Search

It was shown in 1999 that D2 cluster explosion under ultrashort intense laser irradiation can lead to ion energies sufficient to drive nuclear fusion reactions [Ditmire et al. Nature 398, 491]. We show how the use of molecular clusters allows to further enhance the ionic acceleration, allowing one to reach optimal fusion cross sections. A new low density high energy regime is described, in which fusion occurs via a spallation-like process. The process increases with laser intensity up to the relativistic threshold, at which the neutron production is overwhelmed by gamma production resulting from electron acceleration.

We present experimental results concerning the observation and acceleration of negative and positive ions produced from high‐intensity laser beam interactions with deuterated and hydrogenated clusters. The 30‐fs, 10‐Hz rep. rate, 30 TW Ti‐Sa laser of LOA was focused on clusters produced from a pulsed gas nozzle. The novelty of our experimental work was the gas composition (CD4 and CH4), which was used for the cluster formation. Both kinds of ions, negative and positive, were observed with approximately equal energies. The maximum energy of both kinds of ions as well their energy distribution was measured using a Thomson parabola mass spectrometer. Negative and positive ions of H, D, and C were observed with a maximum energy up to 70 keV for a maximum laser beam intensity of 1018 W/cm2. The mass spectrometer allowed us to correlate both the positive and negative ion energy spectrums with the laser beam intensity in order to study the process of cluster explosions, ion acceleration, and negative ion formation in the fs regime. We summarize the more important and new results as follows: (1) only single positive and negative ions were measured on the mass spectrometer; (2) the number of negative ions is equal to the number of positive ions; (3) similar energy spectrums for positive and negative ions were measured; (4) the positive and the negative ions are accelerated in the same radial direction, outwards from the interaction volume; and (5) similar energy spectrums for positive and negative ions at different laser energies (the laser intensity increases by a factor of 5) were measured. There are two possible “scenarios” concerning the process of negative ion formation. The first process is related with the formation of different ion species during the Coulomb explosion of the clusters. The second process for negative ion formation is the double electron attachment caused by collisions between the positive ions with the backing gas. A number of arguments confirm that the second process is more probable. The fact that from the mass spectrometer measurements the value for the more probable energy of the positive ions is the same as the energy at the maximum of the double electron attachment cross section leads to the second process of negative ion formation. For a complete description of our observation, however, we need an efficient physical process for negative ion formation, which will be the subject of our future work. The process of cluster explosion accelerates the D ions sufficiently to produce neutrons from DD nuclear fusion reactions. The efficiency of both the negative ions and the neutron alternative source will be discussed.

International audience; Following the interaction of superintense, short pulse lasers and plasmas, ions can be accelerated to velocities sufficient to drive nuclear fusion reactions, in particular, by the process of Coulomb explosion of clusters [T. Ditmire et al., Nature (London) 398, 491 (1999)]. We show here how short bursts of neutrons can be produced using a jet of low-density deuterated methane clusters. Ion velocity distributions were simultaneously measured by a Thomson parabola mass spectrometer, demonstrating deuteron energies up to 120 keV. We show that, in such conditions, nuclear fusion will occur not only in the hot plasma core, but also in the cold outer region by collision processes.

A laser–plasma interaction experiment was performed in order to match effective conditions for wake-field electron acceleration. A 30-fs pulse was made to interact with a preformed plasma generated via the exploding foil technique from a 1-μm-thick plastic film. The irradiance of the femtosecond pulse in the plasma was 1020 Wcm−2. The interaction conditions were investigated via imaging and spectroscopy at the fundamental and the second harmonic of the laser frequency, both forward and backward. Our data clearly show that conditions suitable for electron acceleration are achieved close to the propagation axis and can be easily reproduced from shot to shot. In contrast, significant growth of instabilities occurs at the boundaries of the interaction region. These observations are consistent with a preliminary evidence of forward acceleration of high-energy electrons. Optical, X-ray, and γ-ray data obtained for different positions of the foil target with respect to the laser focal plane further support this promising scenario.

International audience; We report recent experimental results obtained at LOA on a femtosecond driven collisional x-ray laser. The experiment has consisted in studying the amplification of the 4d^95p-4d^95d line at 41.8 nm in Pd-like xenon. This lasing emission has been observed to dominate the output spectra when focusing a 20 TW 10Hz Ti: sapphire laser system in a few mm gas cell. The influence of several parameters on the x-ray laser emission is presented. A comparison with high harmonic sources is drawn.