Nonlinear optical effects in Raman calibrations of a Thomson scattering system

We present an investigation of the occurrence of stimulated Raman scattering and other nonlinear optical effects during Raman calibrations of Thomson scattering diagnostic systems for magnetic fusion plasmas. When these effects take place, the intensity of the Raman lines is unpredictable, and the calibrations are impossible. In this research Raman scattering from H(2) and D(2) at filling pressures up to 1 atm has been experimentally investigated using the Thomson scattering system of the ETA-BETA II reversed field pinch device. Stimulated Raman Stokes light has been observed at filling pressures above 230 and 500 mbars for H(2) and D(2), respectively, for input laser pulses of 8 J and 30 ns (FWHM) duration. Evidence has been found that the stimulated Raman light does not originate from the observed scattering volume but is detected as light diffused into the vacuum chamber. To explain these results, the Raman gain and the intensity of the stimulated Raman light are calculated, taking into account the multimode structure of the laser beam. We find that significant power conversion from the input laser beam to the Stokes wave takes place near the output window of the vacuum chamber. Part of this radiation is diffused back into the machine, and this part is detected as superimposed on the spontaneous Raman signal. Finally we discuss the Raman calibrations in RFX, a larger plasma device in which the Raman medium will be N(2) at a temperature up to 350°C, and show that a filling pressure of 100 mbar gives a sufficient calibration signal, avoiding any nonlinear effect.