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 H2H2 Cs 4321 θ r r = 105 mm r = 55 mm B 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 measure separation 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

Field index of ISIS dipoles Pole pieces separated by 25mm, with face angles of 8.88°

How the field index varies with pole separation Optimum pole separation = 28.6 mm

How the shape of the field index varies 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

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

Field indices for wide poles with different parameters

Comparing fields off centre 11 degree pole faces, 36 mm separation 14 degree pole faces, mm separation

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. Field index, n

Mapping the good field region Current ISIS poles Wide poles with same face angle as ISIS poles, separated a bit so as not to collide with extract. 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