Instabilities and Feedback

Paper Title Page
TOAC001 Overview of Impedance and Single-Beam Instability Mechanisms 14
  • E. Métral
    CERN, Geneva
  Single-bunch and coupled-bunch instability mechanisms will be reviewed in both longitudinal and transverse planes. The resistive-wall impedance will be discussed in the particular case of the LHC collimators, which reveal a new physical regime. Stabilization by Landau damping, feedbacks, or linear coupling between the transverse planes will also be treated. Benchmarking of analytical predictions with some instability codes will be shown as well as several experimental results.  
TOAC002 Beam Loading Compensation for Super B-Factories 154
  • D. Teytelman
    SLAC, Menlo Park, California
  Funding: Work supported by U.S. Department of Energy contract DE-AC02-76SF00515.

Super B-factory designs under consideration expect to reach luminosities in the 1035 - 1036 range. The dramatic luminosity increase relative to the existing B-factories is achieved, in part, by raising the beam currents stored in the electron and positron rings. For such machines to succeed it is necessary to consider in the RF system design not only the gap voltage and beam power, but also the beam loading effects. The main effects are the synchronous phase transients due to the uneven ring filling patterns and the longitudinal coupled-bunch instabilities driven by the fundamental impedance of the RF cavities. A systematic approach to predicting such effects and for optimizing the RF system design will be presented. Existing as well as promising new techniques for reducing the effects of heavy beam loading will be described and illustrated with examples from the existing storage rings including PEP-II, KEKB, and DAFNE.

TOAC003 Stochastic Cooling for Bunched Beams 310
  • M. Blaskiewicz
    BNL, Upton, Long Island, New York
  Funding: Work performed under the auspices of the U.S. DOE.

A longitudinal stochastic cooling system for RHIC is under construction and partial commissioning is planned for the upcoming run. The state of the system and future plans are discussed.

TOAC004 Experimental Investigation of Beam Breakup in the Jefferson Laboratory 10 kW FEL Upgrade Driver 369
  • C. Tennant, D. Douglas, K. Jordan, L. Merminga, E.P. Pozdeyev, H. Wang
    Jefferson Lab, Newport News, Virginia
  • I.V. Bazarov
    Cornell University, Department of Physics, Ithaca, New York
  • G. Hoffstaetter
    Cornell University, Laboratory for Elementary-Particle Physics, Ithaca, New York
  • S. Simrock
    DESY, Hamburg
  • T.I. Smith
    Stanford University, Stanford, Califormia
  Funding: This work supported by the Office of Naval Research, the Joint Technology Office, the Commonwealth of Virginia, the Air Force Research Laboratory, Cornell University and by DOE Contract DE-AC05-84ER40150.

In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current has been limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovered pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting technology. Experimental observations of the instability at the Jefferson Laboratory 10 kW Free-Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are presented and compared to the predictions of several BBU simulation codes. To further characterize the instability, beam based measurements were made to determine the orientation of the dangerous HOMs. With BBU posing a threat to high current beam operation in the FEL, several suppression schemes were developed. These include direct damping of the dangerous HOMs and appropriately modifying the electron beam optics. Preliminary results of their effectiveness in raising the threshold current for stability are presented.

TOAC005 Coherent Synchrotron Radiation as a Diagnostic Tool for the LCLS Longitudinal Feedback System 428
  • J. Wu, P. Emma, Z. Huang
    SLAC, Menlo Park, California
  Funding: Work is supported by the US Department of Energy under contract DE-AC02-76SF00515.

The Linac Coherent Light Source (LCLS) will be the world's first x-ray free-electron laser (FEL). To ensure the vitality of FEL lasing, a longitudinal feedback system is required together with other diagnostics. In this paper, we study the possibility of using Coherent Synchrotron Radiation (CSR) from the chicane as the diagnostic tool for bunch length feedback. Calculations show that CSR is a good candidate, even for the non-Gaussian, double-horn longitudinal charge distribution. We further check the feasibility for low and high charge options, and also the possibility for detecting the microbunching.