Randschicht-Konfiguration von Tokamaks : Entwicklung und anwendung stochastischer Modelle zur Beschreibung des Neutralgastransports

Any selfconsistent modeling of the plasma in a Tokamak boundary-layer must include neutral particle effects. Important processes involving these particles are dissociation, ionization and charge-exchange. Interactions with the walls must include (ballistic) backscattering, absorption, desorption and reemission. In general, numerical simulation and detailed analytical treatments require strong simplifying assumptions. On the other hand stochastic simulation ( "Monte-Carlo" -simulation) can be done without any majorapproximations in the physical models, but usually at the expense of long computation times. Following the lead of neutron-transport-codes, this worktakes-"- idvantage of both types of simulations by making use of "smoothing effects " on random-variables when conditional expectationf values are calculated. In this Approach, a Monte-Carlo code, which keeps the dimensionality optional, has been developed, so that, depending on symmetry, one-, two- or threedimensional profiles can be computed for up to nine different neutralparticle species. The code is applied to saure models, which are designed to describe the neutral gas behaviour in the Tokamaks : UNITOR (Düsseldorf), ASDEX (Garehing), TEXTOR (Jülich) and JET (Culham, England). The statistical resuits can be transformed numerically into data more suitable for direct comparison with experiments. For example density-profiles of exited H-atoms or charge-exchange spectra, integrated along " lines of sight" (which can be prescibed arbitrarily) can be generated. The code indicates that in ASDEX and TEXTOR, a hot plasma core exists, evidently well separated from the cold boundary laye . However the ion temperature in this core obtained experimentally from the high energy tail of Charge exchange spectra might strongly be over- or underestimated. Theerror in this diagnostic depends an the position of the analyser relative to the sources of cold neutrals (limiters) located in the boundary. For example, in larger Tokamaks (JET), the neutrals emitted from the limiters lead to a Charge-exchange collision freauency which can vary strongly along the line of sight. Nevertheless, the error-sources are reduced to some extent by the relatively masre important volume recombination effects. Another result of the caIculations is that strong anisotropy in the neutral gas profils in the plasma core of small Tokamaks can arise from weak neutral-plasma- interactions. Similar effects in large devices arise from strong local absorption mechanisms. In both cases this anisotropy may increase the local neutral particle energies above the mean ion energy