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Intense lasers interacting with dense targets accelerate relativistic electron beams, which transport part of the laser energy into the target depth. However, the overall laser-to-target energy coupling efficiency is impaired by the large divergence of the electron beam, intrinsic to the laser-plasma interaction. Here we demonstrate that an efficient guiding of MeV electrons with about 30MA current in solid matter is obtained by imposing a laserdriven longitudinal magnetostatic field of 600 T. In the magnetized conditions the transported energy density and the peak background electron temperature at the 60-μm-thick target's rear surface rise by about a factor of five, as unfolded from benchmarked simulations. Such an improvement of energy-density flux through dense matter paves the ground for advances in laser-driven intense sources of energetic particles and radiation, driving matter to extreme temperatures, reaching states relevant for planetary or stellar science as yet inaccessible at the laboratory scale and achieving high-gain laser-driven thermonuclear fusion. [ABSTRACT FROM AUTHOR]

According to particle-in-cell and multi fluid simulations, as a planar shock propagates across an unperturbed classical interface, significant amounts of rearward shocked material are predicted to advect with the shock front over distances that are much larger than an ion-ion collisional mean free path of a shocked ion. This novel mechanism of interface mixing is found to scale strongly with the Mach number (~M4) and produces an ion population bunch that penetrates the upstream material at nearly the shock speed. Possible implications for ignition capsule designs are conjectured. [ABSTRACT FROM AUTHOR]

A kilo-tesla level, quasi-static magnetic field (B-field), which is generated with an intense laser-driven capacitor-coil target, was measured by proton deflectometry with a proper plasma shielding. Proton deflectometry is a direct and reliable method to diagnose strong, mm³-scale laser-produced B-field; however, this was not successful in the previous experiment. A target-normal-sheath-accelerated proton beam is deflected by Lorentz force in the laser-produced magnetic field with the resulting deflection pattern recorded on a radiochromic film stack. A 610630 T of B-field amplitude was inferred by comparing the experimental proton pattern with Monte-Carlo calculations. The amplitude and temporal evolutions of the laser-generated B-field were also measured by a differential magnetic probe, independently confirming the proton deflectometry measurement results. [ABSTRACT FROM AUTHOR]

The properties of collisional shock waves propagating in uniform plasmas are studied with ionkinetic calculations, in both slab and spherical geometry and for the case of one and two ion species. Despite the presence of an electric field at the shock front-and in contrast to the case where an interface is initially present [C. Bellei et al., Phys. Plasmas 20, 044702 (2013)]- essentially no ion reflection at the shock front is observed due to collisions, with a probability of reflection ≲10-4 for the cases presented. A kinetic two-ion-species spherical convergent shock is studied in detail and compared against an average-species calculation, confirming effects of species separation and differential heating of the ion species at the shock front. The effect of different ion temperatures on the DT and D3He fusion reactivity is discussed in the fluid limit and is estimated to be moderately important. [ABSTRACT FROM AUTHOR]

Shock-driven implosions of thin-shell capsules, or "exploding pushers," generate low-density, high-temperature plasmas in which hydrodynamic instability growth is negligible and kinetic effects can play an important role. Data from implosions of thin deuterated-plastic shells with hydroequivalent D3He gas fills ranging from pure deuterium to pure 3He [H. G. Rinderknecht et al., Phys. Rev. Lett. 112, 135001 (2014)] were obtained to evaluate non-hydrodynamic fuel-shell mix mechanisms. Simulations of the experiments including reduced ion kinetic models support ion diffusion as an explanation for these data. Several additional kinetic mechanisms are investigated and compared to the data to determine which are important in the experiments. Shock acceleration of shell deuterons is estimated to introduce mix less than or comparable to the amount required to explain the data. Beam-target mechanisms are found to produce yields at most an order of magnitude less than the observations. [ABSTRACT FROM AUTHOR]

Phenomena consistent with self-collimation (or weak self-focusing) of laser target-normal-sheath-accelerated protons was experimentally observed for the first time, in a specially engineered structure ('lens') consisting of a stack of 300 thin aluminum foils separated by 50 μm vacuum gaps. The experiments were carried out in a 'passive environment,' i.e., no external fields applied, neutralization plasma or injection of secondary charged particles was imposed. Experiments were performed at the petawatt 'PHELIX' laser user facility (E = 100 J, Δt = 400 fs, λ = 1062 nm) at the 'Helmholtzzentrum für Schwerionenforschung-GSI' in Darmstadt, Germany. The observed rms beam spot reduction depends inversely on energy, with a focusing degree decreasing monotonically from 2 at 5.4 MeV to 1.5 at 18.7 MeV. The physics inside the lens is complex, resulting in a number of different mechanisms that can potentially affect the particle dynamics within the structure. We present a plausible simple interpretation of the experiment in which the combination of magnetic self-pinch forces generated by the beam current together with the simultaneous reduction of the repulsive electrostatic forces due to the foils are the dominant mechanisms responsible for the observed focusing/collimation. This focusing technique could be applied to a wide variety of space-charge dominated proton and heavy ion beams and impact fields and applications, such as HEDP science, inertial confinement fusion in both fast ignition and heavy ion fusion approaches, compact laser-driven injectors for a Linear Accelerator (LINAC) or synchrotron, medical therapy, materials processing, etc. [ABSTRACT FROM AUTHOR]

We present experimental results of laser-to-proton conversion efficiency as a function of lateral confinement of the refluxing electrons. Experiments were carried out using the T-Cubed laser at the Center for Ultrafast Optical Science, University of Michigan. We demonstrate that the laser-to-proton conversion efficiency increases by 50% with increased confinement of the target from surroundings with respect to a flat target of the same thickness. Three-dimensional hybrid particle-in-cell simulations using LSP code agree with the experimental data. The adopted target design is suitable for high repetition rate operation as well as for Inertial Confinement Fusion applications. [ABSTRACT FROM AUTHOR]

The energy and angular distributions of the fast electrons predicted by particle-in-cell (PIC) simulations differ from those historically assumed in ignition designs of the fast ignition scheme. Using a particular 3D PIC calculation, we show how the ignition energy varies as a function of source-fuel distance, source size, and density of the pre-compressed fuel. The large divergence of the electron beam implies that the ignition energy scales with density more weakly than the ρ-2 scaling for an idealized beam [S. Atzeni, Phys. Plasmas 6, 3316 (1999)], for any realistic source that is at some distance from the dense deuterium-tritium fuel. Due to the strong dependence of ignition energy with source-fuel distance, the use of magnetic or electric fields seems essential for the purpose of decreasing the ignition energy. [ABSTRACT FROM AUTHOR]

Self-modulated wakefield acceleration was investigated at densities down to ~4 × 1018 cm−3 by propagating the 50 TW 300 fs LULI laser in helium gas jets at lengths up to 1 cm. Long interaction lengths were achieved by closer matching of the initial focal spot size to the matched spot size for these densities. Electrons with energies extending to 180 MeV were observed in broad energy spectra which show some evidence for non-Maxwellian features at high energy. Two-dimensional PIC simulations indicate that the intial laser pulse breaks up into small pulselets that are eventually compressed and focused inside the first few plasma periods, leading to a ‘bubble-like’ acceleration of electron bunches. [ABSTRACT FROM PUBLISHER]

The focusing properties of a laser generated proton beam have been investigated using hemispherical targets in both freestanding and enclosed cone-shaped geometries. The proton trajectories and focusing were strongly affected by the electric fields in the beam, bending the trajectories near the axis. In the cone targets, a sheath field effectively channels the proton beam through the open cone tip, substantially improving the beam focusing from ≈90 μm to ≈55 μm diameter for protons with energies >3 MeV. The proton generation and focusing were modeled using 2D hybrid particle-in-cell simulations, which compared well with the experimental results. Simulations predict further improvement in focusing with more uniform target illumination. These results are of significant interest to proton fast ignition and other high energy density physics applications. [ABSTRACT FROM AUTHOR]

The trajectories of electrons and ions when a hot plasma expands under vacuum are studied in detail from a theoretical point of view and with the aid of numerical simulations. Exact analytic solutions are obtained in multi-dimensions, starting from the solution for the expansion of a quasi-neutral, Gaussian, collisionless plasma in vacuum [D. S. Dorozhkina and V. E. Semenov, Phys. Rev. Lett. 81, 2691 (1998)]. Focusing of laser-accelerated ions with concave targets is investigated with the hybrid particle-in-cell code Lsp. For a given laser energy and pulse duration, a larger laser focal spot is found to be beneficial to focus the ion beam to a smaller focal spot, due both to a geometric effect and to the decrease in the transverse gradient of the hot electron pressure. [ABSTRACT FROM AUTHOR]

The generation of strong, self-generated electric fields (GV/m) in direct-drive, inertial-confinement-fusion (ICF) capsules has been reported [Rygg et al., Science 319, 1223 (2008); Li et al., Phys. Rev. Lett. 100, 225001 (2008)]. A candidate explanation for the origin of these fields based on charge separation across a plasma shock front was recently proposed [Amendt et al., Plasma Phys. Controlled Fusion 51 124048 (2009)]. The question arises whether such electric fields in imploding capsules can have observable consequences on target performance. Two well-known anomalies come to mind: (1) an observed ≈2× greater-than-expected deficit of neutrons in an equimolar D3He fuel mixture compared with hydrodynamically equivalent D [Rygg et al., Phys. Plasmas 13, 052702 (2006)] and DT [Herrmann et al., Phys. Plasmas 16, 056312 (2009)] fuels, and (2) a similar shortfall of neutrons when trace amounts of argon are mixed with D in indirect-drive implosions [Lindl et al., Phys. Plasmas 11, 339 (2004)]. A new mechanism based on barodiffusion (or pressure gradient-driven diffusion) in a plasma is proposed that incorporates the presence of shock-generated electric fields to explain the reported anomalies. For implosions performed at the Omega laser facility [Boehly et al., Opt. Commun. 133, 495 (1997)], the (low Mach number) return shock has an appreciable scale length over which the lighter D ions can diffuse away from fuel center. The depletion of D fuel is estimated and found to lead to a corresponding reduction in neutrons, consistent with the anomalies observed in experiments for both argon-doped D fuels and D3He equimolar mixtures. The reverse diffusional flux of the heavier ions toward fuel center also increases the pressure from a concomitant increase in electron number density, resulting in lower stagnation pressures and larger imploded cores in agreement with gated, self-emission, x-ray imaging data. [ABSTRACT FROM AUTHOR]

Each successive generation of X-ray machines has opened up new frontiers in science, such as the first radiographs and the determination of the structure of DNA. State-of-the-art X-ray sources can now produce coherent high-brightness X-rays of greater than kiloelectronvolt energy and promise a new revolution in imaging complex systems on nanometre and femtosecond scales. Despite the demand, only a few dedicated synchrotron facilities exist worldwide, in part because of the size and cost of conventional (accelerator) technology. Here we demonstrate the use of a new generation of laser-driven plasma accelerators, which accelerate high-charge electron beams to high energy in short distances, to produce directional, spatially coherent, intrinsically ultrafast beams of hard X-rays. This reduces the size of the synchrotron source from the tens of metres to the centimetre scale, simultaneously accelerating and wiggling the electron beam. The resulting X-ray source is 1,000 times brighter than previously reported plasma wigglers and thus has the potential to facilitate a myriad of uses across the whole spectrum of light-source applications. [ABSTRACT FROM AUTHOR]

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of μm-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given. [ABSTRACT FROM AUTHOR]

As the state of the art for high power laser systems increases from terawatt to petawatt level and beyond, a crucial parameter for routinely monitoring high intensity performance is laser spot size on a solid target during an intense interaction in the tight focus regime (<10 μm). Here we present a novel, simple technique for characterizing the spatial profile of such a laser focal spot by imaging the interaction region in third harmonic order (3ωlaser). Nearly linear intensity dependence of 3ωlaser generation for interactions >1019 Wcm-2 is demonstrated experimentally and shown to provide the basis for an effective focus diagnostic. Importantly, this technique is also shown to allow in-situ diagnosis of focal spot quality achieved after reflection from a double plasma mirror setup for very intense high contrast interactions (>1020 Wcm-2) an important application for the field of high laser contrast interaction science. [ABSTRACT FROM AUTHOR]

We present theoretical considerations on the process of ion acceleration with ultra-thin foils irradiated by elliptically polarized, highly intense laser pulses. Very recently the radiation pressure acceleration regime was predicted where mono-energetic ion bunches can be produced with high efficiencies (Klimo et al 2008 Phys. Rev. ST Accel. Beams 11 031301; Robinson et al 2008 New J. Phys. 10 013021). We have studied the process by means of 1D particle-in-cell (PIC) simulations and analytical models and have considered effects of areal mass density of the target and laser ellipticity on the ion acceleration process. For certain target densities and laser parameters the optimum target thickness has been extracted. Peaked ion spectra are found for ellipticity beyond a threshold value of about 0.7. Here, we highlight the drastic difference between linear and circular polarization by movie animations. [ABSTRACT FROM AUTHOR]

Target studies for the proposed High Power Laser Energy Research (HiPER) facility [M. Dunne, Nature Phys. 2, 2 (2006)] are outlined and discussed. HiPER will deliver a 3ω (wavelength λ=0.35 μm), multibeam, multi-ns pulse of about 250 kJ and a 2ω or 3ω pulse of 70–100 kJ in about 15 ps. Its goal is the demonstration of laser driven inertial fusion via fast ignition. The baseline target concept is a direct-drive single shell capsule, ignited by hot electrons generated by a conically guided ultraintense laser beam. The paper first discusses ignition and compression requirements, and presents gain curves, based on an integrated model including ablative drive, compression, ignition and burn, and taking the coupling efficiency ηig of the igniting beam as a parameter. It turns out that ignition and moderate gain (up to 100) can be achieved, provided that adiabat shaping is used in the compression, and the efficiency ηig exceeds 20%. Using a standard ponderomotive scaling for the hot electron temperature, a 2ω or 3ω ignition beam is required to make the hot electron range comparable to the desired size of the hot spot. A reference target family is then presented, based on one-dimensional fluid simulation of compression, and two-dimensional fluid and hybrid simulations of fast electron transport, ignition, and burn. The sensitivity to compression pulse shape, as well as to hot electron source location, hot electron range, and beam divergence is also discussed. Rayleigh–Taylor instability at the ablation front has been addressed by a model and a perturbation code. Simplified simulations of code-guided target implosions have also been performed. [ABSTRACT FROM AUTHOR]

In this study a comparative analysis of iron molecules during aging was performed in locus coeruleus (LC) and substantia nigra (SN), known targets of Parkinson’s Disease (PD) and related disorders. LC and SN neurons, especially the SN pars compacta, degenerate in PD and other forms of parkinsonism. Iron and its major molecular forms, such as ferritin and neuromelanin (NM), were measured in LC and SN of normal subjects at various ages. Iron levels were lower, H-ferritin/iron ratio was higher and the iron content in NM was lower in LC than in SN. Iron deposits were abundant in SN tissue, very scarse in LC tissue and completely absent in pigmented neurons of both SN and LC. In both regions H- and L-ferritins were present only in glia. This suggests that in LC neurons iron mobilization and toxicity is lower than that in SN and is efficiently buffered by NM. Ferritins accomplish the same buffering function in glial cells. [ABSTRACT FROM AUTHOR]

The properties of a proton beam can be investigated by using a stack of radiochromic film imaging the shadows of a mesh placed within the beam path. We present results of a laser-generated proton beam from hemispherical shell targets. The experimental data validate particle-in-cell hybrid modeling using the large-scale plasma code and lead to the understanding of proton focusing relevant to integrated-proton fast-ignition experiments. [ABSTRACT FROM AUTHOR]

Copper activation was used to characterize high-energy proton beam acceleration from near-critical density plasma targets. An enhancement was observed when decreasing the target density, which is indicative for an increased laser-accelerated hot electron density at the rear target-vacuum boundary. This is due to channel formation and collimation of the hot electrons inside the target. Particle-in-cell simulations support the experimental observations and show the correlation between channel depth and longitudinal electric field strength is directly correlated with the proton acceleration. [ABSTRACT FROM AUTHOR]

The claims made in the preceding Comment are categorically refuted. Further evidence to support the conclusions of our original paper is herein provided. [ABSTRACT FROM AUTHOR]

It is shown by means of multi-fluid particle-in-cell simulations that convergence of the spherical shock wave that propagates through the inner gas of inertial confinement fusion-relevant experiments is accompanied by a separation of deuterium (D) and tritium (T) ions across the shock front. Deuterons run ahead of the tritons due to their lower mass and higher charge-to-mass ratio and can reach the center several tens of picoseconds before the tritons. The rising edge of the DD and TT fusion rate is also temporally separated by the same amount, which should be an observable in experiments and would be a direct proof of the 'stratification conjecture' on the shock front [Amendt et al., Phys. Plasmas 18, 056308 (2011)]. Moreover, dephasing of the D and T shock components in terms of density and temperature leads to a degradation of the DT fusion yield as the converging shock first rebounds from the fuel center (shock yield). For the parameters of this study, the second peak in the fusion yield (compression yield) is strongly dependent on the choice of the flux limiter. [ABSTRACT FROM AUTHOR]

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell (PIC) code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum and a divergent angle spectrum (average velocity-space polar angle of 52°). Transport simulations with the PIC-based divergence do not ignite for >1 MJ of fast-electron energy, for a modest (70 μm) standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields ∼50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility have generated fields of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such imploded fields will likely be more compressed in the transport region than in the laser absorption region. When fast electrons encounter increasing field strength, magnetic mirroring can reflect a substantial fraction of them and reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite radius, is presented as one field configuration which circumvents mirroring. [ABSTRACT FROM AUTHOR]

The article offers information on the paper entitled "Bright spatially coherent synchrotron X-rays from a table-top source," by S. Kneip and colleagues on the characterization and realization of ultrafast synchrotron radiation's compact bright source.