16,997 research outputs found
Dynamics of atomic spin-orbit-state wave packets produced by short-pulse laser photodetachment
We analyse the experiment by Hultgren et al. [Phys. Rev. A {\bf 87}, 031404
(2013)] on orbital alignment and quantum beats in coherently excited atomic
fine-structure manifolds produced by short-pulse laser photodetachment of
C, Si and Ge negative ions, and derive a formula that describes the
beats. Analysis of the experimental data enables us to extract the non-coherent
background contribution for each species, and indicates the need for a full
density matrix treatment of the problem
Comment on "Direct photodetachment of F by mid-infrared few-cycle femtosecond laser pulses"
Multiphoton detachment of F by strong few-cycle laser pulses was studied
by Shearer and Monteith using a Keldysh-type approach [Phys. Rev. A 88, 033415
(2013)]. We believe that this work contained errors in the calculation of the
detachment amplitude and photoelectron spectra. We describe the necessary
corrections to the theory and show that the results, in particular, the
interference features of the photoelectron spectra, appear noticeably
different.Comment: 9 pages, 4 figure
Wind tunnel evaluation of a truncated NACA 64-621 airfoil for wind turbine applications
An experimental program to measure the aerodynamic performance of a NACA 64-621 airfoil with a truncated trailing edge for wind turbine applications has been conducted in the Ohio State University Aeronautical and Astronautical Research Laboratory 6 in. by 21 in. pressurized wind tunnel. The blunted or trailing edge truncated (TET) airfoil has an advantage over similar trailing edge airfoils because it is able to streamline a larger spar structure, while also providing aerodynamic properties that are quite good. Surface pressures were measured and integrated to determine the lift, pressure drag, and moment coefficients over angles of attack ranging from -14 to +90 deg at Mach 0.2 and Reynolds numbers of 1,000,000 and 600,000. Results are compared to the NACA 0025, 0030, and 0035 thick airfoils with sharp trailing edges. Comparison shows that the 30 percent thick NACA 64-621-TET airfoil has higher maximum lift, higher lift curve slope, lower drag at higher lift coefficients, and higher chordwise force coefficient than similar thick airfoils with sharp trailing edges
KALwEN: A New Practical and Interoperable Key Management Scheme for Body Sensor Networks
Key management is the pillar of a security architecture. Body sensor networks(BSNs) pose several challenges -- some inherited from wireless sensor networks(WSNs), some unique to themselves -- that require a new key management scheme to be tailor-made. The challenge is taken on, and the result is KALwEN, a new lightweight scheme that combines the best-suited cryptographic techniques in a seamless framework. KALwEN is user-friendly in the sense that it requires no expert knowledge of a user, and instead only requires a user to follow a simple set of instructions when bootstrapping or extending a network. One of KALwEN's key features is that it allows sensor devices from different manufacturers, which expectedly do not have any pre-shared secret, to establish secure communications with each other. KALwEN is decentralized, such that it does not rely on the availability of a local processing unit (LPU). KALwEN supports global broadcast, local broadcast and neighbor-to-neighbor unicast, while preserving past key secrecry and future key secrecy. The fact that the cryptographic protocols of KALwEN have been formally verified also makes a convincing case
The golden circle: A way of arguing and acting about technology in the London ambulance service
This paper analyses the way in which the London Ambulance Service recovered from the events of October 1992, when it implemented a computer-aided despatch system (LASCAD) that remained in service for less than two weeks. It examines the enactment of a programme of long-term organizational change, focusing on the implementation of an alternative computer system in 1996. The analysis in this paper is informed by actor-network theory, both by an early statement of this approach developed by Callon in the sociology of translation, and also by concepts and ideas from Latour’s more recent restatement of his own position. The paper examines how alternative interests emerged and were stabilized over time, in a way of arguing and acting among key players in the change programme, christened the Golden Circle. The story traces four years in the history of the London Ambulance Service, from the aftermath of October 1992 through the birth of the Golden Circle to the achievement of National Health Service (NHS) trust status. LASCAD was the beginning of the story, this is the middle, an end lies in the future, when the remaining elements of the change programme are enacted beyond the Golden Circle
A Constraint on the Organization of the Galactic Center Magnetic Field Using Faraday Rotation
We present new 6 and 20 cm Very Large Array (VLA) observations of polarized
continuum emission of roughly 0.5 square degrees of the Galactic center (GC)
region. The 6 cm observations detect diffuse linearly-polarized emission
throughout the region with a brightness of roughly 1 mJy per 15"x10" beam. The
Faraday rotation measure (RM) toward this polarized emission has structure on
degree size scales and ranges from roughly +330 rad/m2 east of the dynamical
center (Sgr A) to -880 rad/m2 west of the dynamical center. This RM structure
is also seen toward several nonthermal radio filaments, which implies that they
have a similar magnetic field orientation and constrains models for their
origin. Modeling shows that the RM and its change with Galactic longitude are
best explained by the high electron density and strong magnetic field of the GC
region. Considering the emissivity of the GC plasma shows that while the
absolute RM values are indirect measures of the GC magnetic field, the RM
longitude structure directly traces the magnetic field in the central
kiloparsec of the Galaxy. Combining this result with previous work reveals a
larger RM structure covering the central ~2 degrees of the Galaxy. This RM
structure is similar to that proposed by Novak and coworkers, but is shifted
roughly 50 pc west of the dynamical center of the Galaxy. If this RM structure
originates in the GC region, it shows that the GC magnetic field is organized
on ~300 pc size scales. The pattern is consistent with a predominantly poloidal
field geometry, pointing from south to north, that is perturbed by the motion
of gas in the Galactic disk.Comment: Accepted to ApJ. emulateapj style, 14 pages, 15 figure
Thinking beyond the hybrid:“actually-existing” cities “after neoliberalism” in Boyle <i>et al.</i>
In their article, ‘The spatialities of actually existing neoliberalism in Glasgow, 1977 to present’, Mark Boyle, Christopher McWilliams and Gareth Rice (2008) usefully problematise our current understanding of neoliberal urbanism. Our response is aimed at developing a sympathetic but critical approach to Boyle et al's understanding of neoliberal urbanism as illustrated by the Glasgow example. In particular, the counterposing by Boyle et al of a 'hybrid, mutant' model to a 'pure' model of neoliberalism for us misrepresents existing models of neoliberalism as a perfectly finished object rather than a roughly mottled process. That they do not identify any ‘pure’ model leads them to create a straw construct against which they can claim a more sophisticated, refined approach to the messiness of neoliberal urbanism. In contrast, we view neoliberalism as a contested and unstable response to accumulation crises at various scales of analysis
Theory of superradiant scattering of laser light from Bose-Einstein condensates
In a recent MIT experiment, a new form of superradiant Rayleigh scattering
was observed in Bose-Einstein condensates. We present a detailed theory of this
phenomena in which the directional dependence of the scattering rate and
condensate depletion lead to mode competition which is ultimately responsible
for superradiance. The nonlinear response of the system is highly sensitive to
initial quantum fluctuations which cause large run to run variations in the
observed superradiant pulses.Comment: Updated version with new figures,a numerical simulation with
realistic experimental parameters is now included. Featured in September 1999
Physics Today, in Search and Discovery sectio
Motion Induced Radiation from a Vibrating Cavity
We study the radiation emitted by a cavity moving in vacuum. We give a
quantitative estimate of the photon production inside the cavity as well as of
the photon flux radiated from the cavity. A resonance enhancement occurs not
only when the cavity length is modulated but also for a global oscillation of
the cavity. For a high finesse cavity the emitted radiation surpasses radiation
from a single mirror by orders of magnitude.Comment: 4 pages, to appear in Physical Review Letter
- …