Role of core toroidal rotation on the H-mode radial electric field shear, turbulence, and confinement as studied by magnetic braking in the DIII-D tokamak

‘‘Magnetic braking’’ of the plasma toroidal rotation in the high confinement H mode by applied resonant, low m,n=1 static error fields is used in DIII-D [Nucl. Fusion 31, 875 (1991)] as an independent control to evaluate the Er×B stabilization of microturbulence in the plasma core. In the core (ρ≲0.9) of a tokamak, the radial electric field and its shear are dominated by toroidal rotation. The fundamental quantity for shear stabilization of microturbulence is shear in the velocity of the fluctuations v⊥≊Er×B/B⋅B which in the core is v⊥≊vφBθ/ Bφ. With magnetic braking greatly decreasing the toroidal rotation and thus reducing the core radial electric field and shear, far infrared (FIR) measurements of density microturbulence show downshifting in frequency near ρ≊0.8 as a result of the reduced Doppler shift (ω≊kθEr/Bφ) and a factor of 2 increase in the turbulence level (ñ/n)2 in the period between edge localized modes (ELMs). There is also a large reduction in turbulence at an ELM which tends to compensate for the increase in turbulence with reduced radial electric field shear between ELMs. No significant change is found in H-mode plasma energy, confinement time, internal inductance li, density profile, Te profile, or Ti profile. Good H-mode confinement is maintained by the edge (ρ≳0.95) transport barrier where the reversed edge Er and high edge Er shear remain unchanged.