Luminosity Reduction Caused by the Full-Detuning LLRF Scheme on the HL-LHC Crab Cavities

Yamakawa, Emi and Apsimon, Robert and Dexter, Amos and Baudrenghien, Philippe and Calaga, Rama and Galindo Guarch and Galindo Guarch, F. J. (2018) Luminosity Reduction Caused by the Full-Detuning LLRF Scheme on the HL-LHC Crab Cavities. In: IPAC 2018 Proceedings of the 9th International Particle Accelerator Conference :. JACoW, CAN. ISBN 9783954501847

Abstract

The High-Lu­mi­nos­ity LHC (HL-LHC) crab cav­i­ties (CCs) will be in­stalled on both sides of IP1 (ATLAS) and IP5 (CMS) to com­pen­sate for the geo­met­ric lu­mi­nos­ity re­duc­tion due to the cross­ing angle. To cope with the in­creased beam cur­rent (0.55 A DC for LHC, 1.1 A for HL-LHC), the op­er­a­tion of the LLRF sys­tem has been changed: rather than fully com­pen­sat­ing the tran­sient beam load­ing, we allow the phase to vary along the turn (100 ps peak-peak with 1.1 A DC). This has been im­ple­mented at LHC since July 2017. The CCs have high loaded Q (5e5) and the avail­able RF power is in­suf­fi­cient to fol­low the bunch phase mod­u­la­tion. The crab­bing volt­age is not mod­u­lated, caus­ing a phase error w.r.t. the in­di­vid­ual bunch cen­troids, lead­ing to trans­verse kicks of the cen­troids and an asym­met­ric crab­bing of the bunch cores. We pre­sent an an­a­lyt­i­cal model for the re­sult­ing lu­mi­nos­ity re­duc­tion and val­i­date with par­ti­cle track­ing sim­u­la­tions. Due to the sym­me­try of the bunch fill­ing pat­terns for the counter-ro­tat­ing beams, the peak lu­mi­nos­ity is re­duced by only 2% for nom­i­nal HL-LHC pa­ra­me­ters at IPs 1 and 5, which is within tol­er­a­ble lim­its.