Biomedical subjects

F Jenko

Publications and source records attributed to F Jenko.

4 recordsLinked to original sources

PubMed2006

Exact solution of a generalized Kramers-Fokker-Planck equation retaining retardation effects.

In order to describe non-Gaussian kinetics in weakly damped systems, the concept of continuous time random walks is extended to particles with finite inertia. One thus obtains a generalized Kramers-Fokker-Planck equation, which retains retardation effects, i.e., nonlocal couplings in time and space. It is shown that despite this complexity, exact solutions of this equation can be given in terms of superpositions of Gaussian distributions with varying variances. In particular, the long-time behavior of the respective low-order moments is calculated.

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PubMed2006

Anomalous diffusion of inertial, weakly damped particles.

The anomalous (i.e., non-Gaussian) dynamics of particles subject to a deterministic acceleration and a series of "random kicks" is studied. Based on an extension of the concept of continuous time random walks to position-velocity space, a new fractional equation of the Kramers-Fokker-Planck type is derived. The associated collision operator necessarily involves a fractional substantial derivative, representing important nonlocal couplings in time and space. For the force-free case, a closed solution is found and discussed.

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PubMed2002

Prediction of significant tokamak turbulence at electron gyroradius scales.

The experimental conditions under which tokamak turbulence at hyperfine (electron gyroradius) scales is predicted to be significant and observable are described. The first quantitative predictions of fluctuation amplitudes, spectral features, and the associated electron energy transport are presented. A novel theoretical model which quantitatively describes the boundaries of the high-amplitude streamer transport regime is presented and shown to explain the gyrokinetic simulation results. This model uniquely includes consideration of two distinct secondary instabilities.

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PubMed2000

Electron temperature gradient turbulence.

The first toroidal, gyrokinetic, electromagnetic simulations of small scale plasma turbulence are presented. The turbulence considered is driven by gradients in the electron temperature. It is found that electron temperature gradient (ETG) turbulence can induce experimentally relevant thermal losses in magnetic confinement fusion devices. For typical tokamak parameters, the transport is essentially electrostatic in character. The simulation results are qualitatively consistent with a model that balances linear and secondary mode growth rates. Significant streamer-dominated transport at long wavelengths occurs because the secondary modes that produce saturation become weak in the ETG limit.

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