Polarized Beam OperationA polarized beam option has been developed for BT-7, utilizing 3He polarizers immediately before and after the sample. This gives BT-7 full polarized beam capability for experiments where a guide field (only) is applied at the sample position to control the direction of polarization, and thereby the cross sections. The system is fully operational. There are two spin rotators that can be mounted before and after the sample. Alternatively, the 3He itself can be inverted to achieve the alternate spin state before or after the sample. An adjustable guide field at the sample position is under computer control to manipulate the polarization direction perpendicular or parallel to the scattering plane, or in an angular range in the scattering plane.
Each 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.
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 currents 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 are computer controlled. Energizing them appropriately will provide either a vertical guide field at the sample position for the Qperp.P or a horizontal field for Q | | P.
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
Publications that have utilized the polarized beam capability on BT-7
Last modified 19-July-2011 by website owner: NCNR (attn: Jeff Lynn)