[en] The role of fast particles in the edge transport barrier formation is discussed. Analysis of recent experiments on JET which has been carried out in H, D, T and DT mixture with NBI and ICRH heating is presented which supports the idea that the width of the edge transport barrier is controlled by the banana width of the fast beam ions in case of NBI heating and by the banana width of the thermal ions in case of ICRH minority heating. A simple theoretical model which can account for this effect is analysed. (author)

[en] Based on experiments carried out in JET in D:T mixtures varying from 100:0 to 5:95 and those carried out in hydrogen plasmas, the isotopic mass dependence of ELM parameters and the edge pedestal pressure in neutral beam (NBI) and ion cyclotron resonance (ICRF) heated H-mode plasmas is presented. The ELM frequency is found to decrease with the atomic mass number both in ICRH and NBI discharges. However, the frequency in the case of ICRH is about 8 - 10 times higher than in the NBI case. Assuming that ELMs occur at a critical edge pressure gradient, limited by the ballooning instability, the scaling of the maximum edge pressure is most consistent with the assumption that the width of the transport barrier scales as the ion poloidal Larmor radius governed by the average energy of fast ions at the edge. The critical edge pressure in NBI heated discharges increases with the isotopic mass which. is consistent with the higher deduced width of the edge transport, barrier in tritium than in deuterium and hydrogen. The critical edge pressure in ICRH discharges is smaller, presumably, due to the smaller fast-ion contribution to the edge region. As a consequence of the edge pressure scaling with isotopic mass, the edge operational space in the n_e - T_e diagram increases with operation in tritium. If the evidence that the edge pedestal width is governed by the average energy of fast ions in the edge prevails, the pedestal in ITER would be controlled by the slowing down energy spectrum of α-particles in the edge. (author)

[en] During a density ramp-up JET ELMy H-modes typically show a saturation of the core density which may even be followed by a decrease (density limit). At the limit the discharge detaches between ELMs and a recently proposed edge based detachment limit for the separatrix density applies. In this paper a simple picture is proposed for the relation between separatrix and core density which allows us to derive the corresponding limit for the core density. The empirical evidence from JET is discussed and compared with the model predictions. It is shown that the resulting limit for the line averaged density coincides in size and scaling with the empirical Greenwald limit. (orig.)

[en] The role of fast particles in edge transport barrier formation is discussed. Analysis of recent experiments on JET that have been carried out in hydrogen, deuterium and tritium plasmas, and in a DT mixture, with NBI and ICRF heating is presented which supports the idea that the width of the edge transport barrier is controlled by the banana width of the fast beam ions in the case of NBI heating and by the banana width of the thermal ions in the case of ICRF minority heating. A simple theoretical model which can account for this effect is analysed. (author)

[en] One of the key unresolved issues in the design of a future fusion reactor is the power handling capability of the divertor target plates. Earlier we reported on the existence of narrow power deposition profiles in JET, obtained mainly from Langmuir probe measurements. We repeated these measurements in the MkI, MkII and MkIIGB divertor configurations with an upgraded probe system, which allowed us to study the profile shape in more detail. The main results of this study are: In NB heated discharges the electron temperature and power flux at the outer target show a distinct peak of ∼5 mm half-width near the separatrix strike point. The corresponding profiles on the inner target do not show a similar feature. The height of the narrow peak increases with NB heating power and decreases with deuterium and impurity gas puffing. Ion orbit losses are suggested as a possible explanation of the observed profile shape

[en] The injection of an aluminium pellet into the plasma of T-10 was recorded by the surface analysis station WASA. The injected material was deposited only behind the lower slit of the probe. This asymmetry is explained by an E-vector x B-vector rotation of the plasma column and by the assumption that aluminium hits the probe in an ionized state. Estimations of the amount of injected aluminium reaching the WASA probe agree with the experimental results. By contrast, iron, chromium and nickel were deposited behind all four slits, which can only be explained by assuming that most of the intrinsic impurities hit the probe in a neutral state. In addition, the results of the experiment indicate that the gross particle confinement time is smaller than 90 ms. This is considerably shorter than the 150 ms obtained from the relaxation of the soft-X-ray signal. (author)

[en] The paper reviews the findings of the International Workshop on Plasma Edge Theory in Fusion Devices, which was held in Augustusburg, April 26-30, 1988. The workshop mainly concentrated on basic problems of edge plasma modelling, on applications of models to the SOL, on models for the local region near material surfaces and on the behaviour of impurities in the SOL. During the workshop 5 introductory papers and 34 contributed papers were presented. Additionally, a panel discussion on the relations between theory and experiment was held. (orig.)

[en] Recent developments in the understanding of the scrape-off layer structure are reviewed. After the discussion of the general properties of a scrape-off layer formed by an axisymmetric limiter the more common case of non-axisymmetry is analysed. Original results are presented concerning a three-dimensional model of the SOL and related experiments. In the high-density case a specific structure develops, which may be described as a neutral cloud shielding the limiter. The role of a structured SOL for the accountability of energy and particle fluxes is shortly evaluated. (author)

[en] The PSI-1 facility for studying plasma/wall interaction is described. It uses a direct current arc as a plasma generator together with a hollow anode. The plasma expands through the anode into the target chamber and is confined by an axial magnetic field. The design values for the plasma parameters near the target are n_e ≤ 10²⁰ m^-3, T_e ≤ 50 eV, Γ ≤ 6 x 10²³ m^-2 · s^-1, and q ≤ 3.5 MW/m²

[en] The limiter in a tokamak draws significant particle and energy fluxes from the plasma and sharply reduces the interaction between the plasma and the wall, thereby protecting the wall from damage and protecting the plasma from catastrophic contamination. The temperature of a movable limiter in the T-10 tokamak has been measured using the IR emission from the surface of the limiter. When the results of these measurements are interpreted with reference to certain models for the boundary layer at the wall it becomes possible to calculate the heat flux to the limiter. This heat flux is 2--3 kW/cm² in normal discharges and ranges up to 10 kW/cm² in discharges with disruptions. The total power drawn by the limiter is thus comparable to the power deposited in the discharge under these conditions. The measurements reveal that the radial decay of the longitudinal energy flux in the peripheral layer of the plasma column is exponential. The results also yield the constant of this decay. During discharges with disruptions, in the initial and final stages of normal discharges and also when microwave power is injected into a plasma, there is a highly nonuniform heating of the limiter surface. This nonuniformity indicates a definite structure for the peripheral plasma. An overall result of this study is that the limiter can be used successfully in a role in addition to its primary role. Specifically, it can be used as a diagnostic probe in a close contact with the plasma which is not achievable by other means