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| MOP36 |
Preliminary Study on HOM-Based Beam Alignment in the TESLA Test Facility
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117 |
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- N. Baboi, G. Kreps, M. Wendt
DESY, Hamburg
- G. Devanz, R. Paparella
CEA/DAPNIA-SACM, Gif-sur-Yvette Cedex
- O. Napoly
CEA/DSM/DAPNIA, Gif-sur-Yvette
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The interaction of the beam with the higher order modes (HOM) in the TESLA cavities has been studied in the past at the TESLA Test Facility (TTF) in order to determine whether the modes with the highest loss factor are sufficiently damped. The same modes can be used actively for beam alignment. At TTF the beam alignment based on the HOM signals is planned to be studied in the first cryo-module, containing 8 accelerating cavities. One of several modes with higher loss factor will be used. Its polarization has to be determined. The options to use single bunches or bunch trains will be analyzed. The results will be discussed in this paper.
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| MOP37 |
Optimization of Positron Capture in NLC
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120 |
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- Y.K. Batygin
SLAC, Stanford
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In the Next Linear Collider design, the positron capture system includes a positron production target, a flux concentrator, and a linac to accelerate positrons up to 1.9 GeV, the injection energy of the positron pre-damping ring. Two schemes for positron production have been studied: - a conventional approach with a 6.2 GeV electron beam interacting with a high-Z target and
- polarized positron production using polarized photons generated in a helical undulator by a 150 GeV electron beam which then interact with a positron production target.
The capture system has been optimized to insure high positron yield into the 6-dimensional acceptance of the pre-damping ring. Various parameters affecting the positron capture have been analyzed, including: positron deceleration after the flux concentrator, transverse and longitudinal electron beam sizes for positron generation, energy compression after acceleration, etc. As a result of these optimization studies, the positron yield in the conventional scheme has been increased from 1.0 to at least 1.5 and for the polarized positron scheme from 0.25 to 0.30 while maintaining 60% positron polarization.
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Transparencies
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| MOP38 |
Background from Undulator in the Proposed Experiment with Polarized Positrons
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123 |
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- Y.K. Batygin
SLAC, Stanford
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E-166 is a proposed experiment for verification of polarized positron production for linear collider. According to polarized positron source design, high energy electrons pass through helical undulator and produce circularly polarized photons, which interact with tungsten target and produce longitudinally polarized positrons. In the proposed E-166 experiment, 50 GeV beam propagates inside 1m long undulator followed by a drift space of 35 m before interaction with target. Polarized positrons are analyzed by Si-W calorimeter, which is placed along the axis. Polarized positrons are analyzed by CsI calorimeter after reconversion of positrons to photons at the second target. Background is an issue for a considered experiment. GEANT3 simulations were performed to model production of secondary particles from primary electrons hitting undulator. Energy density distribution of background particles at the target and effect of background collimation are discussed.
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| MOP39 |
Positron Transmission and Polarization in E-166 Experiment
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126 |
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- Y.K. Batygin
SLAC, Stanford
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The proposed experiment E-166 at SLAC is designed to demonstrate the possibility of producing longitudinally polarized positrons from circularly polarized photons. Experimental set-up utilizes a low emittance 50 GeV electron beam passing through a helical undulator in the Final Focus Test Beam line of SLAC accelerator. Circularly polarized photons generated by the electron beam in undulator hit a target and produce electron-positron pairs. The purpose of post-target optics is to select the positron beam and to deliver it to a polarimeter keeping positron beam polarization as high as possible. Paper analyzes the positron transmission and polarization both numerically and analytically. The value of positron transmission has a maximum of 3% for positron energy of 7 MeV while positron polarization is around 80%.
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| MOP40 |
A Study Of Coupler-Trapped Modes In X-Band Linacs for the GLC/NLC
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129 |
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- R.M. Jones, V.A. Dolgashev
SLAC/ARDA, Menlo Park, California
- Z. Li
SLAC, Menlo Park, California
- J. Wang
SLAC/ARDB, Menlo Park, California
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Each of the X-band accelerating structures for the GLC/NLC consist of 55 cells which accelerate a train of charged particles. The cells are carefully designed to ensure that the transverse wakefield left behind each bunch does not disrupt the trailing bunches. However, unless attention is paid to the design of the fundamental mode coupler, then a dipole mode is trapped in the region of the coupler and cells. This mode can give rise to severe emittance dilution if care is not taken to avoid a region of resonant growth in the emittance. Here, we present results on HFSS simulations, cold test experimental measurements and beam dynamics simulations arising as a consequence of the mode trapped in the coupler. The region in which the trapped mode has little influence on the beam is delineated.
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| MOP41 |
Emittance-Imposed Alignment and Frequency Tolerances for the TESLA Linear Collider
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132 |
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- N. Baboi
DESY, Hamburg
- R.M. Jones
SLAC/ARDA, Menlo Park, California
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One option in building a future 500 GeV c.m. collider is to use superconducting 1.3 GHz 9-cell cavities. Wakefields excited by the bunch train in the TESLA linac can resonantly drive the beam into unstable operation such that a BBU (Beam Break Up) mode results or at the very least significant emittance dilution occurs. The largest kick factors (proportional to the transverse fields which transversely kick the beam off axis) are found in the first three dipole bands and hence multi-bunch emittance growth is mainly determined from these bands. These higher order dipole modes are damped by carefully orientating higher order mode couplers at the downstream end of the cavities. We investigate the dilution in the emittance of a beam injected with an initial offset from the axis of the cavities. The dependence of beam emittance on systematic errors in the cell frequencies is investigated. We also vary the bunch spacing in order to simulate a systematic frequency error. While scanning the bunch spacing over a wide range, the emittance presents sharp peaks since only few modes contribute effectively to emittance growth. The locations of these peaks sets the frequency tolerances on the structures.
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| MOP42 |
Linac Alignment and Frequency Tolerances from the Perspective of Contained Emittances for the G/NLC
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135 |
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- R.M. Jones
SLAC/ARDA, Menlo Park, California
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We maintain the stable progress of a beam consisting of a train of bunched charges, by a careful design of the geometry of the structures [1]. In practice, the next generation of linear colliders will consist of several tens of thousands of X-band accelerating structures and this will entail inevitable errors in the dimensions and alignments of cells -and groups thereof. These errors result in a dilution of the beam emittance and consequently a loss in overall luminosity of the collider. For this reason it is important to understand the alignment tolerances and frequency tolerances that are imposed for a specified emittance budget. Here we specify an emittance dilution of no more than 10% of the initial value and we track the progress of the beam down the linac whilst accelerating structures (and sub-groups thereof) are misaligned in a random manner and at the same time random frequency are incorporated with structures. This results in tolerances in both frequency errors and sets of alignment errors to be imposed on the structures for a specified emittance dilution.
[1] R.M. Jones, 1997, SLAC NLC-Note 24.
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| MOP43 |
The Impact of Longitudinal Drive Beam Jitter on the CLIC Luminosity
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138 |
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- D. Schulte, E. J. N. Wilson, F. Zimmermann
CERN, Geneva
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In the compact linear collider (CLIC) now under study at CERN, the RF power which accelerates the main beam is provided by decelerating a high current drive beam. Errors in the timing and intensity of the drive beam can turn into RF phase and amplitude errors that are coherent along the whole main linac and the resulting error of the final beam energy, in combination with the limited bandwidth of the beam delivery system, can lead to a significant loss of luminosity. We discuss the stability tolerances that must be applied to the drive beam to avoid this loss. We also examine one of the most important sources of this jitter, which stems from the combination of RF jitter in the drive beam accelerator and subsequent bunch compression. Finally we give details of a potential feedback system that can reduce the drive beam jitter.
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| MOP44 |
Electron-Cloud Effects in the Positron Linacs of Future Linear Colliders
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141 |
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- D. Schulte, A. Grudiev, F. Zimmermann
CERN, Geneva
- K. Oide
KEK, Ibaraki
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Inside the rf structures of positron linacs for future linear colliders, electron multipacting may occur under the combined influence of the beam field and the electromagnetic rf wave. The multipacting could lead to an electron-cloud build up along the bunch train. We present simulation results of this effect for various proposed designs, and discuss possible consequences and eventual countermeasures.
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Transparencies
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| MOP45 |
A Potential Signal for Luminosity Optimisation in CLIC
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144 |
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Luminosity optimisation will be challenging in the compact linear collider (CLIC) studied at CERN. In particular, the signals which can be used for luminosity optimisation need to be identified. The strong beam-beam interaction in CLIC will give rise to the emission of a few megawatts of beamstrahlung; this is a potential candidate for such a signal. In this paper luminosity optimisation using the beamstrahlung is attempted for realistically shaped bunches.
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| FR101 |
Overview of Linear Collider Test Facilities and Results
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827 |
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Linear Collider technology will be recommended by the International Technology Recommendation Panel (ITRP) to the International Linear Collider Steering Committee (ILCSC), soon. Towards this recommendation, many efforts of the developments and the output results of each technology have been made to satisfy the requirements of the technical review committee report (TRC). The test facilities of each linear collider design are the place of the key technology demonstration and realization. The overview of the LC test facilities activities and outputs of TTF, NLCTA, ATF/GLCTA and CTF are summarized and reviewed.
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Transparencies
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