A study for dual-laser Thomson scattering calibrations in RFX-MOD

The dual-laser calibration is a technique for measuring the relative sensitivities of the spectral channels in a Thomson scattering (TS) system. By detecting the spectra scattered by the same plasma volume with two laser pulses of different wavelength, the relative sensitivity coefficients of the spectrometer spectral channels and the plasma electron temperature can be determined simultaneously. The technique exploits the plasma itself as the calibrating light source and therefore it is especially suitable for experiments with limited plasma access such as ITER. The TS system of the RFP experiment RFX-MOD has some characteristics very favourable for a test of the technique. A Nd:YAG ( ! = 1064 nm) and a Nd:YLF ( ! = 1053 nm) laser systems are available as the main and the calibration source. In addition the 4 channels filter polychromators have been designed with two spectral gaps between filters to reject both laser lines. In this paper we discuss the practical aspects of this type of calibration in RFX-MOD and calculate the expected accuracy of the calibration coefficients showing that, in spite of the very small difference between the two wavelengths, the calibration is feasible with good accuracy.