A universal mechanism for plasma blob formation

Bisai, N.; Banerjee, Santanu; Sen, Abhijit

doi:10.1063/1.5082241

Cited by 19 publications

(25 citation statements)

References 20 publications

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“…A single universal criterion for the formation of blobs, which incorporates both poloidal and radial velocity shear was proposed. The theoretical result was found to be in good agreement with two dimensional numerical simulations [14] and also three dimensional simulations [15].…”

Section: Introductionsupporting

confidence: 77%

“…Krasheninnikov et al [13] have attributed the nonlinear Boltzmann factor in the dependence of the electron density on the electrostatic potential to be responsible for the generation of a blob. Recently a unified model incorporating both the scenarios-that is the presence and absence of a poloidal velocity shear, was proposed by Bisai et al [14]. It was argued that in the absence of poloidal velocity shear, one still has the presence of a radial velocity shear that can lead to differential stretching of the streamer, leading to a pinching and breakup of the streamer.…”

Section: Introductionmentioning

confidence: 99%

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Experimental validation of universal plasma blob formation mechanism

et al. 2022

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Anomalous plasma transport in the boundary region of a tokamak plasma is commonly associated with the formation and evolution of coherent density structures known as blobs. Recently, a theory for a universal mechanism of plasma blob formation has been put forward. It is based on a breaking process of a radially elongated streamer due to poloidal and radial velocity shears. The theory is well supported by two-dimensional and three-dimensional numerical simulation results but lacks experimental validation. In this work, we report the first ever experimental validation of this universal criterion by testing it against NSTX data on blobs obtained using the gas-puff imaging (GPI) diagnostic. It is found that the criterion is widely satisfied in most L-mode discharges and may explain the significantly larger number of blob events. We also validate the theoretical criterion against ADITYA Langmuir probe data taken in the scrape-off layer region.

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Section: Introductionsupporting

confidence: 77%

Section: Introductionmentioning

confidence: 99%

Experimental validation of universal plasma blob formation mechanism

et al. 2022

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“…At the same time, gyro-fluid simulations performed in the vicinity of the separatrix indicate that blob formation is not bound to one particular location or instability [10]. A recently proposed universal mechanism suggests that their generation is different in the presence of poloidal velocity shear or of a radial velocity shear in the absence of the former [24].…”

Section: Introductionmentioning

confidence: 95%

Intermittent structures and quasi-stationary equilibrium in a simple magnetized torus in open field line configuration

et al. 2022

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A steady regime dominated by intermittent blob and hole structures is identified in the plasma state of a simple magnetized torus by achieving a quasi-stationary equilibrium using an open magnetic field line configuration. The open helical field line configuration is characterized by a connection length, L_c=2a(B_φ/B_z )≳1750 cm, and pitch ratio, r_B=B_z/B_φ ≲0.01. This is realized by superposing a vertical magnetic fied, B_z, to the toroidal field, B_φ, and the regime is achieved for B_z≤0.4 mT. The combined effect of plasma rotation, arising from a substantial radial electric field, together with an open field line, results in vertically elongated plasma profiles and an asymmetric sheared poloidal flow. The analysis shows the existence of density fluctuations exhibiting universal statistical properties, dominated by non-Gaussian blob events in the edge region and holes in the core plasma, separated by a region ascribed as blob birth zone corresponding to a velocity shear layer. Two-dimensional conditional averaging analyses of fluctuations indicate that blobs form in the sheared layer, when the leading edge of an elongated coherent structure breaks off by differential stretching exerted by the background fluctuating field. Convection of this isolated blob out of the contour corresponding to the maximum radial electric field in the low field side, leads to its ejection while holes move along the same contour driven back into the main plasma. The corresponding potential structure shows counter-rotating E×B velocity field within oppositely charged structures, where the embedded electric field is consistent with the observed structure propagation. A comparison with cross-correlation analysis yields a similar conclusion except for a slight overestimation of the structure size and lifetime.

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“…The basic proposition is that the long scale structures suffer a “pinching off” or a local depletion of density in some regions due to the effect of perpendicular or parallel sheared flows and this eventually leads to their breakup. The physical mechanism for this breakup is analogous to what happens to interchange instability triggered streamer like density structures arising in the edge region of tokamaks that breakup into smaller structures called “blobs” (Bisai et al., 2004, 2019; D'Ippolito et al., 2011) under the influence of sheared flows. The sheared flow in the case of the auroral F region is the

\vec{E} \times \vec{B}

velocity which convects the elongated density structures and also breaks them up through differential stretching.…”

Section: Introductionmentioning

confidence: 98%

Physical Origin of Short Scale Plasma Structures in the Auroral F Region

Bisai

Sen

2021

JGR Space Physics

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ionospheric regions and have been detected using a variety of techniques including radio star scintillations, radio wave propagation, satellite scintillations, radar backscatter, in situ satellite and rocket measurements, total electron content measurements, radar imaging etc. In general, these F region irregularities consist of a wide spectrum of scale lengths ranging from a few kilometers down to a few meters. They are an important concern for communication and navigation systems as the irregularities can cause scintillations in the radio signals and thereby degrade them. Over the years, a number of theoretical studies aided by numerical simulations have elucidated the dynamics of these density irregularity structures. In the low-latitude equatorial region, the plasma density irregularities are associated with the onset of gravity waves seeded Rayleigh Taylor instability in the bottom side of the F region and the subsequent nonlinear formation of plasma bubbles that rise into the upper region of the ionosphere (

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A universal mechanism for plasma blob formation

Cited by 19 publications

References 20 publications

Experimental validation of universal plasma blob formation mechanism

Experimental validation of universal plasma blob formation mechanism

Intermittent structures and quasi-stationary equilibrium in a simple magnetized torus in open field line configuration

Physical Origin of Short Scale Plasma Structures in the Auroral F Region

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