Design and Test of a Thermal Measurement System Prototype for SPIDER Experiment

SPIDER, the full-size prototype of the ITER heating neutral beams (HNBs) radio frequency (RF) ion source, has been designed and is under construction at Consorzio RFX in Padua, Italy. Several thermocouples will be installed in both SPIDER and HNB source to characterize and monitor the possibly severe thermal load on mechanical components. The presence of about 1 MW RF power ionizing the gas and frequent electrical breakdowns on the 100 kV beam accelerator represent a very harsh environment for thermal measurements, both in terms of possible sensors damage and of disturbances induced on the signals. A complete measurement chain prototype, composed of two sensors, conditioning electronics and data acquisition system, was realized to test and validate technologies and design solutions. The prototype was installed and tested inside the BATMAN RF ion source at IPP-Garching, where measurement conditions are very similar to those expected in SPIDER, and in particular high RF power and high voltage breakdowns are present. A careful installation was carried out, with extensive optimization of grounding and shielding configuration, to achieve sufficient ElectroMagnetic Interference immunity. Analog and digital filtering were also implemented on signals to eliminate the residual noise. Results produced during a dedicated experimental campaign on BATMAN have been very successful: 0.5 degrees C measurement global accuracy can be obtained during RF plasma pulses and no damage occurred on sensors or electronic equipments. The paper describes the prototype design detailing the technical solutions implemented, in particular regarding sensors design and installation, signal conditioning, and noise reduction. Hardware and software characteristics of the data acquisition system are presented and discussed. Experimental results obtained are also described and their significance for future work is analyzed.