Spatial and Transient Effects during the Amplification of a Picosecond Pulse Beam by a Nanosecond Pump.

Amplification of a picosecond pulse beam by a lower intensity nanosecond pulse beam was experimentally observed in a flowing plasma. Modifications of intensity distributions in beam focal spots due to nonhomogeneous energy transfer and its transient regime were investigated. The mean transferred power reached 57% of the incident power of the nanosecond pulse beam. An imaging diagnostic allowed the intensity profile of the picosecond pulse beam to be determined, bringing to evidence the spatial nonuniformity of energy transfer in the amplified beam. This diagnostic also enabled us to observe the temporal evolution of the speckle intensity distribution because of the transfer. These results are reproduced by numerical simulations of two complementary codes. The method and the observed effects are important for the understanding of experiments with multiple crossing laser beams in plasmas.

Experimental Investigation of the Collective Raman Scattering of Multiple Laser Beams in Inhomogeneous Plasmas.

Experiments have been performed evidencing significant stimulated Raman sidescattering (SRS) at large angles from the density gradient. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating for the first time this multiple-beam collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. These findings obtained in plasmas conditions relevant of inertial confinement fusion experiments similarly apply to the more complex geometry of these experiments where anomalously large levels of SRS were measured.

Experimental approach to interaction physics challenges of the shock ignition scheme using short pulse lasers.

An experimental program was designed to study the most important issues of laser-plasma interaction physics in the context of the shock ignition scheme. In the new experiments presented in this Letter, a combination of kilojoule and short laser pulses was used to study the laser-plasma coupling at high laser intensities for a large range of electron densities and plasma profiles. We find that the backscatter is dominated by stimulated Brillouin scattering with stimulated Raman scattering staying at a limited level. This is in agreement with past experiments using long pulses but laser intensities limited to 2×10(15) W/cm2, or short pulses with intensities up to 5×10(16) W/cm2 as well as with 2D particle-in-cell simulations.

Exploring the saturation levels of stimulated Raman scattering in the absolute regime.

This Letter reports new experimental results that evidence the transition between the absolute and convective growth of stimulated Raman scattering (SRS). Significant reflectivities were observed only when the instability grows in the absolute regime. In this case, saturation processes efficiently limit the SRS reflectivity that is shown to scale linearly with the laser intensity, and the electron density and temperature. Such a scaling agrees with the one established by T. Kolber et al. [Phys. Fluids B 5, 138 (1993)10.1063/1.860861] and B Bezzerides et al. [Phys. Rev. Lett. 70, 2569 (1993)10.1103/PhysRevLett.70.2569], from numerical simulations where the Raman saturation is due to the coupling of electron plasma waves with ion waves dynamics.

Enhanced propagation for relativistic laser pulses in inhomogeneous plasmas using hollow channels.

The influence of long (several millimeters) and hollow channels, bored in inhomogeneous ionized plasma by using a long pulse laser beam, on the propagation of short, ultraintense laser pulses has been studied. Compared to the case without a channel, propagation in channels significantly improves beam transmission and maintains a beam quality close to propagation in vacuum. In addition, the growth of the forward-Raman instability is strongly reduced. These results are beneficial for the direct scheme of the fast ignitor concept of inertial confinement fusion as we demonstrate, in fast-ignition-relevant conditions, that with such channels laser energy can be carried through increasingly dense plasmas close to the fuel core with minimal losses.

Laser-initiated primary and secondary nuclear reactions in Boron-Nitride.

Nuclear reactions initiated by laser-accelerated particle beams are a promising new approach to many applications, from medical radioisotopes to aneutronic energy production. We present results demonstrating the occurrence of secondary nuclear reactions, initiated by the primary nuclear reaction products, using multicomponent targets composed of either natural boron (B) or natural boron nitride (BN). The primary proton-boron reaction (p + (11)B → 3 α + 8.7 MeV), is one of the most attractive aneutronic fusion reaction. We report radioactive decay signatures in targets irradiated at the Elfie laser facility by laser-accelerated particle beams which we interpret as due to secondary reactions induced by alpha (α) particles produced in the primary reactions. Use of a second nanosecond laser beam, adequately synchronized with the short laser pulse to produce a plasma target, further enhanced the reaction rates. High rates and chains of reactions are essential for most applications.

CR-39 track detector calibration for H, He, and C ions from 0.1-0.5 MeV up to 5 MeV for laser-induced nuclear fusion product identification.

Laser-accelerated ion beams can be used in many applications and, especially, to initiate nuclear reactions out of thermal equilibrium. We have experimentally studied aneutronic fusion reactions induced by protons accelerated by the Target Normal Sheath Acceleration mechanism, colliding with a boron target. Such experiments require a rigorous method to identify the reaction products (alpha particles) collected in detectors among a few other ion species such as protons or carbon ions, for example. CR-39 track detectors are widely used because they are mostly sensitive to ions and their efficiency is near 100%. We present a complete calibration of CR-39 track detector for protons, alpha particles, and carbon ions. We give measurements of their track diameters for energy ranging from hundreds of keV to a few MeV and for etching times between 1 and 8 h. We used these results to identify alpha particles in our experiments on proton-boron fusion reactions initiated by laser-accelerated protons. We show that their number clearly increases when the boron fuel is preformed in a plasma state.

Modification of spatial and temporal gains of stimulated brillouin and raman scattering by polarization smoothing

Significant reductions of stimulated Brillouin (SBS) and Raman (SRS) scattering are measured by adding polarization smoothing (PS) to a random phase plate smoothed beam. The associated plasma waves, as measured by Thomson scattering, are spatially and temporally modified and reveal that the interplay between SBS and SRS has to be taken into account to understand the effect of the smoothing. The results also support the numerical simulations [S. Huller et al., Phys. Plasmas 5, 2706 (1998); R. Berger et al., Phys. Plasmas 6, 1043 (1999)] predicting a reduction of self-focusing with PS, resulting in a decrease of the maximum laser intensity and consequently of SBS and SRS gains.

Enhanced forward scattering in the case of two crossed laser beams interacting with a plasma

The nonlinear enhancement of large-angle forward scattering of two identical laser beams propagating in a preformed plasma has been observed experimentally. The spectral analysis of the forward-scattered light shows two components, one which is unshifted with respect to the initial laser light frequency, and the other which is redshifted by a few angstroms. The redshifted component is found to be strongly enhanced in the case of crossed beam interaction in comparison with that of one beam illumination. Two-dimensional numerical simulations show that this enhancement is due to large-angle forward stimulated Brillouin scattering in which each beam serves as seed for the forward scattering of the other.

First observation of ion acoustic waves produced by the langmuir decay instability

Thomson scattering measurements are presented which demonstrate conclusively the occurrence of the Langmuir decay instability (LDI) in a laser-produced plasma experiment. Both products of the instability, the ion acoustic wave and the electron plasma wave, were simultaneously observed and identified with their spectral characteristics. The secondary decay of the LDI-generated electron plasma wave, into another Langmuir wave and an ion acoustic wave, has been observed for the first time. The connection with growth and saturation of the stimulated Raman instability is discussed.

Observation of ion acoustic waves associated with plasma-induced incoherence of laser beams using Thomson scattering.

We have carried out experiments to investigate the physical processes responsible for the recently discovered phenomenon of plasma-induced incoherence (PII) of a laser beam. Using a Thomson scattering diagnostic, we have observed ion acoustic waves (IAW) having wave vectors transverse to the interaction beam spectral and temporal characteristics of which show a clear correlation with other signatures of PII for various conditions of plasma density and laser intensity. These results support the recent theoretical interpretation for which the IAW result from the coupling between forward stimulated Brillouin scattering and self-focusing of the laser light in PII mechanisms.

[Primary hyperparathyroidism, a differential diagnosis of motor neuron diseases]. / Hyperparathyroïdie primitive, un diagnostic différentiel des maladies du motoneurone.

INTRODUCTION: Motor neuron diseases are always lethal. Other curable causes of neurologic disorders have to be sought. We report an example. EXEGESIS: A 72-year-old man presented a distal weakness and atrophy of the upper extremities. Electromyography showed thenar and hypothenar denervation, without fasciculation. Hypercalcemia led to the discovery of a primary hyperparathyroidism. Five months after parathyroid surgery, there was no worsening. CONCLUSION: Von Recklinghausen and Vical were the first to describe neuromuscular involvement in primary hyperparathyroidism. Faced with symptoms mimicking motor neuron diseases, calcium and phosphorus levels have to be measured because hyperparathyroidism can be cured and neurologic disorders disappear after surgery.

Laser light triggers increased Raman amplification in the regime of nonlinear Landau damping.

Stimulated Raman backscattering (SRS) has many unwanted effects in megajoule-scale inertially confined fusion (ICF) plasmas. Moreover, attempts to harness SRS to amplify short laser pulses through backward Raman amplification have achieved limited success. In high-temperature fusion plasmas, SRS usually occurs in a kinetic regime where the nonlinear response of the Langmuir wave to the laser drive and its host of complicating factors make it difficult to predict the degree of amplification that can be achieved under given experimental conditions. Here we present experimental evidence of reduced Landau damping with increasing Langmuir wave amplitude and determine its effects on Raman amplification. The threshold for trapping effects to influence the amplification is shown to be very low. Above threshold, the complex SRS dynamics results in increased amplification factors, which partly explains previous ICF experiments. These insights could aid the development of more efficient backward Raman amplification schemes in this regime.

Fusion reactions initiated by laser-accelerated particle beams in a laser-produced plasma.

The advent of high-intensity-pulsed laser technology enables the generation of extreme states of matter under conditions that are far from thermal equilibrium. This in turn could enable different approaches to generating energy from nuclear fusion. Relaxing the equilibrium requirement could widen the range of isotopes used in fusion fuels permitting cleaner and less hazardous reactions that do not produce high-energy neutrons. Here we propose and implement a means to drive fusion reactions between protons and boron-11 nuclei by colliding a laser-accelerated proton beam with a laser-generated boron plasma. We report proton-boron reaction rates that are orders of magnitude higher than those reported previously. Beyond fusion, our approach demonstrates a new means for exploring low-energy nuclear reactions such as those that occur in astrophysical plasmas and related environments.