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
The “New Stubby” helicon source
Antenna: 1 turn at 27 MHz, 3 turns at 13 MHz.
Aluminum top plate
Note “skirt”
The top plate reflects the backward wave
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.
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).
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
UCLA
Operating point on “Low-field peak”
Different magnet arrays were calculated
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
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
Discharge with the original magnet
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.
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.
The experimental chamber
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
The SEMion ion energy analyzer
4” diam x 1 cm thick by Impedans, Ltd., Ireland
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.
Gridded and Hall ion thrusters
CATHODE
ANODE
ACCELERATION GRIDS
Electron neutralizer
A helicon thruster
RF
MAGNET COILS
INSULATOR
ANTENNA
PLASMA
DOUBLE LAYER
Double-layer thrustersA review of recent laboratory double layer experiments
Christine Charles, Plasma Sources Sci. Technol. 16 (2007) R1–R25
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%
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
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"
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.
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)
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
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
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.
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
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.
Title