Modelling of confinement degradation in the radiative improved mode caused by a strong gas puff

Confinement deterioration and rollover from the radiative improved (RI) mode to the low (L) confinement mode in plasmas with a strong gas puff in the tokamak TEXTOR-94 are modelled by the one-dimensional transport code RITM (radiation from impurities in the transport model). The anomalous transport coefficients in the plasma core include contributions from the ion temperature gradient and dissipative trapped electron instabilities. This model allows us to reproduce the L-RI bifurcation with impurity seeding in good agreement with experimental observations. Whereas the transport at the plasma edge under the 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 the level of the edge transport must be increased by at least a factor of three in order to reproduce the evolution of the plasma density profile and the effective ion charge during RI-to-L back transitions. An increase of this order is in agreement with reflectometer measurements and could tentatively be explained by the effect of neutrals on resistive drift modes.