Experimental sheath heat transmission factors in diverted plasmas in JET

The heat flux to the target plates q_p is a crucial design parameter for fusion machines. It can be measured by infrared (IR) cameras viewing the target plates. The sheath heat transmission coefficient γ is defined by qp = γkT_ej_target relating q_p to particle fluxes and temperature. Sheath theory predicts γ in the range of 5⋯ 8 We show y determined experimentally from a comparison of q _p from IR with T_e and j_target measured by Langmuir probes for low to high recycling regimes in L-mode with carbon targets. While γ is in the expected range at low recycling, unphysically low values are observed at high recycling. We compare heat flux measured on carbon and tungsten as target material in reproducible discharges. From these data it could be concluded that there is no measurable difference in y for the two materials. We finally discuss the influence of y on simulation results.