francis f. chen- applications of permanent-magnet sources and arrays

8/3/2019 Francis F. Chen- Applications of permanent-magnet sources and arrays

1/50

Applications of

permanent-magnet sources

and arraysFrancis F. Chen

INER, February 24, 2009


2/50

Helicon sources are ICPs with a DC B0

This is a commercialhelicon source madeby PMT, Inc. andsuccessfully used to

etch semiconductorwafers. It requiredtwo large and heavyelectromagnets andtheir power supplies.

Computer chips arenow etched withsimpler sourceswithout a DC B-field.

New applications require larger area coverage.


3/50

Possible uses of large-area plasma processing

Roll-to-roll plastic sheets

Smart windowsOLED displays

Solar cells, mass production Solar cells, advanced


4/50

Distributed helicon source: proof of principle

Power scan at z = 7 cm, 5 mT A, 20 G, 13.56 M Hz,

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30R (cm)

N(1012 cm-3) 3.0

2.5

2.0

1.5

1.0

P(kW)

7-tube m=0 array

ARGON

PROBE

Achieved n > 1.7 x 1012 cm-3, uniform to s3%, but large magnet is required.

F.F. Chen, J.D. Evans, and G.R. Tynan, Plasma Sources Sci. Technol. 10, 236 (2001)


5/50

The problem with small magnets

-10

0

10

20

30

z(cm)

QUARTZ TUBE

PVC PIPE

ANTENNA

MAGNET WINDING

7 cm

5 cm

13 cm

BNC connector

5 mm

17 mm

1 cm

1 cm

10 cm

Internal field

External field

Internal field

External field

A small solenoid Field lines divergetoo rapidly

Annular permanentmagnets have same

problem


6/50

However, the external field can be used

Note that the stagnation point isvery close to the magnet

Place plasma in the externalfield, and eject downwards

Internalfield

Externalfield

Internalfield

Externalfield


7/50

-300

-250

-200

-150

-100

-50

0

50

100

150

0 5 10 15 20 25 30

z (cm)

Bz(G)

Calculated

Measured

Externalfield

Internalfield

0

1

2

3

4

5

6

7

-5 0 5 10 15 20r (cm)

n

(1010cm-3)

Z2, 40Z2, 35

Z2, 30

Z2, 21

Z2, 1

D (cm)

500W, 1 mTorr

The bottom curve is when the tube is

INSIDE the magnet

PM helicons: proof of principle


8/50

Evolution of a multi-tube PM helicon source

1. Antenna design

2. Discharge tube geometry

3. Permanent magnets

4. RF circuitry

Next: construction and testing of Medusa 2

Medusa Medusa 1


9/50

Helicon m = 1 antennas

Only the RH polarized wave is strongly excited

Nagoya Type III antenna:

symmetric, so RH wave isdriven in both directions.

RH helical antenna:RH wave is driven only inthe direction matching theantennas helicity.

This antenna has the highest coupling efficiency


10/50

Why we use an m = 0 antenna

A long antennarequires a long tube,

and plasma goes towall before it gets out.

An m = 0 loop antenna can generateplasma near the exit aperture. Notethe skirt that minimizes eddy currentsin the flange.

Now we have to design thediameter and length of the tube.


11/50

The low-field peak: an essential feature

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

100.0

63.1

39.8

25.1

15.8

10.0

B(G) L=2", 1mTorr, conducting

Low-field peak

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

100.0

63.1

39.8

25.1

15.8

10.0


Low-field peak

The peak occurs when thebackward wave is reflected tointerfere constructively withthe forward wave.

R is the plasma resistance, which determines

the RF power absorbed by the plasma,


12/50

Designing the tube geometry

1

Z

n

a k B

[

w

Adjust a, H, and [RF so that n and B are in desired range.


13/50

This is done with the HELIC codeD.Arnush, Phys. Plasmas 7, 3042 (2000).

Lc is made very large to simulate

injection into a processing chamber.

The code computes the wave fields and theplasma loading resistance Rp vs. n and B


14/50

Choose a peak at low B, mid 1012 cm-3 density

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

100.0

63.1

39.8

25.1

15.8

10.0


Low-field peak


15/50

0.0

0.5

1.0

1.5

2.0

2.5

1E+11 1E+12 1E+13

n (cm-3)

R(ohms)

1000464

215

100

46

22

10

B (G) d = 3", H = 2", 13.56MHz

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

d = 4 in.

d = 3 in.

d = 2 in.

100G, H = 2", 13.56 MHz

Tube diameter

0.0

0.5

1.0

1.5

2.0

2.5

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

H = 3 in.

H = 2 in.

H = 1 in.

100G, d = 3", 13.56 MHz

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

f = 27.12 MHz

f = 13.56 MHz

f = 2 MHz

Typical R(n,B) curves at the low-field peak

Vary the B-field Vary the tube length

Vary the tube diameter Vary the RF frequency


16/50

Final tube design for 13.56 MHz

Material: Pyrex or quartzWith aluminum top


17/50

Reason for maximizing Rp: circuit loss Rc

pin rf

p c

RP PR R

!

:p

p c in rf pc

R

R R P P RR } w

: p c in rf R R P P "" }

10

100

1000

1E+11 1E+12 1E+13n0 (cm-3)

Pin(W)

1000500

200

100

Loss

Prf(W)

No helicon ignition

Unstable equilibrium

Stable equilibrium

Rc = 1.0 ;

10

100

1000

1E+11 1E+12 1E+13n0 (cm-3)

Pin(W)

1000

500

200

100

Loss

Prf(W)

Stable equilibria

Rc = 0.1 ;


18/50

Magnet design for 60-100 G

Vary the outside diameter

Vary the vertical spacing


19/50

Final magnet design

NdFeB material, 3x 5x1 thickBmax = 12 kG

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

-10 -8 -6 -4 -2 0 2 4 6 8 10

0

50

100

150

200

250

300

0 2 4 6 8 10 12

z (in.)

Bz

(G)

0.0

0.52

0.92

r (in.)

D


20/50

RF circuitry

For equal power distribution, the sources are connected in parallel withequal cable lengths. The problem is that the cable lengths, therefore,cannot be short.

The length Z2 and the antenna inductance L are the most critical.


21/50

C1, C2 for N=8, L = 0.8QH, Z1 = 110 cm, Z2 = 90 cm(unless varied)

0

200

400

600

800

1000

1200

1400

1600

0 50 100 150 200Z2 (cm)

C(pF)

C1(S)

C2(S)

0

200

400

600

800

1000

1200

1400

1600

0 0.5 1 1.5 2 2.5 3L (uH)

C(pF)

C1(S)

C2(S)

Allowable values of C1, C2 in match circuit

There is an upper limit to eachantennas inductance L. The range of Z2 can be restrictivefor large arrays


22/50

Layout of 8-tube test module, Medusa 2

Compact configurationStaggered configuration

The spacing is determined from the single-tube density profiles to give 2% uniformity


23/50

Side view

Probe ports

Aluminum sheet

Adjustable height

The source requires only 6 of vertical space abovethe process chamber

Z1Z2


24/50

Medusa 2 in operation at 3 kW CW


25/50

Radial profile between tubes at Z2

0

0.5

1

1.52

2.5

3

3.5

-25 -20 -15 -10 -5 0 5 10 15 20 25r (cm)

n(1011 c

m-3) n

KTe


26/50

UCLA

0 3.5

Compact configuration, 3kW

Side Langmuir probe

Density profiles across the chamber


27/50

UCLA

Density profiles across the chamber

0 7-7 14

Staggered configuration, 3kW

Bottom probe array

0

1

2

3

4

5

-8 -6 -4 -2 0 2 4 6 8

y (in.)

n

(1011

cm-3)

-70714

x (in.)Staggered3kW, D=7",

20mTorr


28/50

An linear array of 15 probes

UCLA

0.375

19.75

4.0"0.25

H. Torreblanca,Multitube helicon source with permanentmagnets, Thesis, UCLA (2008).


29/50

Density profiles along the chamber

Staggered configuration, 2kWBottom probe array

0

1

2

3

4

5

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16x (in.)

n

(1011cm-3)

-3.5

0

3.5

Staggered, 2kW,D=7", 20mTorr

y (in.)


30/50

UCLA

Density profiles along the chamber

Compact configuration, 3kW

Bottom probe array

0

2

4

6

8

10

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16

x (in.)

n(1011c

m-3)

3.5-03.5

Compact, 3kW,D=7", 20mTorr

y (in)

Data by Humberto

Torreblanca, Ph.D.thesis, UCLA, 2008.


31/50

APPLICATION TO LIGHT GASES,

LIKE HYDROGEN


32/50

Hydrogen RnB scans for 13.56 MHz

0.00

0.10

0.20

0.30

0.40

0.50

0.60

1E+10 1E+11 1E+12n (cm-3)

R(ohms)

20

406080

B (G)H = 1.0 in. conducting

13.56 MHz

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1E+10 1E+11 1E+12n (cm-3)

R(ohms)

20

4060

80

B (G)H = 1.5 in. conducting

13.56 MHz

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1E+10 1E+11 1E+12n (cm-3)

R(ohms)

20

4060

80

H = 1.5 in. insulatingB (G)

13.56 MHz

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1E+10 1E+11 1E+12n (cm-3)

R(ohms)

51015

20

B (G) H = 2.0 in. conducting

13.56 MHz

No stable solution for hydrogen. Here, H is distance from antenna to endplate.


33/50

Hydrogen helicons in Medusa 2 tube

0

2

4

6

8

10

12

14

0 20 40 60 80 100

B-field (G)

Lowerhybridfrequency(MHz)

HydrogenArgon

13.56 MHzn = 1E12 cm-3

z

nkk B

[

B w

The lower hybrid frequency [LH) is 6.5 times higher forH than forAr and isnot > [(LH). Need todecrease B to have lower[(LH), but low B means bad coupling, like ICPs. SincekB is same if we keep 2 diam tube, we have to increase [(RF) and change n andkz.

2 2LH c ci[ [! ;


34/50

Meaning of the lower hybrid frequency

The exact lower hybrid frequency [LH

is given by

where ;p is the ion plasma frequency.

The last term is negligible except at very low density, so [LH

is proportional to B/M.

In simple helicons, [ is >> [LH and ;c, so the ions cannot move with the RF. When[LH approaches [RF, the ions will move and contribute to the helicon current. Scime

et al. have seen increased ion temperatures when [ ~ [LH

, but HELIC does not show

any great effect there. At [LH

, the ion and electron orbits B to B look like this:

The blue line is the ion cyclotron orbit, which has

been distorted by the LH wave. The red line is the

orbit of the electron guiding-center E x B drift. The

cyclotron orbits of the electrons is too small to see.

2 2

1 1 1

c cLH p[[

} ; ;


35/50

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

10

30

507090

H = 1.0" conductingB (G)

27.12 MHz

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

2040

6080

H = 1.5" conducting

B (G)

27.12 MHz

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

20

406080

B (G)

H = 1.5 in. insulating27.12 MHz

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

2040

6080

100

H = 3.0 in. conducting27.12 MHz

B (G)

There are stable solutions, but n has to be high, requiring LOTS of power.

Hydrogen RnB scans for 27.12 MHz


36/50

Compare hydrogen at 27.12 MHz with argon at 13.56 MHz

to get an idea of how the discharges behave in the standard 2 diam tube

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

75

50

25

Argon, 13.56 MHzH= 2"

B (G)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

75

50

25

Hydrogen, 27.12 MHzH= 2"

B (G)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

100

7550

Argon, 13.56 MHzH= 3"

B (G)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

125

100

75

Hydrogen, 27.12 MHzH= 3"

B (G)

H is essentially the tube length


37/50

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1E+11 1E+12 1E+13n (cm-3)

R(ohms)

100

80

60

40

20

Argon, 13.56 MHzB (G)

0

1000

2000

3000

4000

0.000 0.005 0.010 0.015 0.020 0.025r (m)

P(r)(arb.)

100G, 1.6E1240 G, 6.3E11

Argon @ 13.56

0

1

2

3

4

-1.00 -0.95 -0.90 -0.85z(m)

P(z)(arb.)

100G, 1.6E12

40 G, 6.3E11

Argon @ 13.56

How does the power deposition look in normalAr discharges?

Here P(z) and P(r) are the power deposition profiles in z and r, and P(k) is the power

spectrum. The cases are at two low-field peaks, and the spectrum is almost a pure mode.

The dashed line is the location of the antenna.

0.000

0.004

0.008

0.012

0.016

0.020

0.024

0.028

0 25 50 75 100k (m-1)

P(k)(arb.)

100G, 1.6E12

40 G, 6.3E11

50G, 3E11

Argon @ 13.56

H

ydrogen, 50G, 3E11@ 27.12 MHz

**


38/50

0

200

400

600

800

1000

1200

1400

0.000 0.005 0.010 0.015 0.020 0.025r (m)

P(r)(arb.)

HydrogenArgon

R = 0.564R = 0.397

0

1

2

3

4

-1.00 -0.95 -0.90 -0.85z(m)

P(z)(arb.)

Hydrogen

Argon

0.000

0.002

0.004

0.006

0.008

0.010

0 20 40 60 80 100k(m-1)

P(k)(arb.)

ArgonHydrogen

This compares the profiles for argon and

hydrogen in the same 2 x 2 tube and at

the same conditions: B = 50G and n = 3

x 1011

cm-3

. However, f = 13.56 MHzfor argon and 27.12 MHz for hydrogen.

Compare similarH and Ar discharges


39/50

0

2000

4000

6000

8000

0 0.005 0.01 0.015 0.02 0.025r (m)

P(r)

1.5", conduct.3.5", insul.

140G, 1.3E12H (in.), endplate

0

0.01

0.02

0.03

0.04

0.05

0 20 40 60 80 100 120 140k (m-1)

P(k)

1.5", conduct.3.5", insul.

140G, 1.3E12 H (in.), endplate

Both are near density peak,but conducting case has pure mode.

Power deposition profiles for two very different cases

P(r) is dominated by the TG mode and does

not vary much.

P(z) peaks near the antenna (dashed line in

each case). High P near endplate is not good,since plasma created there is lost fast.

The k-spectrum is pure forH = 1.5 but has

other modes forH = 3.5, as seen by the

wiggles in the RnB curve on the last page.0

1

2

3

4

5

6

7

-1.00 -0.95 -0.90 -0.85 -0.80z (m)

P(z)

1.5", conduct.

3.5", insul.

140G, 1.3E12

H (in.), endplate

R = 1.41 ;

R = 1.67 ;


40/50

0

1

2

3

4

5

6

-1.00 -0.95 -0.90 -0.85 -0.80z (m)

|Ez|(z)

H = 1.5"

H = 3"

140G, 1.4E12, conducting

R = 1.67;

R = 0.87;


0

1

2

3

4

5

6

7

-1.00 -0.95 -0.90 -0.85 -0.80z (m)

P(z)

H = 1.5 in.

H = 3 in.


140G, 1.4E12, conducting R = 1.67;

R = 0.87;

Comparison of waves in 1.5 in. and 3 in. long tubes

The short tube has higher P(z), but it is high near the endplate. The electric field |Ez|,

however, fits properly , whereas it is too short for the 3 tube. The maximum of Ez at

the endplate causes strong reflection, which gives a higher low-field peak. Thus, the

short tube is better even though a lot of useless ionization occurs near the endplate.

This shows that computing Ez may be the best way to fit the tube length to the half-

wavelength of the helicon wave and optimize the loading.


41/50

Comparison of 3 optimized systems of different diameters

For hydrogen at 27.12 MHz

Tube: 2 diam, 1.5 high

Magnet: 3 x 5, 2 high

Tube: 3 diam, 2 high


Tube: 6 diam, 3 high


Note: antenna inductance has to be adjusted


42/50

APPLICATION TO

SPACECRAFT THRUSTERS


43/50

AHall-effect thruster

It requires an electronneutralizer


44/50

Generation of a double layer

2

0

0 0

rB n

B n r

! !

0 , where -e /e en n e V KT L

L

! |

1/ 2 1/4

0 0

, and thus 1.28n r

e en r

! ! !

The Bohm velocity is reached when L = , and sheath forms


45/50

Potential jump observed by Charles et al.


46/50

B-field in Boswells helicon machine


47/50

Medusa source adapted to VASIMR

The optimized 9-cm diam source is shown with dimensions in cm, together with a NdFeBmagnet designed for 400G at the antenna. D is the distance from the midplane of themagnet to the midplane of the antenna. The magnet is made in two pieces supported by anon-ferrous metal plate. The B-field can be adjusted by changing D either by hand orremotely with a motor.


48/50

A stronger B-field for higher density

Layout of magnet and tube for 600G operation,showing a gas feed line and a DC bias supply.


49/50

A small diam source with for testinghigh-field operation

A 5-cm diam helicon tube and a 600-G magnet designedfor a small overall system diameter.


50/50

Conclusion on spacecraft thrusters

Ambipolar sources can eject ions with automaticspace-charge neutralization.

Helicon sources can generate ions efficiently.

Permanent magnets can reduce the complexity ofhelicon sources. However, for the fields and densities considered

for the VASIMR project, the magnet may be too

large to be practical.

francis f. chen- applications of permanent-magnet sources and arrays

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