Thermal characterization of the SPIDER diagnostic calorimeter

In order to verify the accomplishment of the ITER requirements regarding the maximum allowed heating neutral beam non-uniformity (below ±10%), beamlet divergence and stripping losses, a diagnostic calorimeter, named STRIKE (Short-Time Retractable Instrumented Kalorimeter Experiment), will be exposed to the negative hydrogen ion beam of the ITER ion source prototype SPIDER (Source for the Production of Ions of Deuterium Extracted from an RF plasma) in short pulse operation mode (several seconds). The STRIKE sensor is constituted by 16 unidirectional carbon fiber-carbon matrix (CFC) composite tiles (376 mm × 142 mm × 20 mm), arranged in a 4 × 4 matrix, resembling the arrangement of the SPIDER beamlet groups. By exposing the tiles to the beam and recording their temperature with infra-red cameras the beam energy flux can be retrieved by calorimetry. The requirements of the 1D CFC material include a large thermal conductivity along the tile thickness (one order of magnitude larger than in the other two directions), uniform parameters over the tile surface and the capability to withstand localised heat loads in the order of 10-20 MW/m2. The realization of large CFC tiles satisfying all these requirements required a custom development programme and tests were performed in the last years on many prototype versions from different manufacturers. This contribution gives an overview of the tests performed to assess the thermal properties of the tiles composing the actual STRIKE, ready for installation in the SPIDER vacuum vessel. The spatial uniformity of the parameters and the ratio between the thermal conductivities are assessed by means of a CO2 power laser at Consorzio RFX. Non-linear thermal finite-element simulations are carried out and compared to the experimental data to interpret them and to estimate the thermal conductivities and specific heat.