Status of Single Point Hall Probe Studies in SHMS Q2 Sam Murri, Dave Mack 26 July, 2017
Motivation We would like to be able to scale the SHMS Golden Tune we determine this Fall from 2.2 GeV/c to other momenta with linearity errors less than 2E-4. (See slide 2 of https://hallcweb.jlab.org/doc-private/ShowDocument?docid=876 .) The SHMS tune is most sensitive to the Q2 setting. (2E-4 corresponds to ~4 Gauss near 5 GeV/c, or only 0.8 Gauss near 1 GeV/c.) Assuming the set current error is <= 1E-4, in general the sources of nonlinearity for an SHMS magnet are A small, O(10) Gauss remnant B field in the iron as I 0, A few percent iron saturation as I Imax, An O(1)% change in Effective Field Length as IImax (gotten model-dependently from TOSCA) Hall probe errors at the 1E-3 level, for which we supposedly have correction tables. Ahem.
Taking Q2 Data We went through two bi-polar hysteresis loops with Q2: one loop with 24 data points, and a second one with 14 data points to check the reproducibility. There was a small chance of a quench at full power. So as not to perturb a frail ESR or critical 12 GeV solenoid work in Hall B, our Imax was limited to 70% of 11 GeV/c current. We hoped that magnet history would be erased after ramping to 0.7*Imax. Our set currents were on the downward path of the hysteresis loop: for this coil-dominated magnet where saturation was not expected to be an issue, we stepped downward in coarse steps of 10%, then 5% as we approached the remnant field.
(Still need to check for something more subtle: Q2 Good News Very short settling time: Less than 1 minute after voltage stops changing, the field readings are stable up to a bit-flicker. Reproducibility is excellent: Field measurements at the same current and polarity on different loops are consistent to 5 digits. A little spooky actually. (Since the tempco is 10-4/degC and our measurements took hours, the temperature must be very stable inside the bore of Q2.) Direct cross-talk between Q2 Hall probe and Q3 magnet is negligible. (Still need to check for something more subtle: cross-talk from Q3 magnet to the Q2 power supply when the latter is on. And cross-talk with Q1. )
Uncorrected Q2 Field Residuals Wrt A Line Only includes the first loop and one point for I=0. The take-away: Left plot - Absolute departures from perfect linearity are only ~20 Gauss Right plot - Relatively speaking, nonlinearities are mostly below 2E-3 except at lowest fields (below 1 GeV/c) The Hall probe corrections – when done correctly - should be of this magnitude and might completely change this picture.
Other Good News ==> PS on PS off Zero Offset 1.55 G 2.25 G Assuming approximate symmetry for the + and – polarities, we can determine the iron remnant field and the Hall probe offset at I = 0 without a zeroing can or a degaussing cycle. ==> PS on PS off Zero Offset 1.55 G 2.25 G Remnant Field for +- Polarity, respectively (preliminary since we only cycled to 0.7Bmax) -+11.95 G -+10.05 G Surprisingly, for Q2 we care! Non-surprising results: The remnant field has a reasonable magnitude. We have also confirmed Dave Gaskell’s old HMS observation that the results at Iset = 0 depend on whether the power supply is on/off or +/- polarity. Clearly there is an O(0.1)Amp trickle current at Iset = 0 when the power supply is on. It’s big enough to matter. Surprising result: for + polarity, the remnant field is negative, and vice versa. The working hypothesis is that the thick iron yoke surrounding the coil-dominated Q2 acts like a magnetic mirror.
Q2 Less-Good News The asymmetry between + and – polarity is relatively large (0.2%) even after nominal Hall probe corrections. The corrections for the Q2 Hall probe are too small to account for this, even assuming a sign error. Working hypothesis: the nominal Q3 Hall probe has been installed inside Q2, and vice versa. And the polarity is reversed. Pathetic, but possible. Need another day or two to track this down.
Summary Our goal is to scale the SHMS Golden Tune we determine this Fall at 2.2 GeV/c to other momenta with linearity errors less than 2E-4. Lots of good news, but looks challenging with these Hall probes. What we have learned about Q2 so far: Settling Time (after voltage stabilizes) < 1 minute (transparent to users) Reproducibility (of B at a given Iset over hours) Good to 10-5 (for same Iset and polarity) Hall Probe Offset +1.55 Gauss (nice and small) Remnant Field (after cycling to 0.7Bmax) -+10.05 Gauss (Sign is opposite of nominal polarity, interesting but possibly understandable.) Polarity Asymmetry 2E-3. The only seriously bad news so far. The expectation of polarity symmetry allows us to check the validity of the Hall probe corrections provided by the manufacturer, and have confidence that we’re measuring the true nonlinearity in Field vs Current to few x 1E-4 (rather than getting fooled by the Hall probe nonlinearity). Maybe we’re applying Q2 Hall probe error corrections to the Q3 Hall probe. Work in progress.
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