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"Search
of universality: towards a statistical mechanics of collisionless plasma"
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Date: |
Download-files: |
Time: |
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Thursday,
02 June 2022 |
Video-Recording for any system with MP4-support - Video.mp4 (ca. 536 Mb) |
15:15 – 16:35
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Alexander A. Schekochihin
(Oxford University)
Abstract:
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.
Biography:
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.