emiT Spin Transport Issues for Removing a Be-coated Guide Tube
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K.P. Coulter

05-NOV-01

Following Fred’s suggestion that we remove the last Be-coated guide tube, I looked at how easily we might accommodate the spin transport of the neutrons on this shorter beamline. The simplest approach would be to remove the magnets associated with the removed guide tube and adjust the currents of the remaining magnets. The removed magnets would be Loop 2, Solenoid 3, and Loop 3 (please refer to S. R. Hwang’s dissertation figure 2.6). I had four concerns:

1. Can the field be made adiabatic with minimum changes to the existing magnets?
2. Can the required currents be accommodated by the existing MOSFET power supplies?
3. Will these changes significantly affect the decay region field direction?
4. Is there significant mechanical work involved in removing the guide tube?

To assess the first concern I looked back at calculated fields for the magnets ( we only ever took field maps of the guide fields, not the transport fields). To set the currents of the spin transport magnets I looked at the adiabatic parameter:

Just playing around by hand led to the results in Figure 1. This shows the magnetic fields and the adiabatic parameter for both beamlines. The long beamline case is shown for the currents used in the first run of the experiment and for which a complete Monte Carlo spin transport calculation was done. I was actually able to make the adiabatic parameter smaller in the shortened case (probably because I tweaked the currents a bit more than in the original case). The changes were made to the currents of Loops 2 and 4 and Solenoid 4. They were increased by factors of 1.62, 4.94, and 1.36, respectively.

These increases in current require a bit of shuffling of the MOSFET power supplies, where the biggest concern is to match the voltage supplied by the HP supplies to sets of MOSFET supplies. A few of the sense resistors will also have to be changed. The old configuration is given in Table 1and the new configuration in Table 2. The shifts could easily be done..

Figure 2 shows the polar angle made by the magnetic field with respect to the beam axis. Although the actual angle depends on the alignment of the individual coils and the magnetic environment, it’s obvious that the change in the beamline does not, in and of itself, have a significant effect on the angle in the central 50 cm.

Finally, in Fred’s original email he addressed the necessary mechanical changes required. I include a few comments here for completeness. The individual mounts for magnets and guide tubes and the collimation will not have to be modified at all. Only the support frame and rail will need to be remade. We will need to make the equivalent of the last half of the support frame detailed on Teasdale’s D58 and the rail, detail 3 on D59. I do not think it would be a good idea to shorten the existing ones, we need a fall back option if we’re mistaken about something. My notes show that these items were manufactured at NIST. A priority would be to find out if we have the time to get short ones made, or if we need to go with a complete maxistrut-type construction.

Figure 1. The B fields and adiabatic parameter for each beamline.