Session UI1 - Turbulence, Internal Transport Barriers and Turbulent Transport.
INVITED session, Friday morning, October 31
Kiva, ACC
The related methods of dimensional analysis, similarity, and scale invariance in physics provide a powerful technique for analyzing physical systems. Previous experiments on the JET, DIII-D, and Alcator C-Mod tokamaks have validated the principle of similarity for energy transport in high-temperature plasmas. Recently the dependence of transport on beta, the ratio of the plasma kinetic pressure to the magnetic field pressure, has been measured for H-mode plasmas on DIII-D. Experimentally determining the beta scaling helps to differentiate between various proposed mechanisms of turbulent transport since theories for which ExB transport is dominant show little change with increasing beta up to the ideal ballooning limit, while transport models that invoke electromagnetic effects like magnetic flutter transport generally have a strong, unfavorable beta scaling. These experiments on DIII-D varied the normalized beta from 1.1 to 3.0 in several steps, covering a range from 25% to 85% of the ideal no-wall beta limit, and showed that the measured thermal diffusivities and global energy confinement times (normalized to Bohm) have little dependence on beta. This weak, possibly non-existent, beta scaling of transport confirms previous observations from the DIII-D and JET tokamaks as well as the ATF torsatron. This experimental result is in marked contrast to empirical scaling relations derived from multi-machine H-mode confinement databases, such as the ITER-98(y,2) relation that contains a strong, unfavorable beta dependence. New semi-empirical scaling relations, derived from the confinement databases, that are gyroBohm-like and electrostatic predict that the fusion performance in ITER will optimize at high beta, yielding twice the fusion power as the nominal beta scenario at higher fusion gain.