科研之友
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摘要
Non-diffusive transport, for which the particle mean free path grows nonlinearly in time, is envisaged for many space and laboratory plasmas. In particular, superdiffusion, i.e. proportional to t(alpha) with alpha > 1, can be described in terms of a Levy random walk, in which case the probability of free-path lengths has power-law tails. Here, we develop a direct numerical simulation to reproduce the Levy random walk, as distinct from the Levy flights. This implies that in the free-path probability distribution Psi(x, t) there is a space-time coupling, that is, the free-path length is proportional to the free-path duration. A power-law probability distribution for the free-path duration is assumed, so that the numerical model depends on the powerlaw slope mu and on the scale distance x(0). The numerical model is able to reproduce the expected mean square deviation, which grows in a superdiffusive way, and the expected propagator P(x, t), which exhibits power-law tails, too. The difference in the power-law slope between the Levy flights propagator and the Levy walks propagator is also estimated.
- 出版日期2015-1
