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

In SPIDER, the prototype of the ITER HNB Ion Source (IS), where a 100 keV ion beam is accelerated by an electrostatic accelerator, Thermocouples (TCs) are installed on the components of the IS, 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, expected every 10s or even less. 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. This solution provides electrical insulation during high voltage breakdowns expected up to 20kV. The other sensors are acquired through a commercial Analog Devices solution. The 24 signals from thermal sensors embedded on the GG are acquired by three boards with 8 TC input channels each installed in a box screwed to the frame of the GG together with its isolated power supply based on a 1:1 transformer, able to withstand voltages up to 20kV. Each input signal passes through a low pass filter; any possible voltage transient is stopped by surge protectors before the signal reaches the Cold-Junction Compensated Thermocouple-to-Digital Converter (MAX31856 by Maxim) which digitizes the data from the thermocouples. The data from the MAX31856 devices are collected by a microcontroller which processes and sends them through its internal UART peripheral. The microcontroller TTL UART is connected to a RS-485/RS-422 transceivers which provide 2750 VRMS (for 60s) of galvanic isolation between the serial port and the cable plugged into the output. The most important technical aspect for this custom front-end electronics is its high isolation capability both on the input and on the output sides. This isolation improves communication by breaking ground loops and reduces noise when there are large differences in ground potential between ports. Electro-optical converters are used to send the output to three optical fibers with Peek insulation for the in-vessel path. The signals pass then the vacuum barrier through flange and are fed to an opto-receiver which converts the signal to RS-485 serial format before being acquired by a Moxa port server. The data are here converted to standard Transmission Control Protocol (TCP) 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 Grounded Grid, its commissioning during the start of the operations with SPIDER and some experimental results.