Quasi-Phase Amplification of High-Order Harmonic Generation As a Method for Producing Seed Radiation for X-Ray Free-Electron Lasers

The efficiency and temporal coherence of X-ray free-electron laser radiation can be significantly improved using coherent radiation as a seed. Coherent radiation can be obtained as a result of nonlinear optical frequency conversion of mid-IR femtosecond laser radiation in gas media. A method for increasing the efficiency of high-order harmonic generation due to the effect of quasi-phase matching in media in the form of a set of gas jets is described. It is shown that the harmonic generation efficiency can be significantly increased in a narrow spectral range, whose position depends on the interaction parameters. In this case, the spatial distribution of amplified radiation is a Gaussian.