Crab Compensation for Lhc Beams

An R&D program to establish a road map for the installation of crab cavities in the LHC is rapidly advancing. Both local and global crab schemes are under investigation to develop cavities that will be compatible with the LHC optics and meet the aperture requirements. Space and aperture constraints to accommodate a prototype crab cavity in the LHC along with related optics issues are presented. The design of a prototype TM110 cavity and pertinent RF requirements including impedance estimates and damping are discussed. INTRODUCTION A small angle (∼ 0.3-0.5 mrad) crab scheme has been defined as the first step to realize a full crab crossing scheme for the LHC IR upgrade [1, 2]. The luminosity increase solely from crab cavities is expected to be 12-18% for the nominal LHC with β∗ = 55 cm and 43-62% for the upgrade with β∗ = 25 cm for cavity frequecies of 400800 MHz respectively. Due to space constraints and technical ease, a global scheme at the LHC is considered as the best choice for the first experimental test of crab cavities in hadron colliders. In this scheme the cavities are placed in the accelerating RF section (IR4) to provide head-on collision at one of the interaction points in the LHC. A first mini-workshop (LHC-CC08) took place in February 2008 to discuss several beam dynamics and RF issues and build a global collaboration to establish a road map for the R&D prototype. A large collaboration spanning three continents is now in place to develop an optimized cryomodule compatible with the LHC and IR4 requirements. A tWiki page is set up to maintain a central repository and coordinate the project at an international level [3]. The objective of the repository is to provide easy access to latest developments related to LHC crab cavities and to share information with crab cavity developments from the ILC and light sources for mutual benefit. Regular meetings over the web enable discussions about the latest progress of the cryomodule. In this paper the installation issues at IR4, nominal cavity design, impedance and damping requirements are discussed. APERTURE & COLLIMATION The tight aperture constraints imposed by the LHC collimation system for machine protection and quench prevention leaves very little or no margin for additional aperture [4]. A global crab scheme would approximately re∗This work was partially performed under the auspices of the US Department of Energy, We also acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 ”Structuring the European Research Area” programme (CARE, contract number RII3CT-2003-506395) quire an additional 0.5σ of aperture (see orbits in Fig. 1) to accommodate the tilted bunch. The horizontal retraction