J. P. Matte

Ultrafast x‐ray sources*

Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.

X‐ray spectroscopy of hot solid density plasmas produced by subpicosecond high contrast laser pulses at 1018–1019 W/cm2

Analysis is presented of K‐shell spectra obtained from solid density plasmas produced by a high contrast (1010:1) subpicosecond laser pulse (0.5 μm) at 1018–1019 W/cm2. Stark broadening measurements of He‐like and Li‐like lines are used to infer the mean electron density at which emission takes place. The measurements indicate that there is an optimum condition to produce x‐ray emission at solid density for a given isoelectronic sequence, and that the window of optimum conditions to obtain simultaneously the shortest and the brightest x‐ray pulse at a given wavelength is relatively narrow. Lower intensity produces a short x‐ray pulse but low brightness. The x‐ray yield (and also the energy fraction in hot electrons) increases with the laser intensity, but above some laser intensity (1018 W/cm2 for Al) the plasma is overdriven: during the expansion, the plasma is still hot enough to emit, so that emission occurs at lower density and lasts much longer. Energy transport measurements indicate that approximate...

Interaction of a 1 psec laser pulse with solid matter

Absorption and x‐ray emission results obtained with a 1 psec pulse incident on solid targets with an intensity between 1011 and 1016 W/cm2 are presented and discussed. For I 1015 W/cm2) an evaluation of the plasma parameters is obtained from high resolution keV spectra. Finally, the results are discussed in the light of 1‐D hydrodynamic simulations with time‐dependent atomic physics.

An improvement of the nonlocal heat flux formula

A new heat flux formula valid in strong inhomogeneous laser‐produced plasmas has been derived from the Fokker–Planck equation. The nonlocal treatment of Luciani et al. [Phys. Rev. Lett. 51, 1664 (1983); Phys. Fluids 28, 835 (1985)] is improved by taking into account the electric potential effect. A simple and reliable phenomenological formula is proposed, which is in good agreement with numerical Fokker–Planck computations.

Ion kinetic simulations of the formation and propagation of a planar collisional shock wave in a plasma

Ion kinetic simulations of the formation and propagation of planar shock waves in a hydrogen plasma have been performed at Mach numbers 2 and 5, and compared to fluid simulations. At Mach 5, the shock transition is far wider than expected on the basis of comparative fluid calculations. This enlargement is due to hot ions streaming from the hot plasma into the cold plasma and is found to be limited by the electron preheating layer, essentially because electron–ion collisions slow down these energetic ions very effectively in the cold upstream region. Double‐humped ion velocity distributions formed in the transition region, which are particularly prominent during the shock formation, are found not to be unstable to any electrostatic mode, due to electron Landau damping. At Mach numbers of 2 and below, no such features are seen in velocity space, and there is very little difference between the profiles from the kinetic and fluid simulations.

Modeling and effects of nonlocal electron heat flow in planar shock waves

Electron heat flow was computed in the context of a steadily propagating shock wave. Two problems were studied: a Mach 8 shock in hydrogen, simulated with an ion kinetic code, and a Mach 5 shock in lithium, simulated with an Eulerian hydrodynamic code. The electron heat flow was calculated with Spitzer–Harm classical conductivity, with and without a flux limit, and several nonlocal electron heat flow formulas published in the literature. To evaluate these, the shock’s density, velocity, and ion temperature profiles were fixed, and the electron temperature and heat flow were compared to those computed by an electron kinetic code. There were quantitative differences between the electron temperature profiles calculated with the various formulas. For the Mach 8 shock in hydrogen, the best agreement with the kinetic simulation was obtained with the Epperlein–Short delocalization formula [Phys. Fluids B 4, 2211 and 4190 (1992)], and the Luciani–Mora–Bendib formula [Phys. Rev. Lett. 55, 2421 (1985)] gave good agreement. For the Mach 5 shock in lithium, both of these gave good agreement. The earlier Luciani–Mora–Virmont formula [Phys. Rev. Lett. 51, 1664 (1983)] gave fair agreement, while that of San Martin et al. [Phys. Fluids B 4, 3579 (1992); 5, 1485 (1993)] was even further off than the classical Spitzer–Harm [Phys. Rev. 89, 977 (1953)] formula for thermal conduction. To assess the effect of nonlocal electron heat flow on the shock’s hydrodynamics and ion kinetics, each of the two problems was done with two different electron heat flow models: the classical Spitzer–Harm local heat conductivity, and the Epperlein–Short nonlocal electron heat‐flow formula. In spite of the somewhat different electron temperature profiles, the effect on the shock dynamics was not important.

Numerical and theoretical study of the generation of extreme ultraviolet radiation by relativistic laser interaction with a grating

The generation of harmonics by the interaction of a femtosecond, relativistic intensity laser pulse with a grating of subwavelength periodicity was studied numerically and theoretically. For normal incidence, strong, coherent emission at the wavelength of the grating period and its harmonics is obtained, nearly parallel to the target surface, due to relativistic electron bunches emanating from each protuberance. For oblique incidence (30°), only even harmonics of the grating periodicity are seen, but with an even higher intensity. This is due to constructive interference of the emission from the grating protuberances. The emission along the grating surface is composed of trains of attosecond pulses; therefore there is no need to use a filter. An efficiency greater than 10−4 is obtained for the 24th harmonic. The conversion efficiency is fairly constant when the similarity parameter S=ne/(a0nc)(∝neλL/IL1/2) is held fixed, and is optimum when S≃4. Here, ne and nc are the electron density and the critical de...

Ion acceleration and plasma jets driven by a high intensity laser beam normally incident on thin foils

We study the problem of the radiation pressure acceleration of ions and the formation of plasma jets, driven by a highintensity circularly polarized laser beam normally incident on thin plasma targets. We use an Eulerian Vlasov code to solve the one-dimensional relativistic Vlasov-Maxwell equations for both electrons and ions. We consider the case of a high density plasma with n/ncr = 100, where ncr is the critical density. Three cases are studied with different target thicknesses, to investigate the physical processes involved when decreasing the target thickness from several electron skin depths down to the order of one skin depth. The results show a more important acceleration of the ions when the thickness is decreased. Although we observe in all cases a neutral plasma jet ejected from the back of the target, the evolution of the system which leads to the formation of this neutral plasma jet is different in the three cases considered. In each case, this evolution will be studied in details. Also, a leak or ejection of electrons from the back of the target is observed in the thinnest case treated (thickness of the order of the skin depth), before the formation of the neutral plasma jet, a regime called leaky light sail radiation pressure acceleration. The absence of noise in the Eulerian Vlasov code allows an accurate representation of the phase-space structures of the distribution functions.

Electron kinetic simulations of solid density Al plasmas produced by intense subpicosecond laser pulses. I. Ionization dynamics in 30 femtosecond pulses

The interaction of a 1018 W/cm2, 30 fs laser pulse with solid Al was simulated with the electron kinetic code “FPI” [J. P. Matte et al., Phys. Rev. Lett. 72, 1208 (1994)] in which an improved average ion module was fully coupled to the electron kinetics. It includes electron impact ionization and excitation and their inverse processes: collisional recombination and de-excitation; as well as radiative decay and pressure ionization. We compare to runs without the inverse processes, and also without atomic physics (with 〈Z〉 set to 11). Atomic physics strongly affects the energy balance and the shape of the distribution function. Line radiation is mostly due to three body recombination into excited states after the peak of the pulse, as the plasma cools down. Despite the atomic processes and the high density, strongly non-Maxwellian distribution functions were obtained due to very steep temperature gradients and strong collisional heating, at the peak of the pulse. However, after the pulse, there is a very ra...

Fokker–Planck simulations of hot electron transport in solid density plasma

The transport of hot electrons in solid density plasma created by a high-intensity subpicosecond laser pulse and the resulting heating and ionization of the bulk of plasma are simulated with the electron kinetic code “FPI.” Both the hot and the thermal electrons are treated kinetically. An analysis of the results in terms of a two Maxwellians fit to the numerically obtained distribution function is made.