Calculation of the radiated power in WEST

Abstract

The WEST tokamak is currently equipped with a set of horizontal bolometers to estimate the radiation level of plasma scenarios. The exact relation between bolometer signal and radiating power is derived, highlighting the approximations required to express a proportionality between bolometer signal and radiation level. Further approximations are needed to correct for overlapping lines of sights. They are done via a truncated trapeze approximation. To test the robustness of all these approximations and uncertainties, a synthetic diagnostic basis is constructed coupled to a Monte Carlo ray-tracing tool, and applied to a large set of emissivity phantoms representative of WEST plasma geometries and radiation distributions. It results that the approximations are valid, with uncertainty levels bounded by 5% to 10% depending on the plasma geometry. Application to WEST plasmas shows that the radiated power fraction (related to total power) is about 50% in the first experimental campaigns. A finite radiation level originates from the divertor together with finite radiations from the core. It suggests a detrimental mixture of light impurities and tungsten in the plasma, possibly caused by poor chamber conditioning.