1,528 research outputs found
Comparison of local and global gyrokinetic calculations of collisionless zonal flow damping in quasi-symmetric stellarators
The linear collisionless damping of zonal flows is calculated for quasi-symmetric stellarator equilibria in flux-tube, flux-surface, and full-volume geometry. Equilibria are studied from the quasi-helical symmetry configuration of the Helically Symmetric eXperiment (HSX), a broken symmetry configuration of HSX, and the quasi-axial symmetry geometry of the National Compact Stellarator eXperiment (NCSX). Zonal flow oscillations and long-time damping affect the zonal flow evolution, and the zonal flow residual goes to zero for small radial wavenumber. The oscillation frequency and damping rate depend on the bounce-averaged radial particle drift in accordance with theory. While each flux tube on a flux surface is unique, several different flux tubes in HSX or NCSX can reproduce the zonal flow damping from a flux-surface calculation given an adequate parallel extent. The flux-surface or flux-tube calculations can accurately reproduce the full-volume long-time residual for moderate kx, but the oscillation and damping time scales are longer in local representations, particularly for small kx approaching the system size.</p
Three-Dimensional Shear-Flow Instability Saturation via Stable Modes
Turbulence in three dimensions (D) supports vortex stretching that has
long been known to accomplish energy transfer to small scales. Moreover, net
energy transfer from large-scale, forced, unstable flow-gradients to smaller
scales is achieved by gradient-flattening instability. Despite such enforcement
of energy transfer to small scales, it is shown here not only that the
shear-flow-instability-supplied D-fluctuation energy is largely
inverse-transferred from the fluctuation to the mean-flow gradient, but that
such inverse transfer is more efficient for turbulent fluctuations in D than
in two dimensions (D). The transfer is due to linearly stable eigenmodes
that are excited nonlinearly. The stable modes, thus, reduce both the nonlinear
energy cascade to small scales and the viscous dissipation rate. The
vortex-tube stretching is also suppressed. Up-gradient momentum transport by
the stable modes counters the instability-driven down-gradient transport, which
also is more effective in D than in D (). From unstable modes, these stable
modes nonlinearly receive energy via zero-frequency fluctuations that vary only
in the direction orthogonal to the plane of D shear flow. The more widely
occurring D turbulence is thus inherently different from the commonly
studied D turbulence, despite both saturating via stable modes.Comment: To appear in Physics of Fluid
Comparison of local and global gyrokinetic calculations of collisionless zonal flow damping in quasi-symmetric stellarators
The linear collisionless damping of zonal flows is calculated for
quasi-symmetric stellarator equilibria in flux-tube, flux-surface, and
full-volume geometry. Equilibria are studied from the quasi-helical symmetry
configuration of the Helically Symmetric eXperiment (HSX), a broken symmetry
configuration of HSX, and the quasi-axial symmetry geometry of the National
Compact Stellarator eXperiment (NCSX). Zonal flow oscillations and long-time
damping affect the zonal flow evolution, and the zonal flow residual goes to
zero for small radial wavenumber. The oscillation frequency and damping rate
depend on the bounce-averaged radial particle drift in accordance with theory.
While each flux tube on a flux surface is unique, several different flux tubes
in HSX or NCSX can reproduce the zonal flow damping from a flux-surface
calculation given an adequate parallel extent. The flux-surface or flux-tube
calculations can accurately reproduce the full-volume long-time residual for
moderate , but the oscillation and damping time scales are longer in local
representations, particularly for small approaching the system size.Comment: The following article has been accepted by Physics of Plasmas. After
it is published, it will be found at https://aip.scitation.org/journal/php.
33 pages, 18 figure
The impact of magnetic fields on momentum transport and saturation of shear-flow instability by stable modes
The Kelvin-Helmholtz (KH) instability of a shear layer with an
initially-uniform magnetic field in the direction of flow is studied in the
framework of 2D incompressible magnetohydrodynamics with finite resistivity and
viscosity using direct numerical simulations. The shear layer evolves freely,
with no external forcing, and thus broadens in time as turbulent stresses
transport momentum across it. As with KH-unstable flows in hydrodynamics, the
instability here features a conjugate stable mode for every unstable mode in
the absence of dissipation. Stable modes are shown to transport momentum up its
gradient, shrinking the layer width whenever they exceed unstable modes in
amplitude. In simulations with weak magnetic fields, the linear instability is
minimally affected by the magnetic field, but enhanced small-scale fluctuations
relative to the hydrodynamic case are observed. These enhanced fluctuations
coincide with increased energy dissipation and faster layer broadening, with
these features more pronounced in simulations with stronger fields. These
trends result from the magnetic field reducing the effects of stable modes
relative to the transfer of energy to small scales. As field strength
increases, stable modes become less excited and thus transport less momentum
against its gradient. Furthermore, the energy that would otherwise transfer
back to the driving shear due to stable modes is instead allowed to cascade to
small scales, where it is lost to dissipation. Approximations of the turbulent
state in terms of a reduced set of modes are explored. While the Reynolds
stress is well-described using just two modes per wavenumber at large scales,
the Maxwell stress is not.Comment: 39 pages, 17 figures, preprint forma
Nonlinear mode coupling and energetics of driven magnetized shear-flow turbulence
To comprehensively understand saturation of two-dimensional (D) magnetized
Kelvin-Helmholtz-instability-driven turbulence, energy transfer analysis is
extended from the traditional interaction between scales to include eigenmode
interactions, by using the nonlinear couplings of linear eigenmodes of the
ideal instability. While both kinetic and magnetic energies cascade to small
scales, a significant fraction of turbulent energy deposited by unstable modes
in the fluctuation spectrum is shown to be re-routed to the conjugate-stable
modes at the instability scale. They remove energy from the forward cascade at
its inception. The remaining cascading energy flux is shown to attenuate
exponentially at a small scale, dictated by the large-scale stable modes.
Guided by a widely used instability-saturation assumption, a general
quasilinear model of instability is tested by retaining all nonlinear
interactions except those that couple to the large-scale stable modes. These
complex interactions are analytically removed from the magnetohydrodynamic
equations using a novel technique. Observations are: an explosive large-scale
vortex separation instead of the well-known merger of D, a dramatic
enhancement in turbulence level and spectral energy fluxes, and a reduced
small-scale dissipation length-scale. These show critical role of the stable
modes in instability saturation. Possible reduced-order turbulence models are
proposed for fusion and astrophysical plasmas, based on eigenmode-expanded
energy transfer analyses.Comment: Selected by the editors of Physics of Plasmas as a Featured article.
https://doi.org/10.1063/5.015656
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Implications of large scale shifts in tropospheric NOx levels in the remote tropical Pacific
A major observation recorded during NASA's western Pacific Exploratory Mission (PEM-West B) was the large shift in tropical NO levels as a function of geographical location. High-altitude NO levels exceeding 100 pptv were observed during portions of tropical flights 5-8, while values almost never exceeded 20 pptv during tropical flights 9 and 10. The geographical regions encompassing these two flight groupings are here labeled "high" and "low" NOx regimes. A comparison of these two regimes, based on back trajectories and chemical tracers, suggests that air parcels in both were strongly influenced by deep convection. The low NOx regime appears to have been predominantly impacted by marine convection, whereas the high NOx regime shows evidence of having been more influenced by deep convection over a continental land mass. DMSP satellite observations point strongly toward lightning as the major source of NOx in the latter regime. Photochemical ozone formation in the high NOx regime exceeded that for low NOx by factors of 2 to 6, whereas O3 destruction in the low NOx regime exceeded that for high NOx by factors of up to 3. Taking the tropopause height to be 17 km, estimates of the net photochemical effect on the O3 column revealed that the high NOx regime led to a small net production. By contrast, the low NOx regime was shown to destroy O3 at the rate of 3.4% per day. One proposed mechanism for off-setting this projected large deficit would involve the transport of O3 rich midlatitude air into the tropics. Alternatively, it is suggested that O3 within the tropics may be overall near self-sustaining with respect to photochemical activity. This scenario would require that some tropical regions, unsampled at the time of PEM-B, display significant net column O3 production, leading to an overall balanced budget for the "greater" tropical Pacific basin. Details concerning the chemical nature of such regimes are discussed
Progress in unveiling extreme particle acceleration in persistent astrophysical jets
International audienceExtreme blazars emitting teraelectronvolt photons are ideal targets to study particle acceleration processes. The growing number of such sources has been critical for γ-ray cosmology, studying intergalactic magnetic fields and putting constraints on exotic physics
Long-term monitoring of the radio-galaxy M87 in gamma-rays: joint analysis of MAGIC, VERITAS and Fermi-LAT data
M87 was discovered in the very-high-energy band (VHE, E > 100 GeV) with HEGRA
in 2003, long before its emission was detected in the high-energy band (HE, E >
100 MeV) with Fermi-LAT in 2009, opening the window to a new family of
extragalactic sources with tilted jets. After a series of major VHE flares in
2005, 2008, and 2010, which were detected in multiple bands, the source has
been found in a low activity state, interrupted only by comparatively
smaller-scale flares. MAGIC and VERITAS, two stereoscopic Cherenkov telescope
arrays located at Roque de los Muchachos Observatory (Canary Islands, Spain)
and the Fred Lawrence Whipple Observatory (Arizona, US), have monitored M87
continuously and in coordination for more than 10 years. In this work, we
present the data for 4 years of MAGIC and VERITAS observations corresponding to
2019, 2020, 2021 and 2022. The resulting light curves are shown in daily and
monthly scales where no significant variability is observed. In addition, we
show the first joint analysis using combined event data from the two VHE
instruments and Fermi-LAT to compute the spectral energy distribution
Global Linear and Nonlinear Gyrokinetic Simulations of Tearing Modes
To better understand the interaction of global tearing modes and
microturbulence in the Madison Symmetric Torus (MST) reversed-field pinch
(RFP), the global gyrokinetic code \textsc{Gene} is modified to describe global
tearing mode instability via a shifted Maxwellian distribution consistent with
experimental equilibria. The implementation of the shifted Maxwellian is tested
and benchmarked by comparisons with different codes and models. Good agreement
is obtained in code-code and code-theory comparisons. Linear stability of
tearing modes of a non-reversed MST discharge is studied. A collisionality scan
is performed to the lowest order unstable modes (, ) and shown to
behave consistently with theoretical scaling. The nonlinear evolution is
simulated, and saturation is found to arise from mode coupling and transfer of
energy from the most unstable tearing mode to small-scale stable modes mediated
by the tearing mode. The work described herein lays the foundation for
nonlinear simulation and analysis of the interaction of tearing modes and
gyroradius-scale instabilities in RFP plasmas
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