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Linac Coherent Light Source

The Linac Coherent Light Source (LCLS) is a X-Ray FEL in the Angstrom regime, currently under construction at the Stanford Linear Accelerator Center (SLAC). PBPL participates in this collaboration with other institutes as part of the FEL Physics group. As such we investigate the performance of the Free-Electron Laser, using analytical models and numerical codes. Our main goal is the most accurate prediction of the FEL output power. This includes a consistent set of simulations (start-end simulations) to model the entire beam line from the RF gun to the experimental station of the LCLS beam line.

The basic parameters for the LCLS FEL are:

Wavelength 1.5-15 Angstrom
Electron Energy 4.5-14.3 GeV
Peak Current 3.4 kA
Norm. Emittance 1.2 mm mrad
Energy Spread 0.13-0.47 %
Average beta-function 7-18 m
Pulse Length (FWHM) 230 fs
Repetition Rate 120 Hz
Undulator Type planar
Undulator Period 3 cm
Peak Field 1.32 T
Undulator Length 120 m

Start-end simulations estimate a saturation length of 70 m for the shortest wavelength of 1.5 Angstrom, with about 200 spikes in the radiation profile (Fig.1). The average power level is 4 GW. The residual part of the undulator after saturation yields a small, but additional amplification because spikes slips into parts of the beam, which haven't been spoiled by the FEL amplification.

Fig.1: LCLS Radiation Power along Undulator

Although the current profile is almost rectangular, the envelope of the radiation profile is not (Fig.2). The reason lies in the longitudinal beam parameters such as energy spread or emittance and the strong local impact of the undulator wakefields, dominated by the finite resistivity of the vacuum chamber.

Fig.2: LCLS Radiation Profile at Saturation

The width of the spectrum is slightly larger than expected by the analytical estimate (twice the rho parameter). Start-end simulations show a significant variation of the central beam energy along the bunch, which results in broadening of the bandwidth.

Fig.3: LCLS Radiation Spectrum at Saturation

The evolution of the radiation profile along the undulator can be seen in this animation.

Another branch of our research for LCLS is the possible extension of LCLS in the future. In particular an enhancement of the output power and the reduction of the pulse length below 10 fs are desirable.