Anomalous Transport and Stabilization of Collisionless Drift-Wave Instabilities

Lee, W.W.; Okuda, H.

doi:10.1103/physrevlett.36.870

Cited by 42 publications

(24 citation statements)

References 12 publications

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“…The electron parallel-velocity distribution steepens because of inverse Landau damping. 6 Ion perpendicular temperature T {± increases through the acceleration across the magnetic field associated with the short-wave length turbulence due to the convective cells. Ion parallel temperature T in changes little because ion Landau damping is weak.…”

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Formation of Convective Cells, Anomalous Diffusion, and Strong Plasma Turbulence Due to Drift Instabilities

Cheng

Okuda

1977

Phys. Rev. Lett.

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Large-scale, fully three-dimensional particle simulations have been carried out for the collisionless drift instabilities in a cylindrical geometry. It is found that nonlinear excitation of convective cells (w = 0,& !( = 0) due to drift instabilites gives rise to enhanced turbulence and anomalous plasma diffusion. The observed power spectra have resemblance with the recent measurements in toroidal devices.Here we report our recent results of numerical simulations on collisionless drift instabilities using large-scale fully three-dimensional cylindrical plasma models developed earlier. 1 Theoretical interpretations of the observed results on plasma turbulence and diffusion are also given. Drift-wave turbulence has been of current interest in toroidal confinement devices such as tokamaks, 2 stellarators, 3 and internal ring devices 45 because of the strong correlation between the observed density fluctuations and anomalous plasma transport.The simulation model used is a straight cylinder in a uniform external magnetic field B 0 along the z direction with its length L^/p i = 640, p t = (Tjm i ) 1/2 /^2 i being the ion gyroradius. In the cross section, a 64x64 (L 2 ) spatial grid is used for numerical computation with its physical length L/pi = 32. The plasma is periodic in z while it is surrounded by a conducting wall at the boundary of the cross section. 1 Initially both ions and electrons have Maxwellian velocity distributions with T e /T i = 4. Initial plasma density is taken to be n e (r) = n i (r) = w 0 exp(-4r 2 /a 2 ) with the average density given by Q, e /o) Pe = 5, m { /m. e -100 9 a = L/2; and there is no initial temperature gradient. Seven Fourier modes w = 0, ±1, ±2, and ±3 are kept in the z direction with k z~ 2m/L z .Linear theory predicts that the collisionless drift instability (universal mode) is strongly unstable for n = ±l and becomes stable rapidly with increasing \n\ because of the onset of ion Landau damping. This situation is commonly observed in Q devices. In terms of physical size, the simulation plasma is certainly smaller than a tokamak plasma but may be larger than a conventional Q device; and one can study quite a few problems with the model using realistic plasma parameters.Let us first look at the gross behavior of the instability. Figure 1 shows the particle diffusion, heat transfer, electron velocity distribution, and radial-mode structure associated with the insta-

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confidence: 99%

Formation of Convective Cells, Anomalous Diffusion, and Strong Plasma Turbulence Due to Drift Instabilities

Cheng

Okuda

1977

Phys. Rev. Lett.

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“…14 Due to the existence of high-frequency space-charge waves, the conven tional codes, limited by large noise level as well as by small time steps and small grid sizes, are rather cumbersome for simulating low frequency phenomena with the characteristic length of the order of p" i.e., the ion gyroradius measured at the electron temperature. The development of a gyrokinetic simulation scheme en-4 ables us to mitigate these difficulties.…”

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confidence: 99%

Nonlinear evolution of drift instabilities in the presence of collisions

Federici

Lee

Tang

1987

The Physics of Fluids

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PPPL-2354 DE86 013642Nonlinear evolution of drift instabilities in the presence of electron-ion collisions in a shear-free slab has been studied by using gyrokinetic particle simulation tech niques as well as by solving, both numerically and analytically, model mode-coupling equations. The purpose of the investigation is to determine the mechanisms respon sible for the nonlinear saturation of the instability and for the ensuing steady-state transport. Such an insight is very valuable for understanding drift wave problems in more complicated geometries. The results indicate that the electron E x B convection is the dominant mechanism for saturation. It is also found that the saturation amplitude and the associated quasilinear diffusion are greatly enhanced over their collisionless values as a result of weak collisions. In the highly collisional (fluid) limit, there is an upper bound for saturation with e/T e ~ (<^i/tti)/(k±p,) 2 .The associated quasilinear diffusion, which increases with collisionality, takes the form of D q i ~ -yi/k±, where u>; and 7; are the linear frequency and growth rate, re spectively. In the steady state, the diffusion process becomes stochastic in nature.The relevant mechanisms here are related to the velocity-space nonlinearities and DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED background fluctuations. The magnitude of the diffusion at this stage can be com parable to that of quasilinear diffision in the presance of collisions, and it remains finite even in the collisionless limit.

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“…5,6 Moreover, the simulation approach becomes even m o r e attractive due t o the recent advances in computers and our techniques in numerical modeling of plasmas, which allow us to use rather realistic plasma p a r a m e t e r s in .both…”

Section: Introductionmentioning

confidence: 99%

Numerical simulations of collisionless drift instabilities for low-density plasmas

Lee

Kuo

Okuda

1978

The Physics of Fluids

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Nonlinear behavior of the collisionless drift instabilities are studied for the universal and current driven modes by means of electrostatic particle simulations in two-and-a-half dimensions. Realistic mass ratios of electrons to the ions are used in the simulations, where the guiding-center approximation for the electrons and the exact dynamics for the ions are employed. Several nonlinear effects including the quasi-linear diffusion of the particle density, the frequency shift due to the ambipolar field, the mode competition among the unstable waves, and the quasi-linear diffusion in the velocity space are found to be dominant mechanisms for the saturation. The stabilization of the collisionless drift instabilities by the magnetic shear has also been studied.

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Anomalous Transport and Stabilization of Collisionless Drift-Wave Instabilities

Cited by 42 publications

References 12 publications

Formation of Convective Cells, Anomalous Diffusion, and Strong Plasma Turbulence Due to Drift Instabilities

Formation of Convective Cells, Anomalous Diffusion, and Strong Plasma Turbulence Due to Drift Instabilities

Nonlinear evolution of drift instabilities in the presence of collisions

Numerical simulations of collisionless drift instabilities for low-density plasmas

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