I deeply regret stepping down from the position as Editor-in-Chief of Laser and Particle Beams (LPB), but this has become necessary as my other professional responsibilities have changed. Fortunately, Dieter Hoffmann is an ideal replacement who will provide the leadership needed to maintain the quality and continue the growth of LPB. I am confident that, under his direction, the journal will continue to serve the scientific community as a leading vehicle to exchange and archive forefront research results.The success of LPB to this point has been due to the work and contributions of many people. I learned much from Heinz Hora, the founding editor of LPB, during the 4 year overlap period when I served as Managing Editor. He continued to provide much help and advice after I took over in 1992. The Associate Editors and the Editorial Board have also played an instrumental role in the Journal's operations, ranging from advice about editorial policies to helping with special issues and with Guest Editors. The many authors who have submitted such outstanding manuscripts over the years are, of course, the fundamental reason the journal is so well received in the community. The various reviewers who have given so much time to their work have been integral to maintaining the journal quality. Last, but not least, thanks goes to the Cambridge University Press staff who have so ably handled the proofing, printing and circulation of the Journal.

As early as 1997, we decided to promote a short series of conferences devoted to a field that was showing dramatic progress and in which an increasing number of young researchers were participating. The Euroconference format of the “TMR Programme of the Commission of the European Union” was the most appropriate to fulfill the two objectives of a quasi real-time presentation and discussion of novel results and a tutorial support of young scientists from eminent lecturers active in the field. The success of the two meetings, held in two authentic capitals of the European cultural heritage, went well beyond any optimistic expectation. More than 200 enthusiastic participants, including approximately 100 young scientists, made those events a unique experience of scientific progress and debate. I wish to express my gratitude to Dimitris Charalambidis, who divided with me this experience, to the Scientific Advisory Committee, who guaranteed the high scientific level of the meetings, and to the Local Organizers, who made all this possible. The ULIA participants strongly appreciated the initiative of Laser and Particle Beams to edit two special issues with a collection of selected works presented at the ULIA meetings. Antonio Giulietti was the Scientific Promoter of the ULIA Conferences

The coupled propagation of two electromagnetic waves in plasma is studied to establish the conditions for induced transparency. Induced transparency refers to the situation where both waves propagate unattenuated, although the frequency of one (or both) of them is below the plasma frequency so that it could not propagate in the absence of the other. The effect is due to the interaction of the waves through their beat, which modulates both the electron mass and, by exciting longitudinal plasma oscillations, their number density, and thus the plasma frequency. Starting from a relativistic fluid description, a dispersion relation for plane waves of weakly relativistic intensities is derived, which takes into account the polarization of the waves and the nonlinearities with respect to both their amplitudes. This serves as a basis for the exploration of the conditions for induced transparency and the modes of propagation.

Z pinches have long been interesting to plasma physicists because of the simplicity of their geometry. They were studied first in the 1950s as potential fusion configurations and, later, as intense X-ray sources both for military and civilian applications. The development of pulsed power technologies, some reaching energy and power levels of 3 MJ and 50 TW, respectively, by many organizations around the world has accelerated the interest in z pinches for fusion applications.

The propagation of a 1-ps laser pulse at intensities exceeding 1019 Wcm−2 in a low-density plasma channel was experimentally tested. The channel was produced by a lower intensity preceding pulse of the same duration. Plasma electrons were accelerated during the propagation of the main pulse, and high energy γ-ray detectors were used to detect their bremsstrahlung emission. The γ-ray yield was studied for different channel conditions, by varying the delay between the channel forming pulse and the high intensity pulse. These results are correlated with the interferograms of the propagation region into the plasma.

Z pinches have a long and varied history. Beginning in the 18th century, z pinches have been used to heat plasmas very efficiently. Early in the nuclear fusion program, it was realized that modest currents are required to confine plasma that could produce energy gain. The instability of the confined plasma was convincingly demonstrated in experiments in the 1950s that were performed around the world. These uniformly negative results led to z pinches being dropped as a fusion concept. Recent progress in fast z pinches has reinvigorated the field. We review the field and highlight the recent advances that point the way to a bright future for z pinches.

By this time, we trust that you have received issues from Volume 18 of Laser and Particle Beams which used the new, large-page size, double-column format. As I indicated in an earlier preface, this format provides additional type-space, which will hopefully allow us to speed up publication of articles in LPB.

A series of specialized multidimensional resistive magnetohydrodynamic (MHD) models have been developed to tackle the different phases of evolution of wire array z-pinch implosions. Two-dimensional (r–z) “cold-start” or “wire initiation” simulations of single wires indicate the persistence of a two-component structure with a cold, dense core embedded within a much hotter, low density, m = 0 unstable corona. Cold-start simulations with similar conditions to wires in an array show a general trend in the plasma structure from discrete wires with large m = 0 perturbation amplitudes to partially merged wires with smaller perturbation amplitudes as the number of wires is increased. Two-dimensional (r–θ) simulations then show how the persistence of dense wire cores results in the injection of material between the wires into the interior of the array, generating radial plasma streams which form a precursor plasma upon reaching the axis. Higher-resolution 2-D (r–θ) simulations show similar behavior for large number wire arrays in use at Sandia National Laboratories. This model is also used to predict which modes of implosion are in operation in nested wire array experiments. Separate r–θ plane simulations of the flux of plasma imploding towards the axis from the outer array and the bombardment of the inner array by this flux are presented. Finally, 2-D (r–z) simulations of the Rayleigh–Taylor instability during the final implosion phase are used to illustrate the effect upon the power and duration of the radiation output pulse. The results of low-resolution 3-D resistive MHD simulations are also presented. The need for much higher resolution 3-D simulations of certain aspects of wire array evolution is highlighted.

The magneto-Rayleigh–Taylor (MRT) instability limits the performance of dynamic z pinches. This instability develops at the plasma-vacuum/field interface, growing in amplitude throughout the implosion, thereby reducing the peak plasma velocity and spatial uniformity at stagnation. MRT instabilities are believed to play a dominant role in the case of high wire number arrays, gas puffs and foils. In this article, the MRT instability is discussed in terms of initial seeding, linear and nonlinear growth, experimental evidence, radiation magnetohydrodynamic simulations, and mitigating schemes. A number of experimental results are presented, where the mitigating schemes have been realized. In general, the problem is inherently three dimensional, but two-dimensional simulations together with theory and experiment enhance our physical understanding and provide insight into future load design.

Both dynamic and equilibrium z pinches are mostly described, whether in theory or in simulation, by conventional magnetohydrodynamic (MHD) fluid equations. However there are several key phases in z-pinch behavior when kinetic effects are important. Runaway electrons can occur close to the axis especially in an m = 0 neck or during the subsequent disruption. Large ion-Larmor orbits can, if sufficiently collisionless, lead to stabilizing effects. During a disruption, ion beams can be produced. For deuterium discharges, the interaction of an ion beam with the ambient plasma can lead to a significant neutron yield. If the drift velocity of the current-carrying electrons exceeds a threshold for generating microinstabilities (lower-hybrid or ion-acoustic instabilities), this leads to anomalous resistivity. This can occur not only during a disruption, but also in the low-density (usually outer) plasma boundary of an equilibrium or dynamic pinch. Related to this is the open question of whether current reconnection can occur in fully developed magneto-Rayleigh–Taylor instabilities in the low-density coronal plasma. Two other kinetic effects are new to z pinches. First, there are the collisions of low-density energetic ions from a wire array as they pass close to the axis to form a precursor plasma. Second, there is the possible erosion and ablation of wire cores in the necks of m = 0 coronal current-carrying plasma by flux-limited heat flow with its attendant deviation from a Maxwellian of the isotropic part of the distribution function.

Utilizing an ultraintense (1016 W cm−2) fs laser, the laser/matter interaction of the tetrahedral CH3I molecule is investigated. A mass spectrum and the angular distributions of fragment ions arising from Coulomb explosion of molecular ions, obtained with linearly polarized light, are presented. The distributions for In+ (n ≤ 7), CHm+ (m ≤ 3), Cp+ (p ≤ 4) and H+ ions are all anisotropic and maximal when the polarization lies along the spectrometer axis. The molecule hence seems to behave as a diatomic, with the fragment ions being ejected along the field direction. Also presented are mass spectra of the isomers 1- and 2-nitropropane, which are explosive species, taken for horizontal and vertical polarizations at both 375 and 750 nm. It is shown that femtosecond laser mass spectrometry (FLMS) can be used to distinguish between these two isomers through their differing dissociation patterns. Isomer identification is important for many different applications and FLMS may provide a means of achieving this for a wide range of molecules.

Detailed investigations of the propagation of an ultraintense picosecond laser pulse through preformed plasmas have been carried out. An underdense plasma with peak density around 0.1nc was generated by exploding a thin foil target with an intense nanosecond laser pulse. The formation of plasma channels with an ultraintense laser pulse due to ponderomotive expulsion of elections and the subsequent Coulomb explosion were investigated. The laser transmission through underdense plasmas was measured for a picosecond pulse at intensities above 1019 W/cm2 with and without a plasma channel preformed with an ultraintense prepulse. The energy transmitted through the plasma increased from the few percent transmittance measured in absence of the preformed channel to almost 100% transmission with the channelling to main pulse delay at around 100 ps. The propagation of a relativistic laser pulse through overdense plasmas was also investigated. A well-characterized plasma with an electron density up to 8nc was generated by soft X-ray irradiation of a low-density foam target. The propagation of the laser pulse was observed via X-ray imaging and monitoring the energy transmission through the plasma. Evidence of collimated laser transport was obtained.

Relatively small-scale laser-driven sources of short wavelength radiation covering a range from the extreme ultraviolet to the hard X-ray regime are now available. Because the duration of the X-ray pulses is comparable to, or shorter than the laser pulse width, it is possible to carry out X-ray measurements with picosecond or femtosecond time resolution.

In this work, the interaction of a transversely excited atmospheric (TEA) CO2 laser with tungsten–titanium (W-Ti) alloy deposited on austenitic stainless steel is considered. The W-Ti alloy as a refractory material possesses very good physicochemical characteristics such as thermochemical stability and high melting temperature. Studying of interactions of different energetic particles or laser beams with W-Ti coatings has both application and fundamental importance.

The morphological features of the W-Ti coating, deposited on austenitic stainless steel AISI 316, induced by a TEA CO2 laser after multipulse cumulative laser action, have been considered. The laser pulses with tail (FWHM = 120 ns, tail = 2 μs) and free-tail pulses (FWHM = 80 ns) have been employed. Laser pulses used in the experiment had equal peak power density I = 120 MWcm−2. For the given peak power density, excessive surface changes on the coating were registered. From direct observation on a microscopic scale (OM, SEM), it can be concluded that W-Ti coatings show different behavior under laser irradiation with various temporal pulse shapes.

Experiments to study plasma formation and implosion dynamics of wire array z pinches performed on MAGPIE generator (1.4 MA, 240 ns) at Imperial College are reviewed. Data from laser probing and X-ray radiography show that heterogeneous plasma structure with dense wire cores surrounded by low-density coronal plasma persists in wire arrays for a significant part of the implosion. Early implosion of the coronal plasma produces a precursor plasma column on the array axis, parameters of which depend on the rate of radiative cooling. The seeding of perturbations on the dense core of each wire is provided by nonuniform sweeping of the low-density coronal plasma from the cores by the global J × B force. The spatial scale of these perturbations (∼0.5 mm for Al, ∼0.25 mm for W) is determined by the size of the wire cores (∼0.25 mm for Al, ∼0.1 mm for W). A qualitative change in implosion dynamics, with transition to 0-D-like trajectory, was observed in Al arrays when the ratio of interwire gap to wire core size was decreased to ∼3. In experiments with nested wire arrays, two different modes of operation were identified, both giving significant sharpening of the X-ray pulse (∼10 ns) in comparison with a single array, despite the small number of wires in the arrays (16 outer, 16 inner) and the long implosion time (260 ns).

Characteristics of annular wire array z pinches as a function of wire number and at high wire number are reviewed. The data, taken primarily using aluminum wires on Saturn, are comprehensive. The experiments have provided important insights into the features of wire-array dynamics critical for high X-ray power generation, and have initiated a renaissance in z pinches when high numbers of wires are used. In this regime, for example, radiation environments characteristic of those encountered during the early pulses required for indirect-drive ICF ignition on the NIF have been produced in hohlraums driven by X rays from a z pinch, and are commented on here.

We report on the design and characterization of a grazing-incidence flat-field spectrograph that allows simultaneously the measurement of spectrum, beam divergence, and absolute flux of EUV and soft X-ray radiation for a beam of high-order laser harmonics generated by the interaction between an ultrashort femtosecond laser pulse and a gas jet. The instrument seems a very powerful tool for the understanding of the generation process.

Pulsed-power-driven z-pinch plasmas are an intense source of soft X-ray radiation producing, on the Z facility, about 2 MJ of total radiation for a number of tungsten loads and in the case of a multiwire titanium array over 1 MJ total radiation and about 100 kJ from the titanium K-shell. The production and transport of radiation in these non-LTE plasmas are often modeled assuming some variation of Local Thermodynamic Equilibrium (LTE) in conjunction with radiation diffusion. Since these plasmas are neither in LTE or entirely opaque or transparent these models do not properly predict the emitted radiation spectra and yield. Also, application of these models overestimates the radiation cooling to the extent that the evolving hydrodynamic profiles are significantly different from those that would obtain using the appropriate non-LTE model with a more realistic treatment of the radiation transport. In this investigation, we discuss the production and transport of radiation from the viewpoint of the microscopic collisional and radiative processes and then apply it to z-pinch plasmas. Through the use of examples and illustrations, it is shown that for identical initial load conditions, atomic level structure, and rate coefficients, the models predict different results that affect the dynamic evolution and hydrodynamic history of the plasma. As an example, the emission spectrum is generated using a 1-D radiation MHD model self-consistently coupled to a circuit representing the Sandia Z facility. A comparison is then made between several standard models of ionization dynamics for a multiwire titanium array. Finally, we address some of the issues regarding how the dense plasma environment influences isolated atom structure and processes. These include, for example, atomic level shifts, ionization lowering, collision cross sections, and collision widths. Transition from the isolated-atom to the dressed-particle picture can modify the ionization physics and emission spectra to such an extent that it may challenge our precepts on how best to design loads for the next generation machine.

Collective absorption, so far determined by numerical simulations, is explained in physical terms for cold and warm plasma. After deducing a few general relations for flat targets, interface phase mixing and nonadiabatic electron acceleration in the skin layer are identified as the main physical processes leading to irreversibility, that is, to absorption.

This article reviews the status of experiments directed to understanding the initial phases of exploding wire plasma formation, especially those relevant to the initiation of wire explosions and plasma formation in wire array z-pinch experiments. Thus, although we discuss experiments with ∼100 kA per wire, in which magnetic forces play a major role, our emphasis is on experiments with ∼1 kA per wire, in which magnetic forces appear to be unimportant. With high current (∼100 kA) per wire, the exploding wire consists of a rapidly expanding (1–3 cm/μs) coronal plasma surrounding a dense core that expands much more slowly. The coronal plasma exhibits strong, azimuthally symmetric instabilities driven by the high current, but the dense core appears to be stable, suggesting that it is carrying little of the current. In low-current experiments, the initial wire core expansion rate depends upon the material, the wire size, and whether or not it is coated with an insulator. For bare wires, the core diameter expands at rates which range from ≤0.03 cm/μs (for 25-μm-diameter W) to 0.46 cm/μs (for 25-μm-diameter Ag). This expansion rate increases with the energy deposited resistively in the wire before coronal plasma formation. Furthermore, expansion is more uniform as well as faster for wires in which the deposited energy is comparable to or larger than the vaporization energy. Insulating coatings increase the energy deposition, evidently by forestalling the formation of plasma around a wire. Therefore, wires coated with 1-μm thickness of plastic expand faster (e.g., by a factor of 2 for Ag) than bare wires for all wires tested so far.