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| MOPP034 | Large Scale Linac Simulations Using a Globalised Scattering Matrix Approach | scattering, simulation, linac, dipole | 619 | ||
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A globalised cascaded scattering matrix scheme serves as practical method to simulate the electromagnetic (e.m.) fields in the groups of cavities which constitute the main accelerating structures of a linac. The cascaded scattering matrix technique is a well-proven method which allows realistic fabrication errors to be incorporated in an efficient manner without the necessity to re-mesh the entire geometry. Once the unit cell structures have been determined using a numerical scheme, such as finite element method utilized here, the overall cascaded scattering matrix calculation requires little in the way of computational resources or time and is consequently an efficient means of characterizing the e.m. field. Details of the e.m. field, shunt impedance and trapped modes for large scale linac simulations applied to the baseline and alternate high gradient cavities for the ILC and applications to XFEL are presented.
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| MOPP157 | Critical Magnetic Field Determination of Superconducting Materials | coupling, pick-up, pulsed-power, klystron | 919 | ||
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Using a 11.4 GHz, 50-MW, <1 μs, pulsed power source and a TE013-like mode copper cavity, we have been measuring critical magnetic fields of superconductors for accelerator cavity applications. This device can eliminate both thermal and field emission effects due to a short pulse and no electric field at the sample surface. A model of the system is presented in this paper along with a discussion of preliminary experimental data.
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| TUPC021 | High Bandwidth Wall Current Monitor for CTF3 | resonance, coupling, impedance, shielding | 1092 | ||
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Wall Current Monitors (WCM) are commonly used to observe the time profile and spectra of a particle beam by detecting its image current. For the 3rd CLIC Test Facility (CTF3), a WCM having a very large bandwidth (100kHz-20GHz) is in principle required. This very stringent request was critically reviewed because the low cut-off frequency of 100 kHz is quite outstanding. It was initially chosen because of the bunch train length but, in reality, because of the high frequency cut-off of 20GHz, the low frequency cut-off should rather be related to the maximum expected Missing Bunch Ratio (MBR). The solution that we propose has a low frequency cut-off of 2GHz corresponding to an MBR of 1/6 for 83ps bunch spacing. If needed, it could be lowered to 400MHz (MBR equal to 1/30). That solution has been fully characterized both from an electromagnetic and from a mechanical point of view. The first tests of a prototype are foreseen in February 2008.
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| TUPC142 | Performance of 24 Cavity Vector Sum Controller with Distributed Architecture | controls, klystron, electron, laser | 1401 | ||
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The paper presents the test results of the digital vector sum control applied for 24 superconducting cavities driven by 1 klystron. The controller is based on FPGA chips and consists of multiple processing boards which communicate via optical fiber links. Flexible and scalable distributed architecture was designed and implemented to provide framework for the control algorithms. The tests were performed at FLASH (DESY, Hamburg) facility using ACC4, ACC5 and ACC6 modules. Results were compared to the existing DSP based system.
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| TUPD036 | G4Beamline Simulations for Detector Development | simulation, electron, radiation, photon | 1506 | ||
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In current research programs to develop radiation detection instruments, simplifying assumptions are frequently made in estimating the resolutions and efficiencies attainable by neutron and gamma-ray instruments. Monte Carlo programs (such as Geant4) are capable of realistically modeling such problems, but the technical details of setting up, running, and interpreting the required simulations are beyond the ability of all but the most expert researchers. G4beamline, a program that is an interface to the Geant4 toolkit for the simulation of accelerator beam lines, is being extended to model detectors and related systems needed for applications related to nuclear nonproliferation and other users. The program is flexible, extremely user friendly, and requires no programming by users. Simulations of simple or complex detectors can be setup quickly and are accurately simulated using the power and accuracy of Geant4 for the transport of particles, including scattering, attenuation, interactions, and decays.
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| TUPP038 | On the Longitudinal Coupling Impedance and Transmission Coefficient from Uniform and Hollow Ring Sources | impedance, space-charge, coupling, synchrotron | 1625 | ||
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The longitudinal coupling impedance and the transmission coefficient resulting from a thin ring and from a uniform disk are obtained analytically for a resistive cylindrical beam-pipe of finite wall thickness. The impedances are derived and then compared with the well known corresponding expression for perturbations on a uniform, coasting beam. The transmission coefficients from both sources are found to be exactly the same. Differences do appear in the expressions for the electromagnetic fields within the beam region, and therefore leading to different coupling impedances. By applying the results to parameters relevant for the SIS-18 synchrotron at GSI, it is found that the formula from the ring source underestimates the space-charge impedance at all beam energies and it shows a noticeable deviation from the disk formula for all frequencies. Although their mathematical expressions are different, resistive-wall impedances from the two sources are found to be numerically equal. The space-charge impedances become equal asymptotically only in the so called ultra-relativistic limit.
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A. Al-Khateeb is on leave from Yarmouk University, Irbid, Jordan |
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| TUPP075 | Numerical Studies of Resistive Wall Effects | vacuum, emittance, impedance, electron | 1709 | ||
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In this paper we describe a new numerical code to calculate wakefields of resistive wall geometries. Our code is based on conformal implicit scheme. It allows to estimate wakefields of very short bunches taking into account transitive effects neglected in the European XFEL impedance budget so far.
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| TUPP090 | A Kinetic Model of Multipaction for SRF Cavities for Accelerator Driven Sub-Critical System (ADSS) | electron, simulation, proton, superconductivity | 1741 | ||
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This work simulates multipaction in a 700 MHz elliptical SRF cavity. The cavity design was optimized using SUPERFISH. Then the electromagnetic field was re-computed with FEMLAB, a package using the finite element method, to obtain a more accurate field-mapping, and to make the field values available for computation of multipaction. In the multipacting subroutine, electrons were assumed to be released into the system from various points with different initial parameters. The electrons trajectories were tracked until they hit the cavity surface. Leap-frog scheme was used to solve the Lorentz force equation for primary electrons, as it is easy to use and is accurate up to second order. The position, velocity, phase and kinetic energy of primary electrons at each time step were calculated and stored. An interpolation function was used to calculate secondary emission yield (SEY) at different impact energies. With the emission of secondary electrons, their trajectories too were tracked along with primary electrons, in order to identify parameters responsible for multipaction. By repeating this process for large number of electrons, the multipacting trajectories were identified.
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| WEPP092 | Tuning of Waveguide to Cavity Coupling Coefficient Beta for a PWT Linac and a Photocathode Gun | coupling, gun, linac, simulation | 2734 | ||
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The waveguide to cavity coupling coefficient beta for two types of accelerating structures: a Plane Wave Transformer (PWT) linac and a 1.6 cell photocathode gun has been tuned to obtain critical coupling in both. Analytical calculation of the dimensions of slot required for critical coupling have been done using Gao’s formulation based on Bethe’s theory for hole coupling. While the PWT linac structure, with high inter-cell coupling, shows good agreement between measured and predicted slot dimensions for different values of beta, the agreement is not so good in the photocathode gun on account of poor inter-cell coupling. This paper discusses details of the analytical calculation of slot dimensions for the two structures, their comparison with experimentally measured results, and the procedure adopted for tuning the two structures to critical coupling.
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