Intermittent turbulence and turbulent structures in a linear magnetized plasma

Carter, Troy

doi:10.1063/1.2158929

Cited by 153 publications

(148 citation statements)

References 24 publications

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“…4 in Ref. 17) usually suggest that the scale size of the turbulent structure is proportional to the ion sound radius q s . So, in this experiment, it is expected that the edge turbulence and transport are associated with both the azimuthal flow induced by the E Â B drift and the value of q s .…”

Section: A Overviewmentioning

confidence: 99%

Sheared-flow induced confinement transition in a linear magnetized plasma

et al. 2012

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A magnetized plasma cylinder (12 cm in diameter) is induced by an annular shape obstacle at the Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875(1991]. Sheared azimuthal flow is driven at the edge of the plasma cylinder through edge biasing. Strong fluctuations of density and potential (dn=n $ ed/=kT e $ 0:5) are observed at the plasma edge, accompanied by a large density gradient (L n ¼ jr ln nj À1 $ 2 cm) and shearing rate (c $ 300 kHz). Edge turbulence and cross-field transport are modified by changing the bias voltage (V bias ) on the obstacle and the axial magnetic field (B z ) strength. In cases with low V bias and large B z , improved plasma confinement is observed, along with steeper edge density gradients. The radially sheared flow induced by E Â B drift dramatically changes the cross-phase between density and potential fluctuations, which causes the wave-induced particle flux to reverse its direction across the shear layer. In cases with higher bias voltage or smaller B z , large radial transport and rapid depletion of the central plasma density are observed. Two-dimensional cross-correlation measurement shows that a mode with azimuthal mode number m ¼ 1 and large radial correlation length dominates the outward transport in these cases. Linear analysis based on a two-fluid Braginskii model suggests that the fluctuations are driven by both density gradient (drift wave like) and flow shear (Kelvin-Helmholtz like) at the plasma edge.

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Section: A Overviewmentioning

confidence: 99%

Sheared-flow induced confinement transition in a linear magnetized plasma

et al. 2012

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“…At the edge of magnetic confinement devices, such as tokamaks, [1][2][3] stellarators, 4 reversed field pinches, 5 and linear devices, [6][7][8] a large fraction of the anomalous particle and heat transport is attributed to the presence of blobs. These are intermittent isolated structures where density and temperature are increased with respect to the surrounding plasma.…”

Section: Introductionmentioning

confidence: 99%

“…18 From the experimental point of view, the investigation of the generation mechanism in fusion devices is hampered by the intrinsic difficulty to have diagnostic access in the region of interest with adequate temporal and spatial resolution. To overcome these limitations, a number of investigations of blob physics have recently been undertaken in basic plasma physics devices, [6][7][8] in which full diagnostics access with adequate spatial and temporal resolution can be obtained. Experiments in the linear magnetized helicon device VINETA show a close link between blobs and quasicoherent drift waves.…”

Section: Introductionmentioning

confidence: 99%

Mechanism for blob generation in the TORPEX toroidal plasma

et al. 2008

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The mechanism for blob generation is detailed in the toroidal magnetized plasma of the TORPEX device ͓Fasoli et al., Phys. Plasmas 13, 055902 ͑2006͔͒ using an experimental configuration, which features a plasma region dominated by a coherent wave and a region on the low field side characterized by the propagation of blobs. Predictions from linearized 2D drift-reduced Braginskii equations are compared with experimental data, revealing the interchange nature of the coherent wave. The dynamics of blob formation is investigated using time-resolved measurements of two-dimensional profiles of electron density, temperature, plasma potential and E ϫ B velocity. Blobs form from radially elongated structures associated with the interchange wave. When a blob is generated, the following sequence of events is observed: 1͒ A decrease of local pressure gradient length provides an increase of the interchange mode drive; 2͒ in response, the interchange mode increases in amplitude and expands in the radial direction forming a radially elongated structure from the wave crest; 3͒ the elongated structure is convected by the E ϫ B flow and is eventually sheared off, forming a blob on the low field side. The dependence of the blob amplitude upon the minimum pressure radial scale length before the blob ejection is also investigated.

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“…Recent investigations in plasma physics have revealed the presence of coherent structures of density (also referred to as blobs) in the scrape-off layer of tokamaks [1,3], in simple magnetized torus (SMT) [4,5], and in linear devices [6,7]. In addition to the presence of blobs in SMT, the existence of a shear in the background E Â B flow velocity was recently shown in Ref.…”

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confidence: 99%

“…The velocity shear in a flow is a source of diverse energy-intensive processes (e.g., coherent structures, vortices), which occur in astrophysical objects, in hydrodynamics, in laboratory plasmas, and biological systems. Vortex and coherent structure dynamics are being actively investigated in order to address the recurring dilemma caused by the relationship between the onset and the maintenance of turbulence in shear flows [2].Recent investigations in plasma physics have revealed the presence of coherent structures of density (also referred to as blobs) in the scrape-off layer of tokamaks [1,3], in simple magnetized torus (SMT) [4,5], and in linear devices [6,7]. In addition to the presence of blobs in SMT, the existence of a shear in the background E Â B flow velocity was recently shown in Ref.…”

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confidence: 99%

Dynamics of Plasma Blobs in a Shear Flow

et al. 2008

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The global dynamic of plasma blobs in a shear flow is investigated in a simple magnetized torus using the spatial Fourier harmonics (k-space) framework. Direct experimental evidence of a linear drift in k space of the density fluctuation energy synchronized with blob events is presented. During this drift, an increase of the fluctuation energy and a production of the kinetic energy associated with blobs are observed. The energy source of the blob is analyzed using an advection-dissipation-type equation that includes blob-flow exchange energy, linear drift in k space, nonlinear processes, and viscous dissipations. We show that blobs tap their energy from the dominant E Â B vertical background flow during the linear drift stage. The exchange of energy is unidirectional as there is no evidence that blobs return energy to the flow. In magnetic fusion devices, it is well known that substantial amounts of plasma appearing as coherent structures elongated [1] along the magnetic field lines can propagate to the outer wall, ultimately causing erosion problems. A fair amount is already known about the propagation of these coherent structures; relatively little is understood about their generation and their interaction with the background sheared flow. The velocity shear in a flow is a source of diverse energy-intensive processes (e.g., coherent structures, vortices), which occur in astrophysical objects, in hydrodynamics, in laboratory plasmas, and biological systems. Vortex and coherent structure dynamics are being actively investigated in order to address the recurring dilemma caused by the relationship between the onset and the maintenance of turbulence in shear flows [2].Recent investigations in plasma physics have revealed the presence of coherent structures of density (also referred to as blobs) in the scrape-off layer of tokamaks [1,3], in simple magnetized torus (SMT) [4,5], and in linear devices [6,7]. In addition to the presence of blobs in SMT, the existence of a shear in the background E Â B flow velocity was recently shown in Ref. [5]. In this Letter, we focus on the energy exchange dynamics of blobs with the background flow in the presence of shear. In particular, we present a global analysis in k space of plasma fluctuations in the presence of intermittent and coherent structures also known as plasma blobs in TORPEX [8] with no prior Fourier decomposition in time. This approach, also referred to as non-normal, is generally utilized to better capture the time evolution of these structures in k space. First, within this framework, we show experimental evidence of a linear drift of radial wave numbers in time. This drift is associated with an increase of the kinetic energy of the fluctuations associated with the blob, and is similar to numerical investigations of self-sustained coherent vortices in a shear flow reported by Kim and co-workers [9]. In addition to the drift, we deduce the blob time of flight to the edge of the plasma cross section. Second, the key question pertaining to the source of energy provi...

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Intermittent turbulence and turbulent structures in a linear magnetized plasma

Cited by 153 publications

References 24 publications

Sheared-flow induced confinement transition in a linear magnetized plasma

Sheared-flow induced confinement transition in a linear magnetized plasma

Mechanism for blob generation in the TORPEX toroidal plasma

Dynamics of Plasma Blobs in a Shear Flow

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