Table of contents

This paper presents an overview of results of the 1994/95 experimental campaign on JET with the new pumped divertor and draws implications for ITER in the areas of detached and radiative divertor plasmas, the use of beryllium as a divertor target tile material, the confinement properties of discharges with the same dimensionless parameters (except for the dimensionless Larmor radius) as ITER and the effect of varying the toroidal magnetic field ripple in the ITER relevant range. Discharges with high fusion performance at high current, in steady-state with ELMs and in the ELM-free hot-ion H-mode are also reported. Limits to operations are discussed and projections to D-T performance are made.

The present status and expected performance of the ITER design is presented. The `Outline Design` (July 1994) has been improved from the standpoint of manufacturing, assembly, maintenance, safety and reliability by a joint effort of the Joint Central Team and the Home Teams. Constant plasma performance and strict limitation of the expected costs were required. The resulting `Interim Design` has the following characteristics: the Toroidal Field (TF) coils are provided with casings and radial shear plates; the increased bending stiffness allows a simpler mechanical structure; the number of TF coils has been reduced from 24 to 20. The new Central Solenoid assembly, including two torsion cylinders, now has a weight of 1350 Tonnes. The vacuum vessel is made of SS 316 LN. The blanket consists of a backplate onto which 1 m*2 m large First Wall/Shield modules are mounted. The divertor has 60 cassettes (3 per sector) cooled by straight radial pipes. An improved maintenance concept allows a full divertor changeover in less than six months. The auxilliary heating has been upgraded from 50 to 100 MW to enable H-mode access and to increase the current drive capability. While the above improvements somewhat reduce the margins for ignition (sustained), beta, and burn duration, the physics performance of the Interim Design and of the Outline Design are comparable allowing all ITER goals defined by the Special Working Group 1 (SWG 1) to be achieved.

The completely detached high confinement (CDH) regime established recently in high power ASDEX-Upgrade single null divertor discharges is described. The standard CDH mode scenario with feedback controlled external deuterium and neon puffing is analysed in detail with respect to its divertor detachment and confinement properties. Impurity transport and pumping effects and the density operation windows of the CDH mode are outlined. First results with other impurity radiators (N,Ar) are given. Finally the CDH mode operation space is discussed in terms of main chamber and scrape-off layer plus divertor radiation and possible consequences for future fusion experiments are outlined.

A view of the latest experimental results and progress in the characterization and understanding of edge turbulence is given. The development of new fluctuation analysis techniques has provided a new bridge between experimental measurements and theoretical turbulence models. At the same time, the characterization of fluctuation and turbulent transport has stimulated new experimental developments in the measurement of electron temperature fluctuations; it has been shown that substantial levels of electron temperature fluctuations are present in the boundary region of magnetically confined plasmas. Several driving (curvature, atomic physics, . . .) and damping (electric field) mechanisms of the edge turbulence have been investigated. The development of methods to modify and control plasma turbulence and confinement appears as a very promising area of research. Recent results emphasize the importance of comparative studies between the structure of the turbulence in toroidal confinement systems to test critically edge turbulence models.

Routine tritium operation in TFTR has permitted investigations of alpha particle physics in parameter ranges resembling those of a reactor core. ICRF wave physics in a DT plasma and the influence of isotopic mass on supershot confinement have also been studied. Continued progress has been made in optimizing fusion power production in TFTR, using extended machine capability and Li wall conditioning. Performance is currently limited by MHD stability. A new reversed magnetic shear regime is being investigated with reduced core transport and a higher predicted stability limit.

Atomic beam supported emission spectroscopy is well suited to satisfy the needs of edge diagnostics; in particular, it offers high space- and time resolution and causes almost no perturbation of the plasma. Making use of the species Li, He and H. atomic beams have been developed, in different velocity ranges, by the help of which the most important plasma parameters can be measured. The principles for the measurement of the electron density and temperature profiles of the ion temperature profile and of the radial profiles of highly ionized impurities are explained and the atom sources used at TEXTOR are described. Recent results obtained on TEXTOR for various plasma parameters are presented and critically discussed. We are convinced that plasma turbulence and the two-dimensional character of the plasma edge will be important challenges for the future. Atomic beams may offer solutions only if sufficient efforts are invested into their technical development.

The quasi-helical stellarator, which was invented by Nuhrenberg and Zille in 1988, is a toroidal fusion concept that has the excellent alpha confinement of the axisymmetric tokamak without the disadvantages of major disruptions and current drive. Quasi-helical stellarators demonstrate the flexibility that exists in the tokamak and related toroidal configurations to circumvent critical problems in obtaining fusion power.

Normally the transport observed in tokamak experiments greatly exceeds that of collisional transport theory and this anomalous transport is usually attributed to turbulent fluctuations arising from various micro-instabilities, eg drift waves or pressure gradient driven ballooning modes. However, no theoretical model has yet been proposed which adequately describes all of the many fascinating features tokamak transport displays. In this paper the experimental observations that a transport model would need to satisfy are reviewed. Then the theoretical concepts and tools available for constructing models are outlined. The principal models that have emerged from these considerations are described and the results of detailed comparison with a number of experiments around the world are discussed. Finally the present situation regarding transport modelling is summarised.

Disruptions and related vertical displacement events pose a major problem to the design and operation of future tokamak reactors. The cause and dynamics of disruptions will be described for the many different scenarios that these violent events can follow. Possibilities will be discussed to avoid or at least to ameliorate the damaging effects of disruptions.

The upgraded NB heating power on W7-AS allowed extension of the accessible parameter space. In first experiments, a remarkable increase of the ion temperatures was obtained. The results are discussed on the basis of a detailed transport analysis which confirms the neoclassical nature of the ion transport up to the 1 mfp regime. The maximum NB heating power was applied to investigate beta limits. Although beta values close to the predicted stability limit could be reached, no clear indication of a limit has been found so far. Several experimental observations on W7-AS point to the necessity of a divertor. The feasibility of a divertor making use of the natural islands in an optimized stellarator was investigated, and a possible solution for an island divertor on W7-AS is outlined.

The main physical principles of quasi-stationary plasma accelerators, which are capable to generate super high power plasma streams in long pulses are discussed. The description of QSPA type devices in use in recent experiments, as well as characteristics of the plasma flow in the main acceleration channel and plasma parameters achieved at the outlet of QSPA, are presented here too. Finally, some possibilities of such accelerator applications for fusion and results of their use in divertor plates modeling experiments are described.

DIII-D currently operates with a single- or double-null open divertor and graphite walls. Active particle control with a divertor cryopump has demonstrated density control, efficient helium exhaust, and reduction of the inventory of particles in the wall. Gas puffing of D₂ and impurities has demonstrated reduction of the peak divertor heat flux by factors of 3-5 by radiation. A combination of active cryopumping and feedback-controlled D₂ gas puffing has produced similar divertor heat flux reduction with density control. Experiments with neon puffing have shown that the radiation is equally-divided between a localized zone near the X-point and a mantle around the plasma core. The density in these experiments has also been controlled with cryopumping. These experimental results combined with modeling were used to develop the new radiative divertor for DIII-D. This is a double-null slot divertor with four cryopumps to provide particle control and neutral shielding for high-triangularity advanced tokamak discharges UEDGE and DEGAS simulations, benchmarked to experimental data, have been used to optimize the design.

In Tore Supra, plasma particle exhaust has been studied by means of modular pump limiters, with either throats (ion collection) or vented structures (neutral collection), and with an ergodic divertor. Detailed study of active plasma density control has been performed in these three configurations for ohmic conditions.

The unique flexibility of TCV for the creation of a wide variety of plasma shapes has been exploited to address some aspects of tokamak physics for which the shape may play an important role. The electron energy confinement time in limited ohmic L-mode plasmas whose elongation and triangularity have been varied ( kappa =1.3-1.9, delta =0.1-0.7) has been observed to improve with elongation as kappa ^0.5 but to degrade with triangularity as (1-0.8 delta ), for fixed safety factor. Ohmic H-modes have been obtained in several diverted and limited configurations, with some of the diverted discharges featuring large ELMs whose effects on the global confinement have been quantified. These effects depend on the configuration: in double null (DN) equilibria, a single ELM expels on average 2%, 6% and 2.5% of the particle, impurity and thermal energy content respectively, whilst in single null (SN) configurations, the corresponding numbers are 3.5%, 7% and 9%, indicative of larger ELM effects. The presence or absence of large ELMs in DN discharges has been actively controlled in a single discharge by alternately forcing one or other of the two X-points to lie on the separatrix, permitting stationary density and impurity content (Z_eff=1.6) in long H-modes (1.5 s).

Experimental data and analysis are described which are derived from the study of plasma detachment in the JET Mark I divertor. The paper covers the comparison of carbon and beryllium targets in this regime, the effect of geometry, the need for extrinsic impurities for detachment at high power and the influence of these impurities on plasma purity and confinement.

The paper gives a survey on recent experimental and theoretical investigations on the tokamak TEXTOR concerning the influence of impurities on the plasma behaviour. A broad spectrum of diagnostics allows the measurements of the spatial distributions of densities, temperatures and radiation of impurity and background particles. The plasma density critical for development of the radiative instability observed in the core of ohmically heated plasmas in discharges with high-Z limiters is estimated on the basis of a simple analytical model. The results of modelling by the code RITM of plasmas polluted by low-Z impurities are presented. An interpretation of different stability of OH and additionally heated plasmas to detachment, of improvement of energy confinement with neon seeding is proposed. The importance of impurities in the plasma dynamics in the vicinity of strong sources of impurities through their influence on the electric field is discussed.

Experimental results of high I_p H-mode plasma, high triangularity plasma and radiative divertor are presented from the experimental campaign in 1995. H-mode operation was extended to I_p=4.5 MA. The highest level of stored energy of W~8 MJ was obtained with I_p=3.5-4 MA. The initial experiment of high triangularity plasma was carried out with I_p<or=0.9 MA and 0.3<or= delta <or=0.55. In high triangularity plasma, beta _N limit was increased by 30% and the highest record value of beta _T=2.7% was obtained. With P_NB=23 MW and alternate Neon and D₂ puffing, 50% of the input power was radiated while the good core plasma performance was maintained in I_p=1.2 MA and B_T=2 T, ELMy H-mode plasma.

Electron cyclotron current drive (ECCD) on the 1st and 2nd harmonics of the electron cyclotron resonance (ECR) is considered. Mainly the T-10 experimental results are described and comparison with the experimental data from various devices is performed. Features of ECCD are studied by comparison of Fokker-Planck (FP) calculations with measurements of X-ray spectra in discharges with ECCD. It is shown that ECCD efficiency is in a good agreement with FP calculations. Database obtained on various devices allows to assume that in ITER the ECCD efficiency can be achieved as high as eta _CD=0.3*10²⁰A/Wm².

A discussion of linear dynamics and nonlinear evolution of modes belonging to the shear Alfven branch is presented. Attention is focused to the modes which exist near the gap of the continuous spectrum. A discussion of the relative importance of mode-mode coupling, E*B trapping and stochastic losses is also presented.

The experiments involving the interaction of radio-frequency heating with fast particles injected by neutral beams on TEXTOR are reviewed. The focus point of the present study is the comparison between experiments and theory. Theory involves the modelling of RF diffusion in velocity space, absorption of radio-frequency waves by non-maxwellian populations, computation of drift trajectories to take trapping and first-orbit losses into account. A more specific theoretical development used in this context is the reconstruction of charge exchange spectroscopy spectra. Three types of experiments are examined. The synergetic increase of beam current drive by radio-frequency power, the large neutron production in third harmonic heating of D beam ions and the control of central fast helium concentration by radio-frequency induced diffusion.

Two important effects of MHD fluctuations in the RFP and tokamak are current generation (the dynamo effect) and mode locking. In the T1 and MST RFP experiments new results reveal the mode dynamics underlying these phenomena. In T1 the effect of specific magnetic Fourier modes on the current density profile is evident. In MST, the MHD dynamo term ( delta v* delta B) is measured in the plasma edge, and found to account for the time dependence of the edge current throughout a sawtooth cycle. As edge resistivity is increased in T1 the fluctuation amplitude increases to maintain the dynamo-driven current, as expected from MHD computation. The modes responsible for the dynamo often lock to the local magnetic field error at the vertical cut in MST. The plasma rotation velocity has been measured with a fast Doppler spectrometer to a time resolution of 1 mu s. The plasma rotation and mode phase velocity are remarkably well-correlated, with both slowing, in the presence of an impulsive field error, in a 100 mu s timescale.

The MHD activity giving rise to the beta - and the density limit in ASDEX Upgrade is analyzed. A detailed description of the MHD phenomena occuring prior to and during disruptions is given. The MHD characteristics of the different ELM types occurring in ASDEX Upgrade are described.

Neutral beam systems for the next generation of magnetic fusion devices will be based on negative ions. Developments are progressing steadily, and large negative ion-based systems are under preparation for JT60-U and LHD, and are being considered for ITER. An overview of the physics of the production, acceleration and neutralization of large negative ion beams is given. The present state of the art in R&D is also surveyed.

The injection of pellets into JET sometimes leaves a resonant localised structure commonly known as a snake. Snakes constitute a remarkable phenomenon, having both an intrinsic interest and a relevance to understanding transport. How are snakes formed? What maintains the magnetic island created by the snake? How does the confined density persist? And finally, why don`t such structures arise spontaneously?.

The radar enhanced scattering (RES) diagnostic is shown to be a feasible tool for studying small-scale plasma fluctuations and waves. This method is based on the effect that an RF wave launched into the plasma is considerably slowed down near the hybrid resonance so that the backscattered signal is received with a large time delay. Fairly good spatial and wavenumber resolution of this diagnostic are demonstrated in various laboratory plasmas.

The capability of the Mk. I JET pumped divertor to give access to high performance plasma regimes has been successfully demonstrated. The considerable flexibility afforded by the divertor coil configuration and plasma position control system, together with operation of the torus cryo-pump, have facilitated a very extensive programme of experiments to determine optimum scenarios for maximising the total neutron emission from D-D reactions, fusion product n_D(0) tau _ET_i(0) and equivalent Q_DT. Progress towards sustainable steady-state conditions at high fusion performance is also discussed.