Development and tests of a simple multifoil spectrometer for highly time-resolved line intensity measurements in the RFX experiment

A simple and cost-effective spectrometer dedicated to line-intensity monitoring in the soft X-ray range has been realized, installed and tested in the RFX reversed field pinch experiment for the study of magnetically confined fusion plasmas. The technique of photon energy selection by means of multilayer material filters has been applied to the instrument described here, which, together with the associated electronics, has been designed in an original compact version for RFX. This present version allows on-line monitoring with good time resolution (10 kHz bandwidth) of line emission from carbon and oxygen in their H- and He-line states (which fall within the soft X-ray energy range), but, with modifications of the set of material filters, different wavelength ranges can be chosen and then other lines can be monitored. A detailed description of the instrument is given here and the data analysis technique is presented. Particular attention is dedicated to a critical analysis of the technique and to the discussion of the experimental errors which may be associated with this measurement; new results on the estimate of systematic errors and experimental improvements to eliminate the problem of continuum radiation are presented. As a demonstration of the proper operation of the instrument, data taken under a wide range of conditions in 0.5-0.6 MA RFX plasmas are shown and discussed. Good agreement of the measurements with other spectroscopic diagnostics already operating on the machine is found; by exploiting the high time resolution of the instrument, interesting results concerning the RFP configuration`s dynamics are obtained. Based on this prototype, a multichord diagnostic has been designed for imaging the impurity emission radial profiles.