Ion Source Modelling and Field Index Improvements

by Scott Lawrie

Current goals• Decrease emittance of beam• Study the influence of the sector magnet• Confirm field index, n = 1• Widen magnet poles• Find optimum pole separation and angle• Plot good field region• Make improved poles and test them• …Then move on to the extractor!

The model

Coordinate system layout

5

H2CsCs

4 3 2 1 θ

r

r = 105 mm

r = 55 mmB

B

Cold box top view

Pole piece close-up

Angle of Pole-face

Pole Width

Spacer Thickness

Shim Width

Shim Height

Maximum pole width = 74mm(ISIS standard = 50mm). Allows 1.5mm clearance from bottom of cold box.

Centre of pole face;used to measureseparation of poles

Scan field along curves

Comparison of magnetic fields

8.88 ° faces,24.00mm separation

7.00° faces,19.16mm separation

Calculating the field index

• Sum up the z-components along curve

• Multiply by angle difference integral

dR

dB

B

RRn

R

BB

n

Field index of ISIS dipoles

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

55 60 65 70 75 80 85 90 95 100 105

r (mm)

n

Pole pieces separated by 25mm, with face angles of 8.88°

How the field index varies with pole separation

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

18 20 22 24 26 28 30 32 34 36 38 40

Pole separation / mm

Fie

ld i

nd

exOptimum pole separation = 28.6 mm

How the shape of the field index varies

0.90

0.95

1.00

1.05

1.10

55 60 65 70 75 80 85 90 95 100

Radius in dipole / mm

Fie

ld i

nd

ex

7 degrees

8 degrees

8.5 degrees

8.88 degrees

9 degrees

10 degrees

Pole face angle

Separation of poles modified to give an n = 1 field at R = 80 for each angle.

7 degree poles couldn’t be squeezed close enough together without hitting the extract.

Angle / separation relation for n = 1

18

19

20

21

22

23

24

25

26

27

28

7.0 7.5 8.0 8.5 9.0 9.5 10.0

Pole face angle / Degrees

Po

le s

epar

atio

n /

mm

Field indices for wide poles with fixed 32mmpole separation and varying angle

0.4

0.6

0.8

1.0

1.2

1.4

1.6

50 60 70 80 90 100 110

Radius in sector magnet / mm

Fie

ld i

nd

ex

14.0 degrees

13.0 degrees

12.0 degrees

11.5 degrees

11.0 degrees

10.5 degrees

10.0 degrees

9.0 degrees

8.0 degrees

7.0 degrees

6.0 degrees

Field indices for wide poles with different parameters

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

50 60 70 80 90 100 110

Radius in dipole / mm

Fie

ld in

dex

14 degree faces, 40.87mm separation

11 degree faces, 32mm separation

Comparing fields off centre

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

50 60 70 80 90 100 110

Radius in Dipole / mm

Fie

ld i

nd

ex

At mid-plane

3mm off-centre

6mm off-centre

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

50 60 70 80 90 100 110Radius in Dipole / mm

Fie

ld i

nd

ex

At mid-plane

5mm off-centre

10mm off-centre

11 degree pole faces,36 mm separation

14 degree pole faces,40.87 mm separation

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Mapping the good field regionCurrent ISIS poles

Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract.

Field index, n

Field index, n

Explanation for the cobra-head?

Current ISIS poles with large error

Field index, n

Still to do…

• Check the field map is optimal• Get new poles built• Test new poles• Move on to improving the extract