Fraction Of Laser Energy Research Articles

Effect of fast electrons on the gain of a direct-drive laser fusion target

The results of numerical and theoretical studies of the gain of a direct-drive inertial confinement fusion target, which includes a kinetic description of energy transfer by laser-accelerated fast electrons, are presented. The range of the initial temperature of the fast electrons and the fraction of laser energy contained in these particles were chosen based on the results of recent experiments at the National Ignition Facility. The effect of ‘wandering’ of fast electrons is taken into account which is due to the remoteness of the region of fast electron generation from the ablation surface of the imploded target. As a result a significant fraction of particles do not fall into the compressed part of target. The ‘wandering’ effect leads to a decrease in the negative effect of fast electron generation on the gain of the target.

Characterization of laser ablation of copper in the irradiance regime of laser-induced breakdown spectroscopy analysis

The LIBS signal depends both on the ablated mass and on the plasma excitation temperature. These fundamental parameters depend in a complex manner on laser ablation and on laser–plasma coupling. As several works in the literature suggest that laser ablation processes play a predominant role compared to plasma heating phenomena in the LIBS signal variations, this paper focuses on the study of laser ablation. The objective was to determine an interaction regime enabling to maximally control the laser ablation. Nanosecond laser ablation of copper at 266nm was characterized by scanning electron microscopy and optical profilometry analysis, in air at 1bar and in the vacuum. The laser beam spatial profile at the sample surface was characterized in order to give realistic values of the irradiance. The effect of the number of accumulated laser shots on the crater volume was studied. Then, the ablation crater morphology, volume, depth and diameter were measured as a function of irradiance between 0.35 and 96GW/cm². Results show that in the vacuum, a regular trend is observed over the whole irradiance range. In air at 1bar, below a certain irradiance, laser ablation is very similar to the vacuum case, and the ablation efficiency of copper was estimated at 0.15±0.03atom/photon. Beyond this irradiance, the laser beam propagation is strongly disrupted by the expansion of the dense plasma, and plasma shielding appears. The fraction of laser energy used for laser ablation and for plasma heating is estimated in the different irradiance regimes.

Preheating of a target by laser radiation through plasma and polymer aerogel

Processing of X-ray and optical streak-camera images after brightness amplification 5 proves unambiguously the existence of a preheating of the foil in two-layer targets (polymer aerogel plus Al foil). We observed this effect earlier at the accuracy limit of the experiment. Preheating may be explained by the initial transmission of laser radiation by an inhomogeneous plasma and aerogel remnants. The fraction of laser energy that passed through a microheterogeneous plasma to the foil or shell is small in comparison with the total energy in the pulse, but it comes to the target (foil or shell) at the initial period of the laser pulse and can be significantly affected by the dynamics of plasma and the efficiency of spherical cryogenic thermonuclear targets.

Modeling of vapor-droplet plumes ablated from multiple spots

The study aims at modeling of plume shielding aspects of laser ablation processes with multiple laser pulses applied to multiple targets. The main obstacle with the efficient use of multiple laser pulse technologies is an attenuation of the laser irradiation by previously ablated plumes. Dynamics of plumes is described by the axisymmetric Euler equations describing a vapor-droplet ablated mixture rolling-up in the surrounding ideal gas. For multiple laser pulses, the role of absorption of laser beam by previously ablated plumes is evaluated varying a model parameter that defines the fraction of laser energy absorbed by the ablated mixture. Absorption of laser beam by plume may cause its secondary explosion that cleans the target surface and, subsequently, increases the mass ablated by the consequent pulse. Dynamics of plumes ablated from two targets with possible time delay between two laser hits is investigated as a representative case of multiple targets. Shielding of the surface between targets appears to be significant if the second pulse occurs before the first shock wave passes the second target.

Ultrafast laser-based thermal X-ray sources

The effects of hot electrons on the thermal radiative properties (brightness, duration and spectral shape) and dynamics of solid density plasmas, generated during the interaction of a femtosecond laser and a solid target, are assessed. Line and broadband thermal emissions with duration between 500 fs and 700 fs, have been successfully produced with peak power between 1 and 10 MW, when the fraction of laser energy in the hot electron population was less than 2%, and when the hot electron energy density at the target surface was less than 1 KJ / cm 2 .

Hard x-ray emission in high intensity femtosecond laser–target interaction

Hard x-ray spectra (10–100 keV) created in high contrast, 400 fs, laser pulse interaction with solid targets, have been studied for laser intensities in the 1017–1019 W/cm2 range. The target atomic numbers (Z) extended from Z=13 to Z=73. The measured conversion efficiency at Ag Kα emission line was 10−3% at 5×1018 W/cm2. It has been confirmed that the hot electron temperature increased as (Iλ2)1/3 and the fraction of laser energy in hot electrons follows scaling law of (Iλ2)3/4.

Calorimetric system for recording plasma blowoff and scattered light distributions from laser plasmas

A complete system is described for the measurement of the distribution of energy emanating from laser-produced plasmas in the form of (1) plasma blowoff (ions, electrons, neutral particles, and x rays), and (2) scattered laser light. The detectors consist of small modules containing two differential calorimeters in one package, one for each form of energy. Modular autozeroing amplifiers enable the slowly varying signals to be measured by computer with the aid of CAMAC analog to digital converters. The computer is also capable of absolutely calibrating the units in situ. The programs used to record and display the data, and to calculate the fraction of laser energy absorbed, are described.