Superradiant parametric x-ray emission

We compute a spectrum of parametric x-ray radiation inside a crystal from a bunch of electrons, which is periodically modulated in density. We consider that the bunch of electrons is exiting from an x-ray free-electron laser (XFEL) channel. We demonstrate that in the case of a resonance between the frequency of parametric x-ray radiation and a frequency of modulation of an electron bunch, the sequence of strong quasimonochromatic x-ray pulses is formed—superradiant parametric x-ray emission (SPXE) with frequencies that are multiples of the modulation frequency. The number of photons in the pulse of SPXE in the case of extremely asymmetric diffraction is comparable with the photon number in the pulse of an XFEL. Moreover, the SPXE is directed under the large angle to the electron velocity and every harmonic in the spectrum is emitted under its own angle.

Figure 1

Qualitative scheme of the processes which lead to the generation of SPXE pulses in the XFEL channel.

Figure 2

Scheme of the photon generation in the case of PXR-EAD geometry. $L_b\sim {10}^{-4}\mathrm{cm}$ is the modulated bunch length, $L_{\mathrm{a}\mathrm{b}\mathrm{s}}\sim {10}^{-2}\mathrm{cm}$ is the radiation absorption length.

Figure 3

The incoherent and two harmonics of the coherent pseudophotons spectral densities. The inset demonstrates the zoomed-in peak structure.

Figure 4

The geometry for the dynamical diffraction theory of SPXE.