Experimental Demonstration of a Soft X-Ray Self-Seeded Free-Electron Laser

The Linac Coherent Light Source has added a self-seeding capability to the soft x-ray range using a grating monochromator system. We report the demonstration of soft x-ray self-seeding with a measured resolving power of 2000–5000, wavelength stability of ${10}^{-4}$, and an increase in peak brightness by a factor of 2–5 across the photon energy range of 500–1000 eV. By avoiding the need for a monochromator at the experimental station, the self-seeded beam can deliver as much as 50-fold higher brightness to users.

Figure 1

Schematic of the SXRSS system (not to scale). Shot noise from randomly distributed electrons generates $1–5\mu \mathrm{J}$ of relatively wide bandwidth SASE radiation in the first string of undulators (grey line). The seeding monochromator, consisting of a grating ($G$), three mirrors ($M1,M2,M3$), and an adjustable slit, selects a small bandwidth (green line) while the electron chicane directs the electron bunch around the monochromator and resets the electron beam to shot noise. Finally, the overlap diagnostics (BOD10 and BOD13) coalign electrons and monochromatic x rays in the second half of the FEL. Pulse energies are shown for a nominal electron bunch with a charge of 150 pC, and duration of 100 fs, but other parameter ranges are also possible.

Figure 2

Computer aided design model of the final SXRSS design showing both the optics housing ($\text{Grating}/M1$, Slit, and $M2/M3$) and the electron chicane. In this figure, electrons move right to left to show the details of the optics chambers. The overlap diagnostics are further downstream.

Figure 3

Comparison of seeded (solid red line) and SASE (dashed blue line) spectra at 930 eV for 10 000 shot averages, and seeded simulation (sim.) averaged over 30 shots (dotted-dashed black line). Both seeded and SASE spectra use the slotted foil to produce a 50 fs beam [19]. Seeded spectra are taken after undulator #25 and with the slit retracted. SASE spectra use an optimized configuration with all undulators. Inset shows the fraction of FEL energy contained within an integrated bandwidth when seeding.

Figure 4

Gain length scans showing SASE growth before the seeding monochromator (blue squares, three scans) and seeding growth afterwards (red circles, three scans). During seeded operation, undulator #1 is detuned to keep pulse energy on the grating below $5\mu \mathrm{J}$ (blue star). Black line shows simulated seeding. Green boxes show undulators inserted during seeding; undulator #8 is removed to protect the grating, and undulators #9 and #16 were replaced by the self-seeding chicanes. Measured gain lengths for SASE ($\sim 2\pm 0.2\mathrm{m}$) and seeding ($\sim 1.7\pm 0.2\mathrm{m}$) match simulations. All measurements taken with the gas detector.

Figure 5

Jitter of the seeded mode for 50 fs pulse at 930 eV. (a) A comparison of 20 single-shot seeded (solid light red lines) and SASE (dashed grey lines) spectra shows fluctuations in both peak brightness and central seeded wavelength. Bold lines show average performance for both seeding and SASE. (b) A “mustache” scatter plot shows the FEL pulse energy from the gas detector (GDet) vs measured electron bunch energy for individual shots at 930 eV. (Average electron energy is 4.3 GeV.) Histogram at bottom shows the jitter of the central electron beam energy. The seeding monochromator was set slightly below the average electron energy to match the best lasing condition.