Abstract
Studies of the crab cavities at KEKB revealed that the RF phase could shift by up to 50o within ~50 us 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 behaviour 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 3o. 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.
Highlights
The High Luminosity upgrade for the LHC (HL-LHC) [1] will use crab cavities to compensate for the luminosity reduction at the interaction points (IPs) due to the crossing angle of the counterrotating bunch trains
The pressure is plotted on a logarithmic scale show how pressure varies over several orders of magnitude. As these models are intended to provide an understanding of the order of magnitude of the pressure during a quench and that the pressure will depend on many variables, we conclude that the pressure due to boiling liquid helium (LHe) at 4K will be of the order of 102–103 mbar and for superfluid LHe at 2K, the pressure will be of the order of 10−1–102 mbar
Assuming that the pressure sensitivity of the KEKB crab cavity is similar to the HL-LHC cavity, a pressure spike of the order of 102 mbar would be required, which is consistent with the expected pressure spike from the model
Summary
The High Luminosity upgrade for the LHC (HL-LHC) [1] will use crab cavities to compensate for the luminosity reduction at the interaction points (IPs) due to the crossing angle of the counterrotating bunch trains. The phase and amplitude of the cavity field can change relatively quickly, potentially leading to beam losses throughout the ring. Measurements of the crab cavities installed at KEKB have revealed that the rf phase shifts by up to 50° within 50 μs during a quench; while the cavity voltage is still approximately 75% of its nominal value [3]. Should such a failure occur on the HL-LHC crab cavities, the beam losses could cause substantial damage to the beam line. In this paper we utilize a more realistic rf system model, including low-level rf (LLRF), pressure, Lorentz force, and resistive detuning mechanisms, as well as microphonics and beam loading in order to obtain a more realistic phase and amplitude evolution during crab cavity quenches
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