A self-calibration technique for a Thomson scattering system

A self-calibration technique, based on the use of two lasers of different wavelengths is described. Such a scheme is essential in the presence of inaccessible optical components, as found on large present day machines. In these devices spectral characteristics may change with time, posing a problem for the calibration of a Thomson scattering diagnostic. From a combination of the scattered spectra generated by the two lasers on the same plasma, the temperature and the unknown, slowly varying spectral transmission of the different channels can be determined. A visible wavelength scattering system on a high-temperature plasma is assumed with laser wavelengths of 800 and 530 nm. Details of the analysis procedure and the results of fitting for the unknown spectral calibrations are presented. A statistical analysis of the error in the fitted calibration curve is included. A practical solution, with reference to ITER, is proposed.