• G.V. Stupakov, Z. Huang
  SLAC, Menlo Park, California

Funding: This work was supported by the Department of Energy, contract DE-AC02-76SF00515.

Several ideas have been proposed in the past to "condition" an electron beam prior to the undulator of a Free-Electron Laser (FEL) by increasing each particle's energy in proportion to the square of its transverse betatron amplitude. This conditioning enhances FEL gain by reducing the axial velocity spread within the electron bunch. Nevertheless, a practical solution for beam conditioning remains difficult. In this paper we consider a new approach to condition the beam using nonlinear effects in the RF field. We demonstrate that such effects can generate a radial variation of the particle's energy in the beam, and and calculate the induced energy spread in the limit of weak field. Methods to minimize the emittance growth in such a beam conditioner are also discussed.

• K.L.F. Bane, P. Emma, Z. Huang, H.-D. Nuhn, G.V. Stupakov
  SLAC, Menlo Park, California
• W.M. Fawley
  LBNL, Berkeley, California
• S. Reiche
  UCLA, Los Angeles, California

Funding: Work supported in part by the Office of Science,U.S. Dept. of Energy under Contracts DE-AC02-76F00515 and DE-AC03-76SF0098.

Energy loss due to wakefields within a long undulator, if not compensated by an appropriate tapering of the magnetic field strength, can degrade the FEL process by detuning the resonant FEL frequency. The wakefields arise from the vacuum chamber wall resistivity, its surface roughness, and abrupt changes in its aperture. For LCLS parameters, the resistive component is the most critical and depends upon the chamber wall material (e.g. Cu) and its radius. Of recent interest [1] is the so-called "AC" component of the resistive wake which can lead to strong variations on very short timescales (e.g. ~20 fs). To study the expected performance of the LCLS in the presence of these wakefields, we have made an extensive series of start-to-end SASE simulations with tracking codes PARMELA and ELEGANT, and time-dependent FEL simulation codes GENESIS1.3 and GINGER. We discuss the impact of the wakefield losses upon output energy, spectral bandwidth, and temporal envelope of the output FEL pulse, as well as the benefits of a partial compensation of the time-dependent wake losses obtained with an undulator field taper. We compare these results to those predicted analytically [2].

[1] K.Bane and G. Stupakov, SLAC PUB-10707 (2004). [2] Z. Huang and G. Stupakov, Phys. Rev. ST Accel. Beams 8, 040702 (2005).