Table of contents

The 28th EPS Conference on Controlled Fusion and Plasma Physics was held at the `Madeira Tecnopolo' conference centre in Funchal, Portugal from 18 to 22 June 2001. It was organized by the Centro de Fusão Nuclear (CFN) on behalf of the Contract of Association between the European Atomic Energy Community and the Instituto Superior Técnico (IST), Lisbon and held under the auspices of the Plasma Physics Division of the European Physical Society (EPS). The members of the Local Organizing Committee were drawn from CFN:

C Varandas, Chairman

M E Manso, Vice-Chairman

C Silva, Scientific Secretary

M F Pinto, Administrative Secretary

F Serra

P Varela

A Silva

M Benilov

Following the guidelines of the Board of the EPS Plasma Physics Division, the 2001 Conference included topics from the areas of: tokamaks; stellarators; high-intensity laser-produced plasmas and inertial fusion; alternative magnetic confinement; plasma edge physics; plasma heating and current drive; diagnostics; basic plasma physics; astrophysical and geophysical plasmas; and applied plasma physics. The scientific programme and paper selection were the responsibility of the International Programme Committee, appointed by the Board of the EPS Plasma Physics Division, and whose members were:

D J Campbell, Chairman, EFDA CSU, Garching, Germany

J Drake, APS-PPD, University of Maryland, USA

J Honrubia, Universidad Politecnica, Madrid, Spain

S Karttunen, VTT, Espoo, Finland

S Kuhn, Universität Innsbruck, Austria

S Lebedev, Ioffe Institute, St Petersburg, Russia

B Saoutic, CEA-DRFC, Cadarache, France

T N Todd, UKAEA, Dounreay, United Kingdom

B Unterberg, FZJ, Jülich, Germany

C Varandas, Local Organizing Committee, IST-CFN, Lisbon, Portugal

V Vershkov, Kurchatov Institute, Moscow, Russia

Villard, CRPP-EPFL, Lausanne, Switzerland

F Wagner, Chair, EPS-PPD, IPP Greifswald, Germany

M Watkins, EFDA CSU, Culham, United Kingdom

J Weiland, Chalmers University, Gothenburg, Sweden

O Willi, Heinrich-Heine-Universität, Düsseldorf, Germany

The conference was attended by 620 participants from 34 countries. The Programme Committee selected five review, three tutorial and 23 topical invited lectures. The 580 contributed papers were presented in five poster sessions with 33 also being selected for oral presentation. Additional features of the conference included the Hannes Alfvén Memorial Prize lecture, presented by Professor V D Shafranov, and an evening discussion session at which representatives of the guiding committees of the European Fusion Programme outlined ideas for its development during the 6th European Framework Programme (2003-2006) in view of the possibility of initiating ITER construction during that period. The session allowed members of the European fusion laboratories to exchange ideas with the committee representatives. The conference was also significant for the range and quality of the results from the operation and scientific exploitation of the JET Facilities by the EURATOM Associations under the European Fusion Development Agreement

The four-page contributed papers are published without refereeing on a CD-ROM in the Europhysics Conference Abstracts series, volume 25A, and a provisional collection of the electronically submitted papers has been made available at the conference website (http://www.cfn.ist.utl.pt/EPS2001/fin/). This service has been made available through the ELISE conference submission programme developed by CRPP-EPFL, Lausanne.

29 of the invited papers presented at the conference, together with the Hannes Alfvén Memorial Prize lecture by Professor V D Shafranov, are published in this special issue of Plasma Physics and Controlled Fusion, which will be sent to all registered participants of the conference. These papers were reviewed by members of the Programme Committee according to the normal rules for publication in the journal and are published from the final version submitted by the authors, who remain responsible for their presentation and the accuracy of their papers.

The Guest Editors wish to express their appreciation to the authors and the members of the Programme Committee for their work in preparation and review of the manuscripts, and would like to acknowledge the Publishers for their co-operation in the preparation of this special issue.

D J Campbell and C Varandas

Guest Editors

Professor Vitaly D Shafranov is widely known as one of the acknowledged leaders of the world's scientific community in plasma physics and controlled fusion. His theoretical research on plasma equilibrium and stability made an outstanding contribution to the physics of magnetically confined toroidal plasmas which plays a decisive role for the problem of magnetic fusion.

Professor Shafranov was born in 1929 in the village of Mordvinovo, Ryazaní Province. He graduated from secondary school in 1946 and, being excited by discussions on the perspectives of the applications of nuclear energy, entered the Physics Department of Moscow State University to specialise in atomic physics. Following graduation in late 1951, he came to the Kurchatov Institute, to academician M A Leontovich, who was recently nominated as head of the theoretical group for controlled fusion research. In the first decade of his work, V D Shafranov had already accomplished theoretical achievements in plasma physics, which are now associated with his name: the Kruskal-Shafranov criterion, the Grad-Shafranov equation, the Shafranov shift (of the toroidal magnetic surfaces) and which outlined that new physical object in the laboratory: the toroidal plasma maintained in equilibrium due to interaction of the toroidal current with the external transverse magnetic field.

The first work by V D Shafranov, in co-authorship with M A Leontovich, On the stability of a flexible conductor in the presence of a magnetic field (1952) determined the goal for the experiments initiated by A D Sakharov's proposal on the confinement of a plasma by both the toroidal magnetic field and the inductive electric current, which later led to tokamaks. The approach of that article was improved in the paper On the stability of a cylindrical gaseous conductor in a magnetic field (delivered by L A Artsimovich 1955 6th Int. Symp. Astronomical Union, Stockholm), and in the paper On the stability of a plasma column in the presence of the longitudinal magnetic field and the conducting casing. In these papers the second possible regime of stabilisation by the `frozen' internal longitudinal magnetic field was investigated. Together with S I Braginsky, he also studied the time behaviour of the equilibrium of a high-temperature plasma column in a magnetic field. Several of Shafranov's papers were devoted to the more general problem of the toroidal plasma equilibrium in a magnetic field. In June of 1957, at the 4th International Conference on Ionisation Phenomena in Gases, he presented his paper on an axially symmetric equilibrium equation (now known as the Grad-Shafranov equation), on the virial theorem of the plasma equilibrium in an external magnetic field, and on the formal analogy of such an equilibrium with the toroidal vortex (like the smoke ring) in hydrodynamic flow. In 1958, Shafranov defended his thesis for the degree of Candidate of Science (Russian analogy of PhD), but it immediately earned him the higher scientific degree of a Doctor of Physical and Mathematical Sciences. The results of these investigations were described in Reviews of Plasma Physics 1963 ed M A Leontovich vol 2. His paper on the structure of the collisional shock wave in a plasma is also dated to this period. Some results of this paper were included by Ya B Zelídovich and Yu P Raizer in their book Physics of shock waves and high-temperature hydrodynamic phenomena.

A further series of Shafranov's papers from the first decade of his work was related to the propagation and absorption of electromagnetic waves in high-temperature plasma and to certain instabilities. The original approach to the derivation of the dielectric permeability of the plasma from the correlation function of microcurrents, the collisionless absorption of RF waves in a high-temperature plasma, the radiation damping of charged particles in a plasma with magnetic field, and other aspects, formed the basis of his review paper Electromagnetic waves in a plasma in the series Reviews of Plasma Physics 1963 ed M A Leontovich vol 3.

In the following years, Shafranov concentrated on developing the theory of plasma equilibrium and stability in toroidal magnetic confinement systems. Together with S I Braginsky, as early as 1958, he prepared a paper (number 2500) on estimations of parameters of the idealised toroidal systems with Ohmic heating for the 2nd International Conference on Peaceful Use of Atomic Energy. These estimates were used for choosing the parameters of the future tokamak T-3. In 1962 he derived the formula for the shift of a toroidal plasma of circular cross-section in a tokamak, which stimulated further improvement of plasma parameters. Later he derived a similar formula for the more complicated case of systems without axial symmetry (e.g. for the figure-eight stellarator with a circular cross-section) and for the circular axis, shearless l = 2 stellarator. He also estimated the influence of the plasma torus curvature on the classical thermal conductivity (1965) and derived (with B B Kadomtsev) the formula for classical diffusion in a tokamak plasma taking into account the drift and pinch effects. He was the first to explain the stability of the tokamak plasma against perturbations with high azimuthal modes (1970) and to give a theory for the determination of certain internal equilibrium parameters in tokamaks by external magnetic measurements. Shafranov's review, written in co-authorship with V S Mukhovatov (Nuclear Fusion 1971), which became a handbook on tokamak systems, was very important for world tokamak research. A more detailed review, in co-authorship with L E Zakharov, on the toroidal plasma equilibrium was presented in Reviews of Plasma Physics 1982 ed M A Leontovich vol 11.

The extensive series of papers by Shafranov on plasma equilibrium and stability in the principally steady state toroidal magnetic configurations, both with a rotational transform (stellarators) and without (i.e. with closed magnetic field lines), is essential for the choice of optimal systems for a fusion reactor. His paper Magnetohydrodynamic theory of plasma equilibrium and stability in stellarators: survey of results published in Phys. Fluids 1983, was highly appreciated by those who had been concerned with stellarator research. The most comprehensive review on the theory of stellarators was published, together with V D Pustovitov, in Reviews of Plasma Physics 1984 ed B B Kadomtsev vol 15. An interesting possibility for stabilisation of the MHD instabilities in stellarators by the bootstrap current was discovered together with M I Mikhailov (1989). In the 1990s, V Shafranov concentrated on the innovative stellarator concept associated mainly with the names of A Boozer and J Nuehrenberg. Here, V Shafranov found the invariant vector formulation for the idealised `quasi-symmetry' condition introduced by J Nuehrenberg. The group, headed by Shafranov in his theory department, is now collaborating with various groups working in the field of stellarator system optimisation.

The pioneering theoretical research by V D Shafranov on the physics of plasma confinement in toroidal systems, thanks to its depth, clarity and concreteness, has had substantial impact on many physicists, both theorists and experimentalists, throughout the world.

V D Shafranov created a strong scientific school of theorists working in the field of magnetic plasma confinement, which is recognised in his own country and abroad. Shafranov is the bright successor of M A Leontovich, and it is only natural that since April 1981 he has been at the head of the Department of Plasma Theory at the Institute of Nuclear Fusion, Kurchatov Institute.

In 1981, V D Shafranov was elected a Corresponding Member of the USSR Academy of Sciences. In 1997, he became a full Member of the Academy. For his outstanding contributions to plasma magnetic confinement theory V D Shafranov was awarded the 1971 State Prize of the USSR and the 1984 Lenin Prize.

Since 1983 V D Shafranov has served as editor-in-chief of the journal Fizika Plazmy (translated into English as Plasma Physics Reports). In the eighties he edited twelve issues of Plasma Physics in Russian (Moscow: VINITI Publishing), in the series Itogi Nauki i Tekhniki (Recent Progress in Science and Technology in Russian). V D Shafranov is also editor-in-chief of the series Reviews of Plasma Physics (Dordrecht, Germany and New York: Kluwer Academic and Plenum).

Professor E P Velikhov

President of the Kurchatov Institute

Two problems of toroidal magnetic plasma confinement systems are considered. The first one is connected with advanced helical systems. The peculiarity of geometry of these stellarator systems with the improved confinement is discussed. Some additional ways for further optimization are marked too.

The second problem is concerned with the island structure in toroidal systems. From the very beginning, the theoretical research on plasma equilibrium in stellarators was based mainly on the assumption that the toroidal magnetic surfaces are nested. It means that there is a unique magnetic axis which serves as the edge of the azimuthal coordinate surface θ = const. To avoid the difficulties with such a coordinate in the case of many magnetic axes that result from islands, the tokamak-like representation of the magnetic field B which does not use the coordinate θ is proposed.

Recent progress towards obtaining high density and high confinement in JET as required for the ITER reference scenario at Q = 10 is summarized. Plasmas with simultaneous confinement H_98(y,2) = 1 and densities up to n/n_GW~1 are now routinely obtained. This has been possible (i) by using plasmas at high (δ~0.5) and medium (δ~0.3-0.4) triangularity with sufficient heating power to maintain Type I ELMs, (ii) with impurity seeded plasmas at high (δ~0.5) and low (δ⩽0.2) triangularity, (iii) with an optimized pellet injection sequence, maintaining the energy confinement and raising the density, and (iv) by carefully tuning the gas puff rate leading to plasmas with peaked density profiles and good confinement at long time scales. These high performance discharges exhibit Type I ELMs, with a new and more favourable behaviour observed at high densities, requiring further studies. Techniques for a possible mitigation of these ELMs are discussed, and first promising results are obtained with impurity seeding in discharges at high triangularity. Scaling studies using the new data of this year show a strong dependence of confinement on upper triangularity, density and proximity to the Greenwald limit. Observed MHD instabilities and methods to avoid these in high density and high confinement plasmas are discussed.

The interaction of ultra-intense (and ultra-short) laser beams with plasmas gives rise to a variety of phenomena. The propagation is, in principle, possible in an overdense plasma if the laser intensity is above a threshold fixed by the electron density. However, the solutions describing this so-called relativistically self-induced transparency are subject to violent electron instabilities, as is already the case in an underdense plasma. The growth rates of the instabilities are so large that it appears hopeless to propagate efficiently a high-intensity beam in a cold plasma. The situation is somewhat more favourable in a relativistically hot plasma, where the growth rates of the instabilities are significantly reduced. In any case, the interaction of the laser beam with the plasma results in a strong electron acceleration and heating. The electron acceleration is both due to the plasma waves generated in the plasma by the laser beam and to the laser field itself. In present-day experiments, the fastest electron energy can be in the range of hundreds of MeV. Correlatively, fast ions can be accelerated by the charge separation electric fields, and energetic photons due to bremsstrahlung appear, which in turn can be responsible for photonuclear reactions. The transport and interactions of all these energetic particles in and outside the plasma are interesting for various applications, such as possible laser acceleration of electrons up to the GeV range, high-energy short-duration particle sources, protron radiography, fast-ignition approach to inertial confinement fusion, etc.

ASDEX Upgrade results on crucial issues of H-mode operation are reported, i.e. density profile shape, onset conditions for neoclassical tearing modes (NTM) and smoothing of the power exhaust without losing confinement. Starting from experiments on density peaking at high density, a model for particle transport is developed using the neoclassical particle pinch and assuming that the particle diffusion coefficient D is locally proportional to the heat conductivity χ. This assumption links the D profile to the heat-flux profile due to the stiffness of the temperature profiles. This suggests that density profile shapes of a centrally heated reactor will be significantly flatter than in a device heated by neutral beam injection (NBI). In the discharges with density peaking (3,2)-NTMs develop at significantly smaller values of β_N than in H-modes with rather flat density profiles. Local analysis reveals that for such discharges the density gradient term of the bootstrap current is of the same size as the temperature gradient term. For the experimental temperature (T) and density (n) profiles close to the q = 1.5 surface, the model predictions agree well with the observed onset of the NTM. Finally, we describe the H-mode operation with edge localized modes (ELMs) of type-II in ASDEX Upgrade. This ELM type allows one to smoothen the power exhaust significantly as compared to type-I ELMs without reducing the confinement. Pure type-II ELMy H-modes are so far limited to safety factors q>4.2 at 95% poloidal flux (q₉₅) and high triangularity δ≈0.4, but the latest results indicate that both thresholds may be reduced if the current is increased and the configuration is kept very close to double null.

Recent experimental results in the Large Helical Device have indicated that a large pressure gradient can be formed beyond the stability criterion for the Mercier (high-n) mode. While the stability against an interchange mode is violated in the inward-shifted configuration due to an enhancement of the magnetic hill, the neoclassical transport and confinement of high-energy particle are, in contrast, improved by this inward shift. Mitigation of the unfavourable effects of MHD instability has led to a significant extension of the operational regime. Achievements of the stored energy of 1 MJ and the volume-averaged beta of 3% are representative results from this finding. A confinement enhancement factor above the international stellarator scaling ISS95 is also maintained around 1.5 towards a volume-averaged beta, ⟨β⟩, of 3%. Configuration studies on confinement and MHD characteristics emphasize the superiority of the inward-shifted geometry to other geometries. The emergence of coherent modes appears to be consistent with the linear ideal MHD theory; however, the inward-shifted configuration has reduced heat transport in spite of a larger amplitude of magnetic fluctuation than the outward-shifted configuration. While neoclassical helical ripple transport becomes visible for the outward-shifted configuration in the collisionless regime, the inward-shifted configuration does not show any degradation of confinement deep in the collisionless regime (ν*<0.1). The distinguished characteristics observed in the inward-shifted configuration help in creating a new perspective of MHD stability and related transport in net current-free plasmas. The first result of the pellet launching at different locations is also reported.

The discovery of advanced tokamak scenarios opened new possibilities for the realization of more compact machines with long pulse operation. This requires the simultaneous optimization of the plasma shape, and of the pressure, current and electric field profiles. In this complex process the diagnostics are crucial to understand the plasma physics, to validate the theoretical models and to measure parameters for control purposes.

Here we present results obtained in ASDEX Upgrade showing the great potential of reflectometry for conventional and advanced tokamak investigation, derived from the diagnostic capability to perform a wide range of measurements with high spatial and temporal resolution, both at the edge and plasma core. The abrupt formation of sharp density gradients at the zones where internal and external transport barriers are formed, as well as the modifications of the turbulence parameters in those narrow plasma regions are shown. Density profile changes associated with fast events like ELMs and the L-H transition are resolved. Enhanced fluctuations that may be important for the transport across the edge barrier are also detected by reflectometry. The measurement of the radial profile of E×B velocity on ASDEX Upgrade is discussed and results are compared with v_E×B profiles obtained in W7-AS stellarator.

The first experimental demonstration that edge density profiles from reflectometry can provide accurate and reliable measurements of the temporal evolution of the plasma position is presented. This is essential to prove that reflectometry can be used as an alternative approach to supplement the magnetic measurements, for the control of plasma shape and position in long pulse operation, as it is proposed for ITER. Finally new applications of reflectometry under development are discussed.

Experiments on the DIII-D tokamak have identified a new sustained high-performance operating mode, termed the quiescent double barrier (QDB) regime. The QDB regime combines internal transport barriers (ITBs) with a quiescent, edge localized mode (ELM)-free H-mode edge, termed QH-mode, giving rise to separate core and edge transport barriers. These double barriers have been maintained for {>}3.5 s (~25τ_E), demonstrating a long-pulse, quasi-steady-state capability. The combination of core ITBs and edge H-mode temperature pedestals results in high-performance plasmas; a β_NH₈₉ product of 7 has been maintained for 10 τ_E, other peak (non-simultaneous) parameters include T_i⩽17 keV, β_N⩽2.9% m T MA^-1, H₈₉⩽2.6, β⩽3.8%, τ_E⩽ 160 ms, and DD neutron rate S_n⩽5.5×10¹⁵ s^-1. These results address a major issue with tokamak plasmas: how to sustain long-pulse, high-performance H-mode plasmas without ELMs, yet retaining the density and impurity control hitherto provided by ELMs. In these QDB plasmas ELMs are replaced by continuous benign MHD activity in the edge, which enhances particle transport. A signature of operation with a QH-mode edge appears to be very large radial electric fields in the edge and scrape-off layer (SOL). In the core, simulations and modelling replicate many of the features of the observed transport and fluctuation behaviour, including the ion temperature profile and turbulence correlation lengths. Slow high-Z impurity accumulation (τ⩾500 ms) is observed in the centre of many QDB plasmas, and is the subject of ongoing analysis. To date the QDB regime has only been obtained in plasmas with counter-NBI (injection anti-parallel to the plasma current), and with divertor cryopumping to control the density.

First, the fast ignition of compressed fusion fuel is discussed in terms of the isochoric ignition model and two-dimensional simulations. Non-spherical configurations and deuterium fuel ignited from a deuterium-tritium seed are also investigated. Experiments and simulations for laser fast ignition are also presented, in particular relativistic self-focusing, hole boring, the generation of electron beams as well as filamented electron transport and collective stopping. Finally, recent experimental and numerical work on cone-guided fast ignition and fast ignition by laser-generated proton beams is reviewed.

Several new spherical tokamak (ST) experiments have become operational in the last two years. Results are now becoming available which increase the understanding of tokamak physics, extend confinement and threshold databases for next step devices such as ITER, and show the possibilities offered by the ST as a possible future burning plasma device or fusion power plant.

One of the central physics issues currently targeted by nonlinear gyrokinetic simulations is the role of finite-β effects. The latter change the MHD equilibrium, introduce new dynamical space and time scales, alter and enlarge the zoo of electrostatic microinstabilities and saturation mechanisms, and lead to turbulent transport along fluctuating magnetic field lines. It is shown that the electromagnetic effects on primarily electrostatic microinstabilities are generally weakly or moderately stabilizing. However, the saturation of these modes and hence the determination of the transport level in the quasi-stationary turbulent state can be dominated by nonlinear electromagnetic effects and yield surprising results. Despite this, the induced transport is generally electrostatic in nature well below the ideal ballooning limit.

We present observations of a mechanically forced liquid-metal dynamo experiment and review international efforts in this field. As the magnetic Reynolds number is increased, our measurements show lengthening of the decay time of pulsed magnetic fields. These measurements quantify the approach to magnetic field self-generation. While an external field is applied, large fluctuations occur in the magnetic response of the system. We analyse these fluctuations to characterize the liquid-metal velocity field. This analysis suggests that turbulence strongly influences the approach to self-generation in a forced liquid-metal flow.

The operating space of the TCV tokamak has been extended to higher elongation, κ = 2.8, and higher normalized current, I_N = I_P/aB = 3.6 MA/mT, than were previously attainable. This has been achieved by optimizing the parameters of the vertical position control system and by optimizing the plasma shape. Experimental current and beta limits were found to be consistent with ideal MHD stability calculations, using measured plasma shapes and profiles. The sawtooth period was measured as a function of elongation and sawtooth oscillations are found to disappear, at high elongation, when the internal inductance drops below l_i ∼ 0.69. MHD events at high elongation have been analysed, using magnetic fluctuation measurements. The electron energy confinement time at high elongation, 2.2 < κ < 2.7, shows a strong dependence on density and power flux through the plasma surface, but no statistically significant dependence on κ. Finally, we have applied off-axis, second harmonic, X-mode electron cyclotron resonance heating (ECRH) in order to flatten the current profile and improve vertical stability. This gave access to previously inaccessible domains in κ - I_N space.

W7-AS has recently been equipped with ten open divertor modules in order to experimentally evaluate the island divertor concept. First results are reported in this paper. The new divertors enable access to a new NBI-heated, very high density (up to _e = 3.5 × 10²⁰ m^-3) operating regime with promising confinement properties. The energy confinement time increases steeply with density and then saturates. In contrast, the particle and impurity confinement times decrease with increasing density. This allows full density control and quasi-steady-state operation also under conditions of partial detachment from the divertor targets. Radiated power fractions are low to moderate in attached regimes and reach up to about 90% in detachment scenarios. The radiation always stays peaked at the edge. The extremely high densities necessitated the development of non-standard heating techniques for central heating. For the first time efficient heating of an NBI target plasma by electron Bernstein waves (140 GHz, second harmonic) is achieved. In addition, this heating scenario enables fine tuning of the upstream boundary conditions for divertor operation.

Recent investigations of electron cyclotron current drive (ECCD) in tokamaks and stellarators are the subject of this paper. Values of ECCD efficiency achieved in different machines are compared with theoretical predictions. The role of trapped particles in the reduction of ECCD efficiency is discussed. Recent progress in the measurements of electron cyclotron current profile and in ECCD applications for fully noninductive operation, current profile control and neoclassical tearing mode stabilization is also reviewed.

The modelling of Alfvénic instabilities is discussed from the point of view of mode-conversion, showing how the development of the theory affects the predictions as the limitations of the models are gradually relaxed. Conventional tokamak plasmas are relatively well understood and are used for the case of a kinetic Alfvén eigenmode (AE) to assess the resonant wave-particle interactions along the magnetic field. The large safety factor in the core of deeply reversed shear plasmas and the low magnetic field of spherical tokamaks, however, bring the AEs down into the drift-frequency range; modifications of the spectrum through toroidal mode-conversion then creates a new class of drift-kinetic AEs that could affect the fast particle confinement.Experiments have been carried out to verify these predictions in JET. They confirm the presence of weakly damped modes, which do not follow the usual AEs scaling.

JT-60U has conducted extensive experiments for 17 years to achieve equivalent break-even conditions and to establish the scientific and technological basis of ITER and future DEMO reactors such as SSTR. Two types of advanced scenarios, high β_p H-mode (with weak positive shear) and RS mode (negative shear), are developed in JT-60U. High performance discharges are maintained by full non-inductive current drive, primary with neutral beam current drive. The results are quite promising in demonstrating principle of steady-state tokamak operation. The research clarified many important issues for future studies.

An overview of ongoing activities on heavy ion fusion (HIF) in Russia, Europe, the USA and Japan is presented. In this phase, the activities are oriented towards theoretical and experimental investigations of the state of matter under extreme conditions, experimental and theoretical study on heavy ion beam-plasma interaction, powerful driver issues and development of computer codes for comprehensive numerical simulations of heavy ion driven inertial fusion. In the context of the new-generation facilities at GSI-Darmstadt and ITEP-Moscow, considerations are focused on large synchrotron rings and high current accumulator rings which would be important tools for investigations in the physics of dense plasmas and inertial fusion. Numerous national and international collaborations on HIF research are proceeding effectively towards development of a new energy source that is abundant, safe, environmentally acceptable and economical.

Compact optimized stellarators offer novel solutions for confining high-β plasmas and developing magnetic confinement fusion. The three-dimensional plasma shape can be designed to enhance the magnetohydrodynamic (MHD) stability without feedback or nearby conducting structures and provide drift-orbit confinement similar to tokamaks. These configurations offer the possibility of combining the steady-state low-recirculating power, external control, and disruption resilience of previous stellarators with the low aspect ratio, high β limit, and good confinement of advanced tokamaks. Quasi-axisymmetric equilibria have been developed for the proposed National Compact Stellarator Experiment (NCSX) with average aspect ratio 4-4.4 and average elongation ~1.8. Even with bootstrap-current consistent profiles, they are passively stable to the ballooning, kink, vertical, Mercier, and neoclassical-tearing modes for β > 4%, without the need for external feedback or conducting walls. The bootstrap current generates only 1/4 of the magnetic rotational transform at β = 4% (the rest is from the coils); thus the equilibrium is much less non-linear and is more controllable than similar advanced tokamaks. The enhanced stability is a result of `reversed' global shear, the spatial distribution of local shear, and the large fraction of externally generated transform. Transport simulations show adequate fast-ion confinement and thermal neoclassical transport similar to equivalent tokamaks. Modular coils have been designed which reproduce the physics properties, provide good flux surfaces, and allow flexible variation of the plasma shape to control the predicted MHD stability and transport properties.

This overview summarizes the latest developments in plasma turbulence simulations and their impact on transport modelling. The traditional approach, based on the mixing length estimate, is presented first. The self-regulation of a plasma turbulence through the formation of structures is then emphasized. It is shown that zonal flows and large-scale transport events play an important role. Also the latest results concerning the dimensionless analysis are presented. The implications for transport modelling are then assessed.

Proton imaging is a recently proposed technique for diagnosis of dense plasmas, which favourably exploits the properties of protons produced by high-intensity laser-matter interaction. The technique allows the distribution of electric fields in plasmas and around laser-irradiated targets to be explored for the first time with high temporal and spatial resolution. This leads to the possibility of investigating as yet unexplored physical issues. In particular we will present measurements of transient electric fields in laser-plasmas and around laser-irradiated targets under various interaction conditions. Complex electric field structures have been observed in long-scale laser-produced plasmas, while global target charge-up and growth of electromagnetic instabilities have been detected following ultraintense interactions with solid targets.

A large number of diagnostics have been used to study the performance of gas-filled, plastic-shell direct-drive implosions on the 60-beam OMEGA laser system (Boehly T R et al 1997 Opt. Commun.133 495). These diagnostics allow an estimate of the amount of fuel-pusher mixing in the compressed core. Implosions have primary neutron yields and fuel areal densities that are ~35% and ~100% of the predicted one-dimensional values, respectively. A highly constrained model of the core conditions and fuel-shell mix has been developed. It suggests that there is a `clean' fuel region surrounded by a mixed region, with the clean region accounting for ~70% of the fuel areal density.

In this paper, recent experiments of electron cyclotron plasma heating leading to electron internal transport barriers (ITBs) are summarized pointing out differences and common features. These experiments are characterized by the new condition of dominant electron heating in low collisional plasmas. The role of the central magnetic shear, which has to be weak or negative for an electron-ITB to develop, is pointed out, as well as the possible role of the Shafranov shift or α-stabilization of the turbulence responsible for the anomalous electron transport. Differences with respect to ion-ITB formation conditions are also briefly discussed. The active control of MHD instabilities is another important issue to improve plasma confinement properties. An illustrative example of stabilization of tearing mode by ECRH is reported and discussed.

Recent results of Alfvén wave heating experiments and the characteristics of a new regime of runaway discharges found in Tokamak Chauffage Alfvén Brésilien (TCABR) are discussed. (1) Wave excitation was carried out with one module of the antenna system, with and without a Faraday screen. Evidence of plasma heating was obtained in both cases, for coupled wave powers up to half of the Ohmic power, approximately, without uncontrollable density rise during the RF pulse. The antenna coupling with the plasma seems to have increased when the Faraday screen was removed. (2) The new regime of runaway discharges is produced by initiating the main plasma breakdown without pre-ionization and strongly increasing the neutral gas fuelling at the end of the current ramp-up phase. Consequently, the plasma cools down substantially and switches to a runaway mode in conditions under which the primary (Dreicer) mechanism is strongly suppressed. This new regime of runaway discharges is characterized by strong enhancement of the relaxation oscillations, which are seen in the H_α and ECE emissions, coupled with large spikes in the line density, loop voltage, bolometer, and other diagnostic signals.

The TJ-II stellarator was designed to have a high degree of magnetic configuration flexibility. The rotational transform can be varied between 0.9 and 2.5, and the magnetic well may be changed from -1% to 6%. This flexibility is a powerful tool for physics studies in fusion plasmas. The magnetic-well scan experiments in TJ-II support the view that plasma turbulence displays universality. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the study of transport based on the computation of effective transport coefficients. Comparative studies in TJ-II stellarator and other devices show that fluctuations and E × B sheared flows organize themselves to be close to marginal stability. This property should be considered as a critical test for L-H transition models. The magnetic-configuration scan experiments in TJ-II show the complex interplay between transport and radial electric fields in the proximity of rational surfaces.

Electron transport in tokamaks has many different features which are briefly reviewed. The paper is focused on electron heat transport in conventional tokamak plasmas. An inter-machine comparison indicates that the non-dimensional gradient length of the electron temperature profiles R/L_Te is almost independent of the devices and varies little with plasma parameters. This strongly suggests that electron heat transport is governed by turbulence with a threshold in R/L_Te. This is confirmed by modulation experiments using electron cyclotron heating. Simulations with empirical and physics-based transport models confirm this assumption.

The comprehension of the dynamics of classical and neoclassical tearing modes is a key issue in high-performance tokamak plasmas. Avoiding these instabilities requires a good knowledge of all the physical mechanisms involved in their linear and/or nonlinear onset. Our tridimensional time evolution code XTOR, which solves the full magnetohydrodynamic (MHD) equations including thermal transport, is used to tackle this difficult problem. In this paper, to show the state of art in full-scale nonlinear MHD simulations of tokamak plasmas, we investigate the effect of plasma curvature on the tearing mode dynamics. For a realistic picture of this dynamics, heat diffusion is required in the linear regimes as well, as in the nonlinear regimes. We present a new dispersion relation including perpendicular and parallel transport, and show that it matches the linear and nonlinear regimes. This leads to a new tearing mode island evolution equation including curvature effects, valid for every island size in tokamak plasmas. This equation predicts a nonlinearly unstable regime for tearing instabilities, i.e. a regime which is linearly stable, but where the tearing mode can be destabilized nonlinearly by a finite-size seed island. These theoretical predictions are in good agreement with XTOR simulations. In particular, the nonlinear instability due to curvature effects is reproduced. Our results have an important impact on the onset mechanism of neoclassical tearing modes. They indeed predict that curvature effects lead to a resistive MHD threshold.

Hydrogen isotopes are released from the walls, divertor plates and limiters of fusion devices in different forms, as atoms or molecules. The density and velocity distributions of these particles upon release can heavily influence the boundary plasma, especially through their penetration depth, and, indirectly, the plasma and its confinement properties as a whole. Recent experiments on different tokamaks have brought to light the deep interdependence between atomic and molecular species in this respect. Investigating this complexity calls for sophisticated spectroscopic diagnostics and new data on molecules like D₂ and HD. Hydrogen and deuterium atoms with extremely low energies, definitely below 1 eV, are observed and give indications on the release processes, i.e. on those where molecules are involved. It turns out that corrections to the estimated hydrogen flux may be required - a procedure is proposed in the present work to obtain values for an effectiveS/XB coefficient for atomic hydrogen (≠15), which denotes the ratio of the collisional ionization (S) to the excitation (X) rate coefficients, divided by the branching ratioB. Moreover, attention is drawn to a possible heating mechanism of these very cold atoms by the proton/deuteron background (from 0.2 up to 10 eV). The derived information on the detailed release mechanisms should contribute towards improving the various numerical codes in which neutrals play a role. Eventually, the strong influence of the surface temperature on the ratio of atoms to molecules has to be considered in the choice of materials for plasma-facing components.

Since its launch in 1995, the Solar and Heliospheric Observatory (SOHO) has been revolutionizing our understanding of the Sun. In particular, major advances have been made in revealing the complex and very dynamic nature of the star's outer atmosphere or corona. Through optical, ultraviolet and extreme ultraviolet imagers and spectrometers, SOHO has provided an unparalleled breadth and depth of observational data of this portion of the Sun. This magnetically dominant plasma environment demonstrates numerous magnetic field/plasma interactions over a wide range of lengthscales and timescales. This review outlines how the instruments onboard SOHO diagnose the coronal plasma over an extensive wavelength range. Through high-resolution images and movies, this mission has revealed how the Sun's magnetic field structures the plasma (to form, for example, loops and tornadoes) and how instabilities trigger energy release on incredible scales (from coronal mass ejections). Current thinking on how the corona maintains its temperature in excess of two million degrees is presented. Complementary images from other space-based solar missions (Yohkoh and TRACE) are used to provide further examples of the above. Also, building upon the success of SOHO, future space-based solar missions are profiled.

ITER is planned to be the first fusion experiment operating under reactor-relevant conditions. The following requirements are given: ITER should (1) achieve extended burn in inductively driven plasma at Q ⩾ 10, whilst not precluding the possibility of controlled ignition, (2) aim at demonstrating steady-state operation using non-inductive current drive at Q ⩾ 5, (3) demonstrate availability and integration of essential fusion technologies, and (4) test components for a future reactor including tritium breeding module concepts, with the 14 MeV neutron power load on the first wall ⩾ 0.5 MW m^-2 and fluence ⩾ 0.3 MW am^-2. The ITER tokamak is designed not only to satisfy these requirements but also to have flexibility of plasma operations. This flexibility will provide a wide range of opportunities to develop inductive and non-inductive reactor core plasmas and to study burning plasma physics. Possible operational modes studying physics and technical issues of burning plasmas are overviewed.

The recent progress of the EFDA-JET facility in advanced tokamak (AT) scenario research is reviewed, emphasizing the relevance for the advanced mode of ITER operation. In particular, the key role played by the current density profile from the low-performance prelude phase, to trigger an internal transport barrier (ITB), to the high-performance steady state, to maintain the regime, is illustrated by the recent results obtained on JET with reversed magnetic shear targets. The mitigated ELMy edge behaviour, as well as the neoclassical nature of the particle transport in highly non-inductive ITB discharges, is also reported and discussed. Finally, the first demonstrations of real-time profile control of ITB discharges open the way towards high-performance steady-state AT discharges, provided that current density and pressure profiles are controlled.