105 research outputs found
A Hybrid Gyrokinetic Ion and Isothermal Electron Fluid Code for Astrophysical Plasma
This paper describes a new code for simulating astrophysical plasmas that
solves a hybrid model composed of gyrokinetic ions (GKI) and an isothermal
electron fluid (ITEF) [A. Schekochihin et al., Astrophys. J. Suppl.
\textbf{182}, 310 (2009)]. This model captures ion kinetic effects that are
important near the ion gyro-radius scale while electron kinetic effects are
ordered out by an electron-ion mass ratio expansion. The code is developed by
incorporating the ITEF approximation into {\tt AstroGK}, an Eulerian
gyrokinetics code specialized to a slab geometry [R. Numata et al., J. Compute.
Pays. \textbf{229}, 9347 (2010)]. The new code treats the linear terms in the
ITEF equations implicitly while the nonlinear terms are treated explicitly. We
show linear and nonlinear benchmark tests to prove the validity and
applicability of the simulation code. Since the fast electron timescale is
eliminated by the mass ratio expansion, the Courant--Friedrichs--Lewy condition
is much less restrictive than in full gyrokinetic codes; the present hybrid
code runs times faster than
{\tt AstroGK}\ with a single ion species and kinetic electrons where
is the ion-electron mass ratio. The improvement of
the computational time makes it feasible to execute ion scale gyrokinetic
simulations with a high velocity space resolution and to run multiple
simulations to determine the dependence of turbulent dynamics on parameters
such as electron--ion temperature ratio and plasma beta
Thermal disequilibration of ions and electrons by collisionless plasma turbulence
Does overall thermal equilibrium exist between ions and electrons in a weakly
collisional, magnetised, turbulent plasma---and, if not, how is thermal energy
partitioned between ions and electrons? This is a fundamental question in
plasma physics, the answer to which is also crucial for predicting the
properties of far-distant astronomical objects such as accretion discs around
black holes. In the context of discs, this question was posed nearly two
decades ago and has since generated a sizeable literature. Here we provide the
answer for the case in which energy is injected into the plasma via Alfv\'enic
turbulence: collisionless turbulent heating typically acts to disequilibrate
the ion and electron temperatures. Numerical simulations using a hybrid
fluid-gyrokinetic model indicate that the ion-electron heating-rate ratio is an
increasing function of the thermal-to-magnetic energy ratio,
: it ranges from at to at
least for . This energy partition is
approximately insensitive to the ion-to-electron temperature ratio
. Thus, in the absence of other equilibrating
mechanisms, a collisionless plasma system heated via Alfv\'enic turbulence will
tend towards a nonequilibrium state in which one of the species is
significantly hotter than the other, viz., hotter ions at high
, hotter electrons at low . Spectra of
electromagnetic fields and the ion distribution function in 5D phase space
exhibit an interesting new magnetically dominated regime at high and
a tendency for the ion heating to be mediated by nonlinear phase mixing
("entropy cascade") when and by linear phase mixing
(Landau damping) when $\beta_\mathrm{i}\gg1
Hall magnetohydrodynamics in a relativistically strong mean magnetic field
This Letter presents a magnetohydrodynamic model that describes the
small-amplitude fluctuations with wavelengths comparable to ion inertial length
in the presence of a relativistically strong mean magnetic field.
The set of derived equations is virtually identical to the non-relativistic
Hall reduced magnetohydrodynamics (Schekochihin et al. 2019), differing only by
a few constants that take into account the relativistic corrections.
This means that all the properties of kinetic Alfv\'en turbulence and ion
cyclotron turbulence inherent in the non-relativistic Hall regime persist
unchanged even in a magnetically dominated regime
Comparison of Entropy Production Rates in Two Different Types of Self-organized Flows: B\'{e}nard Convection and Zonal flow
Entropy production rate (EPR) is often effective to describe how a structure
is self-organized in a nonequilibrium thermodynamic system. The "minimum EPR
principle" is widely applicable to characterizing self-organized structures,
but is sometimes disproved by observations of "maximum EPR states." Here we
delineate a dual relation between the minimum and maximum principles; the
mathematical representation of the duality is given by a Legendre
transformation. For explicit formulation, we consider heat transport in the
boundary layer of fusion plasma [Phys. Plasmas {\bf 15}, 032307 (2008)]. The
mechanism of bifurcation and hysteresis (which are the determining
characteristics of the so-called H-mode, a self-organized state of reduced
thermal conduction) is explained by multiple tangent lines to a pleated graph
of an appropriate thermodynamic potential. In the nonlinear regime, we have to
generalize Onsager's dissipation function. The generalized function is no
longer equivalent to EPR; then EPR ceases to be the determinant of the
operating point, and may take either minimum or maximum values depending on how
the system is driven
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