An optimized multifoil soft x-ray spectrometer for the determination of the electron temperature with high time resolution

In RFX a monitor for the measurement of the on-axis electron temperature with high temporal resolution has been designed and operated during the last experimental campaigns. It consists of four soft x-ray (SXR) diodes which observe the plasma through four beryllium filters of different thickness. In this article the main design issues are presented. The measured values of the temperature, computed both with the standard double foil method and with a fit of all the available signals, are reported, correlated with the main plasma parameters and briefly compared with the results of other diagnostics. The main causes of systematic error, the effect of the noise and the reliability of the results in various plasma conditions have been investigated. Dynamic evaluations of the temperature up to a maximum bandwidth of 5 kHz are described, confirming the potentialities of the technique to follow fast phenomena. Possible improvements of the diagnostic and future developments are also pointed out.