Gentlemen,

I have had Mike Swanson construct a simulated BLV beampipe  15" long and 30 mm high, in order to test the electromagnetic skew quad corrector concept.  Ben Poling is presently applying copper strips to it, and connecting so that they conduct in the correct manner.  The strips are 25 mm wide, 0.005" thick and there is a 15 mm gap between them:

The strips are insulated above and below by Kapton tape, so the total thickness is about 0.006", or 150 microns, and the two layers (above and below the beampipe) thus increase the gap by 0.3 mm.

The magnetic field along the diagonal is (x = mm, y = T)

The value of the gradient is 0.238 Gauss/mm, which, if integrated over 2 meters,

gives a total of 576 Gauss, approximately the value needed to correct the whole

skew quad error at a 30 mm gap.  If we leave the permanent magnet skew quad corrector on

the EPU, it also corrects this whole error at 30 mm, so the electrical corrector would need

to do nothing at minimum gap.  However, because the PM corrector does not track the error as a

function of gap, the EM corrector can apply a correction on the correction and null the error that

remains.  The worst case remaining error is about 150 Gauss.

If we were to apply this correction scheme on the new BLV  beampipe, with a minimum magnet

gap of 18.6 mm, and leave the PM corrector in place, it overcorrects the error above 30 mm, but

would undercorrect it between 18.6 and 30 mm, and the EM corrector could be used with the

opposite sign to null the residual error.

Looking at the magnetic field above, there is some octupole superposed on the gradient field, and its

integrated value is 0.27 Gauss/mm^2 over two meters, which is less than James' limit of 0.4 Gauss/mm^2.

But we should probably wait for Zack Wolf to give us test results, to see if we can live with the

amount of octupole.  Of course, if we correct only a 150 Gauss error, not a 570 Gauss error, the octupole

field will be correspondingly smaller.

Roger

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Roger Carr
Stanford Synchrotron Radiation Laboratory
Stanford Linear Accelerator Center
MailStop 69, Building 137, room 208
2575 Sand Hill Road
Menlo Park, CA 94025

Tel:  650-926-3965
Fax:   650-926-4100
Email:  Carr@SLAC.Stanford.edu

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