Modelling enhanced confinement in drift-wave turbulence

Hajjar, R. J.; Diamond, P. H.; Ashourvan, Arash; Tynan, G. R.

doi:10.1063/1.4985323

Cited by 6 publications

(5 citation statements)

References 46 publications

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“…According to the scalings of Eqs. (20) and (24), the particle flux scaling changes from C adia / 1=a with a > 1 to C hydro / ffiffiffiffiffiffiffi ffi 1=a p with a < 1. The turbulent diffusivity v y that relates the vorticity flux to the vorticity gradient also exhibits the same scaling.…”

Section: B Scalings Of Transport Fluxes With Amentioning

confidence: 99%

Dynamics of zonal shear collapse with hydrodynamic electrons

2018

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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.

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Section: B Scalings Of Transport Fluxes With Amentioning

confidence: 99%

Dynamics of zonal shear collapse with hydrodynamic electrons

2018

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“…36 The current model is an extension of that presented in Ref. 42, where the predator/prey relation between DWs and ZFs was derived and validated. 2.…”

Section: Discussionmentioning

confidence: 99%

“…In 2-D turbulent systems, the Rhines' scale, l Rh , defined as the scale beyond which the inverse energy cascade terminates, emerges as an appropriate mixing length. 42 Turbulence simply changes character for l > l Rh , and the plasma dynamics evolve from a turbulence cascade regime to a wave like behavior. In CSDX, the plasma system does not exhibit a sufficiently large dynamical range of energy transfer to observe this transition in turbulence dynamics.…”

Section: The Radial Mixing Length L Mixmentioning

confidence: 99%

The ecology of flows and drift wave turbulence in CSDX: A model

2018

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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 .

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“…The goal of this paper is to understand the evolution of fluctuation-flow ecology (including both hv k i and hv h i). 23,24 This has not been addressed by previous experiments or simulations. The question of what couples the parallel and perpendicular flows in the absence of geometrical mechanisms is open.…”

Section: Introductionmentioning

confidence: 94%

How shear increments affect the flow production branching ratio in CSDX

Diamond

2018

Physics of Plasmas

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The coupling of turbulence-driven azimuthal and axial flows in a linear device absent magnetic shear (Controlled Shear Decorrelation Experiment) is investigated. In particular, we examine the apportionment of Reynolds power between azimuthal and axial flows, and how the azimuthal flow shear affects axial flow generation and saturation by drift wave turbulence. We study the response of the energy branching ratio, i.e., ratio of axial and azimuthal Reynolds powers, PzR/PyR, to incremental changes of azimuthal and axial flow shears. We show that increasing azimuthal flow shear decreases the energy branching ratio. When axial flow shear increases, this ratio first increases but then decreases to zero. The axial flow shear saturates below the threshold for parallel shear flow instability. The effects of azimuthal flow shear on the generation and saturation of intrinsic axial flows are analyzed. Azimuthal flow shear slows down the modulational growth of the seed axial flow shear, and thus reduces intrinsic axial flow production. Azimuthal flow shear reduces both the residual Reynolds stress (of axial flow, i.e., ΠxzRes) and turbulent viscosity (χzDW) by the same factor |⟨vy⟩′|−2Δx−2Ln−2ρs2cs2, where Δx is the distance relative to the reference point where ⟨vy⟩=0 in the plasma frame. Therefore, the stationary state axial flow shear is not affected by azimuthal flow shear to leading order since ⟨vz⟩′∼ΠxzRes/χzDW.

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Modelling enhanced confinement in drift-wave turbulence

Cited by 6 publications

References 46 publications

Dynamics of zonal shear collapse with hydrodynamic electrons

Dynamics of zonal shear collapse with hydrodynamic electrons

The ecology of flows and drift wave turbulence in CSDX: A model

How shear increments affect the flow production branching ratio in CSDX

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