On impurity handling in high performance stellarator/heliotron plasmas

In stellarators and Heliotrons the plasma confining magnetic fields are exclusively produced by external coil systems without induced net currents. Therefore, they offer the intrinsic potential for continuous plasma operation. The large helical device (LHD) and Wendelstein 7-X (W7-X), the latter presently being under construction, already comply with the technical requirements of long pulse operation and focus upon the establishment of high performance long-pulse plasmas. A crucial point is the avoidance of impurity accumulation which can lead to early pulse termination by radiation collapse. Predictions by theory for non-axisymmetric configurations underline the importance of this issue because screening of impurities by ion temperature gradients is not expected in standard ion root plasmas. Trends for increasing impurity confinement with rising densities were indeed found in TJ-II and W7-AS leading to accumulation phenomena at higher densities in CHS, W7-AS and LHD. In W7-AS, the rise of impurity concentration could be stopped by the onset of drag forces in the high density and low temperature scrape-off-layer which flush out impurities and reduce the net impurity influx into the core. Additionally, the impurity core confinement is reduced (HDH-mode). High performance discharges with steep density edge gradients and low edge density do not benefit from this mechanism. Therefore, the utilization of ELMs, which seem to have a similar effect on impurity screening as known from tokamaks, might be a possible way for plasma purification. Degradation of impurity confinement and avoidance of accumulation by increased ECRH heating power was demonstrated at W7-AS as an additional tool against impurity accumulation, but it is not clear whether this effect can be extrapolated to other heating methods. Nevertheless, the exploration of such purification mechanisms is a demanding issue for steady-state operation. (author)