B. Loupias

Inhibition of fast electron energy deposition due to preplasma filling of cone-attached targets

We present experimental and numerical results on the propagation and energy deposition of laser-generated fast electrons into conical targets. The first part reports on experimental measurements performed in various configurations in order to assess the predicted benefit of conical targets over standard planar ones. For the conditions investigated here, the fast electron-induced heating is found to be much weaker in cone-guided targets irradiated at a laser wavelength of 1.057μm, whereas frequency doubling of the laser pulse permits us to bridge the disparity between conical and planar targets. This result underscores the prejudicial role of the prepulse-generated plasma, whose confinement is enhanced in conical geometry. The second part is mostly devoted to the particle-in-cell modeling of the laser-cone interaction. In qualitative agreement with the experimental data, the calculations show that the presence of a large preplasma leads to a significant decrease in the fast electron density and energy flux...

High-Mach number collisionless shock and photo-ionized non-LTE plasma for laboratory astrophysics with intense lasers

We propose that most of the collisionless shocks in the Universe, for example, supernova remnant shocks, are produced because of the magnetic field generated by Weibel instability and its nonlinear process. In order to verify and validate the computational result confirming this theory, we are carrying out model experiments with intense lasers. We are going to make a collisionless counter-streaming plasma with intense laser ablation based on the scaling law to laser plasma with the particle-in-cell simulation resulting in Weibel-mediated shock formation. Preliminary experimental data are shown. The photo-ionization and resultant non-LTE plasma physics are also very important subjects in astrophysics related to mainly compact objects, for example, black hole, neutron star and white dwarf. Planckian radiation with its temperature 80–100 eV has been produced in gold cavity with irradiation of intense lasers inside the cavity. The sample materials are irradiated by the radiation inside the cavity and absorption and self-emission spectra are observed and analyzed theoretically. It is demonstrated how the effect of non-LTE is essential to reproduce the experimental spectra with the use of a precision computational code.

JET FORMATION IN COUNTERSTREAMING COLLISIONLESS PLASMAS

Plasma jet formation was observed in counterstreaming plasmas in a laboratory experiment. In order to model an ambient plasma of astrophysical jets, the counterstreaming plasmas were created by irradiating a double CH-plane target with a high-power laser system. Since the mean free paths of the ions in terms of the counterstreaming motion were larger than the scale length of the experiment, the two-stream interaction of the plasmas was essentially collisionless. The time evolution of the jet collimation was obtained over several shots with different timing by shadowgraphy. When a single CH-plane target was irradiated, no jet collimation was observed. The counterstreaming plasma as an ambient plasma is essential for the jet plasma to collimate.

Laser-driven plasma jets propagating in an ambient gas studied with optical and proton diagnostics

The results of an experiment to propagate laser-generated plasma jets into an ambient medium are presented. The jets are generated via laser irradiation of a foam-filled cone target, the results and characterization of which have been reported previously [Loupias et al., Phys. Rev. Lett. 99, 265001 (2007)] for propagation in vacuum. The introduction of an ambient medium of argon at varying density is seen to result in the formation of a shock wave, and the shock front displays perturbations that appear to grow with time. The system is diagnosed with the aid of proton radiography, imaging the perturbed structure in the dense parts of the shock with high resolution.

Hard x-ray radiography for density measurement in shock compressed matter

In this letter we report on the direct density measurement in a shock compressed aluminum target using hard x-ray radiography. Experimental data employing a molybdenum Kα source at 17.5keV, generated with a short pulse laser are presented. High spatial resolution was obtained thanks to a new design for the backlighter geometry. Density values deduced from radiography are compared to predictions from hydrodynamic simulations, which have been calibrated in order to reproduce shock velocities measured from a rear-side self-emission diagnostic. Our results reveal the great potential of this technique as a diagnostic tool for direct density measurements in dense high-Z opaque materials.

Laser-driven shock waves for the study of extreme matter states

During the last ten years, the ability of high power lasers to generate high energy density shocks has made them a reliable tool to study extreme states of matter. These states of matter are relevant in many important physics areas such as astrophysics, planetology and ICF physics. Here, we present some representative studies performed by using a driven laser shock: melting of iron at pressures relevant for geophysics, developments of new techniques to measure the density of highly compressed matter and a study of a radiative shock.

Astrophysical jet experiments

We present an experimental characterization of jet propagation in an ambient medium. An intense laser (LULI2000) was used to generate the plasma jet using foam filled cone target. We observed, with several diagnostics, a perturbation in the interaction region between the jet and the ambient medium. The effect of the ambient medium on the jet velocity is also presented.

Laboratory investigations on the origins of cosmic rays

We report our recent efforts on the experimental investigations related to the origins of cosmic rays. The origins of cosmic rays are long standing open issues in astrophysics. The galactic and extragalactic cosmic rays are considered to be accelerated in non-relativistic and relativistic collisionless shocks in the universe, respectively. However, the acceleration and transport processes of the cosmic rays are not well understood, and how the collisionless shocks are created is still under investigation. Recent high-power and high-intensity laser technologies allow us to simulate astrophysical phenomena in laboratories. We present our experimental results of collisionless shock formations in laser-produced plasmas.

Propagation of laser-generated plasma jet in an ambient medium

In this work we present experimental research related to laboratory astrophysics using an intense laser. The goal of these experiments is to investigate some of the complex features of young stellar objects and astrophysical outflows, in particular the plasma jet interaction with the interstellar medium. The relevance of these experiments to astrophysics is measured through similarity criteria (scaling laws). These ensure the similarity between the astrophysical object and the laboratory provided that the dimensionless numbers are equivalent. Consequently, measurements of the plasma parameters are crucial to link laboratory research to astrophysics as they are needed for the determination of these dimensionless numbers. In this context, we designed experiments to generate plasma jets using an intense laser, and to study the evolution in vacuum and in an ambient medium.

Recent experiments on electron transport in high-intensity laser matter interaction

We present the results of some recent experiments performed at the LULI laboratory using the 100 TW laser facility concerning the study of the propagation of fast electrons in gas and solid targets. Novel diagnostics have been implemented including chirped shadowgraphy and proton radiography. Proton radiography images did show the presence of very strong fields in the gas probably produced by charge separation. In turn these imply a slowing down of the fast electron cloud as it penetrates in the gas and a strong inhibition of propagation. Indeed chirped shadowgraphy images show a strong reduction in time of the velocity of the electron cloud from the initial value, which is of the order of a fraction of c. We also performed some preliminary experiments with cone targets in order to verify the guiding effect and fast electron propagation in presence of the cone. Finally we compared results obtained by changing the target size.Here we only give a first presentation and preliminary analysis of data, which will be addressed in detail in a following paper.