Measurement of long wavelength turbulence in a Tokamak by extreme far forward scattering

Oscillations induced on the profile of a Gaussian CW CO 2 laser beam by being transmitted through the TOSCA Tokamak plasma are confined within envelopes whose shapes are correctly predicted by theory, permitting plasma fluctuation intensity and wavelength to be deduced as functions of frequency by a curve fitting procedure. The analysis shows spontaneous density fluctuations integrated across the TOSCA minor diameter to be mainly transverse to the magnetic field, with intensity maxima between 40 and 100 kHz and n e ( nu ) varies as nu -2 towards higher frequencies. At 200 kHz n 3 ( nu )/n 3 is about 10 4 times thermal, and the total fluctuation level n 3 /n e is a few percent. Coherent oscillations near 100 kHz, associated with rotating MHD structure, are occasionally observed. Wavelengths are found to lie between 1 and 6 cm, and correspond to kappa perpendicular to rho i in the range 0.2-2.0, pi i being the ion gyroradius for the prevailing plasma conditions.