Creation of hot radiation environments in laser-driven targets

A hot radiation environment, produced by maximizing laser-energy deposition into a small, high- "can," is a platform being developed for investigations of material properties under extreme conditions. In such small targets, almost doubling the laser energy results in only an incremental increase in the x-radiation flux, and almost no increase in the maximum achieved radiation temperature. That most of this additional laser energy is not deposited within the target is a direct consequence of laser-plasma interactions (LPI) outside of the target, which result in high-angle beams never entering the target late in the laser pulse. Accounting for these processes in the modeling results in quantitative agreement for the first time with experiments using very small cans. These findings have provided the scientific foundation for modifying the target geometry to mitigate the LPI and to achieve higher radiation temperatures.     

Direct observation of strong ion coupling in laser-driven shock-compressed targets

In this Letter we report on a near collective x-ray scattering experiment on shock-compressed targets. A highly coupled Al plasma was generated and probed by spectrally resolving an x-ray source forward scattered by the sample. A significant reduction in the intensity of the elastic scatter was observed, which we attribute to the formation of an incipient long-range order. This speculation is confirmed by x-ray scattering calculations accounting for both electron degeneracy and strong coupling effects. Measurements from rear side visible diagnostics are consistent with the plasma parameters inferred from x-ray scattering data. These results give the experimental evidence of the strongly coupled ionic dynamics in dense plasmas.     

Heating of thin foils with a relativistic-intensity short-pulse laser

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Ilambda (2)=2 x 10(18) W microm(2)/cm(2), laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At /=500 fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (approximately 100 eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.     

Intense laser-driven energetic proton beams from solid density targets

The effects of target density on proton acceleration driven by an intense sub-ps laser pulse are investigated using two-dimensional hybrid particle-in-cell simulations. Results show that at higher density the target-normal-sheath acceleration (TNSA) is more effective than shock acceleration for protons from a plastic target. Furthermore a lower-density target is favorable to higher energy of the TNSA protons. Moreover, the longitudinal electric fields at the target surfaces may reveal typical inhomogeneous structures for a long acceleration time. The conversion efficiency of laser energy into particle (electron, proton, and C(+) ion) energy is found to increase with decreasing target density.     

Effect of shock heating on the stability of laser-driven targets

The shock heating of a laser-driven, direct-drive target can determine its stability by affecting Rayleigh-Taylor growth rates through target decompression and ablative stabilization. Measurements indicate that pulses that rise rapidly to 10(14) W/cm(2) produce shock-induced temperatures of approximately 25 eV, whereas more slowly rising pulses show less heating. Analysis of the observed target behavior produced by these two pulses demonstrates that shock heating improves hydrodynamic stability because ablative stabilization increases when the targets are preheated by shocks.     

Analysis of the impedance mismatch effect in foam-solid targets compressed by laser-driven shock waves

The impedance mismatch effect in a two-layer (low density plastic foam, and solid aluminum, respectively) plane target compressed by a laser driven shock wave is considered. In such targets the ablative pressure generated by absorption of laser light in the foam layer is amplified when crossing the foam-aluminum interface. In this paper an analytical model is developed to evaluate the shock pressure in the aluminum layer as a function of the density and thickness of the foam layer and of the laser parameters. The model is in good agreement with previously published experimental results [A. Benuzzi et al., Phys. Plasmas 5, 2827 (1998)]. Received 20 January 2000 and Received in final form 16 May 2000     

Heating and compression of laser-irradiated spherical targets

Heating and dynamics of compression of shell targets irradiated by a nanosecond laser pulse in the “Kal'mar” facility are investigated. Comprehensive procedures are developed and used to investigate the energy balance and the compression dynamics of the target scattered by the radiation plasma at the harmonic frequencies 2ω₀ and 3/2ω₀, and of the spectral distribution of the continuous x radiation. The use of the developed procedures has made it possible to clarify the dependence of the components of the energy balance on the target parameters, to establish new regularities governing the shell motion, to identify the phenomena that take place in the region of the critical and quarter-critical density, and to demonstrate experimentally the possibility of reaching in the “exploding pusher” regime densities much higher than the density of a solid body.     

Three-dimensional dynamics of breakout afterburner ion acceleration using high-contrast short-pulse laser and nanoscale targets

Breakout afterburner (BOA) laser-ion acceleration has been demonstrated for the first time in the laboratory. In the BOA, an initially solid-density target undergoes relativistically induced transparency, initiating a period of enhanced ion acceleration. First-ever kinetic simulations of the BOA in three dimensions show that the ion beam forms lobes in the direction orthogonal to laser polarization and propagation. Analytic theory presented for the electron dynamics in the laser ponderomotive field explains how azimuthal symmetry breaks even for a symmetric laser intensity profile; these results are consistent with recent experiments at the Trident laser facility.     

Imploding shocks in laser-driven fusion

The role of a sequence of imploding spherical and cylindrical shocks is investigated in the context of laser-driven fusion in deuterium-tritium pellets. An approximate analytical treatment of a convergent sequence of shocks is presented within the framework of gas-dynamic equations for self-similar motion. These analytical solutions are compared with the exact numerical solutions. The solutions display an explicit dependence on the relative strength between the successive shocks and the ratio of the final to the initial pressure in the shocks. These solutions are employed to estimate the fusion yield for a given input shock energy.     

Impulse coupling in laser-driven microtargets

We have studied about the thrust imparted to targets of different materials by pulsed TEA CO₂ laser and chopped CW CO₂ laser in air and its dependence on different parameters such as laser intensity and pulse duration. We estimated the impulse-coupling coefficient and compared it with the published results. The mechanism of generation of thrust by laser incident on targets in air is, in effect, combination of those involved in laser ablation in vacuum and laser-induced air detonation.     

Electron-energy bunching in laser-driven soft recollisions

We introduce soft recollisions in laser-matter interaction. They are characterized by the electron missing the ion upon recollision in contrast with the well-known head-on collisions responsible for high-harmonic generation or above-threshold ionization. We demonstrate analytically that soft recollisions can cause a bunching of photoelectron energies through which a series of low-energy peaks emerges in the electron yield along the laser polarization axis. This peak sequence is universal, it does not depend on the binding potential, and is found below an excess energy of one tenth of the ponderomotive energy.     

Impulse-scaling in a laser-driven in-tube accelerator

The laser-driven in-tube accelerator (LITA) is a unique device for laser propulsion. It is characterized by the acceleration of a projectile in a tube. The thrust performance can be improved by exploiting a confinement effect. In the experiment, a 3.0-g projectile is vertically launched, and the momentum coupling coefficient is measured for various monoatomic gases. The measured coupling coefficient is almost proportional to the reciprocal of the speed of sound. The same impulse generation characteristics are obtained in simplified situations that are analyzed based on conservation relations. Received: 26 August 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-22/217-5284, E-Mail: sasoh@ifs.tohoku.ac.jp RID="**" ID="**"Present address: also at Institute of Advanced Aerospace Technology, Seoul National University, Seoul, 151-742, Korea     

Circular dichroism in laser-assisted short-pulse photoionization

A remarkable effect of circular dichroism, i.e., a difference in photoelectron spectra produced by right and left circularly polarized light in two-color multiphoton ionization of atoms, is predicted for the case when the atom is ionized by an extreme ultraviolet or x-ray femtosecond pulse in the field of a strong infrared laser pulse, both pulses being circularly polarized. We show that the sidebands formed in the spectra exhibit different circular dichroism often of different signs both in angle-resolved and angle-integrated experimental conditions. The effect can be used for detecting and measuring circular polarization of x rays in a spectral range where other methods are not effective.     

Quantum signatures in laser-driven relativistic multiple scattering

The dynamics of an electronic Dirac wave packet evolving under the influence of an ultraintense laser pulse and an ensemble of highly charged ions is investigated numerically. Special emphasis is placed on the evolution of quantum signatures from single to multiple scattering events. We quantify the occurrence of quantum relativistic interference fringes in various situations and stress their significance in multiple-particle systems, even in the relativistic range of laser-matter interaction.     

Fusion yield enhancement in magnetized laser-driven implosions

Enhancement of the ion temperature and fusion yield has been observed in magnetized laser-driven inertial confinement fusion implosions on the OMEGA Laser Facility. A spherical CH target with a 10 atm D2 gas fill was imploded in a polar-drive configuration. A magnetic field of 80 kG was embedded in the target and was subsequently trapped and compressed by the imploding conductive plasma. As a result of the hot-spot magnetization, the electron radial heat losses were suppressed and the observed ion temperature and neutron yield were enhanced by 15% and 30%, respectively.     

Amplified spontaneous emission in short-pulse excimer amplifiers

We have found a simple analytical expression which describes the relation between amplified spontaneous emission (ASE) and small-signal gain in short-pulse amplifiers. It is also shown that the contrast of the short pulse to the ASE is weakly dependent on the saturation of the ASE, and influenced mainly by the saturation of the short pulse. The theoretical considerations were verified by measurements.