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damping

     
Paper Title Other Keywords Page
MO201 Linac Coherent Light Source (LCLS) – Accelerator System Overview linac, feedback, simulation, free-electron-laser 7
 
  • P. Krejcik, Z. Huang, J. Wu
    SLAC, Menlo Park, California
  • P. Emma
    SLAC/ARDA, Menlo Park, California
  The Linac Coherent Light Source (LCLS) will be the world's first x-ray free-electron laser (FEL). Pulses of LCLS x-ray FEL will be several orders of magnitude brighter and shorter than most existing sources. These characteristics will enable frontier new science in several areas. To ensure the vitality of FEL lasing, it is critical to preserve the high quality of the electron beam during the acceleration and compression. We will give an overview of the LCLS accelerator system. We will address design essentials and technique challenges to satisfy the FEL requirements. We will report studies on the microbunching instability suppression via a Laser-Heater. The studies clearly prove the necessary of adding the Laser-Heater and show how effectively this Laser-Heater suppresses the instability by enhancing the Landau damping. We will report how to minimize the sensitivity of the final energy spread and the peak current to various system ‘jitters’. To minimize this sensitivity, a feedback system is required together with other diagnostics. With all these considerations, full start-to-end simulations show saturation at 1.5 Å, though the LCLS is expected to be a very challenging machine.  
Transparencies
 
MOP84 First Cryogenic Tests with JLab's new Upgrade Cavities* coupling, impedance, pick-up, higher-order-mode 216
 
  • P. Kneisel, G. Ciovati, G. Myneni, G. Wu
    Jefferson Lab, Newport News, Virginia
  • J. Halbritter
    FZ Karlsruhe, Karlsruhe
  • J. Sekutowicz
    DESY, Hamburg
  For the upgrade of CEBAF to 12 GeV two types of 7-cell cavities have been developed: the High Gradient type (HG) has been optimized with respect to the ratio of Epeak/Eacc and for the Low Loss (LL) type the shunt impedance has been maximized. Each cavity type features four DESY type coaxial HOM couplers and a waveguide input coupler. Design goals for these cavities have been set to Eacc = 20 MV/m with a Q-value at 2.05 K of Q0 = 8·109. A niobium prototype of each cavity has been fabricated at JLab and in a first test the HG cavity has been evaluated at cryogenic temperatures after appropriate buffered chemical polishing. Data for Q(E) were taken at several temperatures after R(T) was measured during initial pump down. In addition the pressure sensitivity as well as the Lorentz force detuning were evaluated. The damping of approximately 20 High Order Modes was measured to verify the room temperature data. Measurements on the LL prototype are in progress. We present in this contribution a summary of measured results of tests we performed on the new proposed shapes of the upgrade cavities.  
 
MOP88 RF Coupler Design for the TRIUMF ISAC-II Superconducting Quarter Wave Resonator coupling, simulation, acceleration, superconductivity 228
 
  • R. L. Poirier, K. Fong, P. Harmer, R.E. Laxdal, A.K. Mitra, I. Sekatchev, B. Waraich, V. Zvyagintsev
    TRIUMF, Vancouver
  An RF Coupler for the ISAC-II medium beta (β=0.058 and 0.071) superconducting quarter wave resonators was designed and tested at TRIUMF. The main goal of this development was to achieve stable operation of superconducting cavities at high acceleration gradients and low thermal load to the helium refrigeration system. The cavities will operate at 6 MV/m acceleration gradient in overcoupled mode at a forward power 200 W at 106 MHz. The overcoupling provides ±20 Hz cavity bandwidth, which improves the stability of the RF control system for fast helium pressure fluctuations, microphonics and environmental noise. Choice of materials, cooling with liquid nitrogen, aluminum nitride RF window and thermal shields insure a small thermal load on the helium refrigeration system by the Coupler. An RF finger contact which causedμdust in the coupler housing was eliminated without any degradation of the coupler performance. RF and thermal calculations, design and test results on the coupler are presented in this paper.  
 
TUP52 Methods for Measuring and Controlling Beam Breakup in High Current ERLs feedback, linac, electron, pick-up 387
 
  • C. Tennant, K. Jordan, E. Pozdeyev, R.A. Rimmer, H. Wang
    Jefferson Lab, Newport News, Virginia
  • S. Simrock
    DESY, Hamburg
  It is well known that high current Energy Recovery Linacs (ERL) utilizing superconducting cavities are susceptible to a regenerative type of beam breakup (BBU). The BBU instability is caused by the transverse deflecting higher-order modes (HOMs) of the cavities which can have high impedance. We present MATLab simulation results for the BBU stability using the analysis tools of control theory. In this framework, methods of experimentally determining the threshold current and the means of suppressing the onset of the instability become more transparent. A scheme was developed to determine the threshold current due to a particular HOM by measuring the decay and rise times of the mode's field in response to an amplitude modulated beam as a function of the average electron beam current. To combat the harmful effects of a particularly dangerous mode, two methods of directly damping HOMs through the cavity HOM couplers were demonstrated. In an effort to suppress the BBU in the presence of multiple, dangerous HOMs, a conceptual design for a bunch-by-bunch transverse feedback system has been developed. By implementing beam feedback, the threshold for instability can be increased substantially.  
 
TUP53 Temporal Profile of the LCLS Photocathode Ultraviolet Drive Laser Tolerated by the Microbunching Instability undulator, simulation, laser, emittance 390
 
  • J. Wu, Z. Huang
    SLAC, Menlo Park, California
  • M. Borland
    ANL, Argonne, Illinois
  • P. Emma
    SLAC/ARDA, Menlo Park, California
  • C. Limborg
    SLAC/SSRL, Menlo Park, California
  The high quality LCLS electron beam generated in the photoinjector is subject to all possible instabilities in the downstream acceleration and compression. The instability can be initiated by any possible density modulation of the electron beam when it is generated at the photocathode. In this note, we prescribe the tolerance on the initial electron beam density modulation possibly introduced by the ultraviolet (uv) laser at the cathode. Our study shows that the initial rms density modulation of the electron beam at the photocathode shall be less than 5 % to ensure the FEL lasing and saturation.  
 
TUP76 Adaptive Feedforward Cancellation of Sinusoidal Disturbances in Superconducting RF Cavities feedback, simulation, superconducting-RF, linac 447
 
  • T.H. Kandil, T.L. Grimm, W. Hartung, H. Khalil, J. Popielarski, J. Vincent, R.C. York
    NSCL, East Lansing, Michigan
  A control method, known as adaptive feedforward cancellation (AFC) is applied to damp sinusoidal disturbances due to microphonics in superconducting RF (SRF) cavities. AFC provides a method for damping internal, and external sinusoidal disturbances with known frequencies. It is preferred over other schemes because it uses rudimentary information about the frequency response at the disturbance frequencies, without the necessity of knowing an analytic model (transfer function) of the system. It estimates the magnitude and phase of the sinusoidal disturbance inputs and generates a control signal to cancel their effect. AFC, along with a frequency estimation process, is shown to be very successful in the cancellation of sinusoidal signals from different sources. The results of this research may significantly reduce the power requirements and increase the stability for lightly loaded continuous-wave SRF systems.  
 
WE102 State of the Art SRF Cavity Performance linac, electron, higher-order-mode, coupling 518
 
  • L. Lilje
    DESY, Hamburg
  The paper will review superconducting RF cavity performance for β=1 cavities used in both linear and circular accelerators. These superconducting cavities are used in two kinds of applications: High current storage rings and efficient high duty cycle linacs. In recent years the performance of those cavities has been improving steadily. High accelerating gradients have been achieved using advanced surface preparation techniques like electropolishing and surface cleaning methods like high pressure water rinsing. High intensity beams can be handled with advanced higher-order-mode damping schemes.  
Transparencies
 
THP31 A Four-Cell Periodically HOM-Damped RF Cavity for High Current Accelerators coupling, acceleration, dipole, impedance 669
 
  • G. Wu, R.A. Rimmer, H. Wang
    Jefferson Lab, Newport News, Virginia
  • J. Sekutowicz
    DESY, Hamburg
  • A. Sun
    ORNL/SNS, Oak Ridge, Tennessee
  A periodically Higher Order Mode (HOM) damped RF cavity is a weakly coupled multi-cell RF cavity with HOM couplers periodically mounted between the cells. It was studied as an alternative RF structure between the single cell cavity and superstructure cavity in high beam current application requiring strong damping of the HOMs. The acceleration mode in this design is the lowest frequency mode (Zero Mode) in the pass band, in contrast to the traditional “π” acceleration mode. The acceleration mode of a four-cell Zero Mode cavity has been studied along with the monopole and dipole HOMs. Some HOMs have been modeled in HFSS with waveguide HOM couplers, which were subsequently verified by MAFIA time domain analysis. To understand the tuning challenge for the weakly coupled cavity, ANSYS and SUPERFISH codes were used to simulate the cavity frequency sensitivity and field flatness change within proper tuning range, which will influence the design of the tuner structure. This paper presents this novel accelerating structure that may be used for variety of accelerator applications.  
 
THP35 Development of a Non-Magnetic Inertial Sensor for Vibration Stabilization in a Linear Collider feedback, linear-collider, collider, acceleration 681
 
  • J. Frisch, A. Chang, V. Decker, L. Hendrickson, T. Markiewicz, R. Partridge, A. Seryi
    SLAC, Menlo Park, California
  • D. Eric, T. Himel
    SLAC/NLC, Menlo Park, California
  One of the options for controlling vibration of the final focus magnets in a linear collider is to use active feedback based on accelerometers. While commercial geophysics sensors have noise performance that substantially exceeds the requirements for a linear collider, they are physically large, and cannot operate in the strong magnetic field of the detector. Conventional nonmagnetic sensors have excessive noise for this application. We report on the development of a non-magnetic inertial sensor, and on a novel commercial sensor both of which have demonstrated the required noise levels for this application.  
Transparencies
 
THP66 Measurement and Control of Microphonics in High Loaded-Q Superconducting RF Cavities linac, beam-loading, superconductivity, feedback 763
 
  • T.L. Grimm, W. Hartung, T.H. Kandil, H. Khalil, J. Popielarski, J. Vincent, R.C. York
    NSCL, East Lansing, Michigan
  • C. Radcliffe
    MSU, East Lansing, Michigan
  Superconducting radio frequency (SRF) linacs with light beam loading, such as the CEBAF upgrade, RIA and energy recovery linacs, operate more efficiently with loaded-Q values >1·107. The narrow band-width puts stringent limits on acceptable levels of vibration, also called microphonics, that detune the SRF cavities. Typical sources of vibration are rotating machinery, fluid fluctuations and ground motion. A prototype RIA 805 MHz v/c=0.47 cryomodule is presently under test in realistic operating conditions [1]. Real-time frequency detuning measurements were made for modulation rates from DC to 1 kHz. At 2 K the maximum frequency deviation was less than 100 Hz peak-to-peak, and was consistent with high loaded-Q operation. The measured modulation spectrum was primarily made up of discrete Fourier components with modulation frequencies less than 80 Hz. Using an accelerometer and helium pressure transducer, the primary sources of vibration were determined to be the high power cryoplant motors and 2 K helium fluctuations. Adaptive feedforward was used to decrease the magnitude of individual Fourier components by four to ten times [2]. Details of the experimental setup and measurements will be presented.

[1] “Experimental Study of an 805 MHz Cryomodule for the Rare Isotope Accelerator”, T.L. Grimm et al., THP70, these proceedings. [2] “Adaptive Feedforward Cancellation (AFC) of Sinusoidal Disturbances in SRF Cavities”, H. Khalil et al., TUP76, these proceedings.

 
Transparencies
 
THP72 A Newly Designed and Optimized CLIC Main Linac Accelerating Structure luminosity, linac, dipole, vacuum 779
 
  • A. Grudiev, W. Wuensch
    CERN, Geneva
  A new CLIC main-linac accelerating-structure design, HDS (Hybrid Damped Structure), with improved high-gradient performance, efficiency and simplicity of fabrication is presented. The gains are achieved in part through a new cell design which includes fully-profiled rf surfaces optimized to minimize surface fields and hybrid damping using both iris slots and radial waveguides. The slotted irises allow a simple structure fabrication in quadrants with no rf currents across joints. Further gains are achieved through a new structure optimization procedure, which simultaneously balances surface fields, power flow, short and long-range transverse wakefields, rf-to-beam efficiency and the ratio of luminosity to input power. The optimization of a 30 GHz structure with a loaded accelerating gradient of 150 MV/m results in a bunch spacing of eight rf cycles and 29% rf-to-beam efficiency. The dependencies of performance on operating frequency, accelerating gradient, and phase advance per cell are shown.  
 
THP85 Test Results of the 3.9 GHz Cavity at Fermilab coupling, emittance, linear-collider, collider 797
 
  • N. Solyak, I. Gonin
    Fermilab, Batavia, Illinois
  • L. Bellantoni, T. Berenc, H. Edwards, M. Foley, N. Khabiboulline, D. Mitchell, A. Rowe
    FNAL, Batavia, Illinois
  Fermilab is developing two types of 3.9 GHz superconducting cavities to improve performances of A0 and TTF photoinjectors. In frame of this project we have built and tested two nine-cell copper models and one 3-cell niobium accelertating cavity and series of deflecting cavities. Properties of the high order modes were carefully studied in a chain of two copper cavities at room temperature. High gradient performance were tested at helium temperature. Achieved gradients and surface resistances are exceed goal parameters. In paper we discuss results of cold tests of the 3-cell accelerating and deflecting cavities.  
 
FR101 Overview of Linear Collider Test Facilities and Results emittance, linac, collider, linear-collider 827
 
  • H. Hayano
    KEK, Ibaraki
  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.  
Transparencies