Velocity shear, turbulent saturation, and steep plasma gradients in the scrape-off layer of inner-wall limited tokamaks

Halpern, Federico David; Ricci, Paolo

doi:10.1088/1741-4326/aa4eb6

Cited by 26 publications

(32 citation statements)

References 25 publications

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“…This interpretation is consistent with a recent theoretical model of the near SOL, presented in Ref. [40], and summarized in the following. In the edge region (inside the LCFS), the radial electric field E r is always negative.…”

Section: Page 18 Of 26 Author Submitted Manuscript -Nf-101877r2supporting

confidence: 93%

“…Non-ambipolar currents, correlated to the presence of a velocity shear, have been measured at the limiter with flush-mounted Langmuir probes. The power in the near SOL ∆P SOL is shown to be correlated to the strength of the velocity shear which, according to a recent theoretical model [40], can affect the turbulence in the near SOL, locally steepening the pressure profiles. The parallel heat fluxes and the non-ambipolar currents measured at the limiter are compared with measurements taken at the Outer MidPlane (OMP) with a reciprocating probe.…”

Section: Resultsmentioning

confidence: 79%

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Understanding and suppressing the near scrape-off layer heat flux feature in inboard-limited plasmas in TCV

et al. 2017

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In inboard-limited plasmas, the Scrape-Off Layer (SOL) shows two regions: the near SOL, extending a few mm from the Last Closed Flux Surface (LCFS), characterized by a steep gradient of the parallel heat flux radial profile, and a far SOL, typically some cm wide, with flatter heat flux profiles. The physics of the near SOL is investigated in TCV with two series of experiments featuring deuterium and helium plasmas, in which the plasma current, density and elongation have been varied. The parallel heat flux profiles are measured on the limiter by means of infrared thermography. For the first time, the near SOL is reported to disappear for low plasma current or at high density, for values of the SOL collisionality ν * SOL corresponding to a conduction-limited regime. The power in the near SOL ∆P SOL is shown to decrease with the normalized Spitzer resistivity ν as ∆P SOL ∝ ν −1 . The floating potential profiles, measured at the limiter using flush-mounted Langmuir probes (LP), show the presence of non-ambipolar currents, and their relation to the presence of a velocity shear layer is discussed. The shearing rate is shown to strictly correlate with the power in the near SOL ∆P SOL , consistently with a recent theoretical model. Measurements of the near SOL on the Low Field Side (LFS) are performed using a reciprocating Langmuir probe (RP). The near SOL is reported to vanish simultaneously at the LFS and at the limiter. The near and far SOL widths are compared with the predictions from existing theoretical models, to which empirical corrections with resistivity and elongation are proposed.

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Section: Page 18 Of 26 Author Submitted Manuscript -Nf-101877r2supporting

confidence: 93%

Section: Resultsmentioning

confidence: 79%

Understanding and suppressing the near scrape-off layer heat flux feature in inboard-limited plasmas in TCV

et al. 2017

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“…We note that the imperfect balance of the three terms in figures 3 and 4 is mainly due to the approximations made in deriving (2.8), (2.13) and (2.16), i.e. the balance given by (2.3) is excellent in the GBS simulations, as pointed out in Halpern & Ricci (2017). The validity of (2.18), or equivalently, the predicted profile of floating potential, equation (2.19), relies on the assumption that V f is the same on both sides of the limiter.…”

Section: Sol Turbulence Simulationsmentioning

confidence: 66%

“…While this model may certainly explain non-ambipolar flows in certain experiments (Strawitch & Emmert 1981), it cannot describe the recent observations made in ITER-relevant tokamak devices: the fact that the magnitude of these sheath currents decreases with plasma collisionality, namely with the normalized collision frequency ν * SOL (Tsui et al 2017). Moreover, it has been recently shown via nonlinear SOL turbulence simulations that both the heat-flux narrow feature and the associated non-ambipolar flows arise even without accounting for the effect of neutrals (Halpern & Ricci 2017).…”

Section: Introductionmentioning

confidence: 93%

Scrape-off-layer current loops and floating potential in limited tokamak plasmas

et al. 2017

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We investigate the question of how plasma currents circulate and close in the scrape-off-layer (SOL) of convection-limited tokamak plasmas. A simplified two-fluid model describes how currents must evacuate charge at the sheaths due to cross-field currents that are not divergence-free. These include turbulence-driven polarization currents and poloidally asymmetric equilibrium diamagnetic currents. The theory provides an estimate for the radial profile of the floating potential, which reveals a dipolar structure like the one observed experimentally. Simulations with a fluid turbulence code provide evidence for the predicted behaviour of currents and floating potential.

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“…The simulation includes the SOL and part of the edge region, where the magnetic field lines lie on closed flux surfaces without intersecting the wall, similarly to [29]. (We remark that we present here for the first time a GBS simulation that includes the SOL and edge regions, as well as the self-consistent neutral-plasma interaction terms.)…”

Section: Self-consistent Turbulence Simulationmentioning

confidence: 99%

Impact of neutral density fluctuations on gas puff imaging diagnostics

Wersal

Ricci

2017

Nucl. Fusion

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A three-dimensional turbulence simulation of the SOL and edge regions of a toroidally limited tokamak is carried out. The simulation couples self-consistently the drift-reduced two-fluid Braginskii equations to a kinetic equation for neutral atoms. A diagnostic neutral gas puff on the low-field side midplane is included and the impact of neutral density fluctuations on light emission investigated. We find that neutral density fluctuations affect the emission. In particular, at a radial distance from the gas puff smaller than the neutral mean free path, neutral density fluctuations are anti-correlated with plasma density, electron temperature, and fluctuations. It follows that the neutral fluctuations reduce the emission in most of the observed region and, therefore, have to be taken into account when interpreting the amplitude of the emission. On the other hand, higher order statistical moments (skewness, kurtosis) and turbulence characteristics (such as correlation length, or the autocorrelation time) are not significantly affected by the neutral fluctuations. At distances from the gas puff larger than the neutral mean free path, a non-local shadowing effect influences the neutral density fluctuations. There, the fluctuations are correlated with the neutral density fluctuations, and the high-order statistical moments and measurements of other turbulence properties are strongly affected by the neutral density fluctuations.

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Velocity shear, turbulent saturation, and steep plasma gradients in the scrape-off layer of inner-wall limited tokamaks

Cited by 26 publications

References 25 publications

Understanding and suppressing the near scrape-off layer heat flux feature in inboard-limited plasmas in TCV

Understanding and suppressing the near scrape-off layer heat flux feature in inboard-limited plasmas in TCV

Scrape-off-layer current loops and floating potential in limited tokamak plasmas

Impact of neutral density fluctuations on gas puff imaging diagnostics

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