Field stability study

Results of magnetic field stability study (August 2014): Media: field_stab.pdf‎

• The field is stable with time well within our 10^-3 accuracy goal. There was a 5 Gauss shift after the power supply trip at the end of day one, although the same power supply setting was restored. The influence of the solenoid field (solenoid current set at 1200A) is at the one Gauss level when the pair spectrometer field is at its nominal 1.8T. However, we measured a ~20 Gauss field in the pair spectrometer when the Solenoid was at 1200A while the pair spectrometer power supply was turned off.

• The Danfysik power supply output readout is accurate. There will be no need for DCCT monitoring during beam operations. Some of the small field changes seem to loosely correlate with small changes in the power supply output readout.

• The Hall probe is stable and correlates well with the NMR probe in spite of their different locations. (The NMR probe is at the center of the magnet. The Hall probe is slightly upstream, just outside the vacuum chamber. We added a 135 offset to the Hall probe field to have it on the same plot).

Field dependence with temperature (August 2014): Media: temp_dep.pdf‎ (For the top plot, the horizontal scale is the measurement number.)

• There is clear field dependence with temperature. However, it is very small (-0.03 Gauss per degree). The Hall probe does not show the dependence because its accuracy is too coarse.

We do not expect changes of temperature much larger than a degree C: To increase the temperature by 4 degrees, we had to close the flow valve by 90%. In conclusion, the temperature effect on the field is well below our accuracy goal and can be neglected. (We remark that, while doing the measurements, we may not have wait for the thermal equilibrium to be fully reached. This is why on the bottom left plot, the last measured point at 36.5 degree does not align well with the other points. Given the smallness of the temperature effect, it was justified to not wait for thermal equilibrium at the cost of a small inaccuracy.)

In conclusion, a Hall probe and the Danfysik power supply readout will be enough to monitor the stability of the field during beam operations. DCCT, NMR probe and temperature monitoring are not necessary.

Details on the procedure and analysis can be found in the report