Hi,
Karen, Jim, and I made a test run of mapping the BLAST magnetic
field this morning. We measured all three components of the magnetic
field at four points in the left sector. Obviously the vertical
component is the most important for BLAST. The attached PDF file
summarises the results for By and compares them to various calculations.
The first four lines of data are the actual measurements. The
other lines are merely to indicate where we may make further
measurements next week to show the sort of fields we will encounter
and the deviations with calculation.
The calculations are:
blast - as used by reconstruction - someone can check the code I
used to generate these numbers (also attached)
bgrid2 - my own interpolation of the bgrid2.blast data file
(does not include holding field)
ideal - Biot-Savard calculation using ideal coil positions
offset - Biot-Savard using coils shifted by 8 mm radially
In my opinion the blast, bgrid2, and ideal calculations are all
consistent. The offset calculation has some discrepancies with the
other calculations but is not wildly off except possibly at the
lowest field values which are not critical.
As for the four measured data points there is a significant
discrepancy will the calculations. Particularly the highest field
value is on the order of 5% different from calculation. The measured
field is greater than calculated which would mean we would
reconstruct it at a lower momentum which I believe is consistent with
our observed systematic shift.
Obviously, this is all very preliminary and many more
measurements will have to be made and the data understood before we
can claim any sort of understanding of the momentum reconstruction.
If necessary we may have to completely re-map the field.
Cheers,
Douglas
26-415
M.I.T. Tel:
+1 (617) 258-7199
77 Massachusetts Avenue Fax: +1
(617) 258-5440
Cambridge, MA 02139, USA E-mail:
hasell@mit.edu
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