Intrinsic rotation generation in ELM-free H-mode plasmas in the DIII-D tokamak—Experimental observationsa)

A detailed description is presented of the experiment reported in [S. H. Müller et al., Phys. Rev. Lett. 106, 115001 (2011)], which reported the first measurements of fluid turbulent stresses in a tokamak H-mode pedestal. Mach probe measurements disclosed a narrow co-current rotation layer at the separatrix, which is also seen in some L-modes [J. A. Boedo et al., Phys. Plasmas 18, 032510 (2011)]. Independent evidence for the existence of the edge co-rotation layer is presented from main-ion rotation measurements by charge-exchange-recombination spectroscopy in comparable helium plasmas. The probe measurements are validated against density and electron temperature profiles from Thomson scattering and in terms of the measured turbulent particle transport, which is consistent with the global density rise. Non-diffusive non-convective angular momentum transport is required by two independent experimental observations: (1) A persistent dip in the rotation profile separates the edge layer from the evolving core region during intrinsic rotation development. (2) The rotation profiles with co- and counter-current neutral beam injection appear well described as the simple sum of a constant intrinsic part and the beam-driven part, also demonstrating the profile-independence of the intrinsic torque. Characteristics of the turbulent fluctuations composing the fluid turbulent stresses are discussed: Up to 0.5 cm inside the separatrix, the low amplitude of the Reynolds stress (<0.05 Nm of torque) is due to both a reduction of the fluctuation amplitudes at the peak of the edge co-rotation layer and weak correlations between the toroidal and radial velocity fluctuations. Further into the core, the correlations increase significantly up to a value of +0.75, resulting in an almost unidirectional character of the turbulent Reynolds stress, generating substantial counter-current torques up to -2 Nm. Additional mechanisms must be present to balance these torques and explain the co-current core-plasma spin-up at a rate of +0.3 Nm.