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

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.

Electron Spin Resonance Magnetometers for Particle Accelerators

Anthony Jean Beaumont

CERN is the European Organization for Nuclear Research located in Geneva, Switzerland. Its main goal is to explore fundamental physics and it exists primarily to provide physicists the necessary tools for doing this, namely accelerators. These tools bring particles to almost the speed of light and then use them in different experiments. The CERN Accelerator Complex includes a chain of interconnected accelerators allowing the acceleration of charged particles up to 6.5 TeV per beam (for protons) in 2018 by the Large Hadron Collider (LHC). Different types of accelerator exist, but at CERN the most common are the synchrotrons. For those, the magnetic field is synchronous with the beam momentum. Magnetic models cannot be always used to predict the field within the required reproducibility, which may be as low as 10^(-5), due to non-linear effects (eddy currents, saturation and hysteresis). Therefore, most of the CERN's synchrotrons are equipped with a real time magnetic field measurement system, the so called "B-Train". The magnetic field measurement devices are composed of absolute field sensors (called "markers"), field tracking sensors (called "induction coils"), an acquisition system, a control system that calculates the magnetic field from the sensors, and a distribution system that delivers the field value to the users. Due to the configuration of the CERN accelerator chain, if one of the injectors fails, the LHC cannot have beam. Consequently, a huge consolidation program of the real time magnetic measurement systems was launched to guarantee operation in the long term. Thus the markers are critical devices for operations at CERN and must be carefully developed to fulfil the high performance and reliability requirements of the B-Train systems. This thesis propose new designs for electron spin resonance (ESR) field markers. We discuss in details the design, the operation, and performance of the ESR sensors based on resonator and oscillator structures including comparison between paramagnetic and ferrimagnetic samples. We propose four field marker levels at 36 mT, 106 mT, 360 mT and 710 mT that correspond to resonance frequencies of 1GHz, 3GHz, 10GHz and 20 GHz. Those measurement values cover most of the marker level requirements of the CERN accelerators and achieving resolution up to 0.1 nT/Hz^(1/2) for field ramps as fast as 5 T/s and field gradients as strong as 12 T/m. We conclude with measurements validation performed on the Proton Synchrotron (PS) and on the Low Energy Ion Ring (LEIR) accelerators.

EPFL2020

Mitigation of intensity limitation in the CERN SPS using a double RF system

Joël Repond, Massimiliano Schwarz

The Super Proton Synchrotron (SPS) at CERN is the injector of the Large Hadron Collider (LHC). Multi-bunch instabilities limit the intensity of the beam that can be accelerated to 450 GeV in the SPS and transferred to the LHC. Without mitigation measures, the threshold of bunch intensity of the longitudinal instability is three times below the nominal bunch intensity of the LHC beam. The High Luminosity LHC project (HL-LHC), which requires a doubling of the nominal bunch intensity, relies on improvement of beam stability in the SPS. A fourth harmonic RF system allows, presently, to stabilize the beam up to nominal LHC intensity. It increases the synchrotron frequency spread inside the bunch, providing more efficient Landau damping of beam instability. However, nonlinearities of the synchrotron frequency distribution inside the bunch pose a limitation on bunch length. This paper explores possible intensity increase in the SPS by studying the effect on beam stability of the voltage ratio between two RF systems. The results are substantiated by beam measurements and particle-tracking simulations. An optimized voltage program of the second RF system during the cycle has been tested in operation and beam stability and the quality have been successfully improved.

2019

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.