Polarized Beam Operation
There are two spin rotators that can be mounted before and after the sample. There is an adjustable guide field that consists of soft iron on top and bottom, with vertical coils at the corners. The spin flipper sits inside. The flipper itself is constructed by winding two flat concentric `coils', one for a horizontal field and one for a vertical compensating field. These are wound with Al wire. The flippers are VERY FRAGILE--DO NOT TOUCH THEM. The Al wire can be easily broken, and/or the coils can be shorted together. Either problem will necessitate rewiring both coils of the flipper, a very time-consuming and tedious task which you won't enjoy (no, there is no technician assigned to BT-2).
There are six HP constant-current power supplies, three for each flipper. One power supply is for the (vertical) guide field, one is for the vertical compensating field, and one is for the horizontal spin-rotating field. The spin-rotation current and the compensating field current are computer controlled in order to scan the currents and maximize the spin-flipper efficiency, and also to adjust the current for the horizontal flipper field to allow varying the incident or final energy.
There are two large Kepco bipolar power supplies to energize the guide field at the sample position, which consists of two separate sets of coils. Both supplies should be run in constant-current mode. Both can be computer controlled. To set the current, use the command ISET 5,10,10 where 5 (or 6) is the number of the power supply (1-4 are for the flippers), 10 is the current (in amps), and the second 10 is the "maximum current". To set it to -10 amps, ISET 5,-10,10). Energizing the supply one way will provide (with the correct current) a vertical guide field at the sample position, while energizing it in the opposite direction will provide a horizontal guide field. This is convenient for measuring with Q^ P and Q | | P. Any direction in between can also be achieved via computer control (either deliberately or inadvertently) .
Typical Values for the Spin Flippers:
Guide field in flipper: ~1 amps, which corresponds to a field of ~10 Oe.
Horizontal field: 1.7 A @ 14.7 meV
Compensating field: 1 A for a guide field of 12 Oe
Measured flipping ratio at 14.7 meV: R = 30 (with Heusler analyzer)
Measured flipping ratio at 5.0 meV: R = 45 (with Heusler analyzer)
Notes and CautionsIn polarized beam operation, don't forget that the Heusler analyzer polarizes in the opposite sense of the supermirror. Thus with neither flipper energized you are measuring the spin-flip cross section.
Last modified 25-August-2003 by website owner: NCNR (attn: )