The results of the numerical simulation of the temporal profile of the seeded FEL output pulses when a 13th-order harmonic pulse are introduced into the undulator(s) to interact with an electron bunch whose durations are (a) 0

<p><strong>Figure 4.</strong> The results of the numerical simulation of the temporal profile of the seeded FEL output pulses when a 13th-order harmonic pulse are introduced into the undulator(s) to interact with an electron bunch whose durations are (a) 0. 2 ps, (b) 0.6 ps and (c) 1 ps. The solid and dashed lines represent respectively the temporal profiles obtained using one undulator (4.5 m length) and those obtained using two undulators (9 m length in total).</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>