Two-Color Radiation Generated in a Seeded Free-Electron Laser with Two Electron Beams

We present the experimental evidence of the generation of coherent and statistically stable two-color free-electron laser radiation obtained by seeding an electron beam double peaked in energy with a laser pulse single spiked in frequency. The radiation presents two neat spectral lines, with time delay, frequency separation, and relative intensity that can be accurately controlled. The analysis of the emitted radiation shows a temporal coherence and a shot-to-shot regularity in frequency significantly enhanced with respect to the self-amplified spontaneous emission.

Figure 1

The electron phase space composed of two beamlets $a$ and $b$ compared to the laser chirped distribution (c). The higher-energy beamlet (a) appears to be fragmented in two parts, due to the space charge effects. Only the zone seeded by the laser takes part to the radiation process. Stars show the regions where the FEL emission is supposed to take place according to the experimental results.

Figure 2

Optical transport setup for the seeding laser. The laser intensity on the horizontal (H) polarization is attenuated with coupling a half-wave plate (HWP) and a splitting polarizer cube (SPC). The plate rotates the polarization and the splitting cube selects only the useful component for the interaction with the electron beam. In addition, a linear step filter (LSF) allows more attenuation steps. Finally, two spherical mirrors focus the laser, with an expected size $\sigma \approx 600\mu \mathrm{m}$, at the center of the first undulator module, where it interacts with the electron beam. With a Rayleigh length $z_r=0.5\mathrm{m}$, the interaction in the next undulator modules can be neglected.

Figure 3

Spectral intensity $I_{\lambda }$ versus wavelength acquired with the fiber spectrometer. The average spectral intensity $⟨I_{\lambda }⟩$ from 100 shots is shown. Seeded FEL spectra are compared for different values of the total seed-laser energy per pulse at the interaction point: (a) $E_0=15\mathrm{nJ}$, (b) $E_0=125\mathrm{nJ}$, and (c) $E_0=810\mathrm{nJ}$. The SASE spectrum (red line) is also pointed out.

Figure 4

Spectral intensity vs wavelength. Sequence of shots without (a) and with (b) seed. Pink lines, average wavelength $⟨{\lambda }_{1,2}⟩$; pink triangles, peak values of each shot.

Figure 5

FROG traces (a) raw, (b) retrieved and (c) simulated with genesis 1.3 using a two energy electron beam with parameters similar to Table 1.

Figure 6

Sequence of the selected FROG time distributions. The pink triangles are the peaks of the distributions.

Figure 7

Comparison between the position of the experimental FEL pulses (blue continuous line) and the two electron beamlets (a) and (b) in time (left) and energy (right) distribution. The seed-laser profile (c) is shown (not in scale). All ordinate scales, showing the intensity, are in arbitrary units except for the current profile $I(A)$ given in ampere. The experimental FEL pulse was measured with FROG and a spectrometer. The magenta dashed curve is the semianalytical simulation made with the parameters of Tables 1 and 2.