4,616 research outputs found
Kinetic simulation of the sheath dynamics in the intermediate radio-frequency regime
The dynamics of temporally modulated plasma boundary sheaths is studied in
the intermediate radio frequency regime where the applied radio frequency and
the ion plasma frequency are comparable. Two kinetic simulation codes are
employed and their results are compared. The first code is a realization of the
well-known scheme, Particle-In-Cell with Monte Carlo collisions (PIC/MCC) and
simulates the entire discharge, a planar radio frequency capacitively coupled
plasma (RF-CCP) with an additional heating source. The second code is based on
the recently published scheme Ensemble-in-Spacetime (EST); it resolves only the
sheath and requires the time resolved voltage across and the ion flux into the
sheath as input. Ion inertia causes a temporal asymmetry (hysteresis) of the
sheath charge-voltage relation; also other ion transit time effects are found.
The two codes are in good agreement, both with respect to the spatial and
temporal dynamics of the sheath and with respect to the ion energy
distributions at the electrodes. It is concluded that the EST scheme may serve
as an efficient post-processor for fluid or global simulations and for
measurements: It can rapidly and accurately calculate ion distribution
functions even when no genuine kinetic information is available
Shearing Interferometer for Quantifying the Coherence of Hard X-Ray Beams
We report a quantitative measurement of the full transverse coherence function of the 14.4 keV x-ray radiation produced by an undulator at the Swiss Light Source. An x-ray grating interferometer consisting of a beam splitter phase grating and an analyzer amplitude grating has been used to measure the degree of coherence as a function of the beam separation out to 30 m. Importantly, the technique provides a model-free and spatially resolved measurement of the complex coherence function and is not restricted to high resolution detectors and small fields of view. The spatial characterization of the wave front has important applications in discovering localized defects in beam line optics
Kinetic Interpretation of Resonance Phenomena in Low Pressure Capacitively Coupled Radio Frequency Plasmas
The kinetic origin of resonance phenomena in capacitively coupled radio
frequency plasmas is discovered based on particle-based numerical simulations.
The analysis of the spatio-temporal distributions of plasma parameters such as
the densities of hot and cold electrons, as well as the conduction and
displacement currents reveals the mechanism of the formation of multiple
electron beams during sheath expansion. The interplay between highly energetic
beam electrons and low energetic bulk electrons is identified as the physical
origin of the excitation of harmonics in the current
Detailed balance in Horava-Lifshitz gravity
We study Horava-Lifshitz gravity in the presence of a scalar field. When the
detailed balance condition is implemented, a new term in the gravitational
sector is added in order to maintain ultraviolet stability. The
four-dimensional theory is of a scalar-tensor type with a positive cosmological
constant and gravity is nonminimally coupled with the scalar and its gradient
terms. The scalar field has a double-well potential and, if required to play
the role of the inflation, can produce a scale-invariant spectrum. The total
action is rather complicated and there is no analog of the Einstein frame where
Lorentz invariance is recovered in the infrared. For these reasons it may be
necessary to abandon detailed balance. We comment on open problems and future
directions in anisotropic critical models of gravity.Comment: 10 pages. v2: discussion expanded and improved, section on
generalizations added, typos corrected, references added, conclusions
unchange
Storage and stability of organic carbon in soils as related to depth, occlusion within aggregates, and attachment to minerals
Conceptual models suggest that stability of organic carbon (OC) in soil depends on the source of plant litter, occlusion within aggregates, incorporation in organomineral complexes, and location within the soil profile. Density fractionation is a useful tool to study the relevance of OC stabilization in aggregates and in association with minerals, but it has rarely been applied to full soil profiles. We aim to determine factors shaping the depth profiles of physically unprotected and mineral associated OC and test their relevance for OC stability across a range of European soils that vary in vegetation, soil types, parent material, and land use. At each of the 12 study sites, 10 soil cores were sampled to 60 cm depth and subjected to density separation. Bulk soil samples and density fractions (free light fractions - fLF, occluded light fractions - oLF, heavy fractions - HF) were analysed for OC, total nitrogen (TN), δ13C, and Δ14C Bulk samples were also incubated to determine CO2 evolution per g OC in the samples (specific mineralization rates) as an indicator for OC stability. Depth profiles of OC in the light fraction (LF-OC) matched those of roots for undisturbed grassland and forest sites, suggesting that roots are shaping the depth distribution of LF-OC. Organic C in the HF declined less with soil depth than LF-OC and roots, especially at grassland sites. The decrease in Δ14C (increase in age) of HF-OC with soil depth was related to soil pH as well as to dissolved OC fluxes. This indicates that dissolved OC translocation contributes to the formation of subsoil HF-OC and shapes the Δ14C profiles. The LF at three sites were rather depleted in 14C, indicating the presence of fossil material such as coal and lignite, probably inherited from the parent material. At the other sites, modern Δ14C signatures and pos sit tive correlations between specific mineralization rates and fLF-OC indicate the fLF is a potentially available energy and nutrient source for subsurface microorganisms throughout the profile. Declining specific mineralization rates with soil depth confirm greater stability of OC in subsoils across sites. The overall importance of OC stabilization by binding to minerals was demonstrated by declining specific mineralization rates with increasing contributions of HF-OC to bulk soil OC, and the low Δ14C values of HF-OC. The stability of HF-OC was greater in subsoils than in topsoils; nevertheless, a portion of HF-OC was active throughout the profile. While quantitatively less important than OC in the HF, consistent older ages of oLF-OC than fLF-OC suggest that occlusion of LF-OC in aggregates also contributes to OC stability in subsoils. Overall, our results indicate that association with minerals is the most important factor in stabilization of OC in soils, irrespective of vegetation, soil type, and land use. © Author(s) 2013.European Unio
Resonant electron heating and molecular phonon cooling in single C junctions
We study heating and heat dissipation of a single \c60 molecule in the
junction of a scanning tunneling microscope (STM) by measuring the electron
current required to thermally decompose the fullerene cage. The power for
decomposition varies with electron energy and reflects the molecular resonance
structure. When the STM tip contacts the fullerene the molecule can sustain
much larger currents. Transport simulations explain these effects by molecular
heating due to resonant electron-phonon coupling and molecular cooling by
vibrational decay into the tip upon contact formation.Comment: Accepted in Phys. Rev. Let
X-ray constraints on the fraction of obscured AGN at high accretion luminosities
The wide-area XMM-XXL X-ray survey is used to explore the fraction of
obscured AGN at high accretion luminosities, , and out to redshift . The sample covers an area
of about and provides constraints on the space density of
powerful AGN over a wide range of neutral hydrogen column densities extending
beyond the Compton-thick limit, . The fraction
of obscured Compton-thin () AGN is estimated
to be for luminosities
independent of redshift. For less luminous sources the fraction of obscured
Compton-thin AGN increases from at to at
. Studies that select AGN in the infrared via template fits to the
observed Spectral Energy Distribution of extragalactic sources estimate space
densities at high accretion luminosities consistent with the XMM-XXL
constraints. There is no evidence for a large population of AGN (e.g. heavily
obscured) identified in the infrared and missed at X-ray wavelengths. We
further explore the mid-infrared colours of XMM-XXL AGN as a function of
accretion luminosity, column density and redshift. The fraction of XMM-XXL
sources that lie within the mid-infrared colour wedges defined in the
literature to select AGN is primarily a function of redshift. This fraction
increases from about 20-30% at z=0.25 to about 50-70% at .Comment: MNRAS accepte
Control of electron dynamics, radical and metastable species generation in atmospheric pressure RF plasma jets by Voltage Waveform Tailoring
Atmospheric pressure capacitively coupled radio frequency discharges operated in He/N2 mixtures and driven by tailored voltage waveforms are investigated experimentally using a COST microplasma reference jet and by means of kinetic simulations as a function of the reactive gas admixture and the number of consecutive harmonics used to drive the plasma. Pulse-type 'peaks'-waveforms, that consist of up to four consecutive harmonics of the fundamental frequency (f = 13.56 MHz), are used at a fixed peak-to-peak voltage of 400 V. Based on an excellent agreement between experimental and simulation results with respect to the DC self-bias and the spatio-temporal electron impact excitation dynamics, we demonstrate that Voltage Waveform Tailoring allows for the control of the dynamics of energetic electrons, the electron energy distribution function in distinct spatio-temporal regions of interest, and, thus, the generation of atomic nitrogen as well as helium metastables, which are highly relevant for a variety of technological and biomedical applications. By tuning the number of driving frequencies and the reactive gas admixture, the generation of these important species can be optimised. The behaviour of the DC self-bias, which is different compared to that in low pressure capacitive radio frequency plasmas, is understood based on an analytical model
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