| Paper | Title | Page |
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| MOPPC001 | Simulation of electron-cloud heat load for the cold arcs of the large hadron collider | 115 |
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| The heat load due to the electron cloud in the Large Hadron Collider (LHC) cold arcs is a concern for its performance near and beyond nominal beam current. We report the results of simulation studies, which examine the electron-cloud induced heat load for different values of low-energy electron reflectivity and secondary emission yield at injection energy, as well as at beam energies of 4 TeV and 7 TeV, for two different bunch spacing: 25 ns and 50 ns. Benchmarking the simulations against heat-load observations at different beam energies and bunch spacing allows an estimate of the secondary emission yield in the cold arcs of the LHC and of its evolution as a function of time. | ||
| MOPPC075 | A Generic Data Model for HeadTail: Design and Implementation with Examples | 307 |
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| HeadTail has been developed in 2002 for the efficient simulation of instabilities and collective effects in large circular accelerators. Since then, the capabilities of the code have been continuously extended and the output data has become increasingly complex and large-scale ranging from the statistical description of single bunches to the statistical description of all slices within bunches up to the dynamics of the full 6D phase space over several thousands of turns. Processing this data in an effective manner and endowing it with a structure that provides a physical concept calls for new and optimised data formats. To meet state-of-the-art standards, the hierarchical data format (HDF5) has been selected as native output data format together with H5Part and XDMF as native data structures. We describe the implementation of the H5Part and the XDMF data structures into HeadTail and show some illustrative examples for data processing. | ||
| TUPPR064 | Time-resolved Shielded-Pickup Measurements and Modeling of Beam Conditioning Effects on Electron Cloud Buildup at CesrTA | 1966 |
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Funding: Work supported by the US National Science Foundation PHY-0734867, PHY-1002467, and the U.S. Department of Energy DE-FC02-08ER41538. The Cornell Electron Storage Ring Test Accelerator program includes investigations into electron cloud buildup in vacuum chambers with various coatings. Two 1.1-m-long sections located symmetrically in the east and west arc regions are equipped with BPM-like pickup detectors shielded against the direct beam-induced signal. They detect cloud electrons migrating through an 18-mm-diameter pattern of holes in the top of the chamber. A digitizing oscilloscope is used to record the signals, providing time-resolved information on cloud development. We present new measurements of the effect of beam conditioning on a newly-installed amorphous carbon coated chamber, as well as on a diamond-like carbon coating. The ECLOUD modeling code is used to quantify the sensitivity of these measurements to model parameters, differentiating between photoelectron and secondary-electron production processes. |
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| WEPPR010 | Comparison between Electron Cloud Build-Up Measurements and Simulations at the CERN PS | 2955 |
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| The build up of an Electron Cloud (EC) has been observed at the CERN Proton Synchrotron (PS) during the last stages of the LHC high intensity beam preparation, especially after the bunch shortening before extraction. A dedicated EC experiment, equipped with two button pick-ups, a pressure gauge, a clearing electrode, and a small dipole magnet, is available in one of the straight sections of the machine. A measurement campaign has been carried out in order to scan the EC build-up of LHC-type beams with different bunch spacing, bunch intensity, and bunch length. Such information, combined with the results from build up simulations, is of relevance for the characterization in terms of Secondary Emission Yield (SEY) of the chamber inner surface. The interest is twofold: this will enable us to predict the EC build up distribution in the PS for higher intensity beams in the frame of the upgrade program, and it will provide validation of the EC simulation models and codes. | ||
| TUPPC086 | Conceptual Design of the CLIC damping rings | 1368 |
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| The CLIC damping rings are designed to produce unprecedentedly low-emittances of 500 nm and 5 nm normalized at 2.86 GeV, in all beam dimensions with high bunch charge, necessary for the performance of the collider. The large beam brightness triggers a number of beam dynamics and technical challenges. Ring parameters such as energy, circumference, lattice, momentum compaction, bending and super-conducting wiggler fields are carefully chosen in order to provide the target emittances under the influence of intrabeam scattering but also reduce the impact of collective effects such as space-charge and coherent synchrotron radiation. Mitigation techniques for two stream instabilities have been identified and tested. The low vertical emittance is achieved by modern orbit and coupling correction techniques. Design considerations and plans for technical system, such as damping wigglers, transfer systems, vacuum, RF cavities, instrumentation and feedback are finally reviewed. | ||
| WEPPR068 | Mitigation of Electron Cloud Instabilities in the LHC Using Sextupoles and Octupoles | 3084 |
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| Coherent electron cloud instabilities pose a serious limitation for luminosity upgrades in the Large Hadron Collider (LHC) at CERN. In particular, when bunch spacings reach below 50 ns, electron cloud formation is enhanced which in turn drives beam instabilities. The beam can be stabilised by shifting the tune and by increasing the tune spread using sextupoles or octupoles, respectively. The resulting values for the chromaticity and the detuning parameters must be selected with care, however, in order not to run into head-tail instabilities or to considerably reduce the dynamic aperture. A simulation study has been launched to estimate the parameters necessary for stabilisation of the beam under the influence of electron clouds. | ||
| WEPPR069 | Measurements and Simulations of Transverse Coupled-Bunch Instability Rise Times in the LHC | 3087 |
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| In the current configuration of the LHC, multibunch instabilities due to the beam-coupling impedance would be in principle a critical limitation if they were not damped by the transverse feedback. For the future operation of the machine, in particular at higher bunch intensities and/or higher number of bunches, one needs to make sure the coupled-bunch instability rise times are still manageable by the feedback system. Therefore, in May 2011 experiments were performed to measure those rise times and compare them with the results obtained from the LHC impedance model and the HEADTAIL wake fields simulation code. At injection energy, agreement turns out to be very good, while a larger discrepancy appears at top energy. | ||
| WEPPR072 | Increasing Instability Thresholds in the SPS by Lowering Transition Energy | 3096 |
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| A new optics for the SPS with lower transition energy was tested experimentally during 2010-2011, showing a significant increase of the single bunch instability thresholds at injection, due to the 3-fold increase of the slip factor. This paper summarizes the series of performed machine studies for different LHC bunch structures and intensities. In particular, the search of the TMCI threshold in the new optics is presented. Observations on the longitudinal multi-bunch stability are compared between the nominal and the low-transition optics. Finally, optics variants with higher vertical tunes are discussed, which can allow to further increase the TMCI and vertical instability thresholds by reducing the vertical beta function. | ||
| WEPPR073 | Effects of an Asymmetric Chamber on the Beam Coupling Impedance | 3099 |
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| The wake function of an accelerator device appears to have a constant term if the geometry of the device is asymmetric or when the beam passes off axis in a symmetric geometry. Its contribution can be significant and has to be taken into account. In this paper a generalized definition of the impedance/wake is presented to take into account also this constant term. An example of a device where the constant term appears is analyzed. Moreover, the impact of a constant wake on the beam dynamics is discussed and illustrated by a HEADTAIL simulation. | ||
| WEPPR074 | Effect of the TEM Mode on the Kicker Impedance | 3102 |
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| The kickers are major contributors to the CERN SPS beam coupling impedance. As such, they may represent a limitation to increasing the SPS bunch current in the frame of a luminosity upgrade of the LHC. The C-Magnet supports a transverse electromagnetic (TEM) mode due to the presence of two conductors. Due to the finite length of the structure this TEM mode affects the impedance below a certain frequency (when the penetration depth in the ferrite becomes comparable to the magnetic circuit length). A theoretical model was developed to take into account also the impedance contribution due to the TEM mode. The model is found to be in good agreement with CST 3D electromagnetic (EM) simulations. It allows for generic terminations in the longitudinal direction. An example of kicker is analyzed taking into account also the external cables. | ||
| WEPPR075 | Monitoring the Progress of LHC Electron-Cloud Scrubbing by Benchmarking Simulations and Pressure-Rise Observations | 3105 |
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| Electron bombardment of a surface has been proven to reduce drastically the secondary electron yield of a material. This technique, known as scrubbing, is the ultimate solution to decrease the negative effects of an electron cloud build-up in any particle accelerator operating with intense beams. Its effectiveness has been already observed at the LHC. Since at the LHC no in-situ secondary-yield measurements are available, it has been necessary to develop a method to infer different key beam-pipe surface parameters by benchmarking simulations and pressure-rise observations. The method developed allows us to monitor the scrubbing process in order to decide on the most appropriate strategies for machine operation. In this paper we present the latest results of applying this method to the LHC in the fall of 2011 and early 2012, including data for the nominal bunch spacing of 25 ns. | ||