The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself-the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of 4 × 10 20 W=cm 2 ). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.
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 ...
A novel design for a compact gamma-ray spectrometer is presented. The proposed system allows for spectroscopy of high-flux multi-MeV gamma-ray beams with MeV energy resolution in a compact design. In its basic configuration, the spectrometer exploits conversion of gamma-rays into electrons via Compton scattering in a low-Z material. The scattered electron population is then spectrally resolved using a magnetic spectrometer. The detector is shown to be effective for gamma-ray energies between 3 and 20 MeV. The main properties of the spectrometer are confirmed by Monte Carlo simulations.
We report on the first demonstration of passive all-optical plasma lensing using a two-stage setup. An intense femtosecond laser accelerates electrons in a laser wakefield accelerator (LWFA) to 100 MeV over millimeter length scales. By adding a second gas target behind the initial LWFA stage we introduce a robust and independently tunable plasma lens. We observe a density dependent reduction of the LWFA electron beam divergence from an initial value of 2.3 mrad, down to 1.4 mrad (rms), when the plasma lens is in operation. Such a plasma lens provides a simple and compact approach for divergence reduction well matched to the mm-scale length of the LWFA accelerator. The focusing forces are provided solely by the plasma and driven by the bunch itself only, making this a highly useful and conceptually new approach to electron beam focusing. Possible applications of this lens are not limited to laser plasma accelerators. Since no active driver is needed the passive plasma lens is also suited for high repetition rate focusing of electron bunches. Its understanding is also required for modeling the evolution of the driving particle bunch in particle driven wake field acceleration.
Temporal overlapping of ultra-short and focussed laser pulses is a particularly challenging task, as this timescale lies orders of magnitude below the typical range of fast electronic devices. Here we present an optical technique that allows for the measurement of the temporal delay between two focussed and ultra-short laser pulses. This method is virtually applicable to any focussing geometry and relative intensity of the two lasers. Experimental implementation of this technique provides excellent quantitative agreement with theoretical expectations. The proposed technique will prove highly beneficial for high-power multiple-beam laser experiments.
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