Generating Single-Spike Hard X-Ray Pulses with Nonlinear Bunch Compression in Free-Electron Lasers

A simple method for generating single-spike hard x-ray pulses in free-electron lasers (FELs) has been developed at the Linac Coherent Light Source (LCLS). This is realized by nonlinear bunch compression using 20-pC bunch charge, demonstrated in the hard x-ray regime at 5.6 and 9 keV, respectively. Measurements show about half of the FEL shots containing a single-spike spectrum. At 5.6-keV photon energy, the single-spike shots have a mean pulse energy of about $10\mu \mathrm{J}$ with 70% intensity fluctuation and the pulse full width at half maximum is evaluated to be at 200-as level.

Figure 1

A sketch of nonlinear bunch compression at the LCLS. At the top is a layout of the LCLS, with main linac $S$-band sections (L1S, L2, L3), one $X$-band linac section (L1X), two bunch compressor chicanes (BC1 and BC2), a dog-leg beam line (DL2), and an undulator. The bottom plots show simulated longitudinal phase space and current profile after BC2 (a), L3 exit (b), and undulator entrance (c), and FEL simulation results, including power profile (d), spectrum (e) and Wigner transformation of the FEL field (f). The FEL photon energy in this simulation is 5.6 keV. Bunch head is to the left.

Figure 2

(left) Spectral measurement using the MIR prism spectrometer versus L2 phase at linear (top, L1X at 19 MV) and nonlinear (bottom, L1X at 12 MV) compression. (right) Two reconstructed current profile examples at minimum achieved pulse duration for linear and nonlinear compression. Electron beam energy was 14 GeV, bunch charge was 20 pC.

Figure 3

Histogram of the number of spikes (left) and measured spectral example (right) at 9 keV versus the L1X voltage: (top) 19 MV; (middle) 15 MV and (bottom) 12 MV. Each setting used 2000 shots. Electron beam energy was 14 GeV, bunch charge was 20 pC.

Figure 4

(left) Histogram of the number of spectral spikes based on 8400 shots. (right) Ten recorded shots of the sorted single-spike x-ray spectra (at 5.6 keV). Electron beam energy was 11.5 GeV, bunch charge was 20 pC. L1X amplitude was 13 MV.