MICROTURBULENCE REDUCTION DURING INTERNAL TRANSPORT BARRIER DISCHARGES IN DIII-D

Increased plasma confinement, temperature and fusion reactivity in neg ative central shear (NCS) discharges in DIII-D are accompanied by redu ced core electrostatic microturbulence. The microturbulence reduces as the local radial electric held and shear increases, consistent with a theoretical model incorporating turbulence stabilization by shear rad ial electric held. Reduced turbulence and the associated anomalous tra nsport reduction leads to further increased radial electric field shea r via a steeper pressure gradient and reduced momentum transport. Duri ng the H-mode phase of a core transport barrier discharge, the microtu rbulence is virtually quenched in the core. Increasing evidence indica tes that the transport barrier initially forms in the plasma interior when E x B shear is large.