Beam Dynamics Studies in Recirculating Machines

The LHeC and the CLIC Drive Beam share not only the high-current beams that make them prone to show instabilities, but also unconventional lattice topologies and operational schemes in which the time sequence of the bunches varies along the machine. In order to asses the feasibility of these projects, realistic simulations taking into account the most worrisome effects and their interplays, are crucial. These include linear and non-linear optics with time dependent elements, incoherent and coherent synchrotron radiation, short and long-range wakefields, beam-beam effect and ion cloud. In order to investigate multi-bunch effects in recirculating machines, a new version of the tracking code PLACET has been developed from scratch. PLACET2, already integrates most of the effects mentioned before and can easily receive additional physics. Its innovative design allows to describe complex lattices and track one or more bunches accordingly to the machine operation, reproducing the bunch train splitting and recombination to and from multiple beamlines. After some initial testing, PLACET2 has been applied to the LHeC Energy Recovery Linac design in order to complete the first end-to-end tracking simulation. The transport of the beam to the dump has been verified in presence of incoherent synchrotron radiation, wakefields and beam-beam effect. Unexpected high radiation losses have been found in specific sections of the lattice, solutions have been proposed to improve both the machine performance and the beam quality. These include a new design of the spreading sections and return arcs based on combined function magnets. The detector bypass, that was originally missing, have now been designed and integrated in the lattice. Tracking simulations have also been performed for PERLE, which have been developed to validate on a smaller scale the technology and the operation of the LHeC. Its performances have been assessed and the design has been consolidated and improved. The work at CTF3 focused on the Combiner Ring. Its length plays a crucial role in the phase structure of the combined beam and must be carefully tuned. The control of the ring length by means of optics scaling, has been measured on the machine and reproduced with the PLACET2 model. Moreover the code has been used to verify a multi-bunch instability that appeared during the commissioning of the ring.

Electron cloud studies for the LHC and future proton colliders

César Octavio Domínguez Sánchez De La Blanca

The Large Hadron Collider (LHC) is the world’s largest and most powerful particle collider. Its main objectives are to explore the validity of the standard model of particle physics and to look for new physics beyond it, at unprecedented collision energies and rates. A good luminosity performance is imperative to attain these goals. In the last stage of the LHC commissioning (2011-2012), the limiting factor to achieving the design bunch spacing of 25 ns has been the electron cloud effects. The electron cloud is also expected to be the most important luminosity limitation after the first Long Shut-Down of the LHC (LS1), when the machine should be operated at higher energy and with 25-ns spacing, as well as for the planned luminosity upgrade (HL-LHC) and future high energy proton colliders (HE-LHC and VHE-LHC). This thesis contributes to the understanding of the electron cloud observations during the first run of the LHC (2010-2012), presents the first beam dynamics analysis for the next generation of high energy hadron colliders, and assists in the prediction of how electron clouds will impact the performance of the future high-luminosity and high-energy machines. In particular, the thesis discusses a method to benchmark pressure measurements at the LHC against electron cloud build-up simulations for identifying the most relevant surface parameters. This method allowed monitoring the effectiveness of LHC “scrubbing runs”, revealing that in the warm regions, the maximum Secondary Electron Yield, δmax, decreased from an initial value of about 1.9 down to about 1.2 (with a low-energy electron reflectivity R ≈ 0.3), thanks to surface conditioning. In addition, the “map formalism”, a good approximation to quickly explain and predict electron cloud effects, has been further developed and applied, for the first time, to optimize the scrubbing process at the LHC. For the HL-LHC, several novel filling schemes have been analyzed in terms of luminosity performance and electron cloud activity. Only a few of them are compatible with an electron cloud activity lower than for the baseline scenario. We highlight a promising option which could be a good fallback scenario in case the electron cloud effects prevent the injection of the baseline beam. This option could also be considered for the nominal LHC after the LS1 if electron cloud turns out to be a serious obstacle. Regarding the future high-energy proton colliders (HE- and VHE-LHC), in the frame of this thesis a performance model was developed to predict the luminosity as a function of time and to optimize the beam parameters, carrying out the first ever performance analysis for these machines. Several scenarios have been considered, including round and flat beams as well as different bunch spacings. The parameters presented in this thesis have been submitted as an input for the most recent update of the European Strategy for Particle Physics. Finally, we also report the electron cloud studies performed for both high energy machines. The large amount of primary photoelectrons generated by synchrotron radiation at these high energies motivates the consideration of high efficiency photon stops as well as other mitigation techniques (e.g. a-C coatings and clearing electrodes). Although for both machines (HE- and VHE-LHC) a tentative bunch spacing of 25 ns has been considered as baseline assumption, the results of this thesis suggest the possibility of going down to 5 ns, since such a beam would present several advantages.

EPFL2013

Beam-dynamics driven design of the LHeC energy-recovery linac

Andrea Latina, Dario Pellegrini

The LHeC is envisioned as a natural upgrade of the LHC that aims at delivering an electron beam for collisions with the existing hadronic beams. The current baseline design for the electron facility consists of a multipass superconducting energy-recovery linac (ERL) operating in a continuous wave mode. The unprecedently high energy of the multipass ERL combined with a stringent emittance dilution budget poses new challenges for the beam optics. Here, we investigate the performances of a novel arc architecture based on a flexible momentum compaction lattice that mitigates the effects of synchrotron radiation while containing the bunch lengthening. Extensive beam-dynamics investigations have been performed with PLACET2, a recently developed tracking code for recirculating machines. They include the first end-to-end tracking and a simulation of the machine operation with a continuous beam. This paper briefly describes the Conceptual Design Report lattice, with an emphasis on possible and proposed improvements that emerged from the beam-dynamics studies. The detector bypass section has been integrated in the lattice, and its design choices are presented here. The stable operation of the ERL with a current up to similar to 150 mA in the linacs has been validated in the presence of single- and multibunch wakefields, synchrotron radiation, and beam-beam effects.

Amer Physical Soc2015

Electron cloud studies for the LHC and future proton colliders

César Octavio Domínguez Sánchez De La Blanca

EPFL2013