Beam Break-up Instability in the CERN PS near Transition

Fast beam losses, due to a vertical coherent instability of high frequency, have been observed in the PS near transition energy, with the high-intensity single-bunch beam for the neutron Time-of-Flight facility (n-ToF). By increasing the longitudinal emittance, the beam could be stabilised. These phenomena can be described by the beam breakup theory, since near transition the longitudinal positions of particles are almost frozen, as in the linac case. Comparison between observations and theory, using Brandt and Gareyte's formula for single-bunch beam breakup in circular accelerators, shows good agreement

Electron Cloud Effects in the CERN PS

The beam-induced electron cloud build-up is one of the major concerns for the SPS and the design of the future LHC. Recently, this effect has been observed also in the PS with the nominal LHC-type beam, consisting of a batch of 72 bunches of 1.1´1011 p/b spaced by 25 ns. The electron cloud induces baseline distortion in electrostatic pick-up signals that is observed, both in the last turns of the PS when the full bunch length is reduced to less than 4 ns, and in the transfer line between the PS and the SPS rings. Experimental observations are presented and compared to simulation results and predictions from theory. Furthermore, possible cures, such as variation of the bunch spacing, inserting gaps in the bunch train and applying weak solenoidal fields, are also discussed

Multi-bunch simulations with HEADTAIL

The HEADTAIL code has been used for many years to study the interaction of a single bunch with a localized or lumped source of electromagnetic perturbation, usually self-induced (impedance, electron cloud or space charge). It models the bunch as macroparticles and at each turn slices up the bunch into several adjacent charged disks, which are made to subsequently interact with the perturbing agent. A first step toward the extension of HEADTAIL to multibunch simulations is presented in this paper. In this case, the bunches themselves are modeled as charged disks and are not sliced, which makes us lose information on the intra-bunch motion but can describe a zero mode interaction between different bunches in a train. The interaction of an SPS bunch train of 72 bunches with the resistive wall is studied as an example

The Proton Beams for the New Time-of-Flight Neutron Facility at the CERN-PS

The experimental determination of neutron cross sections in fission and capture reactions as a function of the neutron energy is of primary importance in nuclear physics. Recent developments at CERN and elsewhere have shown that many fields of research and development, such as the design of Accelerator-Driven Systems (ADS) for nuclear waste incineration, nuclear astrophysics, fundamental nuclear physics, dosimetry for radiological protection and therapy, would benefit from a better knowledge of neutron cross sections. A neutron facility at the CERN-PS has been proposed with the aim of carrying out a systematic and high resolution study of neutron cross sections through Time-Of-Flight (n-TOF) measurement. The facility requires a high intensity proton beam (about 0.7x1013 particles/bunch) distributed in a short bunch (about 25 ns total length) to produce the neutrons by means of a spallation process in a lead target. To achieve these characteristics, a number of complex beam gymnastics have to be performed. All the beam manipulations are presented in this paper as well as some beam dynamics issues encountered during the setting up. The details of the new transfer line used to deliver the beam onto the target are also described

SPS performance with PS2

The upgrade of the PS to the PS2 would allow injection into the SPS at higher energy (up to 70 GeV/c). Possible advantages deriving from a higher injection energy into the SPS include the improvement of space charge at flat bottom, absence of transition crossing for all proton beams and a higher threshold for the horizontal electron cloud coupled bunch instability. Transverse Mode Coupling Instability (TMCI) and vertical Electron Cloud Instability (ECI) thresholds are studied in greater detail. Their dependence on energy is defined in simulations with the HEADTAIL code and the results of this study are presented

Transverse mode coupling instability in the SPS: Headtail simulation and moses calculation

Since 2003, single bunches of protons with high intensity (~ 1.2 1011 protons) and low longitudinal emittance (~ 0.2 eVs) have been observed to suffer from heavy losses in less than one synchrotron period after injection at 26 GeV/c in the CERN Super Proton Synchrotron (SPS) when the vertical chromaticity is corrected (ξy ~ 0). Understanding the mechanisms underlying this instability is crucial to assess the feasibility of an anticipated upgrade of the SPS, which requires bunches of 4 1011 protons. Analytical calculations and particle tracking simulations had already agreed in predicting the intensity threshold of a fast instability. The aim of the present paper is to present a sensitive frequency analysis of the HEADTAIL simulations output using SUSSIX, which brought to light the fine structure of the mode spectrum of the bunch coherent motion. Coupling between the azimuthal modes “-2” and “-3” was clearly observed to be the reason for this fast instability

An update of Zbase, the CERN impedance database

A detailed knowledge of the beam coupling impedance of the CERN synchrotrons is required in order to identify the impact on instability thresholds of potential changes of beam parameters, as well as additions, removals or modifications of hardware. To this end, an update of the impedance database was performed, so that impedance results from theoretical calculations using new multilayer models, impedance results from electromagnetic field simulations and impedance results from bench measurements can be compiled. In particular, the impedance database is now set to separately produce the dipolar and quadrupolar transverse impedance and wakes that the Headtail simulation code needs to accurately simulate the effect of the impedance on the beam dynamics

Tracking Study of the Effect of BPM Impedances in the SPS

Following the observation of a Transverse Mode Coupling Instability (TMCI) in the SPS [1, 2], a systematic estimate of the impedance of the various pieces of equipment installed in the machine has started. In this report the contribution of the Beam Position Monitor trapped modes to the global transverse impedance is considered. The trapped modes have been thus calculated with MAFIA and characterized with their resonator parameters. These impedances have been subsequently fed into the MOSES and HEADTAIL codes in order to evaluate the expected TMCI threshold in the SPS and compare it with the experimental observations