Observation of Time-Domain Modulation of Free-Electron-Laser Pulses by Multipeaked Electron-Energy Spectrum

We present the experimental demonstration of a new scheme for the generation of ultrashort pulse trains based on free-electron-laser (FEL) emission from a multipeaked electron energy distribution. Two electron beamlets with energy difference larger than the FEL parameter $\rho$ have been generated by illuminating the cathode with two ps-spaced laser pulses, followed by a rotation of the longitudinal phase space by velocity bunching in the linac. The resulting self-amplified spontaneous emission FEL radiation, measured through frequency-resolved optical gating diagnostics, reveals a double-peaked spectrum and a temporally modulated pulse structure.

Figure 1

(a) Dependence of the fringe separation on the energy distance between the electron packets for (a) SPARC case, $\lambda =800\mathrm{nm}$, (b) $\lambda =30\mathrm{nm}$, (c) $\lambda =0.15\mathrm{nm}$. Spectrum (b) and temporal shape (c) of the pulse, obtained by GENESIS 1.3, for $N=2$ electron packets (blue, dark curve) and $N=4$ (red, light curve) and simulation parameters in the optical range. The width of the fringes scales as $1/N$.

Figure 2

Single-shot two-color spectra taken with the fiber spectrometer (resolution: 1.2 nm at 800 nm).

Figure 3

Top row: (a) experimental electron phase space and (b) current profiles in the case of beam A. Bottom row: same as top row but for beam B.

Figure 4

Top row, beam A: (a) FROG spectral signal (red, light curve) and spectrometer signal (blue curve, presenting an offset of 10 nm with respect to the FROG trace). (b) Temporal pulse $E$ from FROG (blue, dark line) and start to end simulation by T-STEP and GENESIS 1.3 (violet dotted curve). Bottom row: same as for beam A but for beam B. Wavelength interdistances are evaluated between the arrows.

Figure 5

Top row: (a) experimental, (b) retrieved, and (c) simulated FROG traces in the case of beam A. Bottom row: same as for beam A but for beam B. FROG near-infrared GRENOUILLE: time-bandwidth product $<10$; spectral resolution 0.7 nm at 800 nm; single-shot sensitivity $1\mu \mathrm{J}$.