High-Gain Harmonic-Generation Free-Electron Laser Seeded by Harmonics Generated in Gas

The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (${\lambda }_{\mathrm{rad}}=133\mathrm{nm}$) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators.

Figure 1

Schematic of the SPARC layout. Gun: RF photoinjector (SLAC/BNL/UCLA 1.6 cell S-band RF). AS: Accelerating sections. QT: triplet of quadrupoles. D: dipole. U: undulator section. SM: spherical mirrors, PM: periscope mirrors. Spectrometer: normal incidence grating imaging the variable entrance slit on an UV grade CCD camera (Versarray, 1300B-Princeton Instruments).

Figure 2

Measured spectra of the seed laser, SASE and seeded FEL. The seed spectrum (dashed red line) is multiplied by a factor 10. The SASE spectra in single shot (dotted black line) and averaged over 100 shots (dot-dashed black line) are multiplied by a factor 100. The continuous blue line represents the seeded FEL spectrum averaged over 100 shots. Electron beam parameters are shown in Table (a).

Figure 3

Calculated beam transverse dimensions ${\sigma }_x$ (solid line) and ${\sigma }_y$ (dashed line) vs $z$ along the undulator in the configuration with three modulator sections ($K=1.59$) and three radiators ($K=0.507$) ($3M/3R$). The matching is maintained over the first three modulator sections.

Figure 4

Energy (nJ) vs relative linewidth (%) for 100 experimental shots in the configurations: $3M/3R$ (black squares), $4M/2R$ (red diamonds) and $5M/1R$ (blue circles). Electron beam: Table (b)

Figure 5

Cascaded FEL pulse energy and bandwidth at 133 nm at the end of the undulator in the $5M/1R$, $4M/2R$ and $3M/3R$ configurations; experimental data (black squares) and simulations by GENESIS 1.3 (red stars). Data averaged over 100 shots. Error bars represent $\pm 1$ standard deviation. Electron beam parameter of Table (b). Seed energy: $40\pm 10\mathrm{nJ}$. Simulation data: current $I=49\pm 6\mathrm{A}$ and beam energy $E=176.2\pm 0.35\mathrm{MeV}$ [similar to those of Table (b)], emittance ${ϵ}_{x,y}=0.9\pm 0.25\mathrm{mm}\mathrm{mrad}$ (estimate of the slice parameters based on a 80% charge cut) and energy spread $\Delta E/E={10}^{-4}\pm {10}^{-5}$ (minimum slice energy spread along the longitudinal bunch coordinate). The images in (a),(b),(c) correspond to single shot spectra acquisitions in the various configurations.

Figure 6

Radiation power (solid line, a.u.) and phase profiles (dotted line) on the left side, and $e$-beam phase space (energy $\mathcal{E}$ vs phase $\mathfrak{z}$) in the highlighted region at the end of the six undulator sections, on the right side. (a) Configuration $5M/1R$, (b) $4M/2R$ and (c) $3M/3R$. Simulation with PERSEO [33].