Custom thermocouple input module for sensors on the Grounded Grid of SPIDER

In SPIDER, the prototype of the ITER HNB Ion Source, where a 100 keV ion beam is accelerated by an electrostatic accelerator, Thermocouples (TCs) are installed on the components of the ion source, embedded on the grids, and on the Beam Dump water cooled elements. These measurements are important to monitor the level and the uniformity of heat loads and to protect components under operating conditions that include breakdown events. In particular for TCs on the grounded grid (GG) and for the calorimetry of the Beam Dump, a custom conditioning and acquisition module with isolated power supply and optical signal transmission has been designed and manufactured. The other sensors are acquired through a commercial Analog Devices solution. The 24 signals from TCs embedded on the GG are acquired by three boards with 8 input channels each installed in a box screwed to the frame of the GG together with its isolated power supply, able to withstand voltages up to 20 kV. The acquisition is made by the microcontroller, which selects the channels one by one. These custom boards provide high isolation level on the input with isolated power supply and on the output with optical fiber. The data are sent through three optical fibers and pass the vacuum barrier through flange with optical feedthroughs. Then they are fed to an opto-receiver which converts the signal to RS-485 serial format before being acquired by a port server. The data are here converted to standard Transmission Control Protocol simplifying the acquisition and the storage managed by CODAC (Control, Data Access and Communication) and MDSplus. This paper deals with the description of the developed custom electronic front end for the thermocouples embedded on the GG, its commissioning during the start of the operations with SPIDER and some experimental results.