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In this paper, we report on the acceleration of protons and oxygen ions from tens of micrometer large water droplets by a high-intensity laser in the range of 1020 W/cm2. Proton energies of up to 6 MeV were obtained from a hybrid acceleration regime between classical Coulomb explosion and shocks. Besides the known thermal energy spectrum, a collective acceleration of oxygen ions of different charge states is observed. 3D PIC simulations and analytical models are employed to support the experiential findings and reveal the potential for further applications and studies.

The process of high energy electron acceleration along the surface of grating targets (GTs) that were irradiated by a relativistic, high-contrast laser pulse at an intensity $I=2.5\times 10^{20}~\text{W}/\text{cm}^{2}$ was studied. Our experimental results demonstrate that for a GT with a periodicity twice the laser wavelength, the surface electron flux is more intense for a laser incidence angle that is larger compared to the resonance angle predicted by the linear model. An electron beam with a peak charge of ${\sim}2.7~\text{nC}/\text{sr}$ , for electrons with energies ${>}1.5~\text{MeV}$ , was measured. Numerical simulations carried out with parameters similar to the experimental conditions also show an enhanced electron flux at higher incidence angles depending on the preplasma scale length. A theoretical model that includes ponderomotive effects with more realistic initial preplasma conditions suggests that the laser-driven intensity and preformed plasma scale length are important for the acceleration process. The predictions closely match the experimental and computational results.

We present a study on laser-driven proton acceleration from a hydrogen cluster target. Aiming for the optimisation of the proton source, we performed a detailed parametric scan of the interaction conditions by varying different laser and the target parameters. While the underlying process of a Coulomb-explosion delivers moderate energies, in the range of 100 s of keV, the use of hydrogen as target material comes with the benefit of a debris-free, single-species proton acceleration scheme, enabling high repetition-rate experiments, which are very robust against shot-to-shot fluctuations.

By directing the laser-driven electromagnetic pulses along a helical path, one can achieve a travelling-field accelerator arrangement for simultaneous beam shaping and re-acceleration of laser-accelerated protons. The dynamics of guided acceleration of the transiting protons was studied by varying the length of the helical coil. Experimental data shows that the protons co-moving with the field region exhibit stronger focussing while increasing the coil length, with an increase of kinetic energy due to simultaneous post-acceleration. The net energy gain for a coil of constant pitch however saturates eventually when the post-accelerated protons overtakes the accelerating field region in due course. 3D particle tracing simulation underpins the dynamics of beam transport inside the coil, which highlights the requirement for a variable pitch coil geometry in order to sustain the post-acceleration over an extended coil.

Filamentary ionization tracks have been observed via optical probing inside Al-coated glass targets after the interaction of a picosecond 20-TW laser pulse at intensities above 10 19 W/cm 2. The tracks, up to 700 μm in length and between 10 and 20 μm in width, originate from the focal spot region of the laser beam. Simulations performed with 3D particle-in-cell and 2D Fokker-Planck hybrid codes indicate that the observations are consistent with ionization induced in the glass target by magnetized, collimated beams of high-energy electrons produced during the laser interaction.

Due to their particular properties, the beams of the multi-MeV protons generated during the interaction of ultraintense (I>10(19) W/cm(2)) short pulses with thin solid targets are most suited for use as a particle probe in laser-plasma experiments. The recently developed proton imaging technique employs the beams in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in laser-plasma interaction experiments. In recent investigations carried out at the Rutherford Appleton Laboratory (RAL, UK), a wide range of laser-plasma interaction conditions of relevance for inertial confinement fusion (ICF)/fast ignition has been explored. Among the results obtained will be discussed: the electric field distribution in laser-produced long-scale plasmas of ICF interest; the measurement of highly transient electric fields related to the generation and dynamics of hot electron currents following ultra-intense laser irradiation of targets; the observation in underdense plasmas, after the propagation of ultra-intense laser pulses, of structures identified as the remnants of solitons produced in the wake of the pulse.

A series of experiments recently carried out at the Rutherford Appleton Laboratory investigated various aspects of the laser–plasma interaction in the relativistic intensity regime. The propagation of laser pulses through preformed plasmas was studied at intensities exceeding 1019 W/cm2. The transmission of laser energy through long-scale underdense plasmas showed to be inefficient unless a plasma channel is preformed ahead of the main laser pulse. The study of the interaction with overdense plasmas yielded indication of collimated energy transport through the plasma. The production of fast particles during the interaction with solid density targets was also investigated. The measurements revealed the presence of a small-sized directional source of multi-megaelectron volt protons, which was not observed when a plasma was preformed at the back of the solid target. The properties of the source are promising in view of its use in radiographic imaging of dense matter, and preliminary tests were carried out.

Laser imprinting has been studied and, in particular, saturation of areal density perturbations induced by near single mode laser imprinting was observed. Several issues important for the foam buffered direct drive scheme have been investigated. These studies included measurements of the absolute levels of stimulated Brillouin and Raman scattering observed from laser irradiated low density foam targets, either bare or overcoated with a thin layer of gold. A novel scheme is proposed to increase the pressure in indirectly driven targets. By heating a foam supersonically that is attached to a solid target the pressure generated is not only the ablation pressure but also the combined pressure due to ablation at the foam-foil interface and the heated foam material. Planar brominated plastic foil targets overcoated with a low density foam were irradiated by a soft X ray pulse. The pressure was obtained by comparing the rear side trajectory of the driven target observed by soft X ray radiography with one dimensional radiation hydrodynamic simulations. Observations were also carried out of the transition from supersonic to subsonic propagation of an ionization front in low density chlorinated foam targets irradiated by an intense soft X ray pulse. The diagnostic for these measurements was K shell point projection absorption spectroscopy. In the fast ignitor area the channelling and guiding of picosecond laser pulses through underdense plasmas, preformed density channels and microtubes were investigated. It was observed that a large fraction of the incident laser energy can be propagated. Megagauss magnetic fields were measured, with a polarimetric technique, during and after propagation of intense picosecond pulses in preionized plasmas. Two types of toroidal fields, of opposite orientation, were detected. In addition, the production and propagation of an electron beam through solid glass targets irradiated at intensities above 1019W/cm2 were observed using optical probing techniques.

The absorption of subpicosecond uv laser pulses has been measured at intensities above ${10}^{17}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}.$ High levels of absorption were observed, up to 55% for s polarization and 65% for p polarization. The behavior with angle of incidence and polarization can be interpreted as due to a combination of resonance and collisional absorption, taking place in a plasma with a scale length of the order of a fraction of the laser wavelength.

Ionization fronts have been observed in low density chlorinated foam targets and low density foams confined in gold tubes using time resolved K-shell absorption spectroscopy. The front was driven by an intense pulse of soft x-rays produced by high power laser irradiation. The density and temperature profiles inferred from the radiographs provided detailed measurement of the conditions. The experimental data were compared to radiation hydrodynamics simulations and reasonable agreement was obtained.

Measurements of self-channeling of picosecond laser pulses due to relativistic and ponderomotive expulsion effects have been obtained in preformed plasmas at laser irradiances between 5–9×1018 Wcm−2. The self-focused channel was surrounded by a multi-megagauss magnetic field. The orientation of the field was consistent with a forward going relativistic electron beam propagating along the laser pulse. Self-channeling and magnetic field generation mechanisms were modeled by multidimensional particle-in-cell (PIC) simulations and good agreement was obtained with the experimental observations. Measurements of the channel expansion after the interaction were obtained and the rate of expansion was consistent with a blast wave solution. The level of transmission of an intense laser pulse through such performed density channels was observed to increase significantly compared to the case without a channel. High levels of transmission of an intense laser pulse through microtubes were also observed. The relevance of...

Transonic ionization fronts have been observed in low-density chlorinated foam targets using time-resolved $K$-shell absorption spectroscopy. The front was driven by an intense pulse of soft x rays produced by high-power laser irradiation of a thin foil. The density and temperature profiles inferred from the radiographs provided detailed measurement of the conditions at a number of times. The experimental data were compared to radiation hydrodynamics simulations and reasonable agreement was obtained.

The fast ignitor scheme consists of three stages and relies on ignition of a part of the compressed fusion fuel of an inertialconfinement fusion capsule by an external trigger. A conventional fusio...

Two spatially separated toroidal magnetic fields in the megagauss range have been detected with Faraday rotation during and after propagation of a relativistically intense laser pulse through preionized plasmas. Besides a field in the outer region of the plasma oriented as a conventional thermoelectric field, a field with the opposite orientation closely surrounding the propagation axis is observed, in conditions under which relativistic channeling occurs. A 3D particle-in-cell code was used to simulate the interaction under the conditions of the experiment.

The interaction of short (1−2 ps) laser pulses with solid targets at irradiances of over 1016 Wcm−2, in the presence of a substantial prepulse has been investigated. High absorption of laser energy is found even at high angles of incidence, with evidence for a resonance absorption peak being found for S, P, and circular polarizations. It is considered that this may be a result of refraction and beam filamentation, which causes loss of distinct polarization. Measurements of hard X-ray emission (∼ 100 keV) confirm a resonance absorption type peak at 45−50°, again for all three cases. Typically, 5−15% of the incident light is back-reflected by stimulated Brillouin scatter, with spatially resolved spectra showing evidence of beam hot-spots at high intensity. The possibility that filamentation and refraction of the beam can explain the lack of polarization dependence in the absorption and hard X-ray emission data is discussed.

Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 A of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding ...

Creation of a low density, high temperature plasma buffer between the absorption and ablation layers of a directly driven inertial confinement fusion implosion capsule has been proposed as a means to reduce “early time” imprint from laser nonuniformities. This thermal smoothing blanket might be created from a low density foam layer wrapped around the deuterium–tritium filled microballoon. Preliminary spherical implosion tests of this concept using a polystyrene foam layer surrounding a glass microballoon were performed at the Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)], using a 527 nm drive wavelength. Comparison of capsule yield and imploded core symmetry showed promising improvements in overall target performance, relative to one-dimensional undegraded hydrodynamic simulations, when the foam-buffer layer was present.

Laser driven shock waves propagating through foam targets have been diagnosed by side-on time-resolved x-ray absorption spectroscopy. K-shell absorption spectra of chlorine, doped into the foam targets, were matched to a detailed atomic physics model to infer the temperature profile. The density distribution was obtained from the x-ray transmission through the target. 2D radiation-hydrodynamic modeling of the targets reproduced the experimental results. Preheating of material by radiation from the shocked region was shown to have an important contribution to the measured temperature profile.

Relativistic self-channeling of a picosecond laser pulse in a preformed plasma near critical density has been observed both experimentally and in 3D particle-in-cell simulations. Optical probing measurements indicate the formation of a single pulsating propagation channel, typically of about 5{mu}m in diameter. The computational results reveal the importance in the channel formation of relativistic electrons traveling with the light pulse and of the corresponding self-generated magnetic field. {copyright} {ital 1997} {ital The American Physical Society}

A novel x-ray imaging system based on a microchannel-plate combined with a multi-pinhole camera has been employed to study x-ray emission from laser-produced plasmas in the 0.5–3 keV photon energy range. In particular, the interaction of intense laser radiation with such plasmas, under conditions relevant to inertial confinement fusion, was monitored by recording a sequence of consecutive x-ray images with a temporal resolution of 140 ps and a spatial resolution of 10 μm. These images provide valuable information on the main physical mechanisms involved in the interaction process, including collisional absorption of the incident laser energy and electron thermal conduction.

A quantitative study of refractive whole beam defocusing and small scale breakup induced by optical ionization of subpicosecond and picosecond, 0.25 and 1 {mu}m, laser pulses in gas-jet targets at densities above 1{times}10{sup 19} cm{sup {minus}3} has been carried out. A significant reduction of the incident laser intensity was observed due to refraction from ionization-induced density gradients. The level of refraction measured with optical probing correlated well with the fraction of energy transmitted through the plasma. The numerical and analytical models were found to agree well with experimental observations. {copyright} {ital 1996 The American Physical Society.}

An experimental study of stimulated Brillouin backscatter (B-SBS), produced in a large “homogeneous” underdense plasma using induced spatially incoherent laser light, is presented. The observations reveal that a strong correlation exists between the level of B-SBS and laser filamentation. It is suggested that a filamentory model of B-SBS is needed to better describe the scattering process under conditions of high laser irradiance and large plasma scale lengths.

In this paper we present measurements of energy transport in hot, high-density plasmas produced by picosecond laser interaction with solid targets. The propagation of the ablative heat wave was studied by using X-ray-ultraviolet (XUV) spectroscopy with picosecond temporal resolution. Measurements show that for laser intensities on target above 1016 W/cm2, strong inhibition of heat flux toward the cold target occurs. A detailed modelling of the experimental data is presented in which heat transport and absorption processes are taken into account self-consistently. Finally the role played by lateral transport and self-induced magnetic fields in our experiment is also discussed.

Long scale length plasmas were produced by uniform laser irradiation from opposite sides of metal disks coated on thin plastic stripe targets. The plasma became fully underdense before the end of the 600-ps pulse. The evolution of the plasma was carefully characterized both in electron density and temperature, and the experimental data were compared to the predictions of a one-dimensional hydrodynamics computer code. The electron temperature was obtained from time-resolved x-ray spectra by line-ratio measurements, accounting for opacity effects. Two-dimensional electron density profiles of a large plasma region were obtained by Nomarski interferometry at different times. They showed that the expansion was axially symmetric and reproducible shot by shot. The plasma was found to be substantially free from small-scale density inhomogeneities. The plasma produced with this techique is very suitable for interaction experiments. In particular, laser stimulated plasma instabilities of relevance for laser fusion can be accurately studied in these conditions.

The level of second-harmonic light emitted from a laser-exploded foil plasma at nominal irradiance up to 3·1013 W/cm2 was found to be extremely sensitive to both target position and irradiance on target. Either a small target displacement or a small increase in irradiance resulted in a jump of the 2ω level of more than three orders of magnitude. Correspondingly, a transition was observed from a 2ω source pattern clearly signed by the original laser spot pattern to unstable patterns of filaments whose size is consistent with the maximum growth of the instability.

The effectiveness of incoherence‐induced optical smoothing techniques is studied experimentally and good agreement with theory is found. Using incoherent laser‐drive radiation, the Rayleigh–Taylor instability is investigated in planar foil targets with imposed sinusoidal modulations. Growth rates of up to 60% of the classical value are inferred from the measurements and the nonlinear behavior of the instability is investigated. Growth is observed at all wavelengths tested between 30 and 100 μm.

A novel and simple XUV imaging technique using a laser-produced plasma source is described. A spatial resolution of better than 0.8 mum is demonstrated.

A novel optical line focus system using a lens and off-axis mirror combination which allows a high quality line focus to be obtained has been developed for x-ray laser research. By irradiation of fiber and thin foil targets with a high power laser various experimental observations show the excellent performance of this new scheme.

A novel time-resolved VUV spectrograph is described for use in plasma diagnostics and x-ray laser research. The device uses a grazing incidence flat field grating coupled to a streak camera. (AIP)

The concept of driving a driver/witness‐type plasma wakefield accelerator (PWFA) with quasimonoenergetic double electron bunches from a laser wakefield accelerator (LWFA) is studied. In the quasimonoenergetic LWFA/SMLWFA (self‐modulated LWFA) regime, it is possible to generate multiple quasimonoenergetic electron bunches with durations of only a few fs and distances of only a few tens of fs with a comparably simple experimental setup. In a subsequent high‐density plasma afterburner stage the witness bunch energy can be boosted in the plasma wakefield generated by the driver. Such a hybrid system can increase the maximum energy output of a laser wakefield accelerator and is well suited to study driver/witness plasma accelerator phenomena and can be used as a cost‐effective test‐bed for future high‐energy plasma‐based accelerators.

Dilute plasmas surrounding mass‐limited targets provide sufficient current for influencing strong fields, which are built up due to the interaction of an intense and ultrafast laser pulse. Such situation occurs, where evaporation of the target surface is present. The high‐intensity laser pulse interacts with the quasi‐isolated mass‐limited target and the spatial wings of the intensity distribution account for ionization of the ambient plasma. A fast change of strong electrical fields following intense laser irradiation of water droplets (16 micron diameter) has been measured with proton imaging. An analytical model explains charge transport accounting for the observation.

Beam optimization of laser‐accelerated protons is required to progress towards the development of applications, e.g. for fast ignition of fusion targets or dense plasma radiography. For this, three areas of improvement need to be pursued: increasing the maximum proton energy, enhancing the laser‐to‐protons conversion efficiency and reducing the beam divergence. Here we report on the reasoning which led us to envision limited size target to improve protons acceleration. We point out that pre plasma leakage from the target front side is a critical issue to allow efficient acceleration from such limited size targets and show that with very high temporal contrast laser pulses proton acceleration is improved in such targets.

Recent progress in multilayered mirror technology has allowed the developments of novel imaging systems for probing of high density plasmas in the soft x‐ray regime. The experimental system described here basically consists of a spherical multilayered mirror which reflects a narrow band (∓5 eV) of soft x‐ray radiation in the 50 to 200 eV energy region. The mirror images either the self‐emission of a laser produced plasma and/or the shadow of the expanding plasma generated by a separate laser produced x‐ray source with approximately 50× magnification in the image plane and submicron resolution in the target plane. The time resolution of about 150 ps is obtained with either a gated microchannel plate intensifier as detector or a short pulse x‐ray backlighter. This imaging technique has been applied to several different studies of laser produced plasmas. Measurements of the overdense plasma profiles were carried out by imaging the self‐emission produced by bremsstrahlung. Two‐dimensional x‐ray radiographs of...

A time‐resolved study is presented of stimulated Raman backscatter (B‐SRS) produced in a large‐scale‐length preformed plasma in an intense laser pulse. The first direct experimental results are presented to (i) show that the B‐SRS is generated predominantly in laser filaments and (ii) provide a direct correlation between the spectral and spatial emission profiles of the B‐SRS as a function of interaction irradiance. The ‘‘anomalous’’ B‐SRS spectral features so commonly reported in time‐resolved studies are shown to be the characteristic emission of B‐SRS from filaments. The B‐SRS fraction was also measured as a function of interaction irradiance. The B‐SRS intensity ‘‘threshold’’ and ‘‘saturating’’ behavior recorded can be well described for the present interaction conditions. The onset of B‐SRS is shown to be strongly correlated with the intensity threshold of the filamentation instability. The effect of laser beam ‘‘smoothing’’ techniques on the B‐SRS emission have also been investigated, using either a random phase plate (RPP) or induced spatial incoherence (ISI). The results suggest that their effectiveness in suppressing B‐SRS is strongly associated with their ability to reduce the level of filamentation while still producing a high level of plasma perturbation which can effectively destroy the phase‐matching conditions necessary for B‐SRS.

Due to the envisioned advantages of mass‐limited targets for laser driven ion beams, which are high efficiency and high cut‐off energies, their field dynamics are of special interest. Micro‐water droplets can be used as mass‐limited targets with a high repetition rate. Our investigations show that the surrounding dilute plasma of such liquid spheres influences the interaction. We review our experimental findings together with computer simulations and conclude on the different processes in electron transport and related acceleration fields for mass‐limited targets and foils, respectively.

Self-phase modulation (SPM) in filamentary plasma can explain anomalous large frequency shifts in time-resolved spectra of laser light backscattered from laser plasmas. We have modelled the effect of SPM by assuming a rapid variation in the plasma electron density. This is expected when laser light filaments or self-focuses in plasmas. The first direct experimental observation is presented showing the correlation between laser beam self-focusing and anomalous backscattered stimulated Brillouin (B-SBS) spectra. The B-SBS spectral features recorded are strongly characteristic of those expected when the frequency of the incident laser light is modified by SPM. A consequence of our observation is that SPM should be considered in the interpretation of SBS spectra in laser-plasma interactions where laser self-focusing is occurring. Also, SPM can contribute to the saturation of the SBS process to low levels by effectively destroying the coherence of the SBS interaction.

Optical probing measurements have been made of the uniformity of large underdense plasmas generated by the irradiation of thin foil stripe targets. The generation of such plasmas is of importance of both X-ray laser experiments and laser plasma interaction studies. The results obtained indicate that the plasma expansion can depend critically on the target design. In particular it was found that the presence of a thin formvar substrate, used to support the target, can significantly restrict the plasma expansion. Further evidence of this effect is provided by measurements of the plasma density determined from the stimulated Raman scattering spectrum produced by the interaction of another high power laser beam with the plasma. The results obtained are compared with predictions of a 1-D model and a 2-D hydrodynamic code.

Experimental results are presented for the interaction of an intense laser beam in large preformed, homogeneous underdense plasmas. Parametric instabilities, including filamentation, stimulated Raman (SRS) and stimulated Brillouin scattering (SBS), have been extensively studied. In this paper comparisons are made between coherent and smoothed laser interaction beams for their effectiveness in suppressing these instabilities. Direct observations of the enhanced breakup of the coherent interaction beam due to filamentation are presented. Measurements of the transverse scale lengths of the hot spots, and the level of the intensity modulations of the filamentary structures are given. Further, direct experimental observations of SRS and SBS generated in filaments are presented. These provided conclusive evidence that this instability provides the primary mechanism for the generation of SRS and SBS under such plasma conditions. With the smoothed laser interaction beams, using random phase plates (RPP) and induced spatial incoherence (ISI), the level of filamention was significantly reduced. The backscattered fractions of SRS and SBS with respect to the irradiance of the interaction beam were measured for coherent, RPP and ISI. Reduction of typically a factor of 20 and 500 were recorded using RPP and ISI, respectively, in the backscattering levels of SRS and SBS when compared with the coherent interaction beam. The reductions are attributed primarily to the effectiveness of the beam‐smoothing techniques in suppressing the filamentation instability.

The transport of soft-X-ray radiation through thin foil plastic targets, 0.1 and 5 μm thick, has been studied using time-resolved XUV spectroscopy in the 10 to 70 A spectral wavelength region. An intense source of radiation was produced by overcoating one side of the target with 0.1 μm of gold and irradiating it with green laser light at an intensity between 1·1014 and 5·1014 W cm-2. The experimental results were simulated with a multi-group radiation transport calculation which was coupled to a one-dimensional hydrodynamics model.

The use of a novel diagnostic based on the Moire effect, known as Moire deflectometry, to characterize X-ray laser beams is described. This technique is purely geometric in nature and does not require coherent radiation for its operation. The unique feature of Moire deflectometry is that it provides a fully quantitative laser beam diagnostic which can be operated in any wavelength region of the spectrum. This technique allows the number of transverse modes present in the beam to be determined from the inherent beam divergence. The latter is measured from the degree of reduction in the contrast of the Moire fringes. From the same deflectogram the geometrical-beam behaviour can be determined. This includes both geometrical-beam divergence and ray-aberrations which lead to wavefront distortions.

Thin carbon fibres, coated with aluminium, are irradiated with approximately=120 ps, 0.53 mu m laser pulses yielding a specific absorbed energy of approximately=4 J cm-1 and time-resolved gain measurements on the n=4-3 and n=5-3 transitions of lithium-like Al XI ions in the recombining plasma are made. In particular, gain on the 4f-3d line at 15.4 nm is measured with alpha max approximately=2-3 cm-1 using both single-shot and multi-shot techniques. Gain measurements from multi-shot data on the 5f-3d line at 10.6 nm indicate alpha max approximately=1.5 cm-1. Observations are compared with theoretical calculations, discrepancies are noted and the need for reproducible plasma conditions in low gain-length product data sets is discussed.

XUV amplication at 81 A was observed in a freely expanding adiabatically cooled plasma produced by laser irradiation of LiF coated carbon fibre targets. Measurements included time resolved on- and off-axis spectroscopy, target length variation and the variation of the absorbed laser energy. Detailed analysis of the experimental data has been compared with numerical simulations.

In recent experiments of laser pulse interaction at relativistic intensities with a low density plasma, the proton radiography technique showed evidence of long--lived field structures generated after the self-channeling of the laser pulse. We present 2D particle-in-cell simulations of this interaction regime, where the dynamics of similar structures has been resolved with high temporal and spatial resolution. An axially symmetrical field pattern, resembling both soliton-like and vortex structures, has been observed. A study of the physics of such structures and a comparison with experimental data is in progress., To appear in the Proceedings of the International Workshop "Collective phenomena in macroscopic systems", Villa Olmo, Como, Italy, December 4-6, 2006; 9 pages, 8 figures submitted in GIF format

Laser driven proton beams have been used to diagnose transient fields and density perturbations in laser produced plasmas. Grid deflectometry techniques have been applied to proton radiography to obtain precise measurements of proton beam angles caused by electromagnetic fields in laser produced plasmas. Application of proton radiography to laser driven implosions has demonstrated that density conditions in compressed media can be diagnosed with million electron volt protons. This data has shown that proton radiography can provide unique insight into transient electromagnetic fields in super critical density plasmas and provide a density perturbation diagnostics in compressed matter.

International audience; Experimental investigations of the late-time ion structures formed in the wake of an ultrashort, intense laser pulse propagating in a tenuous plasma have been performed using the proton imaging technique. The pattern found in the wake of the laser pulse shows unexpectedly regular modulations inside a long, finite width channel. On the basis of extensive particle in cell simulations of the plasma evolution in the wake of the pulse, we interpret this pattern as due to ion modulations developed during a two-stream instability excited by the return electric current generated by the wakefield.

Point projection absorption spectroscopy has been used for many years in the measurement of radiative opacity [1]. This powerful diagnostic technique allows both spectral and two-dimensional spatial information to be obtained by radiography of targets using a near point source of x-rays. Recently the authors have applied this technique to measure absorption spectra, not to study opacity, but as a diagnostic of a range of radiation-hydrodynamics phenomena relevant to inertial confinement fusion. Characterization of targets by absorption or emission spectroscopy at frequencies close to the peak of thermal emission is possible but can have problems with self-emission, high background and non-thermal emission and may reduce the size of targets which can be probed. Therefore K-shell absorption spectroscopy was chosen because the line spectra sampled are at much higher frequencies than the peak thermal emission of the targets thus avoiding the problems mentioned above. K-shell spectroscopy also allows the use of crystal spectrometers which tend to be simpler to implement than gratings of similar resolution. Examples of the use of the technique are given. For instance detailed characterization of conditions in shock and radiation waves have enabled models of transonic flows to be tested against experiment. Also the preheating effect of forming a plasma buffer to smooth laser beam imprint has been studied under conditions relevant to direct drive inertial confinement fusion. To infer the target conditions the measured spectra must be compared with synthetic spectra generated by modelling the opacity in the region of the diagnostic lines. The opacity model used in this work is briefly described.

The influence of the plasma density scale length on the production of MeV protons from thin foil targets irradiated at $I{\ensuremath{\lambda}}^{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0ex}{0ex}}\mathrm{W}\mathrm{cm}{}^{\ensuremath{-}2}$ has been studied. With an unperturbed foil, protons with energy $g20\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$ were formed in an exponential energy spectrum with a temperature of $2.5\ifmmode\pm\else\textpm\fi{}0.3\mathrm{MeV}$. When a plasma with a scale length of $100\ensuremath{\mu}\mathrm{m}$ was preformed on the back of the foil, the maximum proton energy was reduced to $l5\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$ and the beam was essentially destroyed. The experimental results are consistent with an electrostatic accelerating mechanism that requires an ultrashort scale length at the back of the target.