Plasma-based accelerators can sustain accelerating gradients on the order of 100 GV/m. If the plasma is not fully ionized, fields of this magnitude will ionize neutral atoms via electron tunneling, which can completely change the dynamics of the plasma wake. Particle-in-cell simulations of a high-field plasma wakefield accelerator, using the OOPIC code [D. L. Bruhwiler et al., Phys. Rev. ST Accel. Beams 4, 101302 (2001)], which includes field-induced tunneling ionization of neutral Li gas, show that the presence of even moderate neutral gas density significantly degrades the quality of the wakefield. The tunneling ionization model in OOPIC has been validated via a detailed comparison with experimental data from the l’OASIS laboratory [W.P. Leemans et al., Phys. Rev. Lett. 89, 174802 (2002)]. The properties of a wake generated directly from a neutral gas are studied, showing that one can recover the peak fields of the fully ionized plasma simulations, if the density of the electron drive bunch is increased such that the bunch rapidly ionizes the gas.

High quantum yield, low transverse energy spread, and prompt response time make GaAs activated to negative electron affinity an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. Here, photoemission from such photocathodes is modeled using detailed Monte Carlo electron transport simulations. Simulations show a quantitative agreement with the experimental results for quantum efficiency, energy distributions of emitted electrons, and response time without the assumption of any ad hoc parameters. This agreement between simulation and experiment sheds light on the mechanism of electron emission and provides an opportunity to design novel semiconductor photocathodes with optimized performance.

The diamond amplifier (DA) is a new device for generating high-current, high-brightness electron beams. Our transmission-mode tests show that, with single-crystal, high-purity diamonds, the peak current density is greater than 400  mA/mm², while its average density can be more than 100  mA/mm². The gain of the primary electrons easily exceeds 200, and is independent of their density within the practical range of DA applications. We observed the electron emission. The maximum emission gain measured was 40, and the bunch charge was 50  pC/0.5  mm². There was a 35% probability of the emission of an electron from the hydrogenated surface in our tests. We identified a mechanism of slow charging of the diamond due to thermal ionization of surface states that cancels the applied field within it. We also demonstrated that a hydrogenated diamond is extremely robust.

In this work we adapt an efficient information integration algorithm to identify the minimal set of potentially relevant Semantic Web data sources for a given query. The vast majority of these sources are files written in RDF or OWL format, and must be processed in their entirety. Our adaptation includes enhancing the algorithm with taxonomic reasoning, defining and using a mapping language for the purpose of aligning heterogeneous Semantic Web ontologies, and introducing a concept of source relevance to reduce the number of sources that we need to consider for a given query. After the source selection process, we load the selected sources into a Semantic Web reasoner to get a sound and complete answer to the query. We have conducted an experiment using synthetic ontologies and data sources which demonstrates that our system performs well over a wide range of queries. A typical response time for a substantial work load of 50 domain ontologies, 80 map ontologies and 500 data sources is less than 2 seconds. Furthermore, our system returned correct answers to 200 randomly generated queries in several workload configurations. We have also compared our adaptation with a basic implementation of the original information integration algorithm that does not do any taxonomic reasoning. In the most complex configuration with 50 domain ontologies, 100 map ontologies and 1000 data sources our system returns complete answers to all the queries whereas the basic implementation returns complete answers to only 28% of the queries.

Abstract. A distributed, end-to-end information integration system that is based on the Semantic Web architecture is of considerable interest to both commercial and government organizations. However, there are a number of challenges that have to be resolved to build such a system given the currently available Semantic Web technologies. We describe here the ISENS prototype system we designed, implemented, and tested (on a small scale) to address this problem. We discuss certain system limitations (some coming from underlying technologies used) and future ISENS development to resolve them and to enable an extended set of capabilities.