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"Search of universality: towards a statistical mechanics of collisionless plasma"
Thursday, 02 June 2022
Video-Recording for any system with MP4-support
- Video.mp4 (ca. 536 Mb)
15:15 – 16:35
Alexander A. Schekochihin
Much of existing plasma physics (and indeed much of the rest of kinetic physics)
is done hovering in the vicinity of a Maxwellian equilibrium, which is the
maximum point of standard Gibbs entropy and is achieved dynamically by means
of two-particle collisions. In this colloquium, I would like to discuss what I believe
to be the next frontier for plasma theoreticians and attempt to grapple with
the fact that many astrophysical plasmas (solar wind being the most accessible
of them) are too collisionless to be Maxwellian (in the sense that their dynamics
occur on shorter timescales than interparticle collisions). The central question
is then whether there exist universal collisionless equilibria, or classes thereof,
and what they are. What is the meaning of entropy in a collisionless plasma?
(Similar questions are asked in galactic dynamics, where the collisionless particles
are stars.) I will discuss some simple ideas, going back to the work of Lynden-Bell
in the 1960s, about the statistical mechanics of a collisionless plasma, leading to a
class of universal collisionless equilibria — these are reminiscent of the equilibria
of Fermi gases, with phase-volume conservation in a collisionless plasma imposing
(an infinite set of) constraints that are analogous to the Pauli exclusion principle.
I will then outline a programme for how one might do to this statistical mechanics
what Boltzmann did to Gibbs: derive a “collisionless collision integral” that
describes the dynamical relaxation of a plasma towards the Lynden-Bell equilibria.
It turns out that in order to make progress in this task, one must understand the
structure of chaotic fluctuations in phase space. Lynden-Bell-like equilibria are
recoverable under some very restrictive assumptions — roughly speaking, when
these fluctuations are treated as structurally similar to a thermal noise. In fact,
they are more likely to behave like fully-fledged turbulence — with phase mixing
(“Landau damping”) and stochastic echoes conspiring to process a constant flux
of energy. What universal equilibria (if any) exist against such a background is a
topic of ongoing research. This talk should be suitable for a general physics
audience from the late-undergraduate level up.
Alexander Schekochihin is Professor of Theoretical Physics at the University of
Oxford and Fellow of Merton College. He did his PhD with Russell Kulsrud in
Astrophysical Sciences at Princeton University, graduating in 2001, then held
postdoctoral fellowships at UCLA, Imperial College (with Steven Cowley) and
Cambridge. He became Lecturer in Plasma Physics at Imperial College in 2006;
in 2008 he moved to Oxford as University Lecturer in Theoretical Astrophysics.
In 2014, he became Professor of Theoretical Physics there, and is now Head of
Theoretical Astrophysics and Plasma Physics Group.
Schekochihin received numerous distinctions, including the
Cecilia Payne-Gaposchkin Medal and Prize for distinguished contributions
to plasma, solar, and space physics from the Institute of Physics in 2019,
and the John Dawson Award for Excellence in Plasma Physics Research
from the American Physical Society, also in 2019.
Schekochihin has made important contributions to plasma astrophysics and
fusion physics, focussing especially on turbulent cascades in magnetized
weakly collisional plasmas. He has also performed early simulations of the
small-scale turbulent dynamo and contributed significantly to the theory
of fast magnetic reconnection. This has important applications to the origin
of cosmic magnetism, heating and emission of X-rays in galaxy clusters,
turbulence and transport in the solar wind and other astrophysical plasmas.
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