2,581 research outputs found
Response to Comment on `Undamped electrostatic plasma waves' [Phys. Plasmas 19, 092103 (2012)]
Numerical and experimental evidence is given for the occurrence of the
plateau states and concomitant corner modes proposed in \cite{valentini12}. It
is argued that these states provide a better description of reality for small
amplitude off-dispersion disturbances than the conventional
Bernstein-Greene-Kruskal or cnoidal states such as those proposed in
\cite{comment
Enhancing proton acceleration by using composite targets
Efficient laser ion acceleration requires high laser intensities, which can
only be obtained by tightly focusing laser radiation. In the radiation pressure
acceleration regime, where the tightly focused laser driver leads to the
appearance of the fundamental limit for the maximum attainable ion energy, this
limit corresponds to the laser pulse group velocity as well as to another limit
connected with the transverse expansion of the accelerated foil and consequent
onset of the foil transparency. These limits can be relaxed by using composite
targets, consisting of a thin foil followed by a near critical density slab.
Such targets provide guiding of a laser pulse inside a self-generated channel
and background electrons, being snowplowed by the pulse, compensate for the
transverse expansion. The use of composite targets results in a significant
increase in maximum ion energy, compared to a single foil target case.Comment: 16 pages, 9 figure
Radiation Pressure Acceleration: the factors limiting maximum attainable ion energy
Radiation pressure acceleration (RPA) is a highly efficient mechanism of
laser-driven ion acceleration, with with near complete transfer of the laser
energy to the ions in the relativistic regime. However, there is a fundamental
limit on the maximum attainable ion energy, which is determined by the group
velocity of the laser. The tightly focused laser pulses have group velocities
smaller than the vacuum light speed, and, since they offer the high intensity
needed for the RPA regime, it is plausible that group velocity effects would
manifest themselves in the experiments involving tightly focused pulses and
thin foils. However, in this case, finite spot size effects are important, and
another limiting factor, the transverse expansion of the target, may dominate
over the group velocity effect. As the laser pulse diffracts after passing the
focus, the target expands accordingly due to the transverse intensity profile
of the laser. Due to this expansion, the areal density of the target decreases,
making it transparent for radiation and effectively terminating the
acceleration. The off-normal incidence of the laser on the target, due either
to the experimental setup, or to the deformation of the target, will also lead
to establishing a limit on maximum ion energy.Comment: 17 pages, 6 figure
Cytotoxic Effects of Hexavalent and Trivalent Chromium on Mammalian Cells In Vitro
The cytotoxic effects of hexavalent (k2Cr2O7) and trivalent (CrCl3) chromium compounds have been studied in cultured hamster fibroblasts (BHK line) and human epithelial-like cells (HEp line)
2D continuous spectrum of shear Alfven waves in the presence of a magnetic island
The radial structure of the continuous spectrum of shear Alfven modes is
calculated in the presence of a magnetic island in tokamak plasmas. Modes with
the same helicity of the magnetic island are considered in a slab model
approximation. In this framework, with an appropriate rotation of the
coordinates the problem reduces to 2 dimensions. Geometrical effects due to the
shape of the flux surface's cross section are retained to all orders. On the
other hand, we keep only curvature effects responsible of the beta induced gap
in the low-frequency part of the continuous spectrum. New continuum
accumulation points are found at the O-point of the magnetic island. The
beta-induced Alfven Eigenmodes (BAE) continuum accumulation point is found to
be positioned at the separatrix flux surface. The most remarkable result is the
nonlinear modification of the BAE continuum accumulation point frequency
Propargyl alcohol as a corrosion inhibitor for AISI 304L stainless steel in hydrochloric acid
Hydrochloric acid solutions (at concentrations of 5 to 28%) are used to acidify or acid-stimulate a rock-reservoir, increasing productivity of an oil well and removing calcareous deposits that lodge inside the carbon steel pipes. Typically, AISI 304 L stainless steel is used to manufacture various components, such as valves, adapters, rings, connectors, and rotors of pumps that are used in these acidifications. In order to guarantee the protection of both the carbon steel and the AISI 304 L, propargyl alcohol can be used as a corrosion inhibitor. The results of mass loss (gravimetric) and electrochemical tests (polarization curve) show a good efficiency of propargyl alcohol as a corrosion inhibitor for AISI 304 L stainless steel in concentrations of 5% (by volume) and 10% (by volume) of hydrochloric acid
Undamped electrostatic plasma waves
Electrostatic waves in a collision-free unmagnetized plasma of electrons with
fixed ions are investigated for electron equilibrium velocity distribution
functions that deviate slightly from Maxwellian. Of interest are undamped waves
that are the small amplitude limit of nonlinear excitations, such as electron
acoustic waves (EAWs). A deviation consisting of a small plateau, a region with
zero velocity derivative over a width that is a very small fraction of the
electron thermal speed, is shown to give rise to new undamped modes, which here
are named {\it corner modes}. The presence of the plateau turns off Landau
damping and allows oscillations with phase speeds within the plateau. These
undamped waves are obtained in a wide region of the plane
( being the real part of the wave frequency and the
wavenumber), away from the well-known `thumb curve' for Langmuir waves and EAWs
based on the Maxwellian. Results of nonlinear Vlasov-Poisson simulations that
corroborate the existence of these modes are described. It is also shown that
deviations caused by fattening the tail of the distribution shift roots off of
the thumb curve toward lower -values and chopping the tail shifts them
toward higher -values. In addition, a rule of thumb is obtained for
assessing how the existence of a plateau shifts roots off of the thumb curve.
Suggestions are made for interpreting experimental observations of
electrostatic waves, such as recent ones in nonneutral plasmas.Comment: 11 pages, 10 figure
Photoperiod-Dependent Expression of MicroRNA in Drosophila.
Like many other insects in temperate regions, Drosophila melanogaster exploits the photoperiod shortening that occurs during the autumn as an important cue to trigger a seasonal response. Flies survive the winter by entering a state of reproductive arrest (diapause), which drives the relocation of resources from reproduction to survival. Here, we profiled the expression of microRNA (miRNA) in long and short photoperiods and identified seven differentially expressed miRNAs (dme-mir-2b, dme-mir-11, dme-mir-34, dme-mir-274, dme-mir-184, dme-mir-184*, and dme-mir-285). Misexpression of dme-mir-2b, dme-mir-184, and dme-mir-274 in pigment-dispersing, factor-expressing neurons largely disrupted the normal photoperiodic response, suggesting that these miRNAs play functional roles in photoperiodic timing. We also analyzed the targets of photoperiodic miRNA by both computational predication and by Argonaute-1-mediated immunoprecipitation of long- and short-day RNA samples. Together with global transcriptome profiling, our results expand existing data on other Drosophila species, identifying genes and pathways that are differentially regulated in different photoperiods and reproductive status. Our data suggest that post-transcriptional regulation by miRNA is an important facet of photoperiodic timing
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