Acceleration of quasineutral plasma blocks by ponderomotive force induced by normally incident short laser pulse is studied here via 1D3V Particle-In-Cell (PIC) simulations. Very high current densities 109–1011 Acm−2 of accelerated ions are observed for maximum laser intensities in the range 1016–1018 Wcm−2 on solid hydrogen target. Ion acceleration process is traced here via evolution of ion density and of ion velocity distribution. Basic parameters of the accelerated plasma blocks are determined from temporally integrated ion distributions. Our results provide more detailed information than the previous analytical estimates (Hora, 2003) and the two-fluid 1D hydrodynamic simulations (Glowacz et al., 2004).

Erratum.

Alfvén's derivation of his current limit is given. It demonstrates that it does not give the maximum possible current of a beam, but the maximum current that can propagate for an indefinite distance and time, from a source, in a charge neutral beam. Furthermore, the value Alfvén obtained applies to a uniform current density and to particles initially moving in the direction of the beam. It is also shown that Alfvén predicted that beams which exceed the limit will filament as a result of the particles that are turned back by the magnetic field. His work is extended to beams with particles that have transverse momentum, to beams with non-uniform current densities, to beams that are not charge neutral and to the time dependent case. These extensions of Alfvén's work are found to require numerical calculations in most cases and to give ambiguous results in some cases. A general formula for the current limit is given based on the conservation of energy. It is calculated for the cases considered previously and found to confirm the accuracy of Alfvén's original estimate. The relevance of the current limit to high intensity laser-solid interactions and fast ignition is then discussed.

(This article appeared in Volume 24, Number 2, Pages 275–282, 2006)

(This article appeared in Volume 24, Number 2, Pages 275–282, 2006)

Below is the correct Reference citation for Breil et al. (2005)

Breil, J., Hallo, L., Maire, P.H. & Olazabal-Loumé, M. (2005). Hydrodynamic instabilities in axisymmetric geometry self-similiar models and numerical simulations. Laser Part. Beams.23, 155–160.

Models for the prediction of electron pedestal temperatures at the edge of type I ELMy H-mode plasmas are developed. These models are based on theory motivated concepts for pedestal width and pressure gradient. The pedestal pressure gradient is assumed to be limited by high n ballooning mode instabilities, where both the first and second stability limits are considered. The effect of the bootstrap current, which reduces the magnetic shear in the steep pressure gradient region at the edge of the H-mode plasma, can result in access to the second stability of ballooning mode. In these pedestal models, the magnetic shear and safety factor are calculated at one pedestal width away from separatrix. The predictions of these models are compared with the experimental electron pedestal temperatures for type I ELMy H-mode discharges obtained from the latest public version (version 3.2) in the International Tokamak Physics Activity Edge (ITPA) Pedestal Database.

We report on the pulsed laser ablation of aluminum (Al) plasma in presence of ambient nitrogen to understand the formation of aluminum nitride (AlN). Formation of carbon nitride (CN) and titanium oxide (TiO) by pulsed laser-ablation of graphite and titanium targets in presence of ambient nitrogen and oxygen is also compared. We discuss the dynamics of plasma expansion based on existing models, shock and drag models, and the plasma gas interface distortion, Rayleigh-Taylor instability at various ambient pressures of nitrogen. Since Rayleigh-Taylor instability may give rise to self-generated magnetic field in the plasma, an attempt is made to understand the mechanism of generation as well as the estimation of this field near the focal spot using the information from the images of the expanding plasma. This is the first time images of the expanding plume are used to estimate self generated magnetic fields. At the irradiance level used in the experiment the field is high very close to the target surface therefore we expect splitting of the energy levels thus giving rise to emissions that may be anisotropic in nature. We discuss the extent of anisotropy by measuring the degree of polarization using emission intensity in optical emission spectrum of selected Al III transition 4s 2S1/2–4p 2P3/2o at 569.6 nm using both nanosecond and picosecond pulses.

An attosecond electron beam generation is studied by an intense short-pulse TEM (1,0) + TEM (0,1)-mode laser with a plasma separator in vacuum. The TEM (1,0) + TEM (0,1)-mode laser has a ring-shaped intensity peak in the radial direction. Electrons are accelerated and compressed near the focus point of the TEM (1,0) + TEM (0,1)-mode laser. However, after the focus point, some electrons move to its deceleration phase of the laser pulse and are decelerated. As a result, a longitudinal velocity deference of electrons generated causes a density lowering. In order to suppress the deceleration and the density lowering, we set an overdense plasma-foil separator before the electrons move to the deceleration phase of the laser pulse. Since only the laser is reflected by the plasma separator, the electrons do not experience the deceleration phase and the density of the electron bunch is kept high after passing through the plasma separator. Consequently, a high-density electron beam is generated and at the same time, the pulse length of the electron bunch becomes sub-femto second.

High-energy protons are generated by focusing an ultrashort pulsed high intensity laser at the Advanced Photon Research Center, JAERI-Kansai onto thin (thickness <10 μm) Tantalum targets. The laser intensities are about 4 × 1018 W/cm2. The prepulse level of the laser pulse is measured with combination of a PIN photo diode and a cross correlator and is less than 10−6. A quarter-wave plate is installed into the laser beam line to create circularly polarized pulses. Collimated high energy protons are observed with CH coated Tantalum targets irradiated with the circularly polarized laser pulses. The beam divergence of the generated proton beam is measured with a CR-39 track detector and is about 6 mrad.

Edward Teller Lectures—Lasers and Inertial Fusion Energy, Heinrich Hora and George H. Miley, eds. Foreword by E.M. Campbell. First edition. Imperial College Press, London, 365 pp. US $63.00 ISBN: 1-86094-468-X

Since 1991, the Edward Teller Medal is awarded to individual researchers in recognition of their respective pioneering experimental or theoretical work in the field of intense laser and particle beam physics, and physics application of high power drivers, which is exactly the field that the journal Laser and Particle Beams covers in great detail. Motivation of this research field is the investigation of properties of high energy density matter with the ultimate goal to achieve inertial fusion in the laboratory under reproducible conditions, and to develop a scientific basis for inertial fusion energy.

Recent particle-in-cell simulations of the stimulated Brillouin backscattering (SBBS) of electromagnetic radiation have shown that even at sub-relativistic intensities (Iλ2 = 1016 Wμm2/cm2) non-drifting solitary waves, “solitons” for short, are easily produced, and remain almost unchanged all along the simulation time, typically for several thousands of optical cycles. They appear in the form of stable local concentrations of electromagnetic radiation trapped inside quasi-neutral density holes. The plasma density inhomogeneity associated with their presence disrupts the resonant SBBS amplification. The cavitation process is accompanied by strong electron and ion heating. The physical characteristics of such solitons are discussed and they are compared with the theoretical predictions of an analytical model for localized solution of the Maxwell equations in warm plasma.

This study deals with the simulation of the experimental study of Roth et al. (2000) on the interaction of energetic Zn projectiles in partially ionized laser produced carbon targets, and with similar type experiments. Particular attention is paid to the specific contributions of the K and L shell target electrons to electron recombination in the energetic Zn ionic projectile. The classical Bohr–Lindhard model was used for describing recombination, while quantum mechanical models were also introduced for scaling the L to K cross-section ratios. It was found that even for a hydrogen-like carbon target, the effect of the missing five bound electrons brings about an increase of only 0.6 charge units in the equilibrium charge state as compared to the cold target value of 23. A collisional radiative calculation was employed for analyzing the type of plasma produced in the experimental study. It was found that for the plasma conditions characteristic of this experiment, some fully ionized target plasma atoms should be present. However in order to explain the experimentally observed large increase in the projectile charge state a very dominant component of the fully ionized plasma must comprise the target plasma. A procedure for calculating the dynamic evolvement of the projectile charge state within partially ionized plasma is also presented and applied to the type of plasma encountered in the experiment of Roth et al. (2000). The low temperature and density tail on the back of the target brings about a decrease in the exiting charge state, while the value of the average charge state within the target is dependent on the absolute value of the cross-sections.

Erratum.

We consider spontaneous thermalization of free photon field (in a vacuum) due to self-interaction, mediated by the virtual e−e+ field at ultra-high concentrations of the electromagnetic energy. That nonlinear, attractive, short-range interaction between photons triggers spontaneous evolution of the initial, low-frequency spectrum toward the maximum-entropy spectral distribution peaking in the gamma range of frequencies. Collimation and the total power of the photon beam (pulse) are hardly affected by the process of spontaneous thermalization. We estimate the threshold intensity that triggers spectral evolution of the photon field, the necessary power of the laser beam, the minimum size of the interaction region, parameters of the fully thermalized photon field, and discuss the near-threshold behavior of the electromagnetic field. Possible applications of thermalized photon beams are suggested, for example, they can serve as the pump field to attain gamma-lasing or facilitate ignition in the fusion pellet.

We propose a focusing mechanism of high-energy ions by an electron cloud produced by a laser interaction with slab plasma. In our 2.5-dimensional (2.5D) particle-in-cell simulations, the laser intensity is 2 × 1020 W/cm2, the laser wavelength λ is 1.053 μm, and the laser spot size is 2.5λ. When the high intensity laser irradiates slab plasma, electrons are accelerated, oscillate around the plasma and produce the electron cloud locally at the sides of the plasma. Because the electrons are localized transversely, a static electric potential is formed to focus ions and at the same time the ions are accelerated longitudinally. Though the longitudinal ion acceleration has been studied well, the ion focusing effect is reported for the first time in this paper. In our calculations, the maximum energy and intensity of the protons are 8.61 MeV and 1.89 × 1017 W/cm2, and the diameter of the proton bunch accelerated are focused to 71.2% of its initial size.

Proton beams, generated in the interaction process of short ultra-intense laser pulses with thin foils, carry imprints of rear side target structures. These intensity patterns, imaged with a particle detector, sometimes show slight deformations. We propose an analytical model to describe these deformations by the spatial shape of a monoenergetic layer of protons in the beginning of free proton propagation. We also present results of simulations, which reproduce the detected structures and allow finally making quantitative conclusions on the shape of the layer. In experiments with electrically conducting targets, the shape is always close to a parabolic one independently on target thickness or laser parameters. Since the protons are pulled by the free electrons, there must be a strong correlation to the electron space charge distribution on the rear side of the illuminated foil. Simulations demonstrate that the deformations in the detected patterns of the proton layers are very sensitive to the initial layer shape. Analyzing spatial structures of the generated proton beams we can indirectly conclude on electron transport phenomena in the overdense part of the target.

The impedance mismatch technique was used for shock pressure amplification in two layered planar foil targets. Numerical simulation results using one-dimensional (1D) radiation hydrocode MULTI in two layer target consisting of polyethylene (CH2)n-aluminium (Al) and polyethylene (CH2)n-gold (Au), show a pressure enhancement of 12 and 18 Mbar, respectively (or a pressure jump of 1.64 and 2.54, respectively), from initial pressure of 7 Mbar in the reference material (polyethylene) using laser intensity of 5 × 1013 Watts/cm2 at 1.064 μm. The simulation data was also corroborated by experiments in our laboratory. Results of laser driven shock wave experiments for pressure enhancement studies in CH2-Al and CH2-Au targets are also presented. A Nd:YAG laser chain (2 J, 1.064 μm wavelength, 200 ps pulse duration FWHM) is used for generating shocks in the planar CH2 foils of thickness varying from 4 to10 μm, and in two layered CH2-Al (or CH2-Au) targets with 8 μm CH2 and 1.5 μm Al or Au .

Laser-beam interaction with expanding plasma was investigated using the PALS high-power iodine-laser system. The interaction conditions are significantly changing with the laser focus spot position. The decisive role of the laser-beam self-focusing, participating in the production of ions with the highest charge states, was proved.

Ion acceleration by lasers is one of the most important innovations in laser-plasma research in recent years. A mechanism that has gained great attention due to the remarkable properties of the accelerated beam is laser acceleration of protons from the rear surface of solid targets. A striking prediction is that these protons are capable of generating images of micro-structures present on this surface. These images might be useful to measure properties of the accelerated beam. In this article, we address the physics of the generation of images of surface structures imprinted into the target back surface with laser-accelerated protons.

A “table-top” high power laser has been used to generate beams of accelerated electrons up to energy of 20 MeV from interactions with underdense plasmas. The energy spectrum of these beams was measured using a magnetic spectrometer and proof-of-principle experiments were performed to evaluate the suitability of these beams for electron radiography applications.

It was reported that the fuel core was heated up to ∼0.8 keV in the fast ignition experiments with cone-guided targets, but they could not theoretically explain heating mechanisms and achievement of such high temperature. Thus simulations should play an important role in estimating the scheme performance, and we must simulate each phenomenon with individual codes and integrate them under the fast ignition integrated interconnecting code project. In the previous integrated simulations, fast electrons generated by the laser-plasma interaction were too hot to efficiently heat the core and we got only 0.096 keV rise of temperature. Including the density gap at the contact surface between the cone tip and the imploded plasma, the period of core heating became longer and the core was heated by 0.162 keV, ∼ 69% higher increment compared with ignoring the density gap effect.