36,879 research outputs found
The Character of Transport Caused by ExB Drift Turbulence
The basic character of diffusive transport in a magnetised plasma depends on
what kind of transport is modelled. ExB turbulence under drift ordering has
special characteristics: it is nearly incompressible, and it cannot lead to
magnetic flux diffusion if it is electrostatic. The ExB velocity is also
related to the Poynting energy flux. Under quasineutral dynamics, electric
fields are not caused by transport of electric charge but by the requirement
that the total current is divergence free. Consequences for well constructed
computational transport models are discussed in the context of a general mean
field analysis, which also yields several anomalous transfer mechanisms not
normally considered by current models.Comment: 31 pages including 2 figures, submitted to Physics of Plasma
Energetic Consistency and Momentum Conservation in the Gyrokinetic Description of Tokamak Plasmas
Gyrokinetic field theory is addressed in the context of a general
Hamiltonian. The background magnetic geometry is static and axisymmetric, and
all dependence of the Lagrangian upon dynamical variables is in the Hamiltonian
or in free field terms. Equations for the fields are given by functional
derivatives. The symmetry through the Hamiltonian with time and toroidal angle
invariance of the geometry lead to energy and toroidal momentum conservation.
In various levels of ordering against fluctuation amplitude, energetic
consistency is exact. The role of this in underpinning of conservation laws is
emphasised. Local transport equations for the vorticity, toroidal momentum, and
energy are derived. In particular, the momentum equation is shown for any form
of Hamiltonian to be well behaved and to relax to its magnetohydrodynamic (MHD)
form when long wavelength approximations are taken in the Hamiltonian. Several
currently used forms, those which form the basis of most global simulations,
are shown to be well defined within the gyrokinetic field theory and energetic
consistency.Comment: RevTeX 4, 47 pages, no figures, revised version updated following
referee comments (discussion more strictly correct/consistent, 4 references
added, results unchanged as they depend on consistency of the theory),
resubmitted to Physics of Plasma
Kepler and the Kuiper Belt
The proposed field-of-view of the Kepler mission is at an ecliptic latitude
of ~55 degrees, where the surface density of scattered Kuiper Belt Objects
(KBOs) is a few percent that in the ecliptic plane. The rate of occultations of
Kepler target stars by scattered KBOs with radii r>10km is ~10^-6 to 10^-4 per
star per year, where the uncertainty reflects the current ignorance of the
thickness of the scattered KBO disk and the faint-end slope of their magnitude
distribution. These occultation events will last only ~0.1% of the planned
t_exp=15 minute integration time, and thus will appear as single data points
that deviate by tiny amounts. However, given the target photometric accuracy of
Kepler, these deviations will nevertheless be highly significant, with typical
signal-to-noise ratios of ~10. I estimate that 1-20 of the 10^5 main-sequence
stars in Kepler's field-of-view will exhibit detectable occultations during its
four-year mission. For unresolved events, the signal-to-noise of individual
occultations scales as t_exp^{-1/2}, and the minimum detectable radius could be
decreased by an order of magnitude to ~1 km by searching the individual
3-second readouts for occultations. I propose a number of methods by which
occultation events may be differentiated from systematic effects. Kepler should
measure or significantly constrain the frequency of highly-inclined, ~10
km-sized KBOs.Comment: 5 pages, 1 figure. No changes. Accepted to ApJ, to appear in the
August 1, 2004 issue (v610
Diurnal variation in harbour porpoise detection ā potential implications for management
Peer reviewedPublisher PD
Negative Differential Resistivity and Positive Temperature Coefficient of Resistivity effect in the diffusion limited current of ferroelectric thin film capacitors
We present a model for the leakage current in ferroelectric thin- film
capacitors which explains two of the observed phenomena that have escaped
satisfactory explanation, i.e. the occurrence of either a plateau or negative
differential resistivity at low voltages, and the observation of a Positive
Temperature Coefficient of Resistivity (PTCR) effect in certain samples in the
high-voltage regime. The leakage current is modelled by considering a
diffusion-limited current process, which in the high-voltage regime recovers
the diffusion-limited Schottky relationship of Simmons already shown to be
applicable in these systems
Measuring emission coordinates in a pulsar-based relativistic positioning system
A relativistic deep space positioning system has been proposed using four or
more pulsars with stable repetition rates. (Each pulsar emits pulses at a fixed
repetition period in its rest frame.) The positioning system uses the fact that
an event in spacetime can be fully described by emission coordinates: the
proper emission time of each pulse measured at the event. The proper emission
time of each pulse from four different pulsars---interpolated as
necessary---provides the four spacetime coordinates of the reception event in
the emission coordinate system. If more than four pulsars are available, the
redundancy can improve the accuracy of the determination and/or resolve
degeneracies resulting from special geometrical arrangements of the sources and
the event.
We introduce a robust numerical approach to measure the emission coordinates
of an event in any arbitrary spacetime geometry. Our approach uses a continuous
solution of the eikonal equation describing the backward null cone from the
event. The pulsar proper time at the instant the null cone intersects the
pulsar world line is one of the four required coordinates. The process is
complete (modulo degeneracies) when four pulsar world lines have been crossed
by the light cone.
The numerical method is applied in two different examples: measuring emission
coordinates of an event in Minkowski spacetime using pulses from four pulsars
stationary in the spacetime; and measuring emission coordinates of an event in
Schwarzschild spacetime using pulses from four pulsars freely falling toward a
static black hole.
These numerical simulations are merely exploratory, but with improved
resolution and computational resources the method can be applied to more
pertinent problems. For instance one could measure the emission coordinates,
and therefore the trajectory, of the Earth.Comment: 9 pages, 2 figures, v3: replaced with version accepted by Phys. Rev.
Targeting determinants of dosage compensation in Drosophila
The dosage compensation complex (DCC) in Drosophila melanogaster is responsible for up-regulating transcription from the single male X chromosome to equal the transcription from the two X chromosomes in females. Visualization of the DCC, a large ribonucleoprotein complex, on male larval polytene chromosomes reveals that the complex binds selectively to many interbands on the X chromosome. The targeting of the DCC is thought to be in part determined by DNA sequences that are enriched on the X. So far, lack of knowledge about DCC binding sites has prevented the identification of sequence determinants. Only three binding sites have been identified to date, but analysis of their DNA sequence did not allow the prediction of further binding sites. We have used chromatin immunoprecipitation to identify a number of new DCC binding fragments and characterized them in vivo by visualizing DCC binding to autosomal insertions of these fragments, and we have demonstrated that they possess a wide range of potential to recruit the DCC. By varying the in vivo concentration of the DCC, we provide evidence that this range of recruitment potential is due to differences in affinity of the complex to these sites. We were also able to establish that DCC binding to ectopic high-affinity sites can allow nearby low-affinity sites to recruit the complex. Using the sequences of the newly identified and previously characterized binding fragments, we have uncovered a number of short sequence motifs, which in combination may contribute to DCC recruitment. Our findings suggest that the DCC is recruited to the X via a number of binding sites of decreasing affinities, and that the presence of high-and moderate-affinity sites on the X may ensure that lower-affinity sites are occupied in a context-dependent manner. Our bioinformatics analysis suggests that DCC binding sites may be composed of variable combinations of degenerate motifs
Coalescing at 8 GeV in the Fermilab Main Injector
For Project X, it is planned to inject a beam of 3 10**11 particles per bunch
into the Main Injector. To prepare for this by studying the effects of higher
intensity bunches in the Main Injector it is necessary to perform coalescing at
8 GeV. The results of a series of experiments and simulations of 8 GeV
coalescing are presented. To increase the coalescing efficiency adiabatic
reduction of the 53 MHz RF is required, resulting in ~70% coalescing efficiency
of 5 initial bunches. Data using wall current monitors has been taken to
compare previous work and new simulations for 53 MHz RF reduction, bunch
rotations and coalescing, good agreement between experiment and simulation was
found. Possible schemes to increase the coalescing efficiency and generate even
higher intensity bunches are discussed. These require improving the timing
resolution of the low level RF and/or tuning the adiabatic voltage reduction of
the 53 MHz.Comment: 3 pp. 3rd International Particle Accelerator Conference (IPAC 2012)
20-25 May 2012, New Orleans, Louisian
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