Table of contents

We report experimental results on the identification of rational surfaces in the Reversed Field Pinch (RFP) plasma core. The position of rational surfaces has been obtained from tomographic measurements of the m = 1 structure which develops during Quasi-Single Helicity (QSH) states. These results allow the estimate of the safety factor profile, which is measured for the first time in a RFP with plasma current ≈1 MA.

Theoretical and heuristic modelling methods are reviewed for studying turbulence phenomena of fluids and plasmas. Emphasis is placed on understanding of effects on turbulence characteristics due to inhomogeneities of field and plasma parameters. The similarity and dissimilarity between the methods for fluids and plasmas are sought in order to shed light on the properties that are shared or not by fluid and plasma turbulence.

New data from the Reversed Field eXperiment (RFX) are presented and analysed, which provide a deeper understanding of pellet experiments in a reversed field pinch. In particular, evidence on the ablation cloud density, ablation rate and homogenization process are given by the measurements of two mid-infra-red interferometers located at different toroidal locations, one of which is at the same poloidal plane as the pellet injector. For each pellet, the measurement of the latter interferometer displays a huge peak, which is due to the crossing of the interferometer chord by the ablation cloud. Its analysis yields information on the cloud dimension and radial density distribution. The typical cloud density is much lower than that measured in tokamaks. Due to such a low density, the stopping power of the ionized part of the ablation cloud is weak. As a result the ablation rate is higher than in tokamaks. Another characteristic of the density increase measured by both of the interferometers is the absence of dense plasma structures propagating for long distances along field lines. This proves that the distance necessary for the ablated material to become incorporated into the plasma is less than about 4 m in the core of the discharge, due to a rapid mixing of the ablated material. Despite the different features displayed by pellet injection experiments in reversed-field pinches and tokamaks, the ablation and homogenization of the deposited material can be described within the same general frame for both magnetic configurations. The differences arise mainly because of the different magnetic field topologies and values of the transport coefficients in the two machines.

Axisymmetric toroidal magnetic configurations relevant to current and future tokamak experiments are analytically modelled taking into account an arbitrary aspect ratio, noncircularity and up-down asymmetry of flux surface shapes. It is shown that equilibrium tokamak magnetic fields can be satisfactorily described using only four geometric parameters, which determine the shape and position of the flux surfaces. These are the Shafranov shift, the elongation, and the parameters of triangularity and of up-down asymmetry. To cover the variety of observed flux surface shapes, three different analytical expansions are provided. Comparison of the modelled with the equilibrium flux surfaces in NSTX (Princeton), JET (Abingdon) and TCV (Lausanne) demonstrates good coincidence.

Three-dimensional (3D) simulations of impurity dynamics in the vicinity of a Tore Supra ergodic divertor neutralizer are presented and compared with spectroscopic observations for a medium density pulse in the high recycling regime. The numerical tool used for the description of impurity generation and transport is a version of the Monte Carlo code BBQ, while the geometry of the magnetic field lines is calculated by use of the MASTOC code. Substantial quantitative consistency is found between the experimental data and our simulations based on recent impurity generation and propagation models. For the given plasma conditions, physical sputtering induced by impact from deuterium ions is found to be the dominant mechanism leading to the carbon contamination of the edge plasma. Chemical sputtering releases a comparable number of carbon impurities from the surface interacting with the plasma but those particles are rapidly re-deposited or pumped away. We find the role of friction of the impurities with the background plasma to be of crucial importance for the determination of the impurity distribution close to the neutralizer. We further find that a flow reversal of the background plasma takes place at the leading edge of the neutralizer with a flow velocity of a few per cent of the ion sound speed in the region that is just clear of the edge.

It is well known that there is an up-down difference in temperature in tokamaks. In the collisional case this difference for ions is on the order of 1/(ω_Biτ)>>1 for a circular cross section tokamak, where ω_Bi and τ_i are the ion cyclotron frequency and collision time, respectively. Here we show that the presence of an X-point amplifies this effect and can lead to effects of order of unity, depending on the ion drift direction. This in turn leads to a strong increase of the pressure gradient in the vicinity of the X-point (if the drift of the ions is in the X-point direction). In the framework of the Alfvén-drift paradigm for the L-H transition, it provides an explanation for the difference in the power threshold for the L-H transition arising from the ion drift direction.

An explanation for the observation of doubly peaked target profiles in JET is presented. It is based on the similarity between experimental results and EDGE2D code modelling with drifts included. The appearance of doubly peaked target profiles is related to the divergence of the radial E×B drift particle flow along the surface of the target.

Coaxial helicity injection has been investigated on the National Spherical Torus Experiment (NSTX). Initial experiments produced 130 kA of toroidal current without the use of the central solenoid. The corresponding injector current was 20 kA. Discharges with pulse lengths up to 130 ms have been produced.

A system of coils, sensors and amplifiers has been installed on the DIII-D tokamak to study the physics of feedback stabilization of low-frequency MHD modes such as the resistive wall mode (RWM). Experiments are being performed to assess the effectiveness of this minimal system and benchmark the predictions of theoretical models and codes. In the last campaign the experiments had been extended to a regime where the RWM threshold is lowered by a fast ramp of the plasma current. In these experiments the onset time of the RWM is very reproducible. With this system, the onset of the RWM had been delayed by up to 100 ms without degrading the plasma performance. The growth rate of the mode increases proportional to the length of the delay, suggesting that the plasma is evolving towards a more unstable configuration. The present results have suggested directions for improving the feedback system, including better sensors and improved feedback algorithms.

The DINA nonlinear time-dependent simulation code has been validated against an extensive set of plasma equilibrium response experiments carried out on the TCV tokamak. Limited and diverted plasmas are found to be well modelled during the plasma current flat top. In some simulations the application of the poloidal field coil voltage stimulation pulse sufficiently changed the plasma equilibrium that the vertical position feedback control loop became unstable. This behaviour was also found in the experimental work, and cannot be reproduced using linear time-independent models. A single null diverted plasma discharge was also simulated from start-up to shut-down and the results were found to accurately reproduce their experimental equivalents. The most significant difference noted was the penetration time of the poloidal flux, leading to a delayed onset of sawtoothing in the DINA simulation. The complete set of frequency stimulation experiments used to measure the open-loop tokamak plasma equilibrium response was also simulated using DINA and the results were analysed in an identical fashion to the experimental data. The frequency response of the DINA simulations agrees with the experimental results. Comparisons with linear models are also discussed in order to identify areas of good and only occasionally less good agreement.

Ion Bernstein wave (IBW) heating has been investigated in the HT-7 superconducting tokamak. Electron heating mode was concentrated on deuterium plasma with an injecting power to 320 kW. The bulk electron temperature showed a significant rise with a heating factor, ΔT_e×n_e/P_RF, up to 9.4 eV×10¹³ cm^-3 kW^-1. The direct electron heating via electron Landau damping from IBW has been demonstrated after intensive RF wall conditioning. Both on-axis and off-axis electron heating were obtained. Electron temperature profile was modified by IBW under different plasma conditions. Edge density showed no sharp drop during IBW heating. Plasma density and temperature remained almost unchanged in front of antenna. But edge temperature and density inside of plasma visibly increased and their profiles became steeper. Particle confinement improvement was observed by IBW.

This conference report summarizes the contributions to the EU-US workshop on Transport in Fusion Plasmas, Transport Barrier Physics held in Varenna, Italy, 4-7 September 2000. The workshop, whose main focus was on transport barriers, was organized into four sections: transport barrier physics, MHD effects concerning barriers, turbulence and core transport. This report is organized in a like manner.

There are corrections to the captions of figures 8 and 9. The text in parentheses at the end of the caption of figure 8 should read: 'Ne puffing'. The corresponding text for figure 9 should read: 'N₂ puffing'.

The book Advanced Space Plasma Physics by Rudolf Treumann and Wolfgang Baumjohann extends the material covered in the companion volume Basic Space Plasma Physics to topics like instabilities and nonlinear plasma phenomena. The book follows the same philosophy as the first volume, namely to introduce general concepts of plasma physics and to discuss their relevance for space plasmas at the same time.

The topics covered in this book are macroscopic and kinetic instabilities (chapters 2­6), magnetic reconnection (chapter 7), wave­particle interaction and weak turbulence (chapters 8 and 9), nonlinear waves (chapter 10), strong turbulence (chapter 11), and collective effects (chapter 12). All these topics are vast and most of them are still acitve areas of research. The authors have done an excellent job in presenting the most important part of the individual topics in a form which should be accessible for someone who has knowledge at the level of the first book. But the more experienced reader will also find this book a useful overview of a field in which the literature is usually very diverse and widely scattered.

A small weakness of the present book is that the authors refer extensively to material and equations presented in the companion volume. Without the first book, it is sometimes difficult to follow the lines of argument made in some chapters. Another point to mention is that, due to the enormous amount of material which has to be covered, the authors sometimes take short-cuts which may not always be transparent to the less experienced reader. One example of this is the discussion of the collisionless tearing mode where the authors suddenly, and without further explanation, use the ideal MHD energy principle to derive a result.

In general, however, this is an excellent book and I enjoyed reading it. It will be a very useful addition to the bookshelf of everyone interested in an introduction to instabilities and nonlinear phenomena relevant for space plasma physics, and I hope that, as for the companion volume Basic Space Plasma Physics, a cheaper paperback edition will soon follow.

Dr T Neukirch School of Mathematics and Statistics, University of St Andrews, St Andrews KY16 9SS

The book Plasma Kinetics in Atmospheric Gases is a worthwhile contribution to the basic phenomena in nitrogen, oxygen and other atmospheric gases. It contains basic introductory chapters on relaxation in translational, rotational (short) and vibrational (extensive) distribution and on the physics of electron excitation and electron distribution functions. In the latter, electron beam excitation (i.e. high electron energies) are included. In the following chapters, much detail follows on spectroscopic constants of various molecules, molecular fragments and ions, on transport coefficients such as diffusion coefficients, relaxation times and rates. The transfer between translational energies and rotational and vibrational energies are treated in this context. Many (electron) excitation rates, electronic lifetimes and rates for chemical reactions for molecules, fragments and ions are given. The background part of the book is completed with an introductory chapter on wall reactions and accommodation coefficients. In this way an enormous wealth of data for nitrogen-, oxygen- and hydrogen-containing molecules can be found in this book.

The book continues with the discussion of discharge physics in pure nitrogen and oxygen, and in mixtures. Some examples are given of applications, of which cleaning of atmospheric gases is, in my view, one of the most important. The many processes which play a role in the ionosphere are another important example of the application of the basic material for nitrogen, oxygen and other molecules. The kinetics which play a role in re-entry problems, and which have a limiting effect on the heat flux, are another example of the application of the plasma chemistry of nitrogen and oxygen gases. In this context fits also a short treatment of acoustic and shock waves, with which the book closes.

The structure of the book is that of separate chapters which are in a logical order and follow naturally from the preceding ones. The chapters are separate in the sense that there are not many cross-references between them and all contain their own reference lists. Most of the reference lists (but not all) are extensive and contain recent references. A particular value of the book is the coverage of Russian references. However, I missed in several chapters some references as, for example, a recent book by Ricard on a related subject and other publications of, for example, French and Japanese groups which work on N₂ and O₂ discharges. This means also that some rates are given without mention of the discussion on them. Nevertheless, in my view this is a very useful book and I will use it extensively. The physics and chemistry of nitrogen- and oxygen-containing plasmas present a great challenge, and much is still unknown and not fully understood, in particular also in combination with surface processes. In this research the book will prove to be a valuable contribution.

Professor D C Schram Department of Physics, Eindhoven University of Technology, 5600 MB Eindhoven

Handbook of Advanced Plasma Processing Techniques has been written as a single introductory text for plasma processing engineers and researchers. Plasma processing is a rapidly mutating field, so up-to-date material is most welcome. The broad range of topics makes the book useful for companies trying to improve their products, in research laboratories where new techniques are sought, in technological universities, and every time researchers and developers wish to broaden their knowledge of plasmas to lateral areas.

After an introduction on the state of the art of plasma material processing, the basic fundamentals of plasma physics are presented in a direct, somewhat simplified, way. This may be useful for beginners, but the expert reader may prefer something more challenging at this point. The chapter on modelling is divided into plasma modelling and reactor design. This second area is further dealt with in a separate chapter. The overview of plasma diagnostic techniques, together with the references quoted therein, gives comprehensive coverage of several diagnostics. Mass spectroscopy is dealt with separately.

In the latter two-thirds, the book is focused much more on processes. Dry etching, micromachining, PCVD, ion beam etching and other processes are discussed with respect to many different materials such as silicon, III­V materials and magnetic materials. Particular attention is given to damage and charging.

The authors are expert and up to date. It is impossible to cover in a single volume all aspects of plasma processing, but it is reasonable to say that this book, together with all the material referenced within, aspires to this goal. Somewhat more American references, rather than European or Russian, are quoted; this is acceptable for the technological section, but rather incomplete for the sections on fundamental plasma physics and diagnostics.

With contributions by several authors, some repetition is unavoidable; this may even help to ensure all chapters are self-consistent. In fact, like any handbook, it is designed more to be consulted than to be read in full. Readers will appreciate the effort of the authors of individual chapters in treating their own work topics and those of the other authors in similar depth.

Given the large amount of information provided, the book is reasonably good value for money.

Dr B M Annaratone Max-Planck Institut für Extraterrestrische Physik D-85740 Garching