Focusing effects in laser-electron Thomson scattering

Abstract

We study the effects of laser pulse focussing on the spectral properties of Thomson scattered radiation. Modelling the laser as a paraxial beam we find that, in all but the most extreme cases of focussing, the temporal envelope has a much bigger effect on the spectrum than the focussing itself. For the case of ultra-short pulses, where the paraxial model is no longer valid, we adopt a sub-cycle vector beam description of the field. It is found that the emission harmonics are blue shifted and broaden out in frequency space as the pulse becomes shorter. Additionally the carrier envelope phase becomes important, resulting in an angular asymmetry in the spectrum. We then use the same model to study the effects of focussing beyond the limit where the paraxial expansion is valid. It is found that fields focussed to sub-wavelength spot sizes produce spectra that are qualitatively similar to those from sub-cycle pulses due to the shortening of the pulse with focussing. Finally, we study high-intensity fields and find that, in general, the focussing makes negligible difference to the spectra in the regime of radiation reaction. * cnharvey@physics.org † mattias.marklund@chalmers.se ‡ amol.holkundkar@pilani.bits-pilani.ac.in

In situ characterization of ultraintense laser pulses

“…The spikes coming from each cycle of the laser field are still clearly distinguishable, although their bases are somewhat broadened. (This is due to that fact that electrons further from the central axis will see a field of lower intensity that those at the centre [39] and this, as we have seen in Figure 6, will effect the angle of emission.) We also note that we only see seven cycles in this plot, compared to 10 for the equivalent plane wave case.…”

Radiation reaction in electron–beam interactions with high-intensity lasers

“…For example, the fact that electron beam energy spectra are expected to broaden due to stochastic effects makes the variance of the spectrum, σ 2 , an attractive signature of the quantum nature of radiation reaction [59,61]. However, such broadening can occur classically in the interaction of an electron beam with a focussed laser pulse, because components of the beam can encounter different intensities and therefore lose different amounts of energy [156]. Thus a crucial role is played by the initial energy spread and size of the incident electron beam [3].…”

Laser pulsing in linear Compton scattering

“…The main adjustments are to modulate the vector potential because of the electron orbit through the three dimensional photon pulse structure and to include the arrival time variation of the individual electrons. A common incident photon spectrum for all of the electrons is no longer possible [26]. Our present intent is to undertake a more general code including such improvements and to publish calculations, including benchmarks, in a future publication.…”