A comparison of fluctuations and transport in the scrape-off layer of a limiter (TEXT) and divertor tokamak (ASDEX)

In the scrape off layer (SOL) of some tokamaks, the detailed measurements of the many quantities necessary to directly determine the fluctuation driven transport are available. We compare the edge fluctuations and transport on two tokamaks, ASDEX, and TEXT, with the objectives of finding any differences which might lead to further understanding of the nature of edge turbulence for ohmically heated tokamaks. ASDEX [R = 1.65 m, a = 0.4 m] is a tokamak operating with a poloidal divertor and copper target plates while TEXT [R = 1.0 m, a = 0.26 m] has a circular poloidal titanium carbide coated carbon limiter. From the point of view of edge transport, the primary differences in ASDEX and TEXT are 1) a separatrix in ASDEX where q becomes large, 2) a factor of 2-3 longer distance to reach a material wall in ASDEX, either through parallel transport to the divertor or perpendicular transport to a wall, and 3) generally a lower level of impurities in ASDEX as compared to TEXT. (orig.)