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Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scales linearly with the laser amplitude, which, however, holds only when the latter is at a moderate level. To overcome this limitation, we propose a scheme using chirped laser pulses irradiating on tenuous gas or plasma targets, which can generate THz pulses with amplitude 10-100 times larger than that from the well-known two-color laser scheme, enabling one to obtain THz field up to 10MV/cm with incident laser at approximately 10(16)W/cm(2).

In order to achieve a high-quality, i.e., monoenergetic, intense ion beam, we propose the use of a double-layer target. The first layer, at the target front, consists of high-Z atoms, while the second (rear) layer is a thin coating of low-Z atoms. The generation of high-quality proton beams from the double-layer target, irradiated by an ultraintense laser pulse, is demonstrated with three-dimensional particle-in-cell simulations.

Abstract: Heat transport in reduced mass targets irradiated with a high-intensity laser pulse was studied. Kα lines from partially ionized chlorine embedded in the middle of a triple-layered plastic target were measured to evaluate bulk electron temperature in the tracer region inside the target. Two groups of Kα lines, one from Cl+–Cl6+ (hereby called “cold Kα”), and the other from Cl9+ and Cl10+ (“shifted Kα”) are observed from different regions within the target. Two-dimensional collisional particle-in-cell simulations show two distinct heating mechanisms occurring concurrently. These processes, which made the target temperature distribution non-uniform, are responsible for producing the two groups of Kα lines in the experiment. The blue shift of cold Kα lines in the experiment is the signature of higher temperatures achieved by the refluxing heating in smaller mass targets. [Copyright &y& Elsevier]

This paper reports an experimental study on preheating of laser-irradiated targets. We performed temperature measurements at the rear surface of laser-irradiated targets under conditions of two different laser wavelengths (0.35 or 0.53 μm) and several intensities (2×1013–1×1014 W/cm2) in order to verify an effect of radiation and nonlocal electron heat transport. The preheating temperature was evaluated by observing self-emission, reflectivity, and expansion velocity at the rear surface of planar polyimide foils. The experimental results show that the x-ray radiation is dominant for preheating for 0.35-μm laser irradiation, but contribution of nonlocal electron heat transport is not negligible for 0.53-μm laser irradiation conditions. [ABSTRACT FROM AUTHOR]

The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Basic and integrated studies are conducted on fast ignition (FI) using various large laser systems. A Peta watt (PW) laser system is used to study the basic elements relevant to FI and can also be injected to a compressed core to test the FI integrated experiment when coupled with a GEKKO twelve laser beam system. Using a spherical target inserted with a Au cone guide for the PW laser pulse, an imploded core is heated up to 1 keV resulting in neutron increase 1000 times more than that without heating pulse. Details of the implosion are examined at the Omega laser system of this type target with indirect implosion scheme and are compared with simulation results. LASNEX simulation indicates that a 400 g/c.c. high density core could be achieved with this scheme at 1.8 MJ laser input. [ABSTRACT FROM AUTHOR]

Investigates the production of plasmas from ultra-short intense laser on x-ray emissions by particle simulations. Integration of the collisional absorption and x-ray bremsstrahlung processes in particle simulation; Utilization of Monte Carlo method in the bremsstrahlung processes integration; Investigation of the photon energy spectrum.

Studies a method of cathode heating using a laser for an electron gun. Practicality of the heating system; Characteristics of the laser-heated gun with a dispenser and thermionic cathodes; Efficiency of direct laser irradiation; Observation that the growth of electron beam emittance was very small in the laser heating.

A new method which suppresses target preheating by hot electrons is proposed. The suppression was verified by means of particle measurements on the rear side of a foil irradiated by a CO2 laser with a thick plate placed in front of the irradiated foil to absorb the energetic electrons which preferentially expand from the critical surface. The results can be explained by the generation of the electrostatic field between the foil and the plate. [ABSTRACT FROM AUTHOR]

Abstract: An analytical study of the relativistic interaction of a linearly-polarized laser-field of ω frequency with highly overdense plasma is presented. Very intense high harmonics are generated produced by relativistic mirrors effects due to the relativistic electron plasma oscillation. Also, in agreement with 1D Particle-In-Cell Simulations (PICS), the model self-consistently explains the transition between the sheath inverse bremsstrahlung (SIB) absorption regime and the heating (responsible for the 2ω electron bunches), as well as the mean electron energy. [Copyright &y& Elsevier]