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The cosmic rays non-resonant streaming instability is believed to be the source of substantial magnetic field amplification. In this work we investigate the effects of the ambient plasma temperature on the instability and derive analytical expressions of its growth rate in the hot, demagnetized regime of interaction. To study its non-linear evolution we perform hybrid-PIC simulations for a wide range of temperatures. We find that in the cold limit about two-thirds of the cosmic rays drift kinetic energy is converted into magnetic energy. Increasing the temperature of the ambient plasma can substantially reduce the growth rate and the magnitude of the saturated magnetic field., Under review in MNRAS

We present the results of studying experimentally the expansion of laser plasma in a strong external magnetic field (with a magnetic flux density of 13.5 T) at various sizes of the region of plasma formation on the surface of a solid-state target. It is shown that when the size of the plasma formation region is smaller than the classical plasma braking radius, a nearly identical topology of plasma flows is observed, which is characterized by the formation of a thin plasma sheet directed along the external magnetic field. If the width of the plasma formation region is comparable with the classical plasma braking radius, an additional plasma sheet starts to be formed.

Aims. EXor-type objects are protostars that display powerful UV-optical outbursts caused by intermittent and powerful events of magnetospheric accretion. These objects are not yet well investigated and are quite difficult to characterize. Several parameters, such as plasma stream velocities, characteristic densities, and temperatures, can be retrieved from present observations. As of yet, however, there is no information about the magnetic field values and the exact underlying accretion scenario is also under discussion. Methods. We use laboratory plasmas, created by a high power laser impacting a solid target or by a plasma gun injector, and make these plasmas propagate perpendicularly to a strong external magnetic field. The propagating plasmas are found to be well scaled to the presently inferred parameters of EXor-type accretion event, thus allowing us to study the behaviour of such episodic accretion processes in scaled conditions. Results. We propose a scenario of additional matter accretion in the equatorial plane, which claims to explain the increased accretion rates of the EXor objects, supported by the experimental demonstration of effective plasma propagation across the magnetic field. In particular, our laboratory investigation allows us to determine that the field strength in the accretion stream of EXor objects, in a position intermediate between the truncation radius and the stellar surface, should be of the order of 100 G. This, in turn, suggests a field strength of a few kilogausses on the stellar surface, which is similar to values inferred from observations of classical T Tauri stars.

We analyze, using experiments and 3D MHD numerical simulations, the dynamics and radiative properties of a plasma ablated by a laser (1 ns, 10$^{12}$-10$^{13}$ W/cm$^2$) from a solid target, as it expands into a homogeneous, strong magnetic field (up to 30 T) transverse to its main expansion axis. We find that as soon as 2 ns after the start of the expansion, the plasma becomes constrained by the magnetic field. As the magnetic field strength is increased, more plasma is confined close to the target and is heated by magnetic compression. We also observe a dense slab that rapidly expands into vacuum after ~ 8 ns; however, this slab contains only ~ 2 % of the total plasma. As a result of the higher density and increased heating of the confined plasma, there is a net enhancement of the total x-ray emissivity induced by the magnetization., 15 pages, 4 figures, Supplementary Information, submitted to PRL

Aims. Investigating the process of matter accretion onto forming stars through scaled experiments in the laboratory is important in order to better understand star and planetary system formation and evolution. Such experiments can indeed complement observations by providing access to the processes with spatial and temporal resolution. A previous investigation revealed the existence of a two-component stream: a hot shell surrounding a cooler inner stream. The shell was formed by matter laterally ejected upon impact and refocused by the local magnetic field. That laboratory investigation was limited to normal incidence impacts. However, in young stellar objects, the complex structure of magnetic fields causes variability of the incident angles of the accretion columns. This led us to undertake an investigation, using laboratory plasmas, of the consequence of having a slanted accretion impacting a young star. Methods. Here, we used high power laser interactions and strong magnetic field generation in the laboratory, complemented by numerical simulations, to study the asymmetry induced upon accretion structures when columns of matter impact the surface of young stars with an oblique angle. Results. Compared to the scenario where matter accretes perpendicularly to the star surface, we observe a strongly asymmetric plasma structure, strong lateral ejecta of matter, poor confinement of the accreted material, and reduced heating compared to the normal incidence case. Thus, slanted accretion is a configuration that seems to be capable of inducing perturbations of the chromosphere and hence possibly influencing the level of activity of the corona.

Collective processes in plasmas often induce microinstabilities that play an important role in many space or laboratory plasma environments. Particularly notable is the Weibel-type current filamentation instability, which is believed to drive the creation of collisionless shocks in weakly magnetized astrophysical plasmas. Here, this instability class is studied through interactions of ultraintense and short laser pulses with solid foils, leading to localized generation of megaelectronvolt electrons. Proton radiographic measurements of both low- and high-resistivity targets show two distinct, superimposed electromagnetic field patterns arising from the interpenetration of the megaelectronvolt electrons and the background plasma. Particle-in-cell simulations and theoretical estimates suggest that the collisionless Weibel instability building up in the dilute expanding plasmas formed at the target surfaces causes the observed azimuthally symmetric electromagnetic filaments. For a sufficiently high resistivity of the target foil, an additional resistive instability is triggered in the bulk target, giving rise to radially elongated filaments. The data reveal the growth of both filamentation instabilities over large temporal (tens of picoseconds) and spatial (hundreds of micrometres) scales. In the interaction of ultraintense, short laser pulses with solid targets, the collisionless Weibel instability is observed. For a sufficiently high resistivity of the target, an additional resistive instability appears.

The collective interaction between intense ion beams and plasmas is studied by simulations and experiments, where an intense proton beam produced by a short pulse laser is injected into a pre-ionized gas. It is found that, depending on its current density, collective effects can significantly alter the propagated ion beam and the stopping power. The quantitative agreement that is found between theories and experiments constitutes the first validation of the collective interaction theory. The effects in the interaction between intense ion beams and background gas plasmas are of importance for the design of laser fusion reactors as well as for beam physics. Keywords: Two stream instabilities, Ultra intense short pulse laser, Proton beam, Wake field, Electron plasma wave, Laser plasma interaction, PACS codes: 52.38.Kd, 29.27.Fh, 52.40.Kh, 52.70.Nc

A superbright X-ray source with a radiation temperature of ~1.2 keV making it possible to create a solid-state plasma whose kinetics is determined by the radiative processes has been implemented under the impact of a 170-TW pulse of the PEARL femtosecond laser facility on an aluminum target with submicron thickness. The diagnostics of the created plasma is performed by X-ray spectral methods using spectral transitions in hollow multicharged ions.

Recent achievements in laboratory astrophysics experiments with high-power lasers have allowed progress in our understanding of the early stages of star formation. In particular, we have recently demonstrated the possibility of simulating in the laboratory the process of the accretion of matter on young stars [G. Revet et al., Sci. Adv. 3, e1700982 (2017)]. The present paper focuses on x-ray spectroscopy methods that allow us to investigate the complex plasma hydrodynamics involved in such experiments. We demonstrate that we can infer the formation of a plasma shell, surrounding the accretion column at the location of impact with the stellar surface, and thus resolve the present discrepancies between mass accretion rates derived from x-ray and optical-radiation astronomical observations originating from the same object. In our experiments, the accretion column is modeled by having a collimated narrow (1 mm diameter) plasma stream first propagate along the lines of a large-scale external magnetic field and then impact onto an obstacle, mimicking the high-density region of the stellar chromosphere. A combined approach using steady-state and quasi-stationary models was successfully applied to measure the parameters of the plasma all along its propagation, at the impact site, and in the structure surrounding the impact region. The formation of a hot plasma shell, surrounding the denser and colder core, formed by the incoming stream of matter is observed near the obstacle using x-ray spatially resolved spectroscopy.

With the much-anticipated multi-petawatt (PW) laser facilities that are coming online, neutron sources with extreme fluxes could soon be in reach. Such sources would rely on spallation by protons accelerated by the high-intensity lasers. These high neutron fluxes would make possible not only direct measurements of neutron capture and β-decay rates related to the r-process of nucleosynthesis of heavy elements, but also such nuclear measurements in a hot plasma environment, which would be beneficial for s-process investigations in astrophysically relevant conditions. This could, in turn, finally allow possible reconciliation of the observed element abundances in stars and those derived from simulations, which at present show large discrepancies. Here, we review a possible pathway to reach unprecedented neutron fluxes using multi-PW lasers, as well as strategies to perform measurements to investigate the r- and s-processes of nucleosynthesis of heavy elements in cold matter, as well as in a hot plasma environment.With the much-anticipated multi-petawatt (PW) laser facilities that are coming online, neutron sources with extreme fluxes could soon be in reach. Such sources would rely on spallation by protons accelerated by the high-intensity lasers. These high neutron fluxes would make possible not only direct measurements of neutron capture and β-decay rates related to the r-process of nucleosynthesis of heavy elements, but also such nuclear measurements in a hot plasma environment, which would be beneficial for s-process investigations in astrophysically relevant conditions. This could, in turn, finally allow possible reconciliation of the observed element abundances in stars and those derived from simulations, which at present show large discrepancies. Here, we review a possible pathway to reach unprecedented neutron fluxes using multi-PW lasers, as well as strategies to perform measurements to investigate the r- an...

Laser irradiation of solid targets can drive short and high-charge relativistic electron bunches over micron-scale acceleration gradients. However, for a long time, this technique was not considered a viable means of electron acceleration due to the large intrinsic divergence (∼50° half-angle) of the electrons. Recently, a reduction in this divergence to 10°–20° half-angle has been obtained, using plasma-based magnetic fields or very high contrast laser pulses to extract the electrons into the vacuum. Here we show that we can further improve the electron beam collimation, down to ∼1.5° half-angle, of a high-charge (6 nC) beam, and in a highly reproducible manner, while using standard stand-alone 100 TW-class laser pulses. This is obtained by embedding the laser-target interaction in an external, large-scale (cm), homogeneous, extremely stable, and high-strength (20 T) magnetic field that is independent of the laser. With upcoming multi-PW, high repetition-rate lasers, this technique opens the door to achieving even higher charges (>100 nC).Laser irradiation of solid targets can drive short and high-charge relativistic electron bunches over micron-scale acceleration gradients. However, for a long time, this technique was not considered a viable means of electron acceleration due to the large intrinsic divergence (∼50° half-angle) of the electrons. Recently, a reduction in this divergence to 10°–20° half-angle has been obtained, using plasma-based magnetic fields or very high contrast laser pulses to extract the electrons into the vacuum. Here we show that we can further improve the electron beam collimation, down to ∼1.5° half-angle, of a high-charge (6 nC) beam, and in a highly reproducible manner, while using standard stand-alone 100 TW-class laser pulses. This is obtained by embedding the laser-target interaction in an external, large-scale (cm), homogeneous, extremely stable, and high-strength (20 T) magnetic field that is independent of t...

The design of ellipsoidal plasma mirrors (EPMs) for the PEARL laser facility is presented. The EPMs achieve a magnification of 0.32 in focal spot size, and the corresponding increase in focused intensity is expected to be about 8. Designing and implementing such focusing optics for short-pulse (

A new experimental set-up designed to investigate the accretion dynamics in newly born stars is presented. It takes advantage of a magnetically collimated stream produced by coupling a laser-generated expanding plasma to a $2\times 10^{5}~{G}\ (20~{T})$ externally applied magnetic field. The stream is used as the accretion column and is launched onto an obstacle target that mimics the stellar surface. This setup has been used to investigate in details the accretion dynamics, as reported in [G. Revet et al., Science Advances 3, e1700982 (2017), arXiv:1708.02528}. Here, the characteristics of the stream are detailed and a link between the experimental plasma expansion and a 1D adiabatic expansion model is presented. Dimensionless numbers are also calculated in order to characterize the experimental flow and its closeness to the ideal MHD regime. We build a bridge between our experimental plasma dynamics and the one taking place in the Classical T Tauri Stars (CTTSs), and we find that our set-up is representative of a high plasma $\beta$ CTTS accretion case.

Magnetized laser-produced plasmas are central to many novel laboratory astrophysics and inertial confinement fusion studies, as well as in industrial applications. Here we provide the first complete description of the three-dimensional dynamics of a laser-driven plasma plume expanding in a 20 T transverse magnetic field. The plasma is collimated by the magnetic field into a slender, rapidly elongating slab, whose plasma-vacuum interface is unstable to the growth of the "classical", fluid-like magnetized Rayleigh-Taylor instability., Comment: Accepted for publication in Physical Review Letters

An experimental platform to study the interaction of two colliding high-velocity (0.01–0.2c; 0.05–20 MeV) proton plasmas in a high strength (20 T) magnetic field is introduced. This platform aims to study the collision of magnetized plasmas accelerated via the Target-Normal-Sheath-Acceleration mechanism and initially separated by distances of a few hundred microns. The plasmas are accelerated from solid targets positioned inside a few cubic millimeter cavity located within a Helmholtz coil that provides up to 20 T magnetic fields. Various parameters of the plasmas at their interaction location are estimated. These show an interaction that is highly non-collisional, and that becomes more and more dominated by the magnetic fields as time progresses (from 5 to 60 ps). Particle-in-cell simulations are used to reproduce the initial acceleration of the plasma both via simulations including the laser interaction and via simulations that start with preheated electrons (to save dramatically on computational expense). The benchmarking of such simulations with the experiment and with each other will be used to understand the physical interaction when a magnetic field is applied. In conclusion, the experimental density profile of the interacting plasmas is shown in the case without an applied magnetic magnetic field, so to show thatmore » without an applied field that the development of high-velocity shocks, as a result of particle-to-particle collisions, is not achievable in the configuration considered.« less

Plasma collisions in magnetic fields play an important role in the acceleration of particles and radiation generation in astrophysical objects and in the high intensity laser plasma interaction. In laser–target interaction the expanding plasmas can be created with the Target Normal Sheath Acceleration mechanism, TNSA. The possibility to observe the Weibel filamentation in two inter-penetrating TNSA plasmas in an ambient magnetic field is studied theoretically and numerically in the present work. Relying on 2D3V Particle-In-Cell simulations and analytical estimates, the effect of the decreasing density profile coupled to the increasing flow velocity on the external magnetic field accumulation and the development of the magnetic vortexes is analyzed. It is demonstrated that with realistic TNSA profiles the plasma collision can lead to magnetic field accumulation and to the development of magnetic vortexes. These results are of interest for interpretation of the experiments which may be carried out in this regime on high intensity laser facilities.

In this work, the possibility of the implementation of impurities in the compositions of solid thick targets irradiated by intense lasers is discussed in order to solve problems of optically-thick plasma diagnostics. Calculations were conducted for relative intensities of oxygen resonance lines (H-like—3p–1s, 4p–1s, 5p–1s, 6p–1s, 7p–1s transitions) in a recombination quasi-stationary model to obtain plasma parameters. In the experiment with 0.6 ns, 40 J laser pulses focused to 600 μm focal spot at solid polyvinylidene chloride target the parameters of plasma jet stopped by solid oxidized Teflon obstacle were studied by means of spatially-resolved x-ray spectroscopy.

High-intensity lasers interacting with solid foils produce copious numbers of relativistic electrons, which in turn create strong sheath electric fields around the target. The proton beams accelerated in such fields have remarkable properties, enabling ultrafast radiography of plasma phenomena or isochoric heating of dense materials. In view of longer-term multidisciplinary purposes (e.g., spallation neutron sources or cancer therapy), the current challenge is to achieve proton energies well in excess of 100 MeV, which is commonly thought to be possible by raising the on-target laser intensity. Here we present experimental and numerical results demonstrating that magnetostatic fields self-generated on the target surface may pose a fundamental limit to sheath-driven ion acceleration for high enough laser intensities. Those fields can be strong enough (~105 T at laser intensities ~1021 W cm–2) to magnetize the sheath electrons and deflect protons off the accelerating region, hence degrading the maximum energy the latter can acquire., Laser-generated ion acceleration has received increasing attention due to recent progress in super-intense lasers. Here the authors demonstrate the role of the self-generated magnetic field on the ion acceleration and limitations on the energy scaling with laser intensity.

International audience; Estimates of typical parameters of accretion flows in the representative intermediate polar EX Hydrae, the polar AM Herculis, and the "hot Jupiter" WASP-12b are presented. Dimensionless parameters of astrophysical systems are compared with those of laboratory experiments on laser ablation in magnetic fields. It is shown that laboratory simulations of astrophysical flows is possible in principle, provided that some adjustment to the magnetic field, plasma density, and plasma velocity are made.

A new type of proton acceleration stemming from large-scale gradients, low-density targets, irradiated by an intense near-infrared laser is observed. The produced protons are characterized by high-energies (with a broad spectrum), are emitted in a very directional manner, and the process is associated to relaxed laser (no need for high-contrast) and target (no need for ultra-thin or expensive targets) constraints. As such, this process appears quite effective compared to the standard and commonly used Target Normal Sheath Acceleration technique (TNSA), or more exploratory mechanisms like Radiation Pressure Acceleration (RPA). The data are underpinned by 3D numerical simulations which suggest that in these conditions a Low Density Collisionless Shock Acceleration (LDCSA) mechanism is at play, which combines an initial Collisionless Shock Acceleration (CSA) to a boost procured by a TNSA-like sheath field in the downward density ramp of the target, leading to an overall broad spectrum. Experiments performed at a laser intensity of 1020 W/cm2 show that LDCSA can accelerate, from ~1% critical density, mm-scale targets, up to 5 × 109 protons/MeV/sr/J with energies up to 45(±5) MeV in a collimated (~6° half-angle) manner.

The collimation of astrophysically-relevant plasma ejecta in the form of narrow jets via a poloidal magnetic field is studied experimentally by irradiating a target situated in a 20 T axial magnetic field with a 40 J, 0.6 ns, 0.7 mm diameter, high-power laser. The dynamics of the plasma shaping by the magnetic field are studied over 70 ns and up to 20 mm from the source by diagnosing the electron density, temperature and optical self-emission. These show that the initial expansion of the plasma is highly magnetized, which leads to the formation of a cavity structure when the kinetic plasma pressure compresses the magnetic field, resulting in an oblique shock [A. Ciardi et al., Phys. Rev. Lett. 110, 025002 (2013)]. The resulting poloidal magnetic nozzle collimates the plasma into a narrow jet [B. Albertazzi et al., Science 346, 325 (2014)]. At distances far from the target, the jet is only marginally magnetized and maintains a high aspect ratio due to its high Mach-number (M∼20) and not due to external magnetic pressure. The formation of the jet is evaluated over a range of laser intensities (1012–1013 W/cm2), target materials and orientations of the magnetic field. Plasma cavity formation is observed in all cases and the viability of long-range jet formation is found to be dependent on the orientation of the magnetic field.

The use of plasmas provides a way to overcome the damage threshold of classical solid-state based optical materials which is the main limitation encountered in producing extreme power laser pulses. In particular one can use plasmas to directly amplify ultra-short laser pulses to very high intensities. Multi-dimensional kinetic simulations and first proof-of-principle experiments show the feasibility of using plasma instabilities involving ion waves, such as stimulated Brillouin backscattering, in a controlled way to transfer energy from a long pump pulse to a short seed pulse and thereby increase the intensity of the latter. Plasma parametric amplification, and the use of plasma mirrors for focusing, is part of the newly developping domain of plasma optics, which eventually will pave the way to Exawatt lasers.

Plasma amplification of low energy, a short (∼100–500 fs) laser pulse by an energetic long (∼10 ps) pulse via strong coupling Stimulated Brillouin Backscattering is investigated with an extensive analysis of one-dimensional particle-in-cell simulations. Parameters relevant to nowadays experimental conditions are investigated. The obtained seed pulse spectra are analyzed as a function of the interaction conditions such as plasma profile, pulses delay, and seed or pulse duration. The factors affecting the amount of energy transferred are determined, and the competition between Brillouin-based amplification and parasitic Raman backscattering is analyzed, leading to the optimization of the interaction conditions.

Ultra intense laser light propagation in a homogeneous overdense plasma was investigated using a plastic foam target filling a polyimide tube. Laser propagation into overdense plasma was measured via Doppler red shift of the reflected laser light from the moving plasma at 0.3-0.4 of speed of light. We also observed strongly collimated electron beam possibly caused by the magnetic field surrounding the plasma channel, and high energy X-rays emitted via synchrotron radiation by the oscillating electrons inside the channel. These features imply that UIL propagates inside the overdense plasma as predicted in PIC calculation, and are very important for direct irradiation scheme of fast ignition. Publisher PDF

This work was supported by Grant No. E1127 from Région Ile de France, the LABEX Plas@Par project, the Grant No. 11‐IDEX‐0004‐02 from Agence Nationale de la Recherche, and Grant No. 001528 from LaserLab-Europe. This work was also supported in part by Grant-in-Aid for Scietific Res (S) (Grant No. 15H05751) of the Japan Society for the Promotion of Science (JSPS). This work was also supported in part by the Ministry of Education and Science of the Russian Federation under Contract No. 14.Z50.31.0007. The ability to produce long-scale length (i.e. millimeter scale-length), homogeneous plasmas is of interest in studying a wide range of fundamental plasma processes. We present here a validated experimental platform to create and diagnose uniform plasmas with a density close or above the critical density. The target consists of a polyimide tube filled with an ultra low-density plastic foam where it was heated by x-rays, produced by a long pulse laser irradiating a copper foil placed at one end of the tube. The density and temperature of the ionized foam was retrieved by using x-ray radiography and proton radiography was used to verify the uniformity of the plasma. Plasma temperatures of 5-10 eV and densities around 1021 cm-3 are measured. This well-characterized platform of uniform density and temperature plasma is of interest for experiments using large-scale laser platforms conducting High Energy Density Physics investigations. Publisher PDF

Plasma-based laser amplification is considered as a possible way to overcome the technological limits of\ud present day laser systems and achieve exawatt laser pulses. Efficient amplification of a picosecond laser\ud pulse by stimulated Brillouin scattering (SBS) of a pump pulse in a plasma requires to reach the self-similar\ud regime of the strongly coupled (SC) SBS. In this Letter, we report on the first observation of the signatures\ud of the transition from linear to self-similar regimes of SC-SBS, so far only predicted by theory and\ud simulations. With a new fully head-on collision geometry, subpicosecond pulses are amplified by a factor\ud of 5 with energy transfers of few tens of mJ. We observe pulse shortening, frequency spectrum broadening,\ud and down-shifting for increasing gain, signatures of SC-SBS amplification entering the self-similar regime.\ud This is also confirmed by the power law dependence of the gain on the amplification length: doubling the\ud interaction length increases the gain by a factor 1.4. Pump backward Raman scattering (BRS) on SC-SBS\ud amplification has been measured for the first time, showing a strong decrease of the BRS amplitude and\ud frequency bandwidth when SBS seed amplification occurs.\ud

The results of experimental and numerical studies of high-intensity proton beam generation driven by a short laser pulse of relativistic intensity are reported. In the experiment, a 350 fs laser pulse of 1.06 or 0.53 μm wavelength and intensity up to 2×1019 Wcm−2 irradiated a thin (0.6–2 μm) plastic (PS) or Au/PS (plastic covered by 0.2 μm Au front layer) target along the target normal. The effect of laser intensity, the target structure and the laser wavelength on the proton beam parameters and laser-protons energy conversion efficiency were examined. Both the measurements and one-dimensional particle-in-cell simulations showed that MeV proton beams of intensity ∼1018 Wcm−2 and current density ∼1012 Acm−2 at the source can be produced when the laser intensity-wavelength squared product I Lλ2 is ∼1019 Wcm−2 μm2 and the laser-target interaction conditions approach the skin-layer ponderomotive acceleration (SLPA) requirements. The simulations also proved that at I Lλ2≥slant 5×1019 Wcm−2 μm2 and λ≤slant 0.53...

The proton radiography technique has been used to investigate the incidence of a 3 ×1019 W/cm2 infrared pulse with a 125 μm-diameter gold wire. The laser interaction is observed to drive the growth of a radial electric field ∼ 1010 V/m on the surface of the wire which rises and decays over a temporal window of 20 ps. Such studies of the ultrafast charging of a solid irradiated at high-intensity may be of relevance to schemes for laser-driven ion acceleration and the fast-ignitor concept for inertial confinement fusion.

This paper reviews recent experimental activity in the area of optimization, control, and application of laser-accelerated proton beams, carried out at the Rutherford Appleton Laboratory and the Laboratoire pour lpsilaUtilisation des Lasers Intenses 100 TW facility in France. In particular, experiments have investigated the role of the scale length at the rear of the plasma in reducing target-normal-sheath-acceleration acceleration efficiency. Results match with recent theoretical predictions and provide information in view of the feasibility of proton fast-ignition applications. Experiments aiming to control the divergence of the proton beams have investigated the use of a laser-triggered microlens, which employs laser-driven transient electric fields in cylindrical geometry, enabling to focus the emitted protons and select monochromatic beamlets out of the broad-spectrum beam. This approach could be advantageous in view of a variety of applications. The use of laser-driven protons as a particle probe for transient field detection has been developed and applied to a number of experimental conditions. Recent work in this area has focused on the detection of large-scale self-generated magnetic fields in laser-produced plasmas and the investigation of fields associated to the propagation of relativistic electron both on the surface and in the bulk of targets irradiated by high-power laser pulses.

The use of laser-accelerated protons as a particle probe for the detection of electric fields in plasmas has led in recent years to a wealth of novel information regarding the ultrafast plasma dynamics following high intensity laser-matter interactions. The high spatial quality and short duration of these beams have been essential to this purpose. We will discuss some of the most recent results obtained with this diagnostic at the Rutherford Appleton Laboratory (UK) and at LULI - Ecole Polytechnique (France), also applied to conditions of interest to conventional Inertial Confinement Fusion. In particular, the technique has been used to measure electric fields responsible for proton acceleration from solid targets irradiated with ps pulses, magnetic fields formed by ns pulse irradiation of solid targets, and electric fields associated with the ponderomotive channelling of ps laser pulses in under-dense plasmas.

The interaction of high-intensity laser pulses with matter releases instantaneously ultra-large currents of highly energetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiments in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channeling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils. Laser-driven impulsive fields at the surface of solid targets can be applied for energy-selective ion beam focusing.

We present a novel technique for focusing and energy selection of high-current, MeV proton/ion beams. This method employs a hollow micro-cylinder that is irradiated at the outer wall by a high intensity, ultra-short laser pulse. The relativistic electrons produced are injected through the cylinder's wall, spread evenly on the inner wall surface of the cylinder, and initiate a hot plasma expansion. A transient radial electric field (107–1010 V/m) is associated with the expansion. The transient electrostatic field induces the focusing and the selection of a narrow band component out of the broadband poly-energetic energy spectrum of the protons generated from a separate laser irradiated thin foil target that are directed axially through the cylinder. The energy selection is tunable by changing the timing of the two laser pulses. Computer simulations carried out for similar parameters as used in the experiments explain the working of the micro-lens.

The interaction of high-intensity laser pulses with matter releases instantaneously ultra-large currents of highly energetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiment in which such phenomena have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channelling in underdense plasmas have been studied with this technique, as also the process of Debye sheath formation and MeV ion front expansion at the rear of laser-irradiated thin metallic foils.

Recent theoretical and experimental studies suggest the possibility of enhancing the efficiency and ease of laser acceleration of protons and ions using underdense or near critical plasmas through electrostatic shocks. Very promising results were recently obtained in this regime. In these experiments, a first ns pulse was focused on a thin target to explode it and a second laser with a high intensity was focused on the exploded foil. The delay between two lasers allowed to control the density gradient seen by the second laser pulse. The transition between various laser ion acceleration regimes depending on the density gradient length was studied. With a laser energy of a few Joules, protons with energies close to the energies of TNSA accelerated protons were obtained for various exploded foils configurations. In the high energy regime (~180 J), protons with energies significantly higher than the ones of TNSA accelerated protons were obtained when exploding the foil while keeping a good beam quality. These results demonstrate that low-density targets are promising candidates for an efficient proton source that can be optimized by choosing appropriate plasma conditions. New experiments were also performed in this regime with gas jets. Scaling shock acceleration in the low density regime to ultra high intensities is a challenge as radiation losses and electron positron pair production change the optimization of the shock process. Using large-scale Particle-In-Cell simulations, the transition to this regime in which intense beams of relativistic ions can be produced is investigated.

One of the main issues of the fast ignitor scheme is the role of fast electron transport in the solid fuel heating. Recent experiments used a new target scheme based on the use of cone to guide the PW laser and enhance the electron production. In this context it is fundamental to understand the physics underlying this new target scheme. We report here recent and preliminary results of ultra-intense laser pulse interaction with three layer targets in presence of the cone or without. Experiments have been performed at LULI with the 100TW laser facility, at intensities up to 3 10 19 W/cm 2 . Several diagnostics have been implemented (2D Kx imaging, Kα spectroscopy and rear side imaging, protons emission) to quantify the cone effect.

The acceleration of ions by ultra-intense lasers has attracted great attention due to the unique properties and the unmatched intensities of the ion beams. In the early days the prospects for applications were already studied, and first experiments have identified some of the areas where laser accelerated ions can contribute to the ongoing inertial confinement fusion (ICF) research. In addition to the idea of laser driven proton fast ignition (PFI) and its use as a novel diagnostic tool for radiography the strong dependence on the electron transport in the target was found to be helpful in investigating the energy transport by electrons in fast ignitor scenarios. More recently an additional idea has been presented to use laser accelerated ion beams as the next generation ion sources, and taking advantage of the luminosity of the beams, to develop a test bed for heavy ion beam driven inertial confinement fusion physics. We review our recent experiments and simulations relevant to ICF research presenting a possible scenario for PFI as well as the prospects for next generation ion sources.

A beam of high energy ions and protons is observed from targets irradiated with intensities up to 5×1019 W/cm2. Maximum proton energy is shown to strongly correlate with laser-irradiance on target. Energy spectra from a magnetic spectrometer show a plateau region near the maximum energy cutoff and modulations in the spectrum at approximately 65% of the cutoff energy. Presented two-dimensional particle-in-cell simulations suggest that modulations in the proton spectrum are caused by the presence of multiple heavy-ion species in the expanding plasma.

Laser–plasma interaction (LPI) physics is one the major issues for the realization of inertial fusion. Parametric instabilities may be driven by the incident laser beams during their propagation in the underdense plasma surrounding the fusion capsule. These instabilities may result in various effects detrimental to a good energy transfer from the laser beams to the target: the backscattering of the incident beams, the generation of energetic electrons which might preheat the fusion fuel, and the spoiling of the laser beam alignment. The control of the linear growth of these instabilities, together with the understanding of their nonlinear saturation mechanisms are therefore of fundamental importance for laser fusion. During the past few years, a series of new concepts have emerged, deeply modifying our approach to LPI physics. In particular, LPI experiments are now carried out with laser beams which are optically smoothed by means of random phase plates. Such beams are characterized inside the plasma by randomly distributed intensity maxima. Filamentation instabilities may locally increase the laser intensity maxima and deplete the electron density, leading to an intricate coupling between various nonlinear processes. One of the most striking features of this intricate coupling is the resulting ability of the plasma to induce additional temporal and spatial incoherence to the laser beams during their propagation. The increased incoherence may in turn reduce the level of backscattering instabilities.

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...

Significantly collimated fast electron beam with a divergence angle 10° (FWHM) is observed when an ultra-intense laser pulse (I = 1014 W/cm2, 300 fs) irradiates a uniform critical density plasma. The uniform plasma is created through the ionization of an ultra-low density (5 mg/c.c.) plastic foam by X-ray burst from the interaction of intense laser (I = 1014 W/cm2, 600 ps) with a thin Cu foil. 2D Particle-In-Cell (PIC) simulation well reproduces the collimated electron beam with a strong magnetic field in the region of the laser pulse propagation. To understand the physical mechanism of the collimation, we calculate energetic electron motion in the magnetic field obtained from the 2D PIC simulation. As the results, the strong magnetic field (300 MG) collimates electrons with energy over a few MeV. This collimation mechanism may attract attention in many applications such as electron acceleration, electron microscope and fast ignition of laser fusion. Publisher PDF

Proton beams driven from short pulse lasers can deposit their energy in material with exceptionally high intensity and drive rapidly evolving plasma conditions and dynamic currents which lead to previously-unseen proton stopping and transport behavior. These effects, such as extended range and narrowing of the heated plasma volume, become more prominent as the beam energy and pulse length increase.

International audience; Since the inception of the laser, there has been a constant push toward increasing the laser peak intensity, as this has lead to opening the exploration of new territories, and the production of compact sources of particles and radiation with unprecedented characteristics. However, increasing the peak laser intensity is usually performed by enhancing the produced laser properties, either by lowering its duration or increasing its energy, which involves a great level of complexity for the laser chain, or comes at great cost. Focusing tightly is another possibility to increase the laser intensity, but this comes at the risk of damaging the optics with target debris, as it requires their placement in close proximity to the interaction region. Plasma devices are an attractive, compact alternative to tightly focus extreme light pulses and further increase the final laser intensity.

International audience; Although bipolar jets are seen emerging from a wide variety of astrophysical systems, the issue of their formation and morphology beyond their launching is still under study. Our scaled laboratory experiments, representative of young stellar object outflows, reveal that stable and narrow collimation of the entire flow can result from the presence of a poloidal magnetic field whose strength is consistent with observations. The laboratory plasma becomes focused with an interior cavity. This gives rise to a standing conical shock from which the jet emerges. Following simulations of the process at the full astrophysical scale, we conclude that it can also explain recently discovered x-ray emission features observed in low-density regions at the base of protostellar jets, such as the well-studied jet HH 154.

International audience; Using hybrid simulations, we investigate the onset of fast reconnection between two cylindrical magnetic shells initially close to each other. This initial state mimics the plasma structure in High Energy Density Plasmas induced by a laser-target interaction and the associated self-generated magnetic field. We clearly observe that the classical quadrupolar structure of the out-of-plane magnetic field appears prior to the reconnection onset. Furthermore, a parametric study reveals that, with a non-coplanar initial magnetic topology, the reconnection onset is delayed and possibly suppressed. The relation between the out-of-plane magnetic field and the out-of-plane electric field is discussed.

International audience; It was recently shown that a promising way to accelerate protons in the forward direction to high energies is to use under-dense or near-critical density targets instead of solids. Simulations have revealed that the acceleration process depends on the density gradients of the plasma target. Indeed, under certain conditions, the most energetic protons are predicted to be accelerated by a collisionless shock mechanism that significantly increases their energy. We report here the results of a recent experiment dedicated to the study of longitudinal ion acceleration in partially exploded foils using a high intensity (similar to 5 x 10(18) W/cm(2)) picosecond laser pulse. We show that protons accelerated using targets having moderate front and rear plasma gradients (up to similar to 8 mu m gradient length) exhibit similar maximum proton energy and number compared to proton beams that are produced, in similar laser conditions, from solid targets, in the well-known target normal sheath acceleration regime. Particle-In-Cell simulations, performed in the same conditions as the experiment and consistent with the measurements, allow laying a path for further improvement of this acceleration scheme. (C) 2014 AIP Publishing LLC.

The intricate spatial and energy distribution of magnetic fields, self-generated during high power laser irradiation (at $I{\ensuremath{\lambda}}^{2}\ensuremath{\sim}1{0}^{13}\ensuremath{-}1{0}^{14}\text{ }\text{ }\mathrm{W}.{\mathrm{cm}}^{\ensuremath{-}2}.\ensuremath{\mu}{\mathrm{m}}^{2}$) of a solid target, and of the heat-carrying electron currents, is studied in inertial confinement fusion (ICF) relevant conditions. This is done by comparing proton radiography measurements of the fields to an improved magnetohydrodynamic description that fully takes into account the nonlocality of the heat transport. We show that, in these conditions, magnetic fields are rapidly advected radially along the target surface and compressed over long time scales into the dense parts of the target. As a consequence, the electrons are weakly magnetized in most parts of the plasma flow, and we observe a reemergence of nonlocality which is a crucial effect for a correct description of the energetics of ICF experiments.

In this paper, we detail the diagnostic technique used to infer the spatially resolved electron temperatures and densities in experiments dedicated to investigate the generation of magnetically collimated plasma jets. It is shown that the relative intensities of the resonance transitions in emitting He-like ions can be used to measure the temperature in such recombining plasmas. The intensities of these transitions are sensitive to the plasma density in the range of 1016–1020 cm−3 and to plasma temperature ranges from 10 to 100 eV for ions with a nuclear charge Zn ∼ 10. We show how detailed calculations of the emissivity of F VIII ions allow to determine the parameters of the plasma jets that were created using ELFIE ns laser facility (Ecole Polytechnique, France). The diagnostic and analysis technique detailed here can be applied in a broader context than the one of this study, i.e., to diagnose any recombining plasma containing He-like fluorine ions.

Of vital importance for Inertial Confinement Fusion (ICF) are the understanding and control of the nonlinear processes which can occur during the propagation of the laser pulses through the underdense plasma surrounding the fusion capsule. The control of parametric instabilities has been studied experimentally, using the LULI six-beam laser facility, and also theoretically and numerically. New results based on the direct observation of plasma waves with Thomson scattering of a short wavelength probe beam have revealed the occurence of the Langmuir decay instability. This secondary instability may play an imporant role in the saturation of stimulated Raman scattering. Another mechanism for reducing the growth of the scattering instabilities is the so-called `plasma-induced incoherence'. Namely, recent theoretical studies have shown that the propagation of laser beams through the underdense plasma can increase their spatial and temporal incoherence. This plasma-induced beam smoothing can reduce the levels of parametric instabilities. One signature of this process is a large increase of the spectral width of the laser light after propagation through the plasma. Comparison of the experimental results with numerical simulations shows an excellent agreement between the observed and calculated time-resolved spectra of the transmitted laser light at various laser intensities.

To test the validity of kinetics for laser-produced plasmas, one would like to measure the X-ray spectrum emitted from a plasma volume whose characteristics are determined by diagnostics that do not rely on interpreting the X-ray spectrum itself. An experimental test bed has been developed at the Janus laser at Lawrence Livermore National Laboratory to simultaneously characterize the electron temperature, the electron density and the X-ray emission from laser-irradiated aluminum dot targets. Thomson scattering, interferometry and pinhole imaging are implemented to achieve this. Further, the X-ray spectrum from 1–2 keV is spatially integrated and is obtained using a flat crystal (PET) and an X-ray streak camera for time resolution. Spectra have been calculated using the HULLAC and FLY atomic physics codes which use the measured density and temperature as input, and DCA which calculates X-ray spectra from the 2-D expanding plasma calculated by a hydrodynamics code. Comparisons of the models with the data will be discussed and future directions will be indicated.