Modelling Fast Response Surface Thermocouple for Plasma Facing Components

Surface eroding thermocouples are developed to measure temperatures and heat fluxes in plasma-facing components of fusion machines, especially in the divertor region. Sensor construction leads to a fast response (8ms) and robust design against heat loads around 10-20MW $cdot text{m}^{-2}$. Electrical design of surface thermocouples is developed considering compensation needs and currents expected in the plasma edge. Improved sensor performance is obtained by enhancing the thermal contact surface between the divertor mono-block and the thermocouple carrier body through the integration of conductive collet and bush, both made of copper. The thermal response is analyzed through a finite element non-linear transient model simulating heating due to a plasma discharge. The usual configuration with voltage (emf) measured between the two thermocouple ribbons is compared with a simplified single-ribbon design in which the emf is measured between the thermocouple sole inner ribbon and the divertor support. Measurement errors are discussed for both single-ribbon and double-ribbon designs. The installation of surface thermocouples is studied at different poloidal positions of the DTT divertor vertical targets. A sensor layout with 4mm poloidal resolution is proposed to map power density peaks of plasma-wall interactions. Surface thermocouple measurements can provide useful local information for studying divertor physics, scaling for development of future machines, monitoring of local conditions to assist controlled terminations of plasma discharges. Installed with other diagnostics, they can provide crosschecks for calibration and data validation. The design, development, and integration procedure herein described can be applied to other plasma-facing component embedded sensors measuring plasma parameters like Langmuir probes.