Metrology of high-order harmonics for free-electron laser seeding

In English We examine the characteristics of high-order harmonics generated with 800 nm, 25 mJ, 160 fs laser pulses in an Ar gas cell with the objective of seeding a free electron laser. We measure the energy per pulse and per harmonic, the energy jitter, the divergence and the position stability of the harmonic beam. We perform ab initio numerical simulations based on integration of the time-dependent Schrodinger equation and of the wave equation within the slowly varying envelope approximation. The results reproduce the experimental measurements to better than a factor of two. The interaction of a frequency comb of harmonic fields with an electron bunch in an undulator is examined with a simple model consisting of calculating the energy modulation owing to the seed-electron interaction. The model indicates that the undulator acts as a spectral filter selecting a given harmonic.