The theoretical and experimental development of stellarators has removed some of the specific deficiencies of this configuration, viz., the limitations in β, the high neoclassical transport, and the low collisionless confinement of α particles. These optimized stellarators can best be realized with a modular coil system. The W7-AS experiment [Plasma Phys. Controlled Fusion 31, 1579 (1989)] has successfully demonstrated two aspects of advanced stellarators, the improved equilibrium and the modular coil concept. Stellarator optimization will much more viably be demonstrated by W7-X [Plasma Physics and Controlled Fusion Research, Proceedings of the 12th International Conference, Nice, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 369], the successor experiment presently under design. Optimized stellarators seem to offer an independent reactor option. In addition, they supplement, in a unique form, the toroidal confinement fusion program, e.g., energy transport is anomalous in stellarators too, but possibly more easily understandable in the frame of existing theoretical concepts than in tokamaks.

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In order to support the operation of ITER and the planned experimental programme an extensive set of plasma and first wall measurements will be required. The number and type of required measurements will be similar to those made on the present-day large tokamaks while the specification of the measurements-time and spatial resolutions, etc-will in some cases be more stringent. Many of the measurements will be used in the real time control of the plasma driving a requirement for very high reliability in the systems (diagnostics) that provide the measurements.The implementation of diagnostic systems on ITER is a substantial challenge. Because of the harsh environment (high levels of neutron and gamma fluxes, neutron heating, particle bombardment) diagnostic system selection and design has to cope with a range of phenomena not previously encountered in diagnostic design. Extensive design and R&D is needed to prepare the systems. In some cases the environmental difficulties are so severe that new diagnostic techniques are required.The starting point in the development of diagnostics for ITER is to define the measurement requirements and develop their justification. It is necessary to include all the plasma parameters needed to support the basic and advanced operation (including active control) of the device, machine protection and also those needed to support the physics programme. Once the requirements are defined, the appropriate (combination of) diagnostic techniques can be selected and their implementation onto the tokamak can be developed. The selected list of diagnostics is an important guideline for identifying dedicated research and development needs in the area of ITER diagnostics.This paper gives a comprehensive overview of recent progress in the field of ITER diagnostics with emphasis on the implementation issues. After a discussion of the measurement requirements for plasma parameters in ITER and their justifications, recent progress in the field of diagnostics to measure a selected set of plasma parameters is presented. The integration of the various diagnostic systems onto the ITER tokamak is described. Generic research and development in the field of irradiation effects on materials and environmental effects on first mirrors are briefly presented. The paper ends with an assessment of the measurement capability for ITER and a forward of what will be gained from operation of the various diagnostic systems on ITER in preparation for the machines that will follow ITER. Performance assessment relative to requirements Design meets requirements S339 A.J.H. Donné et alPhysics Basis [7] and remains essentially the same. However, for ITER, the specific limits have changed. 2.1.2.Measurements needed for plasma control and evaluation. The measurements needed for plasma control and evaluation are naturally directly linked to the experimental programme, and particularly to the operating phase (i.e. H, D or D/T) and the operating scenario (H-mode, hybrid, etc). Since there is expected to be a phased introduction of po...

We show that the modulational instability growth rate of zonal flows is determined directly from the quasilinear wave kinetic equation. We also demonstrate the relation between zonal-flow growth and the cross bispectrum of the high-frequency drift-wave-driven Reynolds stress and the low-frequency plasma potential by explicit calculation. Experimental measurements of the spatiotemporal evolution of the spectrum integrated bicoherence at the L-->H transition near the edge shear layer indicate a modification in the nonlinear phase coupling, which might be linked to the generation of sheared ExB flows.

The three-dimensional magnetic configuration of a stellarator offers two specific mechanisms for a transition to improved particle and energy confinement. One route goes through the so-called electron-root confinement regime, which leads to a reduction of neoclassical transport via strong radial electric fields. In this Letter evidence for a second route is presented. It opens due to the layer of a strongly varying radial electric field which is present in the transitional region from neoclassical electron to ion-root confinement. This type of improvement acts on turbulent transport.

Kinetic effects are important in low-density high-power ECRH discharges, and the electron distribution function can significantly deviate from Maxwellian. The ECRH power deposition is analysed for perpendicular on-axis heating in W7-AS, with different magnetic configurations characterized by either a minimum or a maximum of B on the plasma axis in the RF injection plane. The different heating scenarios are modelled by means of a new bounceaveraged Fokker-Planck code, well suited for the magnetic field geometry close to the plasma axis of W7-AS.The power deposition profile is estimated from the analysis of heat wave propagation stimulated by ECRH power modulation. In general, peaked deposition profiles as predicted from a ray-tracing code are obtained, but with an additional much broader contribution.The broadening of the thermal power deposition profile is assumed to be related to the radial transport by the ∇B drift of locally trapped suprathermal electrons. This is simulated by means of a simple convective Fokker-Planck model. The theoretical predictions are shown to be consistent with the experimental findings.Kinetic effects on the determination of the temperature both by Thomson scattering and by ECE diagnostics are briefly discussed.

The turbulent heat transport in high-temperature fusion plasmas is not understood. It increases with heating power. The physical mechanism of this process is investigated in the W7-AS stellarator. The fundamental question addressed here is whether transport is governed by local plasma parameters or by a global quantity. Three different types of experiments with critical sensitivity are carried out. They can consistently be described on the basis of a non-local dependence of the transport coefficient on the global heating power.

The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.