TRANSPORT BY INTERMITTENT CONVECTION IN THE BOUNDARY OF DIII-D

Measurements by probes and other diagnostics show that the radial transport in the scrape-off-layer (SOL) is partially determined by discrete intermittency events. A magnetic divertor [1] in a tokamak provides heat and particle exhaust and shields the main plasma from impurity contamination. Heat and particles are transported from the plasma core to the edge and where they are, in turn, conducted and convected to the divertor, which is optimized to reduce structural damage and impurity release. The balance between parallel and perpendicular transport in the SOL results in measured SOL radial profiles that are typically near exponential [2] with short (1–3 cm) decay lengths near the separatrix. However, evidence [3–5] that the SOL profiles can be very wide and flat abounds, suggesting larger than usual radial transport. A candidate for this additional edge/SOL transport, intermittency in the fluctuations corresponding to the presence of a significant number of transport events well above the standard deviation, has been documented extensively in linear devices [6,7] stellarators [8] and tokamaks [9–11]. Statistical analysis of the fluctuation in various measured plasma parameters shows a departure from gaussian statistics; a universal feature that has been proven in tokamaks (JET), torsatrons and stellarators [12] (ATF, W7-AS)