Measurements of heat transfer coefficients at low contact pressures for actively cooled bolted armour concepts in Tore Supra

For the future upgrade of inner vessel components (CIEL project) a guard limiter for plasma ramp-up and disruption protection will be installed on the high field side of the vacuum vessel. Among transient heat loads, this structure has to sustain a moderate heat flux in the range of ≤0.5 MW/m2 during quasi steady state operation (1000 s). A bolted carboncarbon (CC) tile is preferred compared with a brazed tile solution due to the expected moderate heat fluxes, costs and the possibility of rapid replacement of individual tiles. Large flat tile assemblies require a sufficient soft and conductive compliant layer enclosed between tile and heat sink in order to avoid thermal contact loss of the assembly during heat loads and therefore minimising the tile surface temperature. The global heat transfer coefficient (Hgl) under vacuum at low contact pressures (0.5–1.5 MPa) between CC and CuCrZr heat sink substrata has been measured in the experimental device, installation of contact heat transfer measurements (ITTAC), using different compliant materials. It appears that the best compliant layer is a graphite sheet (PAPYEX), compared with copper-felt/foam material. As an example, a Hgl number of ∼104 W/m2 K at an average contact pressure of 0.5 MPa has been measured near room temperature between CC (SEP N11) and CuCrZr substrata using a 0.5-mm thick PAPYEX layer. Thermohydraulic calculations (2D) of the guard limiter design show an expected tile surface temperature of about 550°C in steady state regime for an incident heat flux of 0.5 MW/m2.