International audienceWe demonstrate that a beam of x-ray radiation can be generated by simply focusing a single high-intensity laser pulse into a gas jet. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate in the ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a femtosecond pulse of synchrotron radiation, which has keV energy and lies within a narrow (50 mrad) cone angle

, "Quasi-monoenergetic and tunable X-rays from a laser-driven Compton light source" (2013). Donald Umstadter Publications. 92.

Ghebregziabher, I.; Maharjan, C.; Liu, Cheng; Golovin, Grigory V.; Banerjee, Sudeep; Zhang, J.; Cunningham, N.; Moorti, A.; Clarke, S.; and Pozzi, Sara, "MeV-Energy X Rays from Inverse Compton Scattering with Laser-Wakefield Accelerated Electrons" (2013). Donald Umstadter Publications. 87.

A century ago, J. J. Thomson 1 showed that the scattering of low-intensity light by electrons was a linear process (i.e., the scattered light frequency was identical to that of the incident light) and that light's magnetic field played no role. Today, with the recent invention of ultra-high-peakpower lasers 2 it is now possible to create a sufficient photon density to study Thomson scattering in the relativistic regime. With increasing light intensity, electrons quiver during the scattering process with increasing velocity, approaching the speed of light when the laser intensity approaches 10 18 W/cm 2 . In this limit, the effect of light's magnetic field on electron motion should become comparable to that of its electric field, and the electron mass should increase because of the relativistic correction. Consequently, electrons in such high fields are predicted to quiver nonlinearly, moving in figure-eight patterns, rather than in straight lines, and thus to radiate photons at harmonics of the frequency of the incident laser light 3−9 , with each harmonic having its own unique angular distribution 5−7 . In this letter, we report the first ever direct experimental confirmation of these predictions, a topic that has previously been referred to as nonlinear Thomson scattering 7 . Extension of these results to coherent relativistic harmonic generation 10,11 may eventually lead to novel table-top x-ray sources.In this experiment, we used a laser system that produces 400-fs-duration laser pulses at 1.053-µm wavelength with a maximum peak power of 4 TW. The 50-mm diameter laser beam was focused with an f/3.3 parabolic mirror onto the front edge of a supersonic helium gas jet. The focal spot is consisted of a 7-µm FWHM Gaussian spot (containing 60 % of the total energy) and a large (> 100 µm) dim spot. The helium gas was fully ionized by the foot of the laser pulse. A half-wave plate was used to rotate the axis of linear polarization of the laser beam in order to vary the azimuthal angle (φ) of observation. We define θ = 0• as along the direction opposite to that of the laser propagation and φ = 0• as along the axis of linear polarization. In a linearly polarized laser field, electrons move in a figure-eight trajectory lying in the plane defined by the axis of linear polarization and the direction of beam propagation.While the observation of harmonics in laser-plasma (or electron beam) interactions has been made by several groups 12−16 , that alone is insufficient to unambiguously identify nonlinear Thomson scattering and its underlying dynamics. Several other mechanisms might generate continuum or harmonics under our experimental conditions, and, therefore, need to be isolated and discriminated from the signal generated by nonlinear Thomson scattering: (1) continuum generated from self-phase modulation of laser beam in gas, (2) harmonics generated from atomic nonlinear susceptibility of gas or, especially, from the ionization process 17 , (3) continuum generated from (a) (relativistic) self-phase modulation of laser ...