Baffle aperture design study of hollow cathode equipped ion thrusters

A simple theoretical model which can be used as an aid in the design of the baffle aperture region of a hollow cathode equipped ion thruster was developed. An analysis of the ion and electron currents in both the main and cathode discharge chambers is presented. From this analysis a model of current flow through the aperture, which is required as an input to the design model, was developed. This model was verified experimentally. The dominant force driving electrons through the aperture was the force due to the electrical potential gradient. The diffusion process was modeled according to the Bolm diffusion theory. A number of simplifications were made to limit the amount of detailed plasma information required as input to the model to facilitate the use of the model in thruster design. This simplified model gave remarkably consistant results with experimental results obtained with a given thruster geometry over substantial changes in operating conditions. The model was uncertain to about a factor of two for different thruster cathode region geometries. The design usefulness was limited by this factor of two uncertainty and by the accuracy to which the plasma parameters required as inputs to the model were specified

Testing Methods of Surficial Sinkhole Identification Using Remotely Sensed Data

Nearly a quarter of all people rely on karst aquifers for drinking water. In the United States, the Safe Water Drinking Act requires a complete assessment of public water systems\u27 vulnerabilities to contamination. As part of that assessment, watershed boundaries must be delineated, while recharge and supply locations identified. In the context of karst aquifers, surficial karst features, such as sinkholes, can act as a point source of direct recharge to karst aquifers and create vulnerabilities to critical drinking water sources. Historical methods of locating these features are inefficient and depend on basic field investigations, resulting in a clear need for advanced identification methods. To this end, this study focuses on developing more efficient identification methods that use remotely sensed data to locate and map surficial karst features that may require protection. Satellite and unmanned aerial vehicle (UAV) data were used to explore the resolution needed to identify surficial karst feature signatures and the most promising methods for analyzing these data. This study\u27s data included red, green, blue, and near-infrared reflectance rasters, thermal mosaics, and digital surface and terrain models. Spectral and thermal properties were used to filter data that could include karst features. Additionally, digital elevation models were used to explore multiple smoothing methods, image differencing, edge detection, terrain curvature, sink location, and watershed delineation. Findings from the different methods were compared to known karst feature locations. Data with a resolution between 0.5 and 2.5 meters per pixel were found to be ideal for most methods tested. However, vegetation removal, followed by a simple interpolation to fill these areas, created data analysis problems and highlighted the need for other data products, such as LiDAR, that provide accurate elevations of terrain shrouded by vegetation. In the end, it was found that edge detection, mapping curvature, and locating of low points (or sinks) via DEM analyses are all promising methods. It was concluded that by combining multiple methods, detailed digital terrain models could accurately locate many surficial karst features

PIPE Dreams? The Performance of Companies Issuing Equity Privately

Private Investments in Public Equity (PIPEs) have become an important source of financing for young, publicly traded firms whose poor operating performance may limit alternative financing options. We propose that firms are motivated to sell these securities to minimize costs associated with asymmetric information. We find that both the security structure and the investor composition of a PIPE security matter in the subsequent performance of the issuing firm. Poor post-issuance performance is associated with securities where investors obtain significant repricing rights, which protect them from future stock price declines. Furthermore, companies that obtain financing from hedge funds tend to under-perform companies that obtain financing from other institutional investors. We argue that hedge funds act as investors of last resort, playing an important role in the market for young, high-risk firms with substantial asymmetric information. Hedge funds are willing to fund such high-risk companies because they can protect against possible price declines in the issuing companies by either negotiating PIPE securities with repricing rights or by entering into short positions of the underlying stocks of the issuing companies.

Models of planetary rings

The Voyager occultations provide several uniform and high quality data sets for Saturn, Uranus, and Neptune. These data are intercompared, and theoretical models for the particle sizes and the particle transport are developed. The major topics covered include: ring size distribution, torques and resonances, and satellite wakes

Studies on Housing of Pregnant Sows in Groups and Individually

End of Project ReportIntensive methods of sow housing and their effects on health and welfare have become a topic of intense debate. In the EU the use of tethers for pregnant sows must be phased out by 2006. At pre s e n t there are no plans to ban stalls for pregnant sows. In 1997 the EU released a report on sow housing which was critical of stalls but did not recommend a ban (SVC, 1997). Some member countries have introduced more stringent legislation. The UK government has banned the use of both stalls and tethers for pregnant sows fro m January 1, 1999. Sweden has also imposed a ban on both stalls and tethers, and the Netherlands and De n m a rk have announced restrictions on when individual penning may be used. • This study included a comparison of pregnant gilts in loose housing (groups of 4) and in individual stalls from early pregnancy. Behaviour and skin lesions were monitored both during pregnancy and in the farrowing house. Stalled gilts tended to have higher skin lesion scores. Salivary cortisol levels in stalled gilts showed evidence of a chronic stress response. Loose gilts showed more distress when confined in the farrowing crate pre - f a r rowing than did gilts which had been housed in stalls in pregnancy. • Comparison of two group housing systems namely, groups of four with 3.0m2 per sow and groups of eight with 2.2m2 per sow showed a higher level of aggression in the larger group. Both treatments had free-access stalls with full length partitions but the groups of eight had a smaller communal lying area and they spent a greater proportion of their time in the stalls

Helianthus maximiliani and Species Fine-Scale Spatial Pattern Affect Diversity Interactions in Reconstructed Tallgrass Prairies

1. Biodiversity and Ecosystem Function analyses aim to explain how individual spe‐ cies and their interactions affect ecosystem function. With this study, we asked in what ways do species interact, are these interactions affected by species planting pattern, and are initial (planted) proportions or previous year (realized) propor‐ tions a better reference point for characterizing grassland diversity effects? 2. We addressed these questions with experimental communities compiled from a pool of 16 tallgrass prairie species. We planted all species in monocultures and mixtures that varied in their species richness, evenness, and spatial pattern. We recorded species‐specific biomass production over three growing seasons and fit‐ ted Diversity‐Interactions (DI) models to annual plot biomass yields. 3. In the establishment season, all species interacted equally to form the diversity effect. In years 2 and 3, each species contributed a unique additive coefficient to its interaction with every other species to form the diversity effect. These inter‐ actions were affected by Helianthus maximiliani and the species planting pattern. Models based on species planted proportions better‐fit annual plot yield than models based on species previous contributions to plot biomass. 4. Outcomes suggest that efforts to plant tallgrass prairies to maximize diversity ef‐ fects should focus on the specific species present and in what arrangement they are planted. Furthermore, for particularly diverse grasslands, the effort of collect‐ ing annual species biomass data may not be necessary when quantifying diversity effects with DI models

Gonioscopy: A model of the anterior chamber

Gonioscopy: A model of the anterior chambe