Photocathode Quantum Efficiency of Ultra-Thin Cs2Te Layers On Nb Substrates

The quantum efficiencies (QE) of photocathodes consisting of bulk Nb substrates coated with thin films of Cs2Te are reported. Using the standard recipe for Cs2Te deposition developed for Mo substrates (220 {\AA} Te thickness), a QE ~11% - 13% at light wavelength of 248 nm is achieved for the Nb substrates, consistent with that found on Mo. Systematic reduction of the Te thickness for both Mo and Nb substrates reveals a surprisingly high residual QE ~ 6% for a Te layer as thin as 15 {\AA}. A phenomenological model based on the Spicer 3-Step model along with a solution of the Fresnel equations for reflectance, R, leads to a reasonable fit of the thickness dependence of QE and suggests that layers thinner than 15 {\AA} may still have a relatively high QE. Preliminary investigation suggests an increased operational lifetime as well. Such an ultra-thin, semiconducting Cs2Te layer may be expected to produce minimal ohmic losses for RF frequencies ~ 1 GHz. The result thus opens the door to the potential development of a Nb (or Nb3Sn) superconducting photocathode with relatively high QE and minimal RF impedance to be used in a superconducting radiofrequency (SRF) photoinjector.Comment: 12 pages, 3 figure

Noncovariant gauge fixing in the quantum Dirac field theory of atoms and molecules

Starting from the Weyl gauge formulation of quantum electrodynamics (QED), the formalism of quantum-mechanical gauge fixing is extended using techniques from nonrelativistic QED. This involves expressing the redundant gauge degrees of freedom through an arbitrary functional of the gauge-invariant transverse degrees of freedom. Particular choices of functional can be made to yield the Coulomb gauge and Poincar\'{e} gauge representations. The Hamiltonian we derive therefore serves as a good starting point for the description of atoms and molecules by means of a relativistic Dirac field. We discuss important implications for the ontology of noncovariant canonical QED due to the gauge freedom that remains present in our formulation.Comment: 8 pages, 0 figure

Assessing usability of a prototype soft exoskeleton by involving people with gait impairments

Background: Gait impairment is prevalent among many growing clinical populations e.g. people with stroke, incomplete spinal cord injury (iSCI), older adults etc. Such populations may benefit from assistive devices such as exoskeletons to improve their walking ability. XoSoft (www.xosoft.eu) is a soft exoskeleton that is being developed for people with mild to moderate gait impairments to support their mobility by providing physical actuation across joints of the lower extremities. During the design and development of a device like XoSoft, it is crucial that Primary Users (PUs, e.g. patients) are involved and provide insight into their experiences and expectations regarding device usability. However, it is still not standard practice to include PUs in rigorous testing of highly technical exoskeletons. The XoSoft consortium took an iterative design approach to the development of the XoSoft prototypes. Data from usability testing with PUs are informing next iterations of the XoSoft concepts. Purpose: The purpose of this study was to assess the PU experiences of the usability of a XoSoft prototype. This study should also highlight the importance of including PUs during the development of assistive devices. Methods: Eleven participants were recruited (mean age: 73 years, mean height: 166 cm, mean mass: 65 kg). There were three categories of PUs: frail (n=5), stroke (n=1), iSCI (n=5). Participants had no cognitive impairment (Mini Mental State Examination score > 24). The prototype consisted of a leggings-style garment with Velcro straps as anchor points for actuators across the relevant joints (hip, knee, ankle). Actuation and control was provided by a modular pneumatic/sensor controlled system, which was added to the garment in modular fashion based on the PU needs. After independent donning and doffing by the participant, the garment was donned and the Velcro straps placed and secured by a researcher to ensure proper placement. Participants then performed walking tasks with active actuation followed by completion of the System Usability Scale (SUS, Brooke 1996, maximum score = 100). Results: The scores for the SUS ranged from zero to 95 with a median rating of 52.5. The median rating corresponds to an “okay” score. According to the acceptability ranges by Bangor et al. (2008), 3 participants rated the prototype as acceptable, 3 as marginal, and 5 as not acceptable. Conclusions: The scores indicate the need for improvement in the design of future XoSoft prototypes. The large variability in SUS scores indicated that the same device may be rated considerably differently by different users. This highlights the importance of including a variety of potential users of assistive devices during development. Implications: Secondary Users (SUs) such as physical therapists are also involved in the development of XoSoft, providing insight into their own needs and the needs of their patients. The therapists also play a key role in motivating their patients to participate in research and development projects. To ensure that such assistive devices can be integrated into users’ lives and practices, it is essential to have both PUs and SUs actively involved in the technical development

Designing a Regional System of Social Indicators to Evaluate Nonpoint Source Water Projects

A collaborative team has developed a system to measure the social outcomes of nonpoint source water projects as indicators of progress towards environmental goals. The system involves a set of core indicators, additional supplemental indicators, and a process for collecting and using the indicators. This process is supported by methodologies and instruments for data collection, analysis, and reporting that are coordinated and supported through detailed written guidance and an on-line data management tool. Its multi-state scope and application offer a unique opportunity to target, measure, and report interim resource management accomplishments consistently at multiple levels

A search for technosignatures from 14 planetary systems in the Kepler field with the Green Bank Telescope at 1.15-1.73 GHz

Analysis of Kepler mission data suggests that the Milky Way includes billions of Earth-like planets in the habitable zone of their host star. Current technology enables the detection of technosignatures emitted from a large fraction of the Galaxy. We describe a search for technosignatures that is sensitive to Arecibo-class transmitters located within ~420 ly of Earth and transmitters that are 1000 times more effective than Arecibo within ~13 000 ly of Earth. Our observations focused on 14 planetary systems in the Kepler field and used the L-band receiver (1.15-1.73 GHz) of the 100 m diameter Green Bank Telescope. Each source was observed for a total integration time of 5 minutes. We obtained power spectra at a frequency resolution of 3 Hz and examined narrowband signals with Doppler drift rates between +/-9 Hz/s. We flagged any detection with a signal-to-noise ratio in excess of 10 as a candidate signal and identified approximately 850 000 candidates. Most (99%) of these candidate signals were automatically classified as human-generated radio-frequency interference (RFI). A large fraction (>99%) of the remaining candidate signals were also flagged as anthropogenic RFI because they have frequencies that overlap those used by global navigation satellite systems, satellite downlinks, or other interferers detected in heavily polluted regions of the spectrum. All 19 remaining candidate signals were scrutinized and none were attributable to an extraterrestrial source.Comment: 15 pages, 5 figures, accepted for publication in the Astronomical Journa

Purity of Gaussian states: measurement schemes and time-evolution in noisy channels

We present a systematic study of the purity for Gaussian states of single-mode continuous variable systems. We prove the connection of purity to observable quantities for these states, and show that the joint measurement of two conjugate quadratures is necessary and sufficient to determine the purity at any time. The statistical reliability and the range of applicability of the proposed measurement scheme is tested by means of Monte Carlo simulated experiments. We then consider the dynamics of purity in noisy channels. We derive an evolution equation for the purity of general Gaussian states both in thermal and squeezed thermal baths. We show that purity is maximized at any given time for an initial coherent state evolving in a thermal bath, or for an initial squeezed state evolving in a squeezed thermal bath whose asymptotic squeezing is orthogonal to that of the input state.Comment: 9 Pages, 6 Figures; minor errors correcte

Quantum optical dipole radiation fields

We introduce quantum optical dipole radiation fields defined in terms of photon creation and annihilation operators. These fields are identified through their spatial dependence, as the components of the total fields that survive infinitely far from the dipole source. We use these radiation fields to perturbatively evaluate the electromagnetic radiated energy-flux of the excited dipole. Our results indicate that the standard interpretation of a bare atom surrounded by a localised virtual photon cloud, is difficult to sustain, because the radiated energy-flux surviving infinitely far from the source contains virtual contributions. It follows that there is a clear distinction to be made between a radiative photon defined in terms of the radiation fields and a real photon, whose identification depends on whether or not a given process conserves the free energy. This free energy is represented by the difference between the total dipole-field Hamiltonian and its interaction component

Basic functionality of a prototype wearable assistive soft exoskeleton for people with gait impairments : a case study

XoSoft is a soft modular wearable assistive exoskeleton for peo- ple with mild to moderate gait impairments. It is currently being developed by a European Consortium (www.xosoft.eu) and aims to provide tailored and active lower limb support during ambu- lation. During development, user-centered design principles were followed in parallel with the aim of providing functional support during gait. A prototype was developed and was tested for practi- cability, usability, comfort and assistive function (summarized as basic functionality) with a potential end user. The prototype con- sisted of a garment, electromagnetic clutch-controlled elastic bands supporting knee- and hip flexion and a backpack containing the sensor and actuator control of the system. The participant had ex- perienced a stroke and presented with unilateral impairment of the lower and upper extremities. In testing, he donned and doffed the prototype independently as far as possible, and performed walk- ing trials with the system in both active (powered on) and pas- sive (powered off) modes. Afterwards, the participant rated the perceived pressure and various elements of usability. Results high- lighted aspects of the system for improvement during future phases of XoSoft development, and also identified useful aspects of proto- type design to be maintained. The basic functionality of XoSoft could be assumed as satisfactory given that it was the first version of a working prototype. The study highlights the benefits of this participatory evaluation design approach in assistive soft robotics development

Evolutionary connectionism: algorithmic principles underlying the evolution of biological organisation in evo-devo, evo-eco and evolutionary transitions

The mechanisms of variation, selection and inheritance, on which evolution by natural selection depends, are not fixed over evolutionary time. Current evolutionary biology is increasingly focussed on understanding how the evolution of developmental organisations modifies the distribution of phenotypic variation, the evolution of ecological relationships modifies the selective environment, and the evolution of reproductive relationships modifies the heritability of the evolutionary unit. The major transitions in evolution, in particular, involve radical changes in developmental, ecological and reproductive organisations that instantiate variation, selection and inheritance at a higher level of biological organisation. However, current evolutionary theory is poorly equipped to describe how these organisations change over evolutionary time and especially how that results in adaptive complexes at successive scales of organisation (the key problem is that evolution is self-referential, i.e. the products of evolution change the parameters of the evolutionary process). Here we first reinterpret the central open questions in these domains from a perspective that emphasises the common underlying themes. We then synthesise the findings from a developing body of work that is building a new theoretical approach to these questions by converting well-understood theory and results from models of cognitive learning. Specifically, connectionist models of memory and learning demonstrate how simple incremental mechanisms, adjusting the relationships between individually-simple components, can produce organisations that exhibit complex system-level behaviours and improve the adaptive capabilities of the system. We use the term “evolutionary connectionism” to recognise that, by functionally equivalent processes, natural selection acting on the relationships within and between evolutionary entities can result in organisations that produce complex system-level behaviours in evolutionary systems and modify the adaptive capabilities of natural selection over time. We review the evidence supporting the functional equivalences between the domains of learning and of evolution, and discuss the potential for this to resolve conceptual problems in our understanding of the evolution of developmental, ecological and reproductive organisations and, in particular, the major evolutionary transitions