LCABD Collaboration: Work Package 5: Crab Cavity

The most flexible designs of the international linear collider (ILC) beam delivery system in terms of operating parameters, typically have a crossing angle between the electron and positron beamlines greater than about 14 mrad so that electron and positron bunches do not pass through each other's final focusing quadrupole doublets thereby assisting the extraction of spent beams after collision. The baseline option has two interaction points with crossing angles of 20mrad and 2mrad. An alternative option has one crossing angle of 14 mrad. The angle between the electron and positron bunches at the interaction point will cause a luminosity loss unless corrected by an appropriate rotation. The rotation of these bunches is to be performed using two crab cavity systems. A crab cavity is a transverse deflecting RF cavity operated with a 90 degrees phase shift. The development of a crab cavity system is a critical R&D requirement for ILC large crossing angle schemes. The UK has taken a lead role in initial studies to define the type of system that will most readily fit current layouts for the ILC beam delivery system (BDS) and that will meet Global Design Effort (GDE) specifications. A current project has established that the best fit to the ILC crab cavity requirement is to develop a derivative of the CKM 3.9 GHz superconducting cavity. RF phase tolerances for the operation of the cavity have been established. The crab cavity system development now requires optimisation of the superconducting RF dipole cavity, such that its Higher Order Mode (HOM), Lower Order Mode (LOM) and Same Order Mode (SOM) components are sufficiently damped so as to eliminate any unwanted wakefield interaction with the ILC beams. Coupling out this power from the cavity and cryomodule requires development of appropriate coupler and absorber solutions. Microphonics instabilities in the SRF cavity/cryomodule can also deteriorate RF system performance. To mitigate such effects detailed mechanical and thermal analysisis required. Amplitude stability of the crab cavity systems must be better than one part in 10,000 and the relative phase error between the electron and positron crab cavity systems must not be more than 0.07 degrees at 3.9 GHz. To reach this demanding specification it is anticipated that advanced control techiques built on state of the art digital digital processing must be employed and research is needed to push the limit on what is currently achieveable. The high power 3.9 GHz RF system must be characterised in terms of its amplitude and phase stability and subsequent integration with the crab LLRF control system. The entire system needs to be fabricated, tested on a beamline and operation proven in preparation for the Technical Design report of the GDE due in 2010.