The generation of high-energy neutrons using laser-accelerated ions is demonstrated experimentally using the Titan laser with 360 J of laser energy in a 9 ps pulse. In this technique, a short-pulse, high-energy laser accelerates deuterons from a CD 2 foil. These are incident on a LiF foil and subsequently create high energy neutrons through the 7 Li(d,xn) nuclear reaction (Q ¼ 15 MeV). Radiochromic film and a Thomson parabola ion-spectrometer were used to diagnose the laser accelerated deuterons and protons. Conversion efficiency into protons was 0.5%, an order of magnitude greater than into deuterons. Maximum neutron energy was shown to be angularly dependent with up to 18 MeV neutrons observed in the forward direction using neutron time-of-flight spectrometry. Absolutely calibrated CR-39 detected spectrally integrated neutron fluence of up to 8 Â 10 8 n sr À1 in the forward direction.

Short pulse laser interactions at intensities of 2×10(21) W cm(-2) with ultrahigh contrast (10(-15)) on submicrometer silicon nitride foils were studied experimentally by using linear and circular polarizations at normal incidence. It was observed that, as the target decreases in thickness, electron heating by the laser begins to occur for circular polarization leading to target normal sheath acceleration of contaminant ions, while at thicker targets no acceleration or electron heating is observed. For linear polarization, all targets showed exponential energy spreads with similar electron temperatures. Particle-in-cell simulations demonstrate that the heating is due to the rapid deformation of the target that occurs early in the interaction. These experiments demonstrate that finite spot size effects can severely restrict the regime suitable for radiation pressure acceleration.

The formation of a high-energy ion beam suitable for driving nuclear fusion reactions and producing MeV neutrons was investigated. The interaction of intense ultra short laser pulses with a double-layer thin foil for the production of MeV deuterons was studied theoretically and numerically simulated using a two-dimensional electromagnetic particle-in-cell model. The directionality and energy of the deuteron beam, specifically the conversion efficiency of laser energy into deuteron kinetic energy, and deuteron energy and angular distribution functions are studied as a function of peak laser intensity, laser pulse duration and target thickness. A range of parameters was determined for which a highly directional deuteron beam is generated.

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The dynamics of Xe clusters with initial radius between 10 and 100 Å irradiated by an IR subpicosecond laser pulse is investigated. The evolution of the cluster is modeled with a relativistic time-dependent three-dimensional particle simulation model. The focus of this investigation is to understand the energy absorption of clusters and how the absorbed energy is distributed among the various degrees of freedom. The consequence of the initial cluster radius on the absorbed energy, average charge per atom, mean electron and ion energies, ionization, removal of electrons from the cluster, and cluster expansion was studied. The absorbed energy per cluster scales as N 5/3 , and the mean electron and ion energies scale as N 1/3 and N 2/3 , respectively ͑N is the number of atoms per cluster͒. A significant fraction of the absorbed energy ͑ϳ90% ͒ is converted into kinetic energy with comparable contribution to electrons and ions. The energy balance suggests that smaller clusters are more efficient as radiators, while larger clusters are more conducive to particle acceleration. The radiation yield of clusters with initial radius 20-50 Å irradiated by a laser with peak intensity 10 16 W/cm 2 is determined to be 1%-2%.

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2 aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

A relativistic time-dependent three-dimensional particle simulation model has been developed to study the interaction of intense ultrashort KrF (248 nm) laser pulses with small Xe clusters. The trajectories of the electrons and ions are treated classically according to the relativistic equation of motion. The model has been applied to a different regime of ultrahigh intensities extending to 10(21) W/ cm(2). In particular, the behavior of the interaction with the clusters from intensities of approximately 10(15) W/cm(2) to intensities sufficient for a transition to the so-called "collective oscillation model" has been explored. At peak intensities below 10(20) W/cm(2), all electrons are removed from the cluster and form a plasma. It is found that the "collective oscillation model" commences at intensities in excess of 10(20) W/cm(2), the range that can be reached in stable relativistic channels. At these high intensities, the magnetic field has a profound effect on the shape and trajectory of the electron cloud. Specifically, the electrons are accelerated to relativistic velocities with energies exceeding 1 MeV in the direction of laser propagation and the magnetic field distorts the shape of the electron cloud to give the form of a pancake.

A new method for solving the electron Boltzmann equation in spatially inhomogeneous, steady state plasmas by using a multi-term approximation of the expansion of the electron velocity distribution function is presented. This method is a generalization of that approach which has been recently developed to solve the inhomogeneous kinetic equation using the conventional two-term approximation. The objective of using the multi-term approximation is to improve the accuracy of the spatial relaxation treatment of the electrons and to analyse the impact of higher order terms on the relaxation behaviour of both the electron velocity distribution function and the relevant macroscopic electron quantities. In this paper the higher order approximation is used to study the spatial relaxation of plasma electrons in a constant electric field. The investigations are performed for a model plasma, varying in particular some atomic data of the electron collision processes in this model, and for real plasmas. A detailed comparison of the results relevant to the isotropic and the first anisotropic component of the electron velocity distribution function which have been obtained in the multi-term approximation and in the conventional two-term approximation is made. Based on such studies the accuracy already reached concerning the spatial relaxation process by the simpler two-term approximation is critically evaluated.