Heat flux calculation using embedded thermocouple in W-coated CFC tiles in the JET tokamak

JET is currently running experiments with the ITER-like wall (ILW), beryllium in the main chamber and tungsten for the divertor target plates (including a solid tungsten divertor row for the central horizontal target and W-coated CFC for the vertical tiles). Surface temperature measurement remains an essential issue to guarantee plasma operation in acceptable temperature window for each material. In addition to the wide angle IR imaging system (facing reflection and emissivity issues), few embedded thermocouples (TC) located 10 mm below the tile surface are used to measure the bulk temperatures of the vertical CFC tiles (coated with about 20μm of tungsten). We propose here to use an inverse thermal calculation based on Conjugate Gradients Method (CGM) to deduce the surface temperature (thus on the W coating) and deposited heat flux independently from IR absolute measurements. The variations of thermal properties with temperature are taken into account with the real geometry of the tile using 2D finite elements nodes, in the depth and vertical distances. The calculation requires the position of the peak and the 1D-shape of the heat flux deposited on the target, these information are provided by cross comparison between offline magnetic reconstruction and wide angle IR data. The CGM heat flux calculation has been applied to 2.1T/2MA H mode discharge performed with NBI heating (11MW during 4s) with a divertor plasma configuration with the high-field and low-field sides strike points on the vertical targets. The results are Q = 2.5MW/m and Q = 3.5MW/m at the inner (ISP) and outer strike point (OSP). The corresponding surface temperatures and total accumulated energy are of 350°C/12MJ (OSP) and 280°C/5.6MJ (ISP) at the end of the heating phase (lasting 4s). The paper aims to demonstrate the use of embedded TC data (more robust than viewing system in metallic environment during DT operation) and inverse calculation method for surface temperature and power measurements.