Centre Etudes Nucléaires de Bordeaux Gradignan

The production of high energy electrons, protons and heavy ions was observed with ultra high intensity (UHI), 50 joule class lasers a few years ago. This discovery has triggered strong interest for applications in various domains: accelerators, nuclear waste management, medicine, radioisotopes production and inertial fusion. At that time CEA, CNRS and the University Bordeaux 1 have decided to coordinate their efforts in academic research in anticipation of the Megajoule laser installation near Bordeaux. It is in this context that the CENBG group, which was already working at the interface of nuclear and atomic physics, has decided to explore the new possibilities offered by these UHI lasers. Tremendous progress in laser technology and especially the use of the Chirped Pulse Amplification (CPA) allowed the installation of UHI ‘table-top’ lasers. The following figure shows a typical electron energy distribution obtained with such a laser (1 J, 30 fs) at the facility of the Laboratoire d’Optique Appliquée (LOA) in Palaiseau [1]. The laser was focused on a 10 μm thick mylar target with an intensity reaching I=10¹⁹ W/cm².

Electron energy distribution obtained with
a 1 J, 30 fs laser focused (10 µn spot).

The high energy part of the spectrum is characteristic of a Boltzman distribution with a ‘hot temperature’ T_h. The electrons can be converted into photons (by Bremsstrahlung) in order to induce photonuclear reactions. In the presence of very high particle fluxes, these photonuclear reactions are efficiently used to characterize the total electron yield as well as their angular distribution. It has been shown that the high energy electrons are emitted in the forward direction within a cone of only a few degrees.
This experiment proved that a femtosecond laser pulse can generate relativistic electrons, in sufficient quantities to induce photonuclear reactions: 10⁴ fissions/pulse in a 2mm thick uranium target or 10⁵ (γ ,n) reactions in a 4-mm thick copper foil. These activation measurements, combined with GEANT simulations, led to a total amount of 10⁸ electrons above 5 MeV produced in one pulse. This figure is expected to be drastically improved in the future with higher intensity lasers and the optimization of the laser-matter interaction, especially through the control of the pre-plasma due to the CPA.
The large amount of electrons accelerated out of the target gives rise to a tremendous space charge and as a consequence, protons are also extracted and propagate with the moving electrons. Based on activation measurements the intensities of the two beams seem to be correlated.
The opportunity of producing short pulses (less than 1 ns) of high energy particles and the possibility of generating a time synchronized mm³ size plasma open new possibilities for nuclear physics studies. Nuclear excitations in plasmas are being investigated with the kHz laser at the CEntre des Lasers Intenses et Applications (CELIA) facility at Bordeaux [2]. Laser produced proton beams have been used to induce (p,n) reactions on copper plasma at the Laboratoire pour l’Utilisation des Lasers Intenses (LULI) at Palaiseau [3]. These first steps in nuclear excitation studies with UHI lasers now available at LOA, LULI , IOQ Jena or CELIA open the route towards experiments planned with the petawatt laser which should be in operation in 2010 at the Megajoule installation near Bordeaux.

References :

[1] G. Malka et al., Phys. Rev. E. 66 (2002) 066402
[2] F. Gobet et al., Rev. Sci. Inst. 77,093302 (2006)
[3] M. Tarisien et al., European Conf. on Laser Interactions with Matter, Madrid (2006)