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Results are presented from laser-plasma interaction experiments using the VULCAN laser at the Rutherford Appleton Laboratory. Wire targets were used to elucidate the role of the return currents generated by the relativistic electron beam leaving the target at laser intensities up to 5x10(19) W cm(-2). For some shots an additional wire or a foil was placed near the target wire. In other shots, a foil was used as the target with a wire behind. Three main observations were made: (i) Z-pinch behavior in the wires due to the return currents, (ii) optical transition radiation (OTR) at the second harmonic of the laser, and (iii) proton emission. The OTR and the proton emission were observed from both the primary wire target and the adjacent wire. The OTR emission is associated with electron bunches at twice the laser frequency due to ponderomotive JxB acceleration by the laser. The proton emission from the adjacent target was likely due to field emission of electrons by the large potential produced from charging of the primary wire target. The observations agree with simulations using the three-dimensional tree code PEPC and the two-and-one-half-dimensional particle-in-cell code OSIRIS. (C) 2004 American Institute of Physics.

Energetic protons up to 30 MeV have been measured from high intensity laser interactions (⩽5×1019 W/cm2) with solid density plasmas. Up to 1012 protons (> 2 MeV) were observed at the rear of thin aluminum foil targets and measurements of their angular deflection were made. Similar energies were measured from ions produced from the front of the foils. Nuclear activation and track detector measurements suggest that the protons measured at the rear originate from the front surface of the target and are bent by large magnetic fields that exist in the plasma interior, which are likely generated by a laser-produced beam of fast electrons.

An experimental study of optical probing of a dense z-pinch plasma using the MAGPIE (mega-ampere generator for plasma implosion experiments) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)] is reported. The generator was operated with a peak current of 1.1 MA rising in 150 ns (10%–90%). The loads were 33 μm diam carbon fibers. Faraday rotation was used to investigate the distribution of the current flowing in the plasma. A measurable Faraday rotation angle was observed only in a time window from 50 to 60 ns after the current start, due to the fact that this effect depends on a combination of the magnetic-field strength and electron number density. A new type of self-referencing cyclic radial shear interferometer was used to evaluate the plasma density profiles which are necessary for the reconstruction of the current distribution. It was calculated that ∼110 kA was flowing in the plasma at 52 ns after the current start. Shadowgraphy was used to study the dynamics of the plasma and to ...

The interaction of a 1053 nm picosecond laser pulse with a solid target has been studied for focused intensities of up to 1019 W cm-2. The maximum ion energy cutoff Emax (which is related to the hot electron temperature) is in the range 1.0-12.0 MeV and is shown to scale as Emax≈I1/3. The hot electron temperatures were in the range 70-400 keV for intensities up to 5 × 1018 W cm-2 with an indication of a high absorption of laser energy. Measurements of x-ray/y-ray bremsstrahlung emission suggest the existence of at least two electron temperatures. Collimation of the plasma flow has been observed by optical probing techniques. ©7997 American Institute of Physics.

The influence of femtosecond laser pulse chirp on laser-generated longitudinal acoustic strains in Si (100) monocrystal substrates is studied. Degenerate femtosecond pump-probe transient reflectivity measurements are performed using a layered structure of thin Ti transducer film on an Si substrate. Experimental results show that acoustic strains, manifested as strong Brillouin oscillations, are more effectively induced when negatively chirped femtosecond laser pulses pump the transducer. These results are theoretically supported by a modified thermo-mechanical model based on the combination of a revised two-temperature model and elasticity theory that takes into account the instantaneous frequency of the chirped femtosecond laser pump pulses.

An all-optical approach to laser-proton acceleration enhancement is investigated using the simplest of target designs to demonstrate application-relevant levels of energy conversion efficiency between laser and protons. Controlled deposition of laser energy, in the form of a double-pulse temporal envelope, is investigated in combination with thin foil targets in which recirculation of laser-accelerated electrons can lead to optimal conditions for coupling laser drive energy into the proton beam. This approach is shown to deliver a substantial enhancement in the coupling of laser energy to 5-30 MeV protons, compared to single pulse irradiation, reaching a record high 15% conversion efficiency with a temporal separation of 1 ps between the two pulses and a 5 μm-thick Au foil. A 1D simulation code is used to support and explain the origin of the observation of an optimum pulse separation of ∼1 ps. © 2014 Author(s).

The angular distribution of photoelectrons produced by laser‐induced four‐photon excitation in gold metalic targets is reported. Laser‐target interaction time effects have been investigated by using two Nd:YAG lasers emitting at 1.06 μm pulses of 35 ps and 20 ns duration. Both lasers produced four‐photon electron angular distributions of 9°±2° full width at half maximum. This width was independent from the intensity of the picosecond laser up to the breakdown threshold at a laser intensity of 1.4 GW/cm2. In contrast, the distribution produced by the nanosecond pulses was broadened at intensity levels higher than 60 MW/cm2, which is indicative of a significant contribution from thermionic emission. These results are compared to those obtained by D. Charalambidis, E. Hontzopoulos, C. Fotakis, Gy. Farkas, and Cs. Toth [J. Appl. Phys. 65, 2843 (1989)], for single photon excitation by using a KrF excimer laser.

The three dimensional spatiotemporal response of thin metal films surfaces excited by nanosecond laser pulses is investigated in both the thermoelastic and the ablation regimes. An experimental laser whole-field interferometric technique allows for the direct monitoring of the dynamic deformation of a macroscopic area on the surface with ultrahigh lateral resolution. A specially developed three dimension finite element model simulates the laser-surface interaction, predicts the experimentally obtained results, and computes key parameters of matter's thermomechanical response. This method provides a powerful instrument for spatiotemporal behavior of thin-film surfaces under extreme conditions demanded for innovative applications.

Measurements of the bidirectional plasma jets that form at the surface of a solid target during a laser-generated driven magnetic reconnection are presented. Resistivity enhancement of at least 25× the classical Spitzer value is required when applying the Sweet–Parker model of reconnection to reconcile the experimentally observed reconnection time scale. Analytic calculations show that a fast reconnection model, which includes a priori the effects of microturbulent resistivity enhancement, better reproduces the experimental observations.

Reduction of proton acceleration in the interaction of a high-intensity, piosecond laser with a 50-μm aluminum target was observed when 0.1-6 μm of plastic was deposited on the back surface (opposite side of the laser). The maximum energy and number of energetic protons observed at the back of the target were greatly reduced in comparison to pure aluminum and plastic targets of the same thickness. This is attributed to the effect of the interface between the layers. Modeling of the electron propagation in the targets using a hybrid code showed strong magnetic-field generation at the interface and rapid surface heating of the aluminum layer, which may account for the results. © 2006 American Institute of Physics.

Two experiments studying the interaction of high intensity laser pulses (1×1019–5×1020W∕cm2) with underdense plasma are compared. The experiments used lasers that differed in power and focused intensity but had similar pulse duration (∼1ps). Spectroscopic measurements of the forward scattered light (sidebands) near the fundamental laser frequency produced by the self-modulation instability were performed and the energies of electrons accelerated in the interaction are measured and compared. It is found that at high intensities the sideband intensities and the electron energies were not directly correlated, implying that relativistic plasma wave generation is not the most important mechanism for electron acceleration in the ultrahigh intensity regime. Simulation results for the forward scattered spectrum agree well with experimental results.