Experimental study on hard radiation from long laboratory spark discharges in air

The detection of hard radiation emanating from an electrical discharge in air is still a mysterious phenomenon. This thesis focuses on collecting experimental data around spark condition that could lead to the production of energetic photon bursts. Long spark discharges with positive and negative polarity in air are studied. We accurately measure the electrical currents on both electrodes during the formation of the discharge. The Xrays are detected with scintillation detectors, time synchronized with the electrical parameters. Bursts up to several 100 keV photons are observed. The advantage of ‘laboratory lightning’ is the controlled environment that allows to study the distribution of the X-rays in space and time. The experiments are performed in the high voltage laboratory at Eindhoven University of Technology. A 2 MV twelve stage Marx generator, with a standardized lightning impulse with 1.2/50 µs rise/decay time to half-maximum when unloaded, delivers the high voltage air breakdown. A 9 m tall 1:2000 capacitive high voltage divider (part of the waveshaping circuit) is used to monitor the voltage waveform produced by the Marx generator. The generator is connected to a spark gap with two conical electrodes at distances varying between 0.76 and 1.46 m. The current at the grounded electrode is measured by a Pearson current probe. An identical probe around the high voltage electrode was connected through a fiber optical data transmission system for electrical isolation. Electromagnetic disturbance from the discharge itself was reduced to a negligible level in the measurements by proper design of the cables and protection equipment. Fast X-ray detectors with good energy resolution are imperative for reliable X-ray registrations. We use conventional NaI(Tl), nanosecond fast BaF2 and two newly developed LaBr3(Ce+) scintillation detectors, all with suitable photomultiplier integrated. Later the DTU National Space Institute assisted in the experiments with a test Cadmium Zinc Telluride (CZT) semiconductor detector intended for the Atmosphere-Space Interactions Monitor (ASIM) project. In early measurements partial discharges at unexpected positions occurred that could also produce X-rays. This effect could be controlled by covering sharp protrusions with conducting foil. The currents measured through both electrodes differ substantially during the firsts few microseconds. This is caused by the Ramo-Shockley effect. In the development phase of the discharge a charge cloud developed around the high voltage electrode and most of the associated electric field lines end in the environment of the grounded electrode, but not on the electrode. Through this current difference between both electrodes it was possible to tell where burst of X-rays are formed. For both polarities of the high voltage, the bursts of X-rays are associated with the negative streamer formation at the cathode. For positive polarity surges X-ray bursts detected coincide with the onset of the upward negative streamer prior to the bridging of the electrode distance. In the case of negative polarity surges X-ray bursts coincide with the negative streamers immediately at the onset of the spark formation. No X-rays have been detected during the large current of the gap breakdown. At gap breakdown both currents become equal. In a parallel investigation we confirmed that the LaBr3(Ce+) scintillation detector suited best for our experimental study because of the short decay time compared with NaI(Tl) and the better energy resolution compared with BaF2. Still, we found that pile-up of multiple photons and/or electrons can occur in the LaBr3(Ce+) detectors in the 23 ns of the 1/e decay. The LaBr3(Ce+) detector has been calibrated and tested for its linearity at photon energies between 59.5 and 2505 keV, employing photomultiplier bias voltages from 568 up to 1000 V. Preliminary X-ray measurements with a small CZT semiconductor detector gave no conclusive results due to the poor detection events (6 out of 100) obtained. Additional experiments with a larger detector for higher detection rates are recommended for a better understanding of the particle distributions involved. An additional experiment was carried out to confirm the emission of X-ray during the streamer phase of an electrical discharge in air. Streamer filaments were produced in a small streamer-corona reactor with nanosecond high voltage pulses up to 65 kV with an optional 20 kVdc offset. The short high voltage pulse period prevents the streamers to develop into an air breakdown. The LaBr3(Ce+) scintillation detector recorded X-rays from the streamer filaments with very consistent timing of occurrences. This proves that processes near the streamers heads are able to produce X-rays. Various results were presented at a number of international conferences and workshops. Chapter 3 and Chapter 4 have been published in Journal of Physics D: Applied Physics

Impact of foreign investment on household welfare: Evidence from Vietnam

Available online 27 July 2019We examine the impact of foreign direct investment in Vietnam on household and individual welfare and on migration using survey data for the period 2002 to 2016. We find that higher revenue from foreign invested firms measured at province level and normalized on population is associated with a variety of positive outcomes. At household level, income and expenditures per capita are higher and poverty incidence is lower. At individual level, non-farm employment and wages are higher. And at commune level, in-migration is higher. However, although these improvements register as statistically signficant, the magnitudes in economic terms are modest.Anh Tuan Bui, Cuong Viet Nguyen, Thu Phuong Pha

Experimental investigation of AL2O3-EG nanofluid thermal properties and heat exposure stability in closed circuit

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.In the present study, we seek to further advance nanofluid research by simultaneously acquiring the thermal (thermal conductivity) and rheological (dynamic viscosity) properties of surfactant-free 80nm aluminium-oxide (alumina, Al2O3) spheroid gamma nanoparticle dispersions in 99.8% purity ethylene glycol (EG). Samples were diluted into 1, 2.5 and 5% by volume concentrations and pumped through an innovative purpose-developed thermally controlled closed system. Contrary to most studies which focus on individual property analysis through static burst (i.e. short term) data acquisition, concurrent data acquisition was achieved for relatively long testing periods, from 24 to 72 hours. This novel multifaceted approach also permitted to observe the long term effects of heat on colloid stability as well to insure repeatability of the data when applicable.dc201

Novel vascular plexus in the head of a sea snake (Elapidae, Hydrophiinae) revealed by high-resolution computed tomography and histology

Novel phenotypes are often linked to major ecological transitions during evolution. Here, we describe for the first time an unusual network of large blood vessels in the head of the sea snake Hydrophis cyanocinctus. MicroCT imaging and histology reveal an intricate modified cephalic vascular network (MCVN) that underlies a broad area of skin between the snout and the roof of the head. It is mostly composed of large veins and sinuses and converges posterodorsally into a large vein (sometimes paired) that penetrates the skull through the parietal bone. Endocranially, this blood vessel leads into the dorsal cerebral sinus, and from there, a pair of large veins depart ventrally to enter the brain. We compare the condition observed in H. cyanocinctus with that of other elapids and discuss the possible functions of this unusual vascular network. Sea snakes have low oxygen partial pressure in their arterial blood that facilitates cutaneous respiration, potentially limiting the availability of oxygen to the brain. We conclude that this novel vascular structure draining directly to the brain is a further elaboration of the sea snakes' cutaneous respiratory anatomy, the most likely function of which is to provide the brain with an additional supply of oxygen.Alessandro Palci, Roger S. Seymour, Cao Van Nguyen, Mark N. Hutchinson, Michael S. Y. Lee and Kate L. Sander

Transversely-intersecting D-branes at finite temperature and chiral phase transition

We consider Sakai-Sugimoto like models consisting of Dq-Dp-anti-Dp-branes where flavor Dp and anti-Dp-branes transversely intersect color Dq-branes along two (r+1)-dimensional subspaces. For some values of p and q, the theory of intersections dynamically breaks non-Abelian chiral symmetry which is holographically realized as a smooth connection of the flavor branes at some point in the bulk of the geometry created by Dq-branes. We analyze the system at finite temperature and map out different phases of the theory representing chiral symmetry breaking and restoration. For q<5 we find that, unlike the zero-temperature case, there exist two branches of smoothly-connected solutions for the flavor branes, one getting very close to the horizon of the background and the other staying farther away from it. At low temperatures, the solution which stays farther away from the horizon determines the vacuum. For background D5 and D6-branes we find that the flavor branes, like the zero temperature case, show subtle behavior whose dual gauge theory interpretation is not clear. We conclude with some comments on how chiral phase transition in these models can be seen from their open string tachyon dynamics.Comment: 35 pages, 7 figures, minor changes, published versio

Current Opinions on Optimal Management of Basilar Artery Occlusion: After the BEST of BASICS Survey

Background The best management of basilar artery occlusion (BAO) remains uncertain. The BASICS (Basilar Artery International Cooperation Study) and the BEST (Basilar Artery Occlusion Endovascular Intervention Versus Standard Medical Treatment) trials reported neutral results. We sought to understand physicians’ approaches to BAOs and whether further BAO randomized controlled trials were warranted. Methods We conducted an online international survey from January to March 2022 to stroke neurologists and neurointerventionalists. Survey questions were designed to examine clinical and imaging parameters under which clinicians would offer (or rescind) a patient with BAO to endovascular therapy (EVT) or best medical management versus enrollment into a randomized clinical trial. Results Of >3002 invited participants, 1245 responded (41.4% response rate) from 73 countries, including 54.7% stroke neurologists and 43.6% neurointerventionalists. More than 95% of respondents would offer EVT to patients with BAO, albeit in various clinical circumstances. There were 70.0% of respondents who indicated that the BASICS and BEST trials did not change their practice. Only 22.1% of respondents would perform EVT according to anterior circulation occlusion criteria. The selection of patients for BAO EVT by clinical severity, timing, and imaging modality differed according to geography, specialty, and country income level. Over 80% of respondents agreed that further randomized clinical trials for BAO were warranted. Moreover, 45.6% of respondents indicated they would find it acceptable to enroll all trial‐eligible patients into the medical arm of a BAO trial, whereas 26.3% would not enroll. Conclusion Most stroke physicians continue to believe in the efficacy of EVT in selected patients with BAO in spite of BEST and BASICS. There is no consensus on which selection criteria to use, and few clinicians would use anterior circulation occlusion criteria for BAOs. Further randomized clinical trials for BAO are warranted

Endovascular versus Medical Management of Acute Basilar Artery Occlusion: A Systematic Review and Meta-Analysis of the Randomized Controlled Trials

Background and Purpose The optimal management of patients with acute basilar artery occlusion (BAO) is uncertain. We aimed to evaluate the safety and efficacy of endovascular thrombectomy (EVT) compared to medical management (MM) for acute BAO through a meta-analysis of randomized controlled trials (RCTs). Methods We performed a systematic review and meta-analysis of RCTs of patients with acute BAO. We analyzed the pooled effect of EVT compared to MM on the primary outcome (modified Rankin Scale [mRS] of 0–3 at 3 months), secondary outcome (mRS 0–2 at 3 months), symptomatic intracranial hemorrhage (sICH), and 3-month mortality rates. For each study, effect sizes were computed as odds ratios (ORs) with random effects and Mantel-Haenszel weighting. Results Four RCTs met inclusion criteria including 988 patients. There were higher odds of mRS of 0-3 at 90 days in the EVT versus MM group (45.1% vs. 29.1%, OR 1.99, 95% confidence interval [CI] 1.04–3.80; P=0.04). Patients receiving EVT had a higher sICH compared to MM (5.4% vs. 0.8%, OR 7.89, 95% CI 4.10–15.19; P<0.01). Mortality was lower in the EVT group (35.5% vs. 45.1%, OR 0.64, 95% CI 0.42–0.99; P=0.05). In an analysis of two trials with BAO patients and National Institutes of Health Stroke Scale (NIHSS) <10, there was no difference in 90-day outcomes between EVT versus MM. Conclusion In this systematic review and meta-analysis, EVT was associated with favorable outcome and decreased mortality in patients with BAO up to 24 hours from stroke symptoms compared to MM. The treatment effect in BAO patients with NIHSS <10 was less certain. Further studies are of interest to evaluate the efficacy of EVT in basilar occlusion patients with milder symptoms

Dielectrowetting: the past, present and future

Liquid dielectrophoresis is a bulk force acting on dipoles within a dielectric liquid inside a non-uniform electric field. When the driving electrodes are interdigitated, bulk liquid dielectrophoresis is converted to an interface-localised form capable of modifying the energy balance at an interface. When the interface is a solid-liquid one, the wetting properties of a surface are modified and this approach is known as dielectrowetting. Dielectrowetting has been shown to provide the ability to reversibly modify the contact angle of a liquid droplet with the application of voltage, the strength of which is controlled by the penetration depth of the non-uniform field and permittivities of the fluids involved. Importantly, dielectrowetting provides the ability to create thin liquid films, overcoming the limitation of contact angle saturation present in electrowetting. In this paper, we review the development of dielectrowetting - its origins, the statics and dynamics of dielectrowetted droplets, and the applications of dielectrowetting in microfluidics and optofluidics. Recent developments in the field are also reviewed showing the future directions of this rapidly developing field