13,716 research outputs found
Pair plasma cushions in the hole-boring scenario
Pulses from a 10 PW laser are predicted to produce large numbers of
gamma-rays and electron-positron pairs on hitting a solid target. However, a
pair plasma, if it accumulates in front of the target, may partially shield it
from the pulse. Using stationary, one-dimensional solutions of the two-fluid
(electron-positron) and Maxwell equations, including a classical radiation
reaction term, we examine this effect in the hole-boring scenario. We find the
collective effects of a pair plasma "cushion" substantially reduce the
reflectivity, converting the absorbed flux into high-energy gamma-rays. There
is also a modest increase in the laser intensity needed to achieve threshold
for a non-linear pair cascade.Comment: 17 pages, 5 figures. Accepted for publication in Plasma Physics and
Controlled Fusion. Typos corrected, reference update
Longevity and Persistence of Triploid Grass Carp Stocked into the Santee Cooper Reservoirs of South Carolina
This study evaluated longevity and population persistence
of 768,500 triploid grass carp (Ctenopharyngodon idella Valenciennes)
stocked in the 70,000-ha Santee Cooper system in
South Carolina from 1989 through 1996 to control hydrilla
(Hydrilla verticillata (L.f.) Royle)
A modal model for diffraction gratings
A description of an algorithm for a rather general modal grating calculation
is presented. Arbitrary profiles, depth, and permittivity are allowed. Gratings
built up from sub-gratings are allowed, as are coatings on the sidewalls of
lines, and arbitrary complex structure. Conical angles and good conductors are
supported
X-Ray Spectral Variability of Extreme BL Lac AGN H1426+428
Between 7 March 2002 and 15 June 2002, intensive X-ray observations were
carried out on the extreme BL Lac object H1426+428 with instruments on board
the Rossi X-ray Timing Explorer (RXTE). These instruments provide measurements
of H1426+428 in the crucial energy range that characterizes the first peak of
its spectral energy distribution. This peak, which is almost certainly due to
synchrotron emission, has previously been inferred to be in excess of 100 keV.
By taking frequent observations over a four-month campaign, which included
450 ksec of RXTE time, studies of flux and spectral variability on
multiple timescales were performed, along with studies of spectral hysteresis.
The 3-24 keV X-ray flux and spectra exhibited significant variability, implying
variability in the location of the first peak of the spectral energy
distribution. Hysteresis patterns were observed, and their characteristics have
been discussed within the context of emission models.Comment: accepted for publication in Astrophysical Journa
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CIO/CTO Job Roles: An Emerging Organizational Model
The position of Chief Information Systems Officer (CIO) has become a daunting job due to the myriad of business and technical responsibilities assigned to the organization\u27s top information systems (IS) executive. CIOs are being asked to successfully ensure the firm\u27s IS investments are continually aligned with its strategic business objectives, while also planning and maintaining an IT infrastructure that will meet the firm\u27s current and future information processing needs. Many CIOs are finding it extremely difficult to balance the two key roles of technician and businessperson successfully. Many business experts that have analyzed this problem have concluded that the tasks and responsibilities assigned to a typical CIO may be too overwhelming for a single person. This paper advances a formal organizational structure in which the typical responsibilities of the CIO position are re-allocated to two IS executives - the CIO and the Chief Technology Officer (CTO). This paper also describes which responsibilities should be retained by the CIO and which responsibilities should be delegated to the CTO. Finally, a discussion of the challenges and benefits associated with the implementation of CIO/CTO organizational leadership structure is presented
Nonlinear Particle Acceleration in Relativistic Shocks
Monte Carlo techniques are used to model nonlinear particle acceleration in
parallel collisionless shocks of various speeds, including mildly relativistic
ones. When the acceleration is efficient, the backreaction of accelerated
particles modifies the shock structure and causes the compression ratio, r, to
increase above test-particle values. Modified shocks with Lorentz factors less
than about 3 can have compression ratios considerably greater than 3 and the
momentum distribution of energetic particles no longer follows a power law
relation. These results may be important for the interpretation of gamma-ray
bursts if mildly relativistic internal and/or afterglow shocks play an
important role accelerating particles that produce the observed radiation. For
shock Lorentz factors greater than about 10, r approaches 3 and the so-called
`universal' test-particle result of N(E) proportional to E^{-2.3} is obtained
for sufficiently energetic particles. In all cases, the absolute normalization
of the particle distribution follows directly from our model assumptions and is
explicitly determined.Comment: Updated version, Astroparticle Physics, in press, 29 pages, 13
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