ion energy distributions from a permanent-magnet helicon thruster francis f. chen, ucla low...
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![Page 1: Ion Energy Distributions from a Permanent-Magnet Helicon Thruster Francis F. Chen, UCLA Low Temperature Plasma Physics Webinar, January 17, 2014](https://reader035.vdocuments.us/reader035/viewer/2022062721/56649f1c5503460f94c3287e/html5/thumbnails/1.jpg)
Ion Energy Distributions Ion Energy Distributions from a Permanent-Magnet from a Permanent-Magnet
Helicon Thruster Helicon Thruster
Francis F. Chen, UCLA
Low Temperature Plasma Physics Webinar, January 17, 2014
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The “New Stubby” helicon source
Antenna: 1 turn at 27 MHz, 3 turns at 13 MHz.
Aluminum top plate
Note “skirt”
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The top plate reflects the backward wave
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The B-field is from a Neodymium magnet
The magnet is 5” OD, 3” ID, and 1” thick. We use the almost uniform field below the stagnation point.
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The tube was designed with the HELIC code
Lc
a b
h
Loop antenna
Helical antenna
B0
D. Arnush, Role of Trivelpiece-Gould Waves in Antenna Helicon Wave Coupling, Phys. Plasmas 7, 3042 (2000).
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Sample loading curves from HELIC
0.0
0.5
1.0
1.5
2.0
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
200
150
100
50
B (G) 13 MHzH= 2 cm
0.0
0.5
1.0
1.5
2.0
1E+11 1E+12 1E+13n (cm-3)
R (
ohm
s)
200
150
100
50
B (G) 27 MHzH = 1.5 cm
R should be > 1 at operating density
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UCLA
Operating point on “Low-field peak”
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Different magnet arrays were calculated
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Final design: single 3 x 5 x 1” magnet
-16
-14
-12
-10
-8
-6
-4
-2
0
2
4
6
8
-10 -8 -6 -4 -2 0 2 4 6 8 10
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Setting the antenna at 60 G
0
50
100
150
200
250
300
0 2 4 6 8 10 12z (in.)
Bz
(G)
0.92
0.52
0.0
r (in.)
D
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Discharge with the original magnet
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Downstream density vs B and Prf
0
2
4
6
8
10
12
0 25 50 75 100 125 150 175 200B (G)
n1
1
1000
800
600400
200
Prf (W)
This shows that only 30 - 60 G is necessary.
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Only an off-the-shelf magnet is needed
The magnet is 4” OD,
2” ID, and 1/2” thick
The plasma potential is set by grounding the top plate.
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The experimental chamber
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Typical density profiles at Ports 1-3
0
1
2
3
4
5
6
-20 -10 0 10 20r (cm)
n (1
01
1 c
m-3
)
6.8
16.9
27.2
400 W, 60 G(average B)
cm below source
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The SEMion ion energy analyzer
4” diam x 1 cm thick by Impedans, Ltd., Ireland
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The sensor height can be varied continuously
When the sensor is too close to the discharge, it forms an endplate, and the discharge is double-ended.
We know that the discharge is affected because the tuning is changed.
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Gridded and Hall ion thrusters
CATHODE
ANODE
ACCELERATION GRIDS
Electron neutralizer
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A helicon thruster
RF
MAGNET COILS
INSULATOR
ANTENNA
PLASMA
DOUBLE LAYER
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Double-layer thrustersA review of recent laboratory double layer experiments
Christine Charles, Plasma Sources Sci. Technol. 16 (2007) R1–R25
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Cause and location of the “double layer”
1/20½, /s sn n e
1/2 2( / ) ½ ½is s e is is ev c KT M W Mv KT
20 0 0/ / ( / )B B n n r r
F.F. Chen, Phys. Plasmas 13, 034502 (2006)
n
xs x
ne = ni = n
PLASMA
SHEATH
ni
ne
+
ns
PRESHEATH
v = cs
0 , where e /e en n e V KT Maxwellian electrons
Bohm sheath criterion
1/40/ 1.28r r e
A sheath must form here
Single layer forms where r has increased 28%
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Ion energy distribution functions (IEDF)
0
2
4
6
8
10
12
14
-10 -5 0 5 10 15 20 25Voltage
RF
ID (
x107
)
1000
800
600
400
200
Watts 10 mTorr
Expect about 5 the KTe of 1.5-2 eV
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UCLA
Where a diffuse “double layer” would occur
0
50
100
150
200
250
5 10 15 20 25 30z (cm)
B (
G)
Approx. location of "double layer"
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IEDFs vs distance from source
0E+00
5E-07
1E-06
2E-06
2E-06
3E-06
0 5 10 15 20 25Volts
RF
ID
0
6
8
10
10
12
14
cm below tube400W
0E+00
2E-07
4E-07
6E-07
8E-07
0 2 4 6 8 10 12 14Volts
RF
ID
141618202224262830323434
cm below tube400W
1000W @ 34 cm
close to tube further downstream
There is no sign of a double layer jump.
This is probably because the sensor changes the effective length of the discharge.
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IEDFs vs RF power
0
5
10
15
20
-10 -5 0 5 10 15 20Voltage relative to ground
RF
ID (
x107
)
1600140012001000900800700700600500400300200
Prf (W)
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Evidence of ion beam
0.0E+00
5.0E-07
1.0E-06
1.5E-06
2.0E-06
2.5E-06
-10 0 10 20 30 40Voltage
RF
ID
1000
800
600
400
200
Sensor facing upin Port 1
Watts
0E+00
1E-07
2E-07
3E-07
4E-07
5E-07
6E-07
-10 0 10 20 30Voltage
RF
ID
1000
800
600
400
200
Sensor facing downPort 1 Watts
0E+00
2E-07
4E-07
6E-07
8E-07
-10 -5 0 5 10 15 20Voltage
RF
ID
600
600
Sensor facing up/downPort 2
Wattsup
down
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IEDFs vs. pressure
0.0
0.5
1.0
1.5
15 20 25 30 35 40Voltage
RF
ID (
x107
)
400
200
Watts0.5 mTorr
0
2
4
6
8
10
0 5 10 15 20 25 30Voltage
RF
ID (
x107
)
1000
800
600
400
200
Watts 2.5 mTorr
0
2
4
6
8
10
12
-5 0 5 10 15 20 25Voltage
RF
ID (
x107
)
1000
800
600
400
200
Watts 5 mTorr
0
2
4
6
8
10
12
14
-10 -5 0 5 10 15 20 25Voltage
RF
ID (
x107
)
1000
800
600
400
200
Watts 10 mTorr
0
2
4
6
8
-10 -5 0 5 10 15Voltage
RF
ID (
x107
)
1000
800
600
400
Watts 30 mTorr
0
2
4
6
-10 -5 0 5 10 15Voltage
RF
ID (
x107
)
1000
800
600
400
Watts39 mTorr
0
1
2
3
4
-10 -5 0 5 10 15Voltage
RF
ID (
x107
)
1000
800
600
400
Watts60 mTorr
0
2
4
6
8
10
12
14
-10 -5 0 5 10 15 20 25Voltage
RF
ID (
x107
)
1000
800
600
400
200
400
Watts15 mTorr
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Can we increase the ion drift speed?
0
2
4
6
8
10
12
14
-5 0 5 10 15 20Voltage
RF
ID (
x107
) 1000
800
600
400
200
Top plate voltage = 0
Watts
0
2
4
6
8
10
12
14
16
10 15 20 25 30 35 40Voltage
RF
ID (
x107
)
1000
800
600
400
200
Using "car" battery to apply voltage
Top plate voltage = +24Watts
0
2
4
6
8
10
12
14
-10 -5 0 5 10 15Voltage
RF
ID (
x107
)
1000
800
600
400
200
Using "car" battery to apply voltage
Top plate voltage = -24
Watts
Yes! Applying +24V to top plateincreases vi by ~16eV, while applying -24V reduces vi by ~6eV.
The voltage is applied with a Pb-acid battery from an electric scooter.
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Effect of top plate bias
0
2
4
6
8
10
12
14
-10 0 10 20 30 40Voltage
RF
ID (
x107
)
0
24
-24
0
24
-24
Top plate voltage
400W
1000W
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Summary
A small helicon discharge was developed using a permanent magnet for the B-field.
Ions are ejected with a drift velocity of about 5KTe, measured with a retarding- field energy analyzer.
The ion drift can be increased by biasing the top plate of the discharge relative to nearby grounded surfaces.
This device could be developed into a spacecraft thruster.
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Title