1. Some Thoughts on Magnetic Measurements for MICE Michael A. Green Lawrence Berkeley Laboratory Berkeley CA 94720, USA

2. Steps in the MICE Magnetic Measurement Process Magnetic alignment begins during construction. Low current magnetic measurements at 300 K can be used to verify that the magnetic axis is in the correct x-y position and angle, at a time when one can adjust the position of the cold mass. It is planed that the MICE magnets will use self- centering supports. This means that the center between the supports is in the same position at 4 K as it is at 300 K, within ±0.2 mm. The axis angle should not change more that ±0.5 mrad.

3. Magnetic Measurement Steps cont. Measurements at the vendor will verify that the magnet will reach it full design field. The on-axis field will be measured as a function of z. It should be possible to determine the solenoid magnetic axis angle with respect to the magnet bore axis. A field map will not be made at the vendor’s plant. It is recommended that a magnetic field map be made in the regions of the magnet that will see the beam. For the tracker magnet these measurements will be made at FNAL. There is no decision as to where the other magnets will be mapped.

4. Magnetic Measurement Steps cont. Once the MICE channel has been assembled, magnetic measurements of the channel magnets is longer be possible. One cannot measure the effect of current in another MICE magnet on the field within a particular MICE magnet. One must use the calculated to field to determined the effect of one magnet on another magnet in MICE. Jean Michel Rey of Saclay proposed that the field be monitored by three axis magnetic measurement probes (Hall probes) in at least three places around each of the coils in the MICE channel. This method is used to monitor the field in ATLAS. Is this a useful technique for the MICE cooling channel?

5. Is External Field Monitoring Necessary for the MICE Channel Magnets? The field outside of the magnets is relatively low for the tracker magnet and the AFC magnet in the non- flip mode (~0.05 T). Measurement sensitivity may be a problem. The field outside the coupling coil is high (1 to 2 T) and the field outside the AFC magnet in the flip mode is medium (0.2 to 0.35 T). External field monitoring makes sense if there is no iron around the magnets. There will be lumps of iron (compressors, pumps and some other items) that may affect the field outside of MICE, but the field in the MICE channel will not be affected much.

6. External Field Measurements cont. It is not clear that external field measurements are useful for the tracker solenoid. Monitoring of the field in the bore around the tracker is proposed. Lumps of iron outside of MICE can affect the field outside of the coupling coil because the magnetic moment generated by the coil is higher and the coil is closer to the iron. The field outside of the AFC magnet may also be affected by iron. Measuring the external field is useful to determine the channel operating mode and an individual magnet’s polarity. Other than this, field monitoring outside of MICE magnets may not be very useful.

7. External Field Measurements cont. For the tracker module, three axis Hall probes could be installed 120 degrees apart on the outside of the tracker magnet cryostat over the center of each match coil and the center of the long center coil in the spectrometer solenoid. This is in addition to the probes installed on the tracker frame. For the AFC module, a single set of three axis Hall probes could be installed 120 degrees apart on the outside of the AFC cryostat at the center. For the RFCC module, a single set of three axis Hall probes could be installed 120 degree apart on the outside of the magnet cryostat at the center.

8. Concluding Comments Magnetic measurements (both warm and cold) are done at the vendor to determine that the magnet meets the basic specification. Field mapping on all individual magnets should be done in the region where the MICE beam will be. The angle of the magnetic axis with respect to the magnet physical axis must be determined. External field probes are useful for determining the operating mode and the magnet polarity. Knowing the field at the tracker frame is useful.