Nonlinear neoclassical transport in toroidal edge plasmas

Fülöp, Tünde; Helander, P.

doi:10.1063/1.1372179

Cited by 40 publications

(120 citation statements)

References 14 publications

(16 reference statements)

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“…Moreover, in the short neutral mean free path limit the velocity dependence of the neutral distribution function will become the same as that of the ions causing charge exchange collisions of the ions with the neutrals to produce no entropy. For longer neutral mean free paths we expect little entropy production due to the presence of the neutrals based on a self-similar treatment of the neutrals which finds results roughly in agreement with short mean free path results [51].…”

Section: Discussionsupporting

confidence: 74%

Arbitrary poloidal gyroradius effects in tokamak pedestals and transport barriers

Kagan

Catto

2008

Plasma Phys. Control. Fusion

126

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Abstract.A technique is developed and applied for analyzing pedestal and internal transport barrier (ITB) regions in a tokamak by formulating a special version of gyrokinetics. In contrast to typical gyrokinetic treatments, canonical angular momentum is taken as the gyrokinetic radial variable rather than the radial guiding center location. Such an approach allows strong radial plasma gradients to be treated, while retaining zonal flow and neoclassical (including orbit squeezing) behavior and the effects of turbulence. The new, nonlinear gyrokinetic variables are constructed to higher order than is typically the case. The nonlinear gyrokinetic equation obtained is capable of handling such problems as collisional zonal flow damping with radial wavelengths comparable to the ion poloidal gyroradius, as well as zonal flow and neoclassical transport in the pedestal or ITB. This choice of gyrokinetic variables allows the toroidally rotating Maxwellian solution of the isothermal tokamak limit to be recovered. More importantly, we prove that a physically acceptable solution for the lowest order ion distribution function in the banana regime anywhere in a tokamak and, in particular, in the pedestal must be nearly this same isothermal Maxwellian solution. That is, the ion temperature variation scale must be much greater than the poloidal ion gyroradius. Consequently, in the banana regime the background radial ion temperature profile cannot have a pedestal similar to that of plasma density.

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Section: Discussionsupporting

confidence: 74%

Arbitrary poloidal gyroradius effects in tokamak pedestals and transport barriers

Kagan

Catto

2008

Plasma Phys. Control. Fusion

126

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“…This magnitude is larger than allowed by standard neoclassical transport [2][3][4][5] (in this paper, standard neoclassical transport will refer to transport derived assuming the main ion poloidal Larmor radius is much smaller than the perpendicular gradient scale lengths in ion density or temperature, ρ θ,i L ⊥ ). The impurity density asymmetry was present in plasmas with strong electron density gradients, namely H-mode plasmas.…”

Section: Introductionmentioning

confidence: 93%

Poloidal asymmetries in edge transport barriers

et al. 2015

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Measurements of impurities in Alcator C-Mod indicate that in the pedestal region, significant poloidal asymmetries can exist in the impurity density, ion temperature, and main ion density. In light of the observation that ion temperature and electrostatic potential are not constant on a flux surface [Theiler et al., Nucl. Fusion 54, 083017 (2014)], a technique based on total pressure conservation to align profiles measured at separate poloidal locations is presented and applied. Gyrokinetic neoclassical simulations with XGCa support the observed large poloidal variations in ion temperature and density, and that the total pressure is approximately constant on a flux surface. With the updated alignment technique, the observed in-out asymmetry in impurity density is reduced from previous publishing [Churchill et al., Nucl. Fusion 53, 122002 (2013)], but remains substantial (nz,H/nz,L∼6). Candidate asymmetry drivers are explored, showing that neither non-uniform impurity sources nor localized fluctuation-driven transport are able to explain satisfactorily the impurity density asymmetry. Since impurity density asymmetries are only present in plasmas with strong electron density gradients, and radial transport timescales become comparable to parallel transport timescales in the pedestal region, it is suggested that global transport effects relating to the strong electron density gradients in the pedestal are the main driver for the pedestal in-out impurity density asymmetry.

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“…This may indeed occur in strongly rotating plasmas (or even at moderate rotation for heavy impurities) where the centrifugal force induces poloidal asymmetries in the density and electrostatic potential [26]. At the plasma edge, the poloidal variation of the parallel friction between the bulk ions and impurities may also engender density asymmetries [27,28].…”

Section: Theoretical Background and Measurement Principlementioning

confidence: 99%

Indirect measurement of poloidal rotation using inboard–outboard asymmetry of toroidal rotation and comparison with neoclassical predictions

et al. 2013

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An alternative experimental spectroscopic measurement of poloidal plasma rotation in toroidally confined plasmas is proven effective in the TCV tokamak. Charge exchange recombination measurements of the toroidal rotation profile over the full mid-plane plasma diameter are used to infer the complete bi-dimensional flow structure of the intrinsic C 6+ impurity, which includes its poloidal component. For divergence free flows, the difference between the toroidal rotation frequency f t = u t /R at the inboard and outboard locations on the same flux surface is proportional to the poloidal rotation. This indirect measurement provides increased accuracy as the measured quantity f t,in − f t,out ≈ 4qu p /R axis (q is the local safety factor) is larger than the intrinsic uncertainties of a direct spectroscopic measurement of poloidal velocity. The method is applied in a variety of TCV ohmic and electron cyclotron heated L-mode plasmas in the banana-plateau collisionality regime (0.2 < ν * ii < 2.4). In the radial range of normalized poloidal flux ρ ψ < 0.8, an impurity poloidal velocity of u p = 0.5-2.5 km s −1 is observed, always in the electron diamagnetic drift direction. The measurements are compared with neoclassical calculations and they agree in magnitude and sign to within <1 km s −1 .

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Nonlinear neoclassical transport in toroidal edge plasmas

Cited by 40 publications

References 14 publications

Arbitrary poloidal gyroradius effects in tokamak pedestals and transport barriers

Arbitrary poloidal gyroradius effects in tokamak pedestals and transport barriers

Poloidal asymmetries in edge transport barriers

Indirect measurement of poloidal rotation using inboard–outboard asymmetry of toroidal rotation and comparison with neoclassical predictions

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