Ergodic divertor: Present status of understanding

The ergodic divertor (ED) experiments on Tore Supra have demonstrated the capability of open diverters to reach significant results in terms of plasma-wall interaction control. Standard statistical analysis of stochastic systems allows one to derive the diffusion of free particles when the threshold to stochasticity is reached. However, transport analysis modifies this picture owing to boundary effects due to sources and sinks. The energy deposition onto the target plate is shown to be governed by parallel transport over the parallel coherence scale of the temperature field. This transport referred to as laminar transport is very similar to that found in axisymmetric diverters. Using field line tracing (MASTOC code), one can locate precisely the location of the peak in energy deposition. Qualitative agreement with experimental evaluation of the peaking factor is obtained using the radial penetration of field lines given by Mastoc and a WKB approximation.