Generation of coherent two-color pulses at two adjacent harmonics in a seeded free-electron laser

The growing requirements of pump-probe techniques and nonlinear optics experiments greatly promote the studies of two-color free-electron lasers (FELs). We propose a new method to generate coherent two-color pulses in a high-gain harmonic generation (HGHG) FEL. In this scheme, an initial tilted electron beam is sent though the modulator and dispersive section of an HGHG FEL to generate the bunching at harmonics of the seed laser. Then a transverse gradient undulator (TGU) is adopted as the radiator and in such radiator, only two separated fractions of the tilted beam will resonate at two adjacent harmonics of the seed laser and are enabled to emit the coherent two-color pulses simultaneously. The time separation between the two pulses are on the order of hundreds of femtoseconds, and can be precisely controlled by varying the tilted amplitude of the electron beam and/or the transverse gradient of the TGU radiator. Numerical simulations confirm the validity and feasibility of this scheme in the extreme ultraviolet waveband.

Figure 1

Schematic of the proposed scheme for the two-color FEL generation.

Figure 2

The 4th and 5th harmonics’ bunching factors of the electron beam at the exit of the dispersive section.

Figure 3

The time structure (a) and the spectrum (b) of the two-color FEL pulses at the exit of the TGU radiator.

Figure 4

The transverse distribution of the two-color pulses.

Figure 5

The FEL power as a function of the rms seed laser radius. The average energy modulation amplitude is fixed to be $A=10$.

Figure 6

The variations of the time separation (a) and the radiation power (b) of the two-color pulses with the tilted amplitude $\eta$. The transverse gradient is $\alpha =150{\mathrm{m}}^{-1}$.

Figure 7

The variations of the time separation (a) and the radiation power (b) of the two-color pulses with the transverse gradient $\alpha$. The tilted amplitude is $\eta =11.7$.