• J.R. Thompson, G.H. Hoffstaetter
  CLASSE, Ithaca, New York

The production of ultra-short x-rays in Cornell's Energy Recovery Linac (ERL) requires electron bunch lengths of less than 100fs with minimal transverse emittance growth and energy spread. Because the linac consists of two sections separated by an arc, CSR forces limit the bunch length in the linac, and bunch compression has to be installed after acceleration. Creation of such short bunches requires a second order bunch compression scheme with correction of the third order dispersion. In this paper, we discuss possible bunch compression systems and explore the benefits of each using the tracking program TAO including CSR forces. Overall, we find that a FODO compressor utilizing dipole, quadrupole and sextupole magnets can achieve the design goals of the short pulse mode.

• J.R. Thompson, J.M. Byrd
  LBNL, Berkeley, California

Funding: Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Uneven beam fill patterns in storage rings, such as gaps in the fill patterns, leads to periodic, or transient loading of the modes of the RF cavities. We show that an analogous effect can occur in the loading of a dipole cavity mode when the beam passes off the electrical center of the cavity mode. Although this effect is small, it results in a variation of the transverse offset of the beam along the bunch train. For ultralow emittance beams, such as optimized third generation light sources and damping rings, this effect results in a larger projected emittance of the beam compared with the single bunch emittance. The effect is particularly strong for the case when a strong dipole mode has been purposely added to the ring, such as a deflecting, or ‘‘crab'' cavity. We derive an approximate analytic solution for the variation of the beam-induced deflecting voltage along the bunch train. We also show via a tracking simulation the combined effect of the periodic loading of the fundamental and dipole modes.

• J.R. Thompson, J.M. Byrd
  LBNL, Berkeley, California
• W. Höfle, G. Rumolo
  CERN, Geneva

Funding: Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Single and multi-bunch instabilities on bunch trains driven by electron clouds have been observed in the CERN SPS for some years. In this paper, we present initial results to implement a damping system in a computer simulation of a single bunch vertical instability using the HEADTAIL code. The code simulates the interaction between a proton bunch and a uniform electron cloud that has built up inside of the beam pipe. In all simulations we use typical SPS parameter sets for three different values of the beam momentum : 26 GeV/c, 55 GeV/c and 120 GeV/c. The feedback is implemented as a corrective kick calculated from the vertical centroid of each slice of the electron bunch with a one turn delay. The bandwidth of the feedback is varied by filtering the slice information along the bunch. Initial results indicate that the instability can be damped with a minimum bandwidth of 300 MHz with a relatively high gain.