Electron heat transport in improved confinement discharges in DIII-D

In DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] toka mak plasmas with an internal transport barrier (ITB), the comparison of gyr okinetic linear stability (GKS) predictions with experiments in both low an d strong negative magnetic shear plasmas provide improved understanding for electron thermal transport within the plasma. Within a limited region just inside the ITB, the electron temperature gradient (ETG) modes appear to co ntrol the electron temperature gradient and, consequently, the electron the rmal transport. The increase in the electron temperaturegradient with more strongly negative magnetic shear is consistent with the increase in the ETG mode marginal gradient. Closer to the magnetic axis the T-e profile flatte ns and the ETG modes are predicted to be stable. With additional core elect ron heating, FIR scattering measurements near the axis show the presence of high k fluctuations (12 cm(-1)), rotating in the electron diamagnetic drif t direction. This turbulence could impact electron transport and possibly a lso ion transport. Thermal diffusivities for electrons, and to a lesser deg ree ions, increase. The ETG mode can exist at this wave number, but it is c omputed to be robustly stable near the axis. Consequently, in the plasmas w e have examined, calculations of drift wave linear stability do not explain the observed transport near the axis in plasmas with or without additional electron heating, and there are probably other processes controling transp ort in this region. (C) 1999 American Institute of Physics. [S1070-664X(99) 95505-2].