F. Albert

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.

Collimated and Ultrafast X-Ray Beams from Laser-Plasma Interactions

We show that different schemes can be now followed to produce collimated X-ray radiation using laser systems. By focusing intense femtosecond laser light onto a gas jet, electrons of the plasma can be manipulated to generate ultrafast (femtosecond) X-ray radiation in the forward direction along the laser axis. In this chapter we discuss nonlinear Thomson scattering, betatron emission and Compton scattering. In years to come, the rapid development of laser technology will provide more intense laser systems. We can expect to see the creation of bright X-ray beams with a high degree of collimation (< 1 mrad divergence), as well as even shorter pulse durations, down to attosecond time scales. Such sources will provide multidisciplinary scientific communities with unique tools to probe and excite matter.

All optical ultrafast synchrotron hard x‐ray source

Collimated beams of radiation can now be generated in the X‐ray spectral range using laser systems. These new tools may provide efficient probing radiation for the analysis of dense plasmas and ultrafast atomic dynamics phenomena in the matter. Using relativistic laser‐matter interaction, we have shown that X‐ray radiation can be emitted within a collimated 20 mrad cone, in the spectral range of few keV, and with the additional unique properties to be ultrafast (100 fs timescale) and perfectly synchronized with the driving laser system.

Ultrafast X-ray and hard X-ray sources from relativistic laser-matter interaction

The presentation will show that collimated X-ray radiation can now be generated from laser experiments. 20 mrad beams at few keV X-ray energies have been observed from the betatron oscillation of energetic electrons in a plasma wiggler. Further developments will be highlighted. The production of collimated beams of x-ray radiation has been prohibited for a long time since laser systems did not provide the required intensities for that purpose. High order harmonics from atomic ensembles can deliver collimated radiation, but it is limited to the UV and XUV range. Emission from Kα or thermal atomic lines in hot plasmas can reach the x-ray range, but they are fully divergent. Different strategies can be followed to generate collimated X-ray beams, and particularly by wiggling the electrons accelerated from laser-plasma experiments in a laser field, in a plasma wiggler, as well as in a permanent magnet undulator. The presentation will describe these 3 different cases and discuss the experimental results obtained up to now as well as the potentiality of each scheme. Figure 1 shows that X-ray radiation from plasma wiggling can be produced within a collimated 20 mrad cone, in the spectral range of few keV, and with the additional unique properties to be ultrafast (0.1 ps timescale) and perfectly synchronised with the driving laser system. Users from multidisciplinary fields, and in particular for inertial fusion experiments, will find in this conceptually new tool an efficient way to explore and reveal the dense properties of the matter. In this scheme, the ponderomotive force of an intense femtosecond laser pulse can generate --as it propagates in an underdense plasma-a large amplitude wake-field plasma wave. This wave can break, trap plasma electrons, and its large electrostatic field can then accelerate ultrashort pulse duration electron beams to high energies (few hundreds of MeV) over only a millimeter distance scale. The wake has an electron-density depression right behind the laser pulse, leading to the formation of an ion column. This charge displacement results in a strong radial electrostatic field. As the relativistic electrons propagate through these fields, they can undergo oscillations, called betatron oscillations. Like in conventional synchrotron, this oscillatory relativistic transverse motion produces a collimated x-ray beam. However, because the wavelength of the wiggler can be much shorter in a laser-plasma interaction (micrometer scalelength) than in a synchrotron based on permanent magnets (centimeter scalelength), the distance required to produce a bright x-ray beam is much shorter, on the scale of millimeters, and the required energy of the electron beam is also much lower (MeV, rather than GeV). Figure 1: X-ray CCD image of the keV X-ray beam produced from the betatron oscillation of energetic electron beam in a plasma wiggler. References 1) K. Ta Phuoc et al, Phys. Rev. Lett. 19(91), 1950014 (2003) 2) K. Ta Phuoc et al, Physics of Plasmas 12, 023101-8 (2005) 3) A. Rousse et al, Phys. Rev. Lett. 93, 135005 (2004) 4) S. Kiselev et al, Phys. Rev. Lett. 93, 135004 (2004) QTuC3-3-INV