Table of contents

The inclusion of finite pressure in ideal magnetohydrodynamic theory can explain the reversed magnetic shear Alfvén eigenmodes (RSAE) (or Alfvén cascades) that have been observed in several large tokamaks without the need to invoke an energetic particle mechanism for the existence of these modes. The chirping of the RSAE is caused by changes in the minimum of the magnetic safety factor, q_min, while finite pressure effects explain the observed non-zero minimum frequency of the RSAE when q_min has a rational value. Finite pressure effects also play a dominant role in the existence of the downward chirping RSAE branch.

Magnetic triggering of edge localized modes (ELMs) was reported first from TCV in ohmic plasmas showing type-III ELMs. This method, showing successful locking of the ELM frequency to an imposed vertical plasma oscillation, has now also been demonstrated in the ITER-relevant type-I ELM regime in ASDEX Upgrade. Our experiments showed the ELM frequency becoming identical to the driving frequency in steady state for an applied motion of only about twice the value caused by an intrinsic ELM event. Triggered ELMs still showing clear type-I features were found when the plasma down-shift velocity reached its maximum, corresponding to the lowest edge current value. This is the opposite of the behaviour expected from the peeling–ballooning nature attributed to the ELM boundary and to TCV observations. The reason for this behaviour is not yet clear.

The TCV tokamak is equipped with a four-channel ultrasoft x-ray monochromator allowing low resolution (λ/Δλ ∼ 30) measurements in the energy range 200–800 eV. Its main purpose is to simultaneously monitor emission from four of the main Kα resonance lines of highly ionized light impurities such as boron, carbon, nitrogen and oxygen. Wavelength selectivity is achieved using synthetic multilayer mirrors having 30–40 alternate layers with layer periods in the range 3–7 nm, depending on the design wavelength range. Recently the instrument was modified for measuring the radial profiles of Kα and Lyman-α lines from He-like and H-like intrinsic carbon impurities to allow estimates of impurity diffusivities by comparisons with simulations using the impurity transport code STRAHL.

A method commonly used for determining where externally launched power is absorbed inside a tokamak plasma is to examine the temperature response to modulation of the launched power. Strictly speaking, this response merely provides a first good guess of the actual power deposition rather than the deposition profile itself: not only local heat sources but also heat losses and heat wave propagation affect the temperature response at a given position. Making use of this, at first sight non-desirable, effect modulation becomes a useful tool for conducting transport studies. In this paper a minimization method based on a simple conduction–convection model is proposed for deducing the power deposition and transport characteristics from the experimentally measured (electron) energy density response to a modulation of the auxiliary heating power. An L-mode JET example illustrates the potential of the technique.

The theory of the Newcomb equation has been applied to low-n external modes in a tokamak and a method has been developed for computing the stability matrix that gives the change of plasma potential energy due to external modes in terms of the surface values of the perturbations. By using this method, the spectral properties of the ideal external modes have been elucidated, such as the coupling between external modes and internal modes, and the difference in the stability properties between a normal shear tokamak and a reversed shear tokamak. These results will also be useful in the stability analysis of resistive wall modes.

The electron heat transport in low density H-mode plasmas heated by neutral beam injection (NBI) is investigated in ASDEX Upgrade using electron cyclotron heating (ECH) combining both steady-state and transient response analysis by modulating the ECH power. Under these conditions, more than 60% of the NBI power (>3 MW) is delivered to the ions, while approximately 20% (∼1 MW) is delivered to the electrons. In the confinement region, the electron-to-ion temperature ratio, T_e/T_i, varies between 0.5 and 0.7 in the NBI-only phase and between 0.8 and 1.0 when the ECH is also applied. Due to the low collisional coupling, the power in the electron channel is locally more than doubled by applying up to the available 2 MW of ECH, while the power in the ion channel is locally increased by less than 30%. A dependence on the density of the reaction of the plasma parameters to the ECH is observed. For plasmas with average density (defined as 'hot-ion' H-modes), when the ECH is applied, T_e increases, the central T_i drops and the density flattens. These effects disappear with increasing density and are not observed for (defined as 'regular' H-modes). Power balance analysis of both the hot-ion and regular H-modes points to a strong resilient behaviour of the T_e profiles. In the hot-ion cases, the ECH heating induces a strong increase in transport in the ion channel. Power balance and transient response analysis of the regular H-modes are consistent with an inverse scale length transport model with a threshold in , above which the electron heat transport is increased. Comparison with recent studies in pure EC heated L-modes points to a stronger resilience of T_e in the NBI heated H-modes.

Recent experiments in the JFT-2M tokamak have concentrated on studies of the access conditions for H-mode operation without any large ELMs, namely the 'high recycling steady' (HRS) H-mode regime, in terms of the pedestal parameters. The HRS regime is more likely to occur at higher edge density and lower edge temperature, while the ELMy H-mode having large ELMs appears at lower edge density and higher edge temperature. It is found that the ELMy/HRS operational boundary occurs at the normalized electron collisionality of in the plasma edge region, depending slightly on the safety factor at 95% flux surface q₉₅. A key feature of the HRS H-mode is the presence of coherent magnetic fluctuations in the frequency range of the order of 10–100 kHz. It is suggested that the edge MHD activities may keep the edge pressure below a certain level needed to induce a large ELM.

Two reciprocating probe systems, at the same poloidal position at the top of the JET torus but toroidally separated by 180°, have been used to measure parallel flow in the scrape-off layer (SOL) of lower single-null, diverted plasmas. One system uses the entrance slit plates of a retarding field analyser to record upstream and downstream flux densities, whilst the second employs two pins of a nine-pin turbulent transport probe. Measurements have been made for both forward and reversed toroidal field directions. The results from both probe systems are similar.

In the forward field direction, that is with the ion drift direction downwards towards the divertor, a strong parallel flow is measured at the top of the machine in the direction from the outer to the inner divertor. The flow generally has a low value, Mach number M ∼ 0.2, close to the separatrix, but rises in the region of high magnetic shear close to the separatrix to a maximum of M ∼ 0.5 some 20 mm outside the separatrix. In contrast, for a reversed field, the measured flow is small (close to zero) throughout much of the SOL but rises near the separatrix to a value equal in both magnitude and direction to that observed in the forward field. There is thus some symmetry in the flow with respect to field reversal but with a symmetry axis given by a positive offset of around M ∼ 0.2. This paper presents simulations using the EDGE2D/Nimbus code, which predicts very low values of parallel flow Mach number near the probe position. The possibility of impurities released from the probe surfaces increasing the flow velocity is explored using the code.

Rapid changes in the loading resistance of fast wave antennas can limit high power operations of heating systems in the ion cyclotron range of frequencies (ICRF). Although novel matching techniques are being developed to reduce their effects, understanding the physics involved in these variations is of interest to guide and facilitate the design effort. We have studied the dependence of the loading resistance upon plasma parameters for the three ICRF antennas in the Alcator C-Mod tokamak. In contrast with similar studies in JET and Tore Supra, the evanescent decay term was not found to play an important role. The dominant variations could be related to changes in the shape of the electron density profiles in the propagating region. In H-mode, the loading resistance decreases as the density at the top of the pedestal is increased, and increases for higher scrape-off-layer (SOL) densities. This dependence on global plasma parameters is generally identical for the three antennas, up to a proportionality constant, while local changes in front of an individual antennas could explain the residual discrepancy. To link the observations with theory, the surface impedance at the Faraday shield was calculated by solving the wave equation in a slab geometry using experimental radial density profiles. This approach leads to a good agreement with measurements over a wide range of operating conditions in L-mode, ELM-free and EDA H-mode plasmas, and it can be interpreted qualitatively in terms of impedance transformation in the SOL region. Implications for more complex modelling approaches are also discussed.

A thorough investigation of confinement in Frascati Tokamak Upgrade has been carried out on a new database of ohmic, L-mode and advanced scenario discharges (multiple pellet-fuelled, radiation improved and internal transport barriers (ITBs)) obtained with the available auxiliary heating systems, namely electron cyclotron resonant heating, lower hybrid and ion Bernstein wave. A general agreement of the measured τ_E with ITER97 L-mode scaling is found in ohmic and L-mode discharges. An improvement of the energy confinement time (τ_E) of up to about 60% over the ITER97 L-mode scaling has been obtained in ITB discharges, together with a reduction in local electron transport in the region of high pressure gradient, and up to about 30% in pellet-fuelled discharges (where τ_E as large as ∼120 ms have been reached). The linear density dependence of τ_E in ohmic discharges has been found to extend above the saturation density threshold in pellet-fuelled plasmas.

The growing effort in inertial confinement fusion (ICF) research, with the upcoming new MJ class laser facilities, NIF in USA and LMJ in France, and the upgraded MJ z-pinch ZR facility in the USA, makes the appearance of this book by Atzeni and Meyer-ter-Vehn very timely. This book is an excellent introduction for graduate or masters level students and for researchers just entering the field. It is written in a very pedagogical way with great attention to the basic understanding of the physical processes involved. The book should also be very useful to researchers already working in the field as a reference containing many key formulas from different relevant branches of physics; experimentalists will especially appreciate the presence of `ready-to-use' numerical formulas written in convenient practical units.

The book starts with a discussion of thermonuclear reactions and conditions required to achieve high gain in ICF targets, emphasizing the importance of high compression of the D-T fuel, and compares the magnetic confinement fusion and inertial confinement fusion approaches. The next few chapters discuss in detail the basic concepts of ICF: the hydrodynamics of a spherically imploding capsule, ignition and energy gain. This is followed by a thorough discussion of the physics of thermal waves, ablative drive and hydrodynamic instabilities, with primary focus on the Rayleigh--Taylor instability. The book also contains very useful chapters discussing the properties of hot dense matter (ionization balance, equation of state and opacity) and the interaction of laser and energetic ion beams with plasma.

The book is based on and reflects the research interests of the authors and, more generally, the European activity in this area. This could explain why, in my opinion, some topics are covered in less detail than they deserve, e.g. the chapter on hohlraum physics is too brief. On the other hand, the appearance in the book of an interesting chapter on the concept of fast ignition is also a reflection of the research interests of the authors. Altogether, the book is very well written, contains a wealth of useful information and the reviewer highly recommends it to the interested reader.