Table of contents

Plasma fluxes to edge structures, such as limiters, probes and divertor plates, are generally assumed to be due to parallel transport only-and all limiters and divertors have been designed on the basis of this standard model. A number of experimental results have indicated substantial fluxes to surfaces which are parallel, or nearly parallel to B-a result which is inconsistent with the standard model. A new model is presented which includes direct cross-field deposition on edge structures. In the case of limiters and probes this direct deposition process is amplified by a 2D or 3D concentration, or funnelling, effect which is induced in the confined plasma, inboard of the edge structure. As a result, the plasma wetted area of large edge structures such as inner wall limiters is substantially greater than indicated by the standard model-a generally beneficial effect. For sufficiently oblique angles between B and divertor tiles, it may be that heat and particle transport to the tiles is dominated by direct, cross-field transport. This would have major implications for the exploitation of divertor configurations with very large flux expansion and large wetted area

Experimental results from the COMPASS-C tokamak reveal a sharp threshold in amplitude above which externally applied static resonant magnetic perturbations (RMPs) induce stationary magnetic islands. Such islands (in particular, m=2, n=1 islands) give rise to a significant degradation in energy and particle confinement, suppression of the sawtooth oscillation and a large change in the impurity ion toroidal velocity. The observed threshold for inducing stationary (2,1) islands is consistent with a phenomenological resistive MHD model which takes into account plasma rotation (including poloidal flow damping) and externally applied resonant fields. Broadly similar results are found for applied fields other than m=2, n=1. Other results from RMP experiments are also discussed, such as the stabilization of rotating MHD activity, stimulated disruptions and extensions to the disruptive density limit. Finally, the likely effect of field errors on large tokamaks is briefly examined in the light of the COMPASS-C results

Experimental results in the DIII-D tokamak demonstrate that discharges are more robust with regard to external nonaxisymmetric error fields as the plasma fluid rotation is increased, but more sensitive as beta is increased. In a series of discharges, the plasma fluid rotation and beta are varied by the number and type ('tangential' and/or 'perpendicular') of neutral beams. For each discharge, once rotation and plasma kinetic energy are equilibrated, an external coil is used to slowly increase the low m,n=1 resonant, static error field until a locked mode instability occurs. A larger critical error field is needed to nonlinearly induce instability in discharges spinning more rapidly. However, at higher beta, the plasmas become more unstable with increasing beta despite the faster rotation. The beta limit in tokamaks can be reduced by locked modes nonlinearly induced by modest error fields

The broadening of the injected radiofrequency (RF) line in the frequency spectra detected during the lower hybrid (LH) experiment in ASDEX is studied. The parametric decay instabilities (PDI) and the scattering due to density fluctuations are considered as possible causes of the broadening. The analysis of the parametric dispersion relation shows that parametric instabilities driven by ion sound quasi-modes have the highest growth rates, but their RF power threshold is higher than the RF power available in the experiment. This has been confirmed by the negative results of the measurements performed to detect the presence of PDI. Also, measurements of the density fluctuation behaviour during RF power injection have been made. The results obtained from observations with a microwave reflectometer and a Langmuir probe show a correlation of the fluctuations in the scrape-off plasma with the line broadening. It is concluded that the line broadening is due to scattering of the launched waves by density fluctuations whose level is enhanced, via a nonlinear process, by the injected RF power

The high density, low temperature ionized cloud surrounding a pellet moving across a tokamak magnetic field forms a long field aligned 'ablation channel'. This plasma mass shields the pellet from the incident plasma electrons streaming toward the pellet along field lines. Therefore, the shielding capability of the cloud-the effective column ablation density intercepting the incident electrons-depends on the relative cross-field displacement between pellet and ablation channel. This transverse displacement is caused by the J_⊥ × B Lorentz force, where the transverse current density passing through the channel J_⊥ is powered by the motional E-field as seen in the reference frame co-moving with the pellet. It is shown that the currents inside the ablation channel are closed by background field aligned currents carried by outgoing shear Alfven waves. From ∇.J=0, one can obtain a self-consistent equation which describes the electrostatic field near the channel. The transverse force and deflection of the ablation channel are estimated from these results in order to determine the effective shielding length

Results on confinement and turbulence from a set of Ohmic discharges in TORE SUPRA are discussed. Attention is focused on the saturation of the energy confinement time and it is emphasized that this saturation can be explained by a saturation of the electron heat diffusivity. The ion behaviour is indeed governed by dilution and equipartition effects. Although the ion heat transport is never neoclassical, there is no enhanced degradation at saturation. This behaviour is confirmed by turbulence measurements using CO₂ laser coherent scattering. The level of density fluctuations follows the electron heat diffusivity variations with the average density. Waves propagating in the ion diamagnetic direction are always present in turbulence frequency spectra. Thus, the saturation cannot be explained by the onset of an ion turbulence. The existence of ion turbulence in the edge at all densities cannot be excluded. However, this ion feature in scattering spectra could be explained by a Doppler shift associated with an inversion point of the radial electric field at the edge

The magnetic field configuration of the Uragan-3M l=3 torsatron, which has a p=4 multipole vertical magnetic field compensation system, was studied using two methods to map the contours of the magnetic flux surfaces. The first method, the so-called triode method with a constant voltage electron source, measures the current emitted by an open thermoelectron emitter and the portion drawn by a highly transparent grid located in a poloidal cross-section of the torus. The second method involves the use of a conducting luminescent rod which scans the torus cross-section and lights up when struck by electrons emitted by an electron gun. The information on the magnetic surface structure obtained by these two techniques is compared. The characteristics of the two methods are discussed, giving special attention to the triode method because it allows an objective criterion for the quality of the magnetic surface structure to be introduced. It is shown how advantageous both methods are for rapid adjustment and optimization of the magnetic configurations in a stellarator when perturbations are present and what improvements could be achieved on Uragan-3M. Also discussed are experiments on generating electron clouds in Uragan-3M during the ramp-up phase of the magnetic field pulse

The effective radial deposition profile of deuterium pellets injected into the Joint European Torus (JET) and the Tokamak Fusion Test Reactor (TFTR) has been determined from Thomson scattering diagnostic measurements of electron density taken immediately after pellet injection. The pellet ablation rate deduced from these measurements differs from that predicted by conventional pellet ablation theory. The possibility of enhanced radial transport during the ablation process has been examined to determine whether this can explain the difference between theory and experiment, but no evidence has been found to support such an explanation. The temporal evolution of the Balmer alpha light emitted during pellet ablation is found to be different from the effective pellet ablation rate determined from the density profile measurement. The authors conclude that the shielding mechanisms of conventional pellet ablation models have to be modified in order to predict and reproduce the observed effective ablation rate and penetration depth

An experiment in a new regime of ion Bernstein wave (IBW) heating was carried out using 130 MHz high power transmitters in the JIPP T-IIU tokamak. The heating regime utilized the IBW branch between the 3rd and 4th harmonics of the hydrogen ion cyclotron frequencies. This harmonic number is the highest one used in IBW experiments conducted previously. The net radiofrequency (RF) power injected into the plasma is around 400 kW and is limited by the transmitter output power. Core heating of ions and electrons was confirmed in the experiment and density profile peaking was found to be a special feature of IBW heating. Peaking of the density profile was also found when IBWs were injected into neutral beam heated discharges. An analysis, using a transport code with these experimental data, indicates that particle and energy confinement should be improved in the plasma core region upon application of IBW heating. It is also found that the ion energy distribution function observed during IBW heating has a smaller high energy tail than those observed in conventional fast magnetosonic wave ICRF heating regimes. The ion energy distribution function obtained during IBW heating is in reasonable agreement with that calculated using the quasi-linear RF diffusion/Fokker-Planck model

A new method for the analysis of sawtooth heat pulse propagation is presented. The method is based on Fourier analysis of data where the statistical noise is reduced by a suitable noise reduction procedure. Besides the value of the incremental heat diffusivity (χ^inc), information on the magnitude of nondiffusive transport may be extracted, depending on the quality of the data. The method is illustrated using JET data. A comparison is made with two other methods of analysis of sawtooth heat pulse propagation in use at JET, and good agreement is found among the χ^inc values obtained

The dependence of global and local confinement upon plasma current and poloidal field is studied in an analysis of JET data from a series of current ramp experiments. In such experiments the current profile is transiently decorrelated from its steady state shape over a period lasting more than ten confinement times. Global confinement is shown to depend not only on total current, but also on some measure of the current distribution, for example the internal inductance. The origin of this dependence is found in the local analysis to be the dependence of diffusivity on the poloidal field. A weak dependence of diffusivity on shear is observed in the outer half of the plasma. Predictive transport calculations are made in which the electron-ion thermal diffusivities depend on shear; the calculations reproduce the measured temperature profiles to within the measurement errors. Simple mathematical expressions linking global confinement are used to demonstrate that the results from the global and local analysis are fully consistent

The authors present a neutral gas shielding model including the distribution function of particles carrying the heat for ablation. In an ohmically heated tokamak, high energy electrons in the tail part of the Maxwellian with E approximately=6-8 T_e contribute significantly to the ablation process at the pellet surface. A simple scaling for the pellet ablation rate is given which takes into account the Maxwellian electron energy distribution. The pellet penetration depth obtained with the model presented agrees well with the multigroup approximation for an energy distribution during ablation by Houlberg et al. (1988). Pellet ablation dominated by hot ions was obtained in a special NBI heated Heliotron E plasma with an electron temperature much lower than the ion temperature

Vertical asymmetries in soft X-ray emissivity of the order of the inverse aspect ratio = r/R are seen in the core of the tokamak COMPASS-C. The direction, magnitude and scaling of the asymmetries are consistent with neoclassical theory

The effect of electron loss due to the toroidal ripple on the H-mode transition is studied. When energetic electrons exist in tokamaks, for example in the case of electron heating by lower hybrid (LH) waves, the edge electric field may exhibit bifurcation to more positive values. In this state, both the electron loss and the ion loss (such as direct orbit loss) are reduced. The criterion for the transition is derived. Comparison with the H-mode in JT-60 LH plasma shows qualitative agreement

Propagation of the heat pulse arising from a sawtooth crash was studied in Ohmically heated plasmas as a function of density, and in neutral beam heated plasmas with centrally peaked and hollow power deposition profiles at two densities. The sawtooth period became longer with centre heating than with edge heating and with co-injection than with counter-injection. The lengthening of the sawtooth period was consistent with the reduction of the core loop voltage due to heating and noninductive current drive. In Ohmic plasmas, the electron thermal diffusivity χ_e^HPP determined from a time-to-peak analysis decreased inversely proportionally to density for n_e ≲ 2 × 10¹⁹ m^-3