Fig. 1. Schematic drawing of HHG and its beam transportation system for a seeded EUV-FEL. HH pulses are generated in a xenon gas cell with a lens (focal length: 4 m) and separated from the fundamental pulses of the Ti:S laser by the first SiC mirror. A pair of Pt-coated concave mirrors with an 8 m curvature radius is used for the loose focusing HH pulses. The seeding HH pulses are selectively reflected by the second SiC mirror, and fully overlap the electron bunch at the front-end of the first undulator (Undulator 1).

Fig. 2. Schematic drawing of the HHG-seeded FEL system with a timing drift control. This seeded FEL system consists of an SCSS FEL machine (C-band accelerator, magnetic chicane and in-vacuum undulators), a Ti:S laser system (which is the common laser pulse source for an HHG seeding system) and an EOS-based arrival-timing monitor. The EOS arrival-timing monitor is installed before the first undulator. Utilizing the EO probe pulse optically split from the HHG-driving laser pulse, the arrival time difference of the seed laser pulse and the electron bunch is under control and is fixed at the optimal seeding condition. The spectra and pulse energy of the seeded FEL are measured with a single-shot spectrometer and a gas monitor detector at the end of the beamline, respectively. Insets: the spatial profile of the seeding pulse and the electron bunch on a phosphor screen are measured with microchannel plates at the entrance and end of the first undulator. Temporal overlap is roughly checked by a streak camera at sweep ranges from 1 ns down to 50 ps.

Fig. 3. Principle of EOS measurement in the manner of spectral decoding. The probe pulse is linearly chirped and acts as a carrier wave for spectral decoding EOS. In addition, the use of a linear-chirped laser pulse with a flat-top spectrum to probe an ultrafast EO crystal (ZnTe) makes it possible to characterize the temporal bunch charge distribution precisely in real time. The adaptive AO modulator (Fastlite: Dazzler HR45-650-1100) is able to shape both the spectral phase and the intensity distributions of laser pulses with broadband spectra. The EO crystal is set near the electron beam. The linear polarization of the carrier wave changes into an elliptic polarization mainly due to the Pockels effect in the EO crystal under the electric field of the electron bunch. The information on the electron bunch charge distribution is encoded as the intensity modulation in the spectrum, and decoded bunch-by-bunch by a multichannel spectrometer.

Fig. 4. (a) The trigger delay time added to the Candox delay unit by using the home-built feedback system. (b) The relative arrival time of the electron bunch with respect to the optical laser pulse. Both (a) and (b) were measured for 5 h simultaneously.

Fig. 5. Comparison of the typical spectra of FEL pulses with (red line) and without (blue line) seeding HH pulses. The spectral bandwidth of the seeded FEL pulse was 0.06 nm (FWHM).

Fig. 6. Correlation data plot between the normalized intensity and the spectral peak intensity with seeded operation. Here, is the standard deviation of the peak intensities without the HH pulses under the seeding condition of the FEL. Effective seeded FEL pulses are defined as being large as 4 (red) for our user experiments.

Fig. 7. Trend graph of peak intensities of 5,000 FEL pulses in the seeded operations with (blue points) and without (red points) HH pulses. Experimental results of the experiment with feedback in 2012 are shown. The contrast ratio of the peak intensity was improved by a factor of . In 2010, the seeded FEL pulse energy was ^[12]. During our experiments, we achieved a pulse energy of up to at maximum.

Fig. 8. Expected pulse energies of the HHG-seeded HGHG FEL plotted as a function of the photon energy for different harmonic numbers. The harmonics of HGHG were calculated up to the seventh order. In this calculation, the photon energy of the seeding HH pulse was tuned from 30 to 100 eV, with a beam size of 100 m (RMS), pulse length of 5 fs (FWHM) and maximum pulse energy of 15 nJ. The conditions of the seeding HH pulses are feasible even in continuum HHG to realize a wavelength-tunable seeded FEL in the soft x-ray water window region.