Observation of redshifting and harmonic radiation in inverse Compton scattering

Inverse Compton scattering of laser photons by ultrarelativistic electron beam provides polarized x- to $\ensuremath{\gamma}$-ray pulses due to the Doppler blueshifting. Nonlinear electrodynamics in the relativistically intense linearly polarized laser field changes the radiation kinetics established during the Compton interaction. These are due to the induced figure-8 motion, which introduces an overall redshift in the radiation spectrum, with the concomitant emission of higher order harmonics. To experimentally analyze the strong field physics associated with the nonlinear electron-laser interaction, clear modifications to the angular and wavelength distributions of x rays are observed. The relativistic photon wave field is provided by the ps ${\mathrm{CO}}_{2}$ laser of peak normalized vector potential of $0.5l{a}_{L}l0.7$, which due to the quadratic dependence of the strength of nonlinear phenomena on ${a}_{L}$ permits sufficient effects not observed in past 2nd harmonic study with ${a}_{L}\ensuremath{\approx}0.3$ laser [, Phys. Rev. Lett. 96, 054802 (2006)]. The angular spectral characteristics are revealed using $K$-, $L$-edge, and high energy attenuation filters. The observation indicates existence of the electrons' longitudinal motion through frequency redshifting understood as the mass shift effect. Thus, the 3rd harmonic radiation has been observed containing on-axis x-ray component that is directly associated with the induced figure-8 motion. These are further supported by an initial evidence of off-axis 2nd harmonic radiation produced in a circularly polarized laser wave field. Total x-ray photon number per pulse, scattered by 65 MeV electron beam of 0.3 nC, at the interaction point is measured to be approximately ${10}^{9}$.