Prediction of Beam Losses during Crab Cavity Quenches at the HL-LHC

Studies of the crab cavities at KEKB revealed that the rf phase could shift by up to 50° within ∼50 μs during a quench; while the cavity voltage is still at approximately 75% of its nominal amplitude. If such a failure were to occur on the HL-LHC crab cavities, it is likely that the machine would sustain substantial damage to the beam line and surrounding infrastructure due to uncontrolled beam loss before the machine protection system could dump the beam. We have developed a low-level rf system model, including detuning mechanisms and beam loading, and use this to simulate the behavior of a crab cavity during a quench, modeling the low-level rf system, detuning mechanisms and beam loading. We supplement this with measurement data of the actual rf response of the proof of principle double-quarter wave crab cravity during a quench. Extrapolating these measurements to the HL-LHC, we show that Lorentz force detuning is the dominant effect leading to phase shifts in the crab cavity during quenches; rather than pressure detuning which is expected to be dominant for the KEKB crab cavities. The total frequency shift for the HL-LHC crab cavities during quenches is expected to be about 460 Hz, leading to a phase shift of no more than 3°. The results of the quench model are read into a particle tracking simulation, SixTrack, and used to determine the effect of quenches on the HL-LHC beam. The quench model has been benchmarked against the KEKB experimental measurements. In this paper we present the results of the simulations on a crab cavity failure for HL-LHC as well as for the SPS and show that beam loss is negligible when using a realistic low-level rf response.