399 research outputs found
Gain-bandwidth product optimization of heterostructure avalanche photodiodes
A generalized history-dependent recurrence theory for the time-response analysis is derived for avalanche photodiodes with multilayer, heterojunction multiplication regions. The heterojunction multiplication region considered consists of two layers: a high-bandgap Al/sub 0.6/Ga/sub 0.4/As energy-buildup layer, which serves to heat up the primary electrons, and a GaAs layer, which serves as the primary avalanching layer. The model is used to optimize the gain-bandwidth product (GBP) by appropriate selection of the width of the energy-buildup layer for a given width of the avalanching layer. The enhanced GBP is a direct consequence of the heating of primary electrons in the energy-buildup layer, which results in a reduced first dead space for the carriers that are injected into the avalanche-active GaAs layer. This effect is akin to the initial-energy effect previously shown to enhance the excess-noise factor characteristics in thin avalanche photodiodes (APDs). Calculations show that the GBP optimization is insensitive to the operational gain and the optimized APD also minimizes the excess-noise factor
Case Study of Four Homologous Large-Scale Coronal Waves Observed on 2010 April 28 and 29
On 2010 April 28 and 29, the Solar TErrestrial Relations Observatory
B/Extreme Ultraviolet Imager observed four homologous large-scale coronal
waves, the so-called EIT-waves, within 8 hr. All waves emerged from the same
source active region, were accompanied by weak flares and faint coronal mass
ejections, and propagated into the same direction at constant velocities in the
range of ~220-340 km s-1. The last of these four coronal wave events was the
strongest and fastest, with a velocity of 337 +/- 31 km s-1 and a peak
perturbation amplitude of ~1.24, corresponding to a magnetosonic Mach number of
Mms ~ 1.09. The magnetosonic Mach numbers and velocities of the four waves are
distinctly correlated, suggestive of the nonlinear fast-mode magnetosonic wave
nature of the events. We also found a correlation between the magnetic energy
buildup times and the velocity and magnetosonic Mach number
Data analysis and interpretation of UVSP and other experiments on board solar maximum mission
During the period of this contract (February 1 1980 to February 1987) there were two separate efforts involved: one was programmetric, i.e., the coordination of scientific working groups and the organization of workshops in the solar physics discipline; the second was scientific, i.e., to perform research to investigate the fundamental physical mechanisms of the energy and momentum transport from the solar surface to interplanetary space. In the former, 19 workshops, involving 88 scientists were organized. In the latter aspect, the following were investigated: solar flare energy buildup and release, coronal dynamics, energy and momentum transport from lower solar atmosphere to interplanetary space, numerical methods for the calculation of the nonlinear force-free field, and the evolution of the solar magnetic field
Concept for sleeve induction motor with 1-msec mechanical time constant
Conductive sleeve induction motor having a 1-msec mechanical time constant is used with solid-state devices to control all-electric servo power systems. The servomotor rotor inertia is small compared to the maximum force rating of the servo motion, permitting high no-load acceleration
The PAC2MAN mission: A new tool to understand and predict solar energetic events
An accurate forecast of flare and coronal mass ejection (CME) initiation requires precise measurements of the magnetic energy buildup and release in the active regions of the solar atmosphere. We designed a new space weather mission that performs such measurements using new optical instruments based on the Hanle and Zeeman effects. The mission consists of two satellites, one orbiting the L1 Lagrangian point (Spacecraft Earth, SCE) and the second in heliocentric orbit at 1AU trailing the Earth by 80\ub0 (Spacecraft 80, SC80). Optical instruments measure the vector magnetic field in multiple layers of the solar atmosphere. The orbits of the spacecraft allow for a continuous imaging of nearly 73% of the total solar surface. In-situ plasma instruments detect solar wind conditions at 1AU and ahead of our planet. Earth-directed CMEs can be tracked using the stereoscopic view of the spacecraft and the strategic placement of the SC80 satellite. Forecasting of geoeffective space weather events is possible thanks to an accurate surveillance of the magnetic energy buildup in the Sun, an optical tracking through the interplanetary space, and in-situ measurements of the near-Earth environment
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Detailed particle-in-cell simulations on the transport of a relativistic electron beam in plasmas
We present comprehensive two-dimensional (2D) particle-in-cell (PIC) simulations on the transport of a relativistic electron beam in a plasma in the context of fast ignition fusion. The 2D PIC simulations are performed by constructing two different simulation planes and have shown the complete stabilization and destabilization of the Weibel instability due to the beam temperature and background plasma collisions, respectively. Some three-dimensional PIC simulation results on the filamentary structures are also shown thereby further shedding light on the filamentation of the electron beam in plasmas. The linear growth rates of fastest growing mode in the beam-plasma system are compared with a theoretical model developed and are found in good agreement with each other.DFGU. S. Department of Energy DEFG02-04ER41321 DE-FG02-04ER54763Alliance Program of the Helmholtz Association HA216/EMMIPhysic
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The PAC2MAN mission: A new tool to understand and predict solar energetic events
An accurate forecast of flare and coronal mass ejection (CME) initiation requires precise measurements of the magnetic energy buildup and release in the active regions of the solar atmosphere. We designed a new space weather mission that performs such measurements using new optical instruments based on the Hanle and Zeeman effects. The mission consists of two satellites, one orbiting the L1 Lagrangian point (Spacecraft Earth, SCE) and the second in heliocentric orbit at 1AU trailing the Earth by 80° (Spacecraft 80, SC80). Optical instruments measure the vector magnetic field in multiple layers of the solar atmosphere. The orbits of the spacecraft allow for a continuous imaging of nearly 73% of the total solar surface. In-situ plasma instruments detect solar wind conditions at 1AU and ahead of our planet. Earth-directed CMEs can be tracked using the stereoscopic view of the spacecraft and the strategic placement of the SC80 satellite. Forecasting of geoeffective space weather events is possible thanks to an accurate surveillance of the magnetic energy buildup in the Sun, an optical tracking through the interplanetary space, and in-situ measurements of the near-Earth environment
Elastic wave cloaking via symmetrized transformation media
Transformation media theory, which steers waves in solids via an effective
geometry induced by a refractive material (Fermat's principle of least action),
provides a means of controlling vibrations and elastic waves beyond the
traditional dissipative structures regime. In particular, it could be used to
create an elastic wave cloak, shielding an interior region against elastic
waves while simultaneously preventing scattering in the outside domain.
However, as a true elastic wave cloak would generally require nonphysical
materials with stiffness tensors lacking the minor symmetry (implying
asymmetric stress), the utility of such an elastic wave cloak has thus far been
limited. Here we develop a means of overcoming this limitation via the
development of a symmetrized elastic cloak, sacrificing some of the performance
of the perfect cloak for the sake of restoring the minor symmetry. We test the
performance of the symmetrized elastic cloak for shielding a tunnel against
seismic waves, showing that it can be used to reduce the average displacement
within the tunnel by an order of magnitude (and reduce energy by two orders of
magnitude) for waves above a critical frequency of the cloak. This critical
frequency, which corresponds to the generation of surface waves at the
cloak-interior interface, can be used to develop a simple heuristic model of
the symmetrized elastic cloak's performance for a generic problem
Magnetohydrodynamic (MHD) analyses of various forms of activity and their propagation through helio spheric space
Theoretical and numerical modeling of solar activity and its effects on the solar atmosphere within the context of magnetohydrodynamics were examined. Specifically, the scientific objectives were concerned with the physical mechanisms for the flare energy build-up and subsequent release. In addition, transport of this energy to the corona and solar wind was also investigated. Well-posed, physically self-consistent, numerical simulation models that are based upon magnetohydrodynamics were sought. A systematic investigation of the basic processes that determine the macroscopic dynamic behavior of solar and heliospheric phenomena was conducted. A total of twenty-three articles were accepted and published in major journals. The major achievements are summarized
The Unpredictability of the Most Energetic Solar Events
Observations over the past two solar cycles show a highly irregular pattern
of occurrence for major solar flares, gamma-ray events, and solar energetic
particle (SEP) fluences. Such phenomena do not appear to follow the direct
indices of solar magnetic activity, such as the sunspot number. I show that
this results from the non-Poisson occurrence for the most energetic events.
This Letter also points out a particularly striking example of this
irregularity in a comparison between the declining phases of the recent two
solar cycles (1993-1995 and 2004-2006, respectively) and traces it through the
radiated energies of the flares, the associated SEP fluences, and the sunspot
areas. These factors suggest that processes in the solar interior involved with
the supply of magnetic flux up to the surface of the Sun have strong
correlations in space and time, leading to a complex occurrence pattern that is
presently unpredictable on timescales longer than active region lifetimes
(weeks) and not correlated well with the solar cycle itself.Comment: 4 page
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