assessing performance,
stability, thermal, and wear characteristics
–
–
–
–
–
–
–
–
–
–
–
–
Thrust stand
Current probes for measuring discharge current
oscillations
Thermocouples & thermal camera
Far-field ExB, RPA, & emissive probes
Near-field ion current density probe
High-speed discharge chamber Langmuir and
emissive probes (φ,Te)
Flush-mounted wall probes (φ,Te, ji)
Coordinate measuring machine (CMM) for wall
profiles
Quartz Crystal Microbalance (QCM) for measuring
carbon backsputter rate
Residual Gas Analyzer (RGA)
3-axis Gaussmeter
Digital camera
20
Discharge chamber configurations
Unshielded
(US)
Jet Propulsion Laboratory
California Institute of Technology
Magnet ically-Shielded
(MS)
Anode
Anode
Inner wall
Wall
Inner wall
Ring
+Z
Magnet ic Field Line
Boron Nit ride
St ainless St eel
21
Visual observations provide qualitative
evidence of reduced plasma-wall interactions
US
MS
MS
Jet Propulsion Laboratory
California Institute of Technology
• Distinct differences in the
structure of the plasma in
the discharge chamber
were observed.
• These qualitative
observations, were our
first indication that
plasma-wall interactions
were reduced and
magnetic shielding had
been achieved.
Anode is visible when viewed
along the wall.
22
22
410
US
Thrust (mN)
Discharge Current (A)
High-performance maintained in the
magnetically shielded configuration
MS
21
20
MS
390
380
370
19
360
0
1
2
3
4
5
I nner Coil (A)
6
7
0
2100
MS
2000
1950
1900
3
4
5
I nner Coil (A)
6
7
0.63
MS
0.61
0.59
0.57
0
•
2
US
Total Efficiency
2050
•
1
0.65
US
Total I sp (s)
US
400
1
2
3
4
5
I nner Coil (A)
6
7
0
1
2
3
4
5
I nner Coil (A)
6
7
Jet Propulsion Laboratory
California Institute of Technology
• US: 401 mN, 1950 s,
63.5%
• MS: 384 mN (-4.2%),
2000 s (+2.6%), 62.4% (1.7%)
• Efficiency analysis shows:
– Thrust decreased due
to higher plume
divergence angle (+5°)
– Isp increased due to
higher fraction of
multiply-charged ions
(Xe+ decreased from
76% to 58%)
Stability: Discharge current oscillation amplitude increased 25%. Global stability of the
discharge maintained
Thermal: Ring temperatures decreased 60-80 °C (12-16%).
23
Stable operation maintained in the
magnetically shielded configuration
0.64
0.63
3
0.62
0.61
2
0.6
0.59
1
Total Efficiency
Oscillation Amplitude (A)
4
0.58
0.57
0
0.56
0
1
2
3
4
I nner Coil (A)
5
Thruster Configuration
US
MS
6
7
Oscillation Amplitude
(A)
0.8
1.0
Jet Propulsion Laboratory
California Institute of Technology
• Discharge current oscillation
amplitude increased 25%
• Global stability of the discharge
maintained
• 80 kHz modes observed, possibly
linked to cathode oscillations
Breathing-Mode
Frequency (kHz)
14
8
24
Decreased insulator ring temperatures measured
with the magnetically shielded configuration
Thruster Configuration
US
US
MS
MS
Ring Location
Inner
Outer
Inner
Outer
Temperature (°C)
500
510
422
447
Jet Propulsion Laboratory
California Institute of Technology
• Thermal characteristics
essentially unchanged and
may have been improved as
indicated by a 60-80 °C (1216%) decrease in insulator
ring temperatures.
Thermal camera imagery
25
Plasma conditions measured at the wall are consistent with the
predictions of magnetic shielding theory
35
250
25
200
20
150
15
100
10
Plasma pot ent ial
50
5
300
30
250
25
200
20
150
15
100
10
Plasma pot ent ial
50
0
0.8
0.85
z/ Lc
0.9
0.95
1
35
M S, Outer Ring
300
30
250
25
200
20
Plasma pot ent ial
150
15
Elect ron t emperat ure
100
10
50
5
0
0
0.7
0.75
0.8
0.85
z/ Lc
0.9
0.95
1
350
Plasma Pot ent ial (V)
0.75
0
0.7
Elect ron Temperat ure (eV)
Plasma Pot ent ial (V)
350
0
0
0.7
5
Elect ron t emperat ure
Elect ron t emperat ure
Elect ron Temperat ure (eV)
30
35
US, I nner Ring
0.75
0.8
0.85
z/ Lc
0.9
0.95
1
35
M S, I nner Ring
300
30
250
25
200
20
Plasma pot ent ial
150
15
Elect ron t emperat ure
100
10
50
5
0
Elect ron Temperat ure (eV)
300
350
Plasma Pot ent ial (V)
Plasma Pot ent ial (V)
US, Outer Ring
Elect ron Temperat ure (eV)
350
Jet Propulsion Laboratory
California Institute of Technology
0
0.7
0.75
0.8
0.85
z/ Lc
0.9
0.95
1
In the MS configuration, plasma potential was maintained very near the anode potential,
the electron temperature was reduced by 2-3X, and the ion current density was reduced by
at least 2X.
26
After MS testing, insulator rings mostly
covered in carbon deposits
MS Before Testing
Jet Propulsion Laboratory
California Institute of Technology
MS After Testing
• QCM measured carbon backsputter rate of 0.004 μm/h (~2000X less than US
erosion rates)
• Second qualitative observation that magnetic shielding had been achieved.
27
Inner insulator rings from the various trials
Jet Propulsion Laboratory
California Institute of Technology
US magnetic circuit with MS wall geometry
Wall chamfering was NOT the sole cause of the MS case erosion rate reduction
28
Wall erosion in Hall thrusters
Jet Propulsion Laboratory
California Institute of Technology
Xe+
Potential
Boron nitride wall
• Erosion of the boron nitride walls in Hall thrusters is due to high-energy ion
bombardment
• Ions gain energy through potential drops in the bulk plasma and through the
wall sheath
BN
Sheath
Pre-Sheath/Bulk Plasma
29
Carbon deposition and erosion rates
“Undisturbed”
erosion rate
O(1-10) over 50-200 eV
Carbon backsputter rate
Xe+
C
C
Jet Propulsion Laboratory
California Institute of Technology
BN
BN (w/ C)
• Interpretation of carbon deposits complicated by uncertainty in the sputter
yields of carbon and BN under low-energy Xe impact
• BN and C thresholds are in the range of 25-50 eV. The maximum ion energy for
the MS case was 36 eV.
• If YBN/YC is O(1-10) near threshold, erosion rate was less than or equal to
0.004 – 0.08 μm/h, a reduction of 100-2000X from the US case.
30
Coordinate Measuring Machine (CMM)
Erosion Rates
Jet Propulsion Laboratory
California Institute of Technology
8.5 μm/h
US case
Net deposition
MS case
• US rates are typical of Hall thrusters at beginning-of-life (BOL).
• MS rates are below the noise threshold of the CMM.
31
Wall erosion rates reduced by 1000X as computed
from directly measured plasma properties at the wall
Jet Propulsion Laboratory
California Institute of Technology
22
Erosion Rat e (µm/ h)
20
18
Inner Ring US
16
Out er Ring US
14
Inner Ring MS
12
Out er Ring MS
10
1000X
(min)
8
6
4
2
0
0.7
•
•
•
•
•
0.75
0.8
0.85
z/ Lc
0.9
0.95
1
Uncertainty dominated by the ion current density (50%) and sputter yield (30%),
resulting in a combined standard uncertainty of 60%.
Within this uncertainty, US erosion rates are consistent with the CMM data.
For MS case, ion energy is below 30.5 eV threshold (Rubin, 2009) for all but two
locations on the inner wall where 10-13 eV electron temperatures were measured.
Still, the MS erosion rates are at least 1000X below the US case.
Erosion rates calculated this way are independent of facility effects!
32
2D numerical simulation results
Mikellides, I. G., Katz, I., Hofer, R. R., and Goebel, D. M., "Magnetic Shielding of Walls from the
Unmagnetized Ion Beam in a Hall Thruster," Applied Physics Letters 102, 2, 023509 (2013).
Jet Propulsion Laboratory
California Institute of Technology
33
Various rates encountered in these
experiments and other relevant cases
Jet Propulsion Laboratory
California Institute of Technology
34
Throughput range achievable with
magnetically shielded Hall thrusters
• Throughput capability of
magnetically shielded Hall
thrusters is literally off the
charts
– Possible cathode limitations
can be addressed with
redundant cathodes
• Only magnetically shielded
Hall thrusters have the
throughput capability to
meet the most demanding
deep-space missions without
flying extra strings
Jet Propulsion Laboratory
California Institute of Technology
>3,000
H6MS
Demonstrated
141
NEXT
Es mated maximum
NSTAR
Projected from wear rates
102
400
BPT-4000
SPT-100
93
0
Deep-Space Throughput Range
100 - 350 kg/kW
100 200 300 400 500 600 700 800 900 1000
Xenon Throughput (kg/kW)
H6MS: from erosion rate measurements scaled relative to lower
SPT-100 limit of 30 kg/kW.
NEXT: 113 kg/kW demonstrated to date. 141 kg/kW estimated.
NSTAR: 102 kg/kW demonstrated.
BPT-4000: 100 kg/kW demonstrated. 400 kg/kW estimated by
vendor.
SPT-100: poles exposed at 30 kg/kW. 93 kg/kW demonstrated.
35
Summary of MS Investigations at 2000 s Isp
Jet Propulsion Laboratory
California Institute of Technology
•
In a controlled A/B comparison, sixteen diagnostics were deployed to assess the
performance, thermal, stability, and wear characteristics of the thruster in its original
and modified configurations.
•
Practically erosion-free operation has been achieved for the first time in a highperformance Hall thruster
•
Plasma measurements at the walls validate our understanding of magnetic shielding as
derived from the theory. The plasma potential was maintained very near the anode
potential, the electron temperature was reduced by a factor of 2 to 3, and the ion
current density was reduced by at least a factor of 2.
•
Measurements of the carbon backsputter rate, wall geometry, and direct measurement
of plasma properties at the wall indicate the wall erosion rate was reduced by 1000X
relative to the unshielded thruster and by 100X relative to unshielded Hall thrusters late
in life.
Collectively, these changes effectively eliminate wall erosion as a
life limitation or failure mode in Hall thrusters, allowing for new space
exploration missions that could not be undertaken in the past.
36
Magnetic Shielding Investigations
Jet Propulsion Laboratory
California Institute of Technology
• 2010-2011 program established the first principles of magnetic shielding
through a rigorous program of physics-based modeling and detailed laboratory
experiments
– Mikellides, I. G., Katz, I., Hofer, R. R., and Goebel, D. M., "Magnetic Shielding of Walls from the
Unmagnetized Ion Beam in a Hall Thruster," Applied Physics Letters 102, 2, 023509 (2013).
• Success of this program implied substantial growth capability for this
technology to advanced Hall thruster designs
–
–
–
–
Metallic wall thrusters – demonstrated late 2011. Patent pending.
High-power thrusters – NASA-300MS re-design (in progress)
High-voltage thrusters – 2012-2013
High-power density
H6C
37
Jet Propulsion Laboratory
California Institute of Technology
METALLIC-WALLED HALL THRUSTERS
38
H6MS experiments implied significantly reduced
plasma-wall interactions.
Is the wall material still important?
• An extensive set of modeling and experiments have shown that
magnetic shielding radically reduces plasma-wall interactions
• If the plasma is not interacting with the walls, then why make
them out of boron nitride?
–
–
Boron nitride was originally chosen for low secondary electron yield
and low sputtering yield
If these are negligible, then why bother with BN?
• We obtained funding from JPL R&TD to investigate other wall
materials
–
–
–
Selected graphite for the first demonstration
Simple, lightweight, strong, easy to make, …..
Alternative materials will likely also work, provided the material can
tolerate wall temperatures 400-600 C.
39
Jet Propulsion Laboratory
California Institute of Technology
Carbon Wall Thruster (H6C)
Jet Propulsion Laboratory
California Institute of Technology
The Black Edition
40
H6C Operation
Jet Propulsion Laboratory
California Institute of Technology
Looks identical to the H6MS
- plasma is still off the walls
41
H6C Performance
Performance within 1-2% of BN wall results
–
–
Rings float at 5-10 V below the anode potential
Stable operation identical to the H6MS with BN rings observed
0.70
Total-Efficiency
0.65
0.60
0.55
0.50
H6MS?BN
H6MS?Graphite
0.45
H6BL
0.40
2
3
4
5
Inner-Coil-(A)
6
7
2100
420
2050
Total/Isp/(s)
400
Thrust-(mN)
•
Jet Propulsion Laboratory
California Institute of Technology
380
360
H6MS?BN
1950
H6MS?BN
1900
H6MS?Graphite
H6MS?Graphite
H6BL
340
2000
H6BL
1850
320
1800
2
3
4
5
Inner-Coil-(A)
6
7
2
3
4
5
6
Inner/Coil/(A)
42
7
Discharge current oscillations unchanged with
wall material
Jet Propulsion Laboratory
California Institute of Technology
Reduction in wall temperature observed due to emissivity
increase with graphite and a lower deposited power
Jet Propulsion Laboratory
California Institute of Technology
600
Ring.Temperature.(˚C)
500
400
300
200
H6BL
H6MSEBN
H6MSEgraphite
100
0
0
1
2
3
4
Total.Power.(kW)
5
6
7
44
H6C Implications
•
Jet Propulsion Laboratory
California Institute of Technology
Elimination of the boron nitride rings has many advantages for existing Hall
thrusters
– Lower cost
– Simpler thruster fabrication….especially for large high power thrusters
– Easier structural design for vibe/launch loads
•
This innovation could lead to higher power densities
– Thruster power level likely now limited by anode dissipation (radiation)
– Entire channel can now be made of a single piece of material at anode potential
(large radiator)
– Anticipate factor of 2 to 3 times higher power in a given thruster size
• Same 5 kW thruster today turns into a 10-15 kW thruster when needed
•
New thruster designs and capabilities need to be explored
45
Jet Propulsion Laboratory
California Institute of Technology
HIGH-VOLTAGE, MAGNETICALLYSHIELDED HALL THRUSTERS
46
Pathfinding studies of high-voltage operation
demonstrated discharge stability, performance, and
thermal
H6MS
0.690
0.680
0.670
0.660
0.650
0.640
0.630
0.620
0.610
0.600
0.590
6 kW
15 A
Variable Id
800 V, 7-12 kW
200
400
600
800
Discharge Voltage (V)
1000
3200
3150
3100
3050
3000
2950
2900
2850
2800
2750
0.69
0.68
0.67
0.66
0.65
0.64
0.63
0.62
0.61
0.6
0.59
6
7
8
9
10 11 12
Discharge Power (kW)
Efficiency (-)
H6MS - 800 V
Specific Impulse (s)
Total Effic ency
• Studies conducted in 2012
at JPL were the first to
operate a magneticallyshielded thruster at
discharge voltages >400 V
• Demonstrated stable
discharges up to 800 V, 12
kW
• Performance mappings
demonstrated highefficiency operation
• Thermal capability
demonstrated over time
scales of a few hours
Jet Propulsion Laboratory
California Institute of Technology
Isp
Eff
13
47
Magnetic shielding at 3000 s Isp
demonstrated after 100 h wear test
MS Before Testing
Jet Propulsion Laboratory
California Institute of Technology
MS After Testing
• Insulator rings largely coated with backsputtered carbon after 113 h wear test
at 800 V, 9 kW
• QCM measured carbon backsputter rate of 0.0025 μm/h
• Erosion rates ~100-1000X lower than unshielded Hall thrusters
48
Jet Propulsion Laboratory
California Institute of Technology
BACKUP
49
The H6 Design Process
(or, How Most Hall Thrusters are Designed)
Jet Propulsion Laboratory
California Institute of Technology
• In 2006, the H6 design process was a combination semi-empirical design rules
and physics-based design.
• Plasma-based solvers were not used to design for performance or life.
Cathode
Design
Conceptual
Design
Discharge
Chamber
Scaling &
Design
Magnetic
Circuit
Design
Anode
Design
Component
Integration
Mechanical
Design
Fabrication
Acceptance
Testing
Towards an End-to-End Physics-Based Hall
Thruster Design Methodology
Jet Propulsion Laboratory
California Institute of Technology
• Insertion of plasma and erosion models is a major step forward in the design
process that will lead us to an end-to-end physics-based design methodology
Plasma & Erosion Solver
Cathode
Design
Plasma & Erosion Solver
Conceptual
Design
Discharge
Chamber
Scaling &
Design
Magnetic
Circuit
Design
Anode
Design
Component
Integration
Mechanical
Design
Thermal
Design
Fabrication
Acceptance
Testing
What does eliminating life as a constraint on
mission performance enable?
Jet Propulsion Laboratory
California Institute of Technology
• Reduce mission risk by eliminating the dominant thruster failure mode
• Provides the game changing performance required to enable missions that
cannot otherwise be accomplished
– HEOMD missions: human exploration of NEOs and Mars, reusable tugs for cargo
transportation and pre-deployment of assets
– SMD missions: Mars Sample Return, Comet Sample Return, Multiple Asteroid
Rendezvous and Return, and Fast Outer Planet missions.
– DoD Operationally Responsive Space missions
– All-electric orbit transfers from GTO to GEO (commercial, DoD)
• Reduce propulsion system costs by at least one third relative to the State-ofthe-Art (>$20M per string)
• Offers the possibility to realize ultra-high-performance systems
– Increase power density by 2-10X
– Increase specific impulse from 2,000 to 4,000-10,000 s
Near-field ion current density
Jet Propulsion Laboratory
California Institute of Technology
• Wider plume but higher ion current
53
Multiply-charged ion content significantly
increased in the MS configuration
Jet Propulsion Laboratory
California Institute of Technology
54
Multiply-charged ions
Jet Propulsion Laboratory
California Institute of Technology
• Charges states greater than 4 possibly detected for the first time
55
Performance Model
Jet Propulsion Laboratory
California Institute of Technology
56
Efficiency Analysis
Jet Propulsion Laboratory
California Institute of Technology
• Large increases in multiply-charged ion content and decreased
plasma-wall interactions resulted in a 21% reduction in the crossfield electron transport in the magnetically-shielded configuration.
57
Centerline Plasma Diagnostics
400
40
400
40
30
Plasma pot ent ial
250
25
Te dat a
200
20
Te fit
1.2
z/ Lc
1.4
1.6
1.8
2
25
Te dat a
200
20
Te fit
5
50
5
0
0
50
1
Plasma pot ent ial
250
10
10
0.8
30
100
100
0.6
300
15
15
0.4
35
150
150
0
350
0
0.2
0.4
0.6
0.8
1.0
1.2
z/ L c
1.4
1.6
1.8
2.0
• Plasma potential and electron temperature inside the discharge chamber
58
Elect ron Temperat ure (eV)
300
Plasma Pot ent ial (V)
35
Elect ron Temperat ure (eV)
Plasma Pot ent ial (V)
H6US
H6M S
350
0.2
Jet Propulsion Laboratory
California Institute of Technology
Wall Probes
Jet Propulsion Laboratory
California Institute of Technology
59
Insulator rings after testing with the US
magnetic circuit and MS geometry rings
Jet Propulsion Laboratory
California Institute of Technology
Simulation results
• Geometry changes alone were not the sole contributor to the orders of
magnitude reduction in erosion rate
• Simulations show only a 4-8X reduction for this case relative to the US
configuration (consistent with US erosion rates over life of thruster)
60
References
•
•
•
•
•
•
•
•
•
•
•
Jet Propulsion Laboratory
California Institute of Technology
Hofer, R. R. and Gallimore, A. D., "High-Specific Impulse Hall Thrusters, Part 1: Influence of Current Density and
Magnetic Field," Journal of Propulsion and Power 22, 4, 721-731 (2006).
Hofer, R. R. and Gallimore, A. D., "High-Specific Impulse Hall Thrusters, Part 2: Efficiency Analysis," Journal of
Propulsion and Power 22, 4, 732-740 (2006).
Goebel, D. M., Watkins, R. M., and Jameson, K. K., "LaB6 Hollow Cathodes for Ion and Hall Thrusters," Journal of
Propulsion and Power 23, 3, 552-558 (2007).
Mikellides, I. G., Katz, I., Hofer, R. R., Goebel, D. M., De Grys, K. H., and Mathers, A., "Magnetic Shielding of the
Acceleration Channel in a Long-Life Hall Thruster," Physics of Plasmas 18, 033501 (2011).
Goebel, D. M., Jameson, K. K., and Hofer, R. R., "Hall Thruster Cathode Flow Impact on Coupling Voltage and
Cathode Life," Journal of Propulsion and Power 28, 2, 355-363 (2012).
Mikellides, I. G., Katz, I., and Hofer, R. R., "Design of a Laboratory Hall Thruster with Magnetically Shielded
Channel Walls, Phase I: Numerical Simulations," AIAA Paper 2011-5809, July 2011.
Hofer, R. R., Goebel, D. M., Mikellides, I. G., and Katz, I., "Design of a Laboratory Hall Thruster with Magnetically
Shielded Channel Walls, Phase II: Experiments," AIAA-2012-3788, 2012.
Mikellides, I. G., Katz, I., Hofer, R. R., and Goebel, D. M., "Design of a Laboratory Hall Thruster with Magnetically
Shielded Channel Walls, Phase III: Comparison of Theory with Experiment," Presented at the 48th AIAA Joint
Propulsion Conference, AIAA-2012-3789, Atlanta, GA, July 29 - Aug. 1, 2012.
Hofer, R. R., Goebel, D. M., and Watkins, R. M., "Compact High-Current Rare-Earth Emitter Hollow Cathode for
Hall Effect Thrusters," United States Patent No. 8,143,788 (Mar. 27, 2012).
Goebel, D. M., Hofer, R. R., and Mikellides, I. G., "Metallic Wall Hall Thrusters," US Patent Pending, 2013.
Mikellides, I. G., Katz, I., Hofer, R. R., and Goebel, D. M., "Magnetic Shielding of Walls from the Unmagnetized
Ion Beam in a Hall Thruster," Applied Physics Letters 102, 2, 023509 (2013).
61