J. Plasma Fusion Res. SERIES, Vol.5 (2002) ll2-111 Mechanisms of Transport in Radiative lmproved Mode

Improvement of confinement by a deliberate seeding of impurities like neon and argon has been found in many devices. Most intensively this phenomenon was studied in the limiter tokamak TEXTOR, where it was called radiative improved (RI) mode, and in the divertor machine DIII-D. Recent experiments on TFTR, JT-60 and JET have demonstrated that by an optimization of seeding procedure a positive effect of impurities can be achieved in reactor scale devices. Extensive theoretical and modelling activities were performed during past years in order to understand the mechanisms of confinement improvement in Rl-mode. Characteristics of drift instabilities namely the ion temperature gradient (ITG) and dissipate trapped electron (DTE) modes, which provide the main contribution to the anomalous transport in tokamaks, have been analyzed by the code for Gyro-Kinetic Stability. The behavior of non-linear turbulent eddies and vortices was studied in "particle in cell " simulations. Fluid approximation has been applied to asses the effect of impurities on anomalous transport. All these studies predict a reduction of turbulence originated from the most dangerous ITG modes. Computations by a transport code with models for anomalous transport coefficients due to drift micro-instabilities reproduce many peculiarities of Rl-plasmas. Keywords: drift instability, confinement improvement. impurity, turbulence suppressio