(a) Typical examples of the spectral profiles of the 13th harmonic averaged over 30 shots (upper), the single-shot profile of SASE pulse at 61.7 nm (middle) and the single-shot profile of the FEL pulse seeded by the 13th-order harmonic (lower)

<p><strong>Figure 2.</strong> (a) Typical examples of the spectral profiles of the 13th harmonic averaged over 30 shots (upper), the single-shot profile of SASE pulse at 61.7 nm (middle) and the single-shot profile of the FEL pulse seeded by the 13th-order harmonic (lower). (b) Typical examples of the spectral profiles of the 15th-order harmonic averaged over 30 shots, the single-shot profile of SASE pulse at 53.5 nm and the single-shot profile of the FEL pulse seeded by the 15th-order harmonic.</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>