Table of contents

The final success of the ITER project will be determined decisively by the appropriate combination of plasma heating and current drive systems to be chosen in the near future. Motivated by the remarkable progress in the heating area, but also in view of still existing major issues with respect to the requirements of ITER, this paper tries to give a fair assessment of the status and perspectives of the four H&CD systems presently under discussion: electron cyclotron waves (ECW), ion cyclotron waves (ICW), lower hybrid waves (LH) and neutral beam injection (NBI). Following a pertaining analysis of an EU ad hoc group it can be concluded that all systems, in spite of the remarkable achievements at the various fusion experiments, will not meet all ITER requirements and need further development and investigations in all four cases. Particular needs or even severe problems arise from both the more stringent technical constraints (e.g. reduced area available for H&CD installation, larger plasma distance of H&CD components, radiological implications, etc) and the increasing need to also explore steady-state operation eventually in `advanced scenarios' in ITER. Resulting H&CD issues or problems will be addressed, such as not sufficiently qualified RF sources, problems in connection with the plasma-facing RF-coupling structures, or the still unsatisfactory status of negative ion technology in the case of NBI. Stationary current drive (with prescribed radial profile and at acceptable efficiency) and the related various control functions seem the greatest challenge for `advanced' ITER operation and further steps beyond it.

Nuclear fusion research has made considerable progress using tokamaks, and detailed design for a reactor-sized tokamak, ITER, is now under way. On the other hand, the stellarator has been considered to be alternative to the tokamak and there has been considerable progress recently. Plasma heating has been the most important theme of investigation in stellarator research as well as in tokamak research. This paper refers to a few key aspects of stellarator plasma heating in the existing experiments CHS and W7-AS. Efforts to understand the physical meaning of the experimental results in terms of neoclassical theory are also introduced. Throughout this review, the particular importance of particle orbits will be emphasized and the performance of plasma heating in the coming next-generation devices such as LHD and W7-X will be estimated according to the model employed.

Recent progress in the area of ICRF physics is examined and summarized. A very brief summary of simplified theoretical predictions is given. Illustrative examples of experimental results in the area of ICRF heating and fast-wave current drive are given. ICRF heating is found to be highly effective in heating plasmas to high temperatures in modern tokamaks. Fast-wave current drive has been demonstrated in several experiments and the results are in good agreement with theory. Mode-conversion heating and current drive should be effective for profile control, but experimental results are still preliminary.

A review of the two key technologies for the electron cyclotron wave (ECW) system, the gyrotron source and the window, is presented. In both fields, significant progress has been made to meet the requirements of the heating and current drive systems planned for devices such as W7-X or ITER, namely 1 MW-CW (continuous wave) in the frequency range 140-170 GHz. Gyrotrons in this performance range have been tested in long pulse operation (many seconds). With the use of a diamond window, extremely long pulse or true CW operation is possible. Tests presently underway confirm such a promising behaviour of the window, paving the way towards the development of megawatt CW tubes.

In this paper lower hybrid current drive (LHCD) experimental milestones paving the way for future experiments are briefly summarized. The current drive efficiency scaling with the electron temperature is discussed. The role of wave propagation in determining the power deposition profile is stressed and methods are discussed to control the current density profile. Modelling of negative central shear configurations, experimentally obtained by LHCD, are reported. A good agreement is found between the modelling results and the experimental findings, thus showing that a good degree of understanding has been achieved in LHCD theory.

Research in basic plasma physics and in controlled thermonuclear fusion are briefly compared. The development of fusion from the 1958 Geneva conference to present machines is sketched and intervening factors are analysed together with the interplay between smaller and larger thermonuclear machines.

The importance of close ties between universities and fusion laboratories is underlined and the need for regrouping some of the latter, as machine size increases, is examined.

The daunting size of the energy needs of mankind around 2050 as compared with supply and the threats of global warming demonstrate the crucial importance of constructing ITER, the international experimental reactor, very soon now that the necessary physical data have been obtained. It is our deep moral obligation to convince the public at large of the enormous promise and urgency of controlled thermonuclear fusion as a safe, environmentally friendly and inexhaustible energy source.

Recent experiments in D-T plasmas on the JET and TFTR tokamaks have evaluated a wide range of ITER relevant ion cyclotron heating scenarios. Absorption of fast waves at the second-harmonic tritium resonance has provided bulk ion heating in TFTR supershots and electron heating in JET H-mode discharges. In JET, deuterium minority heating has generated 1.7 MW of fusion power with 6 MW of radio frequency power giving a record steady-state Q-value of 0.22. Strong bulk ion heating has been achieved with He³ minority heating with central ion temperatures up to 13 keV being produced in H-modes with a density of 3.6 × 10¹⁹ m^-3. Hydrogen, deuterium and He³ minority heating methods have produced plasmas with normalized confinement times greater than or equal to that required by ITER for ignition. These H-modes are characterized by small-amplitude, high-frequency ELMs, each of which transports less than 1.5% of the plasma energy content to the limiters. The heavy minority scheme of tritium in a deuterium plasma has been demonstrated both as a heating scheme and a generator of suprathermal neutrons. On TFTR mode conversion to an ion Bernstein wave has achieved central bulk ion heating in supershots with target ion temperatures greater than 20 keV.

ITER will require ∼100 MW of auxiliary power for plasma heating, current drive and plasma kinetic control functions. As in most of the present-day tokamaks, these functions will be shared among different heating methods. A (negative-ion) neutral beam injection (NBI) and three RF heating and current drive (H&CD) systems - ion cyclotron (ICRF), electron cyclotron (ECRF) and lower hybrid (LBW) - are under conceptual development, the latter being developed by the European Home Team.

The three RF H&CD Systems, described in this paper, offer together a range of features and services capable of satisfying all ITER requirements. A standardized design approach is adopted for the systems: all launchers are integrated in ITER equatorial ports, feature an all-metal construction, have a very limited impact on the torus layout, and do not significantly modify vacuum and tritium boundaries. They are removable through the port with similar manual and remote handling operations. A discussion of the RF systems design and of the associated R&D programmes is presented.

A review of the present status of electron cyclotron heating and current drive experiments in toroidal fusion devices is presented. In addition to basic heating and current drive studies the review also addresses advances in wave physics and the application of electron cyclotron waves for instability control, transport studies, pre-ionization/start-up assist, etc. A comprehensive overview is given with particular emphasis on recent advances since the major review of Erckmann and Gasparino (1994) ( 36 1869), including results from the latest generation of high-power, high-frequency experiments.

A free-electron maser (FEM) has been built as a pilot experiment for a millimetre-wave source for applications on future fusion research devices such as ITER, the International Tokamak Experimental Reactor. A unique feature of the Dutch fusion FEM is the possibility to tune the frequency over the entire range from 130 to 260 GHz at an output power exceeding 1 MW. In the first phase of the project, the so-called inverse set-up is used. The electron gun is mounted inside the high-voltage terminal. The entire beam line was tested successfully with extremely low loss current, lower than 0.05%. The first generation of millimetre waves was achieved in October 1997. The highest peak power measured so far is 700 kW at 200 GHz. This was achieved with a beam current of 8 A and an acceleration voltage of 1.77 MV. The output power, start-up time and frequency correspond well with the simulation results. The parameter scans for the longitudinal undulator gap, acceleration voltage and reflection coefficient have given a wide range of interesting data of which a few highlights are given.

Purely non-inductive steady-state discharges in tokamak reactors will require a large bootstrap current, I_boot, in order to minimize the amounts of power in external current drive systems. In present day tokamaks, high bootstrap current discharges are produced under various conditions. It has been demonstrated that the on-axis RF heatings are good candidates for producing a large bootstrap current, especially the fast magnetosonic wave direct electron heating (FWEH), for which power deposition is centrally peaked. Moreover, the heating and current drive methods using RF waves could be used as external sources to optimize I_boot through the current profile control. In this paper, the dependence of I_boot on the profiles of the plasma density, temperature, pressure and current density is analysed using a theoretical approach with a matrix formulation. Examples of quasi-steady-state discharges with large bootstrap current fraction using FWEH on Tore Supra are given. Examples of high bootstrap current fractions on reverse shear discharges (TFTR, DIII-D, ...) are also mentioned. Purely non-inductive discharges with improved confinement by combination of FWEH and lower hybrid current drive are analysed and the extrapolation to the so-called `Tore Supra CIEL phase' is reported.

It has been shown that the core energy confinement in tokamaks can be significantly improved by modifying the plasma current profile. Plasma regimes where the transport is reduced to its neoclassical value in the centre of the plasma within a region called the internal transport barrier (ITB) can be achieved with a zero or negative magnetic shear. In this paper the role of the radio frequency (RF) methods: electron cyclotron resonance, ion cyclotron resonance and lower hybrid current drive in producing these ITBs is reviewed. Combined techniques where RF waves are used to modify the magnetic shear together with a central heating method (neutral beam injection or RF) are described. Application of RF methods to high fusion yield plasmas in JET with modified shear are presented together with a discussion on the prospects of achieving steady-state high-performance plasmas. RF techniques are already playing an important role in these `advanced scenarios' and are likely to become indispensable tools for future steady-state tokamak reactors.

This review of recent developments in radio-frequency (RF) theory focusses on the lower (ion-cyclotron) frequency range, although the methods described could be applied to other frequency ranges. The basic RF problem is to solve the wave propagation problem jointly with the evolution of the distribution functions (generally speaking non-Maxwellian) of the particles heated by the RF. We describe the two traditional paths towards such a solution, namely the quasi-uniform magnetic field approach and the Hamiltonian approach. We show that a theory based on the guiding-centre description of the unperturbed particle orbits can unify the two approaches and lead to a fully consistent description of the complete problem, the wave propagation and RF-induced velocity-space diffusion (quasi-linear) parts resting on identical assumptions. This guarantees that basic properties of the wave-particle interactions are preserved and that the power balance between waves and particles is automatically satisfied. The recent evolution of full-wave codes and their comparison is reviewed, and some recent puzzles are pointed out. Recent work on coupling codes is also briefly reviewed.

All high-power ion cyclotron range of frequency (ICRF) heating systems include devices for matching the input impedance of the antenna array to the generator output impedance. For most types of antennas used, the input impedance is strongly time dependent on timescales as rapid as 10^-4 s, while the radio frequency (RF) generators used are capable of producing full power only into a stationary load impedance. Hence, the dynamic response of the matching method is of great practical importance. In this paper, world-wide developments in this field over the past decade are reviewed. These techniques may be divided into several classes. The edge plasma parameters that determine the antenna array's input impedance may be controlled to maintain a fixed load impedance. The frequency of the RF source can be feedback controlled to compensate for changes in the edge plasma conditions, or fast variable tuning elements in the transmission line between the generator output and the antenna input connections can provide the necessary time-varying impedance transformation. In `lossy passive schemes', reflected power due to the time-varying impedance of the antenna array is diverted to a dummy load. Each of these techniques can be applied to a pre-existing antenna system. If a new antenna is to be designed, recent advances allow the antenna array to have the intrinsic property of presenting a constant load to the feeding transmission lines despite the varying load seen by each antenna in the array.

An overview is given on active and passive microwave diagnostics established on fusion experiments. The basic principles of the various techniques are briefly outlined and their limits as well as recent developments are discussed.

Large departures from standard L-mode confinement scaling laws are now being observed in most tokamaks which appear promising for relaxing the plasma current constraint and therefore open the route to high-performance steady-state operation with high bootstrap current fraction. Prospects of the weak or reversed magnetic shear scenarios for steady-state tokamak operation are presented in the light of recent experimental progress to control pressure and current profiles with the combined use of various heating and current drive schemes. Implications of the present experiments on the modelling of future non-inductive operation in existing tokamaks and in ITER are addressed.

Invited papers from the Second Europhysics Topical Conference on Radio Frequency Heating and Current Drive of Fusion Devices

The Second Europhysics Topical Conference on Radio Frequency Heating and Current Drive of Fusion Devices was held at the Koninklijk Hoger Instituut voor Defensie - Institut Royal Supérieur de Défense in Brussels, Belgium, from 20 to 23 January 1998. The conference was organized in honour of Professor P E Vandenplas at the occasion of his becoming Emeritus and his retirement as head of the Laboratory for Plasma Physics of the Royal Military Academy. It started with an academic session honouring Professor Vandenplas at the Brussels Palace of the Academies. The conference was attended by 112 participants from 19 countries. There were 72 contributed papers, 15 invited papers and a special lecture by Professor Vandenplas. Fifteen of the contributed papers were presented orally, the remaining ones as posters.

The contributed papers to the conference were published as volume 22A of the Europhysics Conference Abstract Series. The invited papers are published as this special issue of Plasma Physics and Controlled Fusion, which will be distributed to participants at the meeting and to the regular journal subscribers. Copies of the issue are also available directly from the publisher.

The papers were selected by the Conference Programme Committee, which comprised the following members:

J Jacquinot (Chairman) JET Joint Undertaking, Abingdon, UK

F De Marco ENEA, Frascati, Italy

D Moreau CEA, Cadarache, France

J-M Noterdaeme IPP, Garching, Germany

R Prater GA, San Diego, USA

F C Schueller FOM, Nieuwegein, The Netherlands

G Van Oost LPP/ERM-KMS, Brussels, Belgium

T Watari NIFS, Nagoya, Japan

G H Wolf FZJ, Juelich, Germany

The contributed papers were chosen to present an overview of the advances in the heating and in the generation of non-inductive currents in fusion plasmas by means of radio frequency methods (low frequency, IC, EC and LH&UH). Theory, experiments and technology received comparable emphasis. Special attention was further given to ICRH in D-T plasmas and to RF schemes for plasma profile control.

The invited papers contained in this issue include reviews of wider areas of work as well as specialized in-depth presentations of particular experiments and theories. It is hoped that this collection of papers will form a valuable reference collection, which will be of particular interest to subscribers to Plasma Physics and Controlled Fusion.

J Jacquinot, G Van Oost and R R Weynants Guest editors