Nature of high-Z impurity accumulation in tokamaks

The recent experiments on accumulative behaviour of heavy impurities in TEXTOR with test limiters of molybdenum and tungsten and puffing of xenon are briefly reviewed. The results of the reconstruction of the transport coefficients of high-Z ions in the accumulation stage are presented. They confirm the neoclassical nature of the convective particle transport, leading to peaking of the impurity density. A mechanism triggering accumulation, invoking the temperature dependence of neoclassical flows of impurities, is discussed and the threshold of accumulation obtained is compared with experimental data. Processes which can lead to a saturation of the accumulation, caused by a modification of the flux component proportional to the temperature gradient, are analysed. The results of numerical modelling for experiments in TEXTOR with different durations and intensities of xenon puffing are presented. The role of high-Z ions under reactor conditions is analysed, and it is shown that in a reactor, such as ITER, the discussed heavy impurity driven instabilities should be suppressed.