Sherwood 2015

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Phase mixing vs. turbulence in a drift-kinetic plasma

Author: Alexander Schekochihin
Requested Type: Consider for Invited
Submitted: 2015-02-04 15:52:02

Co-authors: A. Kanekar, G. Hammett, W. Dorland, J. Parker, E. Highcock, P. Dellar, N. Loureiro, C. Staines

Contact Info:
University of Oxford
R. Peierls Centre for Th. Phys
Oxford OX1 4JD, Oxfordshir   00000

Abstract Text:
There are two ways in which (magnetised, drift-kinetic) plasma turbulence can process (free) energy: (i) transfer it via nonlinear mixing to smaller spatial scales - below the ion Larmor scale - and then thermalise it via some form of collisional viscosity (which can involve also a phase-space cascade at sub-Larmor scales) --- this can be thought of as a "fluid" cascade channel, at least as far as drift-kinetic scales are concerned; (ii) transfer it to small scales in (parallel) velocity space and thermalise via collisional velocity-space diffusion --- this is the linear process of phase mixing (or Landau damping). I will discuss how these two thermalisation routes combine. It turns out that a salient feature of phase mixing in a nonlinear turbulent system is that it can be substantially depleted, on average, by "un-phase-mixing" caused by the stochastic version of the plasma echo effect. In order for this depletion to work, quite a lot of phase space needs to be available to the system's free energy, i.e., collisions have to be very weak. Thus, in a very collisionless system, the turbulent cascade may be effectively "fluid" all the way to the ion Larmor scale, whereas at modest collisionalities, it can be significantly drained by linear phase mixing. These results can be understood analytically already by considering the model "solvable" problem of passive chaotic mixing of a kinetic field (work by A. Kanekar, G. Hammett, W. Dorland, N. Loureiro, C. Staines and AAS, to appear). Numerically, they appear to be supported by drift-kinetic simulations of ITG turbulence (work by J. Parker, E. Highcock, P. Dellar and AAS, to appear) and of the compressive electromagnetic fluctuations mixed by Alfvenic turbulence, as characteristic of the solar wind (work by A. Kanekar, W. Dorland and AAS, to appear). Besides providing an attractive paradigm for the phase-space structure of drift-kinetic plasma turbulence, an issue of fundamental theoretical-physical interest, these results raise new and interesting possibilities for further development of Landau-fluid closures for models of turbulent transport.

next/near to W. Dorland, A. Kanekar

March 16-18, 2015
The Courant Institute, New York University