Core turbulence and transport studies on the Texas Experimental Tokamak

Abstract

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 E×B poloidal plasma rotation) and an amplitude that scales inversely with the density scale length Ln. Although consistent with scattering on the power-weighted frequency ω̄, heavy-ion beam probe (HIBP) data typically indicate k̄θρs<0.1 such that ω̄≫ω̄e*+k̄θvE and n̄/ne≪1/k̄θLn. 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*≪1) for a drift-wave spectrum (k̄θρs∼0.3), but not for the low wave numbers seen by the HIBP. While the estimates with k̄θρs∼0.3 predict the scaling of the electron energy flux over a range of discharge conditions with νe*≪1, the model seems unable to predict the magnitude (or phase) of the perturbed particle flux during modulated gas feed experiments. In the outer regions of the plasma where νe*≳1, the calculated quasilinear fluxes are inadequate, even including untrapped particles.