Influence of an external gas puff on the RI-mode confinement properties in TEXTOR

An actual subject of experimental and theoretical studies in present day fusion research is the development of an operational scenario combining simultaneously high confinement, with at least H-mode quality, and high densities, around or above the empirical Greenwald limit. Recently, this subject was studied in TEXTOR radiative improved (RI) mode discharges, in which the seeding of a small amount of impurities is helpful in a transition to the improved confinement stage. It was found that by the careful tailoring of external fuelling and optimisation of the wall conditions it is possible to maintain the H-mode or even higher quality confinement at densities much above Greenwald density limit. However, more intense fuelling, aimed to extend maximal achievable densities, led to the progressive confinement deterioration. The theory explains the transition to the RI-mode as a bifurcation into the stage where the transport governed by the ion temperature gradient (ITG) instability is significantly reduced due to a high density gradient and high value of the effective charge. The numerical studies of an influence of the gas puff intensity on confinement properties of plasma, done with the help of the 1-D transport code RITM, show that the same theory can be used for an explanation of the confinement rollover triggered by a strong gas puff. The code was modified in order to simulate the effect of the gas puff on the confinement properties. The anomalous transport coefficients in the plasma core include contributions from the ITG and dissipative trapped electron (DTE) instabilities. The transport at the plasma edge under RI-mode conditions might be described by the electrostatic turbulence caused by electric currents in the scrape-off layer of the limiter. The present computations show that this assumption for the edge transport does not allow the modeling of an effect of the gas puff intensity on the profiles evolution in agreement with experimental observations. The level of the edge transport must be increased significantly in order to reproduce the evolution of the plasma density and the effective ion charge profiles during confinement degradation caused by a strong gas puff. An increase of this order is in agreement with reflectometer measurements and could tentatively be explained by the effect of neutrals on Drift Resistive Ballooning instability. Although, an increase of the edge transport is a necessary condition for the core confinement deterioration, the latter is produced by the resumption of ITG instability in the plasma core. The present studies show, that the reduction of impurity content of plasma and high positive time derivative of the density provided by a strong gas puff are crucial conditions for the restart of the ITG mode. (orig.)