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

Journal Article · 01 February 2018 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.5018320 · OSTI ID:1524572

Hajjar, R. J. ^[1]; Diamond, P. H. ^[2]; Tynan, G. R. ^[3]

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1. Univ. of California, San Diego, CA (United States). Center for Energy Research
2. Univ. of California, San Diego, CA (United States). Center for Energy Research, Center for Astrophysics and Space Sciences; Center for Fusion Sciences, Southwestern Inst. of Physics, Chengdu, Sichuan (China)
3. Univ. of California, San Diego, CA (United States). Center for Energy Research; Center for Fusion Sciences, Southwestern Inst. of Physics, Chengdu, Sichuan (China)

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 $$\bar{n}$$, and mean flows $$\bar{v}$$y and $$\bar{v}$$z, in addition to fluctuation intensity ε, is presented. Here, $\epsilon =\langle {\tilde{n}}^2+{\left({\nabla }_{\perp }\tilde{\Phi }\right)}^2+{\tilde{v}}_z^2\rangle$ 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 $\langle \tilde{n}{\tilde{v}}_x\rangle$ and Reynolds stresses $\langle {\tilde{v}}_x{\tilde{v}}_y\rangle$ and $\langle {\tilde{v}}_x{\tilde{v}}_z\rangle$ Acoustic coupling breaks parallel symmetry and generates a parallel residual stress Π res xz . The model uses a set of equations to explain the acceleration of $$\bar{v}_y$$ and $$\bar{v}_z$$ via ${\Pi }_{xy}^{res}\propto \nabla \stackrel{-}{n}$ and ${\Pi }_{xy}^{res}\propto \nabla \stackrel{-}{n}$. Flow dynamics in the parallel direction are related to those in the perpendicular direction through an empirical coupling constant σVT. This constant measures the degree of symmetry breaking in the 〈$$k_mk_z$$〉 correlator and determines the efficiency of ∇$$\bar{n}$$ in driving $$\bar{v}_z$$. The model also establishes a relation between ∇$$\bar{v}_y$$ and ∇$$\bar{v}_z$$, via the ratio of the stresses Π res xy and Π res xz . When parallel to perpendicular flow coupling is weak, axial Reynolds power $P_{xz}^{Re}=-\langle {\tilde{v}}_x{\tilde{v}}_z\rangle \nabla {\stackrel{-}{v}}_z$ is less than the azimuthal Reynolds power $P_{xy}^{Re}=-\langle {\tilde{v}}_x{\tilde{v}}_y\rangle \nabla {\stackrel{-}{v}}_y$∇$$\bar{v}_y$$. The model is then reduced to a 2-field predator/prey model where $$\bar{v}_z$$ is parasitic to the system and fluctuations evolve self-consistently. Finally, turbulent diffusion in CSDX follows the scaling: $D_{\text{CSDX}}=D_B{\rho }_{*}^{0.6}$, where D_B is the Bohm diffusion coefficient and ρ_* is the ion gyroradius normalized to the density gradient $|\nabla \stackrel{-}{n}/\stackrel{-}{n}|{}^{-1}$

• Research Organization: Univ. of California, San Diego, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: FG02-04ER54738
• OSTI ID: 1524572
• Alternate ID(s): OSTI ID: 1420214
• Journal Information: Physics of Plasmas, Vol. 25, Issue 2; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English