S. D. Baton

Inhibition of fast electron energy deposition due to preplasma filling of cone-attached targets

We present experimental and numerical results on the propagation and energy deposition of laser-generated fast electrons into conical targets. The first part reports on experimental measurements performed in various configurations in order to assess the predicted benefit of conical targets over standard planar ones. For the conditions investigated here, the fast electron-induced heating is found to be much weaker in cone-guided targets irradiated at a laser wavelength of 1.057μm, whereas frequency doubling of the laser pulse permits us to bridge the disparity between conical and planar targets. This result underscores the prejudicial role of the prepulse-generated plasma, whose confinement is enhanced in conical geometry. The second part is mostly devoted to the particle-in-cell modeling of the laser-cone interaction. In qualitative agreement with the experimental data, the calculations show that the presence of a large preplasma leads to a significant decrease in the fast electron density and energy flux...

Filamentation in long scale length plasmas: Experimental evidence and effects of laser spatial incoherence

Signatures of filamentation have been observed in preformed plasmas using complementary diagnostics: time‐resolved images of the transmitted laser light, dark field imaging, and time‐resolved spectra of Raman light. This last diagnostic clearly shows the presence of small channels inside the plasma with temporal evolution in agreement with the formation of filaments. The filamentary structures disappeared when random phase plates were used in the laser beam. This result is in agreement with a theoretical analysis showing that filamentation does not grow when the speckle size is smaller than the perturbation wavelength which maximizes, in the coherent case, the filamentation growth.

Physics issues for shock ignition

The paper presents theoretical analysis and experimental results concerning the major physical issues in the shock-ignition approach. These are the following: generation of a high amplitude shock in the imploding target, laser–plasma interaction physics under the conditions of high laser intensities needed for high amplitude shock excitation, symmetry and stability of the shock propagation, role of fast electrons in the symmetrization of the shock pressure and the fuel preheat. The theoretical models and numerical simulations are compared with the results of specially designed experiments on laser plasma interaction and shock excitation in plane and spherical geometries.

Stimulated Brillouin scattering in picosecond time scales: Experiments and modeling

This paper presents an experimental and theoretical study of stimulated Brillouin scattering (SBS) in laser produced plasma using a laser pump with a duration of 8–10 psec. The experiments were performed in a preformed plasma to minimize the flow velocity and have the same plasma conditions over a large range of laser intensities. The reflectivity was then compared to theoretical results over an intensity range of 1013–2×1015 W/cm2. A short pulse was used so that the SBS was in the temporally growing regime and saturation was not an issue.

The Generation and Transport of Large Currents in Dense Materials: The Physics of Electron Transport Relative to Fast Ignition

Abstract This paper reviews the physics of extremely high current propagation in dense materials. We consider explicitly the problem of the generation of high-current, high-particle energy propagation arising from laser ionization in otherwise neutral targets. The paper concentrates upon the recent experimental results of measurements of the distribution of the laser-generated fast electrons, both in space as well as in energy. The emphasis is primarily to put into physical context the growing number of experimental observations under widely varying conditions. Little or no effort is made to summarize the theoretical or modeling work because of manuscript size limitations; however, when possible, experimental observations are tied to relevant attempts to model the observed behavior. The fundamental conclusion is that fast electron propagation, at a current density and kinetic energy relevant to fast ignition, is far from a solved problem and that target design for fast ignition will have to play a significant role to overcome some of the emerging physical obstacles.

Subfemtosecond, coherent, relativistic, and ballistic electron bunches generated at ω0 and 2ω0 in high intensity laser-matter interaction

Harmonics of the laser light have been observed from the rear side of solid targets irradiated by a laser beam at relativistic intensities. This emission evidences the acceleration of subfemtosecond electron bunches by the laser pulse in front of the target. These bunches emit coherent transition radiation (CTR) when passing through the back surface of the target. The spectral features of the signal recorded for targets of thicknesses up to several hundred microns are consistent with the electrons being accelerated by both the laser electric field—via vacuum heating and/or resonance absorption,—and the v×B component of the Lorentz force. The spatial study of the radiation shows that the relativistic electrons causing the CTR radiation are coherent and propagate ballistically through the target, originating from a source with a size of the order of the laser focal spot.

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.

Strong absorption, intense forward-Raman scattering and relativistic electrons driven by a short, high intensity laser pulse through moderately underdense plasmas

The propagation of a short and intense laser pulse (1.057 μm, 350 fs, 1017 W/cm2–2×1019 W/cm2) through preformed undercritical plasmas (≈5%–40% of nc) has been experimentally investigated on the 100-TW laser facility at the Laboratoire pour l’Utilisation des Lasers Intenses. The transmission and reflection of the 1 μm laser pulse, the forward- and backward-Raman (respectively, F-SRS and B-SRS) scattered light and the emission of fast electrons are reported. Significant absorption occurs in these plasmas, which is found to increase with the laser intensity. B-SRS is strongly driven at 1017 W/cm2 and gradually decreases at higher intensities. It is shown that the transmission is low and only weakly dependent on the laser intensity. In contrast, the forward Raman scattering continuously increases with the laser intensity, up to 7% of the incident energy at 2×1019 W/cm2 in the lowest density case. The relativistic electrons accelerated in the forward direction appear to be correlated with the F-SRS. The exper...

Proton radiography of laser-driven imploding target in cylindrical geometry

An experiment was done at the Rutherford Appleton Laboratory (Vulcan laser petawatt laser) to study fast electron propagation in cylindrically compressed targets, a subject of interest for fast ignition. This was performed in the framework of the experimental road map of HiPER (the European high power laser energy research facility project). In the experiment, protons accelerated by a picosecond-laser pulse were used to radiograph a 220 μm diameter cylinder (20 μm wall, filled with low density foam), imploded with ∼200 J of green laser light in four symmetrically incident beams of pulse length 1 ns. Point projection proton backlighting was used to get the compression history and the stagnation time. Results are also compared to those from hard x-ray radiography. Detailed comparison with two-dimensional numerical hydrosimulations has been done using a Monte Carlo code adapted to describe multiple scattering and plasma effects. Finally we develop a simple analytical model to estimate the performance of prot...

Fast electron propagation in high density plasmas created by shock wave compression

We present one of the first results of relativistic laser intensities of the transport of fast electrons in high density and warm plasmas. The fast electrons are produced by the interaction of 40 J, 1 ps, 5 × 1019 W cm−2 laser pulses with solid foil targets. A 200 J, 1.5 ns laser focalized over a 500 µm diameter zone on the opposite side of the foil is used to create a shock propagating through and compressing the target to 2-3 times its solid density before the relativistic interaction. For both the solid and the compressed cases, the fast electron transport divergence and range are investigated, via the Kα emission from an embedded copper layer, for a conducting (aluminium) and an insulating (plastic) target material.