Crab cavity development for the Cornell B-factory, CESR-B

The present approach to a CESR B-factory is to divide the desired beam current of 1-2 A into a large number of low-current bunches, which helps reduce detector backgrounds. To fit 230 bunches into the CESR size ring, they need to be very closely spaced, eliminating room for any conventional separators. Instead, small (12 mrad) crossing angle is considered, which may cause harmful coupling between synchrotron and betatron motion. By rotating the bunches before collision (´crabbing´) so they collide head-on, and then rotating them back so they pass through the arcs normally, this dangerous coupling can be eliminated. It is shown that the needed transverse kick of approximately 2 MV can be achieved with one single-cell superconducting cavity, operating in the TM110 mode at 500 MHz, while keeping the surface electric field below 25 MV/m to avoid excessive field emission. The cell design allows all modes higher in frequency than the crab mode to propagate out the beam pipe absorbers. Some 1.5-GHz niobium cavities have been tested to study the multiplicating behavior of the TM110 mode.<>