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The generation of high-order harmonics of femtosecond lasers in rare gas media has recently emerged as a promising tool to produce bright atto- and femtosecond vacuum-ultraviolet and soft x-ray pulses. Theoretical simulations on the High-order Harmonic Generation (HHG) process allow for optimizing experimental sources and for testing seeding of Free Electron Lasers (FELs) with pulses from HHG. In this contribution, Rhyno, a simulation tool to numerically investigate the HHG process, will be presented. The approach is based on the so called Lewenstein model (strong field approximation). The code first calculates the microscopic nonlinear dipole moment of the gas atoms induced by the driving laser field. From this, it computes the macroscopic formation of the harmonics and their propagation through the gas medium taking into account important effects such as self focusing or self defocusing and phase-matching. The program allows for the simulation of HHG signals produced by very short driving laser pulses in the femtosecond range and down to only a few optical cycles. Examples of typical Rhyno outputs (spectra and pulse lengths) will be presented. Rhyno was furthermore used to perform simulations on FELs initiated by a HHG seeding pulse. These simulations predicted that HHG seeding of FELs is not only possible, but can also be used to shorten the experimental setup and to improve the FEL output towards shorter nearly bandwidth-limited pulses with a reproducible FEL spectrum. A comparison to a very recent experimental proof of these predictions as performed at the Spring8 facility in Riken, Japan, will be discussed. |
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