Time-resolved near backscatter imaging system on Laser MegaJoule.

The newly operating near-backscattering imaging (NBI) system on the Laser MegaJoule (LMJ) is briefly described with emphasis on the temporally resolved measurements and their synchronization with the LMJ laser pulse through target shots taken as part of the diagnostic commissioning campaign. The NBI measures the stimulated Brillouin and Raman scattered light around two quadruplets (one inner and one outer) of the upper LMJ hemisphere. The temporal resolution is achieved with a unique system: a specifically designed wide-open optical lens images 40 points of a diffuser onto an array of optical fibers with the scattered light recorded on a multiplexed photodiode array.

Requirements for a 4ω Thomson scattering system on megajoule scale laser facilities.

With the arrival of megajoule class laser facilities, the features of laser-produced plasmas are evolving toward unprecedented high electron temperatures reached in the environment of a cm-scale indirect-drive Hohlraum for a few tens of nanoseconds. In this context, the need for in situ experimental characterization of the plasma parameters becomes critical in order to test hydrodynamics simulations in these novel conditions. Taking advantage of the progress achieved in the last 40 years, Thomson scattering has become a classic diagnostic in the characterization of laser produced plasmas. However, the many beam configuration of the megajoule scale experiments makes the measurements increasingly complex because the Thomson scattering signals produced by the 351 nm heaters themselves dominate the plasma emission around 263 nm, a wavelength range typically of interest when a 4ω Thomson probe is used. This paper reviews the requirements for and the potential of a 4ω Thomson scattering system to be operated on such 351 nm megajoule scale facilities in order to characterize the hot (Te > 3 keV) plasmas produced in the indirect-drive irradiation of a Hohlraum. It is found that the configuration of the diagnostic could be optimized in order to enable the detection of the ion acoustic resonances over a large domain of plasma parameters. The results for the electron plasma wave resonances are also given.

Laser-Plasma Interaction Experiment for Solar Burst Studies.

A new experimental platform based on laser-plasma interaction is proposed to explore the fundamental processes of wave coupling at the origin of interplanetary radio emissions. It is applied to the study of electromagnetic (EM) emission at twice the plasma frequency (2ω_{p}) observed during solar bursts and thought to result from the coalescence of two Langmuir waves (LWs). In the interplanetary medium, the first LW is excited by electron beams, while the second is generated by electrostatic decay of Langmuir waves. In the present experiment, instead of an electron beam, an energetic laser propagating through a plasma excites the primary LW, with characteristics close to those at near-Earth orbit. The EM radiation at 2ω_{p} is observed at different angles. Its intensity, spectral evolution, and polarization confirm the LW-coalescence scenario.

Experimental Evidence of Harnessed Expansion of a High-Z Plasma Using the Hollow Wall Design for Indirect Drive Inertial Confinement Fusion.

The effectiveness of a dome-shaped wall covered by a thin gold foil (hollow wall) [M. Vandenboomgaerde et al., Phys. Plasmas 25, 012713 (2018)PHPAEN1070-664X10.1063/1.5008669] in holding back the high-Z plasma expansion in a gas-filled hohlraum is demonstrated for the first time in experiments reproducing the irradiation conditions of indirect drive at the ignition scale. The setup exploits a 1D geometry enabling record of the complete history of the gold expansion for 8 ns by imaging its emission in multiple x-ray energy ranges featuring either the absorption zones or the thermal emission regions. The measured expansion dynamics is well reproduced by numerical simulations. This novel wall design could now be tailored for the megajoule scale to enable the propagation of the inner beams up to the equator in low gas-filled hohlraum thus allowing the fine-tuning of the irradiation symmetry on the timescale required for ignition.

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.

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 Evidence of the Collective Brillouin Scattering of Multiple Laser Beams Sharing Acoustic Waves.

The indirect-drive scheme to inertial confinement fusion uses a large number of laser beams arranged in a symmetric angular distribution. Collective laser plasma instabilities can therefore develop that couple all the incident laser waves located in a cone to the daughter wave growing along the cone symmetry axis [D. F. DuBois et al., Phys. Fluids B 4, 241 (1992)]. With complementary diagnostics of Thomson scattering and of the scattered light, we demonstrate the occurrence of collective stimulated Brillouin sidescattering driving collective acoustic waves in indirect-drive experiments.

First set of gated x-ray imaging diagnostics for the Laser Megajoule facility.

The Laser Megajoule (LMJ) facility located at CEA/CESTA started to operate in the early 2014 with two quadruplets (20 kJ at 351 nm) focused on target for the first experimental campaign. We present here the first set of gated x-ray imaging (GXI) diagnostics implemented on LMJ since mid-2014. This set consists of two imaging diagnostics with spatial, temporal, and broadband spectral resolution. These diagnostics will give basic measurements, during the entire life of the facility, such as position, structure, and balance of beams, but they will also be used to characterize gas filled target implosion symmetry and timing, to study x-ray radiography and hydrodynamic instabilities. The design requires a vulnerability approach, because components will operate in a harsh environment induced by neutron fluxes, gamma rays, debris, and shrapnel. Grazing incidence x-ray microscopes are fielded as far as possible away from the target to minimize potential damage and signal noise due to these sources. These imaging diagnostics incorporate microscopes with large source-to-optic distance and large size gated microchannel plate detectors. Microscopes include optics with grazing incidence mirrors, pinholes, and refractive lenses. Spatial, temporal, and spectral performances have been measured on x-ray tubes and UV lasers at CEA-DIF and at Physikalisch-Technische Bundesanstalt BESSY II synchrotron prior to be set on LMJ. GXI-1 and GXI-2 designs, metrology, and first experiments on LMJ are presented here.

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.

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.

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.

Effect of the laser wavelength on the saturated level of stimulated Brillouin scattering.

Equivalent stimulated Brillouin backscattering (SBS) saturation levels have been measured in the interaction with 0.527 and 0.351 microm laser beams demonstrating that the initial interaction wavelength is not influencing the final saturation levels. Experiments have been performed at the two wavelengths in similar interaction conditions obtained by preforming the plasma from a solid target with a creation beam converted at the same wavelength as the interaction beam. This produces an almost exponential density profile from vacuum to the critical density of the interaction beam in which large SBS gains are reached.

Laser smoothing and imprint reduction with a foam layer in the multikilojoule regime.

This Letter presents first experimental results of the laser imprint reduction in fusion scale plasmas using a low-density foam layer. The experiments were conducted on the LIL facility at the energy level of 12 kJ with millimeter-size plasmas, reproducing the conditions of the initial interaction phase in the direct-drive scheme. The results include the generation of a supersonic ionization wave in the foam and the reduction of the initial laser fluctuations after propagation through 500 mum of foam with limited levels of stimulated Brillouin and Raman scattering. The smoothing mechanisms are analyzed and explained.

Thomson-scattering study of the subharmonic decay of ion-acoustic waves driven by the Brillouin instability.

Thomson scattering (TS) has been used to investigate the two-ion decay instability of ion acoustic waves generated by stimulated Brillouin scattering in an underdense CH plasma. Two complementary TS diagnostics, spectrally and spatially resolved, demonstrate the occurrence of the subharmonic decay of the primary ion acoustic wave into two secondary waves. The study of the laser intensity dependence shows that the secondary ion acoustic waves are correlated with the SBS reflectivity saturation, at a level of a few percent.

Strong reduction of the degree of spatial coherence of a laser beam propagating through a preformed plasma.

A strong reduction of the spatial coherence of a laser beam after its propagation through a plasma has been measured using a Fresnel biprism interferometer. The laser beam was diffraction limited; the coherence width was reduced from 40 mm in vacuum down to a few mm with the plasma. Numerical results based on a paraxial model exhibit a coherence degree close to the experimental one; they also prove the importance of taking into account the nonlocal transport effects in numerical simulations for such plasma conditions.

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.

Langmuir decay instability cascade in laser-plasma experiments.

Thomson scattering has been used to investigate the nonlinear evolution of electron plasma waves (EPWs) generated by stimulated Raman scattering (SRS). Two complementary diagnostics demonstrate the occurrence of the cascade of Langmuir decay instabilities (LDI). The EPW wave-number spectrum displays an asymmetric broadening towards small wave numbers, interpreted as a signature of the secondary EPWs produced in the LDI cascade. The number of cascade steps is in agreement with the broadening of the associated ion-acoustic-waves' spectra. The total energy transferred in the EPWs cascade is found to be either less than or of the same order of magnitude as the energy of the primary EPW.