Photovoltaic power systems workshop

Discussions are presented on apparent deficiencies in NASA planning and technology development relating to a standard power module (25-35 kW) and to future photovoltaic power systems in general. Topics of discussion consider the following: (1) adequate studies on power systems; (2) whether a standard power system module should be developed from a standard spacecraft; (3) identification of proper approaches to cost reduction; (4) energy storage avoidance; (5) attitude control; (6) thermal effects of heat rejection on solar array configuration stability; (7) assembly of large power systems in space; and (8) factoring terrestrial photovoltaic work into space power systems for possible payoff

Theory of continuum percolation III. Low density expansion

We use a previously introduced mapping between the continuum percolation model and the Potts fluid (a system of interacting s-states spins which are free to move in the continuum) to derive the low density expansion of the pair connectedness and the mean cluster size. We prove that given an adequate identification of functions, the result is equivalent to the density expansion derived from a completely different point of view by Coniglio et al. [J. Phys A 10, 1123 (1977)] to describe physical clustering in a gas. We then apply our expansion to a system of hypercubes with a hard core interaction. The calculated critical density is within approximately 5% of the results of simulations, and is thus much more precise than previous theoretical results which were based on integral equations. We suggest that this is because integral equations smooth out overly the partition function (i.e., they describe predominantly its analytical part), while our method targets instead the part which describes the phase transition (i.e., the singular part).Comment: 42 pages, Revtex, includes 5 EncapsulatedPostscript figures, submitted to Phys Rev

Characterisation and statistical analysis of breakdown data for a corona-stabilised switch in environmentally-friendly gas mixtures

Characterisation of a corona-stabilised switch in the single-shot regime, including triggering range, delay times and jitter is reported, over the pressure range 0-3 bar gauge, as a continuation of work from similar characterisation with this switch filled with SF6 with different gap spacings. When filled with mixtures of HFO-1234ze and N2, the breakdown voltage can be increased by up to ~306% and ~191% under negative and positive polarity, respectively, of that using 100% N2. These results were achieved with gas mixtures consisting of 80% N2 and 20% HFO-1234ze, by pressure. The maximum negative polarity triggering range was 13.6 kV, comparable to that achieved previously using SF6. The measured delay time and calculated jitter was generally found to increase with increasing pressure, and with increasing percentage (from 5% to 20%) of HFO-1234ze in the gas mixtures. Von Laue statistical analysis of time-to-breakdown data showed that both the formative time and statistical time increased with increasing pressure, and with increasing percentage of HFO-1234ze in the gas mixtures. The formative time under negative polarity was significantly longer than that for positive polarity. The results indicate that HFO-1234ze may be considered as a suitable candidate to replace SF6 for switching applications, although there are some operational observations that require further investigation

Spontaneous symmetry breaking: exact results for a biased random walk model of an exclusion process

It has been recently suggested that a totally asymmetric exclusion process with two species on an open chain could exhibit spontaneous symmetry breaking in some range of the parameters defining its dynamics. The symmetry breaking is manifested by the existence of a phase in which the densities of the two species are not equal. In order to provide a more rigorous basis to these observations we consider the limit of the process when the rate at which particles leave the system goes to zero. In this limit the process reduces to a biased random walk in the positive quarter plane, with specific boundary conditions. The stationary probability measure of the position of the walker in the plane is shown to be concentrated around two symmetrically located points, one on each axis, corresponding to the fact that the system is typically in one of the two states of broken symmetry in the exclusion process. We compute the average time for the walker to traverse the quarter plane from one axis to the other, which corresponds to the average time separating two flips between states of broken symmetry in the exclusion process. This time is shown to diverge exponentially with the size of the chain.Comment: 42 page

Glass transition and effective potential in the hypernetted chain approximation

We study the glassy transition for simple liquids in the hypernetted chain (HNC) approximation by means of an effective potential recently introduced. Integrating the HNC equations for hard spheres, we find a transition scenario analogous to that of the long range disordered systems with ``one step replica symmetry breaking''. Our result agree qualitatively with Monte Carlo simulations of three dimensional hard spheres.Comment: 7 pages, 7 figures, Revtex fil

The Serums Tool-Chain:Ensuring Security and Privacy of Medical Data in Smart Patient-Centric Healthcare Systems

Digital technology is permeating all aspects of human society and life. This leads to humans becoming highly dependent on digital devices, including upon digital: assistance, intelligence, and decisions. A major concern of this digital dependence is the lack of human oversight or intervention in many of the ways humans use this technology. This dependence and reliance on digital technology raises concerns in how humans trust such systems, and how to ensure digital technology behaves appropriately. This works considers recent developments and projects that combine digital technology and artificial intelligence with human society. The focus is on critical scenarios where failure of digital technology can lead to significant harm or even death. We explore how to build trust for users of digital technology in such scenarios and considering many different challenges for digital technology. The approaches applied and proposed here address user trust along many dimensions and aim to build collaborative and empowering use of digital technologies in critical aspects of human society

Stringy Instantons and Quiver Gauge Theories

We explore contributions to the 4D effective superpotential which arise from Euclidean D3 branes (``instantons'') that intersect space-filling D-branes. These effects can perturb the effective field theory on the space-filling branes by nontrivial operators composed of charged matter fields, changing the vacuum structure in a qualitative way in some examples. Our considerations are exemplified throughout by a careful study of a fractional brane configuration on a del Pezzo surface.Comment: 30 pages, 4 figures; v2: reference added; v3: confusing minor error in axion charges fixed (thanks to D. Green for pointing it out

Statistical analysis of impulsive flashover voltages across solid-air interfaces

Within the pulsed power industry, a key factor determining the achievable output voltage of a HV system is the flashover voltage of the insulating parts. Statistical analysis of the breakdown voltages associated with solid-gas interfaces can reveal useful information to aid system designers in the selection of solid materials. However, it is important to test the applicability of the distribution being applied, to ensure that the fitting parameters obtained are truly representative of the distribution of the data. Normal, lognormal, 2-parameter Weibull and 3-parameter Weibull cumulative distribution functions (CDF) were plotted, to enable extraction of the specific fitting parameters associated with each distribution. The CDF for each statistical method has been plotted alongside the empirical cumulative distribution function (ECDF), found from the flashover voltages recorded during experimental testing. The distribution of best fit was then analysed by using the Kolmogorov-Smirnov (K-S) test, in order to determine the CDF that best represented the ECDF. Maximum values have been compared to the α = 0.05, K-S critical value, in order to reject or accept the null hypothesis based on how the data fits the specified distributions. This will facilitate a comparison between different statistical distributions, applied to experimental data on breakdown/flashover voltages of gas-solid interfaces, generated at a fixed pressure, and different levels of R

The effect of relative humidity on the flashover strength of solid insulation

This paper informs on the flashover strength of 3 materials: Delrin (Polyoxymethylene), HDPE (High Density Polyethylene) and Ultem (Polyetherimide) with 'smooth' and 'knurled' surface finishes, in zero-grade air at -0.5, 0 and 0.5 bar gauge, and at 90% relative humidity (RH). All tests were completed using a 10-stage Marx generator, producing HV impulses with a nominal 100/700 ns waveshape. Each test conformed with the ASTM D3426-97 standard of 'step up' testing, to find the average flashover voltage for each set of conditions. The electrode system with each dielectric material demonstrated a decrease in breakdown voltage as the RH was increased. In high humidity environments, the knurled surface finish was seen to have a positive effect on the flashover strength of Delrin and Ultem samples, compared to a smooth, machined surface. Increasing pressure yielded an increase in the flashover strength also. For samples with a smooth surface finish, a decrease in flashover strength was found as the permittivity of the material increased, irrespective of humidity and pressure. However, a knurled surface finish had a much more erratic effect on flashover strength, with no discernible trend with increasing material permittivit

Effect of surface deviation of solid insulation on impulsive flashover voltages under varying environmental conditions

In pulsed power engineering, the modification of dielectric surfaces is a potential method in order to increase the flashover strength of solid insulation. In this work, dielectric materials are subjected to a knurled finish, where small indentations are machined on to the surface resulting in quick consistent modification. These flashover voltages from the knurled dielectric spacers have been compared to spacers with a ‘smooth’ machined finish. The three materials which have been tested are HDPE (High Density Polyethylene), Delrin (Polyoxymethylene) and Ultem (Polyetherimide). The materials were tested under a 100/700 ns impulse voltage. Cylindrical spacers made of these materials were located in the center of a parallel-plane electrode setup in air, which provided a quasi-uniform field distribution. Breakdown tests published in this work were performed in a sealed container at air pressures of −0.5, 0 and 0.5 bar gauge, with relative humidity levels of 90% RH