Title: Transport Barrier Formation on HBT-EP
Description: The physics of the biasing induced L-H transition and the mechanism for E×B shear flow suppression of turbulence are investigated on HBT-EP. Detailed measurements of the transverse length scales, behavior, and propagation direction of the edge turbulence match what is expected for the ion temperature gradient (ITG) mode. In the scrape-off layer (SOL), radially propagating blob-filament turbulence is identified and characterized, with velocities, sizes, and distributions comparable to measurements on other devices. Through systematic studies of the effect of applied shear flow on the turbulence, it is found that the E×B suppression of turbulence matches what is expected by the spectral shift model [Staebler et al. 2013 Phys. Rev. Lett. 110 055003]. Namely, the application of shear flow tilts the turbulent eddies and shifts the mean radial wavenumber ⟨kr⟩ of the turbulence spectrum from near zero to finite values, leading to a reduction in the turbulence intensity. The investigation also shows that both the decorrelation model and quench rule are able to reproduce the measured reduction of the turbulence intensity with applied shear flow when appropriate parameters are chosen.

However, the decorrelation model fails to explain the increase in the shear-wise correlation length measured with increasing applied shear, and the quench rule fails to capture the suppression of the turbulence to a finite intensity at high shear. It is found that the same shearing effect that tilts the eddy structures and shifts ⟨kr⟩, enhances the gradient in the Reynolds stress at the edge and suppresses the blob-filament turbulence in the SOL. Although the biasing levels leading up to the transition are shown to enhance the Reynolds stress in a radially varying manner, it is found that the high flow shear in the H-mode state completely quenches the Reynolds stress. A careful examination of the spatial structure and temporal dynamics of the forcing terms in both dithering and one-step transitions reveals that the biasing induced L-H transition is caused by a reduction in poloidal viscosity at high flow velocity, in agreement with neoclassical theory. Nevertheless, the Reynolds force is measured to be comparable to the force from the electrode current, allowing the turbulence driven stress to work synergistically (or antagonistically) with forces from the probe to achieve the critical poloidal flow velocities.

The similarities between the transition criteria on HBT-EP and other devices indicate that reduction of poloidal viscosity leading to the transition to improved confinement regimes may be a universal trait among toroidal confinement devices. The application of resonant magnetic perturbations (RMPs) is shown to both reduce the Reynolds stress and increase the biasing threshold for the transition. The observed reduction in the Reynolds stress stems from a reduction in the intensity of the underlying turbulence; namely, a decrease in the amplitude of velocity fluctuations in regions where the Reynolds stress is high without an applied RMP. This study has therefore expanded the current understanding of transport barrier formation in magnetic confinement devices.