37,344 research outputs found
Focussing effects in laser-electron Thomson scattering
We study the effects of laser pulse focussing on the spectral properties of
Thomson scattered radiation. Modelling the laser as a paraxial beam we find
that, in all but the most extreme cases of focussing, the temporal envelope has
a much bigger effect on the spectrum than the focussing itself. For the case of
ultra-short pulses, where the paraxial model is no longer valid, we adopt a
sub-cycle vector beam description of the field. It is found that the emission
harmonics are blue shifted and broaden out in frequency space as the pulse
becomes shorter. Additionally the carrier envelope phase becomes important,
resulting in an angular asymmetry in the spectrum. We then use the same model
to study the effects of focussing beyond the limit where the paraxial expansion
is valid. It is found that fields focussed to sub-wavelength spot sizes produce
spectra that are qualitatively similar to those from sub-cycle pulses due to
the shortening of the pulse with focussing. Finally, we study high-intensity
fields and find that, in general, the focussing makes negligible difference to
the spectra in the regime of radiation reaction.Comment: 14 pages, 17 figure
Insensitive control technology development
THe investigation of two insensitive controller synthesis techniques was reported. The finite dimensional inverse approach produces a time varying insensitive controller and/or parameter identifier by constructing inverse functions derived from a finite number of input output pair relationships. The MD/IM concept relies on the information matrix theory that was developed in the estimation and identification field. The MD/IM synthesis technique is based on the hypothesis that minimizing the information matrix will reduce system identifiability and consequently system sensitivity to uncertain parameters. The controllers designed with both techniques were evaluated on a realistic C-5A aircraft flight control problem. Results indicate that the FDI controller is more suited to trajectory type problems because of its time varying nature. The MD/IM controller performed as well as the top-rated controllers of the initial effort and has direct application to aircraft flight control problems
Monstrous branes
We study D-branes in the bosonic closed string theory whose automorphism
group is the Bimonster group (the wreath product of the Monster simple group
with Z_2). We give a complete classification of D-branes preserving the chiral
subalgebra of Monster invariants and show that they transform in a
representation of the Bimonster. Our results apply more generally to self-dual
conformal field theories which admit the action of a compact Lie group on both
the left- and right-moving sectors.Comment: 31 pages, references adde
Video-rate computational super-resolution and integral imaging at longwave-infrared wavelengths
We report the first computational super-resolved, multi-camera integral
imaging at long-wave infrared (LWIR) wavelengths. A synchronized array of FLIR
Lepton cameras was assembled, and computational super-resolution and
integral-imaging reconstruction employed to generate video with light-field
imaging capabilities, such as 3D imaging and recognition of partially obscured
objects, while also providing a four-fold increase in effective pixel count.
This approach to high-resolution imaging enables a fundamental reduction in the
track length and volume of an imaging system, while also enabling use of
low-cost lens materials.Comment: Supplementary multimedia material in
http://dx.doi.org/10.6084/m9.figshare.530302
Dissociation dynamics of fluorinated ethene cations:\ud from time bombs on a molecular level to double-regime dissociators\ud
The dissociative photoionization mechanism of internal energy selected CHF, 1,1-CHF, CHF and CF cations have been studied in the 13−20 eV photon energy range using imaging photoelectron photoion coincidence spectroscopy. Five predominant channels have been found; HF loss, statistical and non-statistical F loss, cleavage of the C–C bond post H or F-atom migration, and cleavage of the C=C bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E, of the daughter ions have been determined. Both CHF and 1,1-CHF are veritable time bombs with respect to dissociation via HF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for CHF and CF involves an atom migration across the C=C bond, giving CF–CHF and CF–CF, respectively, which then dissociate to form CHF and CF. The nature of the F-loss pathway has been found to be bimodal for CHF and 1,1-CHF, switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of CF is found to be comprised of two regimes. At high internal energies, a long-lived excited electronic state is formed, which loses an F atom in a non-statistical process and undergoes statistical redistribution of energy among the nuclear degrees of freedom. This is followed by a subsequent dissociation. In other words only the ground electronic state phase space stays inaccessible. The accurate E of CF and CF formation from CF together with the now well established ∆Hº of CF yield self-consistent enthalpies of formation for the CF, CF, CF, and CF species
Worldsheet Instanton Corrections to the Kaluza-Klein Monopole
The Kaluza-Klein monopole is a well known object in both gravity and string
theory, related by T-duality to a "smeared" NS5-brane which retains the
isometry around the duality circle. As the true NS5-brane solution is localized
at a point on the circle, duality implies that the Kaluza-Klein monopole should
show some corresponding behavior. In this paper, we express the Kaluza-Klein
monopole as a gauged linear sigma model in two dimensions and show that
worldsheet instantons give corrections to its geometry. These corrections can
be understood as a localization in "winding space" which could be probed by
strings with winding charge around the circle.Comment: 20 pages, REVTeX, v2: minor equation correctio
Sustainability, transport and design: reviewing the prospects for safely encouraging eco-driving
Private vehicle use contributes a disproportionately large amount to the degradation of the environment we inhabit. Technological advancement is of course critical to the mitigation of climate change, however alone it will not suffice; we must also see behavioural change. This paper will argue for the application of Ergonomics to the design of private vehicles, particularly low-carbon vehicles (e.g. hybrid and electric), to encourage this behavioural change. A brief review of literature is offered concerning the effect of the design of a technological object on behaviour, the inter-related nature of goals and feedback in guiding performance, the effect on fuel economy of different driving styles, and the various challenges brought by hybrid and electric vehicles, including range anxiety, workload and distraction, complexity, and novelty. This is followed by a discussion on the potential applicability of a particular design framework, namely Ecological Interface Design, to the design of in-vehicle interfaces that encourage energy-conserving driving behaviours whilst minimising distraction and workload, thus ensuring safety
Noncommutative Solitons and Intersecting D-Branes
We construct intersecting D-branes as noncommutative solitons in bosonic and
type II string theory. ``Defect'' branes which are D-branes containing bubbles
of the closed string vacuum play an important role in the construction.Comment: 17 pages, harvmac; published version with added clarification
Design-for-test structure to facilitate test vector application with low performance loss in non-test mode.
A switching based circuit is described which allows application of voltage test vectors to internal nodes of a chip without the problem of backdriving. The new circuit has low impact on the performance of an analogue circuit in terms of loss of bandwidth and allows simple application of analogue test voltages into internal nodes. The circuit described facilitates implementation of the forthcoming IEEE 1149.4 DfT philosophy [1]
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Hand pollination to increase seed-set of red helleborine Cephalanthera rubra in the Chiltern Hills, Buckinghamshire, England
In 2007 and in previous years, as part of ongoing attempts to improve red helleborine Cephalanthera rubra seed-set, hand pollination of florets has been undertaken at a small colony of this species in Buckinghamshire, southern England. Natural pollination rarely occurs (one mature pod recorded in 10 years) at this site. In 2007, hand pollination resulted in the production of four seed pods, of which one withered and died. Upon ripening, the three remaining pods were removed for attempted micropropagation of the seeds. Ongoing conservation management has probably benefited the solitary bee Chelostoma campanularum which now appears fairly plentiful at the site, but despite the presence of this red helleborine flower visitor, natural pollination remains virtually unrecorded at this locality; field observations suggest that C.campanularum is in fact probably not large enough to act as an effective red helleborine pollinator as it can slip in and out of the flowers without removing the pollinia, unlike it larger relative C.fuliginosum, absent from the UK but which is a known pollinator of red helleborine in continental Europe
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