Optimization of the LCLS X-Ray FEL Output Performance in the Presence of Strong Undulator Wakefields

The Linac Coherent Light Source (LCLS) Free-Electron Laser will operate in the wavelength range of 1.5 to 15 Angstroms. Energy loss due to wakefields within the long undulator 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 material (e.g. Cu) and its radius. To study the expected performance in the presence of these wakefields, we make a series of start-to-end simulations with tracking codes PARMELA and ELEGANT and time-dependent FEL simulation codes Genesis 1.3 and Ginger. We discuss the impact of the wakefield on output energy, spectral band-width, and temporal envelope of the output FEL pulse, as well as the benefits of a partial compensation obtained with a slight z dependent taper in the undulator field. We compare these results to those obtained by decreasing the bunch charge or increasing the vacuum chamber radius. We also compare our results to those predicted in concurrent analytical work.

INTRODUCTION

The Linac Coherent Light Source (LCLS) is a X-ray Free-Electron Laser (FEL), currently under construction at SLAC [1]. It is designed to operate in the wavelength range of 1.5–15 Ångstrom. Unlike alternative designs of X-ray FELs (e.g.[2]), the gap of the LCLS undulator modules is fixed and the wavelength is tuned by adjusting the electron beam energy. The vacuum chamber, which fits between the undulator poles, has an inner radius of 2.5 mm, which causes strong undulator wakefields. The change in the electron energy by these wakefields disrupts the FEL performance by pushing electrons off-resonance. Because the wakefields are not constant but depend on the position along the electron bunch, the effect cannot be compensated globally by an adjustment of the undulator field (taper).