Nature of high-Z impurity accumulation in Tokamaks

Recent experiments in TEXTOR on accumulative behavior of heavy impurities released from test-limiters of Mo and W and by puffing of Xe are shortly reviewed. The transport coefficients of high-Z ions in the accumulation stage, reconstructed from the bolometric measurements of radiation losses, are presented. They confirm the neo-classical nature of the convective particle transport which causes peaking of the impurity density. A mechanism triggering accumulation, invoking the temperature dependence of neo-classical flows of impurities, is discussed and the calculated threshold of the accumulative instability is compared with experimental data. Changes in the flux component proportional to the temperature gradient, caused by the increase of the impurity concentration, are analyzed and it is shown that they can lead to saturation of the accumulation. The possible nature of MHD-modes excited by peaking of the impurity density and flatiening of the temperature in the plasma core is discussed. Results of numerical modeling for experiments in TEXTOR with xenon puffing of different duration and intensity are presented. The role of high-Z ions under reactor conditions is analyzed and it is shown that in lTER the instabilities driven by heavy impurities should be suppressed