Experimental constraints on transport

Characterization of the cross-field energy transport in magnetic confinement experiments in a manner applicable to the accurate assessment of future machine performance continues to be a challenging goal. Experimental results from the DIII-D tokamak in the areas of dimensionless scaling and non-diffusive transport represent progress toward this goal. Dimensionless scaling shows how beneficial the increase in machine size and magnetic field is for future devices. The experiments on DIII-D are the first to determine separately the electron and ion scaling with normalized gyroradius ρ*; the electrons scale as expected from gyro-Bohm class theories, while the ions scale consistently with the Goldston empirical scaling. This result predicts an increase in transport relative to Bohm diffusion as ρ* will decrease in future devices. The existence of distinct ρ* scalings for ions and electrons cautions against a physical interpretation of one-fluid or global analysis. The second class of experiments reported here is the first to demonstrate the existence of non-diffusive energy transport. Electron cyclotron heating was applied at the half radius; the electron temperature profile remains substantially peaked. Power balance analysis indicates that heat must flow in the direction of increasing temperature, which is inconsistent with purely diffusive transport. The dynamics of electron temperature perturbations indicates the presence, in the heat flux, of a term dependent on temperature rather than its gradient. These two observations strongly constrain the types of model which can be applied to cross-field heat transport. (author). 23 refs, 5 figs