The PMT (Trikon) MØRI source

Commercial helicon sources need heavy magnetsand cannot cover large substrates

Solution 1: Use distributed sources

ROTATING PROBE ARRAY

PERMANENT MAGNETS

3"

DC MAGNET COIL

18"

However, still need a large electromagnet

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

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30R (cm)

N (

101

2 cm

-3) 3.0

2.5

2.0

1.5

1.0

P(kW)

7-tube m=0 array

ARGON

Experiment at PMT,ca 1995

UCLA

12.7 cm

7.6 cm

PLASMA

Material: NdFeB

Bmax = 12 kG

The magnets are dangerous!

Solution 2: Use permanent magnets

The innovation involves two parts:

1. Novel use of PMs2. Use of the low-field peak effect.

The field reverses at a stagnation point very close to the magnet.

Plasma created inside the rings follows the field lines and cannot be ejected.

Internal field

External field

The field of annular permanent magnets

Proof of principle on 3” diam tube

Gate Valve

To Turbo Pump

34 cm

36 cm

D

Z1

Z2

-300

-250

-200

-150

-100

-50

0

50

100

150

0 5 10 15 20 25 30

z (cm)

Bz

(G)

Calculated

Measured

External field

Internal field

0

1

2

3

4

5

6

7

-5 0 5 10 15 20r (cm)

n (

101

0cm

-3)

Z2, 40

Z2, 35

Z2, 30

Z2, 21

Z2, 1

D (cm)

500W, 1 mTorr

The bottom curve is when the tube is INSIDE the magnet

Mechanism of the Low Field Peak

UCLA

0 1,r

z

ne nk

k B a k B

Basic helicon relations

Constructive interference of reflected wave

Design of discharge tube: maximize the loading resistance

Calculations are done using the HELIC code of D. Arnush

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 (

oh

ms)

100.0

63.1

39.8

25.1

15.8

10.0

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

Low-field peak

2.120.25

4.00

2.25

Lesker QF50 cover plate

Lesker QF50 centering ringParker 2-330 Viton O-ring

Corning #237580 tubing, 63.5 mm OD, 4.8 mm wall

Parker 2-239 Viton O-ring

Dimensions are in inches

Final design of “stubby” discharge tube

Medusa 2: An 8-tube linear test array

UCLA

165 cm

53.3 cm

17.8

17.8

17.8

17.8 cm73.7 cm

8.9 cm

x

y

Top view

The array source is vertically compact

UCLA

The magnets can be made in two pieces so that they hold each other on an aluminum sheet.

Once placed, the magnets cannot easily be moved, so for testing we use a wooden support.

165 cm

30 cm

15 cm

Side view

Probe ports

Wooden frame for safe storage

UCLA

The wooden magnet frame is used in testing

UCLA

64"

21"

3.5"

12"

Wooden magnet support

2.00

5.00

3.75

2.5

24 ea. 2.5 x 5.5 x 1/2"4 ea. 2.5 x 64 x ¾”

1 ea. 21 x 64 x 1/2”16 ea. 1/4” diam dowels

Antenna connections

UCLA

For CW operation, all connections were solidly soldered, and RG/393 teflon-insulated cable was used. Cable

connectors cannot take the startup voltage.

Matching circuit for N tubes in parallel

UCLA

R, L

R, L

R, L

R, L

PS

N loads

Z2 - short cables

Distributor

Z1Z2

Z1 - long cable

C1C2

Matching ckt. 50W

The problem with array sources is that the cable lengths cannot be short. The match circuit cannot be

close to all the tubes.

C1, C2 for N=8, L = 0.8H, Z1 = 110 cm, Z2 = 90 cm

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)

Matching sets limits on antenna inductance and cable lengths.There is a “sweet spot” for tube design when RF is considered.

A water-cooled, 50-W, low resistance,rectangular transmission line

Medusa 2 in operation

UCLA

UCLA

0

2

4

6

8

10

-8 -6 -4 -2 0 2 4 6 8y (in)

n (1

011 cm

-3)

Z1, x = 0Z1, x = 3.5Z2, x = 0Z2, x = 3.5

Compact 3kW, D=7", 20mTorr

0 3.5

Compact configuration, 3kW

Side Langmuir probe

Density profiles across the chamber

<< 4” below tubes

<< 7” below tubes

UCLA

0

1

2

3

4

5

-8 -6 -4 -2 0 2 4 6 8y(in)

n (1

011 c

m-

- 70714

x (in)Staggered3kW, D=7", 20mTorr

Density profiles across the chamber

0 7-7 14

Staggered configuration, 3kW

Bottom probe array

UCLA

0

1

2

3

4

5

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

n (

10

11 c

m-

-3.503.5

Staggered, 3kW, D=7", 20mTorr y (in)

Density profiles along the chamber

Staggered configuration, 3kW

Bottom probe array

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 (1

011 c

m-

-3.503.5

Compact, 3kW, D=7", 20mTorr y (in)

A compact, stackable module

The match circuit fits on top of the array