8,251 research outputs found
A Plasma Lens for Magnetron Sputtering
A plasma lens, consisting of a solenoid and potential-defining ring electrodes, has been placed between a magnetron and substrates to be coated. Photography reveals qualitative information on excitation, ionization, and the transport of plasma to the substrate
Paper #2-16 BIODIVERSITY MANAGEMENT AND TECHNOLOGY Prepared by the Technology Subgroup
and Oil Resources, also approved the making available of certain materials used in the study process, including detailed, specific subject matter papers prepared or used by the studyâs Task Groups and/or Subgroups. These Topic and White Papers were working documents that were part of the analyses that led to development of the summary results presented in the reportâs Executive Summary and Chapters. These Topic and White Papers represent the views and conclusions of the authors. The National Petroleum Council has not endorsed or approved the statements and conclusions contained in these documents, but approved the publication of these materials as part of the study process. The NPC believes that these papers will be of interest to the readers of the report and will help them better understand the results. These materials are being made available in the interest of transparency. The attached paper is one of 57 such working documents used in the study analyses. Also included is a roster of the Subgroup that developed or submitted this paper. Appendix C of the final NPC report provides a complete list of the 57 Topic and Whit
Principal Value Resummation
We present a new resummation formula for the Drell-Yan cross section. The
formal resummation of threshold corrections in Drell-Yan hard-scattering
functions produces an exponent with singularities from the infrared pole of the
QCD running coupling. Our reformulation treats such `infrared renormalons' by a
principal value prescription, analogous to a modified Borel transform. The
resulting expression includes all large threshold corrections to the hard
scattering function as an asymptotic series in , but is a finite
function of . We find that the ambiguities of the resummed perturbation
theory imply the presence of higher twist corrections to quark-antiquark
hard-scattering functions that begin at . This suggests an
important role for higher twist in the phenomonolgy of hadron-hadron inclusive
cross sections. We also discuss the numerical evaluation of the exponent and
its asymptotic perturbation series for representative values of .Comment: 28 pages, ITP-SB-93-6
Modulation of charge-density waves by superlattice structures
We discuss the interplay between electronic correlations and an underlying
superlattice structure in determining the period of charge density waves
(CDW's), by considering a one-dimensional Hubbard model with a repeated
(non-random) pattern of repulsive (U>0) and free (U=0) sites. Density matrix
renormalization group diagonalization of finite systems (up to 120 sites) is
used to calculate the charge-density correlation function and structure factor
in the ground state. The modulation period can still be predicted through
effective Fermi wavevectors, k_F*, and densities, and we have found that it is
much more sensitive to electron (or hole) doping, both because of the narrow
range of densities needed to go from q*=0 to \pi, but also due to sharp
2k_F*-4k_F* transitions; these features render CDW's more versatile for actual
applications in heterostructures than in homogeneous systems.Comment: 4 pages, 5 figures, to appear in Phys Rev
A SCUBA survey of Orion, the low-mass end of the core mass function
We have re-analysed all of the SCUBA archive data of the Orion star-forming
regions. We have put together all of the data taken at different times by
different groups. Consequently we have constructed the deepest submillimetre
maps of these regions ever made. There are four regions that have been mapped:
Orion A North & South, and Orion B North & South. We find that two of the
regions, Orion A North and Orion B North, have deeper sensitivity and
completeness limits, and contain a larger number of sources, so we concentrate
on these two. We compare the data with archive data from the Spitzer Space
Telescope to determine whether or not a core detected in the submillimetre is
pre-stellar in nature. We extract all of the pre-stellar cores from the data
and make a histogram of the core masses. This can be compared to the stellar
initial mass function (IMF). We find the high-mass core mass function follows a
roughly Salpeter-like slope, just like the IMF, as seen in previous work. Our
deeper maps allow us to see that the core mass function (CMF) turns over at ~
1.3 Mo, about a factor of 4 higher than our completeness limit. This turnover
has never previously been observed, and is only visible here due to our much
deeper maps. It mimics the turnover seen in the stellar IMF at ~ 0.1 Mo. The
low-mass side of the CMF is a power-law with an exponent of 0.35 +/- 0.2, which
is consistent with the low-mass slope of the young cluster IMF of 0.3 +/- 0.1.
This shows that the CMF continues to mimic the shape of the IMF all the way
down to the lower completeness limit of these data at ~ 0.3 Mo.Comment: 9 pages, inc. 6 figures (+ Appendix; 1 Table = 6 pages
Recent advances in metallic glass nanostructures: synthesis strategies and electrocatalytic applications
Recent advances in metallic glass nanostructures (MGNs) are reported, covering a wide array of synthesis strategies, computational discovery, and design solutions that provide insight into distinct electrocatalytic applications. A brief introduction to the development and unique features of MGNs with an overview of top-down and bottom-up synthesis strategies is presented. Specifically, the morphology and structural analysis of several examples applying MGNs as electrodes are highlighted. Subsequently, a comprehensive discussion of commonly employed kinetic parameters and their connection with the unique material structures of MGNs on individual electrocatalytic reactions is made, including the hydrogen evolution reaction, oxygen reduction reaction, and alcohol (methanol or ethanol) oxidation reaction. Finally, a summary of the challenges and perspective on the future research and development relevant to MGNs as electrocatalysts is provided.317FAPESP â FUNDAĂĂO DE AMPARO Ă PESQUISA DO ESTADO DE SĂO PAULO2017/11958â
Magnetic field generation in finite beam plasma system
For finite systems boundaries can introduce remarkable novel features. A well
known example is the Casimir effect [1, 2] that is observed in quantum
electrodynamic systems. In classical systems too novel effects associated with
finite boundaries have been observed, for example the surface plasmon mode [3]
that appears when the plasma has a finite extension. In this work a novel
instability associated with the finite transverse size of a beam owing through
a plasma system has been shown to exist. This instability leads to distinct
characteristic features of the associated magnetic field that gets generated.
For example, in contrast to the well known unstable Weibel mode of a beam
plasma system which generates magnetic field at the skin depth scale, this
instability generates magnetic field at the scales length of the transverse
beam dimension [4]. The existence of this new instability is demonstrated by
analytical arguments and by simulations conducted with the help of a variety of
Particle - In - Cell (PIC) codes (e.g. OSIRIS, EPOCH, PICPSI). Two fluid
simulations have also been conducted which confirm the observations.
Furthermore, laboratory experiments on laser plasma system also provides
evidence of such an instability mechanism at work
Performance Limits of Stochastic Sub-Gradient Learning, Part II: Multi-Agent Case
The analysis in Part I revealed interesting properties for subgradient
learning algorithms in the context of stochastic optimization when gradient
noise is present. These algorithms are used when the risk functions are
non-smooth and involve non-differentiable components. They have been long
recognized as being slow converging methods. However, it was revealed in Part I
that the rate of convergence becomes linear for stochastic optimization
problems, with the error iterate converging at an exponential rate
to within an neighborhood of the optimizer, for some and small step-size . The conclusion was established under weaker
assumptions than the prior literature and, moreover, several important problems
(such as LASSO, SVM, and Total Variation) were shown to satisfy these weaker
assumptions automatically (but not the previously used conditions from the
literature). These results revealed that sub-gradient learning methods have
more favorable behavior than originally thought when used to enable continuous
adaptation and learning. The results of Part I were exclusive to single-agent
adaptation. The purpose of the current Part II is to examine the implications
of these discoveries when a collection of networked agents employs subgradient
learning as their cooperative mechanism. The analysis will show that, despite
the coupled dynamics that arises in a networked scenario, the agents are still
able to attain linear convergence in the stochastic case; they are also able to
reach agreement within of the optimizer
Solar wind collisional heating
To properly describe heating in weakly collisional turbulent plasmas such as
the solar wind, inter-particle collisions should be taken into account.
Collisions can convert ordered energy into heat by means of irreversible
relaxation towards the thermal equilibrium. Recently, Pezzi et al. (Phys. Rev.
Lett., vol. 116, 2016, p. 145001) showed that the plasma collisionality is
enhanced by the presence of fine structures in velocity space. Here, the
analysis is extended by directly comparing the effects of the fully nonlinear
Landau operator and a linearized Landau operator. By focusing on the relaxation
towards the equilibrium of an out of equilibrium distribution function in a
homogeneous force-free plasma, here it is pointed out that it is significant to
retain nonlinearities in the collisional operator to quantify the importance of
collisional effects. Although the presence of several characteristic times
associated with the dissipation of different phase space structures is
recovered in both the cases of the nonlinear and the linearized operators, the
influence of these times is different in the two cases. In the linearized
operator case, the recovered characteristic times are systematically larger
than in the fully nonlinear operator case, this suggesting that fine velocity
structures are dissipated slower if nonlinearities are neglected in the
collisional operator
- âŠ