550 research outputs found
Dissipation-Scale Turbulence in the Solar Wind
We present a cascade model for turbulence in weakly collisional plasmas that
follows the nonlinear cascade of energy from the large scales of driving in the
MHD regime to the small scales of the kinetic Alfven wave regime where the
turbulence is dissipated by kinetic processes. Steady-state solutions of the
model for the slow solar wind yield three conclusions: (1) beyond the observed
break in the magnetic energy spectrum, one expects an exponential cut-off; (2)
the widely held interpretation that this dissipation range obeys power-law
behavior is an artifact of instrumental sensitivity limitations; and, (3) over
the range of parameters relevant to the solar wind, the observed variation of
dissipation range spectral indices from -2 to -4 is naturally explained by the
varying effectiveness of Landau damping, from an undamped prediction of -7/3 to
a strongly damped index around -4.Comment: 6 pages, 2 figures, accepted for publication in AIP Conference
Proceedings on "Turbulence and Nonlinear Processes in Astrophysical Plasmas
Molecular Cloning and Heterologous Expression of the Dehydrophos Biosynthetic Gene Cluster
SummaryDehydrophos is a vinyl phosphonate tripeptide produced by Streptomyces luridus with demonstrated broad-spectrum antibiotic activity. To identify genes necessary for biosynthesis of this unusual compound we screened a fosmid library of S. luridus for the presence of the phosphoenolpyruvate mutase gene, which is required for biosynthesis of most phosphonates. Integration of one such fosmid clone into the chromosome of S. lividans led to heterologous production of dehydrophos. Deletion analysis of this clone allowed identification of the minimal contiguous dehydrophos cluster, which contained 17 open reading frames (ORFs). Bioinformatic analyses of these ORFs are consistent with a proposed biosynthetic pathway that generates dehydrophos from phosphoenolpyruvate. The early steps of this pathway are supported by analysis of intermediates accumulated by blocked mutants and in vitro biochemical experiments
Astrophysical Gyrokinetics: Basic Equations and Linear Theory
Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of
astrophysical plasmas, including accretion disks, the solar wind, and the
interstellar and intracluster medium. On small scales, this turbulence is often
expected to consist of highly anisotropic fluctuations with frequencies small
compared to the ion cyclotron frequency. For a number of applications, the
small scales are also collisionless, so a kinetic treatment of the turbulence
is necessary. We show that this anisotropic turbulence is well described by a
low frequency expansion of the kinetic theory called gyrokinetics. This paper
is the first in a series to examine turbulent astrophysical plasmas in the
gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations
and explore the linear properties of gyrokinetics as a prelude to nonlinear
simulations. The linear dispersion relation for gyrokinetics is obtained and
its solutions are compared to those of hot-plasma kinetic theory. These results
are used to validate the performance of the gyrokinetic simulation code {\tt
GS2} in the parameter regimes relevant for astrophysical plasmas. New results
on global energy conservation in gyrokinetics are also derived. We briefly
outline several of the problems to be addressed by future nonlinear
simulations, including particle heating by turbulence in hot accretion flows
and in the solar wind, the magnetic and electric field power spectra in the
solar wind, and the origin of small-scale density fluctuations in the
interstellar medium.Comment: emulateapj, 24 pages, 10 figures, revised submission to ApJ:
references added, typos corrected, reorganized and streamline
Chiral Hierarchies, Compositeness and the Renormalization Group
A wide class of models involve the fine--tuning of significant hierarchies
between a strong--coupling ``compositeness'' scale, and a low energy dynamical
symmetry breaking scale. We examine the issue of whether such hierarchies are
generally endangered by Coleman--Weinberg instabilities. A careful study using
perturbative two--loop renormalization group methods finds that consistent
large hierarchies are not generally disallowed.Comment: 22 pp + 5 figs (uuencoded and submitted separately),
SSCL-Preprint-490; FERMI-PUB-93/035-
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Implementation of AI/Deep Learning Disruption Predictor into a Plasma Control System
This paper reports on advances to the state-of-the-art deep-learning
disruption prediction models based on the Fusion Recurrent Neural Network
(FRNN) originally introduced a 2019 Nature publication. In particular, the
predictor now features not only the disruption score, as an indicator of the
probability of an imminent disruption, but also a sensitivity score in
real-time to indicate the underlying reasons for the imminent disruption. This
adds valuable physics-interpretability for the deep-learning model and can
provide helpful guidance for control actuators now that it is fully implemented
into a modern Plasma Control System (PCS). The advance is a significant step
forward in moving from modern deep-learning disruption prediction to real-time
control and brings novel AI-enabled capabilities relevant for application to
the future burning plasma ITER system. Our analyses use large amounts of data
from JET and DIII-D vetted in the earlier NATURE publication. In addition to
when a shot is predicted to disrupt, this paper addresses reasons why by
carrying out sensitivity studies. FRNN is accordingly extended to use many more
channels of information, including measured DIII-D signals such as (i) the
n1rms signal that is correlated with the n =1 modes with finite frequency,
including neoclassical tearing mode and sawtooth dynamics, (ii) the bolometer
data indicative of plasma impurity content, and (iii) q-min, the minimum value
of the safety factor relevant to the key physics of kink modes. The additional
channels and interpretability features expand the ability of the deep learning
FRNN software to provide information about disruption subcategories as well as
more precise and direct guidance for the actuators in a plasma control system
Kepler eclipsing binary stars. VII. the catalogue of eclipsing binaries found in the entire Kepler data set
The primary Kepler Mission provided nearly continuous monitoring of ~200,000 objects with unprecedented photometric precision. We present the final catalog of eclipsing binary systems within the 105 deg2 Kepler field of view. This release incorporates the full extent of the data from the primary mission (Q0-Q17 Data Release). As a result, new systems have been added, additional false positives have been removed, ephemerides and principal parameters have been recomputed, classifications have been revised to rely on analytical models, and eclipse timing variations have been computed for each system. We identify several classes of systems including those that exhibit tertiary eclipse events, systems that show clear evidence of additional bodies, heartbeat systems, systems with changing eclipse depths, and systems exhibiting only one eclipse event over the duration of the mission. We have updated the period and galactic latitude distribution diagrams and included a catalog completeness evaluation. The total number of identified eclipsing and ellipsoidal binary systems in the Kepler field of view has increased to 2878, 1.3% of all observed Kepler targets
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