Development of readout electronics for thermocouples on NIO1 beam source

A small radiofrequency (RF) ion source, named NIO1 (Negative Ion Optimization, phase 1), has been recently installed at Consorzio RFX, in Padua. The NIO1 source is capable of producing about 130 mA of negative hydrogen ions accelerated at 60 kV and represents a useful test bench for negative ions source optimization. The exploitation of this device is included in the framework of the activities for SPIDER and MITICA, the prototypes of the negative ion source and of the whole neutral beam injector for ITER experiment, respectively. As in SPIDER and MITICA, thermal measurements are performed in NIO1 by several thermocouples. These measurements are indeed important for the evaluation of the thermal load distribution, plasma generation and beam extraction uniformities, for the monitoring of different experimental conditions, and for the verification of the reliability of source and beam-line components in the expected range of temperatures. The paper deals with the development of a new electronic front-end for acquisition and transmission of the thermal signals from the thermocouples, preserving signal integrity. The system was designed in particular to minimize the electromagnetic noise, which is expected to affect the low amplitude thermocouple signals. Each thermocouple signal is filtered and then fed to a Cold- Junction Compensated Thermocouple-to-Digital Converter, which performs the cold-junction compensation, amplifies and digitizes the signal. The output data format is signed 14-bit, SPIcompatible. The converter temperature resolution is 0.25°C, allowing readings temperatures in the noticeable range from - 270°C to 1800°C. The microcontroller (PIC®) installed on the board manages eight converters in a multiplexer scheme. Data from the eight input channels are packaged and sent via a RS232 serial interface. The electrical isolation between input and output circuitry is guaranteed by two optocouplers, which act as electrical barriers between the receiver and transmitter pins of the PIC thus isolating the input and the output of the board. The tests on NIO1 have allowed to characterize the circuit and in particular to verify its reliability during RF plasma production. The paper presents the design solutions and the preliminary tests of the front-end electronics, discussing its performance with respect to the RF noise immunity.