Improved performance of the Thomson scattering system in RFX

Abstract

A Thomson scattering diagnostic is used in the RFX reversed field pinch experiment to measure single pulse, multipoint electron temperature (Te) profiles. Recent significant improvements have provided reliable and routine operation of the diagnostic. Custom developed CAMAC charge analog-to-digital converters have been introduced, which sample the plasma light 170 ns after the laser light and have greater sensitivity. The spectrometer configuration and the calibration procedure have been optimized. These upgrades have finally led to better fitted scattering spectra and more regular estimates of Te profiles, allowing the measurement of temperatures as low as 20 eV. The introduction of a second spectrometer has recently added a further ten positions to the original ten point profile, resolving the often steep Te edge gradient. On axis values are in good agreement with other Te measurements from SXR diagnostics (either SiLi and double filter method). Te profiles are typically rather flat, but some dependence on the plasma parameters, like density and pinch parameter, has been drawn. Significant Te profile modifications have been observed during transient events like pulsed poloidal current drive and pellet injection. The diagnostic will be completed soon, allowing production of 27 point profiles, with the introduction of the third spectrometer. Double pulse operation of the laser is in progress.