| Paper |
Title |
Page |
| MOPP065 |
Microwave Transmission Measurement of the Electron Cloud Density in the Positron Ring of PEP-II
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694 |
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- M. T.F. Pivi, A. Krasnykh
SLAC, Menlo Park, California
- J. M. Byrd, S. De Santis, K. G. Sonnad
LBNL, Berkeley, California
- F. Caspers, T. Kroyer, F. Roncarolo
CERN, Geneva
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Clouds of low energy electrons in the vacuum beam pipes of accelerators of positively charged particle beams present a serious limitation for operation of these machines at high currents. Because of the size of these accelerators, it is difficult to probe the low energy electron clouds over substantial lengths of the beam pipe. We have developed a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave which is independently excited and transmitted over a section of the accelerator. We infer the absolute phase shift with relatively high accuracy from the phase modulation of the transmission due to the modulation of the electron cloud density from a gap in the positively charged beam. We have used this technique for the first time to measure the average electron cloud density over a 50 m straight section in the positron ring of the PEP-II collider at the Stanford Linear Accelerator Center. We have also measured the variation of the density by using low field solenoid magnets to control the electrons.
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| TUPP029 |
Beam Coupling Impedance Measurement and Mitigation for a TOTEM Roman Pot
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1598 |
| |
- M. Deile, F. Caspers, T. Kroyer, M. Oriunno, E. Radermacher, A. Soter
CERN, Geneva
- F. Roncarolo
UMAN, Manchester
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The longitudinal and transverse beam coupling impedance of the first final TOTEM Roman Pot unit has been measured in the laboratory with the wire method. For the evaluation of transverse impedance the wire position has been kept constant, and the insertions of the RP were moved asymmetrically. With the original configuration of the RP, resonances with fairly high Q values were observed. In order to mitigate this problem, RF-absorbing ferrite plates were mounted in appropriate locations. As a result, all resonances were sufficiently damped to meet the stringent LHC beam coupling impedance requirements.
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| TUPP049 |
Experimental Electron Cloud Studies in the CERN Proton Synchrotron
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1655 |
| |
- E. Mahner, F. Caspers, T. Kroyer
CERN, Geneva
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Indications for a beam-induced electron cloud build-up are observed since 2000 for the nominal LHC beam in the PS to SPS transfer line and during the last turns before ejection from the PS. A new electron cloud setup was designed, built, and installed in the PS. It contains shielded button-type pickups, a dipole magnet, a vacuum gauge, and a dedicated stripline electrode to experimentally verify the beneficial effect of electron cloud clearing electrodes. During the 2007 run, the electron cloud effect was also clearly observed in the PS and efficient electron cloud suppression has been obtained for negative and positive bias voltages on the clearing electrode. Here, we present electron cloud measurements with different filling patterns and bunch spacings in the PS.
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| TUPP061 |
Comparison between Laboratory Measurements, Simulations and Analytical Predictions of the Resistive Wall Transverse Beam Impedance at Low Frequencies
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1679 |
| |
- F. Roncarolo
UMAN, Manchester
- F. Caspers, T. Kroyer, E. Métral
CERN, Geneva
- B. Salvant
EPFL, Lausanne
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The prediction of the resistive wall transverse beam impedance at the first unstable betatron line (8 kHz) of the CERN Large Hadron Collider (LHC) is of paramount importance for understanding and controlling the related coupled-bunch instability. Until now only novel analytical formulas were available at this frequency. Recently, laboratory measurements and numerical simulations were performed to crosscheck the analytical predictions. The experimental results based on the measurement of the variation of a probe coil inductance in the presence of i) sample graphite plates, ii) stand-alone LHC collimator jaws and iii) a full LHC collimator assembly are presented in detail. The measurement results are compared to both analytical theories and simulations. In addition, the consequences for the understanding of the LHC impedance are discussed.
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| TUPP063 |
Characterization of the ATLAS Roman Pots Beam Coupling Impedance and Mechanics
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1685 |
| |
- F. Roncarolo, R. M. Jones
UMAN, Manchester
- F. Caspers, B. Di Girolamo, T. Kroyer
CERN, Geneva
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At the LHC, four Roman Pot (RP) type detectors will be installed on both sides of the ATLAS experiment with the aim of measuring elastic scattering at very small angles and determining the absolute luminosity at the interaction point. During dedicated LHC runs, the detectors will be positioned at about 1 mm from the nominal beam orbit. Numerical simulations and laboratory measurements were carried out to characterize the RP impact on the total LHC beam coupling impedance. The measurement results assess the effectiveness of RF-absorbing ferrite plates that have been mounted in convenient locations in order to damp high Q resonances of the RP structure. In addition, we review the RP mechanics emphasizing the accuracy and reproducibility of the positioning system.
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| TUPP066 |
CERN SPS Impedance in 2007
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1691 |
| |
- E. Métral, G. Arduini, T. Bohl, H. Burkhardt, F. Caspers, H. Damerau, T. Kroyer, H. Medina, G. Rumolo, M. Schokker, E. N. Shaposhnikova, J. Tuckmantel
CERN, Geneva
- R. Calaga
BNL, Upton, Long Island, New York
- B. Salvant
EPFL, Lausanne
- B. Spataro
INFN/LNF, Frascati (Roma)
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Each year several measurements of the beam coupling impedance are performed in both longitudinal and transverse planes of the CERN Super Proton Synchrotron to keep track of its evolution. In parallel, after the extensive and successful campaign of identification, classification and cure of the possible sources of (mainly longitudinal) impedance between 1998 and 2001, a new campaign (essentially for the transverse impedance this time) has started few years ago, in view of the operation of the SPS with higher intensity for the LHC luminosity upgrade. The present paper summarizes the results obtained from the measurements performed over the last few years and compares them to our predictions. In particular, it reveals that the longitudinal impedance is reasonably well understood and the main contributors have already been identified. However, the situation is quite different in the transverse plane: albeit the relative evolution of the transverse impedance over the last few years can be well explained by the introduction of the nine MKE kickers necessary for beam extraction towards the LHC, significant contributors to the SPS transverse impedance have not been identified yet.
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| TUPP068 |
Bench Measurements of the Low Frequency Transverse Impedance of the CERN LHC Beam Vacuum Interconnects with RF Contacts
|
1697 |
| |
- B. Salvant
EPFL, Lausanne
- F. Caspers, E. Métral
CERN, Geneva
- F. Roncarolo
UMAN, Manchester
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The low frequency longitudinal and transverse impedances of the CERN Large Hadron Collider (LHC) have to be specifically minimized to prevent the onset of coherent instabilities. The LHC beam vacuum interconnects were designed as Plug In Modules (PIMs) with RF contacts to reduce their coupling impedances, but the resulting contact resistance is a concern, as this effect is difficult to estimate. High sensitivity measurements of the transverse impedance of a PIM at low frequency using a coil probe are presented. In particular, the increase of the transverse impedance of the PIM when it is elongated to its operating position is discussed in detail. Finally, the issue of non-conforming contact resistance is also addressed.
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| WEPP056 |
Aperture Restriction Localisation in the LHC Arcs using an RF Mole and the LHC Beam Position Measurement System
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2644 |
| |
- O. R. Jones, J. Albertone, S. Bartolome-Jimenez, C. Boccard, T. Bogey, P. B. Borowiec, E. Calvo, F. Caspers, M. Gasior, J. L. Gonzalez, B. Jenninger, L. K. Jensen, T. Kroyer, S. Weisz
CERN, Geneva
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Ensuring that the two 27km beam pipes of the LHC do not contain aperture restrictions is of utmost importance. Most of the ring is composed of continuous cryostats, so any intervention to remove aperture restrictions when the machine is at its operating temperature of 1.9K will require a substantial amount of time. On warming-up the first cooled sector, several of the sliding contacts which provide electrical continuity for the image current between successive sections of the vacuum chamber were found to have buckled into the beam pipe. This led to a search for a technique to verify the integrity of a complete LHC arc (~3km) before any subsequent cool-down. In this paper the successful results from using a polycarbonate ball fitted with a 40MHz RF transmitter will be presented. Propulsion of the ball is achieved by sucking filtered air through the entire arc, while its progress is traced every 54m via the LHC beam position measurement system which is auto-triggered by the RF transmitter on passage of the ball. Reflectometry at frequencies in the 4-8 GHz range can cover the gaps between beam position monitors and could therefore be used to localise a ball blocked by an obstacle.
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| THPC081 |
RF Wire Compensator of Long-range Beam-beam Effects
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3173 |
| |
- U. Dorda, F. Caspers, T. Kroyer, F. Zimmermann
CERN, Geneva
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The dynamic aperture of the proton beam circulating in the Large Hadron Collider (LHC) is expected to be limited by up to 120 long-range beam-beam encounters. In order to perfectly compensate the LHC long-range beam-beam effect for nominal as well as for so-called 'PACMAN' bunches, i.e. bunches at the start or end of a bunch train, the wire compensator strength should be adjusted for each bunch individually. Here an RF-based compensator is proposed as a practical solution for the PACMAN compensation. We show that this approach also allows relaxing the power and precision requirements compared with those of a pulsed DC device, to a level within the state-of-the-art of RF technology. Furthermore it allows the use of a passive circulator in the tunnel close to the beam and thus a significantly reduction of the transmission line length and of the resulting multiple reflection issues. Simulations, issues related to RF phase noise and first experimental results from laboratory models as well as from a wire-compensator prototype installed in the CERN Super Proton Synchrotron (SPS) are presented.
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