Turbulent fluxes and entropy production rate

Garbet, X.; Dubuit, N.; Asp, E.; Sarazin, Y.; Bourdelle, C.; Ghendrih, Ph.; Hoang, G. T.

doi:10.1063/1.1951667

Cited by 91 publications

(110 citation statements)

References 39 publications

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“…Since we are interested in turbulent relaxation, we focus on the generation of the "mean field" entropy, 17 S 0 ϵ −͐d⌫͗f͘ln͗f͘, where ͗f͘ is a coarse grained mean distribution function. By decomposing f = ͗f͘ + ␦f, one can approximate the coarse grained entropy as…”

Section: A Formulationmentioning

confidence: 99%

“…We formulate the entropy budget for the turbulent relaxation process by calculating the time evolution of the mean field entropy. 17 The mean field entropy is the part of entropy defined using only the mean field distribution function as S 0 ϵ −͐d⌫͗f͘ln͗f͘, which evolves due to the action of turbulence. Note that S 0 is defined in terms of coarse grained fields.…”

Section: Introductionmentioning

confidence: 99%

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On the efficiency of intrinsic rotation generation in tokamaks

2010

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A theory of the efficiency of the plasma flow generation process is presented. A measure of the efficiency of plasma self-acceleration of mesoscale and mean flows from the heat flux is introduced by analogy with engines, using the entropy budget defined by thermal relaxation and flow generation. The efficiency is defined as the ratio of the entropy destruction rate due to flow generation to the entropy production rate due to ٌT relaxation ͑i.e., related to turbulent heat flux͒. The efficiencies for two different cases, i.e., for the generation of turbulent driven E ϫ B shear flow ͑zonal flow͒ and for toroidal intrinsic rotation, are considered for a stationary state, achieved by balancing entropy production rate and destruction rate order by order in O͑k ʈ / k Ќ ͒, where k is the wave number. The efficiency of intrinsic toroidal rotation is derived and shown to be e IR ϳ͑Mach͒ th 2 ϳ 0.01. The scaling of the efficiency of intrinsic rotation generation is also derived and shown to bewhich suggests a machine size scaling and an unfavorable plasma current scaling which enters through the shear length.

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Section: A Formulationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

On the efficiency of intrinsic rotation generation in tokamaks

2010

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“…This condition corresponds to a state of minimum entropy production. 29 Any density and temperature perturbations that occur in "stationary" profiles represent entropy waves 30 that propagate in the direction of the curvature heat flux.…”

Section: Introductionmentioning

confidence: 99%

Turbulent fluctuations during pellet injection into a dipole confined plasma torus

et al. 2017

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We report measurements of the turbulent evolution of the plasma density profile following the fast injection of lithium pellets into the Levitated Dipole Experiment (LDX) [Boxer et al., Nat. Phys. 6, 207 (2010)]. As the pellet passes through the plasma, it provides a significant internal particle source and allows investigation of density profile evolution, turbulent relaxation, and turbulent fluctuations. The total electron number within the dipole plasma torus increases by more than a factor of three, and the central density increases by more than a factor of five. During these large changes in density, the shape of the density profile is nearly “stationary” such that the gradient of the particle number within tubes of equal magnetic flux vanishes. In comparison to the usual case, when the particle source is neutral gas at the plasma edge, the internal source from the pellet causes the toroidal phase velocity of the fluctuations to reverse and changes the average particle flux at the plasma edge. An edge particle source creates an inward turbulent pinch, but an internal particle source increases the outward turbulent particle flux. Statistical properties of the turbulence are measured by multiple microwave interferometers and by an array of probes at the edge. The spatial structures of the largest amplitude modes have long radial and toroidal wavelengths. Estimates of the local and toroidally averaged turbulent particle flux show intermittency and a non-Gaussian probability distribution function. The measured fluctuations, both before and during pellet injection, have frequency and wavenumber dispersion consistent with theoretical expectations for interchange and entropy modes excited within a dipole plasma torus having warm electrons and cool ions.

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“…͑7͒, which usually gives rise to an inward flux in the poloidally symmetric case ͑positive R / L nz 0 ͒, is the one proportional to ͗D͘ originating from the curvature drift. [16][17][18] Note that ͗D͘ is reduced or even negative if the impurities accumulate on the inboard side ͑see …”

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confidence: 99%

Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas

Fülöp

Moradi

2011

Physics of Plasmas

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The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that if the density of the impurity ions is poloidally asymmetric then the zero-flux impurity density gradient is significantly reduced and even a sign change in the impurity flux may occur if the asymmetry is sufficiently large. This effect is most effective in low shear plasmas with the impurity density peaking on the inboard side and may be a contributing factor to the observed outward convection of impurities in the presence of radio frequency heating. ͓doi:10.1063/1.3569841͔The presence of impurities in fusion plasmas has a significant effect on fusion performance. Accumulation of impurities in the core would have detrimental effect on fusion reactivity due to increased radiation losses and plasma dilution. On the other hand the presence of impurities at the edge can be beneficial, and seeded impurities are often used to create a radiative belt in order to reduce the heat loads on the walls. Therefore, it is of great value to understand the physics mechanisms governing the impurity transport, and to identify plasma conditions in which impurity accumulation in the core can be avoided.Experimental observations on several tokamaks show that the impurity distribution over the flux surface is often poloidally asymmetric. [1][2][3][4][5] The asymmetries are usually most pronounced for heavy impurities. In the plasma core, where the collisionality is low, in-out asymmetries can arise due to toroidal rotation or the presence of radio frequency ͑RF͒ heating. In the case of rotating plasmas, the poloidal asymmetry of the impurity ion distribution is due to the centrifugal force, which causes the impurities to accumulate on the outboard side of flux surfaces. 1,2 In the case of RF heated plasmas, the asymmetry is a result of the increase of the hydrogen-minority density on the outboard side. These particles tend to be trapped on the outside of the torus and the turning points of their orbits drift toward the resonance layer due to the heating. The poloidal asymmetry in the hydrogenminority density gives rise to an electric field that pushes the other ion species to the inboard side. In the case of highlycharged impurities, this effect is amplified by their large charge Z ͑Ref. 3͒. In the tokamak edge, where the plasma is sufficiently collisional, steep radial pressure or temperature gradients give rise to an in-out asymmetry. 6 Neoclassical theory also predicts an up-down asymmetry, which is caused by the ion-impurity friction. 7 Recent experiments have shown that auxiliary heating can influence the impurity convective flux. For example in JET, it has been observed that with ion cyclotron resonance heating ͑ICRH͒ the accumulation of high-Z impurities can be avoided. 8,9 The physical mechanism by which the change of the direction of the impurity convective velocity occurs has not yet been clearly identified, in spite of the various efforts that have been made. [9][10][11] I...

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Turbulent fluxes and entropy production rate

Cited by 91 publications

References 39 publications

On the efficiency of intrinsic rotation generation in tokamaks

On the efficiency of intrinsic rotation generation in tokamaks

Turbulent fluctuations during pellet injection into a dipole confined plasma torus

Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas

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