The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today's lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ε > 500 MeV) with an intense laser pulse (a 0 > 10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γ rays), consistent with a quantum description of radiation reaction. The generated γ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy ε crit > 30 MeV.

We review known and discuss new signatures of high-intensity Compton scattering assuming a scenario where a high-power laser is brought into collision with an electron beam. At high intensities one expects to see a substantial redshift of the usual kinematic Compton edge of the photon spectrum caused by the large, intensity-dependent effective mass of the electrons within the laser beam. Emission rates acquire their global maximum at this edge while neighboring smaller peaks signal higher harmonics. In addition, we find that the notion of the center-of-mass frame for a given harmonic becomes intensity dependent. Tuning the intensity then effectively amounts to changing the frame of reference, going continuously from inverse to ordinary Compton scattering with the center-of-mass kinematics defining the transition point between the two.

We demonstrate that charged particles in a sufficiently intense standing wave are compressed toward, and oscillate synchronously at, the antinodes of the electric field. We call this unusual behavior anomalous radiative trapping (ART). We show using dipole pulses, which offer a path to increased laser intensity, that ART opens up new possibilities for the generation of radiation and particle beams, both of which are high energy, directed, and collimated. ART also provides a mechanism for particle control in high-intensity quantum-electrodynamics experiments.

We report on the generation of a narrow divergence (θ ≈ 2.5 mrad), multi-MeV (EMAX = 18 MeV) and ultra-high brilliance (≈ 2 × 10 19 photons s −1 mm −2 mrad −2 0.1% BW) γ-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a0 ≈ 2). The spectrum of the generated γ-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature.The generation of high-quality Multi-MeV γ-ray beams is an active field of research due to the central role of these beams not only in fundamental research [1], but also in extremely important practical applications, which include cancer radiotherapy [2,3], active interrogation of materials [4], and radiography of dense objects [5]. As an example, Giant Dipole Resonances of most heavy nuclei occur in an energy range of [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], exciting photofission of the nucleus.Different mechanisms have been proposed to generate γ-ray beams with these properties, including bremsstrahlung emission, synchrotron emission, and Compton scattering. Bremsstrahlung sources are routinely used for medical applications, and exploit electron beams accelerated by linear accelerators (LINAC) [7]. Laser-driven bremsstrahlung sources, whereby the electron beam is generated via laser-wakefield acceleration (LWFA) [8] have also been recently reported [5,9,10]. However, the relatively broad divergence and source size limit the maximum brightness achievable with this technique and a more promising physical mechanism in this respect has been identified in Compton scattering. Laserdriven electron beams with energy per particle of the order of the GeV are now routinely available in the laboratory [8], allowing for the possibility of all-optical and compact Compton-scattering sources [11,12].Previous investigations of laser-driven Compton scattering have mostly focused on the linear regime, i.e. whenever the dimensionless intensity of the laser pulse is less than 1 (a 0 < 1, whereby a 0 = eE L /(m e ω L c), with E L and ω L being the laser electric field and central frequency, respectively, and m e being the electron rest mass) [13,14] and report on γ-ray energies ranging from a few hundreds of keV [13] up to 3-4 MeV [14]. Three main factors can in principle be modified in order to increase the energy of the generated photons: the electron Lorentz factor (γ e ), the laser photon energy ω L , or the laser intensity a 0 . The mean energy of the generated photons can in fact be estimated as:with f (a 0 ) ≈ 1 for a 0 1 andLiu and collaborators recently reported on an increase in photon energy (up to 8-9 MeV) by frequency converting the scattering laser up to its second harmonic (thus increasing ω L in Eq. 1) [15]. However, using a higher laser frequency for scattering significantly ...

We test current numerical implementations of laser-matter interactions by comparison with exact analytical results. Focusing on photon emission processes, it is found that the numerics accurately reproduce analytical emission spectra in all considered regimes, except for the harmonic structures often singled out as the most significant high-intensity (multiphoton) effects. We find that this discrepancy originates in the use of the locally constant field approximation.