SixTrack V and runtime environment

SixTrack is a single-particle tracking code for high-energy circular accelerators routinely used at CERN for the Large Hadron Collider (LHC), its luminosity upgrade (HL-LHC), the Future Circular Collider (FCC) and the Super Proton Synchrotron (SPS) simulations. The code is based on a 6D symplectic tracking engine, which is optimized for long-term tracking simulations and delivers fully reproducible results on several platforms. It also includes multiple scattering engines for beam{matter interaction studies, as well as facilities to run the integrated simulations with external particle matter interaction codes. These features differentiate SixTrack from general-purpose, optics-design software. The code recently underwent a major restructuring to merge the advanced features into a single branch, such as multiple ion species, interface with external codes and high-performance input/output. This restructuring also removed a large number of compilation flags, instead enabling/disabling the functionality with runtime options. In the process, the code was moved from Fortran 77 to Fortran 2018 standard, also allowing and achieving a better modularization. Physics models (beam-beam effects, Radio-Frequency (RF) multipoles, current carrying wires, solenoid and electron lenses) and methods (symplecticity check) have also been reviewed and refined to offer more accurate results. The SixDesk runtime environment allows the user to manage the large batches of simulations required for accurate predictions of the dynamic aperture. SixDesk supports several cluster environments available at CERN as well as submitting jobs to the LHC@Home volunteering computing project, which enables volunteers contributing with their hardware to CERN simulation. SixTrackLib is a new library aimed at providing a portable and flexible tracking engine for single- and multi-particle problems using the models and formalism of SixTrack. The library is able to run in CPUs as well as graphical processing units (GPUs). This contribution presents the status of the code, summarizes the main existing features and provides details about the main development lines SixTrack, SixDesk and SixTrackLib.

Space Charge Effects and Advanced Modelling for CERN Low Energy Machines

Adrian Oeftiger

The strong space charge regime of future operation of CERN's circular particle accelerators is investigated andmitigation strategies are developed in the framework of the present thesis. The intensity upgrade of the injector chain of Large Hadron Collider (LHC) prepares the particle accelerators to meet the requirements of the High-Luminosity LHC project. Producing the specified characteristics of the future LHC beams imperatively relies on injecting brighter bunches into the Proton Synchrotron Booster (PSB), the downstream Proton Synchrotron (PS) and eventually the Super Proton Synchrotron (SPS). The increased brightness, i.e. bunch intensity per transverse emittance, entails stronger beam self-fields which can lead to harmful interaction with betatron resonances. Possible beamemittance growth and losses as a consequence thereof threaten to degrade the beam brightness. These space charge effects are partly mitigated by the upgrade of the PSB and PS injection energies. Nevertheless, the space charge tune spreads of the future injector beams are found to exceed the values reached by present LHC or other intense fixed target physics beams. This thesis project comprises three key tasks: detailed modelling of space charge effects, measurement at the CERN machines and mitigation of space charge impact. Throughout the course of this thesis, the simulation tool PyHEADTAIL has been developed and extended to model 3D space charge effects in circular accelerators across the wide energy range from PSB to SPS. The implementation for hardwareaccelerating GPU architectures enables extensive studies, especially when employing the self-consistent particle-in-cell algorithm. The implemented models have been benchmarked with analytical results for space charge beam dynamics. In particular, the spectra of quadrupolar pick-ups - which provide a direct measurement method for the space charge tune shift - have been simulated and compared with the derived theory. The space charge situation at the SPS injection plateau has been extensively investigated in the course of comprehensive measurement studies, resulting in the identification of an optimal working point region for the SPS. The interplay of space charge and the horizontal quarter-integer resonance has been scrutinised in measurement, theory and simulation. Last but not least, a new LHC beam type with a hollow longitudinal phase space distribution has been developed for the PSB and proved to substantially mitigate space charge impact on the PS injection plateau.

EPFL2016

Impact of high energy high intensity proton beams on targets: Case studies for Super Proton Synchrotron and Large Hadron Collider

Juan Blanco Sancho, Rudiger Schmidt

The Large Hadron Collider (LHC) is designed to collide two proton beams with unprecedented particle energy of 7 TeV. Each beam comprises 2808 bunches and the separation between two neighboring bunches is 25 ns. The energy stored in each beam is 362 MJ, sufficient to melt 500 kg copper. Safety of operation is very important when working with such powerful beams. An accidental release of even a very small fraction of the beam energy can result in severe damage to the equipment. The machine protection system is essential to handle all types of possible accidental hazards; however, it is important to know about possible consequences of failures. One of the critical failure scenarios is when the entire beam is lost at a single point. In this paper we present detailed numerical simulations of the full impact of one LHC beam on a cylindrical solid carbon target. First, the energy deposition by the protons is calculated with the FLUKA code and this energy deposition is used in the BIG2 code to study the corresponding thermodynamic and the hydrodynamic response of the target that leads to a reduction in the density. The modified density distribution is used in FLUKA to calculate new energy loss distribution and the two codes are thus run iteratively. A suitable iteration step is considered to be the time interval during which the target density along the axis decreases by 15%-20%. Our simulations suggest that the full LHC proton beam penetrates up to 25 m in solid carbon whereas the range of the shower from a single proton in solid carbon is just about 3 m (hydrodynamic tunneling effect). It is planned to perform experiments at the experimental facility HiRadMat (High Radiation Materials) at CERN using the proton beam from the Super Proton Synchrotron (SPS), to compare experimental results with the theoretical predictions. Therefore simulations of the response of a solid copper cylindrical target hit by the SPS beam were performed. The particle energy in the SPS beam is 440 GeV while it has the same bunch structure as the LHC beam, except that it has only up to 288 bunches. Beam focal spot sizes of sigma = 0.1, 0.2, and 0.5 mm have been considered. The phenomenon of significant hydrodynamic tunneling due to the hydrodynamic effects is also expected for the experiments.

2012

Possible Mitigations of Longitudinal Intensity Limitations for HL-LHC Beam in the CERN SPS

Joël Repond

The Super Proton Synchrotron (SPS) at CERN is the injector of the Large Hadron Collider (LHC), the world's largest particle collider. The High-Luminosity LHC (HL-LHC) project is a major step forward in the improvement of the LHC performances and it requires a doubling of the nominal bunch intensity of the current LHC beam. In the SPS, multi-bunch instabilities and particle losses limit the beam intensity that can be accelerated to 450 GeV/c and transferred to the LHC. Without mitigation measures, the bunch intensity threshold for longitudinal instabilities is three times below the nominal intensity of the LHC beam. Moreover, the present limited RF power is not sufficient to accelerate beams with intensities well above nominal without substantial particle losses and a reduction of the RF voltage available for the beam at the flat top energy. The SPS will undergo significant upgrades but they may not be sufficient to ensure the stability of the HL-LHC beam. The objectives of this doctoral research are to study the longitudinal intensity limitations of the LHC proton beam in the SPS and to find possible mitigation measures to ensure the beam stability and quality at HL-LHC intensity. Beam measurements and particle simulations are used in conjunction with analytical estimations to study the multi-bunch instabilities during the cycle in the SPS. This work attempts to identify the main sources of instabilities and beam quality degradation. Possible scenarios of mitigation measures are investigated to explore the future beam parameters achievable after upgrades. The effects on beam stability of the foreseen RF upgrade, the double RF operation and the reduction of various longitudinal beam-coupling impedances are analysed in detail. The scenario of a lower-harmonic RF system in the SPS, for particle losses reduction, is also studied.