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Abstract
Electron transport has been measured in CDX-U using two separate perturbative techniques. Gas modulation at the plasma edge introduces cold-pulses which propagate towards the plasma center, providing time-of-flight information leading to a determination of χe as a function of radius. Sawteeth at the q = 1 radius (r/a ∼ 0.15) induce heat-pulses which propagate outward towards the plasma edge, providing a complementary time-of-flight based χe profile measurement. This work represents the first localized measurement of χe in a spherical torus.
It is found that χe = 1–2 m2/s in the plasma core (r/a < 1/3), increasing by an order of magnitude or more outside of this region. Furthermore, the χe profile exhibits a sharp transition near r/a = 1/3, indicating a possible transport barrier. Spectral and profile analyses of the soft x-ray, scanning interferometer, and edge probe data show no evidence of a significant magnetic island in the high χe region.
In support of the electron transport experiments, a multichannel Thomson scattering system has been designed and constructed, providing the first electron profile information in CDX-U. The edge cold-pulse experiments make extensive use of the EBW electron temperature diagnostic, the sawtooth heat-pulse measurements are made with the soft x-ray array, and χe profiles are compared with Te and ne profiles from the Thomson scattering system.
