Philippe Nicolai

Physics of giant electromagnetic pulse generation in short-pulse laser experiments.

In this paper we describe the physical processes that lead to the generation of giant electromagnetic pulses (GEMPs) at powerful laser facilities. Our study is based on experimental measurements of both the charging of a solid target irradiated by an ultra-short, ultra-intense laser and the detection of the electromagnetic emission in the GHz domain. An unambiguous correlation between the neutralization current in the target holder and the electromagnetic emission shows that the source of the GEMP is the remaining positive charge inside the target after the escape of fast electrons accelerated by the ultra-intense laser. A simple model for calculating this charge in the thick target case is presented. From this model and knowing the geometry of the target holder, it becomes possible to estimate the intensity and the dominant frequencies of the GEMP at any facility.

Dynamic model of target charging by short laser pulse interactions.

A model providing an accurate estimate of the charge accumulation on the surface of a metallic target irradiated by a high-intensity laser pulse of fs-ps duration is proposed. The model is confirmed by detailed comparisons with specially designed experiments. Such a model is useful for understanding the electromagnetic pulse emission and the quasistatic magnetic field generation in laser-plasma interaction experiments.

Coupled hydrodynamic model for laser-plasma interaction and hot electron generation.

We present a formulation of the model of laser-plasma interaction (LPI) at hydrodynamical scales that couples the plasma dynamics with linear and nonlinear LPI processes, including the creation and propagation of high-energy electrons excited by parametric instabilities and collective effects. This formulation accounts for laser beam refraction and diffraction, energy absorption due to collisional and resonant processes, and hot electron generation due to the stimulated Raman scattering, two-plasmon decay, and resonant absorption processes. Hot electron (HE) transport and absorption are described within the multigroup angular scattering approximation, adapted for transversally Gaussian electron beams. This multiscale inline LPI-HE model is used to interpret several shock ignition experiments, highlighting the importance of target preheating by HEs and the shortcomings of standard geometrical optics when modeling the propagation and absorption of intense laser pulses. It is found that HEs from parametric instabilities significantly increase the shock pressure and velocity in the target, while decreasing its strength and the overall ablation pressure.

Deterministic model for the transport of energetic particles: Application in the electron radiotherapy

A new deterministic method for calculating the dose distribution in the electron radiotherapy field is presented. The aim of this work was to validate our model by comparing it with the Monte Carlo simulation toolkit, GEANT4. A comparison of the longitudinal and transverse dose deposition profiles and electron distributions in homogeneous water phantoms showed a good accuracy of our model for electron transport, while reducing the calculation time by a factor of 50. Although the Bremsstrahlung effect is not yet implemented in our model, we propose here a method that solves the Boltzmann kinetic equation and provides a viable and efficient alternative to the expensive Monte Carlo modeling.

Investigation of high intensity laser proton acceleration with underdense targets

In the last few years, intense research has been conducted on laser-accelerated ion sources and their applications. Recently, experiments have shown that a gaseous target can produce proton beams with characteristics comparable to those obtained with solid targets. In underdense laser proton acceleration, volume effects dominate the acceleration, while in target normal sheath acceleration, the electric field value is directly related to the electron surface density. Using Particle-In-Cell simulations, we have studied in detail the effect of an underdense density gradient on proton acceleration with high intensity lasers. Underdense laser ion acceleration strongly depends on the length, the shape and the amplitude of the density gradient and on the laser pulse shape. The accelerated proton beam characteristics in the shock-like regime are very promising.

X-ray Spectroscopy of Hot Dense Plasmas: Experimental Limits^ Line Shifts & Field Effects

High‐resolution x‐ray spectroscopy is capable of providing complex information on environmental conditions in hot dense plasmas. Benefiting from application of modern spectroscopic methods, we report experiments aiming at identification of different phenomena occurring in laser‐produced plasma. Fine features observed in broadened profiles of the emitted x‐ray lines and their satellites are interpreted using theoretical models predicting spectra modification under diverse experimental situations.

Fast electron transport and induced heating in aluminium foils

Beams of fast electrons have been generated from the ultra-intense laser interaction with Aluminium foil targets. The dynamics of the fast electrons propagation and the level of induced in-depth heating have been investigated using the optical emission from the foils rear side. Important yields of thermal emission, consequence of high target temperatures, were detected for targets thinner than 50 μm. We precisely characterized the targets in-depth temperature profile in order to reproduce the emission yields. At shallow depth, we show the important heating (estimated to > 100 eV till 15 μm depth) has a resistive origin upon the neutralizing return current. For deeper regions, because of the bulk component divergence, the fast electron energy losses and induced heating are due to collisions. Coupling the model to the experimental measurements, we were able to quantify the bulk of the fast electron population, corresponding to 35% of the laser energy and a 500 keV temperature.

High performance modelling of the transport of energetic particles for photon radiotherapy

This work consists of the validation of a new Grid Based Boltzmann Solver (GBBS) conceived for the description of the transport and energy deposition by energetic particles for radiotherapy purposes. The entropic closure and a compact mathematical formulation allow our code (M1) to calculate the delivered dose with an accuracy comparable to the Monte-Carlo (MC) codes with a computational time that is reduced to the order of few minutes without any special processing power requirement. A validation protocol with heterogeneity inserts has been defined for different photon sources. The comparison with the MC calculated depth-dose curves and transverse profiles of the beam at different depths shows an excellent accuracy of the M1 model.

Generation of high pressures by short-pulse low-energy laser irradiation

The goal of this paper is twofold: first, we demonstrate shock generation with ultra-short (24 fs) and low-energy (J) laser pulse, following the energy deposition in the target by fast electrons; second, we show that such shocks can be used to provide information on compressed matter. For a target with 50 mu m thickness we have clearly inferred the formation of a shock wave with pressure >= 100 Mbar. We have also measured the color temperature of the emitting target rear side at breakout time (T approximate to 0.6 eV), which is in good agreement with predictions from equation-of-state models (SESAME tables) and hydrodynamic simulations.

Publisher's Note: Dynamic model of target charging by short laser pulse interactions [Phys. Rev. E 92, 043107 (2015)].

This corrects the article DOI: 10.1103/PhysRevE.92.043107.