Table of contents

This special issue presents a collection of refereed plenary and review papers presented at the 12th International Congress on Plasma Physics held in Nice, France, 25–29 October, 2004 (ICPP2004).

The primary aims of ICPPs are:

• To advance all fields of plasma physics worldwide.

• To promote strong linkage with other areas of science in order to highlight the interdisciplinary character of the field.

• To strongly encourage participation by all members of the plasma physics community, in particular by young plasma physicists, women plasma physicists and plasma physicists from developing countries.

The 12th ICPP was organized on behalf of the International Advisory Committee (IAC). About 330 papers (invited and contributed) were presented. These papers covered the four topics:

• Plasma applications

• Space and astrophysical plasmas

• Fundamental plasma physics

• Fusion plasmas

Most of the contributions are available online at the following address: http://hal.ccsd.cnrs.fr/ICPP2004/en/.

The Congress was underwritten by the Association Euratom–CEA. The organizers wish to thank the main sponsors: the International Union of Pure and Applied Physics (IUPAP), the Commissariat à l'Energie Atomique, the Region Provence Alpes Côtes d'Azur, the city of Nice and the French Ministry of Foreign Affairs.

For this special issue, the Programme Committee selected 41 speakers from about 100 proposals received from the IAC, to present invited talks. Some of them provided a written paper. Refereeing of these papers was conducted by the guest editors, following the normal refereeing standards of the journal.

Theoretical investigations as well as experimental observations conclusively show the formation of different classes of nonlinear waves and structures in weakly and strongly coupled dusty plasmas. Our objective here is to describe the underlying physics and mathematical details of these nonlinear waves and structures (particularly, solitary waves and shock structures), and to pinpoint the theoretical models which explain the experimental observations.

A classical interatomic potential model has been developed for systems consisting of hydrogen, carbon and nitrogen atoms. By using the new potential model, molecular dynamics simulations of low energy beam injections are carried out for an organic polymer substrate. Dose dependence of the net sputtering yields of a poly(1,4-phenylene) substrate by radical beam injections is obtained with the injection energy of 25 eV. Simulation results suggest that injected carbon or nitrogen atoms contribute to the formation of possible passivation layers.

In thermonuclear fusion research using magnetic confinement, the tokamak is the leading candidate for achieving the conditions required for a reactor. An international experiment, ITER, is proposed as the next essential and critical step on the path to demonstrating the scientific and technological feasibility of fusion energy. ITER is to produce and study plasmas dominated by self-heating. This would give unique opportunities to explore, in reactor relevant conditions, the physics of α-particle heating, plasma turbulence and turbulent transport, stability limits to the plasma pressure and exhaust of power and particles. Important new results obtained in experiments, theory and modelling, enable an improved understanding of the physical processes occurring in tokamak plasmas and give enhanced confidence that ITER will achieve its goals. In particular, progress has been made in research to raise the performance of tokamaks, aimed to extend the discharge pulse length towards steady-state operation (advanced scenarios). Standard tokamak discharges have a current density increasing monotonically towards the centre of the plasma. Advanced scenarios, on the other hand, use a modified current density profile. Different advanced scenarios range from (i) plasmas that sustain a central region with a flat current density profile (zero magnetic shear), capable of operating stationary at high plasma pressure, to (ii) discharges with an off-axis maximum of the current density profile (reversed magnetic shear in the core), able to form internal transport barriers, to increase the confinement of the plasma. The physics of advanced tokamak discharges is described, together with an overview of recent results from different tokamak experiments. International collaboration between experiments aims to provide a better understanding, control and optimization of these plasmas. The ability to explore advanced scenarios in ITER is very desirable, in order to verify the results obtained in experiments today and to demonstrate the potential to significantly increase the economic attractiveness of the tokamak.

We review our recent studies on structure formation in pure electron plasmas confined in magneto-electric traps. Two-dimensional dynamics of strongly magnetized plasmas is studied in terms of a two-dimensional vortex composed of high-density clumps and a low-density distribution in the background (BG). By controlling the initial distributions we examine in detail the fundamental processes where the clumps form highly ordered geometrical configurations (vortex crystals). Experimental studies of the formation of three-dimensional density structures have been initiated with a newly built strong-magnetic-field (2.2 T) trap operatable with various potential profiles including a square-well potential (Malmberg trap) and harmonic potential (Mohri trap). In the Mohri trap, Penning-type distributions are observed to form as a result of interaction of string-shaped distributions of electrons. The density distributions observed in the relaxation process are quite different from those observed in a square-well potential most probably due to three-dimensional deformations and merger of the string distributions. The equilibrium distributions generally consist of sets of spheroidal distributions, a core distribution plus halo distributions. Observationally the halo distributions appear to be associated with the amount of excessive angular momentum provided through the injection of off-axial electron strings. Like the BG vorticity distribution in the two-dimensional merger of clumps, the halo may act as the absorber of momentum in the course of the merging process of strings to create the core distribution. In the Penning-like distributions we have also observed the accumulation of negative ions replacing the electrons in time scales much longer than thermal relaxation.

A study of phase space coherent structures in the plasma wave–particle interaction is presented. The study is based on a reduced single wave model (SWM) of the Vlasov–Poisson system. The reduced model describes the weakly nonlinear dynamics of generic electrostatic instabilities and incorporates the self-consistent wave–particle interaction through a mean field that couples the resonant particles to the amplitude of a single wave potential. Following a brief review of the SWM, we show numerical evidence of trapped and untrapped hole–clump dipole states for both symmetric and weakly asymmetric initial conditions. The rotation (in the trapped case) and translation (in the untrapped case) of the dipole manifest as periodic (in the case of symmetric states) and quasi-periodic (in the case of asymmetric states) time dependences of the potential that gives rise to self-consistent chaos. The role of chaotic mixing and hyperbolic–elliptic bifurcations in the relaxation of initial conditions far from equilibrium is also discussed.

Physical properties of hot dense matter at hundreds gigapascal pressures are considered. The new experimental results on pressure ionization of hot matter generated by multiple shock compression of hydrogen and noble gases are presented. The low-frequency electrical conductivity of shock-compressed hydrogen, helium and xenon plasmas was measured. A sharp increase in the electrical conductivity of strongly nonideal plasma was recorded and theoretical models were invoked to describe this increase.

Relativistic solitons are self-trapped, finite-sized, electromagnetic waves of relativistic intensity that propagate without diffraction spreading. They have been predicted theoretically within the relativistic fluid approximation, and have been observed in multi-dimensional particle-in-cell simulations of laser pulse interaction with the plasma. Solitons were observed in laser irradiated plasmas with the proton imaging technique as well. This paper reviews many theoretical results on relativistic solitons in electron–ion plasmas.

One of the main objectives of LHD is to extend the plasma confinement database for helical systems and to demonstrate such extended plasma confinement properties to be sustained in the steady state. Among the various plasma parameter regimes, the study of confinement properties in the collisionless regime is of particular importance. Electron cyclotron resonance heating (ECRH) has been extensively used for these confinement studies of LHD plasma from the initial operation. The system optimizations including the modification of the transmission and antenna system are performed with special emphasis on the local heating properties. As a result, a central electron temperature of more than 10 keV with an electron density of 0.6 × 10¹⁹ m⁻³ is achieved near the magnetic axis. The electron temperature profile is characterized by a steep gradient similar to those of an internal transport barrier observed in tokamaks and stellarators. The 168 GHz ECRH system demonstrated efficient heating at densities more than 1.0 × 10²⁰ m⁻³. The continuous wave ECRH system is successfully operated to sustain a 756 s discharge.

A review of recent experiments on the MAGPIE generator (1 MA, 250 ns) aimed at studying the implosion dynamics of wire array Z-pinches is presented. The first phase of implosion is dominated by the gradual ablation of stationary wire cores and gradual redistribution of the array mass by the precursor plasma flow. It is found that the rate of wire ablation depends on the magnitude of the global (collective) magnetic field of the array, and increases with the field. The existence of the modulation of the ablation rate along the wires leads to the presence of a 'trailing' mass left behind by the imploding current sheath. The trailing mass provides an alternative path for the current, reducing the force available for compression of the pinch at stagnation. The observed dependence of the ablation rate on inter-wire separation suggests an explanation for the existence of the optimal wire number maximizing the x-ray power. Axially resolved spectroscopy shows the presence of the x-ray 'bright' spots (< 150 µm) emitting intense continuum radiation.

The so-called explosive electron emission observed as individual packets or avalanches of electrons is shown to occur on microexplosions at the cathode. This avalanche of electrons is called an ecton. Electron current in an ecton starts flowing as a result of the overheating of the metal because of the high energy density (10⁴ J g⁻¹), and the current stops as the emission zone cools off. Ectons occur in plasma-involving phenomena, such as electrical discharges in vacuum, cathode spots of vacuum arcs, volumetric discharges in gases, pseudosparks, coronas, unipolar arcs, etc.

Plasma-based ion implantation (PBII), invented in 1987, can now be considered as a mature technology for thin film modification. After a brief recapitulation of the principle and physics of PBII, its advantages and disadvantages, as compared to conventional ion beam implantation, are listed and discussed. The elaboration of thin films and the modification of their functional properties by PBII have already been achieved in many fields, such as microelectronics (plasma doping/PLAD), biomaterials (surgical implants, bio- and blood-compatible materials), plastics (grafting, surface adhesion) and metallurgy (hard coatings, tribology), to name a few. The major advantages of PBII processing lie, on the one hand, in its flexibility in terms of ion implantation energy (from 0 to 100 keV) and operating conditions (plasma density, collisional or non-collisional ion sheath), and, on the other hand, in the easy transferrability of processes from the laboratory to industry. The possibility of modifying the composition and physical nature of the films, or of drastically changing their physical properties over several orders of magnitude makes this technology very attractive for the elaboration of innovative materials, including metastable materials, and the realization of micro- or nanostructures. A review of the state of the art in these domains is presented and illustrated through a few selected examples. The perspectives opened up by PBII processing, as well as its limitations, are discussed.

Two different types of microdischarge-integrated plasma sources have been operated at around the atmospheric pressure range. The discharge characteristics were diagnosed by optical emission spectroscopy (OES), laser absorption spectroscopy (LAS) and microwave transmission (MT) techniques. The dynamic spatiotemporal behaviour of excited atoms was analysed using OES and LAS and the temporal behaviour of the electron density was estimated using the MT method. In Ar and Xe/Ne gases, waveforms of the MT signal followed the current waveform in the rise period and lasted longer according to the recombination losses. However, in He the waveform followed the density of metastable atoms, reflecting the production of a large amount of electrons by the Penning ionization process with impurities. The estimated peak electron density in those plasma sources is of the order of 10¹² cm⁻³, and the metastable atom density can reach 10¹³ cm⁻³. Thus, it is suggested that these sources can be potentially applied to convenient material processing tools of large area operated stably at atmospheric pressure.

In this paper, we analyse the morphology and chemistry of carbonaceous dust particles previously proposed as a candidate analogue material for dust in the diffuse interstellar medium (ISM). The particles were polymerized in a low pressure capacitively coupled radiofrequency discharge in mixtures of argon/acetylene and helium/acetylene. Infrared (IR) spectra of our dust particles reveal the strong presence of aliphatic features, both stretching and bending modes, which are very similar to astronomical data from diffuse interstellar dust. A comparison between the IR spectra obtained using argon or helium as buffer gases reveals the strong presence of C = C double bonds (skeletal modes) when helium is used. The presence of these skeletal modes is clearly visible through the broad absorption feature at 1600 cm⁻¹ (6.25 µm). The origin of this feature in our laboratory analogue is related to the change in the plasma parameters due to the variation of the buffer gas used under the same discharge conditions. Observational data of interstellar polycyclic aromatic hydrocarbons also exhibit a feature at this wavelength.

High energy density (HED) physics refers broadly to the study of macroscopic collections of matter under extreme conditions of temperature and density. The experimental facilities most widely used for these studies are high-power lasers and magnetic-pinch generators. The HED physics pursued at these facilities is still in its infancy, yet new regimes of experimental science are emerging. Examples from astrophysics include work relevant to planetary interiors, supernovae, astrophysical jets and accreting compact objects (such as neutron stars and black holes). In this paper, we review a selection of recent results in this new field of HED laboratory astrophysics and provide a brief look ahead to the coming decade.

The existence of magnetic fields is a mandatory requirement for the onset of most nonthermal phenomena in cosmological sources, especially gamma-ray burst sources and relativistic jet sources. The processes leading to the magnetization of the intergalactic medium are not yet known. Large-scale structures in the universe, like filaments and sheets of galaxies, evolve by the gravitational collapse of initially overdense regions giving rise to an intense relative motion of fully ionized gaseous matter and strong gaseous shock structures. We investigate analytically and numerically the generation of magnetic fields in the intergalactic medium by Weibel-type instabilities involving interpenetrating electron streams. Because of the hot temperatures of the intergalactic medium the investigation of the Weibel instability is based on the now available covariantly correct dispersion theory of linear waves, and thus improves on the existing non-relativistic treatments in the literature. These primordial Weibel magnetic fields may serve as cosmological seed fields for even stronger magnetic fields in cosmological sources.

During the past centuries, industrial processes and energy conversion plants have shown no or little care for environmental quality. The result is a huge accumulation of pollution and hazardous by-products, left as a heritage for the present and future generations. Recuperation of by-products or thermal energy is not only motivated by cost saving, but also by resource saving considerations. Environmental awareness is more than staying within the lines of the existing regulations.

By the application of a plasma based system to a wide range of possible feedstocks which are CO₂ neutral, a clean syngas of high caloric value is produced from the organic substances simultaneously with a non-leachable vitrified lava from the inorganic substances. The results will provide the advanced technology for the environmentally friendly treatment of hazardous wastes, biomass and low grade fuel. The driving force behind the task is to give priority to environmental quality at affordable costs. Thus, the investigation of ways to increase the efficiency of the process is very important. A plasma based remediation system is the only technology that prevents undesired pollution in the by-products and end product (such as syngas or other gases). The problem to be solved is twofold: recuperate clean energy from waste and renewables without pollution at affordable costs. Such a technique fulfils the objectives of sustainable development.

Today, one of the main reasons that restricts the use of plasma based methods is the cost of electrical energy. The crucial element is the plasma torch performance. Hence, the physics of modern plasma torches is addressed in detail. The optimistic scenario holds the promise to provide 10–15% of the energy needs for the European Union (EU). Thus, the investigation of ways to increase the efficiency of the process is very important.

Particle simulation of plasmas, employed since the 1960s, provides a self-consistent, fully kinetic representation of general plasmas. Early incarnations looked for fundamental plasma effects in one-dimensional systems with ∼10²–10³ particles in periodic electrostatic systems on computers with ≲100 kB memory. Recent advances model boundary conditions, such as external circuits to wave launchers, collisions and effects of particle–surface impact, all in fully relativistic three-dimensional electromagnetic systems using ∼10⁶–10¹⁰ particles on massively parallel computers. While particle codes still enjoy prominance in a number of basic physics areas, they are now often used for engineering devices as well.

For wave–particle interaction studies, the one-dimensional beam-plasma system can be advantageously replaced by a travelling wave tube (TWT). This allowed us to carefully check the saturation mechanisms by the trapping of a cold electron beam, or by diffusion in the broad spectrum excited by a warm beam. While the former case is well understood, the latter case raises the question of the unsolved quasilinear paradox. When the beam intensity is low enough to ensure that beam-plasma instabilities are ruled out, a beam of test electrons is propagated in the TWT. By recording the test beam energy distribution at the output of the tube, we report the first experimental observation of the resonant domain of a single wave and of the overlap of the resonance domains responsible for the transition to large scale chaos common to many Hamiltonian systems. The fractal structure of phase space associated with this transition can also be explored.

Dusty plasma liquids are formed by suspending negatively charged micro-metre sized particles in a low pressure discharge background. They are good candidates for studying the generic spatio-temporal dynamical behaviours at the kinetic level through direct optical video-microscopy due to the sub-mm interparticle spacing and the slow time scale. In this paper, starting from the basic picture of the avalanche type stick–slip cooperative hopping process under the interplay of mutual coupling and stochastic thermal agitation, we briefly review our recent study on particle micro-motion and the associated structural rearrangement. The effects of mesoscopic confinement and external slow shear drive are also discussed.

A limited overview is given of the theoretical understanding of edge plasmas in fusion devices. This plasma occupies the thin region between the hot core plasma and material walls in magnetic confinement devices. The region is typically formed by a change in magnetic topology from close magnetic field lines (i.e. the core region) and open field lines that contact material surfaces (i.e. the scrape-off layer (SOL)), with the most common example being magnetically diverted tokamaks. The physics of this region is determined by the complex interaction of plasma with neutral species in the presence of plasma turbulence, and impurity radiation is often an important component. Recent advances in modelling strong, intermittent micro-turbulent edge-plasma transport are given, as is the closely coupled self-consistent evolution of the edge-plasma profiles in tokamaks. In addition, selected new results are given for the characterization of edge-plasmas behaviour in the areas of edge-pedestal relaxation and SOL transport via edge-localized modes, impurity formation including dust and magnetic field-line stochasticity at the edge of tokamaks.

Ultracold neutral plasmas are formed by photo-ionizing laser-cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1 to 1000 K and the ion temperature is around 1 K. The density can approach 10¹¹ cm⁻³. Fundamental interest stems from the possibility of creating strongly coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr⁺²S_1/2 → ²P_1/2 transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photo-ionization.

Linear and nonlinear wave phenomena and their influence on the discharge performance are considered in the helicon plasma (HP) and magnetized inductively coupled plasma (MICP). Magnetic field configuration is shown to result in strong variation in the efficiency of plasma production caused by alterations of the character of wave processes rather than by the particle confinement. Effects of magnetic configuration are found to be dominant in operation of the compact HP with permanent magnet. Low frequency turbulence in the megahertz range is revealed to be inherent for both the HP and MICP and to depend strongly on the magnetic configuration. Enhanced axial electric field arising in front of the metal end plate in the HP is argued to be a potential source of the nonlinear effects.

Understanding the mechanisms responsible for particle transport is of the utmost importance for magnetized fusion plasmas. A peaked density profile is attractive to improve the fusion rate, which is proportional to the square of the density, and to self-generate a large fraction of non-inductive current required for continuous operation.

Experiments in various tokamak devices (ASDEX Upgrade, DIII-D, JET, TCV, TEXT, TFTR) indicate the existence of a turbulent particle pinch. Recently, such a turbulent pinch has been unambiguously identified in Tore Supra very long discharges, in the absence of both collisional particle pinch and central particle source, for more than 4 min (Hoang et al 2003 Phys. Rev. Lett. 90 155002). This turbulent pinch is predicted by a quasilinear theory of particle transport (Weiland J et al1989 Nucl. Fusion29 1810), and confirmed by non-linear turbulence simulations (Garbet et al 2003 Phys. Rev. Lett. 91 035001) and general considerations based on the conservation of motion invariants (Baker et al 2004 Phys. Plasmas11 992). Experimentally, the particle pinch is found to be sensitive to the magnetic field gradient in many cases (Hoang et al 2004 Phys. Rev. Lett. 93 135003, Zabolotsky et al2003 Plasma Phys. Control. Fusion45 735, Weisen et al2004 Plasma Phys. Control. Fusion46 751, Baker et al2000 Nucl. Fusion40 1003), to the temperature profile (Hoang et al 2004 Phys. Rev. Lett. 93 135003, Angioni et al2004 Nucl. Fusion44 827) and also to the collisionality that changes the nature of the microturbulence (Angioni et al 2003 Phys. Rev. Lett. 90 205003, Garzotti et al2003 Nucl. Fusion43 1829, Weisen et al 2004 31st EPS Conf. on Plasma Phys. (London) vol 28G (ECA) P-1.146, Lopes Cardozo N J 1995 Plasma Phys. Control. Fusion37 799). The consistency of some of the observed dependences with the theoretical predictions gives us a clearer understanding of the particle pinch in tokamaks, allowing us to predict more accurately the density profile in ITER.

We give a brief review of different particle-in-cell (PIC) simulation methods used for studying the magnetized plasma–wall transition (PWT), and present new results on the steady state multi-ion-component PWT.

We discuss the dust particle dynamics in tokamak edge plasmas, with special emphasis on dust particle transport in the sheath and plasma recycling regions. We demonstrate that being dragged by plasma flows in the vicinity of the material surface, dust particles can be accelerated to speeds of ∼10³–10⁴ cm s⁻¹. The opposite direction of plasma recycling flow as well as the frictional forces at the inner and outer divertor legs, propel the dust particles in opposite toroidal directions depending on their location. The interactions of a dust particle with a corrugated surface or plasma turbulence can cause it to exit the recycling region and fly through the scrape-off layer plasma towards the tokamak core. It is conceivable that dust formation in and transport from the divertor region can play an important role in core plasma contamination. However, even then, the dust particle density around the separatrix is ∼10⁻² cm⁻³, which makes it difficult to detect.

Sterilization of bio-medical materials using radio frequency (RF) excited inductively coupled plasmas (ICPs) has been investigated. A double ICP has been developed and studied for homogenous treatment of three-dimensional objects. Sterilization is achieved through a combination of ultraviolet light, ion bombardment and radical treatment. For temperature sensitive materials, the process temperature is a crucial parameter. Pulsing of the plasma reduces the time average heat strain and also provides additional control of the various sterilization mechanisms. Certain aspects of pulsed plasmas are, however, not yet fully understood. Phase resolved optical emission spectroscopy and time resolved ion energy analysis illustrate that a pulsed ICP ignites capacitively before reaching a stable inductive mode. Time resolved investigations of the post-discharge, after switching off the RF power, show that the plasma boundary sheath in front of a substrate does not fully collapse for the case of hydrogen discharges. This is explained by electron heating through super-elastic collisions with vibrationally excited hydrogen molecules.

This paper includes a brief overview of the contributions and results of several groups working in plasma focus (PF) devices around the world in the last few years. In section 2 a summary of the most important results of the dense transient plasma research programme of the Comisión Chilena de Energía Nuclear is presented. An approach to an integrated vision of the present PF research status is presented in the next section. Some parameters that remain practically constant in PF devices that operate in a wide range of energies from 1 MJ to tens of joules are discussed. These parameters ('plasma energy density parameter' and 'drive parameter') are used as a design tool to achieve an ultra-miniature pinch focus device operating at energies less than 1 J. Preliminary results of such an ultra-miniature device are presented. Applications to non-destructive tests, detection of substances, pulsed radiation in biology and material sciences are also briefly discussed in this paper.