Transcript Slide 1

Erosion Characterization via Ion Power
Deposition Measurements in a 6-kW Hall
Thruster
Hall Thruster Overview
Hall thrusters are an electric propulsion device typically
used on satellites for orbit station-keeping, but is a
promising option for deep-space missions that require
an efficient, long-lasting propulsion system.
Large measured sheath potentials (~ 5Te) indicate the
presence of high-energy electrons and a thermalization
process that supports the use of Hobbs and Wesson.
0
Ion Current Density in the
Near-field Plume
Ion power deposition can
be calculated with the
measured ion current
density and estimated
ion energy, which is
found from the plasma
potential and electron
temperature. Average
power to the walls was found to be 11% of the discharge
power, with excess power being measured at 150 and
500 V.
Large ion currents and
ion energies were
found at low and high
discharge voltages. At
high
voltage,
the
acceleration
zone
recedes and exposes
the wall to more
current
and
high
energy ions. At low
voltage, the ion beam
diverges more readily
and sheath energies
are higher.
150 V
400
300
1000
2000
3000
4000
Discharge Power [W]
5000
10 mg/s Anode MFR
20 mg/s Anode MFR
30 mg/s Anode MFR
400
450
500
Average Ion Enegy to Walls [%]
By simulating various
conditions to characterize
the effective area increase
as a function of bias
voltage, the ion saturation
regime can be corrected
to recover the “true” ion
saturation current.
Large Beam Energies
10
Large Sheath Energies
5
10 mg/s Anode MFR
20 mg/s Anode MFR
30 mg/s Anode MFR
150
200
250
300
350
Discharge Voltage [V]
400
450
500
0
5
10
15
20
Plume Divergence Angle [deg]
25
Future Work
The measured ion current densities and energies can be
used to estimate the wall erosion rate at each position.
The sputtering yield is the volumetric rate of erosion per
unit charge to the surface, and is dependent upon ion
impact energy, incident angle, and wall material. This
yield is usually derived from limited experimental data
and contains large uncertainties.
The data from this study must be compared to current
simulation results in order to validate/refine existing wall
physics and erosion models. The hybrid-PIC code
HPHall-2 is used at JPL to simulate channel and nearfield physics of Hall thrusters.
In particular, near-wall
plasma
properties
and
erosion
rate
predictions must be
compared
between
experiment
and
simulation.
The
existing sheath and
sputtering
models
must then be refined
to
better
match
observations.
600
25
Ion Current Density
Sputtering Yield
400
15
200
10
5
-0.16
-0.12
-0.08
-0.04
0.00
Position Along Wall [Channel Lengths from Exit Plane]
3
80x10
60
Ion Power Deposition Density
Recession (Erosion) Rate
50
60
40
30
20
20
10
0
0
-0.16
-0.12
-0.08
-0.04
0.00
Position Along Wall [Channel Lengths from Exit Plane]
250
Future flush-mounted
probe studies may
also be done to
enhance the current
data set at desired
operating conditions.
35
30
Ion Current Density
Electron Temperature
200
25
150
20
100
15
10
50
5
0
0
-1.0
-0.8
-0.6
-0.4
-0.2
Position Along Wall [Channel Lengths from Exit Plane]
0.0
Electron Temperature [eV]
40
Wall Recession Rate [mm/khr]
Ion Power Deposition Density [W/m ]
20
Ion Current Density [A/m ]
30
2
2
800
3
Ion Current Density [A/m ]
-3
35x10
The estimated wall
erosion rate exhibits
the expected shape
based on observed
profiles.
It also
loosely follows the
calculated
ion
power
deposition
density,
although
this does not strictly
hold in all instances.
The recession rate
is ~10X greater than
expected, indicating
the sputtering model
requires refinement.
Physical
Probe Radius
Extrapolation of Plasma
Properties
Due to the limited size of the interrogation zone,
extrapolation of the data set to the entire channel length
was performed using fitting functions. The functions
were derived from proper fits to other data taken within
the channel that had higher spatial resolution and a
wider range.
Plasma and floating
potentials
were
extrapolated using
sigmoid
functions,
while
electron
temperature
was
calculated using the
difference between
the two potentials.
Ion current density
was
extrapolated
using a combination
of Gaussian and
Lorentzian functions.
250
Measured Plasma Potential
Extrapolated Plasma Potential
Measured Floating Potential
Extrapolated Floating Potential
Extrapolated Electron Temperature
200
150
30
25
20
100
15
50
10
Electron Temperature [eV]
Sputtering Predictions
1000
Effective
Probe Radius
30
Potential [V]
300
350
Discharge Voltage [V]
0
2
A similar angle can be defined within the channel by
comparing the total ion current that hits the walls to
that exiting the
40
thruster. This angle
is shown to have a 30
rough
correlation 20
with the plume 10
Data Points
Line Fit
divergence angle.
0
0
-1.0
-0.5
0.0
0.5
1.0
1.5
Position Along Wall [Channel Lengths from Exit Plane]
800
2
250
0
Relevant properties near the wall were measured: ion
current density; electron temperature; floating potential;
and plasma potential. These properties were then used
to characterize the ion power deposited onto the
channel walls as well as to predict erosion rates.
Ion beam spreading in the plume is characterized by the
divergence angle (see above). The larger the axial
component of the ion beam, the smaller the spreading
and the lower the divergence angle. The divergence
angle can be deduced from ion current density
measurements in the thruster plume.
The flush-mounted nature
of the probes requires a
dedicated model of sheath
expansion in the ion
saturation
regime
to
account for the special
geometry and boundary
conditions.
Ion Current Density [A/m ]
200
15
Probes
50
Plasma in Contact with Larger Portion of Wall
Sputtering Yield [mm /C]
Five Langmuir probes were flush-mounted onto each
channel wall, concentrated near the thruster exit plane.
Data were taken across nine operating conditions
under a wide range of discharge voltages and powers.
40
6000
40
150
Experimental Setup
20
30
Electron Temperature [eV]
Sheath Expansion Model
d
100
0
Figure from: Reid, B. M. and Gallimore, A. D., "Langmuir Probe Measurements
in the Discharge Channel of a 6-kW Hall Thruster," Presented at the 44th
AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA-20084920, Hartford, CT, July 21 - 23, 2008.
10
200
20
While significant advancements have been made in
understanding wall physics, there is a lack of
experimental validation. Measurements of plasma
parameters at the wall
are best obtained using
Langmuir probes flushmounted along the
channel walls, since
traditional
methods
cannot
obtain
the
required proximity (see
right).
-200
0
Average Power = 10.9%
60
20
Experimental Data
no SEE (Vsheath = -5.27Te)
Hobbs and Wesson (BN)
Hobbs and Wesson (W)
-250
Internal Divergence Angle [deg]
Total Ion Current to Walls [%]
80
-150
500
100
High Beam Divergence
-100
500 V
600
0
A primary failure mechanism of Hall thrusters is
erosion of the discharge channel wall by ion
bombardment. Present characterization of erosion
involves long, expensive life-testing which will become
cost prohibitive in the future. Thus, a comprehensive
model of Hall thruster erosion and channel wall
physics would facilitate rapid lifetime predictions.
-50
Sheath Potential [V]
Ion Power Deposition to
Channel Walls
0
Research Motivation
Determination of sheath potentials is complicated by
the presence of secondary electron emission from the
wall. The Hobbs and Wesson solution describes the
sheath potential under space-charge limited emission.
Emission from the opposing wall could traverse the
channel and “cancel” out part of the emission effects.
Rohit Shastry, Professor Alec D. Gallimore, and Dr. Richard R. Hofer
Total Ion Power to Walls [W]
Typical Hall thrusters consist of four primary
components: an anode; cathode; discharge channel;
and magnetic circuit. Electrons emitted from the
cathode migrate towards the anode, but get trapped by
the applied magnetic field. The resulting electric field
and magnetic field cause the electrons to drift
azimuthally. Neutral gas, typically xenon, is injected
through the anode and is ionized by the trapped
electrons. The ions are then accelerated out of the
channel by the electric field, forming thrust.
Sheath Potentials
Measured Ion Current Density
Extrapolated Ion Current Density
600
400
200
0
-1.0
-0.5
0.0
0.5
1.0
1.5
Position Along Wall [Channel Lengths from Exit Plane]
Questions? Contact R. Shastry at
[email protected]