912 research outputs found
Transverse self-fields within an electron bunch moving in an arc of a circle
As a consequence of motions driven by external forces, self-fields (which are
different from the static case) originate within an electron bunch. In the case
of magnetic external forces acting on an ultrarelativistic beam, the
longitudinal self-interactions are responsible for CSR (Coherent Synchrotron
Radiation)-related phenomena, which have been studied extensively. On the other
hand, transverse self-interactions are present too. At the time being, existing
theoretical analysis of transverse self-forces deal with the case of a bunch
moving along a circular orbit only, without considering the situation of a
bending magnet with a finite length. In this paper we propose an
electrodynamical analysis of transverse self-fields which originate, at the
position of a test particle, from an ultrarelativistic electron bunch moving in
an arc of a circle. The problem will be first addressed within a two-particle
system. We then extend our consideration to a line bunch with a stepped density
distribution, a situation which can be easily generalized to the case of an
arbitrary density distribution. Our approach turns out to be also useful in
order to get a better insight in the physics involved in the case of simple
circular motion and in order to address the well known issue of the partial
compensation of transverse self-force.Comment: 23 pages, 14 figure
Optical absorption in semiconductor quantum dots: Nonlocal effects
The optical absorption of a single spherical semiconductor quantum dot in an
electrical field is studied taking into account the nonlocal coupling between
the field of the light and the polarizability of the semiconductor. These
nonlocal effects lead to a small size anf field dependent shift and broadening
of the excitonic resonance which may be of interest in future high precision
experiments.Comment: 6 pages, 4 figure
FAST-DB: A novel solid-state approach for diffusion bonding dissimilar titanium alloy powders for next generation critical components
Titanium alloy components are subjected to challenging conditions in high performance applications, consisting of complex loads and thermal gradients. To improve the performance and efficiency of such components, it is desirable to introduce different microstructures into subcomponent regions which cannot be achieved by the conventional melt-wrought processing route. Instead, a solid-state consolidation route using dissimilar titanium alloy powders is proposed. In this study commercially pure Ti (CP-Ti), Ti-6Al-4V (Ti-6-4), and Ti-5Al-5Mo-5V-3Cr (Ti-5553) have been diffusion bonded using field assisted sintering technology (FAST) for dwell times of 10, 20, and 60 min. The effectiveness of FAST for diffusion bonding (DB) of dissimilar alloy powders has led to the authors terming this hybrid process as "FAST-DB". Excellent bond integrity was produced with no cracking, unbonded regions or voids in the dissimilar bond combinations, at all dwell times. Furthermore, reliable control and prediction of the bond characteristics and degree of elemental diffusion was demonstrated through commercial thermodynamic software. Elemental diffusion was characterised across the bonds and the chemical diffusion bond width increased linearly with dwell time. Peak hardness values occurred directly on the interface for Ti-5553 and CP-Ti bonds, whilst slightly into the Ti-6-4 alloy for the Ti-5553 and Ti-6-4 bonds, which can be attributed to fine scale alpha at the interface. In Ti-6-4 with CP-Ti bonds, a smooth transition was observed. Mechanical testing demonstrated that the FAST-DB interface had excellent structural integrity, with necking and fracture occurring in the lower strength alloy
A comparison of covered vs bare expandable stents for the treatment of aortoiliac occlusive disease
ObjectiveThis trial was conducted to determine if covered stents offer a patency advantage over bare-metal stents in the treatment of aortoiliac arterial occlusive disease.MethodsThe Covered Versus Balloon Expandable Stent Trial (COBEST), a prospective, multicenter, randomized controlled trial, was performed involving 168 iliac arteries in 125 patients with severe aortoiliac occlusive disease who were randomly assigned to receive a covered balloon-expandable stent or bare-metal stent. Patient demographic data, clinical signs and symptoms, TransAtlantic Inter-Society Consensus (TASC) classification, and preprocedure and postprocedure ankle-brachial index measurements were recorded. The primary end points included freedom from binary restenosis and stent occlusion of the treated area, as determined by ultrasound imaging or quantitative visual angiography, or both. Postprocedural follow-up was at 1, 6, 12, and 18 months.ResultsAortoiliac lesions treated with a covered stent were significantly more likely to remain free from binary restenosis than those that were treated with a bare-metal stent (hazard ratio [HR], 0.35; 95% confidence interval (CI), 0.15-0.82; P = .02). Freedom from occlusion was also higher in lesions treated with covered stents than in those treated with a bare-metal stent (HR, 0.28; 95% CI, 0.07-1.09); however, this did not reach statistical significance (P = .07). Subgroup analyses demonstrated a significant difference in freedom from binary restenosis for covered stents in TASC C and D lesions compared with a bare stent (HR, 0.136; 95% CI, 0.042-0.442). This difference was not demonstrated for TASC B lesions (HR, 0.748; 95% CI, 0.235-2.386).ConclusionsCOBEST demonstrates covered and bare-metal stents produce similar and acceptable results for TASC B lesions. However, covered stents perform better for TASC C and D lesions than bare stents in longer-term patency and clinical outcome
Scattering of light and atoms in a Fermi-Dirac gas with BCS pairing
We theoretically study the optical properties of a Fermi-Dirac gas in the
presence of a superfluid state. We calculate the leading quantum-statistical
corrections to the standard column density result of the electric
susceptibility. We also consider the Bragg diffraction of atoms by means of
light-stimulated transitions of photons between two intersecting laser beams.
Bardeen-Cooper-Schrieffer pairing between atoms in different internal levels
magnifies incoherent scattering processes. The absorption linewidth of a
Fermi-Dirac gas is broadened and shifted. Bardeen-Cooper-Schrieffer pairing
introduces a collisional local-field shift that may dramatically dominate the
Lorentz-Lorenz shift. For the case of the Bragg spectroscopy the static
structure function may be significantly increased due to superfluidity in the
nearforward scattering.Comment: 13 pages, 6 figures; to appear in PR
Exercise Training Reduces Liver Fat and Increases Rates of VLDL Clearance, but not VLDL Production in NAFLD
Context Randomised controlled trials in non-alcoholic fatty liver disease (NAFLD) have shown that regular exercise, even without calorie restriction, reduces liver steatosis. A previous study has shown that 16 weeks supervised exercise training in NAFLD did not affect total VLDL kinetics.
Objective To determine the effect of exercise training on intrahepatocellular fat (IHCL) and the kinetics of large triglyceride-(TG)-rich VLDL1 and smaller denser VLDL2 which has a lower TG content.
Design A 16 week randomised controlled trial.
Patients 27 sedentary patients with NAFLD.
Intervention Supervised exercise with moderate-intensity aerobic exercise or conventional lifestyle advice (control).
Main outcome Very low density lipoprotein1 (VLDL1) and VLDL2-TG and apolipoproteinB (apoB) kinetics investigated using stable isotopes before and after the intervention.
Results In the exercise group VO2max increased by 31±6% (mean±SEM) and IHCL decreased from 19.6% (14.8, 30.0) to 8.9% (5.4, 17.3) (median (IQR)) with no significant change in VO2max or IHCL in the control group (change between groups p<0.001 and p=0.02, respectively). Exercise training increased VLDL1-TG and apoB fractional catabolic rates, a measure of clearance, (change between groups p=0.02 and p=0.01, respectively), and VLDL1-apoB production rate (change between groups p=0.006), with no change in VLDL1 -TG production rate. Plasma TG did not change in either group.
Conclusion An increased clearance of VLDL1 may contribute to the significant decrease in liver fat following 16 weeks of exercise in NAFLD. A longer duration or higher intensity exercise interventions may be needed to lower plasma TG and VLDL production rate
Assessing the sensitivity of seagrass bed biotopes to pressures associated with marine activities.
This project was commissioned to generate an improved understanding of the sensitivities of
seagrass habitats to pressures associated with human activities in the marine environment -
to provide an evidence base to facilitate and support management advice for Marine
Protected Areas; development of UK marine monitoring and assessment, and conservation
advice to offshore marine industries.
Seagrass bed habitats are identified as a Priority Marine Feature (PMF) under the Marine
(Scotland) Act 2010, they are also included on the OSPAR list of threatened and declining
species and habitats, and are a Habitat of Principle Importance (HPI) under the Natural
Environment and Rural Communities (NERC) Act 2006, in England and Wales.
The purpose of this project was to produce sensitivity assessments with supporting evidence
for the HPI, OSPAR and PMF seagrass/Zostera bed habitat definitions, clearly documenting
the evidence behind the assessments and any differences between assessments.
Nineteen pressures, falling in five categories - biological, hydrological, physical damage,
physical loss, and pollution and other chemical changes - were assessed in this report.
Assessments were based on the three British seagrasses Zostera marina, Z. noltei and
Ruppia maritima. Z. marina var. angustifolia was considered to be a subspecies of Z.
marina but it was specified where studies had considered it as a species in its own rights.
Where possible other components of the community were investigated but the basis of the
assessment focused on seagrass species.
To develop each sensitivity assessment, the resistance and resilience of the key elements
were assessed against the pressure benchmark using the available evidence. The
benchmarks were designed to provide a ‘standard’ level of pressure against which to assess
sensitivity. Overall, seagrass beds were highly sensitive to a number of human activities:
• penetration or disturbance of the substratum below the surface;
• habitat structure changes – removal of substratum;
• physical change to another sediment type;
• physical loss of habitat;
• siltation rate changes including and smothering; and
• changes in suspended solids.
High sensitivity was recorded for pressures which directly impacted the factors that limit
seagrass growth and health such as light availability. Physical pressures that caused
mechanical modification of the sediment, and hence damage to roots and leaves, also
resulted in high sensitivity.
Seagrass beds were assessed as ‘not sensitive’ to microbial pathogens or ‘removal of target
species’. These assessments were based on the benchmarks used. Z. marina is known to
be sensitive to Labyrinthula zosterae but this was not included in the benchmark used.
Similarly, ‘removal of target species’ addresses only the biological effects of removal and not
the physical effects of the process used. For example, seagrass beds are probably not
sensitive to the removal of scallops found within the bed but are highly sensitive to the
effects of dredging for scallops, as assessed under the pressure penetration or disturbance
of the substratum below the surface‘. This is also an example of a synergistic effect
Assessing the sensitivity of seagrass bed biotopes to pressures associated with marine activities
between pressures. Where possible, synergistic effects were highlighted but synergistic and
cumulative effects are outside the scope off this study.
The report found that no distinct differences in sensitivity exist between the HPI, PMF and
OSPAR definitions. Individual biotopes do however have different sensitivities to pressures.
These differences were determined by the species affected, the position of the habitat on the
shore and the sediment type. For instance evidence showed that beds growing in soft and
muddy sand were more vulnerable to physical damage than beds on harder, more compact
substratum. Temporal effects can also influence the sensitivity of seagrass beds. On a
seasonal time frame, physical damage to roots and leaves occurring in the reproductive
season (summer months) will have a greater impact than damage in winter. On a daily
basis, the tidal regime could accentuate or attenuate the effects of pressures depending on
high and low tide. A variety of factors must therefore be taken into account in order to
assess the sensitivity of a particular seagrass habitat at any location.
No clear difference in resilience was established across the three seagrass definitions
assessed in this report. The resilience of seagrass beds and the ability to recover from
human induced pressures is a combination of the environmental conditions of the site,
growth rates of the seagrass, the frequency and the intensity of the disturbance. This
highlights the importance of considering the species affected as well as the ecology of the
seagrass bed, the environmental conditions and the types and nature of activities giving rise
to the pressure and the effects of that pressure. For example, pressures that result in
sediment modification (e.g. pitting or erosion), sediment change or removal, prolong
recovery. Therefore, the resilience of each biotope and habitat definitions is discussed for
each pressure.
Using a clearly documented, evidence based approach to create sensitivity assessments
allows the assessment and any subsequent decision making or management plans to be
readily communicated, transparent and justifiable. The assessments can be replicated and
updated where new evidence becomes available ensuring the longevity of the sensitivity
assessment tool. The evidence review has reduced the uncertainty around assessments
previously undertaken in the MB0102 project (Tillin et al 2010) by assigning a single
sensitivity score to the pressures as opposed to a range.
Finally, as seagrass habitats may also contribute to ecosystem function and the delivery of
ecosystem services, understanding the sensitivity of these biotopes may also support
assessment and management in regard to these.
Whatever objective measures are applied to data to assess sensitivity, the final sensitivity
assessment is indicative. The evidence, the benchmarks, the confidence in the
assessments and the limitations of the process, require a sense-check by experienced
marine ecologists before the outcome is used in management decisions
Superluminal Signals: Causal Loop Paradoxes Revisited
Recent results demonstrating superluminal group velocities and tachyonic
dispersion relations reopen the question of superluminal signals and causal
loop paradoxes. The sense in which superluminal signals are permitted is
explained in terms of pulse reshaping, and the self-consistent behavior which
prevents causal loop paradoxes is illustrated by an explicit example.Comment: 6 pages, 3 figure
Novel high strength titanium-titanium composites produced using field-assisted sintering technology (FAST)
To increase the strength of titanium alloys beyond that achievable with α-β microstructures, alternative reinforcing methods are necessary. Here, field-assisted sintering technology (FAST) has been used to produce a novel Ti-5Al-5Mo-5V-3Cr (Ti-5553) metal-matrix-composite (MMC) reinforced with 0-25 wt.% of a ∼2 GPa yield strength TiFeMo alloy strengthened by ordered body-centred cubic intermetallic and ω phases. The interdiffusion region between Ti-5553 and TiFeMo particles was studied by modelling, electron microscopy, and nanoindentation to examine the effect of graded composition on mechanical properties and formation of α, intermetallic, and ω phases, which resulted in a > 200 MPa strengthening benefit over unreinforced Ti-5553
Effect of thermally grown oxides on colour development of stainless steel
Stainless steels are used in an array of applications where their ability to take on colour is of
aesthetic value. Although thermal colouring (heat tinting) is known, particularly in terms of its effect
on welds, the surface development of thermally coloured stainless steel has not been considered in
detail. This paper demonstrates the colour development of a bright annealed 304 stainless steel
with time at temperature. The colour development has been quantified using colour spectrophotoscopy
and the colour related to the growth of surface oxide features by scanning electron
microscopy and the chemical profiles by X-ray photoelectron spectroscopy (XPS). There is a clear
development of both the oxide thickness and chemistry, with increasing time with small nodules
being the main features seen across the surface of the samples with time. The oxides have been
shown to comprise of an outer iron rich oxide and an inner chromium/iron spinel layer
- …