Spectral distribution of the free electron laser (FEL) pulses at 53.4 nm obtained with the 15th-order harmonic seed pulses after accumulating the date for 3000 shots of FEL

<p><strong>Figure 3.</strong> Spectral distribution of the free electron laser (FEL) pulses at 53.4 nm obtained with the 15th-order harmonic seed pulses after accumulating the date for 3000 shots of FEL. In the presentation of the 3000 spectra recorded during the period of 100 s, the spectra of 30 consecutive shots are averaged, and the 100 averaged spectra are plotted.</p> <p><strong>Abstract</strong></p> <p>By introducing 13th- (61.7 nm) and 15th-order harmonics (53.4 nm) of femtosecond laser pulses at 800 nm into an undulator of SCSS (SPring-8 Compact SASE Source) test accelerator at RIKEN, these harmonic pulses were amplified by a factor of more than 10<sup>2</sup> with a high contrast ratio through the interaction between accelerated electron bunches and the harmonic pulses. From numerical simulations of the amplification processes of high-order harmonic pulses in the undulator, optimum conditions of the electron bunch duration interacting with the high-order harmonic pulses were investigated for generating full-coherent and intense pulses in the extreme ultraviolet wavelength region.</p>