Error estimation in the histogram Monte Carlo method

We examine the sources of error in the histogram reweighting method for Monte Carlo data analysis. We demonstrate that, in addition to the standard statistical error which has been studied elsewhere, there are two other sources of error, one arising through correlations in the reweighted samples, and one arising from the finite range of energies sampled by a simulation of finite length. We demonstrate that while the former correction is usually negligible by comparison with statistical fluctuations, the latter may not be, and give criteria for judging the range of validity of histogram extrapolations based on the size of this latter correction.Comment: 7 pages including 3 postscript figures, typeset in LaTeX using the RevTeX macro packag

Evaluation of the use of on-board spacecraft energy storage for electric propulsion missions

On-board spacecraft energy storage represents an under utilized resource for some types of missions that also benefit from using relatively high specific impulse capability of electric propulsion. This resource can provide an appreciable fraction of the power required for operating the electric propulsion subsystem in some missions. The most probable mission requirement for utilization of this energy is that of geostationary satellites which have secondary batteries for operating at high power levels during eclipse. The study summarized in this report selected four examples of missions that could benefit from use of electric propulsion and on-board energy storage. Engineering analyses were performed to evaluate the mass saved and economic benefit expected when electric propulsion and on-board batteries perform some propulsion maneuvers that would conventionally be provided by chemical propulsion. For a given payload mass in geosynchronous orbit, use of electric propulsion in this manner typically provides a 10% reduction in spacecraft mass

Strong spectral evolution during the prompt emission of GRB 070616

Swift has revealed features in GRB early light curves, such as steep decays and X-ray flares, whose properties are consistent with an internal origin though they are far from understood. The steep X-ray decay is often explained using the curvature effect; however a significant number of GRBs display strong spectral evolution during this phase, and a new mechanism must be invoked to explain this. Of particular interest are the longest duration GRBs in which the early emission can be studied in most detail. Here we present data for GRB 070616, in which the prompt emission shows a complex multipeaked structure, leading to one of the longest prompt emission durations ever recorded. We take advantage of extensive coverage of such a long burst by all Swift instruments. Combining data from Swift and Suzaku we study the evolution of the prompt emission spectrum, following the temporal variability of the peak energy and spectral slope.Comment: 4 pages, 2 figures (Fig 1 in colour), contributed talk, submitted to the proceedings of Gamma Ray Bursts 2007, Santa Fe, New Mexico, November 5-9 200

Some implications of sampling choices on comparisons between satellite and model aerosol optical depth fields

The comparison of satellite and model aerosol optical depth (AOD) fields provides useful information on the strengths and weaknesses of both. However, the sampling of satellite and models is very different and some subjective decisions about data selection and aggregation must be made in order to perform such comparisons. This work examines some implications of these decisions, using GlobAerosol AOD retrievals at 550 nm from Advanced Along-Track Scanning Radiometer (AATSR) measurements, and aerosol fields from the GEOS-Chem chemistry transport model. It is recommended to sample the model only where the satellite flies over on a particular day; neglecting this can cause regional differences in model AOD of up to 0.1 on monthly and annual timescales. The comparison is observed to depend strongly upon thresholds for sparsity of satellite retrievals in the model grid cells. Requiring at least 25% coverage of the model grid cell by satellite data decreases the observed difference between the two by approximately half over land. The impact over ocean is smaller. In both model and satellite datasets, there is an anticorrelation between the proportion <i>p</i> of a model grid cell covered by satellite retrievals and the AOD. This is attributed to small <i>p</i> typically occuring due to high cloud cover and lower AODs being found in large clear-sky regions. Daily median AATSR AODs were found to be closer to GEOS-Chem AODs than daily means (with the root mean squared difference being approximately 0.05 smaller). This is due to the decreased sensitivity of medians to outliers such as cloud-contaminated retrievals, or aerosol point sources not included in the model

6D Muon Ionization Cooling with an Inverse Cyclotron

A large admittance sector cyclotron filled with LiH wedges surrounded by helium or hydrogen gas is explored. Muons are cooled as they spiral adiabatically into a central swarm. As momentum approaches zero, the momentum spread also approaches zero. Long bunch trains coalesce. Energy loss is used to inject the muons into the outer rim of the cyclotron. The density of material in the cyclotron decreases adiabatically with radius. The sector cyclotron magnetic fields are transformed into an azimuthally symmetric magnetic bottle in the center. Helium gas is used to inhibit muonium formation by positive muons. Deuterium gas is used to allow captured negative muons to escape via the muon catalyzed fusion process. The presence of ionized gas in the center may automatically neutralize space charge. When a bunch train has coalesced into a central swarm, it is ejected axially with an electric kicker pulse.Comment: Five pages. LaTeX, three postscript figure files. To appear in the AIP Conference Proceedings for COOL05: International Workshop on Beam Cooling, Galena, IL, 18-23 Sept. 200

Orbital thermal analysis of lattice structured spacecraft using color video display techniques

A color video display technique is demonstrated as a tool for rapid determination of thermal problems during the preliminary design of complex space systems. A thermal analysis is presented for the lattice-structured Earth Observation Satellite (EOS) spacecraft at 32 points in a baseline non Sun-synchronous (60 deg inclination) orbit. Large temperature variations (on the order of 150 K) were observed on the majority of the members. A gradual decrease in temperature was observed as the spacecraft traversed the Earth's shadow, followed by a sudden rise in temperature (100 K) as the spacecraft exited the shadow. Heating rate and temperature histories of selected members and color graphic displays of temperatures on the spacecraft are presented

Excimer lasers

The results of a two-year investigation into the possibility of developing continuous wave excimer lasers are reported. The program included the evaluation and selection of candidate molecular systems and discharge pumping techniques. The K Ar/K2 excimer dimer molecules and the xenon fluoride excimer molecule were selected for study; each used a transverse and capillary discharges pumping technique. Experimental and theoretical studies of each of the two discharge techniques applied to each of the two molecular systems are reported. Discharge stability and fluorine consumption were found to be the principle impediments to extending the XeF excimer laser into the continuous wave regime. Potassium vapor handling problems were the principal difficulty in achieving laser action on the K Ar/K2 system. Of the four molecular systems and pumping techniques explored, the capillary discharge pumped K Ar/K2 system appears to be the most likely candidate for demonstrating continuous wave excimer laser action primarily because of its predicted lower pumping threshold and a demonstrated discharge stability advantage

Modeling of a Cantilever-Based Near-Field Scanning Microwave Microscope

We present a detailed modeling and characterization of our scalable microwave nanoprobe, which is a micro-fabricated cantilever-based scanning microwave probe with separated excitation and sensing electrodes. Using finite-element analysis, the tip-sample interaction is modeled as small impedance changes between the tip electrode and the ground at our working frequencies near 1GHz. The equivalent lumped elements of the cantilever can be determined by transmission line simulation of the matching network, which routes the cantilever signals to 50 Ohm feed lines. In the microwave electronics, the background common-mode signal is cancelled before the amplifier stage so that high sensitivity (below 1 atto-Farad capacitance changes) is obtained. Experimental characterization of the microwave probes was performed on ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or topographical signals can be realized using different reflection modes of the probe.Comment: 7 figure

Modelling patient flow and outcomes in community healthcare − a fluid approximation of a stochastic queueing system

An ambition of UK healthcare policy in recent decades has been to deliver more care in the community. However, questions remain over the impact of shifting services from acute settings closer to patient homes. This is complicated by a lack of comparable measures, nationally and locally, for evaluating quality across differing community services. In this project we develop a novel patient flow model to aid the evaluation of community services which incorporates patient outcomes. The model includes patient flow dynamics common to community care such as the use of multiple services and possible re-use of services. We represent outcomes as groups which patients may move between during a course of care, which are used to model differentiated service. The model provides insight into the performance of an interrelated healthcare services. We extend a first order fluid approximation of a stochastic queueing system with service reuse to include multiple patient groups. In considering differentiated service, we implement a novel method for dynamically allocating servers across parallel queues, overcoming problems of server inactivity. Furthermore, we develop a new measure of “the flow of outcomes” to evaluate how individual services contribute to the output of outcomes from a system of care over time