Core turbulence and transport studies on the Texas Experimental Tokamak

Recent experiments on the Texas Experimental Tokamak (TEXT) [Plasma Phys. Controlled Fusion 27, 1335 (1985)] have focused on identifying the drives and transport significance of the turbulence in the interior of discharges in the neo-Alcator confinement regime. Far-infrared (FIR) scattering consistently observes density fluctuations indicative of electron drift waves, i.e., ωk∼ωe*+kθvE, where vE≡-Er/B (the electron diamagnetic frequency Doppler-shifted by the ExB poloidal plasma rotation) and an amplitude that scales inversely with the density scale length Ln. Although consistent with scattering on the power-weighted frequency bar ω, heavy-ion beam probe (HIBP) data typically indicate bar kθρs<0.1 such that bar ω much-gt bar ωe*+bar kθvE and bar n/ne much-lt 1/bar kθLn. Experiments with a modulated gas feed and/or electron-cyclotron resonance heating (ECRH) seem to rule out ∇Te as the turbulent drive, although little evidence for ∇ne is apparent either. In fact, the interior fluctuations seen by the HIBP seem to depend more on edge conditions than local gradients. The quasilinear fluxes due to dissipative trapped electrons are found to be sufficient to account for the equilibrium fluxes in the region of low collisionality (νe*much-lt 1) for a drift-wave spectrum (bar kθρs∼0.3), but not for the low wave numbers seen by the HIBP