This paper presents a theory for the collapse of the edge zonal shear layer, as observed at the density limit at low b. This paper investigates the scaling of the transport and mean profiles with the adiabaticity parameter a, with special emphasizes on fluxes relevant to zonal flow (ZF) generation. We show that the adiabaticity parameter characterizes the strength of production of zonal flows and so determines the state of turbulence. A 1D reduced model that self-consistently describes the spatiotemporal evolution of the mean density n, the azimuthal flow v y , and the turbulent potential enstrophy e ¼ hðñ À r 2/ Þ 2 =2i-related to fluctuation intensity-is presented. Quasi-linear analysis determines how the particle flux C n and vorticity flux P ¼ Àv y r 2 v y þ P res scale with a, in both hydrodynamic and adiabatic regimes. As the plasma response passes from adiabatic (a > 1) to hydrodynamic (a < 1), the particle flux C n is enhanced and the turbulent viscosity v y increases. However, the residual flux P res-which drives the flow-drops with a. As a result, the mean vorticity gradient r 2 v y ¼ P res =v y-representative of the strength of the shear-also drops. The shear layer then collapses and turbulence is enhanced. The collapse is due to a decrease in ZF production, not an increase in damping. A physical picture for the onset of collapse is presented. The findings of this paper are used to motivate an explanation of the phenomenology of low b density limit evolution. A change from adiabatic (a ¼ k 2 z v 2 th =ðjxj ei Þ > 1) to hydrodynamic (a < 1) electron dynamics is associated with the density limit.

This study traces the emergence of sheared axial flow from collisional drift-wave turbulence with broken symmetry in a linear plasma device-the controlled shear decorrelation experiment. As the density profile steepens, the axial Reynolds stress develops and drives a radially sheared axial flow that is parallel to the magnetic field. Results show that the nondiffusive piece of the Reynolds stress is driven by the density gradient, results from spectral asymmetry of the turbulence, and, thus, is dynamical in origin. Taken together, these findings constitute the first simultaneous demonstration of the causal link between the density gradient, turbulence, and stress with broken spectral symmetry and the mean axial flow.

Detailed measurements of intrinsic axial flow generation parallel to the magnetic field in the CSDX linear plasma device with no axial momentum input are presented and compared to theory. The results show a causal link from the density gradient to drift-wave turbulence with broken spectral symmetry and development of the axial mean parallel flow. As the density gradient steepens, the axial and azimuthal Reynolds stresses increase and radially arXiv:1805.03705v1 [physics.plasm-ph] 9 May 2018

This paper describes the ecology of drift wave turbulence and mean flows in the coupled drift-ion acoustic wave plasma of a CSDX linear device. A 1D reduced model that studies the spatiotemporal evolution of plasma mean density n, and mean flows v y and v z , in addition to fluctuation intensity e, is presented. Here, e ¼ hñ 2 þ ðr ?/ Þ 2 þṽ 2 z i is the conserved energy field. The model uses a mixing length l mix inversely proportional to both axial and azimuthal flow shear. This form of l mix closes the loop on total energy. The model self-consistently describes variations in plasma profiles, including mean flows and turbulent stresses. It investigates the energy exchange between the fluctuation intensity and mean profiles via particle flux hñṽ x i and Reynolds stresses hṽ xṽy i and hṽ xṽz i. Acoustic coupling breaks parallel symmetry and generates a parallel residual stress P res xz. The model uses a set of equations to explain the acceleration of v y and v z via P res xy / r n and P res xy / r n. Flow dynamics in the parallel direction are related to those in the perpendicular direction through an empirical coupling constant r VT. This constant measures the degree of symmetry breaking in the hk m k z i correlator and determines the efficiency of r n in driving v z. The model also establishes a relation between r v y and r v z , via the ratio of the stresses P res xy and P res xz. When parallel to perpendicular flow coupling is weak, axial Reynolds power P Re xz ¼ Àhṽ xṽz ir v z is less than the azimuthal Reynolds power P Re xy ¼ Àhṽ xṽy ir v y. The model is then reduced to a 2-field predator/prey model where v z is parasitic to the system and fluctuations evolve self-consistently. Finally, turbulent diffusion in CSDX follows the scaling: D CSDX ¼ D B q 0:6 ? , where D B is the Bohm diffusion coefficient and q ? is the ion gyroradius normalized to the density gradient jr n= nj À1 .

The results of modeling studies of an enhanced confinement in the drift wave turbulent plasma of the CSDX linear device are presented. The mechanism of enhanced confinement is investigated here using a reduced 1D, time-dependent model, which illustrates the exchange of enstrophy between two disparate scale structures: the mesoscale flow and profile, and the turbulence intensity fields. Mean density, mean vorticity, and turbulent potential enstrophy are the variables for this model. Total potential enstrophy is conserved in this model. Vorticity mixing occurs on a scale length related to an effective Rhines' scale of turbulence, and shrinks as both density and vorticity gradients steepen. Numerical results obtained from solution of the model agree well with the experimental data from CSDX showing: (i) a steepening of the mean density profile, indicating a radial transport barrier formation, (ii) the development of a radially sheared azimuthal flow velocity that coincides with the density steepening and initiates a turbulence quench, and (iii) negative Reynolds work values, indicating that fluctuations drive the shear flow. These observations as the magnitude of the magnetic field B increases are recovered using purely diffusive expressions for the vorticity and density fluxes. A new dimensionless turbulence parameter R DT-defined as the ratio of the integrated potential enstrophy transfer from turbulence to the flow, to the integrated potential enstrophy production due to relaxation of the density gradient is introduced as a turbulence collapse indicator that detects when the enhanced confinement state is triggered. Published by AIP Publishing.