Optical studies of the pre-breakdown mechanism in dielectronic liquids.

PhDThe work described in this thesis provides an optical study of pre-breakdown events in liquid dielectrics. A small scale rig employing a 50SI test cell with viewing windows, as part of a high voltage co-axial line, enabled short rise time pulses to be applied to a non-uniform test gap. For the liquid dielectric, changes of refractive index resulting from applied voltage were rendered visible by means of a Schlieren optical system. A high speed image converter camera enabled rapidly changing pre-breakdown phenomena to be photographically recorded at framing speeds up to 107 per second. Scattered light photographs were taken by orthogonal flash illumination under both pulse and direct voltage conditions, allowing large format macrophotography. Using a piezoelectric transducer placed within the test cell, and a photomultiplier to view the gap region, a relationship has also been established between the generation of mechanical waves, light scintillation and conduction current pulses. From the photographic records and conventional parameter measurements there exists strong evidence for the presence of a gaseous phase in processes leading to the electrical breakdown of liquid dielectrics even under pulse conditions

Analysis of a novel non-contacting waveguide backshort

A new non-contacting waveguide backshort has been developed for millimeter and submillimeter wave frequencies. The design consists of a metal bar with rectangular or circular holes cut into it, which is covered with a dielectric (mylar) layer to form a snug fit with the walls of a waveguide. Hole geometries are adjusted to obtain a periodic variation of the guide impedance on the correct length scale, in order to produce efficient reflection of RF power. It is a mechanically rugged design which can be easily fabricated for frequencies from 1 to 1000 GHz and is thus a sound alternative to the miniaturization of conventional non-contacting shorts. To aid in high-frequency design, a rigorous full-wave analysis has been completed, which will allow variations of the size, number and spacing of the holes to be easily analyzed. This paper will review the backshort design and the method developed for theoretical characterization, followed by a comparison of the experimental and numerical results. Low frequency models operating from 4-6 GHz are shown to demonstrate return loss of greater than -0.2 dB over a 33 percent bandwidth. The theory is in good agreement with measured data

Sequential Phase-Shifted Model Predictive Control for multicell power converters

© 2017 IEEE. This paper proposes a sequential Phase-Shifted Model Predictive Control (PS-MPC) strategy for multicell power converters. The key novelty of this proposal lies in the way the predictive control strategy is formulated to fully exploit a phase-shifted pulse width modulation (PS-PWM) stage. Normally, when using a linear controller along with a standard PS-PWM stage, the modulator receives the same duty cycle for all the internal carriers. In contrast, by means of an appropriate choice of synchronized models for each carrier, the proposed predictive controller obtains independent optimal duty cycles for each carrier in a sequential manner. This allows one to formulate the optimal control problem to govern not only the output current but also the internal floating capacitor voltages, similarly to the finite-control-set MPC (FCS-MPC) case. As a result, the proposed sequential PS-MPC can attain a faster floating voltage balancing dynamic when compared to a standard PS-PWM implementation. Moreover, it generates a fixed-spectrum in the steady state with a constant commutation rate, which outperforms a standard FCS-MPC strategy. Simulation results of the proposed sequential PS-MPC strategy governing a single-phase four-level flying capacitor converter are presented to verify its dynamic and steady-state performance

Uptake and distribution of nickel and other metals in the hyperaccumulator Berkheya coddii

Berkheya coddii is a fast-growing, high biomass nickel (Ni) hyperaccumulator plant that has recently attracted attention for its possible use in the phytoextraction of Ni. The mechanisms of Ni accumulation, however, are not well understood in this plant. Plants were grown hydroponically in varying Ni concentrations to assess the uptake and distribution of Ni, and other metals, at the whole plant level. X-ray microanalyses (EDXA) of frozen hydrated tissues were conducted to determine the distribution of Ni at the cellular level in the leaves. Most Ni was found in the shoots, especially in the leaves. Leaf Ni concentration increased with age, whereas older stem sections had lower Ni concentrations than new growth. EDXA analyses revealed that the cuticle of the upper epidermis had a significantly higher Ni concentration than the rest of the leaf. The Ni concentrations in the other leaf tissues were not significantly different. This pattern of distribution contrasts sharply with some other hyperaccumulator species that commonly show a preferential accumulation of Ni and other metals in the vacuoles of the epidermal cells

A Supersoft X-Ray Binary in the Small Magellanic Cloud

Photometric observations of the supersoft X-ray source 1E 0035.4-7230 obtained during two years reveal that the very blue optical counterpart (V (maximum)=20.2, B-V=-0.15, U-B=-1.06) undergoes nearly sinusoidal variations with a period of 0.1719256 days and an amplitude of Delta V~0.3 mag. ROSAT observations show the X-rays vary with approximately the same period. However, either the X-ray minimum precedes the optical minimum by about a quarter cycle or there is a small period difference between the two wavelength regions. We consider that this X-ray source is a close binary, with the optical light coming primarily from an accretion disk surrounding the compact star. Optical spectra show weak, variable He II (4686 Angstrom) emission which probably originates in this disk. Possible interpretations of the light curve are discussed, including X-ray heating of the secondary star. The very broad minimum in the X-ray light curve suggests the X-rays may be scattered in a large accretion disk corona (ADC) which is partially occulted, probably by an azimuthally irregular bulge on the disk rim. If this system lies at the distance of the Small Magellanic Cloud, it radiates near the Eddington luminosity.Comment: 10 pages, 1 table and 3 (of 4) figures as uuencoded compressed Postscript. Also available at http://www.dao.nrc.ca/DAO/SCIENCE/science.htm

Examining the symptom-level specificity of negative problem orientation in a clinical sample

This article was originally published in Cognitive Behaviour Therapy. The version of record can be found here: http://www.tandfonline.com/doi/abs/10.1080/16506073.2014.987314Given the equivocal state of the literature as to the symptom-level specificity of the cognitive variable labeled negative problem orientation (NPO), we targeted NPO–symptom relations. A clinical sample (N = 132) of adults diagnosed with an anxiety disorder, mood disorder, or obsessive-compulsive disorder completed self-reports of NPO and symptom types (worry, depression, obsessive-compulsive, panic, and social anxiety). Symptom-level specificity was examined using a combination of zero-order and regression analyses that controlled for the other assessed symptoms. Results were that NPO shared small to moderate correlations with the symptoms. Regression results indicated that NPO only shared unique associations with worry, depression, and social anxiety. In the analyses, NPO clustered particularly strongly with worry. The present results provide support for conceptualizing NPO as a cognitive variable common to emotional disorders, but not as related equivalently to all disorders within this category

The Social Interaction Anxiety Scale (SIAS) and the Social Phobia Scale (SPS): A comparison of two short-form versions.

This article may not exactly replicate the final version published in the American Psychological Association journal. It is not the copy of record. The version of record can be found here: http://dx.doi.org/10.1037/a0037313The widespread use of Mattick and Clarke’s (1998) Social Interaction Anxiety Scale (SIAS) and Social Phobia Scale (SPS) led two independent groups of researchers to develop short-forms of these measures (Fergus, Valentiner, McGrath, Gier-Lonsway, & Kim, 2012; Peters, Sunderland, Andrews, Rapee, & Mattick, 2012). This three-part study examined the psychometric properties of Fergus et al.’s and Peters et al.’s short-forms of the SIAS and SPS using an American nonclinical adolescent sample in Study 1 (N = 98), American anxiety disorder patient sample in Study 2 (N = 117), and both a South Korean college student sample (N = 341) and an American college student sample (N = 550) in Study 3. Scores on both sets of short-forms evidenced adequate internal consistency, inter-item correlations, and measurement invariance. Scores on Fergus et al.’s short-forms, particularly their SIAS short-form, tended to capture more unique variance in scores of criterion measures than did scores on Peters et al.’s short-forms. Implications for the use of these two sets of short-forms are discussed

Disappearance of Elliptic Flow: A New Probe for the Nuclear Equation of State

Using a relativistic hadron transport model, we investigate the utility of the elliptic flow excitation function as a probe for the stiffness of nuclear matter and for the onset of a possible quark-gluon-plasma (QGP) phase-transition at AGS energies 1 < E_Beam < 11 AGeV. The excitation function shows a strong dependence on the nuclear equation of state, and exhibits characteristic signatures which could signal the onset of a phase transition to the QGP.Comment: 11 pages, 4 Postscript figures, uses epsf.sty, submitted to Physical Review Letter