• 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

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.

• G. Hoffstaetter, S.A. Belomestnykh, J.S.-H. Choi, Z. Greenwald, M. Liepe, H. Padamsee, D. Sagan, C. Song, R.M. Talman, M. Tigner
  Cornell University, Laboratory for Elementary-Particle Physics, Ithaca, New York
• I.V. Bazarov, K.W. Smolenski
  Cornell University, Ithaca, New York
• D.H. Bilderback, M.G. Billing, S.M. Gruner, Y. Li, C.K. Sinclair
  Cornell University, Department of Physics, Ithaca, New York

We describe the status of plans to build an Energy-Recovery Linac (ERL) X-ray facility at Cornell University. This 5 GeV ERL is an upgrade of the CESR ring that currently powers the Cornell High Energy Synchrotron Source (CHESS). Due to its very small electron-beam emittances, it would dramatically improve the capabilities of the light source and result in X-ray beams orders of magnitude better than any existing storage ring light source. The emittances are based upon simulations for currents that are competitive with ring-based sources. The ERL design that is presented has to allow for non-destructive transport of these small emittances. The design includes a series of X-ray beamlines for specific areas of research. As an upgrade of the existing storage ring, special attention is given to reuse of many of the existing ring components. Options of bunch compression are discussed, tolerances for emittance growth are specified, and simulations of the beam-breakup instability and methods of increasing its threshold current are shown. This planned upgrade illustrates how other existing storage rings could be upgraded as ERL light sources with vastly improved beam qualities.