Beam-Shape Effects in Nonlinear Compton and Thomson Scattering

TL;DR

This paper analyzes how finite-beam geometries and focusing affect the emission spectra in nonlinear Compton and Thomson scattering, distinguishing classical and quantum effects, and proposes realistic parameters for experimental observation.

Contribution

It provides a detailed classical and quantum analysis of emission spectra modifications due to beam geometry and focusing, including a scaling law for spectral density and experimental feasibility.

Findings

Focusing broadens the emission spectrum into a continuum.

Higher harmonics are visible only at moderate focusing levels.

A scaling law aids in averaging over electron phase space.

Abstract

We discuss intensity effects in collisions between beams of optical photons from a high-power laser and relativistic electrons. Our main focus are the modifications of the emission spectra due to realistic finite-beam geometries. By carefully analyzing the classical limit we precisely quantify the distinction between strong-field QED Compton scattering and classical Thomson scattering. A purely classical, but fully covariant, calculation of the bremsstrahlung emitted by an electron in a plane wave laser field yields radiation into harmonics, as expected. This result is generalized to pulses of finite duration and explains the appearance of line broadening and harmonic substructure as an interference phenomenon. The ensuing numerical treatment confirms that strong focussing of the laser leads to a broad continuum while higher harmonics become visible only at moderate focussing, hence lower intensity. We present a scaling law for the backscattered photon spectral density which facilitates averaging over electron beam phase space. Finally, we propose a set of realistic parameters such that the observation of intensity induced spectral red-shift, higher harmonics, and their substructure, becomes feasible.