Study of low-frequency density fluctuations in the PETULA B tokamak by sub-millimetre laser interferometry

A submillimetre laser interferometry technique with a CO2 - pumped CH3I laser at 447 μm wavelength is employed for the first time to study low-frequency (up to 100 kHz) electron-density fluctuations in a tokamak. Digitally implemented spectral analysis method is used to investigate the characteristics of fluctuations in both low-density (approximately 1.5 x 1013cm(3)) and high-density (approximately 6-7 x 1013cm-3) discharges in the absence as well as in the presence of the lower hybrid waves. It is found that the lower-hybrid wave influences the spectrum significantly in a manner to stabilize the coherent waves excited during the ohmic heating phase. The spectral index n of the fluctuations, corresponding to the relation P approximately fsup(-n) in the power spectrum, is estimated during the ohmic heating period to be about 3.5 in the low-density case, while it is approximately 5 in the high-density regime