Signal processing and calibration electronics for the metal foil bolometers of RFX

Abstract

A new signal processing and calibration electronics, expressly designed to be used together with the tomographic diagnostic's metal foil bolometers of RFX (a Reversed Field eXperiment for the study of thermonuclear fusion), is presented in detail. The detectors are Wheatstone bridges, which are unbalanced by the incident radiation originating from the plasma. In this version, a suppressed-carrier amplitude modulation technique is implemented using 50 kHz carrier frequency amplifiers, which permit a signal bandwidth of 10 kHz and a voltage amplification up to 5000. Directly after amplification, filtering and demodulation, the analog signals are sampled in the torus hall and digitized. The digitized signal is optically sent to the VME acquisition boards with a TAXI (Transparent Asynchronous Xmitter/receiver Interface) link. A customized circuitry has also been developed for the absolute calibration of the gain settings and the bandwidths of the electronics. This paper contains a detailed description of the circuits, the results of the bench tests and the analysis of the performances of both the amplification and the calibration electronics. The results obtained during RFX operation, the nature of the noise present in the experimental conditions and possible upgrades of the system are also shown and discussed.