Table of contents

This special issue of Plasma Physics and Controlled Fusion comprises refereed papers contributed by invited speakers at the 31st European Physical Society Conference on Plasma Physics. The conference was jointly hosted by the Rutherford Appleton Laboratory, by the EURATOM/UKAEA Fusion Association and by Imperial College London, where it took place from 28 June to 2 July 2004. The overall agenda for this conference was set by the Board of the Plasma Physics Division of the European Physical Society, chaired by Friedrich Wagner (MPIPP, Garching) and his successor Jo Lister (CRPP, Lausanne). It built on developments in recent years, by further increasing the scientific diversity of the conference programme, whilst maintaining its depth and quality. A correspondingly diverse Programme Committee was set up, whose members are listed below.

The final task of the Programme Committee has been the preparation of this special issue. In carrying out this work, as in preparing the scientific programme of the conference, the Programme Committee formed specialist subcommittees representing the different fields of plasma science. The chairmen of these subcommittees, in particular, accepted a very heavy workload on behalf of their respective research communities. It is a great pleasure to take this opportunity to thank: Emilia R Solano (CIEMAT, Madrid), magnetic confinement fusion; Jürgen Meyer-ter-Vehn (MPQ, Garching), laser-plasma interaction and beam plasma physics; and Jean-Luc Dorier (CRPP, Lausanne), dusty plasmas. The relatively few papers in astrophysical and basic plasma physics were co-ordinated by a small subcommittee which I led. Together with Peter Norreys (RAL, Chilton), we five constitute the editorial team for this special issue. The extensive refereeing load, compressed into a short time interval, was borne by the Programme Committee members and by many other experts, to whom this special issue owes much. We are also grateful to the Local Organizing Committee chaired by Henry Hutchinson (RAL, Chilton), and to the Plasma Physics and Controlled Fusion journal team (Institute of Physics Publishing, Bristol), for their work on this conference.

At the 2004 European Physical Society Conference on Plasma Physics, plenary invited speakers whose talks spanned the entire field were followed, each day, by multiple parallel sessions which also included invited talks. Invited speakers in both these categories were asked to contribute papers to this special issue (the contributed papers at this conference, and at all recent conferences in this series, are archived at http://epsppd.epfl.ch). The Programme Committee is very grateful to the many invited speakers who have responded positively to this request. Invited papers appear here in their order of presentation during the week beginning 28 June 2004; this ordering provides an echo of the character of the conference, as it was experienced by those who took part.

Programme Committee 2004

Professor Richard Dendy UKAEA Culham Division, UK Chairman and guest editor

Dr Jean-Luc Dorier Centre de Recherches en Physique des Plasmas, Lausanne, Switzerland (Co-ordinator of dusty plasmas and guest editor)

Professor Jürgen Meyer-ter-Vehn Max-Planck-Institut für Quantenoptik, Garching, Germany (Co-ordinator of laser-plasma interaction and beam plasma physics and guest editor)

Dr Peter Norreys Rutherford Appleton Laboratory, Chilton, UK (Scientific Secretary and guest editor)

Dr Emilia R Solano CIEMAT Laboratorio Nacional de Fusión, Madrid, Spain ( Co-ordinator of magnetic confinement fusion and guest editor)

Dr Shalom Eliezer Soreq Nuclear Research Centre, Israel

Dr Wim Goedheer FOM-Instituut voor Plasmafysica, Rijnhuizen, Netherlands

Professor Henry Hutchinson Rutherford Appleton Laboratory, Chilton, UK

Professor John Kirk Max-Planck-Institut für Kernphysik, Heidelberg, Germany

Dr Raymond Koch École Royale Militaire/Koninklijke Militaire School, Brussels, Belgium

Professor Gerrit Kroesen Technische Universiteit Eindhoven, Netherlands

Dr Martin Lampe Naval Research Laboratory, Washington DC, USA

Dr Jo Lister Centre de Recherches en Physique des Plasmas, Lausanne, Switzerland

Dr Paola Mantica Istituto di Fisica del Plasma, Milan, Italy

Professor Tito Mendonça Instituto Superior Técnico, Lisbon, Portugal

Dr Patrick Mora École Polytechnique, Palaiseau, France

Professor Lennart Stenflo Umeå Universitet, Sweden

Professor Paul Thomas CEA Cadarache, Saint-Paul-lez-Durance, France

Professor Friedrich Wagner Max-Planck-Institut f̈r Plasmaphysik, Garching, Germany

Professor Hannspeter Winter Technische Universität Wien, Austria

The Hannes Alfvén Prize of the European Physical Society for Outstanding Contributions to Plasma Physics (2004) has been awarded to Jack Connor, Jim Hastie and Bryan Taylor `for their seminal contributions to a wide range of issues of fundamental importance to the success of magnetic confinement fusion, including: the development of gyro-kinetic theory; the prediction of the bootstrap current; dimensionless scaling laws; pressure-limiting instabilities, and micro-stability and transport theory'.

Jack Connor, Jim Hastie and Bryan Taylor form one of the most successful teams of theoretical physicists in the history of magnetic confinement fusion. They have made important contributions individually, but their greatest discoveries have mostly been accomplished jointly, either in pairs or as a team involving all three. Their early work, in the 1960s, included the development of the gyro-kinetic theory for fine-scale plasma instabilities, which today forms the basis of the most advanced turbulence simulation codes in tokamak and stellarator research. The theoretical prediction of the bootstrap current, made in 1970–71 was not confirmed experimentally for over a decade but is now regarded as crucial to the success of the tokamak as a steady-state fusion power source. Their work on collisional transport also included the prediction of impurity ion accumulation, which is observed in internal transport barriers and is a key concern for long-pulse tokamak operation. The relativistic threshold for runaway electrons, identified in 1975, forms the basis of the most recent tokamak disruption mitigation schemes. In the late 1970s, the team developed the theory for ballooning instabilities, which provided an important ingredient in the `Troyon–Sykes' β-limit—an expression that is still used as a guide to the performance of tokamaks and in the design of ITER. Ballooning mode theory has also contributed to the understanding of instabilities in space plasmas such as magnetospheres and the solar corona. Finally, coming right up to date, the ballooning mode is thought to be a key ingredient in edge-localized modes (ELMs), which are a main issue for ITER, and ballooning stability is an important feature of modern stellarators. In the late 1970s and through the 1980s, the concept of dimensionless scaling laws was introduced and developed (following work by Kadomtsev), enabling scalings for transport coefficients to be derived without tackling all the details of the plasma turbulence. The same ideas are still used today to provide various constraints on confinement scaling laws, for example, on which the ITER design is largely based. The linear theory of toroidal drift waves was also developed by the team during this period, and into the 1990s. Key results on the role of shear damping in toroidal geometry, the identification of modes with extended radial correlation lengths, and the role of flow shear in reducing these correlation lengths (and hence transport) were deduced. All of these are key ideas that are often components in theoretical models for tokamak confinement and the generation of transport barriers.

This laudation can only address a small number of the areas in which this formidable team of theoretical plasma physicists have made great contributions to our understanding of magnetically confined plasmas. It is appropriate and timely that their contributions are recognized as they approach the end of their careers.

This paper describes work on an important class of plasma instabilities in toroidal confinement systems. These are instabilities with large mode number in the toroidal direction but long wavelength parallel to the magnetic field. After a brief historical introduction the now standard method—the ballooning representation—for calculating such modes is described. This method is remarkably successful for most stationary plasmas, but breaks down for configurations with low magnetic shear or with significant sheared rotational flow. These are two areas of great current interest because of their association with 'transport barriers' in tokamaks. Some extensions of ballooning theory for dealing with these situations are described and some preliminary results, in particular showing the stabilizing effect of sheared rotation, are presented.

Petawatt (PW) lasers are unique tools to study plasmas under extreme conditions. There are many applications for these plasmas that potentially have an impact on a wide range of scientific disciplines. A number of these are highlighted here in this review including: fast ignition of fusion targets; high brightness x-ray harmonic generation from oscillating plasma surfaces and the production of super-strong magnetic fields. This is a rich field of investigation, and space prevents a detailed discussion of some of these fascinating topics, including electron and ion acceleration processes that were highlighted at the London conference. Fortunately, they are presented elsewhere in other invited papers in this special issue.

Internal transport barriers (ITBs) can provide high tokamak confinement at modest plasma current. This is desirable for operation with most of the current driven non-inductively by the bootstrap mechanism, as currently envisaged for steady-state power plants. Maintaining such plasmas in steady conditions with high plasma purity is challenging, however, due to MHD instabilities and impurity transport effects. Significant progress has been made in the control of ITB plasmas: the pressure profile has been varied using the barrier location; q-profile modification has been achieved with non-inductive current drive, and means have been found to affect density peaking and impurity accumulation. All these features are, to some extent, interdependent and must be integrated self-consistently to demonstrate a sound basis for extrapolation to future devices.

Recent progress in the physics of fast ignition of fusion targets is reviewed here. Fundamental studies on hot electron energy transport show that the scheme looks promising if the heating pulse can be guided close enough to a compressed core. The idea of using cone-guided compression was first demonstrated experimentally under a Japan–UK collaboration. The use of the gold cone was extremely successful and showed a 10³ neutron increase out of CD target implosion with a 300 J/0.5 ps enforced heating laser pulse. The heated temperature was close to 1 keV. In order to increase the temperature to 10 keV, a 10 kJ PW⁻¹ laser system is necessary. Osaka University has started constructing such a laser system.

It is now widely believed that low frequency turbulence developing from small-scale instabilities is responsible for the phenomenon of anomalous transport generally observed in magnetic confinement fusion experiments. The micro-instabilities are driven by gradients of equilibrium density, ion and electron temperatures and magnetic field strength. Gyrokinetic theory is based on the Vlasov–Maxwell equations and, consistent with the ordering, averages out the fast particle gyromotion, reducing the phase space from 6 to 5 dimensions. Solving the resulting equations is a non-trivial task. Difficulties are associated with the magnetic confinement geometry, the strong disparities in space and time scales perpendicular and parallel to B, the different time scales of ion and electron dynamics, and the complex nonlinear behaviour of the system. The main numerical methods are briefly presented together with some recent developments and improvements to the basic algorithms. Recent results are shown, with emphasis on the roles of zonal E × B flows, of parallel nonlinearity and of toroidal coupling on the saturation of ion temperature gradient (ITG) driven turbulence in tokamaks.

Major industrial plasma processes operating close to atmospheric pressure are discussed. Applications of thermal plasmas include electric arc furnaces and plasma torches for generation of powders, for spraying refractory materials, for cutting and welding and for destruction of hazardous waste. Other applications include miniature circuit breakers and electrical discharge machining. Non-equilibrium cold plasmas at atmospheric pressure are obtained in corona discharges used in electrostatic precipitators and in dielectric-barrier discharges used for generation of ozone, for pollution control and for surface treatment. More recent applications include UV excimer lamps, mercury-free fluorescent lamps and flat plasma displays.

The very large stellarator experiments LHD (operating) and W7X (under construction) move stellarator-confined plasmas into the near-reactor regime. Continuing experiments on smaller devices operating at heating powers from kilowatts to a few megawatts are exploring the effects of magnetic configuration stability and turbulence on plasma confinement to improve stellarator performance and our understanding of general toroidal confinement physics. Key issues being explored are the relation of rational magnetic surfaces and magnetic configuration characteristics such as helical ripple to plasma transport, confinement scaling and turbulence. The robust macroscopic stability of currentless stellarator plasma is a major contributing factor to these studies. Many of the phenomena most clearly evident in stellarators are increasingly implicated in tokamak experiments as well.

Within the framework of the eddy-damped quasi-normal Markovian approximation for incompressible isotropic magnetohydrodynamic (MHD) turbulence a prediction for the inertial range scaling of the residual energy spectrum, , is obtained. This scaling, while in contradiction to earlier theoretical results, is shown to be in agreement with high-resolution direct numerical simulations of nonhelical decaying MHD turbulence. The underlying phenomenology states a dynamic quasi-equilibrium of the small-scale turbulent dynamo and the Alfvén effect.

Single-crystal nanoparticles of silicon, several tens of nanometres in diameter, may be suitable as building blocks for single-nanoparticle electronic devices. Previous studies of nanoparticles produced in low-pressure plasmas have demonstrated the synthesis of nanocrystals 2–10 nm diameter but larger particles were amorphous or polycrystalline. This work reports the use of a constricted, filamentary capacitively coupled low-pressure plasma to produce single-crystal silicon nanoparticles with diameters between 20 and 80 nm. Particles are highly oriented with predominantly cubic shape. The particle size distribution is rather monodisperse. Electron microscopy studies confirm that the nanoparticles are highly oriented diamond-cubic silicon.

The Rayleigh–Taylor instability (RTI) of the inner surface of an inertial confinement fusion shell is studied through high-resolution two-dimensional numerical simulations. The instability is seeded by a mass displacement introduced in the simulations at the end of the implosion coasting stage. Analysis of single-mode, small-amplitude perturbations confirms that ablation caused by electron conduction and fusion alpha-particles causes significant growth reduction of all modes and stabilization of high-l modes. Different measures of the instability are discussed and compared with modified Takabe-like expressions. Large-amplitude multi-mode simulations are performed to study the effects of RTI on ignition and burn. RTI perturbations reduce the size of the central hot spot and delay ignition. For a few different perturbation spectra the dependence of fusion yield on the initial perturbation root mean square amplitude is studied.

Anomalous transport in tokamaks is generally attributed to turbulent fluctuations. Since a large variety of modes are potentially unstable, a wide range of short-scale fluctuations should be measured, with wavenumbers from kρ_i ∼ 0.1 to kρ_i ≫ 1. In the Tore Supra tokamak, a light scattering experiment has made possible fluctuation measurements in the medium- and high-k domains where a transition in the k-spectrum is observed: the fluctuation level decreases much faster than usual observations, typically with a power law S(k) ≡ k⁻⁶. A scan of the ion Larmor radius shows that the transition wavenumber scales with ρ_i around kρ_i ∼ 1.5. This transition indicates that a characteristic length scale should be involved to describe the fluctuation nonlinear dynamics in this range. The resulting very low level of fluctuations at high-k does not support a strong effect of turbulence driven by the electron temperature gradient. For this gyroradius scan, the characteristics of turbulence also exhibit a good matching with predictions from gyro-Bohm scaling: the typical scale length of turbulence scales with the ion Larmor radius, the typical timescales with a/c_s; the turbulence level also scales with ρ_i, according to the mixing length rule.

The French Commissariat à l'Energie Atomique (CEA) began the construction of the Laser Megajoule (LMJ), a 240-beam laser facility, at the CEA Laboratory CESTA near Bordeaux. The LMJ will be a cornerstone of the CEA 'Programme Simulation', the French analog of the US Stockpile Stewardship Program.

The LMJ is designed to deliver 2 MJ of 0.35 µm light to targets for high energy density physics experiments and to ultimately obtain ignition and propagating burn with DT targets in the laboratory.

The Scientific conception and system design was completed in 1999 and was followed by the Demonstration of an Engineering Prototype which was achieved in early 2003 with the operation of one beam of the Ligne d'Intégration Laser (LIL) at CESTA, with 9.5 kJ of UV light (0.35 µm) in less than 9 ns from a single laser beam.

The realization phase of the LMJ facility was initiated in March of 2003 with the start of construction of the building and the target chamber.

This paper will present results from the commissioning phase of the LIL program in 2003 and 2004. The activation and commissioning of all eight beamlines of LIL over the next 2 years will be part of determining the final specifications and integration and commissioning plans for the LMJ, which is expected to demonstrate first light performance through 240 beams by 2009.

Recently, the dynamic ergodic divertor (DED) of TEXTOR has been studied in an m/n = 3/1 set-up which is characterized by a relatively deep penetration of the perturbation field. The perturbation field creates (a) a helical divertor, (b) an ergodic pattern and/or (c) excitation of tearing modes, depending on whether the DED current is static, rotating in the co-current direction or in the counter-current direction. Characteristic divertor properties such as the high recycling regime or enhanced shielding have been studied. A strong effect of the ergodization is spin up of the plasma rotation, possibly due to the electric field at the plasma edge. Tearing modes are excited in a rather reproducible way and their excitation threshold value, their motion and their reduction due to the ECRH/ECCD have been studied. The different scenarios are characterized by strong modifications of the toroidal velocity profile and by a reduced or enhanced radial transport.

Plasma transport and energy dissipation in the driven dynamic magnetosphere are intermittent (bursty), and occur on a range of spatiotemporal scales. System observables such as geomagnetic indices, and auroral images, show evidence of scaling in the statistics of these events. Taken together these are hallmarks of a complex system. Here we underline the importance of robustness in these statistical signatures with respect to variability in the drive, and of bursty transport as opposed to intermittent structures, as key signatures of nonlinear complex avalanching systems. Power law statistics of bursty events do not necessarily require an underlying nonlinearity.

In this work we characterize a low-power radio-frequency atmospheric plasma (plasma needle) in terms of dissipated (input) and emitted power per unit surface (power outflux). The plasma is a non-thermal source, used for treatment of biological tissues and other vulnerable surfaces. A calibrated thermal probe is used to determine the power emitted from the plasma towards treated surfaces. Transmission of the emitted plasma power through various media (solid layers, fluids and physiological media) is studied for a broad range of plasma conditions. These data give insight into various contributions to the power outflux (thermal conduction, radiation and energetic species), as well as the penetration depth of the plasma into treated objects. The power outflux is shown to be a very important parameter, which determines the performance of the plasma tool. For the effectiveness and reproducibility of the process the power outflux is much more important than the nominal power setting. Thus, a thermal probe should become a standard control unit in surface processing reactors.

The bubble regime of electron acceleration in ultra-relativistic laser plasma is considered. It has been shown that the bubble can produce ultra-short dense bunches of electrons with quasi-monoenergetic energy spectra. The first experiment in this regime done at LOA has confirmed the peaked electron spectrum (Faure J et al 2004 Nature at press). The generated electron bunch may have density an order of magnitude higher than that of the background plasma. The bubble is able to guide the laser pulse over many Rayleigh lengths, thus no preformed plasma channel is needed for high-energy particle acceleration in the bubble regime. In this work we discuss a simple analytical model for the bubble fields as well as the scaling laws.

In the burning fusion plasmas of next step devices such as ITER (2001 ITER-FEAT Outline Design Report IAEA/ITER EDA/DS/18 (Vienna: IAEA) p 21), the majority of the heating of the fusing fuel will come from the plasma self-heating by fusion born α -particles. Recent advances in theoretical understanding, together with the development of new diagnostic techniques, make this a timely opportunity to survey the role of energetic particles in fusion plasmas and how it projects to future burning plasma devices.

Recent theoretical work on magnetic reconnection in hot plasma confinement devices is reviewed. The presentation highlights the common aspects of reconnection phenomena, and current research trends are emphasized. Progress in understanding the dynamics of slowly evolving modes of the tearing family, based on advanced analytic techniques and numerical simulation, as well as of faster modes that lead to internal disruptions, is reported.

Advanced tokamak (AT) research in DIII-D seeks to provide a scientific basis for steady-state high performance operation in future devices. These regimes require high toroidal beta to maximize fusion output and high poloidal beta to maximize the self-driven bootstrap current. Achieving these conditions requires integrated, simultaneous control of the current and pressure profiles and active magnetohydrodynamic stability control. The building blocks for AT operation are in hand. Resistive wall mode stabilization by plasma rotation and active feedback with non-axisymmetric coils allows routine operation above the no-wall beta limit. Neoclassical tearing modes are stabilized by active feedback control of localized electron cyclotron current drive (ECCD). Plasma shaping and profile control provide further improvements. Under these conditions, bootstrap supplies most of the current. Steady-state operation requires replacing the remaining inductively driven current, mostly located near the half radius, with non-inductive external sources. In DIII-D this current is provided by ECCD, and nearly stationary AT discharges have been sustained with little remaining inductive current. Fast wave current drive is being developed to control the central magnetic shear. Density control, with divertor cryopumps, of AT discharges with ELMing H-mode edges facilitates high current drive efficiency at reactor relevant collisionalities. An advanced plasma control system allows integrated control of these elements. Close coupling between modelling and experiment is key to understanding the separate elements, their complex nonlinear interactions, and their integration into self-consistent high performance scenarios. This approach has resulted in fully non-inductively driven plasmas with β_N ≤ 3.5 and β_T ≤ 3.6% sustained for up to 1 s, which is approximately equal to one current relaxation time. Progress in this area, and its implications for next-step devices, will be illustrated by results of these and other recent experiment and simulation efforts.

We summarize our current understanding of the optimization of PIN solar cells produced by plasma enhanced chemical vapour deposition from silane–hydrogen mixtures. To increase the deposition rate, the discharge is operated under plasma conditions close to powder formation, where silicon nanocrystals contribute to the deposition of so-called polymorphous silicon thin films. We show that the increase in deposition rate can be achieved via an accurate control of the plasma parameters. However, this also results in a highly defective interface in the solar cells due to the bombardment of the P-layer by positively charged nanocrystals during the deposition of the I-layer. We show that decreasing the ion energy by increasing the total pressure or by using silane–helium mixtures allows us to increase both the deposition rate and the solar cells efficiency, as required for cost effective thin film photovoltaics.

The Rayleigh–Taylor (RT) instability with material ablation through an unstable interface is the key physics that determines the success or failure of inertial fusion energy (IFE) generation, as the RT instability potentially quenches ignition and burn by disintegrating the IFE target. We present two suppression schemes of the RT growth without significant degradation of the target density. The first scheme is to generate a double ablation structure in high-Z doped plastic targets. In addition to the electron ablation surface, a new ablation surface is created by x-ray radiation from the high-Z ions. Contrary to the previous thought, the electron ablation surface is almost completely stabilized by extremely high flow velocity. On the other hand, the RT instability on the radiative ablation surface is significantly moderated. The second is to enhance the nonlocal nature of the electron heat transport by illuminating the target with long wavelength laser light, whereas the high ablation pressure is generated by irradiating with short wavelength laser light. The significant suppression of the RT instability may increase the possibility of impact ignition which uses a high-velocity fuel colliding with a preformed main fuel.

An overview is given of the experimental method, the analysis technique and the results for trace tritium experiments conducted on the JET tokamak in 2003. Observations associated with events such as sawtooth collapses, neo-classical tearing modes and edge localized modes are described. Tritium transport is seen to approach neo-classical levels in the plasma core at high density and low q₉₅, and in the transport barrier region of internal transport barrier (ITB) discharges. Tritium transport remains well above neo-classical levels in all other cases. The correlation of the measured tritium diffusion coefficient and convection velocity for normalized minor radii r/a = [0.65, 0.80] with the controllable parameters q₉₅ and plasma density are found to be consistent for all operational regimes (ELMy H-mode discharges with or without ion cyclotron frequency resonance heating, hybrid scenario and ITB discharges). Scaling with local physics parameters is best described by gyro-Bohm scaling with an additional inverse beta dependence.

A short review of different approaches to calculating the ion drag force is presented. First, a concise summary of the previously published results of the pair collision approach is given. Then, the recently proposed linear kinetic approach is discussed and generalized. The results of the two approaches are compared, and the importance of the self-consistent kinetic consideration is highlighted. In conclusion, the applicability of the approaches is analysed and unresolved issues are briefly discussed.

Internal transport barriers (ITBs), marked by a steep density profile, even stronger peaking in the pressure profile and reduction of core transport are obtained in Alcator C-Mod. They are induced by the use of off-axis D(H) ICRF (ion cyclotron range of frequencies) power deposition. They also arise spontaneously in Ohmic H-mode plasmas once the H-mode lasts for several energy confinement times. Recent studies have explored the limits for forming, maintaining and controlling these plasmas. The C-Mod provides a unique platform for studying such discharges: the high density (up to 8 × 10²⁰ m⁻³) causes the ions and electrons to be tightly coupled by collisions with T_i/T_e = 1, and the plasma has no internal particle or momentum sources. The ITBs formed in both Ohmic and ICRF heated plasmas are quite similar regardless of the trigger method. Control of impurity influx and heating of the core plasma in the presence of the ITB have been achieved with the addition of central ICRF power, in both Ohmic H-mode and ICRF induced ITBs. Control of the radial location of the transport barrier is achieved through manipulation of the toroidal magnetic field and plasma current. A narrow region of decreased electron thermal transport, as determined by sawtooth heat pulse analysis, is found in these plasmas as well. Transport analysis indicates that reduction of the particle diffusivity in the barrier region allows the neoclassical pinch to drive the density and impurity accumulation in the plasma centre. Examination of the gyro-kinetic stability indicates that the density and temperature profiles of the plasma core are inherently stable to long-wavelength drift mode driven turbulence at the onset time of the ITB, but that the increasing density gradients cause the trapped electron mode to play a role in providing a control mechanism to ultimately limit the density and impurity rise in the plasma centre.

Experimental results of vertical oscillations of dust grains in the driven electrode sheath of a capacitive RF plasma are presented. Below a certain pressure, waves suddenly start to propagate in the lower planes of a plasma crystal. Further decreasing the pressure causes the waves to spread in the entire dust cloud. The waves are found to propagate at constant frequency but with a higher phase velocity as the pressure is reduced. A mechanism based on the ion–dust two-stream instability is proposed to explain the observations. From the numerical solution the wave frequency is found to be highly dependent on the value chosen for the dust charge and so gives a diagnostic method for determining the charge on the dust.

In laser fusion, the coupling and the propagation of the laser beams in the plasma surrounding the pellet must be well controlled for to succeed in producing a high energy level. To achieve thermonuclear ignition and high gain, the coupling efficiency must be as high as possible, the uniformity of the energy deposition must be very good and the fast electron generation must be minimized. This implies a deep understanding of the laser–plasma interaction mechanisms to keep the nonlinear processes at a low level. Important advances in laser–plasma interaction physics have been achieved thanks to the converging efforts of the experimental and theoretical approaches.

Among the different studies of the last few years, we will report results on three themes which are important for future fusion experiments. The first concerns the ability of plasmas to induce temporal and spatial incoherence to the laser beams during their propagation. Beam smoothing, beam spraying and increased incoherence may in turn reduce the level of backscattering instabilities.

In laser fusion, multiple beams are used to irradiate the target. The effect of the overlap of the laser beams on parametric instabilities may complicate the problem. Not only is there the interplay between instabilities driven by one beam, but also the interplay between instabilities driven by different beams. In the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) experiment, although the overall stimulated Brillouin scattering (SBS) reflectivity was reduced, a well-defined resonance of the amplitude of ion acoustic waves (IAWs) associated with SBS has been observed for waves propagating along the bisecting direction between two laser beams. Energy transfer between two identical laser beams has been observed and correlated with plasma induced incoherence.

The nonlinear saturation of stimulated scattering instabilities is a fundamental ingredient of the understanding of the observed and future reflectivity levels. Using Thomson scattering, the decay of the primary IAWs associated with SBS in secondary IAWs has been observed and correlated with the saturation of SBS.

The experiments were performed with the six-beam laser facility of the LULI at Ecole Polytechnique, using one or two 1.053 µm interaction beams in the nanosecond regime and a well-characterized preformed plasma. Multiple diagnostics, including Thomson scattering of a probe beam with spatial, temporal and spectral resolution, were used. The comparison between the experimental results and numerical simulations is used to improve the physics included in the codes.

Within the framework of MHD modelling the RFP is shown to develop turbulent or laminar regimes switching from the former to the latter in a continuous way depending on the strength of dissipative forces (the higher they are the more laminar is the corresponding regime). In either of these cases interesting features can be observed such as the occurrence of quasi-periodic relaxation events involving reconnection processes, or the formation of stationary helical symmetric configurations. The first case corresponds to the conventional turbulent dynamo in the RFP where perturbations with multiple helical harmonic content are present. The second case corresponds to a global single helical deformation of the current channel. This simpler configuration is associated with a laminar electrostatic dynamo field and may also be found as a solution of a helical Ohmic equilibrium problem where a finite beta is necessary. The continuity of the transition between the two regimes suggests that the simple helical symmetric solution can provide a fruitful intuitive description of the RFP dynamo in general. Many of the MHD predictions are in good agreement with experimental findings and suggest possible improvements for the confinement properties of the RFP configuration.

There is considerable interest in the propagation dynamics of intense neutrino beams in a background dispersive medium such as dense plasmas, particularly in the search for a mechanism to explain the dynamics of type II supernovae. Neutrino interactions with matter are usually considered as single particle interactions. All the single particle mechanisms describing the dynamical properties of neutrinos in matter are analogous with the processes involving single electron interactions with a medium such as Compton scattering, Cerenkov radiation, etc. However, it is well known that beams of electrons moving through a plasma give rise to a new class of processes known as collective interactions, such as two stream instabilities, which result in either the absorption or generation of plasma waves. Employing the relativistic kinetic equations for neutrinos interacting with dense plasmas via the weak force, we explore collective plasma streaming instabilities driven by neutrino beams. We examine the anomalous transfer between neutrinos and dense plasma via excitation of electron plasma waves. The nonlinear coupling between an intense neutrino beam and a plasma reveals the presence of two regimes, a hydrodynamic regime and a kinetic regime. The latter is responsible for Landau damping or growth of electron plasma waves. In dense fusion stellar plasmas neutrino Landau damping can play a significant role as an additional stellar plasma cooling process. Another interesting result is an asymmetry in the momentum balance imported by the neutrinos to the core of the exploding star due to symmetry breaking by the collapsed star's magnetic fields. This results in a directed velocity of the resulting neutron star or pulsar, explaining the so called 'birth' velocity.

Recent investigations of the wall pumping and saturation during tokamak discharges have been summarized, focusing on wall saturation in long pulse (30 s) ELMy H-mode discharges in JT-60U. The ELMy H-mode discharges in divertor tokamaks such as ASDEX-Upgrade and JT-60U showed large deuterium retention (N_wall = (3–5) × 10²² D) and a large retention ratio (R_wall = 0.3–0.5) for cases with large gas puffing into the vacuum vessel. In short discharges of ASDEX-Upgrade, the deuterium retention and the retention ratio were increased with gas puff flux. On the other hand, in some long-pulse discharges of JT-60U, the net wall pumping flux decreased to zero during the later phase, i.e. wall saturation was observed. The net wall pumping flux in the later phase changed the operation history, which determined the variation of the deuterium retention. Wall pumping at low temperature regions in the divertor or private dome rather than co-deposition with carbon is important in explaining the large change in deuterium retention in the series of discharges.

Under saturated wall conditions, significant changes were not seen in SOL and divertor plasmas. However, increases in the recycling flux and carbon generation were locally observed in the private flux region. The increases in particle recycling and carbon influx were larger than those expected on increasing the surface temperature of the dome tiles. Understanding the carbon generation process in long-pulse discharge is important for controlling the divertor plasma, while the increase in carbon contamination in the main plasma was small under the attached divertor condition.

Large amplitude Alfvén waves are frequently found in magnetized space and laboratory plasmas. Our objective here is to discuss the linear and nonlinear properties of dispersive Alfvén waves (DAWs) in a uniform magnetoplasma. We first consider finite frequency (ω/ω_ci) and ion gyroradius effects on inertial and kinetic Alfvén waves, where ω_ci is the ion gyrofrequency. Next, we focus on nonlinear effects caused by DAWs. Such effects include plasma density enhancement and depression by the Alfvén wave ponderomotive force, electron Joule heating by the thermal Alfvén wave force, the generation of zonal flows due to the shear Alfvén wave mode couplings as well as the formation of localized Alfvénic structures and Alfvénic vortices. The relevance of our investigation to the appearance of nonlinear Alfvén waves in the Earth's auroral acceleration region, in the solar corona and in the large plasma device at UCLA is discussed.

The results of numerical simulations of particles structure (ordering) and dynamics (diffusion constant) in a 'liquid plasma' state of strongly coupled dusty plasmas are considered. Static and dynamical properties of microparticle systems in the liquid state have been studied in experiments under microgravity conditions. The comparison of the experimental results with the results of numerical simulations of Debye systems was used to estimate the effective coupling parameter which governs particles structural and dynamical properties. This diagnostics tool allows one to estimate particle charge and plasma screening length which are very important characteristics of dusty plasmas.

Here, a model for the nonlinear Rayleigh–Taylor instability (RTI) of a steady ablation front based on a sharp boundary approximation is presented. The model includes the effect of mass ablation and represents a basic tool for investigating many aspects of the nonlinear ablative RTI relevant to inertial confinement fusion. The single mode analysis shows the development of a nonlinear exponential instability for wave numbers close to the linear cutoff. Such a nonlinear instability grows at a rate faster than the linear growth rate and leads to saturation amplitudes significantly larger than the classical value 0.1λ. We also found that linearly stable perturbations with wave numbers larger than the linear cutoff become unstable when their initial amplitudes exceed a threshold value. The shedding of long wavelength modes via mode coupling is much greater than predicted by the classical RTI theory. The effects of ablation on the evolution of a front of bubbles is also investigated and the front acceleration is computed.

Some techniques with a long tradition in the plasma technology field have already been successfully applied to research in plasma–wall interactions of fusion devices. They have produced important advances in the control of particle and energy exhaust. In this paper, the possible application of these techniques to the problem of tritium inventory control in fusion reactors with carbon-based plasma facing materials, as in ITER, is proposed. It is based on a critical analysis of relevant information obtained in the field of hard CN film deposition and consists of the use of chemical scavengers for the inhibition of tritium-rich carbon-film formation in hidden areas of the divertor. The practical implementation of the technique, however, requires a detailed knowledge of the physio-chemical processes involved, and, to date, experiments in cold and divertor plasmas have been performed. Very recent experiments in the ASDEX Upgrade device have shown that the injection of nitrogen in the sub-divertor region can lead to a drastic decrease in the level of deposited material with no significant effects in the performance of the main plasma. This and other findings are interpreted in the light of recent results from laboratory and divertor plasma experiments and the extrapolation to new divertor scenarios is discussed.

Laboratory astroplasma physics experiments advance both our astrophysics and plasma physics knowledge. Contemporary high-energy, high-power laser technology enables us to reproduce in the laboratory the conditions of temperature and pressure that are met in extreme stellar environments. The focus is on experiments designed to address key aspects of the plasma physics occurring in supernova remnants. In this approach, a plasma physics model of the astrophysical object is identified and then scaled, and applied to a laboratory experiment. This offers the possibility of detailed measurements, which can be repeated as the input conditions are altered. Results from a scaled experiment designed to address aspects of collisionless plasma interaction in a young supernova remnant are presented. This experimental study is based on the interaction of two millimetre-scale counter-streaming laser-produced plasmas, created from exploded thin plastic foils in an intense transverse magnetic field. The dynamics of the two plasmas and their interaction are studied with, and without, magnetic fields, through spatially and temporally resolved measurements of the electron density.

Ten years of R&D on Hall plasma thrusters (HPTs) in former USSR led to successful tests in space as early as 1972. Nevertheless, they remain actively investigated within the perspective of a routine use in space technologies. This long story underlines the contrast between a rather simple concept and a complex physical background, up to now not fully understood in a quantitative way. This lack of knowledge limits the development of fully predictive simulation codes. Industrial developments are, nevertheless, under way, with new goals in terms of thruster performances. Illustrations of such developments are given, but this paper is mainly focused on a review of recent results and open questions underlined through a coordinated research programme.

Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.

A 'hybrid' scenario for ITER is defined through its objectives: a large fusion yield for a long time duration. In many tokamaks, discharges characterized by a stationary current density profile, enclosing a large volume of low magnetic shear with q₀ near 1, have achieved improved confinement and higher beta limits. Their extrapolation to ITER from existing data corresponds to the ITER hybrid scenario. These discharges are characterized by soft MHD events. Physics issues relevant to the existence and extrapolation of this scenario will be addressed. New JET experiments with a large component of RF heating have answered some of these issues: injected momentum is not essential, hybrid scenarios are achievable at low ρ^*, hybrid regimes have been achieved with ITER-relevant T_e/T_i and they are compatible with a very low edge activity/low pressure pedestal. Data from pure RF discharges in other tokamaks (FTU, TS, TCV) seem to confirm that a large volume of low magnetic shear with q₀ close to 1 is the key to achieving hybrid scenarios. Issues needing resolution in the extrapolation to ITER are discussed. The present understanding provides encouraging prospects for the use of this scenario in ITER.

In a two-dimensional liquid melted from a triangular lattice, the accumulation of constructive perturbation from thermal noise and other external slow drive can cause stick–slip type particle hopping over the caging barrier formed by the surrounding particles, which in turn distort the order lattice structure and generate topological defects. Through the strong mutual coupling, these nonlinear threshold-type micro-excitations of fast particles and topological defects usually occur in the form of avalanche type clusters involving a small number of sites. The dusty plasma liquid formed by suspending negatively charged micrometre sized particles in a low pressure discharge background turns out to be a good candidate to study the above spatio-temporal dynamical behaviours at the kinetic level through direct optical video microscopy. In this paper, we review our recent studies on this issue and compare the generic behaviours with other nonlinear coupled complex system excited by noise and other slow drives.

Resistive magneto-hydrodynamic (MHD) simulations are used to evaluate the influence of three-dimensional inhomogeneities on x-ray power production in wire array Z-pinches. In particular, we concentrate on simulations of wire array Z-pinch experiments on the MAGPIE generator at Imperial College. An initial temperature perturbation is used to stimulate variations in wire core ablation rates that result in a highly non-uniform final implosion. Results indicate that x-ray power production is governed by the symmetry of the implosion surface and by the rate at which current can transfer to the axis through a three-dimensional debris field that trails behind the main implosion. The peak power is ultimately limited by the growth of MHD instabilities in the stagnated pinch. The individual contributions of the implosion kinetic energy, compression of the stagnated pinch, ohmic heating and MHD instabilities to the radiation yield are quantified. The onset of m = 1 instabilities is found to provide an efficient mechanism for dissipation of the magnetic energy surrounding the stagnated pinch. The formation of a helical plasma column not only allows the magnetic field to do work in driving an expansion of the helix but also enhances the ohmic heating by elongating the path of the current through the pinch. The effect of these energy sources combined is to increase the radiation yield to typically 3½ times the kinetic energy of the implosion. Simulations of arrays with different wire numbers, wire material and with nested arrays are used to examine the mechanisms that influence the peak soft x-ray power. In the simulations, peak power can be increased by: increasing the number of wires (which improves the implosion symmetry), by increasing the atomic number of the material (which increases the compressibility of the plasma) and by using a nested inner array (which brings the mass and the current to the axis more efficiently than a single array).

Low aspect ratio plasmas in devices such as the mega ampere spherical tokamak (MAST) are characterized by strong toroidicity, strong shaping and self fields, low magnetic field, high beta, large plasma flow and high intrinsic E × B flow shear. These characteristics have important effects on plasma behaviour, provide a stringent test of theories and scaling laws and offer new insight into underlying physical processes, often through the amplification of effects present in conventional tokamaks (e.g. impact of fuelling source and magnetic geometry on H-mode access). The enhancement of neoclassical effects makes MAST ideal for the study of particle pinch processes and neoclassical resistivity corrections, which can be assessed with unique accuracy. MAST data have an important influence on scaling laws for confinement and H-mode threshold power, exerting strong leverage on the form of these scaling laws (e.g. scaling with aspect ratio, beta, magnetic field, etc). The high intrinsic flow shear is conducive to transport barrier formation by turbulence suppression. Internal transport barriers are readily formed in MAST with both co- and counter-NBI, and electron and ion thermal diffusivities have been reduced to the ion neoclassical level. The strong variation in toroidal field (∼ × 5 in MAST) between the inboard and outboard plasma edges, provides a useful test of edge models prompting, for example, a comparison of inboard and outboard scrape-off-layer transport to highlight magnetic field effects. Low aspect ratio plasmas are also an ideal testing ground for plasma instabilities, such as neoclassical tearing modes, edge localized modes (ELMs) and Alfvén eigenmodes, which are readily generated due to the supra-Alfvénic ion population. Examples of how MAST is providing new insights into such instabilities (e.g. ELM structure) are described.

This paper presents an experimental study on the nature, the dimensions and the timescale of the perturbation introduced by radiofrequency (rf) biasing of areas adjacent to the plasma. The analysis of the rf sheath, and of the charging of particles in it, has disclosed a levitation force on particles, which is substantially different from the dc one often used in complex plasmas. Experimentally, the rf heavily loaded sheath presents characteristics completely different from the normal case V_rf ⩽ V_dc. Regions of extra ionization and complex electrostatic structures arise. These have been visualized by nanoparticles grown in the plasma. A variety of equilibrium positions for a controlled number of microparticles (injected) can be achieved by fine balancing of dc and rf on a pixel with the neighbouring sheath kept under control. In certain situations gravity is completely compensated, allowing the study of three-dimensional clusters. The motion of clusters from 4 to about 100 particles is simultaneously monitored by a three-dimensional visualization based on two laser lights modulated in intensity. This method enables the study of time-varying effects, such as transitions and vibrations, as well as the study of static structures and lattice defects. At pressures below 40 Pa in large clusters a poloidal motion appears.

A variety of techniques are being tested on ASDEX Upgrade for avoiding or reducing the divertor energy load caused by ELMs. Quiescent H-mode operation has been improved by operation with recently boronized vessel walls. Z_eff values in QH-modes are now ∼2.5, comparable with those found in ELMy H-modes of similar density. Type II ELM operation has been generated in discharges with ICRF additional heating alone, thus demonstrating its compatibility with reactor-relevant low core momentum and particle input. Furthermore, Type II ELM operation at low (<1) values of edge collisionality has been achieved using strong plasma shaping and high input power. Modification of the ELM frequency and of the ELM energy has been demonstrated using pellet injection, supersonic pulsed gas injection, magnetic triggering via vertical position oscillations and modulated edge electron cyclotron heating and current drive. ELM control using pellet pace making is now being incorporated into integrated scenarios that aim to provide energy and particle exhaust compatible with ITER requirements while maximizing the core confinement.

Dusty plasmas are unusual states of matter where the interactions between the dust grains can be collective and are not a sum of all pair particle interactions. This state of matter is appropriate to form non-linear dissipative collective self-organized structures. It is found that the potential around the grains can be over-screened leading to a new phenomenon—collective attraction of pairs of large charge grains of equal sign. The grain clouds can self-contract and their collapse is terminated at distances where the interaction becomes repulsive. The homogeneous dusty plasma distribution is universally unstable to form structures. The potential of the collective attraction is proportional to the square of the dimensionless parameter P = n_dZ_d/n_i, where n_d and n_i are the average dust and ion densities, respectively, and Z_d is the dust charge in units of electron charge. The collective attraction is determined by finite grain size and by the presence of absorption of plasma flux on grains. The physics of attraction is related to the space charge accumulation caused by collective flux disturbances. The collective attraction operates for systems with size larger than the mean free path for ion–dust absorption, the condition met in many existing low temperature dusty plasma experiments, in edge plasmas of fusion devices and in space dusty plasmas. The collective attraction exceeds the previously known non-collective attraction such as shadow attraction or wake attraction. The collective attraction can be responsible for pairing of dust grains (this process is completely classical in contrast to the known pairing in superconductivity) and can serve as the main process for the formation of more complicated dust complexes up to dust-plasma crystals. The equilibrium structures formed by collective attraction have universal properties and can exist in a limited domain of parameters (similar to the equilibrium balance known for stars). The balance conditions for dust structures include not only the pressure effects but also the ion drag, grain charging and collective plasma flux effects. The non-linear effects in collective attraction are relatively small for astrophysical conditions and for edge fusion plasmas. For plasmas used in plasma etching and in plasma crystal experiments the non-linear effects in attraction are important. The distance between the grains for the first attraction well is in rough agreement with the observed separation of grains in dust-plasma crystals. The results of the numerical simulations of the final stationary configurations with appropriate wall boundary conditions are in rough agreement with recent results of structure observation in the experiments on the International Space Station.

Some applications of fast ions driven by a short (⩽1 ps) laser pulse (e.g. fast ignition of ICF targets, x-ray laser pumping, laboratory astrophysics research or some nuclear physics experiments) require ion beams of picosecond (or shorter) time durations and of very high ion current densities (∼10¹⁰ A cm⁻² or higher). A possible way of producing ion beams with such extreme parameters is ballistic focusing of fast ions generated by a target normal sheath acceleration (TNSA) mechanism at relativistic laser intensities. In this paper we discuss another method, where the production of short-pulse ion beams of ultrahigh current densities is possible in a planar geometry at subrelativistic laser intensities and at a low energy (⩽1 J) of the laser pulse. This method—referred to as skin-layer ponderomotive acceleration (S-LPA)—uses strong ponderomotive forces induced at the skin-layer interaction of a short laser pulse with a proper preplasma layer in front of a solid target. The basic features of the high-current ion generation by S-LPA were investigated using a simplified theory, numerical hydrodynamic simulations and measurements. The experiments were performed with subjoule 1 ps laser pulses interacting with massive or thin foil targets at intensities of up to 2 × 10¹⁷ W cm⁻². It was found that both in the backward and forward directions highly collimated high-density ion beams (plasma blocks) with current densities at the ion source (close to the target) approaching 10¹⁰ A cm⁻² are produced, in accordance with the theory and numerical calculations. These ion current densities were found to be comparable to (or even higher than) those estimated from recent short-pulse TNSA experiments with relativistic laser intensities. Apart from the simpler physics of the laser–plasma interaction, the advantage of the considered method is the low energy of the driving laser pulses allowing the production of ultrahigh-current-density ion beams with a high repetition rate. It opens a prospect for unique tabletop experiments in various fields of physical and technological research.

This paper is an overview of recent results relating to turbulent particle and heat transport, and to the triggering of internal transport barriers (ITBs). The dependence of the turbulent particle pinch velocity on plasma parameters has been clarified and compared with experiment. Magnetic shear and collisionality are found to play a central role. Analysis of heat transport has made progress along two directions: dimensionless scaling laws, which are found to agree with the prediction for electrostatic turbulence, and analysis of modulation experiments, which provide a stringent test of transport models. Finally the formation of ITBs has been addressed by analysing electron transport barriers. It is confirmed that negative magnetic shear, combined with the Shafranov shift, is a robust stabilizing mechanism. However, some well established features of internal barriers are not explained by theory.

This paper reviews the concepts, recent progress and current challenges for realizing the tremendous electric fields in relativistic plasma waves for applications ranging from tabletop particle accelerators to high-energy physics. Experiments in the 1990s on laser-driven plasma wakefield accelerators at several laboratories around the world demonstrated the potential for plasma wakefields to accelerate intense bunches of self-trapped particles at rates as high as 100 GeV m⁻¹ in millimetre-scale gas jets. These early experiments have been followed in the current decade by experiments that are advancing on several fronts—increasing the accelerated charge (to the several nanocoulomb level), producing higher transverse beam quality (to the mm mrad normalized emittance level) and accessing new physics regimes at higher laser power. Several groups are engaged in pursuing two key challenges for laser wakefield accelerators—producing beams with small energy spread and extending the interaction length from millimetres to centimetres and beyond. Major breakthroughs on both fronts have occurred in the past year. In parallel with the progress in laser-driven wakefields, particle-beam driven wakefield accelerators are making large strides. A series of experiments using the 30 GeV beam of the Stanford Linear Accelerator has demonstrated high-gradient acceleration of electrons and positrons in metre-scale plasmas as well as key scaling laws for a 'plasma afterburner', a concept for doubling the energy of a high-energy collider in a few tens of metres of plasma. In addition to wakefield acceleration, these and other experiments have demonstrated the rich physics bounty to be reaped from relativistic beam–plasma interactions. This includes plasma lenses capable of focusing particle beams to the highest energy density ever produced, collective radiation mechanisms capable of generating high-brightness x-ray beams, collective refraction of particles at a plasma interface and acceleration of intense proton beams from laser-irradiated foils.

Small particles with sizes between a few nanometers and a few 10 µm (dust) are formed in fusion devices by plasma–surface interaction processes. Though it is not a major problem today, dust is considered a problem that could arise in future long pulse fusion devices. This is primarily due to its radioactivity and due to its very high chemical reactivity. Dust formation is particularly pronounced when carbonaceous wall materials are used. Dust particles can be transported in the tokamak over significant distances. Radioactivity leads to electrical charging of dust and to its interaction with plasmas and electric fields. This may cause interference with the discharge but may also result in options for particle removal. This paper discusses some of the multi-faceted problems using information both from fusion research and from low-temperature dusty plasma work.