Inception and propagation of positive streamers in high-purity nitrogen: effects of the voltage rise-rate

Controlling streamer morphology is important for numerous applications. Up to now, the effect of the voltage rise rate was only studied across a wide range. Here we show that even slight variations in the voltage rise can have significant effects. We have studied positive streamer discharges in a 16 cm point-plane gap in high-purity nitrogen 6.0, created by 25 kV pulses with a duration of 130 ns. The voltage rise varies by a rise rate from 1.9 to 2.7 kV ns−1 and by the first peak voltage of 22 to 28 kV. A structural link is found between smaller discharges with a larger inception cloud caused by a faster rising voltage. This relation is explained by the greater stability of the inception cloud due to a faster voltage rise, causing a delay in the destabilization. Time-resolved measurements show that the inception cloud propagates slower than an earlier destabilized, more filamentary discharge. This explains that the discharge with a faster rising voltage pulse ends up being shorter. Furthermore, the effect of remaining background ionization in a pulse sequence has been studied, showing that channel thickness and branching rate are locally affected, depending on the covered volume of the previous discharge

Slow decay of radiation after a pulsed streamer discharge in pure nitrogen

Light emission and electrical characteristics in the early post-discharge of a high purity nitrogen streamer have been investigated. Up to the millisecond regime, both light emission and current are significant, while the voltage has decayed after several tens of microseconds. The corresponding decay time constants are 240 µs and 580 µs for the current and radiance, respectively, versus 3.8 µs for the voltage decay. This suggests that energy transfer to high vibrational levels of N2 (X 1 Σ, ν) and high population of metastable N2 (A3 Σ+ ) species are important in sustaining the discharge

THE RELATIONSHIP OF SOCIOECONOMIC STATUS AND RELIGIOUS FAITH AFFILIATION TO BLACK STUDENT RETENTION AT A PREDOMINANTLY WHITE INSTITUTION

This study was conducted to examine the relationships, if any, between socioeconomic status (SES), religious faith affiliation, and retention of Black students in a predominantly White institution. The 2002 University New Student Census, a questionnaire given to all students during summer orientation, was used to secure a sample of Black students. Students who selected the "Black, African American, Negro" option ONLY were considered for the study. Socioeconomic Status was separated into three groups: Father's level of education; Mother's level of education; and total parental income. A student was considered retained if they returned for the Fall 2003 semester. Also, participants' religious preference was self-reported using the choices offered in the survey. The study used chi-square analyses, because the nature of the data is categorical. The data suggest that there were no statistically significant findings using SES, religious faith affiliation, and retention; yet there were trends that further research could explain

Robótica Educativa

La Facultad de Ingeniería Eléctrica (FIE) de la Universidad Tecnológica de Panamá (UTP), con el interés de mantenerse a la vanguardia del crecimiento tecnológico ha integrado tres áreas importantes en su nuevo Club de Mecatrónica FIE

On the Semantics and Ontology of Race: Constructivism against Realism

This paper is an essay in philosophical ontology tasked with defending a constructionist approach to race. Anthony Appiah, in his In My Father's House: Africa in the Philosophy of Culture , argues that the concept of race is an “empty concept” precisely because it is both a semantic and an ontological fiction. He advises us to segregate race to the realm of the vague and fuzzy because all racial discourse, which he claims must transcendentally presuppose a biological notion of race in order to be meaningful, seductively misleads us to the extent that such discourse presupposes that the term "race" names a natural kind. While identifying the concept of race as deficient, he challenges us to learn “to think beyond race.” His rebuke of race as a dysfunctional concept takes the form of casting race as a virus spreading the contagion of bad thinking, that is, delusional, even paranoid thinking. This paper argues that while Appiah's theoretical interrogation of the concept of race is mainly accurate, race, nevertheless, is meaningful from a socio-cultural perspective . Discourse about race need not require that races exist in the sense of being natural kinds. Race, I shall be arguing, is a construction, i. e., human agents with varied interests, purposes, and linguistic resources devise the conditions of meaningfulness for the term "race". The general structure of the paper is as follows. I first present a critical discussion of Appiah’s view on race by examining his realist view of meaning and reference. Next, I critically review his main arguments against the concept of race. In part two of the essay, I offer a discussion of constructionism. I describe this position, offer examples, and say why it more adequately accommodates a defense of the concept of race by treating it as a socio-historical construction rather than a scientific natural kind. It is obvious that I challenge Appiah’s realist view that a meaningful concept is one that is intelligible and that the deciding grounds of the intelligibility is a matter of a concept referring to something that has existence. Meaningful concepts refer to objects whereas meaningless concepts do not refer to anything that has being

Three-dimensional modelling of thrust-controlled foreland basin stratigraphy

In this thesis a tectono-sedimentary forward model has been presented, devised to simulate sediment erosion and deposition in a coupled drainage basin - foreland system, as well as accumulating a three-dimensional stratigraphy. The aim of the research was to investigate which features recorded in the foreland basin architecture are diagnostic of the balance and interplay between two main external forcings: repeated tectonic activity and eustatic sea-level variation. Special attention has been paid to differences in stacking patterns of fluvial depositional systems and the character of the sequence-bounding unconformities. The sediment yield generated by fluvial bedrock erosion in the orogenic drainage basins has the typically asymmetrical shape of a response curve (Chapter 2). Yield gradually increases during tectonic activity, and declines exponentially during tectonic quiescence. Syn-tectonic yields are increasing, but they are not sufficient to completely fill the accommodation space created in the adjacent foreland basin due to flexural response upon active tectonic thrust loading. As a result and counter-intuitively, deposition of alluvial fan gravels in the foreland basin retreats during tectonic activity, whereas progradation is characteristic of phases of tectonic quiescence and reduced flexural subsidence. A history of pulsating tectonic activity is reflected in the alluvial architecture of the basin as a succession of coarsening-up, prograding gravel sheets that laterally connect during quiescence, and alternate with basin-wide onlap of fine-grained sediments marking renewed tectonic activity. The experiments of Chapter 3 show that there are two contrasting types of sequence boundaries developed in the alluvial stratigraphy when a eustatic sea-level variation is superposed upon the alternation of tectonic activity and subsidence: A) During intervals of tectonic activity, eustatic fall and rise of sea level form prograding, shallowing up sequences, which are bounded by Type-2 unconformities and subsequent flooding surfaces. The syn-tectonic, high flexural subsidence rates prohibit the sea level to drop below the delta break in slope, safeguarding the stratigraphy from severe incision. B) During intervals of tectonic quiescence, Type-1 unconformities are formed, because eustatic falls now drop below the delta break in slope, as they are no longer compensated by the subsidence component in relative sea level. Because multiple eustatic sea-level cycles may occur during a quiescence interval, the resulting Type-1 unconformity at the base of the delta-top sheet sandstone can be a composite and therefore poor time marker. The suites of amalgamating, axial channel belts that characterize this delta top sheet sandstone have a preference for the depression between the basin-margin alluvial fans and the conical delta surface that was formed during a previous tectonic phase. Similar suites of amalgamating axial channel belts are created when the foreland basin is detached from its substratum by a hinterland-dipping sole thrust and transformed into a thrust-sheet top basin (Chapter 4). In the light of these model results the Eocene Castissent Formation in the Pyrenean Tremp Basin, previously interpreted as a incised valley system, is explained as a phase of moderate tectonic reduction of the accommodation space. This mechanism explains the continuous marine influence on the delta plain simultaneous with a forced regression and increased sandstone interconnectedness at the cost of fine-grained intervals

Streamer knotwilg branching; sudden transition in morphology of positive streamers in nitrogen

We describe a peculiar branching phenomenon in positive repetitive streamer discharges in high purity nitrogen. We name it knotwilg branching after the Dutch word for a pollard willow tree. In a knotwilg branching a thick streamer suddenly splits into many thin streamers. Under some conditions this happens for all streamers in a discharge at about the same distance from the high-voltage electrode tip. At this distance, the thick streamers suddenly bend sharply and appear to propagate over a virtual surface surrounding the high-voltage electrode, rather than following the background electric field lines. From these bent thick streamers many, much thinner, streamers emerge that roughly follow the background electric field lines, creating the characteristic knotwilg branching. We have only found this particular morphology in high purity nitrogen at pressures in the range 50 to 200 mbar and for pulse repetition rates above 1 Hz; the experiments were performed for an electrode distance of 16 cm and for fast voltage pulses of 20 or 30 kV. These observations clearly disagree with common knowledge on streamer propagation. We have analyzed the data of several tens of thousands of discharges to clarify the phenomena. We also present some thoughts on how the ionization of the previous discharges could concentrate into some pre-ionization region near the needle electrode and create the knotwilg morphology, but we present no final explanation

Measurement of the temperature of an ultracold ion source using time-dependent electric fields

We report on a measurement of the characteristic temperature of an ultracold rubidium ion source, in which a cloud of laser-cooled atoms is converted to ions by photo-ionization. Extracted ion pulses are focused on a detector with a pulsed-field technique. The resulting experimental spot sizes are compared to particle-tracking simulations, from which a source temperature $T = (1 \pm 2)$ mK and the corresponding transversal reduced emittance $\epsilon_r = 7.9 X 10^{-9}$ m rad $\sqrt{\rm{eV}}$ are determined. We find that this result is likely limited by space charge forces even though the average number of ions per bunch is 0.022.Comment: 8 pages, 11 figure