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Experimental simulations of galaxy-forming conditions using lasers show that the Biermann battery generates seed magnetic fields, which turbulence can amplify to affect galaxy evolution. Experimental astrophysical magnetic fields In an example of the relatively young field of laboratory astrophysics, researchers at the LULI 2000 laser facility in Palaiseau, near Paris, have tackled the question of how magnetic fields are produced in an astrophysical environment. The standard model assumes that galactic magnetic fields arise from the amplification of 'seed' fields through dynamo or turbulent processes. It is thought that the tiny seed fields are induced by a thermally generated electronic field through the Biermann battery effect. The feasibility of this mechanism was demonstrated in a system mimicking the conditions associated with astrophysical shocks in the intergalactic medium. Seed fields were indeed produced by the Biermann effect. When the results are scaled to the intergalactic medium, where turbulence can act on timescales in the region of 700 million years, amplification to the observed levels of galactic magnetic fields becomes possible. The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations 1 , 2 , 3 . Although other mechanisms may also operate 4 , 5 , currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks 6 associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10 −21  gauss (refs 7 , 8 ). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations 9 , 10 , astrophysical environments can effectively be reproduced in the laboratory 11 , 12 . Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields 13 , 14 sufficiently to affect galaxy evolution.

We examine the impact of several factors on electron acceleration by a laser pulse and the resulting electron energy gain. Specifically, we consider the role played by: (1) static longitudinal electric field, (2) static transverse electric field, (3) electron injection into the laser pulse, and (4) static longitudinal magnetic field. It is shown that all of these factors lead, under certain conditions, to a considerable electron energy gain from the laser pulse. In contrast with other mechanisms such as wakefield acceleration, the static electric fields in this case do not directly transfer substantial energy to the electron. Instead, they reduce the longitudinal dephasing between the electron and the laser beam, which then allows the electron to gain extra energy from the beam. The mechanisms discussed here are relevant to experiments with under-dense gas jets, as well as to experiments with solid-density targets involving an extended pre-plasma.

Highly anisotropic, beam-like neutron emission with peak flux of the order of 109 n/sr was obtained from light nuclei reactions in a pitcher-catcher scenario, by employing MeV ions driven by a sub-petawatt laser. The spatial profile of the neutron beam, fully captured for the first time by employing a CR39 nuclear track detector, shows a FWHM divergence angle of ∼ 70 ° , with a peak flux nearly an order of magnitude higher than the isotropic component elsewhere. The observed beamed flux of neutrons is highly favourable for a wide range of applications, and indeed for further transport and moderation to thermal energies. A systematic study employing various combinations of pitcher-catcher materials indicates the dominant reactions being d(p, n+p)1H and d(d,n)3He. Albeit insufficient cross-section data are available for modelling, the observed anisotropy in the neutrons' spatial and spectral profiles is most likely related to the directionality and high energy of the projectile ions.

The radiation pressure of next generation ultra-high intensity (>1023 W cm−2) lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these laser pulses with matter. Here we show that these effects may lead to the production of an extremely dense (∼1024 cm−3) pair-plasma which absorbs the laser pulse consequently reducing the accelerated ion energy and laser to ion conversion efficiency by up to 30%-50% and 50%-65%, respectively. Thus we identify the regimes of laser-matter interaction, where either ions are efficiently accelerated to high energy or dense pair-plasmas are produced as a guide for future experiments.

Laser plasma electron acceleration from the interaction of an intense femtosecond laser pulse with an isolated microparticle surrounded by a low-density gas is studied here. Experiments presented here show that optimized plasma tailoring by introducing a pre-pulse boosts parametric instabilities to produce MeV electron energies and generates electron temperatures as large as 200 keV with the total charge being as high as 350 fC/shot/sr, even at a laser intensity of a few times 10 16  Wcm −2 . Corroborated by particle-in-cell simulations, these measurements reveal that two plasmon decay in the vicinity of the microparticle is the main contributor to hot electron generation.

While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field Bseed can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, Bgen ∼ −2πBseedΔx/λ0 (in the large spot size limit), where Δx is the target thickness and λ0 is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a μm-thick laser-irradiated target with a kilotesla-level seed magnetic field.

We report on the successful implementation and characterization of a cryogenic solid hydrogen target in experiments on high-power laser-driven proton acceleration. When irradiating a solid hydrogen filament of 10 μm diameter with 10-Terawatt laser pulses of 2.5 J energy, protons with kinetic energies in excess of 20 MeV exhibiting non-thermal features in their spectrum were observed. The protons were emitted into a large solid angle reaching a total conversion efficiency of several percent. Two-dimensional particle-in-cell simulations confirm our results indicating that the spectral modulations are caused by collisionless shocks launched from the surface of the the high-density filament into a low-density corona surrounding the target. The use of solid hydrogen targets may significantly improve the prospects of laser-accelerated proton pulses for future applications.

Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. The inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs. Brown dwarfs are small stars that are believed to be made of a warm dense plasma that cannot support hydrogen fusion as larger stars do. Here, the authors present a method for studying the properties, such as resistivity, of warm dense plasmas in the laboratory.

A key issue in realising the development of a number of applications of high-intensity lasers is the dynamics of the fast electrons produced and how to diagnose them. We report on measurements of fast electron transport in aluminium targets in the ultra-intense, short-pulse (<50 fs) regime using a high resolution temporally and spatially resolved optical probe. The measurements show a rapidly (≈0.5 c ) expanding region of Ohmic heating at the rear of the target, driven by lateral transport of the fast electron population inside the target. Simulations demonstrate that a broad angular distribution of fast electrons on the order of 60° is required, in conjunction with extensive recirculation of the electron population, in order to drive such lateral transport. These results provide fundamental new insight into fast electron dynamics driven by ultra-short laser pulses, which is an important regime for the development of laser-based radiation and particle sources.

MeV temperature electrons are typically generated at laser intensities of 10 18 W cm −2 . Their generation at non-relativistic intensities (~10 16 W cm −2 ) with high repetition rate lasers is cardinal for the realization of compact, ultra-fast electron sources. Here we report a technique of dynamic target structuring of micro-droplets using a 1 kHz, 25 fs, millijoule class laser, that uses two collinear laser pulses; the first to create a concave surface in the liquid drop and the second, to dynamically-drive electrostatic plasma waves that accelerate electrons to MeV energies. The acceleration mechanism, identified as two plasmon decay instability, is shown to generate two beams of electrons with hot electron temperature components of 200 keV and 1 MeV, respectively, at an intensity of 4 × 10 16 Wcm −2 , only. The electron beams are demonstrated to be ideal for single shot high resolution (tens of μ m) electron radiography. Typically, mJ lasers generate 50 keV temperature plasma electrons. Here, the authors use mJ laser pulses to chisel a liquid droplet surface and generate an electron temperature of 1 MeV, a feat previously possible only with 100 times more powerful lasers.