Density fluctuation

A spatially scanning microwave scattering system operating in the extraordinary mode at 60 GHz is used to study small scale density fluctuations in TFTR plasmas. Heterodyne measurements of density fluctuation in the wave number range 2.0 cm-1≤k≤25 cm-1 are made at various radial and poloidal locations. Power spectra at r/a ∼0.3 for linear and saturated ohmic, L-mode, and supershot plasmas have strong similarities. In all cases the highest fluctuation amplitude occurs at the longest resolvable scale lengths corresponding to kρ<0.5. Estimates of the local density fluctuation level shows general agreement with the mixing length estimates, δne∼1/(kLn). However, direct comparison of ohmic and L-mode plasmas with the same Ip, BT, ne(r), and Te(r)/Te(0) indicate that δne/ne for strongly heated L-mode plasmas is several times higher than the ohmic level. In ohmic plasmas no features have been observed propagating in the ion diamagnetic direction; however, plasma rotation is probably shifting all spectral features substantially toward the electron side. (author) 6 refs., 5 figs