Identification and characterization of high order incoherent space charge driven structure resonances in the CERN Proton Synchrotron

Space charge is typically one of the performance limitations for the operation of high intensity and high brightness beams in circular accelerators. In the Proton Synchrotron (PS) at CERN, losses are observed for vertical tunes above $Q_y=6.25$, especially for beams with large space charge tune shift. The work presented here shows that this behaviour is associated to structure resonances excited by space charge due to the highly symmetric accelerator lattice of the PS, typical for first generation alternating gradient synchrotrons. Experimental studies demonstrate the dependency of the losses on the beam brightness and the harmonic of the resonance, and simulation studies reveal the incoherent nature of the resonance. Furthermore, the calculation of the Resonance Driving Terms (RDT) generated by the space charge potential shows that the operational working point of the PS is surrounded by multiple space charge driven incoherent resonances. Finally, measurements and simulations on both lattice driven and space charge driven resonances illustrate the different behaviour of the beam loss depending on the source of the resonance excitation and on the beam brightness

Working point and resonance studies at the CERN Proton Synchrotron

The Proton Synchrotron (PS) is the oldest yet the most versatile particle accelerator operating at CERN. Having accelerated a multitude of different particle species within the last five decades, it is today used to define the longitudinal structure of the proton beams going into collision in the Large Hadron Collider (LHC), and thus constitutes an integral part of the LHC injector chain. Around 2020 the LHC will be subject to an upgrade to significantly increase the number of collisions at the interaction points. The beam parameters demanded by the High Luminosity LHC (HL-LHC) will, as a result, require substantial improvements of the pre-accelerators, which are currently being studied within the LHC Injectors Upgrade (LIU) project. The increase of luminosity will be accompanied by an increase of beam intensity, which might result in instabilities appearing on the injection flat bottom of the PS. Transverse Head-Tail instabilities have already been observed on operational LHC beams and an alternative stabilizing mechanism for this type of instability is currently being studied. It consists of reducing the mode number of the transverse oscillation by changing linear chromaticity and in succession completely damping the instability by a damper system with appropriate bandwidth. However, nowadays at the PS there is no chromaticity correction scheme implemented at low energy. Special circuits mounted on top of the main magnet poles - the Pole Face Windings (PFW) - could account for that, but so far they are only used to control the betatron tunes and linear chromaticities at high energy. The first part of this thesis is therefore dedicated to extensive studies concerning the correction of betatron tunes, linear and higher order chromaticities by exploitation of the intrinsic opportunities these special circuits offer at low energy. An additional limitation of the PS for high-brightness and high-intensity beams is the presence of beam destructive betatron resonances, which restrict the choice of the injection working point and the maximum acceptable tune spread. This is especially the case for the double batch injection for LHC beams: four bunches are kept at injection energy for 1.2 seconds, leaving enough time for degradation of the transverse beam characteristics in case the space charge induced tune spread causes the beam to touch stop bands of different resonances. Detailed knowledge of the working point plane is thus necessary in order to choose both transverse tunes in an area sufficiently free of resonances. To improve the current working point control scheme, the influence of the PFW on the machine resonances is examined in the second part of this thesis, leading to a deeper understanding of the limits of the PS

Beam Dynamics Studies for High-Intensity Beams in the CERN Proton Synchrotron

With the discovery of the Higgs boson, the existence of the last missing piece of the Standard Model of particle physics (SM) was confirmed. However, even though very elegant, this theory is unable to explain, for example, the generation of neutrino masses, nor does it account for dark energy or dark matter. To shed light on some of these open questions, research in fundamental particle physics pursues two complimentary approaches. On the one hand, particle colliders working at the high-energy frontier, such as the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN), located in Geneva, Switzerland, are utilized to investigate the fundamental laws of nature. Alternatively, fixed target facilities require high-intensity beams to create a large flux of secondary particles to investigate, for example, rare particle decay processes, or to create neutrino beams. This thesis investigates limitations arising during the acceleration of high-intensity beams at the CERN Proton Synchrotron (PS). The studies presented are aimed at reducing beam loss occurring during the injection and extraction processes, which cause high radioactive activation of the PS ring. The minimization of beam loss is essential to allow maintenance, repair or exchange of crucial accelerator equipment, especially for the production of more intense proton beams, which will eventually be required by future experimental facilities. The first part of this thesis focuses on an intra-bunch oscillation phenomenon, which is observed immediately after injection of high-intensity beams and causes undesirable beam loss. The oscillations are experimentally characterized, detailed simulation studies are presented and the underlying mechanism, namely the interaction between the beam and the self-induced electromagnetic fields in the surrounding vacuum chamber, is explained. The second part of this thesis sets out the way to making the Multi-Turn Extraction (MTE), a novel scheme based on advanced concepts of non-linear beam dynamics, an operational replacement of the Continuous Transfer (CT) process. Experimental studies stressing the susceptibility of the MTE technique to fluctuations of the magnetic field are discussed, and the results of 6D time-dependent simulations are explained. Furthermore, a redesign of the extraction process itself is presented. The design of a new extraction bump, which is required by the installation of a passive absorber to protect the magnetic extraction septum, is set out. In addition, improved non-linear extraction optics are presented, which allow the reduction of beam loss at extraction to the expected design values of less than 2%. The entirety of the presented studies played a crucial role in concluding the MTE commissioning process. Since September 2015, the MTE scheme has successfully replaced the CT extraction, leading to a significant reduction of the activation of the PS ring

Beam-Based Measurement of Skew-Sextupole Errors in the CERN Proton Synchrotron

During Proton Synchrotron (PS) commissioning in 2021, large beam losses were observed when crossing the 3Qy resonance if the Beam Gas Ionization (BGI) profile monitor was enabled. This indicated the presence of a strong skew-sextupole source in this instrument. Beam-based measurements of the skew sextupole component in the BGI magnet were performed, in order to benchmark the BGI magnetic model and to provide quantitative checks of sextupole corrections determined empirically to minimize the beam-losses. In this contribution, results of the successfully performed measurements are presented, including tune feed-down, chromatic coupling and resonance driving terms

Linear and non-linear optics measurements in PS using turn-by-turn BPM data

For the first time, the optics of the CERN Proton Synchrotron (PS) was measured using turn-by-turn BPM data of forced betatron oscillations excited with an AC dipole. We report results of phase advance and beta-beating measurements. Linear coupling was globally minimised along the machine by measuring and correcting coupling resonance driving terms. Finally, non-linear properties of the ring were probed looking at third- and fourth-order resonance driving terms and amplitude detuning

Influence of the Alignment of the Main Magnets on Resonances in the CERN Proton Synchrotron

During the Long Shutdown 1 seven out of the one hundred combined function PS main magnets were removed from the tunnel to conduct maintenance. After reinstallation, the main magnets were aligned to the reference positions and within the first week of operation of the accelerator, a beam-based re-alignment campaign was performed to reduce the excursions of the closed orbit. In order to further investigate and understand the source of betatronic resonances, which, already in 2011, were found to be excited by the bare machine, tune diagram measurements before and after this beam-based magnet alignment were conducted. In both cases the same resonances as in 2011 were found to be present; however, after the alignment, an overall increase of their strengths was observed. In this paper we present the corresponding measurement results and discuss the direct impact on the daily operation of the accelerator

Space charge driven resonances in the CERN PS

In the CERN Proton Synchrotron space charge driven resonances are excited around the operational working point due to the periodicity of the optics functions. In this paper, the resonances are studied using analytical methods, i.e. the evaluation of the resonance driving terms connected to the space charge potential of Gaussian distributions. Furthermore, the resonances are characterized in measurements and simulations for various beams. The beams considered are different in terms of brightness, in order to study the dependence of the resonance strength on the space charge force

Longitudinal Microwave Instability Study at Transition Crossing with Ion Beams in the CERN PS

The luminosity of lead ion collisions in the Large Hadron Collider (LHC) was significantly increased during the 2018 ion run by reducing the bunch spacing from 100 ns to 75 ns, allowing to increase the total number of bunches. With the new 75 ns variant, three instead of four bunches are generated each cycle in the Low Energy Ion Ring (LEIR) and the Proton Synchrotron (PS) with up to 30% larger intensity per bunch. The beam was produced with satisfactory quality but at the limit of stability in the injectors. In particular, the minimum longitudinal emittance in the PS is limited by a strong longitudinal microwave instability occurring just after transition crossing. The uncontrolled blow-up generates tails, which translate into an unacceptably large satellite population following the RF manipulations prior to extraction from the PS. In this paper, instability measurements are compared to particle simulations using the latest PS impedance model to identify the driving impedance sources. Moreover, means to mitigate the instability are discussed