Development of core ion temperature gradients and edge sheared flows in a helicon plasma device investigated by laser induced fluorescence measurements

Thakur, Saikat Chakraborty; Gosselin, J. J.; McKee, John; Scime, Earl; Sears, Stephanie; Tynan, G. R.

doi:10.1063/1.4960824

Cited by 17 publications

(16 citation statements)

References 32 publications

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“…The axial velocity, v z , was measured by a Mach In previous studies, we found that Mach probe measurements can give spuriously large axial flows 22 which were later found to be inconsistent with laser-induced fluorescence (LIF). 27 This overestimation of parallel Mach number is found to be related to shadowing effects in Mach probes. 30 In this study, we used small enough tips (D probe ≈ 3 mm) to avoid probe shadowing effects and we verified that the mean flow profile measured by the Mach probe agreed with LIF measurements of the same ion flow taken in the same plasma conditions.…”

Section: Methodsmentioning

confidence: 97%

“…More details on this device can be found in previous publications. 21,22,27 A horizontal scanning probe was used to record basic plasma information such as ion saturation currents and floating potentials at port α that is about 1 m downstream from the helicon source. The probe array is a combination of Mach and Langmuir probes and is capable of measuring the axial and radial plasma velocities simultaneously (Fig.…”

Section: Methodsmentioning

confidence: 99%

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Generation of parasitic axial flow by drift wave turbulence with broken symmetry: Theory and experiment

Hong

Hajjar

et al. 2018

Physics of Plasmas

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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

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Section: Methodsmentioning

confidence: 97%

Section: Methodsmentioning

confidence: 99%

Generation of parasitic axial flow by drift wave turbulence with broken symmetry: Theory and experiment

Hong

Hajjar

et al. 2018

Physics of Plasmas

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“…In addition, LIF measurements in CSDX show that the axial flow is well within the subsonic limit. 31 Thus, the radial gradient of the parallel velocity reported as contributing to an inward particle flux in Ref. 32 does not contribute to such a flux here, as a parallel shear flow instability simply cannot be triggered.…”

Section: Structure Of the 3-field Reduced Modelmentioning

confidence: 64%

Modelling enhanced confinement in drift-wave turbulence

et al. 2017

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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.

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“…Experimental results show that when drift waves dominate, the turbulence production due to the release of free energy in r n, can excite secondary parallel flows. 22,25,30,38 Theoretical studies also show that both axial and zonal flows are driven by turbulence, particularly by the non-diffusive residual stress in both expressions for hṽ xṽy i and hṽ xṽz i. 21,23,32,39,40 Reference 26 investigates the relation between the axial and azimuthal flows and turbulence, and formulates a new zonal momentum balance theorem for the coupled drift-ion acoustic waves.…”

Section: B the Vorticity Flux The Perpendicular Reynolds Stress Anmentioning

confidence: 99%

“…r as a function of the density fluctuationsñ normalized by the average plasma density n 42 l 0 ' 2:3q 0:6 s L 0:3 n : (38) This suggests that the CSDX turbulent plasma diffusion coefficient scales like…”

Section: A Case Of a Purely Azimuthal Shearmentioning

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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Development of core ion temperature gradients and edge sheared flows in a helicon plasma device investigated by laser induced fluorescence measurements

Cited by 17 publications

References 32 publications

Generation of parasitic axial flow by drift wave turbulence with broken symmetry: Theory and experiment

Generation of parasitic axial flow by drift wave turbulence with broken symmetry: Theory and experiment

Modelling enhanced confinement in drift-wave turbulence

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

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