Philippe Baudrenghien

arXiv2019

Recent developments in LLRF and its controls at CERN Linac4

At the end of the Large Hadron Collider's Run 2, CERN's Proton Injector Linac 2, commissioned in 1978, delivered its final beam in December 2018. For Run 3, from March 2021, a new H$^{-}$ will take over the role: Linac4. The machine has been producing test beams since 2013 and in 2016 it reached its design 160 MeV energy with 20 mA beam current. Since then several improvements have been made which enhance the LLRF performance and stability. In this paper the structure of the machine and the control system are presented. Problems arising from different RF station types are described and our solutions explained, along with new features recently added to the LLRF. An Adaptive Feed Forward implemented as a flexible hybrid hardware-software solution is described, and first results of this application running on a laboratory test stand are presented. Addition of the set point modulation needed for longitudinal phase-space painting is discussed. Finally, software tools for automation of setting up, monitoring and operations are described. As the hardware and low-level software are still reaching maturity, these are only briefly introduced here.

physics.acc-ph↗

arXiv2018

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 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.

physics.acc-ph↗