Review of experimental achievements with stochastic boundaries

Control of the plasma-wall interaction is a difficult challenge to cope with in controlled fusion research. A straightforward idea consists of destroying the magnetic configuration at the plasma edge by field line stochastization: this can easily be achieved by applying helical magnetic perturbations. Interestingly enough, most of the physics properties stem directly from the description of the chaotic nature of the field lines. This is readily expected whenever parallel transport largely exceeds transverse anomalous transport, a standard property for most boundary plasmas. While quasilinear theory gives access to the poloidally and radially averaged values of heat and particle energy transport, it can then be shown that the understanding of energy transport through the stochastic boundary and especially power deposition onto the target plates requires a non-averaged calculation. Indeed, ergodization is limited by the bounded length of the field lines themselves before they hit a plasma facing component within the tokamak chamber. The so-called ergodic layer is consequently surrounded by a generalized scrape-off layer: the laminar zone. Towards the plasma core, the vanishing ergodization is shown to give rise to an internal transport barrier. Outstanding experimental results such as impurity screening and increased radiation at reduced core contamination indicate that transport monitored by a controlled stochasticity provides a powerful means to tackle major issues of fusion physics. Significant advances in ergodic divertor physics give us confidence that this open divertor concept is an alternative to the closed axisymmetric divertor which relies heavily on mechanical baffling of neutrals. Experimental evidence is given by a worldwide family of devices such as Tore Supra, TEXT, TEXTOR and CSTN-II.