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Mean free path

About: Mean free path is a research topic. Over the lifetime, 4412 publications have been published within this topic receiving 114418 citations.


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Journal ArticleDOI
R. Chodura1
TL;DR: The potential gradient length in the sheath in front of the target is small as compared to the mean free path length; the velocity distributions of ions and electrons as determined by the electric field are far from being Maxwellians there.
Abstract: The scrape off layer (SOL) of a confined plasma (Fig. 1a) is in many respects in a state far from, thermodynamic equilibrium: The energy diffusing outward from the plasma core across the separatrix is condensed in the thin SOL to a strong energy flux along the magnetic field. The recycling of cold neutrals from the limiter or divertor target gives rise to a strong temperature gradient from the hot midplane region to the relatively cold recycling zone. For long mean free path length in the plasma relative to the extension of the recycling zone the thermalization in the recycling zone may be incomplete. The potential gradient length in the sheath in front of the target is small as compared to the mean free path length; the velocity distributions of ions and electrons as determined by the electric field are far from being Maxwellians there. Thus the follwing topics will be discussed in this paper: Parallel transport (electron heat flow, ion viscosity) Recycling Sheath structure (grazing incidence of magnetic field, secondary electrons). Boundary conditions for fluid models Numerical illustrations for these kinetic effects are given from a 1d particle-in-cell code including Coulomb collisions [1]. Not treated are kinetic effects connected with instabilities, perpendicular transport, neutrals and impurities.

42 citations

Journal ArticleDOI
TL;DR: In this paper, the mass attenuation coefficients (MAC) have been calculated at different photon energies between 0.015 and 10 MeV using MCNPX general-purpose of Monte Carlo code.

42 citations

Journal ArticleDOI
TL;DR: In this article, the heating-frequency dependence of the apparent thermal conductivity in a semi-infinite body with periodic planar surface heating is explained by an analytical solution to the Boltzmann transport equation.
Abstract: The heating-frequency dependence of the apparent thermal conductivity in a semi-infinite body with periodic planar surface heating is explained by an analytical solution to the Boltzmann transport equation. This solution is obtained using a two-flux model and gray mean free time approximation and verified numerically with a lattice Boltzmann method and numerical results from the literature. Extending the gray solution to the nongray regime leads to an integral transform and accumulation-function representation of the phonon scattering spectrum, where the natural variable is mean free time rather than mean free path, as often used in previous work. The derivation leads to an approximate cutoff conduction similar in spirit to that of Koh and Cahill [Phys. Rev. B 76, 075207 (2007)] except that the most appropriate criterion involves the heater frequency rather than thermal diffusion length. The nongray calculations are consistent with Koh and Cahill's experimental observation that the apparent thermal conductivity shows a stronger heater-frequency dependence in a SiGe alloy than in natural Si. Finally these results are demonstrated using a virtual experiment, which fits the phase lag between surface temperature and heat flux to obtain the apparent thermal conductivity and accumulation function.

42 citations

Journal ArticleDOI
Mark Nelkin1
TL;DR: In this article, a variational expression for the decay constant is formulated and evaluated for trial solutions of the Maxwellian form for the neutron spectrum, for small deviations from thermal equilibrium, an expression of the form proposed by von Dardel is obtained.

41 citations

Journal ArticleDOI
TL;DR: The corrections due to ion-carrier gas interaction to the particle- ion recombination rate are shown to remain finite even for very small particles, whereas in the case of particle-ion repulsion the contribution of ion-molecular collisions to the rate of particle charging is suppressed in the free-molescule regime.
Abstract: A new flux-matching theory is formulated and applied to the study of particle charging by ions. Assuming that the ion-particle interaction includes the Coulomb + polarization forces the collisionless kinetic equation is solved and the ion concentration profile in the free-molecule zone (at the distances less than the ion mean free path) is found. This profile is then matched to that derived from the solution of the diffusion equation, which describes the ion transport outside the free-molecule zone. Three matching parameters are introduced: the ion flux, the matching distance, and the ion density at the matching distance; and three conditions are formulated for fixing these parameters: (i) the constancy of the total ion flux, (ii) the continuity of the ion concentration profile, and (iii) the continuity of the derivative of the ion concentration profile. The charging efficiencies are expressed in terms of their free-molecule values, the ion diffusivity in the carrier gas, and the ion thermal velocity. This approach is applied for calculating the efficiencies of particle charging in the transition regime (the particle size is comparable to the ion mean free path and the Coulomb length). The corrections due to ion-carrier gas interaction to the particle-ion recombination rate aremore » shown to remain finite even for very small particles, whereas in the case of particle-ion repulsion the contribution of ion-molecular collisions to the rate of particle charging is suppressed in the free-molecule regime.« less

41 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202375
2022207
2021134
2020114
2019113
201887