Beta scaling of transport on the DIII-D Tokamak: Is transport electrostatic or electromagnetic?

Abstract

Determining the scaling of transport with beta (β), the ratio of the plasma kinetic pressure to the magnetic pressure, helps to differentiate between various proposed theories of turbulent transport since mechanisms that are primarily electrostatic show little change in transport with increasing β, while primarily electromagnetic mechanisms generally have a strong unfavorable β scaling. Experiments on the DIII-D tokamak [J.L. Luxon, Nucl. Fusion 42, 614 (2002)] have measured the β scaling of heat transport with all of the other dimensionless parameters held constant in high confinement mode (H-mode) plasmas with edge localized modes. A four point scan varied β from 30% to 85% of the ideal ballooning stability limit (normalized beta from 1.0 to 2.8) and found no change in the normalized confinement time, i.e., Bτth∝β−0.01±0.09. The measured thermal diffusivities, normalized to the Bohm diffusion coefficient, also did not vary during the β scan to within the experimental uncertainties, whereas the normalized helium particle transport decreased with increasing β. The H-mode pedestal β varied in concert with the core β and showed no signs of saturation. This weak, possibly nonexistent, β scaling of transport favors primarily electrostatic mechanisms such as E×B transport, and is in marked disagreement with the strong unfavorable β dependence contained in empirical scaling relations derived from multimachine H-mode confinement databases.