R. Fitour

Femtosecond x rays from laser-plasma accelerators

Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent, and femtosecond radiation. In this article, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons are reviewed. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications.

Demonstration of the ultrafast nature of laser produced betatron radiation

This Letter aims to demonstrate the ultrafast nature of laser produced betatron radiation and its potential for application experiments. An upper estimate of the betatron x-ray pulse duration has been obtained by performing a time-resolved x-ray diffraction experiment: The ultrafast nonthermal melting of a semiconductor crystal (InSb) has been used to trigger the betatron x-ray beam diffracted from the surface. An x-ray pulse duration of less than 1ps at full width half-maximum (FWHM) has been measured with a best fit obtained for 100fs FWHM.

Analysis of wakefield electron orbits in plasma wiggler

In relativistic laser plasma interaction, electrons can be simultaneously accelerated and wiggled in an ion cavity created in the wake of an intense short pulse laser propagating in an underdense plasma. As a consequence of their motion, the accelerated electrons emit an intense x-ray beam called laser produced betatron radiation. Being an emission from charged particles, the features of the betatron source are directly linked to the electrons trajectories. In particular, the radiation is emitted in the direction of the electrons velocity. In this article we show how an image of electrons orbits in the wakefield cavity can be deduced from the structure of x-ray spatial profiles.

Full characterization of a laser-produced keV x-ray betatron source

This paper presents the complete characterization of a kilo-electron-volt laser-based x-ray source. The main parameters of the electron motion (amplitude of oscillations and initial energy) in the laser wakefield have been investigated using three independent methods relying on spectral and spatial properties of this betatron x-ray source. First we will show studies on the spectral correlation between electrons and x-rays that is analyzed using a numerical code to calculate the expected photon spectra from the experimentally measured electron spectra. High-resolution x-ray spectrometers have been used to characterize the x-ray spectra within 0.8–3 keV and to show that the betatron oscillations lie within 1 µm. Then we observed Fresnel edge diffraction of the x-ray beam. The observed diffraction at the center energy of 4 keV agrees with the Gaussian incoherent source profile of full width half maximum <5 µm, meaning that the amplitude of the betatron oscillations is less than 2.5 µm. Finally, by measuring the far field spatial profile of the radiation, we have been able to characterize the electrons trajectories inside the plasma accelerator structure with a resolution better than 0.5 µm.

Imaging electron trajectories in laser wakefield cavity using betatron x-ray radiation

We demonstrate that betatron x-ray radiation provides a direct imaging of electrons trajectories accelerated in laser wakefields. Electron excursions down to 0.7 ¿m ± 0.2 ¿tm have been measured in our parameter regime.

Control and mapping of x-ray emission in a laser-plasma accelerator

We show that the control and the mapping of the x-ray emission reveals unique features of the laser-plasma accelerator physics, including strong correlations between electron and x-ray beams, and density-dependence of electron injection position.

Temporal characterization and flux improvement of the x-ray betatron source

In this paper, we present the first temporal characterization of betatron X-ray radiation. Results obtained from time resolved x-ray diffraction experiments, for which the ultra fast phase transition of non thermal melting of InSb was used, indicates that the x-ray pulse duration is less than 1 ps. We then propose a novel technique to improve the spectral and flux properties of the x-ray source. The energy and the flux can be enhanced when the electron beam propagates and oscillates in a tailored plasma density profile.