Ion Energy Distributions from a Permanent-Magnet Helicon Thruster Francis F. Chen, UCLA Low Temperature Plasma Physics Webinar, January 17, 2014.

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.

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