An IVA (inductive voltage adder) research programme at AWE began with the construction of a small scale IVA test bed named LINX and progressed to building PIM (Prototype IVA Module). The work on PIM is geared towards furnishing AWE with a range of machines operating at 1 to 4 MV that may eventually supersede, with an upgrade in performance, existing machines operating in that voltage range. PIM has a water dielectric Blumlein of 10 ohms charged by a Marx generator. This has been used to drive either one or two 1.5 MV inductive cavities and fitting a third cavity may be attempted in the future. The latest two cavity configuration is shown which requires a split oil coax to connect the two cavities in parallel. It also has a laser triggering system for initiating the Blumlein and the prepulse reduction system fitted to the output of the Blumlein. A short MITL (magnetically insulated transmission line) connects the cavities, via a vacuum pumping section, to a chamber containing an e-beam diode test load.
Surfactant-templated silica thin films are potentially important materials for applications such as chemical sensing. However, a serious limitation for their use in aqueous environments is their poor hydrolytic stability. One convenient method of increasing the resistance of mesoporous silica to water degradation is addition of alumina, either doped into the pore walls during material synthesis or grafted onto the pore surface of preformed mesophases. Here, we compare these two routes to Al-modified mesoporous silica with respect to their effectiveness in decreasing the solubility of thin mesoporous silicate films. Direct synthesis of templated silica films prepared with Al/Si = 1:50 was found to limit film degradation, as measured by changes in film thickness, to less than 15% at near-neutral pH over a 1 week period. In addition to suppressing film dissolution, addition of Al can also cause structural changes in silica films templated with the nonionic surfactant Brij 56 (C{sub 16}H{sub 33}(OCH{sub 2}CH{sub 2}){sub n{approx}10}OH), including mesophase transformation, a decrease in accessible porosity, and an increase in structural disorder. The solubility behavior of films is also sensitive to their particular mesophase, with 3D phases (cubic, disordered) possessing less internal but more thickness stability than 2D phases (hexagonal), as determined with ellipsometric measurements. Finally, grafting of Al species onto the surface of surfactant-templated silica films also significantly increases aqueous stability, although to a lesser extent than the direct synthesis route.
We demonstrate a voltage tunable two-color quantum-well infrared photodetector (QWIP) that consists of multiple periods of two distinct AlGaAs/GaAs superlattices separated by AlGaAs blocking barriers on one side and heavily doped GaAs layers on the other side. The detection peak switches from 9.5 {micro}m under large positive bias to 6 {micro}m under negative bias. The background-limited temperature is 55 K for 9.5 {micro}m detection and 80 K for 6 {micro}m detection. We also demonstrate that the corrugated-QWIP geometry is suitable for coupling normally incident light into the detector.
We demonstrate the presence of a resonant interaction between a pair of coupled quantum wires, which are formed in the ultrahigh mobility two-dimensional electron gas of a GaAs/AlGaAs quantum well. The coupled-wire system is realized by an extension of the split-gate technique, in which bias voltages are applied to Schottky gates on the semiconductor surface, to vary the width of the two quantum wires, as well as the strength of the coupling between them. The key observation of interest here is one in which the gate voltages used to define one of the wires are first fixed, after which the conductance of this wire is measured as the gate voltage used to form the other wire is swept. Over the range of gate voltage where the swept wire pinches off, we observe a resonant peak in the conductance of the fixed wire that is correlated precisely to this pinchoff condition. In this paper, we present new results on the current- and temperature-dependence of this conductance resonance, which we suggest is related to the formation of a local moment in the swept wire as its conductance is reduced below 2e{sup 2}/h.
Analytical instrumentation such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) provides a tremendous quantity of data since an entire mass spectrum is saved at each pixel in an ion image. The analyst often selects only a few species for detailed analysis; the majority of the data are not utilized. Researchers at Sandia National Laboratory (SNL) have developed a powerful multivariate statistical analysis (MVSA) toolkit named AXSIA (Automated eXpert Spectrum Image Analysis) that looks for trends in complete datasets (e.g., analyzes the entire mass spectrum at each pixel). A unique feature of the AXSIA toolkit is the generation of intuitive results (e.g., negative peaks are not allowed in the spectral response). The robust statistical process is able to unambiguously identify all of the spectral features uniquely associated with each distinct component throughout the dataset. General Electric and Sandia used AXSIA to analyze raw data files generated on an Ion Tof IV ToF-SIMS instrument. Here, we will show that the MVSA toolkit identified metallic contaminants within a defect in a polymer sample. These metallic contaminants were not identifiable using standard data analysis protocol.
The maximum contact map overlap (MAX-CMO) between a pair of protein structures can be used as a measure of protein similarity. It is a purely topological measure and does not depend on the sequence of the pairs involved in the comparison. More importantly, the MAX-CMO present a very favorable mathematical structure which allows the formulation of integer, linear and Lagrangian models that can be used to obtain guarantees of optimality. It is not the intention of this paper to discuss the mathematical properties of MAX-CMO in detail as this has been dealt elsewhere. In this paper we compare three algorithms that can be used to obtain maximum contact map overlaps between protein structures. We will point to the weaknesses and strengths of each one. It is our hope that this paper will encourage researchers to develop new and improve methods for protein comparison based on MAX-CMO.
We consider the convergence properties of a non-elitist self-adaptive evolutionary strategy (ES) on multi-dimensional problems. In particular, we apply our recent convergence theory for a discretized (1,{lambda})-ES to design a related (1,{lambda})-ES that converges on a class of seperable, unimodal multi-dimensional problems. The distinguishing feature of self-adaptive evolutionary algorithms (EAs) is that the control parameters (like mutation step lengths) are evolved by the evolutionary algorithm. Thus the control parameters are adapted in an implicit manner that relies on the evolutionary dynamics to ensure that more effective control parameters are propagated during the search. Self-adaptation is a central feature of EAs like evolutionary stategies (ES) and evolutionary programming (EP), which are applied to continuous design spaces. Rudolph summarizes theoretical results concerning self-adaptive EAs and notes that the theoretical underpinnings for these methods are essentially unexplored. In particular, convergence theories that ensure convergence to a limit point on continuous spaces have only been developed by Rudolph, Hart, DeLaurentis and Ferguson, and Auger et al. In this paper, we illustrate how our analysis of a (1,{lambda})-ES for one-dimensional unimodal functions can be used to ensure convergence of a related ES on multidimensional functions. This (1,{lambda})-ES randomly selects a search dimension in each iteration, along which points generated. For a general class of separable functions, our analysis shows that the ES searches along each dimension independently, and thus this ES converges to the (global) minimum.
We have investigated InAs quantum dots (QD) formed on GaAs(1 0 0) using metal-organic chemical vapor deposition. Through a combination of room temperature photoluminescence and atomic force microscopy we have characterized the quantum dots. We have determined the effect of growth rate, deposited thickness, hydride partial pressure, and temperature on QD energy levels. The window of thickness for QD formation is very small, about 3 {angstrom} of InAs. By decreasing the growth rate used to deposit InAs, the ground state transition of the QD is shifted to lower energies. The formation of optically active InAs QD is very sensitive to temperature. Temperatures above 500 C do not form optically active QDs. The thickness window for QD formation increases slightly at 480 C. This is attributed to the thermal dependence of diffusion length. The AsH{sub 3} partial pressure has a non-linear effect on the QD ground state energy.