[en] The radiation behaviour close to the density limit (DL) of ohmically and NI-heated D₂ discharges in the old and the new ASDEX divertor configuration, with uncoated and with boronized vessel walls, is investigated. At q_a > 3, the DL is usually preceded by a Marfe. At the onset of the Marfe, the Marfe limit (MaL), main chamber radiation RAD is about 40% of the heating power and divertor radiation DIV is about 20%. During the Marfe, RAD reaches 70% and DIV falls to about 10%. With boronized walls and only at very high q_a (q_a∼6.0), the Marfe can become quasi-stationary. The DL is influenced but not caused by radiation. At q_a < 2.6, the DL is presumably connected with MHD instabilities. A frequently observed DL minimum at q_a = 2.1 is not due to radiation

[en] Experimental data on impurity transport in the boundary plasma is reviewed. Cross-field and parallel transport have been considered both inside and outside the last closed flux surface. Reliable experimental data are relatively scarce and there are difficulties in interpreting data, frequently due to a poorly defined impurity source function. Nevertheless, progress is being made. Cross-field impurity diffusion coefficients are generally of the same order as those of the main plasma species. Parallel transport is consistent with classical collision rates and can be reasonably well described as a diffusive process. There is evidence, however, that under conditions of high impurity injection the background plasma is so perturbed that impurity penetration is much increased

[en] The helium removal and exhaust in ITER tokamak are discussed. The possibility of the enhancement of the radiation of light impurities is investigated. A model for the calculation of radiation power from the ITER divertor is proposed. The results show that in the non corona model a fraction of the power entering into the ITER divertor region can be reradiated in the case of carbon divertor plates. The time relaxation mode of the ITER divertor region operation can be calculated for Be-seeded and C-seeded plasmas

[en] The requirements for radiation cooling in a tokamak are discussed and experimental evidence for a plasma with a cold radiating boundary - the detached plasma - is presented as an example. The radiation characteristics is analyzed, and in particular the non-coronal conditions at the plasma boundary are considered by introducing the radiation potential in order to express the radiation in terms of impurity fluxes rather than in densities. The possibilities for radiation control by intrinsic and injected impurities are examined and some experimental examples are presented

[en] In tokamaks, the maximum density is most commonly limited by a major disruption. These disruptions are caused by impurity radiation; high-Z radiation from the plasma center can directly create an MHD unstable temperature/current profile. Low-Z radiation from the plasma edge causes the temperature profile to contract, again leading to a current profile that is MHD unstable (leading to a disruption). In the case of low-Z radiation from the plasma edge, the temperature profile becomes unstable to contraction at or slightly before the condition where the radiated power equals the input power. The cause of the rise in edge radiation is most likely due to the nonlinear rise in edge density resulting from a loss in central plasma particle confinement as the density limit antin_e/KJ=1 is approached. This edge-radiation-limited density limit is reduced by a high Z_eff and increased by the addition of auxiliary input power

[en] The standard neoclassical theory of both the banana and the plateau regimes is revised incorporating an anomalous transport in it. This results in a new model explaining the growth of the poloidal rotation at the plasma edge in a tokamak. The ramifications of the theory important for experiments on tokamaks are emphasized

[en] The workshop was dedicated to the realization and stability of a cold layer at the plasma edge for fusion reactors. The subjects of the communications presented were: impurity transport, and control, plasma boundary layers, power balance, radiation control and modifications, limiter discharges, tokamak density limit, Asdex divertor discharges, thermal stability of a radiating diverted plasma, plasma stability, auxiliary heating in Textor, detached plasma in Tore Supra, poloidal divertor tokamak, radiation cooling, neutral-particle transport, plasma scrape-off layer, edge turbulence

[en] Radiation control in poloidal divertor tokamaks is considered on the basis of experimental and theoretical work done at Garching. The divertor benefits with respect to impurity control are outlined, and closed and open divertor configurations are compared. ASDEX experimental results concerning impurity transport and radiation effects are summarized together with appropriate theoretical models (closed divertor). Two-dimensional multifluid simulations of the impurity and radiation behavior in the edge region are presented for an open model x-point geometry. Various edge scenarios assuming nickel as wall or target material are investigated and active divertor radiation control by external neon puffing is demonstrated. Problems arising from poloidal and toroidal asymmetry are discussed

[en] The observations made during ion cyclotron heating in Textor under detached plasma conditions are summarized. The characterization of the detachment and the conditions under which it occurs are described. The changes in the density profile and in the electron temperature profile are discussed. The deterioration of the confinement by detachment can be related to the observed changes in density and temperature scale lengths. The energy confinement time is found to be lower than in attached plasmas with the same limiter radius and the same line density

[en] A modelling procedure of the edge plasma is proposed. It is applied to the JET experimental data from pulse number 15797, which has a short H-mode. A classical plasma transport model is presented. The forces acting on the impurities are calculated. A heat transport equation and an alternative formulation when convection dominates conduction are presented. The model and the experimental results are compared. The results suggest the shielding of carbon impurities released at the plate and that the effect of the power at the scrape-off layer is not strong