R. Jafer

Effects of front surface plasma expansion on proton acceleration in ultraintense laser irradiation of foil targets

The properties of beams of high energy protons accelerated during ultraintense, picosecond laser-irradiation of thin foil targets are investigated as a function of preplasma expansion at the target front surface. Significant enhancement in the maximum proton energy and laser-to-proton energy conversion efficiency is observed at optimum preplasma density gradients, due to self-focusing of the incident laser pulse. For very long preplasma expansion, the propagating laser pulse is observed to filament, resulting in highly uniform proton beams, but with reduced flux and maximum energy.

Fast-electron transport in cylindrically laser-compressed matter

Experimental and theoretical results of relativistic electron transport in cylindrically compressed matter are presented. This experiment, which is a part of the HiPER roadmap, was achieved on the VULCAN laser facility (UK) using four long pulses beams (~4 × 50 J, 1 ns, at 0.53 µm) to compress a hollow plastic cylinder filled with plastic foam of three different densities (0.1, 0.3 and 1 g cm−3). 2D simulations predict a density of 2–5 g cm−3 and a plasma temperature up to 100 eV at maximum compression. A short pulse (10 ps, 160 J) beam generated fast electrons that propagate through the compressed matter by irradiating a nickel foil at an intensity of 5 × 1018 W cm−2. X-ray spectrometer and imagers were implemented in order to estimate the compressed plasma conditions and to infer the hot electron characteristics. Results are discussed and compared with simulations.

Effects of laser prepulses on laser-induced proton generation

Low-intensity laser prepulses (<10(13) W cm(-2), nanosecond duration) are a major issue in experiments on laser-induced generation of protons, often limiting the performances of proton sources produced by high-intensity lasers (approximate to 10(19) W cm(-2), picosecond or femtosecond duration). Depending on the intensity regime, several effects may be associated with the prepulse, some of which are discussed in this paper: (i) destruction of thin foil targets by the shock generated by the laser prepulse; (ii) creation of preplasma on the target front side affecting laser absorption; (iii) deformation of the target rear side; and (iv) whole displacement of thin foil targets affecting the focusing condition. In particular, we show that under oblique high-intensity irradiation and for low prepulse intensities, the proton beam is directed away from the target normal. Deviation is towards the laser forward direction, with an angle that increases with the level and duration of the ASE pedestal. Also, for a given laser pulse, the beam deviation increases with proton energy. The observations are discussed in terms of target normal sheath acceleration, in combination with a laser-controllable shock wave locally deforming the target surface.

Laser-driven cylindrical compression of targets for fast electron transport study in warm and dense plasmas

Fast ignition requires a precise knowledge of fast electron propagation in a dense hydrogen plasma. In this context, a dedicated HiPER (High Power laser Energy Research) experiment was performed on the VULCAN laser facility where the propagation of relativistic electron beams through cylindrically compressed plastic targets was studied. In this paper, we characterize the plasma parameters such as temperature and density during the compression of cylindrical polyimide shells filled with CH foams at three different initial densities. X-ray and proton radiography were used to measure the cylinder radius at different stages of the compression. By comparing both diagnostics results with 2D hydrodynamic simulations, we could infer densities from 2 to 11 g/cm3 and temperatures from 30 to 120 eV at maximum compression at the center of targets. According to the initial foam density, kinetic, coupled (sometimes degenerated) plasmas were obtained. The temporal and spatial evolution of the resulting areal densities a...

Surface morphology modifications of human teeth induced by a picosecond Nd:YAG laser operating at 532 nm

The interaction of an Nd:YAG laser, operating at 532 nm with 40 ps pulse duration, with human teeth was studied. The results show that teeth were significantly modified at an energy fluence of about 11 J/cm 2 . Various surface morphologies of enamel and dentine were recorded. Features on enamel include crater (conical form) in the central part and cauliflower morphology at the periphery, whereas on dentine the crater looks like a stretched dome between sharp edges. The behavior of the enamel-dentine junction area showed different morphology with respect to both tooth enamel and dentine alone. Finally, the junction channel showed a removal of collagen fibers and the formation of a needle-like bottom structure. Generally, this investigation showed that the picosecond Nd:YAG laser can ablate a tooth surface practically instantaneously, implying that large tooth surfaces can be processed in short time.

Proton radiography of a laser-driven cylindrical implosion

A recent experiment was performed at the Rutherford Appleton Laboratory (UK) to study fast electron propagation in cylindrically compressed targets, a subject of interest for fast ignition. This experiment was performed in the framework of the experimental road map of the Hiper project (the European High Power laser Energy Research facility Project). In this experiment, protons accelerated by a pecosecond laser pulse have been used to radiograph a 220 μm‐diameter, 20 μm‐wall cylinder filled with 0.1 g/cc foam, imploded with ∼200 J of green laser light in 4 symmetrically incident beams of pulse length 1 ns. Point projection proton backlighting was used to measure the compression degree as well as the stagnation time. Results were compared to those from hard X‐ray radiography. Finally, Monte Carlo simulations of proton propagation in the cold and in the compressed targets allowed a detailed comparison with 2D numerical hydro simulations.

X-ray diagnostics of fast electrons propagation in high density plasmas obtained by cylindrical compression

We report on X-ray diagnostics results from an experiment on fast electrons propagation in cylindrically compressed targets. It was performed on the VULCAN TAW laser facility at RAL (UK) using four long pulses (1ns, 70 J each at 2ω) to compress a cylindrical polyimide target filled with CH foam at 3 different initial densities. The cylindrical geometry allows us to reach temperatures and densities higher than those obtained in planar geometry compression. 2D hydrodynamic simulations predicted a core density range from 4 to 8 g/cm3 and a core temperature from 30 eV up to 175 eV at maximum compression. An additional short laser pulse (10 ps, 160 J at ω) was focused on a Ni foil at one of the cylinder edges in order to generate a fast electrons current propagating along the compressed target. A X-ray radiography diagnostic was implemented in order to estimate the core plasma conditions of the compressed cylinder. Moreover two Bragg X-ray spectrometers collected the Kα fluorescence from the target so as to determine the variations of fast electrons population during the compression.

Proton radiography of a cylindrical laser-driven implosion

A recent experiment was performed at the Rutherford Appleton Laboratory (UK) to study fast electron propagation in cylindrically compressed targets, a subject of interest for fast ignition. In this experiment, protons accelerated by a picosecond laser pulse have been used to radiograph a 220 µm diameter cylinder (10 µm wall filled with 0.1 g/cc foam), imploded with _ 200 J of green laser light in 4 symmetrically incident beams of wavelength and pulse length 1 ns. Point projection proton backlighting was used to measure the compression degree as well as the stagnation time. Results were also compared to those from a hard X-ray radiography diagnostics. Finally, Monte Carlo simulations of proton propagation in the cold and in the compressed targets allowed a detailed comparison with 2D numerical hydro simulations.