Observation of redshifting and harmonic radiation in inverse Compton scattering

Abstract

Inverse Compton scattering of laser photons by ultrarelativistic electron beam provides polarized x-to γ-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 CO 2 laser of peak normalized vector potential of 0.5 < a L < 0.7, 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 ≈ 0.3 laser [M. Babzien et al., 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 .

Single shot, double differential spectral measurements of inverse Compton scattering in the nonlinear regime

“…This divergence is of course eliminated in practice by homogeneous broadening effects. The predictions of this straightforward, heuristic scattering model are consistent with the results of a full L-W calculation [22], as displayed in Fig. 5, which uses the maximum photon energy and redshift parameters encountered in the scenario of Figs.…”

“…3. As can be seen from the data and the predictions of the particle trackingbased, Liénard-Wiechert (L-W) simulations [22] of the experimental scenario, utilizing Gaussian laser intensity and electron beam density profiles (whose physical significance is examined below), one observes a very large bandwidth spectrum at a given θ y , even for the less energetic, lower a 0 case. The details of the wavelength spectral shape at constant θ y revealed in this DDS image are central to the current investigation, and are analyzed further in the following discussion.…”

“…The methodology employed here, as well as the push to considerably higher laser intensities (∼2.5 times larger than those of Ref. [22]), thus provides a much more detailed look into the electrodynamic processes involved in nonlinear inverse Compton scattering than previously obtained. As such, this investigation has, importantly, introduced a powerful new experimental method for understanding the intense laser-relativistic electron beam interaction, and also in the process also shed considerable light on the spectral structure of ICS in this limit, which is highly relevant to applications.…”

“…[22], with the notable exception of an increase in laser peak power. In the experiments reported here the electron beam parameters were: ∼3 ps rms pulse length, ∼1.3 mm-mrad normalized emittance, 0.3 nC total charge, and E 0 ¼ 65 MeV.…”

“…Pursuant to the above described physics motivations, a recent experimental study performed at the Brookhaven National Laboratory Accelerator Test Facility (BNL ATF) has examined the relevant aspects of the nonlinear ICS interaction [22], with cases where a L ∼ 0.6 showing strong evidence of both redshifting, as well as harmonic scattering up to third order. These experiments [23,24] employed metallic foils that exploit K-edge absorption to produce low-pass and bandpass filters.…”