962 resultados para SEMICONDUCTOR CDS
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Spatiotemporal chaos is predicted to occur in n-doped semiconductor superlattices with sequential resonant tunneling as their main charge transport mechanism. Under dc voltage bias, undamped time-dependent oscillations of the current (due to the motion and recycling of electric field domain walls) have been observed in recent experiments. Chaos is the result of forcing this natural oscillation by means of an appropriate external microwave signal.
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Over the past years there has been considerable interest in the growth of single crystals both from the point of view of basic research and technological application. With the revolutionary emergence of solid state electronics which is based on single crystal technolo8Ys basic and applied studies on crystal growth and characterization _have gained a-more significant role in material science. These studies are being carried out for single crystals not only of semiconductor and other electronic materials but also of metals and insulators. Many organic crystals belonging to the orthorhombic class exhibit ferroelectric, electrooptic, triboluminescent and piezoelectric properties. Diammonium Hydrogen Citrate (DAHC) crystals are reported to be piezoelectric and triboluminescent /1/. Koptsik et al. /2/ have reported the piezoelectric nature of Citric Acid Monohydrate (CA) crystals. And since not much work has been done on these crystals, it has been thought useful to grow and characterize these crystals. This thesis presents a study of the growth of these crystals from solution and their defect structures. The results of the microindentation and thermal analysis are presented. Dielectric, fractographic, infrared (IR) and ultraviolet (UV) studies of DAHC crystals are also reported
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Laser engineering is an area in which developments in the existing design concepts and technology appear at an alarming rate. Now—a-days, emphasis has shifted from innovation to cost reduction and system improvement. To a major extent, these studies are aimed at attaining larger power densities, higher system efficiency and identification of new lasing media and new lasing wavelengths. Todate researchers have put to use all the ditferent Forms of matter as lasing material. Laser action was observed For the first time in a gaseous system - the He-Ne system. This was Followed by a variety of solidstate and gas laser systems. Uarious organic dyes dissolved in suitable solvents were found to lase when pumped optically. Broad band emission characteristics of these dye molecules made wavelength tuning possible using optical devices. Laser action was also observed in certain p-n junctions of semiconductor materials and some of these systems are also tunable. The recent addition to this list was the observation of laser action from certain laser produced plasmas. The purpose of this investigation was to examine the design and Fabrication techniques of pulsed Nitrogen lasers and high power Nd: Glass laserso Attempt was also made to put the systems developed into certain related experiments
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The work reported in this thesis is the preparation, and the structural, electrical and optical properties of reactively evaporated lead sulphide and tin telluride thin films. The three temperature method had been used for the preparation of these semiconductor thin films. In this preparation technique constituent elements are evaporated from separate sources with the substrate kept at a particular temperature. when one of the constituent element is a gas near room temperature, the method is often called reactive evaporation. It has been found for many materials that a stoichiometric interval exists with a limited range of flux and substrate temperature. Usually this technique is used for the preparation of thin films of high melting point compounds or of materials which decompose during evaporation. Tin telluride and lead sulphide are neither high melting point materials nor do they decompose on melting. But even than reactive evaporation offers the possibility of changing the ratios of the flux of the constituent elements within a wide range and studying its effect on the properties of the films
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Semiconductor photocatalysis has received much attention during last three decades as a promising solution for both energy generation and environmental problems. Heterogeneous photocatalytic oxidation allows the degradation of organic compounds into carbon dioxide and water in the presence of a semiconductor catalyst and UV light source. The •OH radicals formed during the photocatalytic processes are powerful oxidizing agents and can mineralise a number of organic contaminants. Titanium dioxide (TiO2), due to its chemical stability, non-toxicity and low cost represents one of the most efficient photocatalyst. However, only the ultraviolet fraction of the solar radiation is active in the photoexcitation processes using pure TiO2 and although, TiO2 can treat a wide range of organic pollutants the effectiveness of the process for pollution abatement is still low. A more effective and efficient catalyst therefore must be formulated. Doping of TiO2 was considered with the aim of improving photocatalytic properties. In this study TiO2 catalyst was prepared using the sol-gel method. Metal and nonmetal doped TiO2 catalysts were prepared. The photoactivity of the catalyst was evaluated by the photodegradation of different dyes and pesticides in aqueous solution. High photocatalytic degradation of all the pollutants was observed with doped TiO2. Structural and optical properties of the catalysts were characterized using XRD, BET surface area, UV-Vis. DRS, CHNS analysis, SEM, EDX, TEM, XPS, FTIR and TG. All the catalysts showed the anatase phase. The presence of dopants shifts the absorption of TiO2 into the visible region indicating the possibility of using visible light for photocatalytic processes.
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Fluorescence is a powerful tool in biological research, the relevance of which relies greatly on the availability of sensitive and selective fluorescent probes. Nanometer sized fluorescent semiconductor materials have attracted considerable attention in recent years due to the high luminescence intensity, low photobleaching, large Stokes’ shift and high photochemical stability. The optical and spectroscopic features of nanoparticles make them very convincing alternatives to traditional fluorophores in a range of applications. Efficient surface capping agents make these nanocrystals bio-compatible. They can provide a novel platform on which many biomolecules such as DNA, RNA and proteins can be covalently linked. In the second phase of the present work, bio-compatible, fluorescent, manganese doped ZnS (ZnS:Mn) nanocrystals suitable for bioimaging applications have been developed and their cytocompatibility has been assessed. Functionalization of ZnS:Mn nanocrystals by safe materials results in considerable reduction of toxicity and allows conjugation with specific biomolecules. The highly fluorescent, bio-compatible and water- dispersible ZnS:Mn nanocrystals are found to be ideal fluorescent probes for biological labeling
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Photoluminescence (PL) spectroscopy is an optical technique that has emerged successful in the field of semiconductor material and device characterization. This technique is quite a powerful one which gives idea about the defect levels in a material, the band gap of the material, composition as well as material quality. Over the recent years it has received an elevation as a mainstream characterization technique. This thesis is an attempt to characterize each individual layer used in a thin film solar cell with special focus on the electrical properties. This will be highly beneficial from the lab as well as industrial point of view because electrical measurements generally are contact mode measurements which tend to damage the surface. As far as a thin film solar cell is concerned, the constituent layers are the transparent conducting oxide (TCO), absorber layer, buffer layer and top electrode contact. Each layer has a specific role to play and the performance of a solar cell is decided and limited by the quality of each individual layer. Various aspects of PL spectroscopy have been employed for studying compound semiconductor thin films [deposited using chemical spray pyrolysis (CSP)] proposed for solar cell application. This thesis has been structured in to seven chapters
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Cache look up is an integral part of cooperative caching in ad hoc networks. In this paper, we discuss a cooperative caching architecture with a distributed cache look up protocol which relies on a virtual backbone for locating and accessing data within a cooperate cache. Our proposal consists of two phases: (i) formation of a virtual backbone and (ii) the cache look up phase. The nodes in a Connected Dominating Set (CDS) form the virtual backbone. The cache look up protocol makes use of the nodes in the virtual backbone for effective data dissemination and discovery. The idea in this scheme is to reduce the number of nodes involved in cache look up process, by constructing a CDS that contains a small number of nodes, still having full coverage of the network. We evaluated the effect of various parameter settings on the performance metrics such as message overhead, cache hit ratio and average query delay. Compared to the previous schemes the proposed scheme not only reduces message overhead, but also improves the cache hit ratio and reduces the average delay
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Semiconductor physics has developed significantly in the field of re- search and industry in the past few decades due to it’s numerous practical applications. One of the relevant fields of current interest in material science is the fundamental aspects and applications of semi- conducting transparent thin films. Transparent conductors show the properties of transparency and conductivity simultaneously. As far as the band structure is concerned, the combination of the these two properties in the same material is contradictory. Generally a trans- parent material is an insulator having completely filled valence and empty conduction bands. Metallic conductivity come out when the Fermi level lies within a band with a large density of states to provide high carrier concentration. Effective transparent conductors must nec- essarily represent a compromise between a better transmission within the visible spectral range and a controlled but useful electrical con- ductivity [1–6]. Generally oxides like In2O3, SnO2, ZnO, CdO etc, show such a combination. These materials without any doping are insulators with optical band gap of about 3 eV. To become a trans- parent conductor, these materials must be degenerately doped to lift the Fermi level up into the conduction band. Degenerate doping pro- vides high mobility of extra carriers and low optical absorption. The increase in conductivity involves an increase in either carrier concen- tration or mobility. Increase in carrier concentration will enhance the absorption in the visible region while increase in mobility has no re- verse effect on optical properties. Therefore the focus of research for new transparent conducting oxide (TCO) materials is on developing materials with higher carrier mobilities.
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Multi-component reactions are effective in building complex molecules in a single step in a minimum amount of time and with facile isolation procedures; they have high economy1–7 and thus have become a powerful synthetic strategy in recent years.8–10 The multicomponent protocols are even more attractive when carried out in aqueous medium. Water offers several benefits, including control over exothermicity, and the isolation of products can be carried out by single phase separation technique. Pyranopyrazoles are a biologically important class of heterocyclic compounds and in particular dihydropyrano[2,3-c]pyrazoles play an essential role in promoting biological activity and represent an interesting template in medicinal chemistry. Heterocyclic compounds bearing the 4-H pyran unit have received much attention in recent years as they constitute important precursors for promising drugs.11–13 Pyrano[2,3-c]pyrazoles exhibit analgesic,14 anti-cancer,15 anti-microbial and anti-inflammatory16 activity. Furthermore dihydropyrano[2,3-c]pyrazoles show molluscidal activity17,18 and are used in a screening kit for Chk 1 kinase inhibitor activity.19,20 They also find applications as pharmaceutical ingredients and bio-degradable agrochemicals.21–29 Junek and Aigner30 first reported the synthesis of pyrano[2,3-c]pyrazole derivatives from 3-methyl-1-phenylpyrazolin-5-one and tetracyanoethylene in the presence of triethylamine. Subsequently, a number of synthetic approaches such as the use of triethylamine,31 piperazine,32 piperidine,33 N-methylmorpholine in ethanol,34 microwave irradiation,35,36 solvent-free conditions,37–39 cyclodextrins (CDs),40 different bases in water,41 γ -alumina,42 and l-proline43 have been reported for the synthesis of 6-amino-4-alkyl/aryl-3-methyl- 2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles. Recently, tetraethylammonium bromide (TEABr) has emerged as mild, water-tolerant, eco-friendly and inexpensive catalyst. To the best of our knowledge, quaternary ammonium salts, more specifically TEABr, have notbeen used as catalysts for the synthesis of pyrano[2,3-c]pyrazoles, and we decided to investigate the application of TEABr as a catalyst for the synthesis of a series of pyrazole-fused pyran derivatives via multi-component reactions
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There is an enormous demand for chemical sensors in many areas and disciplines including chemistry, biology, clinical analysis, environmental science. Chemical sensing refers to the continuous monitoring of the presence of chemical species and is a rapidly developing field of science and technology. They are analytical devices which transform chemical information generating from a reaction of the analyte into an measurable signal. Due to their high selectivity, sensitivity, fast response and low cost, electrochemical and fluorescent sensors have attracted great interest among the researchers in various fields. Development of four electrochemical sensors and three fluorescent sensors for food additives and neurotransmitters are presented in the thesis. Based on the excellent properties of multi walled carbon nanotube (MWCNT), poly (L-cysteine) and gold nanoparticles (AuNP) four voltammetric sensors were developed for various food additives like propyl gallate, allura red and sunset yellow. Nanosized fluorescent probes including gold nanoclusters (AuNCs) and CdS quantum dots (QDs) were used for the fluorescent sensing of butylated hydroxyanisole, dopamine and norepinephrine. A total of seven sensors including four electrochemical sensors and three fluorescence sensors have been developed for food additives and neurotransmitters.
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Im Informationszeitalter erhalten die Neuen Medien eine immer größer werdende Bedeutung. Lange Zeit galten gedruckte Informationen – häufig in Form von Büchern und Zeitschriften – als eigentliche Informationsquelle in den Bibliotheken. Mehr und mehr nehmen jedoch mittlerweile auch multimediale Anwendungen breiteren Raum ein, gilt es doch, sich den veränderten Lese- und insbesondere Informationsgewohnheiten anzupassen. Insbesondere sollen elektronische Informationsquellen und digitale Literatur erschlossen und bereitgestellt werden. Im Zuge dieser Entwicklung wurde durch die hessische Ministerin für Wissenschaft und Kunst am 11.11.1998 in der Gesamthochschulbibliothek Kassel eine neugeschaffene Multi-Media-Thek (MMT) eröffnet. In dieser Multimediathek sollen die Bibliotheksbenutzer die Möglichkeit erhalten, die Neuen Medien zu erkunden und gewinnbringend für ihre Fragestellungen einzusetzen. Neben den mittlerweile weiter verbreiteten Internetarbeitsplätzen haben die Benutzer Gelegenheit, an speziell ausgestatteten Arbeitsplätzen Audio- und Videoanwendungen zu testen. Die zur Verfügung stehenden Arbeitsplätze sind plattformübergreifend gestaltet, neben Windows NT-Rechnern können Macintosh-, Java- und Linux-Rechner benutzt werden. Alle PC-Systeme verfügen über hochauflösende Grafik- und Soundkarten, allerdings müssen zum Abhören von Audio-Medien, DVD’s oder Multimedia-CD’s entsprechende Kopfhörer aufgesetzt werden. Über WinCenter können auch auf Linux- und Macintosh-Rechnern CD-Applikationen aus dem GHB-internen CD-Server angewählt werden. Zur Auswahl stehen derzeit 140 Datenbanken, die zudem ergänzt werden durch entsprechende Datenbankzugriffe auf den Silverplatter-Server der GHB. In separaten Klein-Arbeitsräumen können Audio-Cassetten und Video-Bänder benutzt werden. Natürlich lassen sich auch herkömmliche Dias und Microfiches in den Räumen der MMT betrachten und Ausdrucke über den Reader-Printer bzw. über das Bibliotheksnetz auf einen zentralen Benutzer-Drucker (OCE-Drucksystem mit Entgeldregelung) machen. Als Auslegestelle für DIN-Normen kann im DIN-Katalog über Internet recherchiert und das Ergebnis der Recherche direkt im DIN-Bestand angesehen werden. In einem speziell eingerichteten Macintosh-Pool sind mittels Scanner, Grafikbearbeitungssoftware und OCR-Möglichkeit weitere Bedingungen für multimediales Arbeiten geschaffen. Sie sollen bei entsprechendem Bedarf um digitale Bildbearbeitung (Photo, Video) gegebenenfalls ergänzt werden. Die Bibliothek bietet in den Räumen der Multimediathek entsprechende Schulungen zur effektiven Nutzung der Rechnerausstattung an.
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In now-a-days semiconductor and MEMS technologies the photolithography is the working horse for fabrication of functional devices. The conventional way (so called Top-Down approach) of microstructuring starts with photolithography, followed by patterning the structures using etching, especially dry etching. The requirements for smaller and hence faster devices lead to decrease of the feature size to the range of several nanometers. However, the production of devices in this scale range needs photolithography equipment, which must overcome the diffraction limit. Therefore, new photolithography techniques have been recently developed, but they are rather expensive and restricted to plane surfaces. Recently a new route has been presented - so-called Bottom-Up approach - where from a single atom or a molecule it is possible to obtain functional devices. This creates new field - Nanotechnology - where one speaks about structures with dimensions 1 - 100 nm, and which has the possibility to replace the conventional photolithography concerning its integral part - the self-assembly. However, this technique requires additional and special equipment and therefore is not yet widely applicable. This work presents a general scheme for the fabrication of silicon and silicon dioxide structures with lateral dimensions of less than 100 nm that avoids high-resolution photolithography processes. For the self-aligned formation of extremely small openings in silicon dioxide layers at in depth sharpened surface structures, the angle dependent etching rate distribution of silicon dioxide against plasma etching with a fluorocarbon gas (CHF3) was exploited. Subsequent anisotropic plasma etching of the silicon substrate material through the perforated silicon dioxide masking layer results in high aspect ratio trenches of approximately the same lateral dimensions. The latter can be reduced and precisely adjusted between 0 and 200 nm by thermal oxidation of the silicon structures owing to the volume expansion of silicon during the oxidation. On the basis of this a technology for the fabrication of SNOM calibration standards is presented. Additionally so-formed trenches were used as a template for CVD deposition of diamond resulting in high aspect ratio diamond knife. A lithography-free method for production of periodic and nonperiodic surface structures using the angular dependence of the etching rate is also presented. It combines the self-assembly of masking particles with the conventional plasma etching techniques known from microelectromechanical system technology. The method is generally applicable to bulk as well as layered materials. In this work, layers of glass spheres of different diameters were assembled on the sample surface forming a mask against plasma etching. Silicon surface structures with periodicity of 500 nm and feature dimensions of 20 nm were produced in this way. Thermal oxidation of the so structured silicon substrate offers the capability to vary the fill factor of the periodic structure owing to the volume expansion during oxidation but also to define silicon dioxide surface structures by selective plasma etching. Similar structures can be simply obtained by structuring silicon dioxide layers on silicon. The method offers a simple route for bridging the Nano- and Microtechnology and moreover, an uncomplicated way for photonic crystal fabrication.
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Im Vordergrund dieser Arbeit stehen die Synthesen des Azobenzol-4-trichlorsilans sowie des Bis(4-azobenzol)disulfids, ausgehend von einfachen und kommerziell erhältlichen Verbindungen. Moleküle, aus denen sich diese Verbindungen synthetisieren lassen, sind die Iodderivate des Azobenzols, welche über die Kondensation von Benzolaminen (Anilinen) und Nitrosobenzolen dargestellt wurden, aber auch über die altbewährte Azokupplung. Insgesamt wurden 19 neue Azobenzolderivate, das neue [(4-Aminophenyl)ethinyl]ferrocen und das neue Bis[4-(4'-bromazobenzol)]disulfid synthetisiert und charakterisiert. Außerdem wurden 13 neue Kristallstrukturen erzeugt. Mit den synthetisierten Molekülen wurden Substrat-Adsorbat-Systeme gebildet. Als Substrate wurden oberflächenoxidiertes Silizium und Gold gewählt. Die Präparation dieser sogennanten selbstorganisierten Monolagen (SAMs) bzw. der kovalent gebundenen Monolagen im Falle der Trichlorsilylderivate (CAMs) wurde eingehend studiert. Das Azobenzol wurde als photoschaltbare Einheit gewählt, da es bereits Kern zahlreicher Untersuchungen war und als solcher als guter und zuverlässiger Baustein für reversible photoschaltbare Systeme etabliert ist. Zur Charakterisierung Schichten und zur Untersuchung ihres photoresponsiven Verhaltens sowie sowie zur Untersuchung der Schichtbildung selbst wurden mehrere physikalische Messmethoden angewandt. Die Schichtbildung wurde mit SHG (optische Frequenzverdopplung) verfolgt, die fertigen Schichten wurden mit XPS (Röntgen-Photonen-Spektroskopie) und NEXAFS (Nahkanten-Röntgen-Absorptions-Feinstruktur) untersucht, um Orientierung und Ordnung der Moleküle in der Schicht zu ermitteln. Das Schaltverhalten wurde mit Ellipsometrie und durch Messungen des Wasserkontaktwinkels beobachtet. Durch Variation der Endgruppe des Azobenzols ist es möglich, die Oberflächeneigenschaften einstellen gezielt zu können, wie Hydrophobie, Hydrophilie, Komplexierungsverhalten oder elektrische Schaltbarkeit. Dies gelingt durch Gruppen wie N,N-Dimethylamino-, Methoxy-, Ethoxy-, Octyloxy-, Dodecyloxy-, Benzyloxy-, Methyl-, Trifluormethyl-, Pyridyl-, Phenylethinyl- und Ferrocenyl-Restgruppen, um nur eine Auswahl zu nennen. Einerseits wurde Silizium als Substrat gewählt, da es wegen seiner Verwendung in der Halbleiterindustrie ein nicht uninteressantes Substrat darstell und die Möglichkeiten der kovalenten Anbindung von Trichlorsilanen aber auch Trialkoxysilanen auch gut untersucht ist. Andererseits wurden auch Untersuchungen mit Gold als Substrat angestellt, bei dem Thiole und Disulfide die bevorzugten Ankergruppen bilden. Während sich auf Gold sogenannte SAMs bilden, verleiht die kovalente Siloxanbindung den CAMs auf Silizium eine besondere Stabilität.
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Excimerlaser sind gepulste Gaslaser, die Laseremission in Form von Linienstrahlung – abhängig von der Gasmischung – im UV erzeugen. Der erste entladungsgepumpte Excimerlaser wurde 1977 von Ischenko demonstriert. Alle kommerziell verfügbaren Excimerlaser sind entladungsgepumpte Systeme. Um eine Inversion der Besetzungsdichte zu erhalten, die notwendig ist, um den Laser zum Anschwingen zu bekommen, muss aufgrund der kurzen Wellenlänge sehr stark gepumpt werden. Diese Pumpleistung muss von einem Impulsleistungsmodul erzeugt werden. Als Schaltelement gebräuchlich sind Thyratrons, Niederdruckschaltröhren, deren Lebensdauer jedoch sehr limitiert ist. Deshalb haben sich seit Mitte der 1990iger Jahre Halbleiterschalter mit Pulskompressionsstufen auch in dieser Anwendung mehr und mehr durchgesetzt. In dieser Arbeit wird versucht, die Pulskompression durch einen direkt schaltenden Halbleiterstapel zu ersetzen und dadurch die Verluste zu reduzieren sowie den Aufwand für diese Pulskompression einzusparen. Zudem kann auch die maximal mögliche Repetitionsrate erhöht werden. Um die Belastung der Bauelemente zu berechnen, wurden für alle Komponenten möglichst einfache, aber leistungsfähige Modelle entwickelt. Da die normalerweise verfügbaren Daten der Bauelemente sich aber auf andere Applikationen beziehen, mussten für alle Bauteile grundlegende Messungen im Zeitbereich der späteren Applikation gemacht werden. Für die nichtlinearen Induktivitäten wurde ein einfaches Testverfahren entwickelt um die Verluste bei sehr hohen Magnetisierungsgeschwindigkeiten zu bestimmen. Diese Messungen sind die Grundlagen für das Modell, das im Wesentlichen eine stromabhängige Induktivität beschreibt. Dieses Modell wurde für den „magnetic assist“ benützt, der die Einschaltverluste in den Halbleitern reduziert. Die Impulskondensatoren wurden ebenfalls mit einem in der Arbeit entwickelten Verfahren nahe den späteren Einsatzparametern vermessen. Dabei zeigte sich, dass die sehr gebräuchlichen Class II Keramikkondensatoren für diese Anwendung nicht geeignet sind. In der Arbeit wurden deshalb Class I Hochspannungs- Vielschicht- Kondensatoren als Speicherbank verwendet, die ein deutlich besseres Verhalten zeigen. Die eingesetzten Halbleiterelemente wurden ebenfalls in einem Testverfahren nahe den späteren Einsatzparametern vermessen. Dabei zeigte sich, dass nur moderne Leistungs-MOSFET´s für diesen Einsatz geeignet sind. Bei den Dioden ergab sich, dass nur Siliziumkarbid (SiC) Schottky Dioden für die Applikation einsetzbar sind. Für die Anwendung sind prinzipiell verschiedene Topologien möglich. Bei näherer Betrachtung zeigt sich jedoch, dass nur die C-C Transfer Anordnung die gewünschten Ergebnisse liefern kann. Diese Topologie wurde realisiert. Sie besteht im Wesentlichen aus einer Speicherbank, die vom Netzteil aufgeladen wird. Aus dieser wird dann die Energie in den Laserkopf über den Schalter transferiert. Aufgrund der hohen Spannungen und Ströme müssen 24 Schaltelemente in Serie und je 4 parallel geschaltet werden. Die Ansteuerung der Schalter wird über hochisolierende „Gate“-Transformatoren erreicht. Es zeigte sich, dass eine sorgfältig ausgelegte dynamische und statische Spannungsteilung für einen sicheren Betrieb notwendig ist. In der Arbeit konnte ein Betrieb mit realer Laserkammer als Last bis 6 kHz realisiert werden, der nur durch die maximal mögliche Repetitionsrate der Laserkammer begrenzt war.