Abstract
In a laser-pumped x-ray free-electron laser (FEL) an intense laser field replaces the magnetic wiggler field of a conventional FEL. Depending on the intensity and quality of both the electron beam and pump laser, the Thomson backscattered radiation can be coherently amplified. In a conventional FEL the generation of x rays requires electron beam energies in the multi-GeV range. In a laser-pumped x-ray FEL, electron beam energies in the multi-MeV range would be sufficient. To generate coherent x rays with this mechanism a number of physics and technology issues must be addressed. Foremost among these are the stringent requirements placed on the electron beam quality and brightness as well as on the pump laser. The seed radiation for the laser-pumped FEL is the laser-induced spontaneous radiation. The evolution of incoherent radiation into coherent radiation as well as the power gain lengths associated with the coherent x rays are analyzed and discussed. There is excellent agreement between our analytical results and GENESIS simulations for the radiated power, gain length, conversion efficiency, linewidth, and saturation length. These issues, as well as others, necessary to achieve coherent amplified x rays in a laser-pumped FEL are discussed. While a coherent x-ray source would have a number of attractive features, the requirements placed on both the electron beam and pump laser are extremely challenging.
Highlights
The free-electron laser (FEL) can, in principle, generate coherent, polarized, short pulses of x rays for numerous applications in research
While a coherent x-ray source would have a number of attractive features, the requirements placed on both the electron beam and pump laser are extremely challenging
The power gain length, saturation length, linewidth, and conversion efficiency have been calculated for the laser-pumped FEL
Summary
The free-electron laser (FEL) can, in principle, generate coherent, polarized, short pulses of x rays for numerous applications in research. Analysis of stimulated emission from relativistic electrons interacting with an electromagnetic pump was presented and discussed in [27] This analysis was limited to the low-gain, thermal beam regime. The requirements on the electron beam quality and the pump laser power are demanding, for x-ray generation Since these early studies there have been a number of papers that have considered employing electromagnetic pumps in FELs [29,30,31,32,33]. For electron beams of sufficiently high quality, with energies of $6 MeV and peak currents of 500 A, we find that coherent x rays at 20 A# can be generated with power gain lengths of $300 m, saturation lengths of $0:4 cm, and conversion efficiencies of $0:01%. The pump laser and electron beam propagate in opposite directions along the z axis
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