Dusty plasma cavities: probe-induced and natural

A comprehensive exploration of regional dust evacuation in complex plasma crystals is presented. Voids created in 3D crystals on the International Space Station have provided a rich foundation for experiments, but cavities in dust crystals formed in ground-based experiments have not received as much attention. Inside a modified GEC RF cell, a powered vertical probe was used to clear the central area of a dust crystal, producing a cavity with high cylindrical symmetry. Cavities generated by three mechanisms are examined. First, repulsion of micrometer-sized particles by a negatively charged probe is investigated. A model of this effect developed for a DC plasma is modified and applied to explain new experimental data in RF plasma. Second, the formation of natural cavities is surveyed; a radial ion drag proposed to occur due to a curved sheath is considered in conjunction with thermophoresis and a flattened confinement potential above the center of the electrode. Finally, cavity formation unexpectedly occurs upon increasing the probe potential above the plasma floating potential. The cavities produced by these methods appear similar, but each are shown to be facilitated by fundamentally different processes.Comment: 10 pages, 12 figure

Dusty Plasma Correlation Function Experiment

Dust particles immersed within a plasma environment, such as those in protostellar clouds, planetary rings or cometary environments, will acquire an electric charge. If the ratio of the inter-particle potential energy to the average kinetic energy is high enough the particles will form either a "liquid" structure with short-range ordering or a crystalline structure with long range ordering. Many experiments have been conducted over the past several years on such colloidal plasmas to discover the nature of the crystals formed, but more work is needed to fully understand these complex colloidal systems. Most previous experiments have employed monodisperse spheres to form Coulomb crystals. However, in nature (as well as in most plasma processing environments) the distribution of particle sizes is more randomized and disperse. This paper reports experiments which were carried out in a GEC rf reference cell modified for use as a dusty plasma system, using varying sizes of particles to determine the manner in which the correlation function depends upon the overall dust grain size distribution. (The correlation function determines the overall crystalline structure of the lattice.) Two dimensional plasma crystals were formed of assorted glass spheres with specific size distributions in an argon plasma. Using various optical techniques, the pair correlation function was determined and compared to those calculated numerically.Comment: 6 pages, Presented at COSPAR '0

Magnetic Field Structure around Low-Mass Class 0 Protostars: B335, L1527 and IC348-SMM2

We report new 350 micron polarization observations of the thermal dust emission from the cores surrounding the low-mass, Class 0 YSOs L1527, IC348-SMM2 and B335. We have inferred magnetic field directions from these observations, and have used them together with results in the literature to determine whether magnetically regulated core-collapse and star-formation models are consistent with the observations. These models predict a pseudo-disk with its symmetry axis aligned with the core magnetic field. The models also predict a magnetic field pinch structure on a scale less than or comparable to the infall radii for these sources. In addition, if the core magnetic field aligns (or nearly aligns) the core rotation axis with the magnetic field before core collapse, then the models predict the alignment (or near alignment) of the overall pinch field structure with the bipolar outflows in these sources. We show that if one includes the distorting effects of bipolar outflows on magnetic fields, then in general the observational results for L1527 and IC348-SMM2 are consistent with these magnetically regulated models. We can say the same for B335 only if we assume the distorting effects of the bipolar outflow on the magnetic fields within the B335 core are much greater than for L1527 and IC348-SMM2. We show that the energy densities of the outflows in all three sources are large enough to distort the magnetic fields predicted by magnetically regulated models.Comment: Accepted for publication in The Astrophysical Journa

Pulsation models for the roAp star HD 134214

Precise time-series photometry with the MOST satellite has led to identification of 10 pulsation frequencies in the rapidly oscillating Ap (roAp) star HD 134214. We have fitted the observed frequencies with theoretical frequencies of axisymmetric modes in a grid of stellar models with dipole magnetic fields. We find that, among models with a standard composition of $(X,Z) = (0.70,0.02)$ and with suppressed convection, eigenfrequencies of a $1.65\,{\rm M}_\odot$ model with $\log T_{\rm eff} = 3.858$ and a polar magnetic field strength of 4.1kG agree best with the observed frequencies. We identify the observed pulsation frequency with the largest amplitude as a deformed dipole ($\ell = 1$) mode, and the four next-largest-amplitude frequencies as deformed $\ell = 2$ modes. These modes have a radial quasi-node in the outermost atmospheric layers ($\tau \sim 10^{-3}$). Although the model frequencies agree roughly with observed ones, they are all above the acoustic cut-off frequency for the model atmosphere and hence are predicted to be damped. The excitation mechanism for the pulsations of HD 134214 is not clear, but further investigation of these modes may be a probe of the atmospheric structure in this magnetic chemically peculiar star.Comment: 9 pages, 6 figures; accepted for publication in MNRA

An update on female-controlled methods for HIV prevention: Female condom, microbicides and cervical barriers

Worldwide, nearly 5 million new HIV infections occurred in 2005 with more than 3 million of these in sub-Saharan Africa. Despite declines in the HIV prevalence in some sub-Saharan African countries, many continue to be heavily affected.1 In Zimbabwe data have shown a decrease in prevalence from 22.1% to 20.1% since 2003, yet there is no sign of a similar trend in South Africa, where the adult prevalence rate is 18.8%.2 National-level prevalence rates, however, may not reveal the full impact of the epidemic on different populations. For example, 30.2% of pregnant women in South Africa attending public antenatal clinics in 2005 were HIV positive, with rates varying by province from 15.7% in the Western Cape to 39.1% in KwaZulu-Natal.3 In addition, female youth aged 15 - 24 years were three times more likely to be infected than young men in the same age group.4 Women are increasingly bearing the burden of the epidemic. Of the 39 million people worldwide living with HIV/AIDS, half are women. UNAIDS estimates that of the new infections expected to occur between 2002 and 2010, 70% will be among women in the developing world.5 Several factors account for women\'s higher risk of infection, including biological, socio-cultural and economic factors. For instance, the female reproductive tract is more susceptible to HIV infection than the male reproductive system, and young women are at highest risk of HIV infection due to an immature physiology. Further, sexual violence and gender inequalities frequently play a role in women\'s and girls\' ability to practise safer sex.6 Current HIV prevention methods are male-initiated or require a male partner\'s co-operation, leaving women without sufficient means to protect themselves from infection. New female-controlled prevention methods are urgently needed to reduce women\'s and girls\' risk of HIV infection. This article provides an update on currently available methods and others being tested and/or developed to provide women and girls with more HIV prevention options. Southern African Journal of HIV Medicine Vol. 7 (4) 2006: pp. 7-1

Risk reduction in pesticide application - A conceptual framework

Pesticides are heavily criticised by environmental groups and others who argue that their inefficient application pollutes our environment. They are, however, still essential to support food production and human well being. The complex process of transferring a pesticide from a container to the biological target requires a holistic approach to look at all sources of pesticide losses to reduce risks in application. As the majority of pesticides are applied as sprays, a conceptual framework of the spray application process from spray planning (e.g. dose selection, timing, treatment plan), mixing, filling and cleaning, spray atomisation, transport and deposition, weathering and run off is considered. This is an attempt to help identify those areas of the spray application process of most importance for the protection of the environment and human health

On-demand or Spot? Selling the cloud to risk-averse customers

In Amazon EC2, cloud resources are sold through a combination of an on-demand market, in which customers buy resources at a fixed price, and a spot market, in which customers bid for an uncertain supply of excess resources. Standard market environments suggest that an optimal design uses just one type of market. We show the prevalence of a dual market system can be explained by heterogeneous risk attitudes of customers. In our stylized model, we consider unit demand risk-averse bidders. We show the model admits a unique equilibrium, with higher revenue and higher welfare than using only spot markets. Furthermore, as risk aversion increases, the usage of the on-demand market increases. We conclude that risk attitudes are an important factor in cloud resource allocation and should be incorporated into models of cloud markets.Comment: Appeared at WINE 201