Plasma issues associated with the use of electrodynamic tethers

The use of an electrodynamic tether to generate power or thrust on the space station raises important plasma issues associted with the current flow. In addition to the issue of current closure through the space station, high power tethers (equal to or greater than tens of kilowatts) require the use of plasma contactors to enhance the current flow. They will generate large amounts of electrostatic turbulence in the vicinity of the space station. This is because the contactors work best when a large amount of current driven turbulence is excited. Current work is reviewed and future directions suggested

Plasma contactors for use with electodynamic tethers for power generation

Plasma contactors are proposed as a means of making good electrical contact between biased surfaces such as found at the ends of an electrodynamic tether and the space environment. The plasma contactor emits a plasma cloud which facilitates the electrical connection. The physics of this plasma cloud is investigated for contactors used as electron collectors. The central question addressed is whether the electrons collected by a plasma contactor come from the far field or by ionization of local neutral gas. This question is important because the system implications are different for the two mechanisms. It is shown that contactor clouds in space will consist of a spherical core possibly containing a shock wave. Outside of the core the cloud will expand anisotropically across the magnetic field leading to a turbulent cigar shape structure along the field. This outer region is itself divided into two regions by the ion response to the electric field. A two-dimensional theory for the outer regions of the cloud is developed. The current voltage characteristic of an Argon plasma contactor cloud is estimated for several ion currents in the range of 1 to 100 Amperes. It is suggested that the major source of collected electrons comes by ionization of neutral gas while collection of electrons from the far field is relatively small

Observations Outside the Light-Cone: Algorithms for Non-Equilibrium and Thermal States

We apply algorithms based on Lieb-Robinson bounds to simulate time-dependent and thermal quantities in quantum systems. For time-dependent systems, we modify a previous mapping to quantum circuits to significantly reduce the computer resources required. This modification is based on a principle of "observing" the system outside the light-cone. We apply this method to study spin relaxation in systems started out of equilibrium with initial conditions that give rise to very rapid entanglement growth. We also show that it is possible to approximate time evolution under a local Hamiltonian by a quantum circuit whose light-cone naturally matches the Lieb-Robinson velocity. Asymptotically, these modified methods allow a doubling of the system size that one can obtain compared to direct simulation. We then consider a different problem of thermal properties of disordered spin chains and use quantum belief propagation to average over different configurations. We test this algorithm on one dimensional systems with mixed ferromagnetic and anti-ferromagnetic bonds, where we can compare to quantum Monte Carlo, and then we apply it to the study of disordered, frustrated spin systems.Comment: 19 pages, 12 figure

Entropy and Entanglement in Quantum Ground States

We consider the relationship between correlations and entanglement in gapped quantum systems, with application to matrix product state representations. We prove that there exist gapped one-dimensional local Hamiltonians such that the entropy is exponentially large in the correlation length, and we present strong evidence supporting a conjecture that there exist such systems with arbitrarily large entropy. However, we then show that, under an assumption on the density of states which is believed to be satisfied by many physical systems such as the fractional quantum Hall effect, that an efficient matrix product state representation of the ground state exists in any dimension. Finally, we comment on the implications for numerical simulation.Comment: 7 pages, no figure

Significant reduction in arc frequency biased solar cells: Observations, diagnostics, and mitigation technique(s)

A variety of experiments were performed which identify key factors contributing to the arcing of negatively biased high voltage solar cells. These efforts have led to reduction of greater than a factor of 100 in the arc frequency of a single cell following proper remediation procedures. Experiments naturally lead to and focussed on the adhesive/encapsulant that is used to bond the protective cover slip to the solar cell. An image-intensified charge coupled device (CCD) camera system recorded UV emission from arc events which occurred exclusively along the interfacial edge between the cover slip and the solar cell. Microscopic inspection of this interfacial region showed a bead of encapsulant along this entire edge. Elimination of this encapsulant bead reduced the arc frequency by two orders of magnitude. Water contamination was also identified as a key contributor which enhances arcing of the encapsulant bead along the solar cell edge. Spectrally resolved measurements of the observable UV light shows a feature assignable to OH(A-X) electronic emission, which is common for water contaminated discharges. Experiments in which the solar cell temperature was raised to 85 C showed a reduced arcing frequency, suggesting desorption of H2O. Exposing the solar cell to water vapor was shown to increase the arcing frequency. Clean dry gases such as O2, N2, and Ar show no enhancement of the arcing rate. Elimination of the exposed encapsulant eliminates any measurable sensitivity to H2O vapor

Teaching and assessing consultation skills: an evaluation of a South African workshop on using the Leicester Assessment Package

BackgroundThe consultation is at the very centre of clinical practice. It is in the meeting between doctor and patient that the story is told (and in good practice properly heeded) and decisions are made about the cause and treatment of the patient's problem. Following one year of supervised internship, South African doctors are required to do a year of community service and these doctors mostly work in understaffed peripheral hospitals. A substantial component of this work is unsupervised consultations with patients suffering from new or complex continuing diseases. On graduation, these doctors therefore require a high level of consultation competence. They must be able to make accurate diagnoses and manage patients' problems reliably and efficiently.The Leicester Assessment Package (LAP) was originally developed to assess the consultation competence of general practitioners in the UK. It was subsequently adapted for use in undergraduate teaching. In 2002, the LAP was presented at a medical education conference in South Africa. As a result, the Department of Family Medicine at Pretoria University began using the LAP in the teaching and formative assessment of the consultation skills of senior students in outpatient clinics. In 2003, the University of the Witwatersrand introduced a four-year graduate entry medical curriculum. The Centre for Health Care Education was interested in assessing whether the LAP would be suitable for the summative assessment of the consultation performance of students during their third and four years of the new curriculum.A workshop course was organised to train senior clinicians from the Universities of Pretoria and the Witwatersrand in the use of the LAP as a means of teaching and assessing the consultation performance of South African medical students.MethodTwenty-two experienced South African medical educators participated in a three-day workshop. Their attitudes to the LAP and the forms of teaching that its use promotes were analysed by responses to pre- and post-workshop questionnaires with Likert-scale and free-text questions.ResultsThe participants were positive about the LAP at the end of the workshop. They all believed that it was a useful instrument, and a majority would apply this method in their own departments. There were continuing reservations about the feasibility of the method and some respondents felt it would require some adaptation, particularly to the criteria for awarding grades.ConclusionsThe workshop participants learnt to use an instrument developed in the United Kingdom that encourages an analytical approach to the assessment and teaching of consultation skills. They believed it would be useful in the contexts in which they worked.For full text, click here:SA Fam Pract 2006;48(3):14-14

Statistics of Partial Minima

Motivated by multi-objective optimization, we study extrema of a set of N points independently distributed inside the d-dimensional hypercube. A point in this set is k-dominated by another point when at least k of its coordinates are larger, and is a k-minimum if it is not k-dominated by any other point. We obtain statistical properties of these partial minima using exact probabilistic methods and heuristic scaling techniques. The average number of partial minima, A, decays algebraically with the total number of points, A ~ N^{-(d-k)/k}, when 1<=k<d. Interestingly, there are k-1 distinct scaling laws characterizing the largest coordinates as the distribution P(y_j) of the jth largest coordinate, y_j, decays algebraically, P(y_j) ~ (y_j)^{-alpha_j-1}, with alpha_j=j(d-k)/(k-j) for 1<=j<=k-1. The average number of partial minima grows logarithmically, A ~ [1/(d-1)!](ln N)^{d-1}, when k=d. The full distribution of the number of minima is obtained in closed form in two-dimensions.Comment: 6 pages, 1 figur

Preliminary results of flight tests of vortex attenuating splines

Flight tests have been conducted to evaluate the effectiveness of a wingtip vortex attenuating device, referred to as a spline. Vortex penetrations were made with a PA-28 behind a C-54 aircraft with and without wingtip splines attached and the resultant rolling acceleration was measured and related to the roll acceleration capability of the PA-28. Tests were conducted over a range of separation distances from about 5 nautical miles (n. mi.) to less than 1 n. mi. Preliminary results indicate that, with the splines installed, there was a significant reduction in the vortex induced roll acceleration experienced by the PA-28 probe aircraft, and the distance at which the PA-28 roll control became ineffective was reduced from 2.5 n. mi. to 0.6 n. mi., or less. There was a slight increase in approach noise (approximately 4 db) with the splines installed due primarily to the higher engine power used during approach. Although splines significantly reduced the C-54 rate of climb, the rates available with four engines were acceptable for this test program. Splines did not introduce any noticeable change in the handling qualities of the C-54

Tip Splittings and Phase Transitions in the Dielectric Breakdown Model: Mapping to the DLA Model

We show that the fractal growth described by the dielectric breakdown model exhibits a phase transition in the multifractal spectrum of the growth measure. The transition takes place because the tip-splitting of branches forms a fixed angle. This angle is eta dependent but it can be rescaled onto an ``effectively'' universal angle of the DLA branching process. We derive an analytic rescaling relation which is in agreement with numerical simulations. The dimension of the clusters decreases linearly with the angle and the growth becomes non-fractal at an angle close to 74 degrees (which corresponds to eta= 4.0 +- 0.3).Comment: 4 pages, REVTex, 3 figure

Report from space plasma science

Space plasma science, especially plasma experiments in space, is discussed. Computational simulations, wave generation and propagation, wave-particle interactions, charged particle acceleration, particle-particle interactions, radiation transport in dense plasmas, macroscopic plasma flow, plasma-magnetic field interactions, plasma-surface interactions, prospects for near-term plasma science experiments in space and three-dimensional plasma experiments are among the topics discussed