Table of contents

Results of experiments investigating the performance of the JET Mark IIA divertor are reported and compared with the performance of its Mark I predecessor. The principal effect of reducing the divertor width (increasing closure) was to increase pumping for both deuterium and impurities while reducing upstream neutral pressure. Neither the orientation of the divertor target relative to the divertor plasma nor the width of the divertor had a major influence on core plasma performance in ELMy H modes. Changing the core triangularity and thus the edge magnetic shear modifies the ELM frequency in ELMy H mode plasmas, thereby changing the peak divertor power loading. The integrated performance of the core and divertor plasmas is reviewed with a view to extrapolation to the requirements of ITER. The confinement of JET ELMy H modes with hot, medium density edges is good (H₉₇ ≈ 1) and follows a gyro-Bohm scaling. The impurity content of these discharges is low and within the ITER requirements. When an attempt is made to raise the density with deuterium gas fuelling, the ELM frequency increases and the confinement, especially in the edge, decreases. Good confinement can be achieved in JET either by producing a large edge pedestal, typically in discharges with NB heating or by centrally peaked heating with ICRH schemes. Large amplitude type I ELMs, which are present in all discharges with a large edge pedestal, would result in unacceptable divertor plate erosion when scaled to ITER. Since the power deposition profile due to α heating in ITER is calculated to be intermediate between the JET NB and RF heating profiles, it is likely that operation in ITER with small ELMs in order to reduce first wall loading will result in degraded confinement compared with present day scaling laws.

In current large tokamaks, non-intrinsic seeded impurities have been used to produce divertor power loads which would be considered acceptable when extrapolated to ITER. Many devices have achieved the goals of high fractional radiated powers, small frequent ELMs and detachment which are characteristic of radiative H mode regimes. The influence of divertor geometry on these characteristics is described. It has been a matter of concern that the Z_eff associated with the seeded impurities may exceed that allowable in ITER and also that the degradation in energy confinement may be unacceptable. Confidence can only be built in the prediction of these parameters in ITER if reliable scalings are available for impurity content and energy confinement which have a sound physics basis. Work is described at JET in this area whilst using multimachine data to characterize the size scaling and provide a context for the JET data. Predicted levels for the impurity content of seeded ITER plasmas appear to be of marginal acceptability. Discharges run in the JET Mark I, Mark IIA and Mark IIAP divertors are compared and indicate that increased divertor closure has brought relatively minor benefits in highly radiative discharges. The acceptability of the energy confinement of radiation for ITER remains unclear. Dimensionless parameter scaling experiments have been conducted in which β, q₂₅, fractional radiated power and Z_eff are held constant for a range of ρ_*. The price paid for high edge radiation and small ELMs appears to be a 25% loss in total stored energy as a result of edge pedestal degradation. However, the underlying energy confinement scaling may still be consistent with gyro-Bohm scaling, which would give an adequate margin for ITER. This conclusion is, however, sensitive to the scaling of confinement with collisionality, which is difficult to determine due to the coupling between ρ_* and ν_* which is a consequence of radiation dominated regimes.

Impurity production and plasma impurity concentrations are surveyed over a wide range of operating conditions but with emphasis on ELMy H mode operation using NB heating. The Mark I divertor campaigns with carbon and beryllium targets are compared with the Mark II campaign, which had carbon targets and a more closed divertor geometry. The beryllium target campaign has a significantly lower central impurity concentration than that with carbon targets, although the Z_eff value is still dominated by carbon. The divertor geometry appears to have little effect on the central impurity level, but it is found that Z_eff and the carbon and nickel concentrations are correlated with the ELM frequency, with maxima at a frequency of around 5-15 Hz. This occurs whether the ELM frequency is varied by changing the plasma triangularity or the input power, or by increasing the fuelling rate. The absolute impurity influxes from the inner and outer targets and from the inner wall have been estimated from the C II and C III line intensities. These fluxes are compared with the central carbon concentration, and an attempt is made to correlate the influxes and the concentrations in the confined plasma, both in their time evolution and in their absolute values.

The Wigner distribution is introduced as an appropriate tool for processing data from fusion plasma diagnostics whose signals have a time varying frequency spectrum, and is thus presented as a particularly suited form for the time-frequency analysis of transients and other non-stationary phenomena. Its effectiveness is illustrated by applying it to the problem of electron density profile measurement through broadband microwave reflectometry in fusion devices, its advantages being demonstrated via a detailed comparison with the well known short time Fourier transform spectrogram. In particular, the Wigner distribution is used in a novel application as a means not only to retrieve from reflectometry data the instantaneous frequency needed for profile inversion, but also to provide an accurate representation of reflectometry signals in the time-frequency plane. Further, in a careful discussion stressing its benefits relative to more standard approaches based on the quadrature signal, the analytic signal is proposed as being appropriate to routinely extract from reflectometry data an unambiguously defined set of instantaneous amplitude, phase and frequency. The problems associated with the fact that digitized reflectometry data give rise to discrete time signals are properly handled, and the different estimates for the instantaneous frequency obtained from the Wigner distribution and the spectrogram by calculating their first frequency moments and mean frequencies, as well as by locating their peaks, are also compared.

The aim of these investigations is the description of the power flux on roof-like limiters near the LCFS. Here, the specific subject is the toroidal pump limiter ALT-II of TEXTOR-94. Similar to other plasma facing objects, the surface of ALT-II is shaped to enhance the plasma wetted area; this is achieved by reducing the angle of incidence of the magnetic field to less than 1° for the first 10 mm of the SOL. This small angle of incidence enhances all effects of toroidal non-uniformity as given, for example, by magnetic field ripple. Extensive modelling explains well the observed heating pattern on the limiter surface due to the ripple effect. In contrast to the expectations from density and temperature distributions in the SOL and at the edge of the confined region, an excessive power density is deposited on the first few millimetres near the tip of the limiter roof. Physical effects which could cause this phenomenon are discussed.

The fundamental properties of the anomalous inward drift and the width of the steep gradient zone in H mode plasmas are explored. A special version of the 1.5-D BALDUR transport code is used to determine the profiles of the electron heat diffusivity and v_in/D by transport analysis. The strong rise with radius of v_in/D in the edge region is explained by a linear dependence on the neutral deuterium density n₀, resulting in a new scaling expression v_in(x)/D(x) = F₀Z_eff(x)n₀(x)2x/(ρ_wx_s²). Applying this in simulations reproduces the empirical fit of the v_in/D profile not only in the edge plasma but also in the bulk plasma. Modellings with this scaling yield the observed flattening of density profiles with rising line averaged density. The decreasing penetration of deuterium atoms to the core causes a decline of the inward drift. The new scaling is shown to be compatible with gas oscillation experiments, while n₀-independent scalings are not. This further explains the strong density profile peaking and rise of v_in/D during and after pellet injection by the increase in neutral density. The width of the steep gradient zone is found to be connected with the penetration of neutrals at the edge and the presence of high inward drift velocities. The anomalous inward drift is attributed to ion dynamics, i.e. to the friction between fluctuating deuterons and deuterium atoms diffusing inward. A more general v_in/D scaling including impurity effects is presented.

The theory of tearing mode stabilization in toroidal plasmas by RF driven currents that are modulated in phase with the island rotation is investigated. A time-scale analysis of the phenomena involved indicates that transient effects, such as finite time response of the driven currents, island rotation during the power pulses and the inductive response of the plasma, are intrinsically important. A dynamical model of such effects is developed, based on a 3-D Fokker-Planck code coupled to both the electric field diffusion equation and the island evolution equation. Extensive applications to both ECCD and LHCD in ITER are presented.

The early phase of the neoclassical (3,2) + (2,2) tearing mode at ASDEX Upgrade has been investigated in order to study the seed island. Sawtooth crashes or fishbones can trigger this mode, and in few cases it appeared spontaneously.

The perturbations in ion temperature, density and parallel velocity resulting from sawtooth disruptions in TFTR are measured with a novel diagnostic. The local ion thermal diffusivity, particle diffusivity and parallel momentum diffusivity are determined in a high power discharge at r/a = 0.64 by fitting the observed pulses to a simple model of the radial diffusive propagation of heat, particles and momentum caused by the crash. The incremental ion thermal diffusivity, χ_i^inc, is found to be similar in amplitude to the ion and electron thermal diffusivities obtained from a steady state 1-D power balance analysis, and the particle and parallel momentum diffusivities are found to be an order of magnitude smaller than χ_i^inc.