4,272 research outputs found
Numerical design of resonator coils for high field magnetic resonance imaging
In the past, design of transmit/receive coils for magnetic resonance imaging
was performed using simple lumped circuit models; however, as developments in
technology have allowed for increasingly high magnetic fields, the frequency at
which the imaging must be performed has increased to the hundreds of megahertz,
causing technical difficulties to arise. This project seeks to overcome the problem of
inhomogeneous magnetic excitation through a redesign of the resonator coil based in
electromagnetic theory. Several designs were attempted, with the primary
modifications being in the dielectric filling and length of the resonator.
Since the lumped circuit model is no longer valid at today’s high fields, it
must be replaced by a full-wave electromagnetic solution. Therefore, all simulations
for this project were performed with a FDTD program previously developed at Ohio
State for this purpose.
The simulations have shown that a significant increase in homogeneity can be
achieved by loading the resonator with a high permittivity dielectric. This in turn
lowers the resonant frequency, so the cavity length must be shortened to raise the
frequency back to normal values. The best case design found here is with a 10 cm
resonator filled with relative dielectric of 60. A better design can likely be found with
a resonator of approximately 8 cm in length, but this was unable to be tested due to
technical difficulties and time constraints. The project also confirms that dielectric
loading of a certain region will cause the field peak to shift towards the load. This
could be useful for locating the field peak at a certain point in the head in order to
obtain an MRI image in just that area
Climatic effects of 1950-2050 changes in US anthropogenic aerosols - Part 2: Climate response
We investigate the climate response to changing US anthropogenic aerosol sources over the 1950–2050 period by using the NASA GISS general circulation model (GCM) and comparing to observed US temperature trends. Time-dependent aerosol distributions are generated from the GEOS-Chem chemical transport model applied to historical emission inventories and future projections. Radiative forcing from US anthropogenic aerosols peaked in 1970–1990 and has strongly declined since due to air quality regulations. We find that the regional radiative forcing from US anthropogenic aerosols elicits a strong regional climate response, cooling the central and eastern US by 0.5–1.0 °C on average during 1970–1990, with the strongest effects on maximum daytime temperatures in summer and autumn. Aerosol cooling reflects comparable contributions from direct and indirect (cloud-mediated) radiative effects. Absorbing aerosol (mainly black carbon) has negligible warming effect. Aerosol cooling reduces surface evaporation and thus decreases precipitation along the US east coast, but also increases the southerly flow of moisture from the Gulf of Mexico resulting in increased cloud cover and precipitation in the central US. Observations over the eastern US show a lack of warming in 1960–1980 followed by very rapid warming since, which we reproduce in the GCM and attribute to trends in US anthropogenic aerosol sources. Present US aerosol concentrations are sufficiently low that future air quality improvements are projected to cause little further warming in the US (0.1 °C over 2010–2050). We find that most of the warming from aerosol source controls in the US has already been realized over the 1980–2010 period
Transverse Energy Measurement in Au+Au Collisions by the STAR Experiment
Transverse energy () has been measured with both of its components,
namely hadronic () and electromagnetic () in a common
phase space at mid-rapidity for 62.4 GeV Au+Au collisions by the STAR
experiment. production with centrality and is studied
with similar measurements from SPS to RHIC and is compared with a final state
gluon saturation model (EKRT). The most striking feature is the observation of
a nearly constant value of GeV from AGS, SPS to RHIC. The
initial energy density estimated by the boost-invariant Bjorken hydrodynamic
model, is well above the critical density for a deconfined matter of quarks and
gluons predicted by lattice QCD calculations.Comment: 4 pages, 10 figures, Presented in Quark Matter 2008, Jaipur, India.
To be published in Indian Journal of Physic
Antenna Element Design Using Characteristic Mode Analysis: Insights and Research Directions
[EN] This article provides a comprehensive review of recent applications of characteristic mode analysis (CMA) to innovative antenna element designs, including multi port, circularly polarized, wideband, reconfigurable, and dielectric resonator antennas (DRAs). Emphasis is placed on the interpretation of the characteristic modes (CMs) for those unfamiliar with the method and physical insights gained from
the characteristic eigenvalues and eigenvectors of an antenna. In addition, we review CMA-based design strategies and specific design examples that highlight the application of CMA to vari ous types of antennas. Ultimately, this article seeks to dem onstrate the value of CMA-based design insights for antenna engineering and look toward promising new research directions for CMA and antenna research.Adams, JJ.; Genovesi, S.; Yang, B.; Antonino Daviu, E. (2022). Antenna Element Design Using Characteristic Mode Analysis: Insights and Research Directions. IEEE Antennas and Propagation Magazine. 64(2):32-40. https://doi.org/10.1109/MAP.2022.3145718324064
Phenomenology of single spin asymmetries in p(transv. polarized)-p -> pion + X
A phenomenological description of single transverse spin effects in
hadron-hadron inclusive processes is proposed, assuming a generalized
factorization scheme and pQCD hard interactions. The transverse momentum, k_T,
of the quarks inside the hadrons and of the hadrons relatively to the
fragmenting quark, is taken into account in distribution and fragmentation
functions, and leads to possible non zero single spin asymmetries. The role of
k_T and spin dependent quark fragmentations -- the so-called Collins effect --
is investigated in details in p(transv. polarized)-p -> pion + X processes: it
is shown how the experimental data could be described, obtaining an explicit
expression for the spin asymmetry of a polarized fragmenting quark, on which
some comments are made. Predictions for other processes, possible further
applications and experimental tests are discussed.Comment: 20+1 pages, LaTeX, 6 eps figures, uses epsfig.sty. Version v2: Some
sentences rephrased and comments added throughout the paper; one reference
added; no changes in results and figures. Final version to be published in
Phys. Rev.
Recommended from our members
Climatic Effects of 1950-2050 Changes in US Anthropogenic Aerosols - Part 1: Aerosol Trends and Radiative Forcing
We calculate decadal aerosol direct and indirect (warm cloud) radiative forcings from US anthropogenic sources over the 1950–2050 period. Past and future aerosol distributions are constructed using GEOS-Chem and historical emission inventories and future projections from the IPCC A1B scenario. Aerosol simulations are evaluated with observed spatial distributions and 1980–2010 trends of aerosol concentrations and wet deposition in the contiguous US. Direct and indirect radiative forcing is calculated using the GISS general circulation model and monthly mean aerosol distributions from GEOS-Chem. The radiative forcing from US anthropogenic aerosols is strongly localized over the eastern US. We find that its magnitude peaked in 1970–1990, with values over the eastern US (east of 100° W) of −2.0 W m−2 for direct forcing including contributions from sulfate (−2.0 W m−2), nitrate (−0.2 W m−2), organic carbon (−0.2 W m−2), and black carbon (+0.4 W m−2). The uncertainties in radiative forcing due to aerosol radiative properties are estimated to be about 50%. The aerosol indirect effect is estimated to be of comparable magnitude to the direct forcing. We find that the magnitude of the forcing declined sharply from 1990 to 2010 (by 0.8 W m−2 direct and 1.0 W m−2 indirect), mainly reflecting decreases in SO2 emissions, and project that it will continue declining post-2010 but at a much slower rate since US SO2 emissions have already declined by almost 60% from their peak. This suggests that much of the warming effect of reducing US anthropogenic aerosol sources has already been realized. The small positive radiative forcing from US BC emissions (+0.3 W m−2 over the eastern US in 2010; 5% of the global forcing from anthropogenic BC emissions worldwide) suggests that a US emission control strategy focused on BC would have only limited climate benefit.Engineering and Applied Science
Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 2: Climate response
We investigate the climate response to changing US anthropogenic aerosol sources over the 1950–2050 period by using the NASA GISS general circulation model (GCM) and comparing to observed US temperature trends. Time-dependent aerosol distributions are generated from the GEOS-Chem chemical transport model applied to historical emission inventories and future projections. Radiative forcing from US anthropogenic aerosols peaked in 1970–1990 and has strongly declined since due to air quality regulations. We find that the regional radiative forcing from US anthropogenic aerosols elicits a strong regional climate response, cooling the central and eastern US by 0.5–1.0 °C on average during 1970–1990, with the strongest effects on maximum daytime temperatures in summer and autumn. Aerosol cooling reflects comparable contributions from direct and indirect (cloud-mediated) radiative effects. Absorbing aerosol (mainly black carbon) has negligible warming effect. Aerosol cooling reduces surface evaporation and thus decreases precipitation along the US east coast, but also increases the southerly flow of moisture from the Gulf of Mexico resulting in increased cloud cover and precipitation in the central US. Observations over the eastern US show a lack of warming in 1960–1980 followed by very rapid warming since, which we reproduce in the GCM and attribute to trends in US anthropogenic aerosol sources. Present US aerosol concentrations are sufficiently low that future air quality improvements are projected to cause little further warming in the US (0.1 °C over 2010–2050). We find that most of the warming from aerosol source controls in the US has already been realized over the 1980–2010 period
Orbital Instabilities in a Triaxial Cusp Potential
This paper constructs an analytic form for a triaxial potential that
describes the dynamics of a wide variety of astrophysical systems, including
the inner portions of dark matter halos, the central regions of galactic
bulges, and young embedded star clusters. Specifically, this potential results
from a density profile of the form , where the radial
coordinate is generalized to triaxial form so that . Using the resulting analytic form of the potential, and the
corresponding force laws, we construct orbit solutions and show that a robust
orbit instability exists in these systems. For orbits initially confined to any
of the three principal planes, the motion in the perpendicular direction can be
unstable. We discuss the range of parameter space for which these orbits are
unstable, find the growth rates and saturation levels of the instability, and
develop a set of analytic model equations that elucidate the essential physics
of the instability mechanism. This orbit instability has a large number of
astrophysical implications and applications, including understanding the
formation of dark matter halos, the structure of galactic bulges, the survival
of tidal streams, and the early evolution of embedded star clusters.Comment: 50 pages, accepted for publication in Ap
Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 1: Aerosol trends and radiative forcing
We use the GEOS-Chem chemical transport model combined with the GISS general circulation model to calculate the aerosol direct and indirect (warm cloud) radiative forcings from US anthropogenic sources over the 1950–2050 period, based on historical emission inventories and future projections from the IPCC A1B scenario. The aerosol simulation is evaluated with observed spatial distributions and 1980–2010 trends of aerosol concentrations and wet deposition in the contiguous US. The radiative forcing from US anthropogenic aerosols is strongly localized over the eastern US. We find that it peaked in 1970–1990, with values over the eastern US (east of 100° W) of −2.0 W m[superscript −2] for direct forcing including contributions from sulfate (−2.0 W m[superscript −2]), nitrate (−0.2 W m[superscript −2]), organic carbon (−0.2 W m[superscript −2]), and black carbon (+0.4 W m[superscript −2]). The aerosol indirect effect is of comparable magnitude to the direct forcing. We find that the forcing declined sharply from 1990 to 2010 (by 0.8 W m−2 direct and 1.0 W m[superscript −2] indirect), mainly reflecting decreases in SO[subscript 2] emissions, and project that it will continue declining post-2010 but at a much slower rate since US SO[subscript 2] emissions have already declined by almost 60 % from their peak. This suggests that much of the warming effect of reducing US anthropogenic aerosol sources may have already been realized by 2010, however some additional warming is expected through 2020. The small positive radiative forcing from US BC emissions (+0.3 W m[superscript −2] over the eastern US in 2010) suggests that an emission control strategy focused on BC would have only limited climate benefit
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