Horizontal Crossing Angle: Crab cavity?

The design principle is the smallest crossing angle as possible because of the highest luminosity without a crab cavity. The present angle of $\pm$4mrad is limited by synchrotron radiation background emitted from collimated beam. If we use a crab cavity, the luminosity increases by about 40%. For NLC, SLAC assumes the perfect performance for a crab cavity, then the crossing angle is $\pm 10$mrad for easier extraction of disrupted beams, whose magnitude is limited by a 4 Tesla (detector-)solenoidal field. Phase stability of the crab cavity must be less than $0.2^\circ$, thus we need a prototype cavity. Since KEK-B factory uses a crab cavity, it may be the prototype(?).