D field mapping

Two NMR probes are located at fixed points inside of the D magnet as monitors; one of them will remain there not only during the mapping but also during the experiment. It is possible to normalize the field map with this NMR probe.

Our field mapping plan for the D magnet is as follows:

1. Excitation curve (BI curve) measurement,
2. Downstream mapping,
3. Upstream fringe field region mapping,
4. Three magnets (Q1, Q2 and D) simultaneous excitation to check the interference between Q1 and Q2.
5. Two magnets (Q2 and D) simultaneous excitation to check the interference between Q2 and D.

The excitation curve was measured at the center of the magnet (figure 34). In order to operate the magnet on the same hysteresis curve, an initialization procedure (the current was changed from zero to the maximum for three times) was performed before the measurements. Measured points and TOSCA calculated points are plotted.

The field mapping of the HKS D magnet is now in process. The mapping is being carried out with a mesh size of 40 mm for inner part and 20 mm for the fringe region. The field mapper can measure 300$\times$800$\times$200 mm$^3$ volume at one setting, and thus we divided the mapping region into 19 segments for downstream and 5 segments for upstream (figure 35).

The position of the field mapping system relative to the magnet is calibrated by searching a cone shaped permanent magnet which was set at reference positions on the pole face. The angle of the probes to the magnet pole face was measured by the laser displacement detector (KEYENCE LC-2440) which can measure the distance from the pole face to the Hall probe with an accuracy of 0.2$\mu$m.

It is also planned to examine the interference between the three magnets, since they are placed very close to each other and such an interference could be a problem for a high resolution spectrometer. The interference effects between magnets were evaluated to some extent with the TOSCA 3D field calculation which showed shortening of the effective field boundaries of the Q1 and Q2 magnets. It is, thus, necessary to examine the interference experimentally and also to establish the magnet excitation procedures. After the single D field mapping, the Q1, Q2 and D magnets will be aligned in the real experiment configuration and they will be excited to the operational fields. Several current setting procedures (Which magnet will be excited first? Is the concurrent excitation better?) will be tested to determine the best one. At first, Q1-Q2 interference will be measured from upstream and then, Q1 will be removed for the field mapper to access Q2-D interference region.

In this way, thorough field measurement of the HKS magnets will be carried out before they are shipped to Jlab. At Jlab, the magnetic field of HKS-D will be measured at several points to check whether the result in Japan is reproduced in order to confirm the accuracy of the assembly.