Heat flux measurements and modeling in the RFX-mod experiment

Knowledge of edge plasma transport parameters and plasma edge phenomena is a key element in the design of the first wall for a magnetically confined fusion experiment. In RFX-mod heat flux measurement and edge transport modeling have been done to improve the understanding of this aspect. Heat flux deposition profiles have been evaluated from infrared temperature measurements of graphite limiters. They were inserted up to 12 mm into the plasma of ohmically heated discharges with Ip=0.6÷1.0 MA, ne=0.5÷3·1019 m-3 (n/nG<0.7) and total power of about 15 MW. Strong asymmetries in temperature increase have been measured in poloidal direction at low density between the electron and the ion drift side and smaller ones in toroidal direction when q(a)?0. The poloidal asymmetry has been associated to the presence of superthermal electrons [1] while the toroidal one has been less clearly identified as due to the small toroidal extension of the insertable limiter. To account the 2D deposition nature of heat load on the surface of the employed limiters a simple 3D code has been developed to evaluate heat flux from temperature data. In this way at the deeper limiter insertions a heat flux decay length of about 2 mm and 2.5 mm has been evaluated in electron and ion drift sides. Modelling of the evaluated heat fluxes has been done using the SOLEDGE2D-EIRENE edge code [2]. This fluid code is well suited for the RFX-mod wall limiter configuration because, thanks to the implemented penalization technique, the computational domain can be extended up to the entire first wall. Edge modeling has shown that measured decay lengths are compatible with energy diffusion coefficients smaller than those commonly evaluated at plasma edge; the cause of the reduced diffusion in the SOL will be discussed in the paper.