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The development, characterization and implementation of a water (H 2 O) droplet source for laser–plasma interactions at high-repetition rates are reported. The system is capable of generating a continuous stream of water droplets for several hours and was tested to operate continuously as a target source. Based on glass capillaries, a detailed parameter scan on the automated pulling process is presented, allowing a fine-tuning of the shape of the capillaries and therefore the diameter of the droplets.

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.

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...

With the latest configuration, the Ti:Sa laser system ARCTURUS (Düsseldorf University, Germany) operates with a double-chirped pulse amplification (CPA) architecture delivering pulses with an energy of 7 J before compression in each of the two high-power beams. By the implementation of a plasma mirror system, the intrinsic laser contrast is enhanced up to $10^{-12}$ on a time scale of hundreds of picoseconds, before the main peak. The laser system has been used in various configurations for advanced experiments and different studies have been carried out employing the high-power laser beams as a single, high-intensity interaction beam ( $I\approx 10^{20}~\text{W}/\text{cm}^{2}$ ), in dual- and multi-beam configurations or in a pump–probe arrangement.

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

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.

We report on the experimental observation of the instability of a plasma shell, which formed during the expansion of a laser-ablated plasma into a rarefied ambient medium. By means of a proton radiography technique, the evolution of the instability is temporally and spatially resolved on a timescale much shorter than the hydrodynamic one. The density of the thin shell exceeds that of the surrounding plasma, which lets electrons diffuse outward. An ambipolar electric field grows on both sides of the thin shell that is antiparallel to the density gradient. Ripples in the thin shell result in a spatially varying balance between the thermal pressure force mediated by this field and the ram pressure force that is exerted on it by the inflowing plasma. This mismatch amplifies the ripples by the same mechanism that drives the hydrodynamic nonlinear thin-shell instability (NTSI). Our results thus constitute the first experimental verification that the NTSI can develop in colliding flows. Funding agencies: EPSRC [EP/I031766/1, EP/K022415/1, EP/I029206/1, SFB-TR18, GRK1203, ENE2013-45661-C2-1-P, PEII-2014-008-P]; Vetenskapsradet [Dnr 2010-4063]; Triangle de la Physique RTRA network (ULIMAC)

Proton radiography using laser-driven sources has been developed as a diagnostic since the beginning of the decade, and applied successfully to a range of experimental situations. Multi-MeV protons driven from thin foils via the Target Normal Sheath Acceleration mechanism, offer, under optimal conditions, the possibility of probing laser-plasma interactions, and detecting electric and magnetic fields as well as plasma density gradients with ~ps temporal resolution and ~ 5–10 µm spatial resolution. In view of these advantages, the use of proton radiography as a diagnostic in experiments of relevance to Inertial Confinement Fusion is currently considered in the main fusion laboratories. This paper will discuss recent advances in the application of laser-driven radiography to experiments of relevance to Inertial Confinement Fusion. In particular we will discuss radiography of hohlraum and gasbag targets following the interaction of intense ns pulses. These experiments were carried out at the HELEN laser facility at AWE (UK), and proved the suitability of this diagnostic for studying, with unprecedented detail, laser-plasma interaction mechanisms of high relevance to Inertial Confinement Fusion. Non-linear solitary structures of relevance to space physics, namely phase space electron holes, have also been highlighted by the measurements. These measurements are discussed and compared to existing models.

Multi-Mev proton beams generated by target normal sheath acceleration (TNSA) during the interaction of an ultra intense laser beam (I≥1019 W/cm2) with a thin metallic foil (thickness of the order of a few tens of microns) are particularly suited as a particle probe for laser plasma experiments. The proton imaging technique employs a laser-driven proton beam in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in such experiments. The proton probing technique has been applied in experiments of relevance to inertial confinement fusion (ICF) such as laser heated gasbags and laser-hohlraum experiments. The data provides direct information on the onset of laser beam filamentation and on the plasma expansion in the hohlraum’s interior, and confirms the suitability and usefulness of this technique as an ICF diagnostic.

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 chirped pulse amplification (CPA) technique has opened new perspectives in the radiation-matter interaction studies using ultra-short laser pulses at ultra-relativistic intensities. In particular the original idea, proposed by Tajima and Dawson, of accelerating electrons by the huge electric fields of plasma waves which develop in the wake of a laser pulse propagating in a plasma, become feasible. Some laboratories all over the world have produced by such a technique collimated electron busts of hundreds of MeV along acceleration lengths of a few hundreds of microns. In other experiments, using thin solid targets, intense bursts of energetic protons have been at the same time detected. The proton acceleration mechanism is essentially based on the Coulomb force appearing at the thin solid target surface as a consequence of the previous escape of the energetic electrons from the target. In the paper some experimental results will be presented as well as the opportunities the INFN PLASMONX project will offer in this research field at LNF.

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.

Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron (RE) beams and their losses are proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode-number magneto-hydro-dynamics (MHD) modes. It is hypothesized that the RE beam is formed in the central plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of plasmas with the safety factor $q(0) the most stable RE beams are formed by the outermost intact magnetic surface located between the magnetic surface $q=1$ and the closest low-order rational surface $q=m/n>1~(q=5/4,q=4/3,\dots )$. The thermal quench (TQ) time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current quench (CQ) stage is due to the particle transport in a stochastic magnetic field. The RE beam current is modelled as a sum of a toroidally symmetric part and a small-amplitude helical current with a predominant $m/n=1/1$ component. The REs are lost due to two effects: (i) by outward drift of electrons in a toroidal electric field until they touch the wall and (ii) by the formation of a stochastic layer of REs at the beam edge. Such a stochastic layer for high-energy REs is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to the wall. The effect of external resonant magnetic perturbations on RE loss is discussed. A possible cause of the sudden MHD signals accompanied by RE bursts is explained by the redistribution of runaway current during the resonant interaction of high-energetic electron orbits with the $m/n=1/1$ MHD mode.

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.

A new physical mechanism of formation of runaway electron (RE) beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between $q=1$ and the closest low--order rational magnetic surfaces [$q=5/4$ or $q=4/3$, \dots]. It results in that runaway electron beam current has a helical nature with a predominant $m/n=1/1$ component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high--energy electrons with the $m/n=1/1$ MHD mode are discussed., Comment: 5 pages, 4 figures, one table. arXiv admin note: text overlap with arXiv:1501.01404

International audience; A method for obtaining quantitative information about electric field and charge distributions from proton imaging measurements of laser-induced plasmas is presented. A parameterised charge distribution is used as target plasma. The deflection of a proton beam by the electric field of such a plasma is simulated numerically as well as the resulting proton density, which will be obtained on a screen behind the plasma according to the proton imaging technique. The parameters of the specific charge distributions are delivered by a combination of linear regression and nonlinear fitting of the calculated proton density distribution to the measured optical density of a radiochromic film screen changed by proton exposure. It is shown that superpositions of spherical Gaussian charge distributions as target plasma are sufficient to simulate various structures in proton imaging measurements, which makes this method very flexible.

High power lasers are a tool that can be used to determine important parameters in the context of Warm Dense Matter, i.e. at the convergence of low-temperature plasma physics and finite-temperature condensed matter physics. Recent results concerning planet inner core materials such as water and iron are presented. We determined the equation of state, temperature and index of refraction of water for pressures up to 7 Mbar. The release state of iron in a LiF window allowed us to investigate the melting temperature near the inner core boundary conditions. Finally, the first application of proton radiography to the study of shocked material is also discussed.

Energetic ion beams are produced during the interaction of ultrahigh-intensity, short laser pulses with plasmas. These laser-produced ion beams have important applications ranging from the fast ignition of thermonuclear targets to proton imaging, deep proton lithography, medical physics, and injectors for conventional accelerators. Although the basic physical mechanisms of ion beam generation in the plasma produced by the laser pulse interaction with the target are common to all these applications, each application requires a specific optimization of the ion beam properties, that is, an appropriate choice of the target design and of the laser pulse intensity, shape, and duration.

Low-frequency, relativistic, subcycle solitary waves are found in two-dimensional and three-dimensional particle-in-cell (PIC) numerical simulations, as a result of the interaction of ultrashort, high-intensity laser pulses with plasmas. Moreover, nondrifting, subcycle relativistic electromagnetic solitons have been obtained as solutions of the hydrodynamic equations for an electron–ion warm plasma, by assuming the quasi-neutrality character of the plasma response. In addition, the formation of long-living macroscopic soliton-like structures has been experimentally observed by means of the proton imaging diagnostics. Several common features result from these investigations, as, for example, the quasi-neutral plasma response to the soliton radiation, in the long-term evolution of the system, which leads to the almost complete expulsion of the plasma from the region where the electromagnetic radiation is concentrated, even at subrelativistic field intensity. The results of the theoretical investigations are reviewed with special attention to these similarities.

One of the most exciting results recently obtained in the ultraintense interaction research area is the observation of beams of protons with energies up to several tens of megaelectron volts, generated during the interaction of ultraintense picosecond pulses with solid targets. The particular properties of these beams (high brilliance, small source size, high degree of collimation, short duration) make them of exceptional interest in view of diagnostic applications. In a series of experiments carried out at the Rutherford Appleton Laboratory (RAL) and at the Lawrence Livermore National Laboratory (LLNL), the laser-produced proton beams have been characterized in view of their application as a particle probe for high-density matter, and applied to diagnose ultraintense laser–plasma interactions. In general, the intensity cross section of a proton beam traversing matter will be modified both by collisional stopping/scattering, and deflections caused by electric/magnetic fields. With a suitable choice of irradiation geometry and target parameters, the proton probe can be made mainly sensitive to the electric field distribution in the object probed. Therefore, point projection proton imaging appears as a powerful and unique technique for electric field detection in laser-irradiated targets and plasmas. The first measurements of transient electric fields in high-intensity laser-plasma interactions have been obtained with this technique.

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.

Detailed investigations of the propagation of an ultraintense picosecond laser pulse through preformed plasmas have been carried out. An underdense plasma with peak density around 0.1nc was generated by exploding a thin foil target with an intense nanosecond laser pulse. The formation of plasma channels with an ultraintense laser pulse due to ponderomotive expulsion of elections and the subsequent Coulomb explosion were investigated. The laser transmission through underdense plasmas was measured for a picosecond pulse at intensities above 1019 W/cm2 with and without a plasma channel preformed with an ultraintense prepulse. The energy transmitted through the plasma increased from the few percent transmittance measured in absence of the preformed channel to almost 100% transmission with the channelling to main pulse delay at around 100 ps. The propagation of a relativistic laser pulse through overdense plasmas was also investigated. A well-characterized plasma with an electron density up to 8nc was generated by soft X-ray irradiation of a low-density foam target. The propagation of the laser pulse was observed via X-ray imaging and monitoring the energy transmission through the plasma. Evidence of collimated laser transport was obtained.

A detailed investigation of many aspects of the physics of laser–plasma interaction at very high laser intensities is required in order to assess the feasibility and the promise of the fast ignitor scheme for inertial confinement fusion. Relevant results, obtained in a series of experiments carried out at the Rutherford Appleton Laboratory, Chilton (UK) and at the Centre d'Etudes Atomique, Limeil Valenton (France), are presented and discussed here. In particular, the formation of plasma channels was observed following the propagation of relativistically intense, ps laser pulses through underdense plasmas. The channels persist long after the interaction, and their expansion has been measured. Efficient guiding of ultraintense laser pulses, both through preformed density channels and through solid guides, has been demonstrated. Finally, indication of collimated fast electron propagation through solid targets has been obtained from the observation of filamentary ionization tracks in laser irradiated solid targets.

We present results of the first laser-driven proton acceleration experiment with two ultrashort laser pulses at the Arcturus laser facility in Dusseldorf. The Ti:sapphire laser provides two laser pulses (30fs, 2.3J; 30fs, 3.7J) which reaches focused intensities up to 1020 W/cm2. The second pulse has a contrast of 108 on a picosecond timescale. A plasma mirror enhances the contrast of the first pulse by three orders of magnitude on a picosecond timescale. The scheme of the experiment is as follows. The first laser pulse irradiates a thin foil and accelerates ions according to the target normal sheath acceleration mechanism. 10s of femtoseconds after the first pulse the second pulse is interacting with the expanding plasma and enhances the acceleration process of the ions. In the experiment the pulses were spatially overlapping on a 5μm thin Titanium foil. The delay between the pulses was varied from 10s to 100s of femtoseconds. As a main diagnostic a Thomson Parabola was employed in combination with a micro-channel-plate. In a second part of the experiment a time-and-space-resolved-interferometer (TASRI) was used to determine the electron density and temperature at the rear side of the foil with a 3μm spatial and 3ps temporal resolution. Two regimes were observed, which were only present in the two-beam configuration. One regime is dominated by a large number of different (up to 13) ion species - namely carbon, nitrogen, oxygen and titanium ions. But in this regime only a relatively low maximum proton energy was observed. In contrast to this is the other regime, where only a few weak ion species were present. In return protons with relatively high energy could be measured. The proton maximum cut-off energy in this regime was distinctly higher compared to the single-beam configuration. First simulations with the particle-in-cell code EPOCH indicate the enhanced acceleration mechanism is driven by a second push of the electrons from the second laser pulse.

The effective action of strong electrical fields on a beam of protons passing through a laser irradiated thin foil has been investigated. The energy distribution function of protons propagating along the surface normal changes in a pronounced way, exhibiting a gap in the spectrum accompanied by up to two local maxima. The temporal behavior is set into context with expectations derived from the evolution of strong electrical fields at the plasma-vacuum interface, usually being considered responsible for fast ion acceleration during the initial stage of laser driven plasma expansion. Our investigation reveals complex field effects in thin foils when irradiated with intense and ultra-short pulses with a very high temporal contrast. The experiments were performed with a laser accelerated proton beam, the probe, traversing a “plasma slab” created by ultra-short ( 80fs), high-intensity (~ 1 × 1019 W/cm2) laser irradiation of a 30 nm to 800 nm thick foil. Laser pulses with different temporal contrast and pulse duration have been used, both for the probe and for the plasma slab creation (the pump). An analytical model is discussed to approach an understanding of the observation.

International audience; Using ion carbon beams generated by high intensity short pulse lasers we perform measurements of single shot mean charge equilibration in cold or isochorically heated solid density aluminum matter. We demonstrate that plasma effects in such matter heated up to 1 eV do not significantly impact the equilibration of carbon ions with energies 0.045-0: 5 MeV/nucleon. Furthermore, these measurements allow for a first evaluation of semiempirical formulas or ab initio models that are being used to predict the mean of the equilibrium charge state distribution for light ions passing through warm dense matter. DOI: 10.1103/PhysRevLett.110.135003

We present experimental results for laser pulse compression due to wakefield excitation in the relativistic regime using the spectral phase interferometry for direct electric-field reconstruction technique for complete pulse characterization. The results indicate that the pulse was compressed to a 10-fs duration which was verified by fully relativistic three-dimensional particle-in-cell simulations. Optimized modeling based on a hydrodynamic code was also done, showing the possibility of producing laser pulses of few-cycle duration.

Summary form only given. Collisionless shocks are relevant to a variety of astrophysical scenarios, such as the generation of highly energetic particles and cosmic rays during supernova explosions, and have recently been the focus of several laboratory plasma investigations employing high-power lasers. We present here a study of the generation and evolution of collisionless shocks in tenuous plasma, carried out at the VULCAN laser facility, Rutherford Appleton Laboratory. The shocks are generated during the expansion of a warm plasma, produced after the irradiation of a thin solid foil by a long (∼1ns) and intense (∼1015 W/cm2) laser pulse, into a tenuous (∼ 1016 cm−3), non magnetized background plasma. Shock structures are seen to develop at the interface between the two plasmas. The shocks are probed via a proton projection imaging (PPI) technique [1], which allows one to measure the electric field distribution at the shock front with high spatial resolution (∼ µm), and temporally resolve its propagation within a ps scale. The temporal evolution of electrostatic shocks in ambient plasma has been observed and the associated electric field has been reconstructed in a number of different conditions. In particular, we have identified in the data the transition from an unipolar field profile, typical of a double layer structure, into a bipolar electric field, and the subsequent formation of a collisionless, electrostatic shock. This structure is then seen to propagate in the ambient plasma with a Mach number ∼3.5. A PIC simulation supports the existence of high Mach number (∼3.3) shocks launched by collision of plasma clouds of equal electron and ion temperatures [2].

We investigate the properties of a laser-plasma electron accelerator as a bright source of keV x-ray radiation. During the interaction, the electrons undergo betatron oscillations and from the carefully measured x-ray spectrum the oscillation amplitude of the electrons can be deduced which decreases with increasing electron energies. From the oscillation amplitude and the independently measured x-ray source size of $(1.8\ifmmode\pm\else\textpm\fi{}0.3)\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ we are able to estimate the electron bunch diameter to be $(1.6\ifmmode\pm\else\textpm\fi{}0.3)\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$.

The development of current instabilities behind the front of a cylindrically expanding plasma has been investigated experimentally via proton probing techniques. A multitude of tubelike filamentary structures is observed to form behind the front of a plasma created by irradiating solid-density wire targets with a high-intensity (I~10 19 W/cm²), picosecond-duration laser pulse. These filaments exhibit a remarkable degree of stability, persisting for several tens of picoseconds, and appear to be magnetized over a filament length corresponding to several filament radii. Particle-in-cell simulations indicate that their formation can be attributed to a Weibel instability driven by a thermal anisotropy of the electron population. We suggest that these results may have implications in astrophysical scenarios, particularly concerning the problem of the generation of strong, spatially extended and sustained magnetic fields in astrophysical jets. Refereed/Peer-reviewed

The dynamics of magnetic fields with amplitude of several tens of Megagauss, generated at both sides of a solid target irradiated with a high intensity (? 1019W/cm2) picosecond laser pulse, has been spatially and temporally resolved using a proton imaging technique. The amplitude of the magnetic fields is sufficiently large to have a constraining effect on the radial expansion of the plasma sheath at the target surfaces. These results, supported by numerical simulations and simple analytical modeling, may have implications for ion acceleration driven by the plasma sheath at the rear side of the target as well as for the laboratory study of self-collimated high-energy plasma jets., Accepted for publication in Physical Review Letters

A review of the proton radiography technique will be presented. This technique employs laser-accelerated laminar bunches of protons to diagnose the temporal and spatial characteristic of the electric and magnetic fields generated during high-intensity laser-plasma interactions. The remarkable temporal and spatial resolution that this technique can achieve (of the order of a picosecond and a few microns respectively) candidates this technique as the preferrable one, if compared to other techniques, to probe high intensity laser-matterinteractions. © 2012 American Institute of Physics.

The experimental evidence of the correlation between the initial electron density of the plasma and electromagnetic soliton excitation at the wake of an intense (10(19) W/cm(2)) and short (1 ps) laser pulse is presented. The spatial distribution of the solitons, together with their late time evolution into post-solitons, is found to be dependent upon the background plasma parameters, in agreement with published analytical and numerical findings. The measured temporal evolution and electrostatic field distribution of the structures are consistent with their late time evolution and the occurrence of multiple merging of neighboring post-solitons. (C) 2011 American Institute of Physics. [doi:10.1063/1.3625261]

The physics of laser-plasma interactions has undergone dramatic improvements in recent years. By directing a multi-TW, ultrashort laser pulse onto a thin foil or a gas jet, it is nowadays possible to produce multi-MeV proton, ion and electron beam s. Although much progress has been made in characterizing and improving the quality of such laser-generated beams, it is still an untouched issue whether the laser-generated beams are or can be spin-polarized and, thus, whether laser-based polarized sourc es are conceivable. To this end, one may either think of a selection of certain spin states through the huge magnetic field gradients that are inherently generated in the laser-generated plasmas, or of pre-polarized target particles which maintain their p olarization during the rapid acceleration procedure. We have developed a method to measure the degree of polarization of protons that have been accelerated at the 200 TW laser facility ARCturus at Duesseldorf University. As a next step, measurements with unpolarized H(2) (for proton acceleration) and (3)He gas (for (3)He ions) are planned and, finally, pre-polarized (3)He will be used.

Slowly evolving, regularly spaced patterns have been observed in proton projection images of plasma channels drilled by intense ($\ensuremath{\gtrsim}{10}^{19}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$) short ($\ensuremath{\sim}1\text{ }\text{ }\mathrm{ps}$) laser pulses propagating in an ionized gas jet. The nature and geometry of the electromagnetic fields generating such patterns have been inferred by simulating the laser-plasma interaction and the following plasma evolution with a two-dimensional particle-in-cell code and the probe proton deflections by particle tracing. The analysis suggests the formation of rows of magnetized soliton remnants, with a quasistatic magnetic field associated with vortexlike electron currents resembling those of magnetic vortices.

The onset of filamentation, following the interaction of a relatively long (${\ensuremath{\tau}}_{L}\ensuremath{\simeq}1\text{ }\text{ }\mathrm{ns}$) and intense (${I}_{L}\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) laser pulse with a neopentane filled gas bag target, has been experimentally studied via the proton radiography technique, in conditions of direct relevance to the indirect drive inertial confinement fusion scheme. The density gradients associated with filamentation onset have been spatially resolved yielding direct and unambiguous evidence of filament formation and quantitative information about the filamentation mechanism in agreement with previous theoretical modelings. Experimental data confirm that, once spatially smoothed laser beams are used, filamentation is not a relevant phenomenon during the heating laser beams propagation through typical hohlraum gas fills.

An ultracompact laser-plasma-generated, fs-scale electron double bunch system can be injected into a high-density driver/witness-type plasma wakefield accelerator afterburner stage to boost the witness electrons monoenergetically to energies far beyond twice their initial energy on the GeV scale. The combination of conservation of monoenergetic phase-space structure and fs duration with radial electric plasma fields E(r) similar to 100 GV/m leads to dramatic transversal witness compression and unprecedented charge densities. It seems feasible to upscale and implement the scheme to future accelerator systems.

The interaction of an extremely intense laser beam with the dense plasma generated on a solid target can lead tocoherent electron dynamics at the vacuum plasma interface which causes the emission of ultra short light burstswithin each optical cycle of the driving laser pulse. Therefore, the resulting pulse train consists of high harmonicsof the laser fundamental frequency. We report on the, to our knowledge, “rst observation of surface-harmonicsup to the 42nd order without any contrast improvement by a plasma mirror. In the spectral region from the25th to the 40th order approximately 10 7 photons were detected in a solid angle of 4 ·10 Š 4 sr.Keywords: surface harmonics, attosecond pulses, laser-plasma interaction 1. INTRODUCTION Phase locked dynamics of electrons in laser “elds can lead to the emission of attosecond light bursts and opennovel opportunities to probe matter at on previously unaccessible short timescales. 1,2 High-harmonic generationin gases is a well-known way to generate these short pulses.

The interaction of a $3\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ laser pulse with a metallic wire has been investigated using proton radiography. The pulse is observed to drive the propagation of a highly transient field along the wire at the speed of light. Within a temporal window of 20 ps, the current driven by this field rises to its peak magnitude $\ensuremath{\sim}{10}^{4}\text{ }\text{ }\mathrm{A}$ before decaying to below measurable levels. Supported by particle-in-cell simulation results and simple theoretical reasoning, the transient field measured is interpreted as a charge-neutralizing disturbance propagated away from the interaction region as a result of the permanent loss of a small fraction of the laser-accelerated hot electron population to vacuum.

We have used time and space resolved interferometry to measure the dynamics of a plasma expanding off the surface of a solid target. This allows accessing 1) the time- and space-resolved dynamics of the fast electrons (laser-accelerated from solids and expanding into vacuum from the rear target surface), and of the energy partition into bulk (cold) electrons and 2) the expansion velocity of an isochorically heated target, from which information about the heating process can be retrieved.

Experimental evidence of plasma jets ejected from the rear side of thin solid targets irradiated by ultraintense (> 10(19) W cm(-2)) laser pulses is presented. The jets, detected by transverse interferometric measurements with high spatial and temporal resolutions, show collimated expansion lasting for several hundreds of picoseconds and have substantially steep density gradients at their periphery. The role played by radiation pressure of the laser in the jet formation process is highlighted analytically and by extensive two-dimensional particle-in-cell simulations.

The propagation in a rarefied plasma (${n}_{e}\ensuremath{\lesssim}{10}^{15}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$) of collisionless shock waves and ion-acoustic solitons, excited following the interaction of a long (${\ensuremath{\tau}}_{L}\ensuremath{\sim}470\text{ }\text{ }\mathrm{ps}$) and intense ($I\ensuremath{\sim}{10}^{15}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$) laser pulse with solid targets, has been investigated via proton probing techniques. The shocks' structures and related electric field distributions were reconstructed with high spatial and temporal resolution. The experimental results were interpreted within the framework of the nonlinear wave description based on the Korteweg--de Vries--Burgers equation.

Absorption measurements on solid conducting targets have been performed in s and p polarization with ultrashort, high-contrast Ti:sapphire laser pulses at intensities up to 5x10{16}W/cm{2} and pulse duration of 8 fs. The particular relevance of the reported absorption measurements lies in the fact that the extremely short laser pulse interacts with matter close to solid density during the entire pulse duration. A pronounced increase of absorption for p polarization at increasing angles is observed reaching 77% for an incidence angle of 80 degrees . Simulations performed using a 2D particle in cell code show a very good agreement with the experimental data for a plasma profile of L/lambda approximately 0.01.

Ion acceleration from solid targets irradiated by high-intensity pulses is a burgeoning area of research, currently attracting large interest worldwide. The properties of laser-driven ion beams (high brightness and laminarity, high-energy cut-off, ultrashort burst duration) are, in several respects, markedly different from those of 'conventional' accelerator beams. In view of these properties, laser-driven ion beams have the potential to be employed in a number of innovative applications in the scientific, technological and medical areas. The scope of this paper is to briefly review the state of the art in this area and discuss in this context some recent results obtained by our group and collaborators, aimed at studying the acceleration mechanism, optimizing the source properties and applying the beams as a diagnostic tool in scientific experiments. An application that has already produced important results is the use of laser-accelerated protons as a particle probe for the detection of electric and magnetic fields in plasmas. Examples of these results will be presented and discussed.