ELM driven divertor target currents on TCV

There is currently both considerable interest in the physics of ELM transport in the scrape-off layer (SOL) and concern over the impact of the ELM power and particle loads on the divertor targets of future fusion reactors. This paper describes some experimental observations of the reaction of target floating potentials and currents during ELMS in TCV, relying principally on fast measurements of these parameters using tile embedded Langmuir probe arrays. Clear evidence is presented for rapid modifications in the local target floating potentials occurring long before the characteristic rise of hydrogenic excitation emission due to local recycling provoked by the arrival at the target of the ELM ion flux. This precursor activity appears to be synchronous with the growth of MHD modes in the main chamber. Simple conditional averaging is used to derive a 'coherent ELM' and thus generate the radial distribution of the current to probes held at the target potential. At some locations, the coherent ELM can also be used to estimate the time evolution of the local target electron temperature, density and power flux, even though these quantities are not directly measured. The time delays between the reactions of currents, floating potentials and derived temperature are consistent with the expections of recently published kinetic simulations of ELM energy transport down a one-dimensional SOL plasma. The strong potential variations observed during the ELM are the result of current flows at the targets. These currents are generally of opposite sign at inner and outer divertors and are thus consistent, at least in part, with a thermoelectric origin in which the driven current is produced by an in/out divertor temperature asymmetry near the strike point of nearly a factor 2, probably due to the generation of divertor asymmetries by the ELM heat pulse. Such asymmetries are commonly observed in low to medium density L-modes for the particular TCV magnetic equilibrium studied in this paper. The total current balance during the ELM is satisfied only to within a factor 2, so that, whilst some of the driven current flows parallel to field lines in the SOL, there is an apparent additional negative current to the inner divertor during the ELM whose origin remains unexplained. It might, however, be due, in part, to increased plasma-wall interaction in the main chamber during the ELM event.