Signal processing and calibration electronics for the SXR tomographic diagnostic of the RFX fusion experiment

The signal processing electronics supporting the soft x-ray tomography of the reversed field experiment (RFX) is described. Current from Silicon diodes, of the order of several hundred picoamperes in the most severe cases, is amplified by the I-V stage and then, after further amplification and filtering, digitized at a frequency of 1 MHz. The digits are optically sent to VME boards in the acquisition room, which provide standard tomographic inversions. Performance of the circuits (maximum transimpedance 10⁸, maximum bandwidth of about 200 kHz) and their noise behavior have been tested extensively during various experimental campaigns of RFX. A new prototype, with a higher transimpedance of 10⁹, has already been developed, tested on the bench, and successfully operated on RFX. A remotely controlled calibration unit, designed to automatically determine amplifier gains and the bandwidths to within a few percents, is also presented together with the most significant results obtained in the calibration of the complete system.