Externally-driven H-mode studies in CCT

Test particle radial flux surface excursions are reduced during the H-mode. Particle transport is reduced by a factor of 10 in the H-mode, but energy confinement increases are small. In the H-mode the evolution of poloidally resolved turbulent statistics are not explained by published theory. Turbulent momentum transport leads to a concentration of poloidal momentum within the transport barrier, and compressibility leads to poloidal shock-like phenomena. The electron distribution functions may be modified by this shock, leading to kinetic instabilities. A physics-based understanding of the H-mode must therefore include toroidal effects combined with an adequate treatment of particle orbits, plasma compressibility and associated kinetic effects, and at least a two-species model of turbulent transport. The results suggest that rapid poloidal core-plasma rotation could form core transport barriers without reliance on fluid shear or reversed magnetic shear effects.