Observation of a localized transition from edge to core density turbulence in the TFTR tokamak

A localized transition zone is observed between turbulent, long-wavelength density fluctuations (${\mathit{k}}_{\mathrm{\ensuremath{\perp}}}$2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) in the core and edge of beam-heated discharges in TFTR. The core turbulence is unimodal while the edge has two counterpropagating modes, both of which are spectrally distinct from the core fluctuations. The fluctuation amplitude in the core scales with the global energy confinement time while in the edge it does not. These observations suggest that distinct modes are responsible for turbulence in the two regions and that only the core mode is directly related to global confinement.