Breakdown processes in HID lamps : exploration of various key aspects

This thesis presents the results of experimental and modelling studies of breakdown processes in near-atmospheric pressure noble gasses. The motivation came from the lighting industry - our goal was to provide a better understanding of the breakdown phenomena in conditions typical for a mid-pressure high-intensity discharge (HID) lamp. However, the research can be used in a broader spectrum of applications involving breakdown, for example in high power electronics, in removal of unwanted electric charges, photocopying, handling waste, UV generation or surface treatment. We focused our research to mid-pressure (0.1 to 1 bar) discharges in argon and xenon with varying conditions that will prove to greatly influence the breakdown process. First, we examined the effect the dielectric surfaces have on the breakdown process. A pin-to-pin electrode geometry was placed in close vicinity of a flat dielectric in an argon atmosphere of pressure varying between 0.1 and 1 bar. We used positive pulsed voltages on the charged electrode (rise time varying between 47 and 100 V/ns), observed the development and measured the speed of the discharge forming on the dielectric surface and in the gas between the electrode tips. Our results prove that surface discharges use propagation and growth mechanisms that are in some aspects different from the discharges that form in the gas. The effect of the voltage form on the breakdown process was subsequently studied. Lowering of the breakdown voltage of lamps is a constant goal to be met, and it has already been observed that substituting pulsed voltages for AC in the 100-kHz range brings significant improvements. We performed electrical and optical measurements of the breakdown parameters and explained why AC breakdown works on lower voltages than pulsed breakdown. The differences between the discharges in different gasses were explained, along with the influience of voltage frequency on the breakdown process and UV- and Kr85-related effects. Statistical lag times were calculated for different parameters. A valuable contribution to the understanding of the AC ignition process was done by using computer simulations. A fluid model and a cylindrically symmetric 2D geometry with a pin-to-pin electrode configuration was used to simulate a 700 mbar argon discharge. After simulating pulsed ignition in free gas and proving the importance of metastables on the discharge growth, the AC breakdown process in a lamp-like geometry was also simulated at frequencies between 60 kHz and 1 MHz. The main finding of this part of the research was the explanation why AC breakdown requires lower voltage than pulsed breakdown. We also explained the influence of the voltage frequency observed in experiments. The final part of the research considered the influence of external structures ("antennae") on the breakdown process in AC discharges. Antennae are thin metallic formations on the outer surface of the lamp burner. An EM model was used to examine the influence of different antenna structures on the electric field enhancement in the lamp in a static case. We have also done a series of experiments on lamps, showing that the antennae significantly lower the breakdown voltage. The last part of the thesis shows how antennae work, why the active ones work better than the passive ones, and the reason behind the observed differences in the workings of the passive antennae

The impingement of a kHz helium atmospheric pressure plasma jet on a dielectric surface

A parametric study of the impingement of a helium kHz atmospheric pressure plasma jet on a flat glass surface was performed by means of time-resolved intensified charge-coupled device imaging. The development of the plasma on the target is linked to the plasma evolution in the source and governed by the power supply. The glass surface takes part in the elongation of the plasma jet by the virtue of two mechanisms: the local enhancement of the electric field and the supply of pre-deposited charge. The evidence for the pre-deposited charge is the formation of a sheath on the glass surface, and the faint discharge formed on the glass surface during the negative voltage slope starting at the maximum of the negative current peak. The influence of the gas flow dynamics taking into account various gas flows, incident angles and distances is more important for the behaviour of the discharge on the surface than the voltage amplitude or the geometry of the source. The capacitance of the target strongly modifies the interaction with the plasma jet and increases the deposited surface charge density, featuring a streamer-like propagation mechanism in the case of high electric field enhancement at the surface

Nonlinear Growing Caps

Engineers capture growth strains in two ways, reflecting the inherent bending-stretching nature of shells: by a strain gradient through the thickness or by an average in-plane value. We analyse their interaction by assuming a uniform displacement and growth-strain field in shells with elastic spring supports and a radial force applied to their outer boundary. The increased degree of statical indeterminancy enriches the variety of existing solutions and we distinguish two in-plane actuation modes which can induce Gaussian curvature via radially varying quadratic expansions in either the circumferential or radial direction. Using a Rayleigh-Ritz approach, we find closed-form solutions of the Föppl-von Kármán shell equations for the buckling thresholds, bistable limits and the post-buckled shape, which show good agreement with finite element reference solutions and available results from the literature. Moreover, we show that ‘natural’ growth modes, which evoke a change of shape without incurring elastic strain energy, can be achieved by employing quadratic radial expansions only. Additionally, we study unsupported shells subjected to higher-order actuation distributions, which give rise to natural growth modes with varying wavenumbers. Our approach dramatically simplifies an otherwise non-trivial general solution, and may be applied in novel generations of smart materials with actively tunable material properties.Friedrich-Ebert-Foundatio

Increasing signal-to-noise ratio in over-determined Mueller matrices

This work investigates how the signal-to-noise ratio (SNR) of an over-determined Mueller matrix can be improved by changing the method of calculation. Specifically, our investigation focused on comparing SNRs achieved using the vector methodology from the field of partial Mueller polarimetry, and the matrix methodology. We use experimentally derived measurements from an investigation into the time-varying signal produced by the Mueller matrix of an electro-optic Bismuth Silicon Oxide (BSO) crystal undergoing cyclical impact of a Helium plasma ionisation wave. Our findings show that the vector methodology is superior to the matrix methodology, with a maximum SNR of 7.54 versus 4.97. We put forth that the superiority of the vector methodology is due to its greater flexibility, which results in the Mueller matrix being calculated with better condition matrices, and higher levels of SNR in the intensity measurements used for calculation.</p

Mode-selective coupling of coherent phonons to the Bi2212 electronic band structure

Cuprate superconductors host a multitude of low-energy optical phonons. Using time- and angle-resolved photoemission spectroscopy, we study coherent phonons in Bi$_{2}$Sr$_{2}$Ca$_{0.92}$Y$_{0.08}$Cu$_{2}$O$_{8+\delta}$. Sub-meV modulations of the electronic band structure are observed at frequencies of $3.94\pm 0.01$ and $5.59\pm 0.06$ THz. For the dominant mode at 3.94 THz, the amplitude of the band energy oscillation weakly increases as a function of momentum away from the node. Theoretical calculations allow identifying the observed modes as CuO$_{2}$-derived $A_{1g}$ phonons. The Bi- and Sr-derived $A_{1g}$ modes which dominate Raman spectra in the relevant frequency range are absent in our measurements. This highlights the mode-selectivity for phonons coupled to the near-Fermi-level electrons, which originate from CuO$_{2}$ planes and dictate thermodynamic properties.Comment: 7 pages, 3 figure

Annexin A1 Tethers Membrane Contact Sites that Mediate ER to Endosome Cholesterol Transport

Membrane contact sites between the ER and multivesicular endosomes/bodies (MVBs) play important roles in endosome positioning and fission and in neurite outgrowth. ER-MVB contacts additionally function in epidermal growth factor receptor (EGFR) tyrosine kinase downregulation by providing sites where the ER-localized phosphatase, PTP1B, interacts with endocytosed EGFR before the receptor is sorted onto intraluminal vesicles (ILVs). Here we show that these contacts are tethered by annexin A1 and its Ca2+-dependent ligand, S100A11, and form a subpopulation of differentially regulated contact sites between the ER and endocytic organelles. Annexin A1-regulated contacts function in the transfer of ER-derived cholesterol to the MVB when low-density lipoprotein-cholesterol in endosomes is low. This sterol traffic depends on interaction between ER-localized VAP and endosomal oxysterol-binding protein ORP1L, and is required for the formation of ILVs within the MVB and thus for the spatial regulation of EGFR signaling

Breakdown in mm-sized discharges : modifying the electric field

Due to the small size of the gas gap in micro- and millimeter sized discharges, the presence of a metallic structure in its vicinity profoundly influences the breakdown process. This is a drawback because it makes electrical probing impossible, but can also be an advantage because it allows control over the electric field in the discharge reactor. Optical and electrical measurements were performed in an argon atmosphere at 0.3 or 0.7 bar. A pin-pin geometry was used, with 4 or 7mm between the electrode tips. We found that both active and passive structures influence breakdown, and we demonstrated the differences between the two types and their effects on the breakdown process