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beam-loading

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MO2001 Status of the CLIC Test Facility (CTF3) linac, acceleration, klystron, extraction 11
 
  • G. Geschonke
    CERN, Geneva
  The CTF3 project, being built within the frame-work of an international collaboration involving more than 12 institutions, is advancing as planned. To date, the electron linac with its sub-harmonic bunching system, the magnetic chicane for bunch-length variations, and the Delay Loop have been installed. The 1.5 GHz sub-harmonic bunching system with fast phase switching allows the longitudinal position of the bunches to be changed every 140 ns. This phase-coded beam has been successfully injected into the Delay Loop using an RF deflector and bunch interleaving of 140 ns long sub-bunch trains which double the bunch repetition frequency has been demonstrated in the extraction line. In addition to its role as a test bed for the CLIC RF power source, CTF3 is being used as a source of high-power RF at 30 GHz for the testing of CLIC accelerating structures. In this power-generating mode, about 100 MW of 30 GHz power is routinely extracted from the beam half-way up the linac by special-purpose power-extracting structures and transported to the high-gradient test area by low-loss waveguides. This paper describes the overall status of the CTF3 project and outlines the plans for the future.  
 
MOP002 Efficient Long-Pulse, Fully Loaded CTF3 Linac Operation linac, acceleration, klystron, gun 31
 
  • P. Urschütz, H.-H. Braun, R. Corsini, S. Doebert, E. Jensen, F. Tecker
    CERN, Geneva
  An efficient RF to beam energy transfer in the accelerating structures of the drive beam is on of the key points of the Compact Linear Collider (CLIC) RF power source. For this, the structures are fully beam-loaded, i.e. the accelerating gradient is nearly zero at the downstream end of each structure. In this way, about 96% of the RF energy can be transferred to the beam. To demonstrate this mode of operation, 1500 ns long beam pulses are accelerated in six fully loaded structures in the CLIC Test Facility (CTF3) Linac. In the paper we present the results of experimental studies on this mode of operation, compare them with theoretical predictions and discuss its potential use in CLIC.  
 
MOP025 Study on High-Current Multi-Bunch Beam Acceleration for KEKB Injector Linac klystron, linac, acceleration, simulation 91
 
  • M. Yoshida, H. Katagiri, Y. Ogawa
    KEK, Ibaraki
  The KEKB injector linac is usually operated to accelerate only two 10 nC electron bunches to generate positron, since more bunch cannot be equalized the beam energy using the conventional pulse compressor (SLED) and the simple phase modulation. The aim of this study is to find how to accelerate more bunches without any modification of high power RF distribution. One way is that a part of the acceleration units is used to compensate the beam energy difference. On the other hand, the recent electron linac is designed for the multi-bunch operation by compensating the beam loading. And this beam loading compensation method is usually realized by combining the output power of two or more klystrons. However our linac system consists of one 50 MW klystron in one acceleration unit, and eight klystrons are driven by a 100kW klystron. Another way to realize the multi-bunch acceleration in our linac is using the amplitude modulation of the klystron. This is realized using the I-Q modulation of the low level RF considering the non-linear characteristics of the total amplification system including klystrons. Further we developed a FPGA board with 100 MHz DACs and ADCs to realize this.  
 
MOP057 A Fault Recovery System for the SNS Superconducting Cavity Linac SNS, linac, klystron, proton 174
 
  • J. Galambos, S. Henderson, Y. Zhang
    ORNL, Oak Ridge, Tennessee
  One of the advantages for the change of the Spallation Neutron Source (SNS) linac from copper to superconducting cavities, was the possibility of fault tolerance. Namely, the ability to rapidly recover from a cavity failure, retune the downstream cavities with minimal user disruption. While this is straightforward for electron machines, where beta is constant, it is more involved for the case of proton machines, where the beta changes appreciably throughout the Superconducting Linac (SCL). For SNS when the SCL is first turned on, each cavity’s RF amplitude and phase (relative to the beam) are determined with a beam based technique. Using this information a model calculated map of arrival time and phase setpoint for each cavity is constructed. In the case of cavity failure(s) the change in arrival time at downstream cavities can be calculated and the RF phases adjusted accordingly. Typical phase adjustments are in the 100 – 1000 degree range. This system has been tested on the SNS SCL in both controlled tests and a need based instance in which more than 10 cavity amplitudes were simultaneously reduced. This scheme and results will be discussed.  
 
TUP032 Comparison of SNS Superconducting Cavity Calibration Methods SNS, acceleration, controls, pick-up 315
 
  • Y. Zhang, I. E. Campisi, P. Chu, J. Galambos, S. Henderson, D.-O. Jeon, K.-U. Kasemir, A. P. Shishlo
    ORNL, Oak Ridge, Tennessee
  Three different methods have been used to calibrate the SNS superconducting cavity RF field amplitude. Two are beam based and the other strictly RF based. One beam based method uses time-of-flight signature matching (phase scan method), and the other uses the beam-cavity interaction itself (drifting beam method). Both of these methods can be used to precisely calibrate the pickup probe of a SC cavity and determine the synchronous phase. The initial comparisons of the beam based techniques at SNS did not achieve the desired precision of 1% due to the influence of calibration errors, noise and coherent interfaces in the system. To date the beam-based SC cavity pickup probe calibrations agree within approximately 4%, comparable to the conventional RF calibrations.  
 
TUP048 Beam-Loading Effect in the Normal-Conducting ILC Positron Source Pre-Accelerator positron, focusing, linac, linear-collider 355
 
  • V. V. Paramonov
    RAS/INR, Moscow
  • K. Floettmann
    DESY, Hamburg
  Significant positron bunch charge (several nC) in the ILC Positron Source results in high pulse beam loading for normal-conducting accelerating structures in Positron Pre-Accelerator (PPA). Time interval between bunches (~ 300 ns) is not negligibly small in comparison with accelerating structure time constant (rise time for Standing Wave (SW) or filling time for Traveling Wave (TW) options). As the result, beam loading effect has particularities both from stored energy acceleration regime and continuous beam loading one. Taking into account particular PPA beam structure, beam loading effect is estimated for the present ILC base line parameters, both for SW and TW PPA options. Possible solutions for beam loading compensation are discussed.  
 
TUP071 Beam-Loading Effects on Phase Scan for the Superconducting Cavities linac, simulation, SNS, impedance 418
 
  • D.-O. Jeon, S. Henderson, S.-H. Kim, Y. Zhang
    ORNL, Oak Ridge, Tennessee
  When the beam is passing through superconducting cavities, it excites beam induced field in cavities. A systematic study was performed to study the beam loading effects by the nonrelativistic beam for β = 0.81 superconducting cavities of the SNS linac. The analysis indicates that the induced field level is quite close to the estimation and its effect on the phase scan is consistent with the model.  
 
TUP084 Drifting Beam Application for SNS Superconducting Cavity Setting SNS, controls, linac, Spallation-Neutron-Source 454
 
  • P. Chu, Y. Zhang
    ORNL, Oak Ridge, Tennessee
  A software application for tuning superconducting linac cavity has been developed and tested at the Spallation Neutron Source (SNS). The application is based on the drifting beam method and the XAL online model. The drifting beam method and the application were proved to be consistent with other cavity tuning method during the SNS commissioning runs. Detail algorithm and data acquisition for the application will be presented.  
 
THP005 Digital Control of Cavity Fields in the Spallation Neutron Source Superconducting Linac controls, linac, SNS, feedback 571
 
  • H. Ma, M. S. Champion, M. T. Crofford, K.-U. Kasemir, M. F. Piller
    ORNL, Oak Ridge, Tennessee
  • A. Brandt
    DESY, Hamburg
  • L. R. Doolittle, A. Ratti
    LBNL, Berkeley, California
  Control of the pulsed RF cavity fields in the Spallation Neutron Source (SNS) superconducting Linac uses both the real-time feedback regulation and the pulse-to-pulse adaptive feed-forward compensation. This control combination is required to deal with the typical issues associated with superconducting cavities, such as the Lorentz force detuning, mechanical resonance modes, and cavity filling. The all-digital implementation of this system provides the capabilities and flexibility necessary for achieving the required performance, and to accommodate the needs of various control schemes. The low-latency design of the digital hardware has successfully produced a wide control bandwidth, and the developed adaptive feed forward algorithms have proved to be essential for the controlled cavity filling, the suppression of the cavity mechanical resonances, and the beam loading compensation. As of this time, all 96 LLRF systems throughout the Linac have been commissioned and are in operation.  
 
THP006 Performance of a Digital LLRF Field Control System for the J-PARC Linac controls, feedback, linac, klystron 574
 
  • S. Michizono, S. Anami, Z. Fang, S. Yamaguchi
    KEK, Ibaraki
  • T. Kobayashi
    JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
  • H. Suzuki
    JAEA, Ibaraki-ken
  Twenty high power klystrons are installed in the J-PARC linac. The requirements for the rf field stabilities are ±1% in amplitude and ±1 deg. in phase during a 500 us flat-top. In order to satisfy these requirements, we adopt the digital feedback and feed-forward system with FPGAs and a commercial DSP board. The FPGAs (Virtex-II 2000) enable a fast PI control for a vector sum of two cavity fields. The measured stability during rf pulse was ±0.15% in amplitude and ±0.15 deg in phase. The tuner control was successively operated by a way of the DSP board by measuring the phase difference between the cavity input wave and the cavity field. Beam loading effects were emulated using a beam-loading test box. By proper feed-forward, the rf stability was less than ±0.3% and ±.15 deg.  
 
THP040 New Concept of Small Delay Line Type RF Pulse Compressor Using Coupled Cavities klystron, simulation, linac, coupling 667
 
  • M. Yoshida
    KEK, Ibaraki
  I propose a new concept for the RF pulse compressor using the coupled cavities to make a small delay line. This new concept is a hybrid scheme of a cavity type and a delay line type of the RF pulse compressor. The delay line produces the pulse compression outputs through resultant RF beat between two inputs connected both ports of the coupled cavities. The time constant of the beat is matched to the time constant of the power flow of the coupled cavities. Further the special test stand for the coupled cavities was developed to easily adjust the resonant frequency of such high-Q coupled cavities.  
 
THP046 Status of 3.9-GHz Deflecting-Mode (CRAB) Cavity R&D dipole, luminosity, simulation, positron 682
 
  • L. Bellantoni, H. Edwards, M. Foley, T. K. Khabiboulline, D. V. Mitchell, A. M. Rowe, N. Solyak
    Fermilab, Batavia, Illinois
  • C. Adolphsen
    SLAC, Menlo Park, California
  • G. Burt, A. C. Dexter
    Cockcroft Institute, Lancaster University, Lancaster
  • P. Goudket
    CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire
  • T. W. Koeth
    Rutgers University, The State University of New Jersey, Piscataway, New Jersey
  The superconducting 3.9GHz deflecting mode cavity design which has been under development as a beam slice diagnostic is planned for use as the ILC crab cavity. We describe the applications and review the status of the R & D, giving both prototype test results and computational studies of beam interaction.  
 
THP058 Proposed LLRF Improvements for Fermilab 201.25 MHz Linac linac, feedback, coupling, pick-up 713
 
  • T. A. Butler, E. Cullerton, V. Tupikov
    Fermilab, Batavia, Illinois
  The Fermilab Proton Plan, tasked to increase the intensity and reliability of the Proton Source for 10 or more years of operation, has identified the Low Level RF (LLRF) system as the critical component to be upgraded in the Linac. The current 201.25 MHz Drift Tube Linac was designed and built over 30 years ago and does not meet the higher beam quality demands required under the new Proton Plan. Measurement data, used to characterize the system, has been collected as input for a new computer model of the system. This model shows what improvements can be made by replacing the LLRF system to improve beam quality. The model includes RF driver amplifiers, a 5 MW 7835 triode power amplifier, the high voltage switch tube modulator, and the drift tube cavity. Complete system gain and bandwidth characterization data has been collected for the 7835 triode power amplifier, modulator and RF driver stages. This model will be a useful analysis tool for present and future Linac system upgrades.