Particle diffusion coefficient at the edge of RFX

Abstract

The radial particle diffusion coefficient D of the RFP experiment RFX has been derived from particle balance in the local scrape-off layer created by an instrumented limiter inserted into the edge region. The parallel particle flux incident on the limiter and its spatial decay are measured by Langmuir probes, whereas the connection lengths between the limiter and the wall are inferred from the images of a CCD camera with a H α and N II filter, and ion sources are taken into account. The measured values of D are in the range 10–20 m 2/s for plasma current in the range 300–400 kA. The radial profile of D exhibits a maximum close to the wall and it decreases to lower values at r/ a around 0.90. The value of D and its profile at the edge appear to be consistent with those derived from the electrostatic turbulence driven particle flux and from the local density profile. The results are discussed in terms of the diffusion mechanism in the edge plasma and in the scrape-off layer, by assessing the perturbation produced by the limiter insertion.