Dual-laser, self-calibrating Thomson scattering measurements in RFX-MOD

A self-calibrating Thomson scatting technique, based on using two lasers of different wavelength, is tested in RFX-MOD. The method employs the available Nd:YAG and Nd:YLF laser systems fired in sync through the same plasma volume. The combination of these scattered spectra are used to simultaneously obtain the self-calibrating measurements of the electron temperature Te, the electron density ne, and the relative calibration coefficients of the polychromator spectral channels sensitivity Ci. Despite the small difference in the wavelength of the lasers scattered spectra, previous study has shown that reliable measurements of the calibration coefficients should be obtained with a reasonable number of plasma shots. This dual-laser method can be practically applied to most fusion devices that employ Thomson scattering, and can provide continuous online monitoring of the spectral transmission of the entire detection system. This may prove essential in larger devices such as ITER, where access to the optical components for traditional calibration is difficult, cumbersome, and much less frequent. To the authors' knowledge, this technique has never been successfully demonstrated before.