Optics Meeting Mar 7 2023 230PM ET
From Moller Wiki
- magnetic field study - mean radial location on GEMs (Kate) 
David, Paul S, Ciprian, Juliette, Andrew, Zuhal, Vassu, Sayek, Michael D., Jim
- (Kate) With the "asymmetric" field map, as well as the "worst case" field map, our reconstruction algorithms still converge without issue. Conclusion: we our algorithm will work even for non-symmetric field maps - good! The worst case field map is expected to have a significant dipole component to the B-field, which according to Juliette would bend electrons that were on the beam axis upwards and to the left (not sure what happens to electrons in the acceptance). When using the fit parameters extracted using from the ideal ("default") field map, but reconstructing data obtained with the asymmetric field map, the results are almost unchanged. When using these same ideal fit parameters but reconstructing data with obtained with the worst case field map, we start to see effect on the reconstructed theta - the mean value of the reconstructed theta disagrees with the actual theta at the target by 0.15 mrad which is 3% of the smallest scattering angle in the acceptance. So, we hope we can find diagnostics to identify that the field map was "off" from ideal. Kate will investigate if the effect varies with individual holes, and will plot residuals for theta and phi for individual holes.
- (Kate) Comparing images of the hole patterns in R,phi at the first GEM plane for individual holes we see clear diagnostics. For holes 71 and 72, we see a significant shift (order 5 mm) between the results for the worst case vs. ideal field, mainly in the r-direction for hole 71 (aside: Kate should check if the number for 71 and 72 matches her diagram). This gives us two diagnostics: 1) comparison with the r position (and maybe phi) for the hole's image between what is measured and what is expected from the ideal field map and 2) comparison of the images in the data between symmetric holes (71 and 72) and looking differences in where they appear in r. About half the holes in the sieve show shifts from nominal with the worst case field of at least this size (i.e. > 5 mm). This provides a very clear set of diagnostics. Can see smaller (1-2 mm) shifts in the case of the "asymmetric" field map. Also clear diagnostics for this field map: holes 21 and 51 (same r,phi, but in different sectors), 31 and 53, 23 and 62, which are at the same radii but different phi offsets in different sectors.
- (all) Notes and ideas:
- We will get an "as built" field map, based on various coil survey, etc. information, so we should not have to start with an "ideal" field map
- SAM data (effect on the unscattered beam) will provide a useful independent diagnostic of, especially, dipole components to the field map. Beam dump viewers also should help at a crude level. Ultimately field map should produce measured tracking data and SAM distributions.
- Calibrating GEM positions - can be done by projecting to the thin quartz detectors and seeing the detector "edges" in tracks that caused hits in the thin quartz. Similar using the HVMAPs. Similar using the upstream scanner (for that sector). We should also check to see if tracks in the GEMs can project to the DBMs.
- We should repeat Kate's studies with other beam energies (1 pass and 3 pass), to see which is most sensitive to bad B-field.
- We need to update the hole numbering scheme to match the sector numbers as defined by the spectrometer group.
- It would be interesting to see what we can learn from turning off (or down) the current in particular segments of the spectrometer (e.g. things like setting all of TM3 to half current).
- (all) To do before ordering sieve (i.e. time-sensitive):
- We need to check to see if the 12C/Moller electron separation would be compromised for any of the holes with a "worst case" field map.
- Need to see if our present hole pattern is "blind" to any particular field distortions; if so, we might want to add holes or change the pattern. How to check? a) For the "worst case", we can simply rotate the Sieve sector-by sector (7 separate simulations) and see if in any of the sector orientations fail to give "smoking gun" diagnostics of the field being off. b) then we need the spectrometer group to generate TOSCA field maps for other plausible distorted field configurations.
- We should decide if we can add a few holes to the sieve pattern that are not useful for the optics, but which may be useful just for looking for field distortions - perhaps at large phi and/or small R. It won't matter if the images on the GEM plane from those holes include events from both Moller and 12C elastics.
Meeting ID: 972 5975 5403