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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MOPP006 |
Machine Induced Backgrounds for FP420
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559 |
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- R. Appleby, K. M. Potter, F. Roncarolo, G. J. Sellers
UMAN, Manchester
- I. Azhgirey, I. Baishev, I. L. Kurochkin, V. Talanov
IHEP Protvino, Protvino, Moscow Region
- M. Ruspa
INFN-Torino, Torino
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The LHC FP420 collaboration is assessing the feasibility of installing forward proton detectors at 420m from the ATLAS and/or CMS interaction points. Such detectors aim at measuring diffracted protons, which lost less than 2% of their longitudinal momentum. The success of this measurement requires a very good understanding of the charged and neutral particle environment in the detector region in order to avoid the signal being swamped as well as for detector survivability. This background receives contributions from beam-gas interactions, halo particles surviving from the Betatron and momentum cleaning systems and secondary showers produced by particles from the 14TeV collision region striking the beampipe upstream of the FP420 detectors. In this paper, such background sources are reviewed, and the expected background rates calculated.
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TUPP029 |
Beam Coupling Impedance Measurement and Mitigation for a TOTEM Roman Pot
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1598 |
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- 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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TUPP061 |
Comparison between Laboratory Measurements, Simulations and Analytical Predictions of the Resistive Wall Transverse Beam Impedance at Low Frequencies
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1679 |
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- 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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TUPP062 |
Beam Coupling Impedance Studies on LHC FP420 Multi-pocket Beam Pipe Prototype
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1682 |
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- F. Roncarolo, R. Appleby, R. M. Jones
UMAN, Manchester
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The LHC FP420 collaboration is assessing the feasibility of installing forward proton detectors 420m from the ATLAS and/or CMS interaction points. The latest prototype of a FP420 station consists of a modified LHC beam pipe in which two pockets hosting the detectors introduce an abrupt cross-section variation of the pipe. During the FP420 proposed operation, each station is moved towards the beam as close as 3 mm (~ 10 σx). The impact on the LHC beam coupling impedance has been evaluated with a laboratory wire measurement and a suite of numerical simulations. In addition, we describe a proposed modification of the beam pipe design which minimizes the impedance of the resonances without compromising the FP420 detector signal to background ratio.
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TUPP063 |
Characterization of the ATLAS Roman Pots Beam Coupling Impedance and Mechanics
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1685 |
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- 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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TUPP068 |
Bench Measurements of the Low Frequency Transverse Impedance of the CERN LHC Beam Vacuum Interconnects with RF Contacts
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1697 |
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- 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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