Recent far infrared collective scattering measurements on the DIII-D tokamak during H mode operation have provided a more complete understanding of the L to H transition, and have confirmed the interpretation of previous reflectometry data. The frequency spectra of fluctuations measured during the neutral beam heated L and H mode discharges in DIII-D are dominated by E × B Doppler shifts. This fact is utilized to enhance the spatial localization of scattering data through knowledge of the measured variation in the radial electric field, Er. Scattering data interpreted in this way indicate a rapid (≈100 μs) reduction in the edge density fluctuation level coincident with the L to H transition, thereby quantifying and confirming the interpretation of previously obtained reflectometer data. The reduction in the RMS electron density fluctuation level (ne) is ⩾50%. Additionally, the presence of increased electric field shear is observed in the region where the fluctuations are strongly suppressed, as determined from the reduction and broadening of the scattered auto-power spectrum associated with the plasma edge inside the separatrix. Finally, the poloidal wavenumber spectrum has been characterized across the L to H mode transition, indicating that there are no significant modifications to the spectral width over the H mode parameter range investigated
The physics of transport barrier formation in the PBX-M H-mode