Innershell femtosecond x-ray lasers pumped by Larmor radiation and characteristics of Larmor radiation

High-repetition-rate, femtosecond x-ray lasers would be useful for dynamical study of ultra-fast phenomena in nature. One of routes to get fs x-ray lasers is to use inner-shell processes in atomic and ionic systems. In this paper, the two inner-shell schemes recently proposed will be reviewed and compared in detail. One of important issues using inner-shell schemes is fast and intense x-ray pumping source. One of good candidate sources for that purpose is Larmor radiation produced by electrons under an intense fs laser field. The relativistic, nonlinear Thomson scattering by an electron of an intense laser field is investigated in computer simulation. Under a laser field with a pulse duration of 20 fs Full Width Half Maximum and an intensity of 1020 W/cm2, the motion of an electron is highly relativistic and generates an ultra-short radiation of 2 attoseconds with photon energies of 100 to 600 eV. An interesting modulated structure of the spectrum is observed and analyzed. A radiation produced by the zigzag motion of an electron under a linearly polarized laser has better characteristics than by a helical motion under a circularly polarized laser pulse in terms of an angular divergence and an energy spectrum.