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The current work is focused on the study of two surface modification plasma processes, (i) the active screen plasma nitriding (ASPN) and nitrocarburizing (ASPNC) for the hardening of ferrous surfaces and (ii) the microwave plasma assisted chemical vapor deposition (MW-PACVD) for the synthesis of single crystal and doped diamond. Conventional and active screen plasma nitriding processes have been investigated in a cylindrical, industrial scale ASPN reactor with a volume of about 1 m3, using low-pressure pulsed dc H2-N2 plasmas with admixtures of CH4 or CO2. The experiments were carried out (i) with the plasma at an internal model probe, (ii) with the plasma at the active screen (floated model probe) and (iii) with the plasma at the active screen and an additional plasma at the biased model probe. For deeper insights in ASPN and ASPNC processes, a laboratory scale plasma nitriding monitoring reactor, PLANIMOR, has been constructed. The main feature of this reactor is the linear configuration of the electrode setup combined with a tubular glass vessel, overcoming the experimental disadvantages of cylindrical laboratory scale ASPN reactors. With the help of infrared laser absorption spectroscopy (IRLAS) the rotational temperature of the stable molecules in the gas phase and the concentrations of the precursor, CH4, and the reaction products (NH3, HCN, C2H2, C2H4, CO, CH3) could be determined in both reactors, depending on the plasma power, the gas mixture, the plasma at the model probe and the admixture of CH4. Furthermore, the admixture of CO2 as the carbon containing precursor has been studied in the ASPN reactor leading to an additional reaction product H2O. The concentration of the molecular species has been found being in a range of 1012 to 1016 molecules cm-3. Also optical emission spectroscopy (OES) has been applied during the studies for analyzing the emission of the plasmas in the nitriding and nitrocarburizing processes. A similar behavior of the plasma chemistry in PLANIMOR comparing to that in the ASPN reactor has been found. Beside the plasma chemical investigations, both reactors have been used for the treatment of C15 steel samples. These samples have been analyzed with the help of GDOES resulting in the elements profile of the treated surfaces. It has been found that samples treated in PLANIMOR reach comparable nitriding results as samples treated in the ASPN reactor. Another focus of interest during the investigations about plasma nitrocarburizing has been the application of a carbon containing screen electrode as carbon source. For this purpose the carbon containing precursor and the steel screen have been substituted by a meshed carbon electrode, acting as the active screen. This change of the setup leads to a decrease of the NH3 production by a factor of 2.5 and an increase of the concentrations of HCN by a factor of 30 and of C2H2 by a factor of 70. The investigations of MW-PACVD processes used for diamond layer deposition have been carried out in a jacketed stainless steel reactor (JR), dedicated to the deposition of single crystalline diamond under high pressure and plasma power conditions. Using H2-plasmas with admixtures of CH4 and B2H6, the experiments were carried out in order to analyze the dependence of the plasma chemistry on several parameters, such as plasma power, pressure and gas mixture, in a wide pressure (p = 25…270 mbar) and power range (P = 0.6…4 kW). Using IRLAS the concentrations of six molecular species (B2H6, CH4, C2H2, C2H4, C2H6, CH3) have been monitored. With the help of OES the concentration of atomic boron could be determined. The concentrations of the detected molecular and atomic species were found to be in a range of 1010 to 1017 cm-3. With the help of the line-ratio-method the rotational temperature of the stable molecules has been determined. The temperature increased with pressure and power from 340 to 425 K. Using the Doppler broadening of the absorption line of CH3 at ν = 612,413 cm-1, the gas temperature has found to be Tg = (2000 ± 200) K under lower pressure and power conditions. For the H2-CH4 gas mixture, the experimental obtained molecular densities have been compared to those of a 1D-radial thermochemical model. The calculated radial densities have been integrated axially. For the same range the chemical processes in JR have been compared with those in a bell-jar (BJ) reactor. The hydrocarbon chemistry in JR has found to be similar to that in a BJ reactor.
The aim of this thesis is to concentrate on the investigation of these ROS&RNS composition distribution and their production pathways in the gas phase produced by a plasma jet. By understanding the physical mechanisms behind the generation of the ROS&RNS a precise tuning and design of the composition distribution in the gas phase can be achieved. One crucial physical parameter is the dissipated power inside the plasma. Only if this parameter is known a meaningful comparison of different feed gas settings is possible. Therefore, a concept for measuring the dissipated power inside the plasma for the modified micro-scaled atmospheric pressure plasma jet( µAPPJ) is designed. Additionally, due to achievements within this thesis it is now possible to ignite a homogeneous discharge in argon and helium within the geometry of the µAPPJ. The used feed gas is a determining factor concerning the electron energy distribution function and consequently influencing the production mechanism of the ROS&RNS. First of all, the electrical characterisation of the modified µAPPJ was performed including the alpha-to-gamma transition. It is shown that the alpha-to-gamma transition power is increasing with increasing frequency. For the first time it is now feasible to investigate the influence of the dissipated power on the neutral gas temperature, the metastable atom densities and the ROS&RNS production for the modified µAPPJ with argon and helium as feed gas. Due to the possibility of changing the feed gas and controlling the dissipated power a fundamental insight into the production mechanism of the ROS&RNS generated by the plasma jet is achieved. With rising dissipated power the temperature and the metastable densities as well as the ozone and nitrogen dioxide concentrations are increasing. By adding molecular oxygen and nitrogen to the feed gas of a plasma jet the ROS&RNS composition can be tuned. However, also the dissipated power is changed by the small amount of admixtures. Due to the developed dissipated power measurements within this thesis it was possible to disentangle the influence of the admixture on the power and on the ROS&RNS production. If the dissipated power is fixed for the µAPPJ with argon and helium feed gas, respectively, the highest amount of ozone was measured with oxygen admixture in an argon discharge, the highest amount of dinitrogen pentoxide with nitrogen admixture in an argon discharge and the highest amount of nitrogen dioxide with nitrogen admixture in a helium discharge. Beyond the influence of the dissipated power and the molecular admixture on the ROS&RNS production the feed gas temperature is a crucial parameter for the corresponding chemical reactions. By changing this parameter the distribution of ozone and nitrogen dioxide can be tuned precisely in such a way that with increasing temperature the ozone density goes down and the nitrogen dioxide density rises. Another determinant for the ROS&RNS composition produced by an atmospheric pressure plasma jet is the influence of ambient air. If the ambient air is changing from pure nitrogen to pure oxygen atmosphere the ozone density produced by the plasma jet is increasing. For the same conditions the nitrogen dioxide has a maximum at an oxygen-to-nitrogen ratio of 1:1. To avoid the influence of the ambient air on the reactive species production the afterglow of the µAPPJ was prolonged with a glass tube. By increasing the amount of molecular admixtures to the feed gas with each in equal quantities a totally different ROS&RNS composition can be obtained compared without the glass tube. It figures out that for small molecular admixtures the reactive species composition is nitrogen dominated and for higher admixtures it is oxygen dominated. Consequently, by shielding the ambient air from the active effluent and by admixing molecular oxygen and nitrogen the ROS&RNS composition can be designed.
Im ersten Teil der Arbeit wird der erfolgreiche Aufbau einer Diagnostik zur quantitativen Bestimmung von Oberflächenladungsdichten beschrieben. Das Messprinzip bedient sich des elektro-optischen Pockelseffekts eines BSO-Kristalls, der in der Entladungszelle als Dielektrikum eingesetzt ist. Diese Methode arbeitet zeitlich und lateral aufgelöst, was die Untersuchung der Dynamik von Oberflächenladungen auf drei verschiedenen Zeitskalen ermöglicht. Die erste Zeitskala liegt in der Größenordnung von einigen 100 ns. Damit kann erstmals die Deposition von elektrischer Ladung auf einer dielektrischen Oberfläche während eines Entladungsdurchbruchs beobachtet werden. Die Deposition beginnt im Zentrum eines zuvor deponierten Ladungsspots. Die Polarität der neudeponierten Ladung ist der des ursprünglichen Ladungsspots entgegengesetzt. Die Folge ist, dass die absolute Ladungsdichte im Zentrum im Verlauf einiger hundert Nanosekunden kleiner wird als in den Randbereichen. Der Umladungsprozess wird so lange fortgesetzt, bis das elektrische Feld der neu deponierten Ladungen dem äußeren Feld so stark entgegenwirkt, dass die Spannung zur Aufrechterhaltung der Entladung unterschritten wird und die Entladung erlischt. Die zweite untersuchte Zeitskala liegt in der Größenordnung der Periodendauer der externen Spannung. Im Nulldurchgang der Spannung liegen zeitlich stationäre Ladungsdichteverteilungen auf dem Dielektrikum vor. Die Geometrie eines mittleren Ladungsspots wird in Abhängigkeit der anliegenden Spannungen und des Gasdrucks untersucht. Einerseits ist der Spotradius abhängig von den Ionisationsprozessen im Volumen, weil die Dichte der Raumladungen die Stärke des Elektronenfokus in das Innere der Entladung steuert. Andererseits wird die Spotbildung durch eine laterale Drift von Ladungsträgern kurz vor der Oberfläche aufgrund des elektrischen Feldes deponierter Ladungsträger beeinflusst. Die dritte untersuchte Zeitskala liegt in einer Größenordnung von Sekunden. Im Fall einer initial homogenen Oberflächenladungsverteilung nimmt die mittlere Ladungsdichte in einer Größenordnung von Sekunden monoton ab. Dieser Prozess stellt einen Ladungsabbau dar, dessen zeitliches Verhalten durch zwei überlagerte Exponentialfunktionen beschreiben ließ. Dadurch werden zwei Ladungsträgerpopulationen im BSO angenommen, die verschieden abgebaut werden. Im Fall einer initial inhomogenen Ladungsdichteverteilung wird ein Transport elektrischer Ladung auf der BSO-Oberfläche in einer Größenordnung von Sekunden beobachtet. Es wird weiterhin erstmals die durch einen Atmosphärendruck-Plasmajet deponierten Ladungen auf BSO zeitaufgelöst gemessen. Die zeitliche Entwicklung der Oberflächenladungen kann mit der Messung des elektrischen Stroms an einer der Ringelektroden des Jets korreliert werden. Dadurch wird geschlossen, dass der Ladungsaustauch nicht direkt durch einen Bullet verursacht wird. Er erzeugt stattdessen einen elektrisch leitfähigen Kanal zwischen der Düse des Jets zur BSO-Oberfläche. Infolgedessen kann Ladung, die sich auf der Innenseite der Jetkapillare befindet, auf den BSO-Kristall transportiert werden. Im zweiten Teil der Arbeit werden Kenngrößen entwickelt, die den Ordnungszustand einer aus Einzelobjekten zusammengesetzten Entladungsstruktur quantitativ beschreiben. Die Kenngrößen werten dabei die laterale Leuchtdichteverteilung der Entladungsemisssion, u.a. auf Basis der Tripel-Korrelationsfunktion. Dabei werden zwei separate Bifurkationsspannungen zwischen einer hexagonalen und einer ungeordneten Anordnung beobachtet: Bei der Verringerung der Spannung wird zunächst der Bifurkationspunkt der azimutalen Ordnung durchlaufen und anschließend der Bifurkationspunkt der radialen Ordnung. Die Systeme gehen jeweils in einen Zustand geringerer Ordnung über. Die Ursache des Ordnungsverlusts ist das zunehmende Fehlen von Entladungsspots, was im Mittel zu einer geringeren Wechselwirkung der Spots untereinander führt und das System an Freiheitsgraden gewinnt. Im dritten Teil dieser Arbeit wird erstmals ein Ansatz verfolgt, der die Steuerung lateral strukturierter Entladungen ermöglicht. Dafür wurde ein Aufbau konstruiert, bei dem ein gekühlter Halbleiter als Dielektrikum in der Entladungszelle dient. Dessen externe Beleuchtung führt bei einer anliegenden Spannung zu einer Änderung des Spannungsteilerverhältnisses der kapazitiven Elemente und schließlich zu einer lokalen Erhöhung der Spannung über dem Entladungsraum. Die Größe und Leuchtintensität der durch die Beleuchtung gezündeten Entladung ist stark abhängig von der beleuchteten Fläche, der Leistungsdichte der Beleuchtung und der anliegenden Spannung.
The realistic description of the physical processes in quantum optical systems requires careful investigation of the interplay between quantum dissipation and entanglement generation. In this thesis, we have considered from a microscopical perspective the entanglement generation in semiconductor microcavities at short times, the dissipative evolution of the quantum harmonic oscillator towards a stationary state, and the nonclassical properties of the asymptotic states of different photonic systems. In our description of two-dimensional semiconductor microcavities we showed that two different pump configurations can be used to stimulate parametric scattering processes between polaritons that lead to the generation of internal polariton entanglement. A moving polariton induces an ultrafast electric polarisation as a source of light that serves as a probe of the internal entanglement properties. The identification of the nonclassical correlations of the emitted photons is based on entanglement witnesses that can also be used for the quantification of entanglement, e.g., in terms of the Schmidt number. The simultaneous creation of multiple branch entangled photon pairs renders it possible to generate an arbitrary number of entangled qubit states. By adjusting the number of pump beams and their spectral properties, one can optimize the Bell-type correlations within one ore more of those entangled qubits. Quantum dissipation can be studied in a microscopic setting with the well known model of a central oscillator coupled linearly to a bath of harmonic oscillators. We showed that equilibration of the central oscillator is the generic behaviour, which is prevented only in situations in which the classical oscillator equation of motion possesses undamped oscillatory solutions. Because of its localised spectral function, the infinite linear harmonic chain is an example for this behaviour. Thermalisation of the central oscillator depends on additional conditions. Equipartition of kinetic and potential energies requires the weak damping limit but is independent on the initial condition. The initial bath preparation enters the asymptotic temperature. Essential for the thermalisation of several oscillators is, that the asymptotic temperature is independent of the central oscillator frequency, which is fulfilled if the initial bath energy distribution matches that of a thermal state. Nevertheless, because this condition involves the sum of kinetic and potential energy, full thermalisation is possible in environments with nonthermal individual energy distributions, even in those far from thermal equilibrium. We showed, that even in the absence of full thermalisation the fluctuations of the central oscillator follow a generalised fluctuation dissipation theorem that reduces to the well known thermal result whenever the central oscillator thermalises in the strict sense. Photonic systems such as two-level emitters in a cavity or semiconductor microcavities are employed in quantum optics applications. The realistic theoretical description of the physical processes requires the use of methods from quantum optics as well as fromthe field of quantum dissipation. Our focus was on the correct theoretical description of the emission from systems with strong coupling. The analysis of the light generated by emitters in a cavity reveals a non-trivial dependence of the photon statistics on the light-matter coupling and temperature. Clearly identifiable parameters regimes with sub- and super-Poissonian photon statistics appear at strong and ultrastrong coupling, and lie immediately next to each other. We provided an approximate rule to relate the emission characteristics for a single emitter to those obtained for few emitters under an appropriate scaling of the emitter-cavity coupling. In accordance with this rule, the generation of noncassical light is easier with more emitters. The outright failure of the quantum optical master equation at predicting any of the features observed in the emission statistics shows that using the correct master equation is essential in all situations. Including internal dissipation channels we showed that a continuously driven semiconductor microcavity generates entangled light even at infinitely large times. The entanglement generation is thus robust against decoherence under realistic experimental conditions. Because the pair correlations between polaritons can sustain over long times and distances in these solid-state devices, a microcavity is a highly efficient source of entangled light and therefore well suited for quantum optics applications.
In the last decade a new domain has developed in plasma physics: plasma medicine. Despite the successes that have already been achieved in this exciting new field, the interaction of plasmas with “biological materials” is not yet fully understood. Further investigations in particular with respect to the properties of the applied plasmas sources are therefore essential in order to decode this complex interaction process. Currently, a great variety of different discharge types are used in plasma medical investigation which are generally are operated in noble gases like helium and argon or with dry air. In the present work, the main focuses is on the diagnostics of reactive oxygen and nitrogen species (RONS) resulting from the plasma chemistry of an argon radio-frequency (RF) atmospheric pressure plasma jet (APPJ) and its interaction with the ambient atmosphere. To conduct this study, a commercially available plasma device, so-called kinpen is used due to its technical development maturity and its accessibility on the market. As a method of choice, diagnostic techniques are based on optical spectroscopy known to be a reliable tool to investigate plasmas. Consequently, three complementary optical laser diagnostics, namely quantum cascade laser absorption spectroscopy (QCLAS), laser induced fluorescence (LIF) and planar single shot LIF (PLIF), have been successfully applied to the plasma jet itself or its effluent. All of these diagnostics offer a high species selectivity and an excellent spatial and temporal resolution. They are used in this work for i) the characterization of the plasma chemical dynamics with respect to the generation of biological active RONS – in particular for the case of N2 and O2 admixtures. ii) the measurement of the NO density profile in the plasma effluent iii) the investigation of the flow characteristics of the neutral gas component (laminar vs. turbulent) and its influence on the plasma chemistry. Numerical analysis have been carried out in collaboration with PLASMANT (University of Antwerp) via kinetic simulations of the entire plasma chemistry. Expectingly, atomic oxygen (O) and nitric oxide (NO) turn out to be precursors of ozone (O3) and nitric dioxide (NO2). However, it was intriguing to unveil that atomic oxygen and nitrogen metastable (N2(A)) play together a key part --as intermediate species-- in the generation of more stable RONS, e.g. NO. The absolute density of NO space resolved was measured by LIF and absolutely calibrated molecular beam mass spectrometer. LIF was used to determine relative density of OH radical in the plasma plume. 2D-LIF was used to investigate the gas flow pattern with OH as a flow tracer. The results are discussed in details and show different operating mode of the jet, e.g. laminar or turbulent and that the plasma influences these regimes. The first detection and relative measurement by LIF of nitrogen metastable (N2(A)) produced by an argon APPJ is also shortly reported in this work. The outcome of this thesis will bring new insights in the field of argon APPJs chemistry and its interaction with the ambient atmosphere which can be valuable to support plasma modelling and to consider for the applications in plasma medicine.
Ein System zu Abscheidung intermetallischer Cu-Ti basierter Schichten durch Magnetronsputtern wurde entworfen, aufgebaut und plasmadiagnostisch charakterisiert. Die duale, extern schaltbare Leistungselektronik mit hoher Parallelkapazität erlaubt den Pulsbetrieb beider Magnetrons gegeneinander mit beliebiger Frequenz f, Tastgrad t_a/T und Pulsverzögerung t_d. Auf diese Weise kann neben konventionellen Pulsmodi (dual-MS: f = 4.6 kHz, t_a/T = 50 %) das Hochenergieimpulsmagnetronsputtern (dual-HiPIMS: f = 100 Hz, t_a/T = 1 %) realisiert werden. Außerdem können die unterschiedlichen Sputterausbeuten von Ti und Cu durch individuell einstellbare mittlere Entladungsströme kompensiert werden. Die Entladungscharakteristika zeigen besonders hohe temporäre Ströme (I > 50 A) während HiPIMS. Langmuir-Sondenmessungen bestätigen höhere Elektronendichten (n_e = 10^18 m^-3) und eine breitere Elektronenenergieverteilung im Vergleich zu dual-MS. Als Folge kommt es zur verstärkten Ionisierung und Anregung von Plasmaspezies, nachgewiesen durch optische Emissionsspektroskopie (OES). Spektral integrierte OES wurde zur Beschreibung der räumlich und zeitlichen Entladungsentwicklung herangezogen, während mittels Gegenfeldanalysator die für die Schichtbildung wichtige Ionengeschwindigkeitsverteilung zeitaufgelöst erfasst wurde. Die gewonnenen Schichten wurden röntgenographisch analysiert und deren Eigenschaften in Hinblick auf ihre Bildung unter verschiedenen Entladungsmodi gedeutet. Schichtdicke, Kristallinität und Dichte zeigen eine klare Abhängigkeit vom Entladungsmodus mit vorwiegend höherer Güte durch HiPIMS. Die Variation des mittleren Cu-Entladungsstromes erlaubt Einfluss auf die Schichtzusammensetzung, maßgeblich für praktische Anwendungen. Des Weiteren wurden Kompositschichten auf Basis von Cu-Nanopartikeln (Cluster) eingebettet in einem dielektrischen Matrixmaterial (TiO2) synthetisiert und untersucht. Zunächst wurde das Cu-Clusterwachstum durch ein neues Buffergas-Pulsverfahren zeitaufgelöst untersucht und die damit einhergehende Performance der Partikelquelle auf Basis eines einfachen Modells evaluiert. Als Resultat kann die Größen-/Massenverteilung und damit die Eigenschaften der Cluster auf einfache Weise beeinflusst werden. Schließlich wurden die Cu-TiO2-Kompositschichten in einem eigens angefertigten Co-Depositionssystem erstellt. Dabei werden die aus der Gasaggregationsquelle emittierten Cu-Cluster simultan zu reaktiv (mit O2) gesputtertem Ti abgeschieden. Separat abgeschiedene Cu-Cluster zeigen vorwiegend polykristallines fcc-Cu, dass an Luft oder unter Zugabe von molekularem O2 oberflächlich Cu2O ausbildet. Während auch das separat reaktiv abgeschiedene Ti Titandioxid (TiO2) bildet, weist das Cu im Nanokomposit grundlegende strukturelle Unterschiede auf: Im Komposit liegt kein metallisches Cu mehr vor, stattdessen ist es vollständig zu CuO konvertiert. Dies ist auf die Anwesenheit eines reaktiven Sauerstoffplasmas beim Co-Depositionsprozess zurückzuführen. Es konnte gezeigt werden, dass molekularer O2 lediglich eine diffusionsbegrenzte Menge Cu2O erzeugt, während entladungsaktivierte Sauerstoffspezies zur völligen Durchoxidation der Cu-Cluster zu CuO führt. Dies ist eine wichtige Erkenntnis für die zukünftige Herstellung ähnlicher Komposite.
Magnetic reconnection is a ubiquitous phenomenon observed in a wide range of magnetized plasmas from magnetic confinement fusion devices to space plasmas in the magnetotail. The process enables the release of accumulated magnetic energy by rapid changes in magnetic topology, heating the plasma in the vicinity of the reconnection site, generating fast particles and allowing a wealth of instabilities to grow. This thesis reports on the results from a newly constructed linear, cylindrical and modular guide field reconnection experiment with highly reproducible events, VINETA.II. A detailed analysis of the reconnecting current sheet properties on a macroscopic and microscopic scale in time and space is presented. In the experiment, four parallel axial wires create a figure-eight in-plane magnetic field with an X-line along the central axis, as well as an axial inductive field that drives magnetic reconnection. Particle-in-cell simulations show that the axial current is limited by sheaths at the boundaries and that electrostatic fields along the device axis always set up in response to the induced electric field. Current sheet formation requires an additional electron current source, realized as a plasma gun, which discharges into a homogeneous background plasma created by a rf antenna. The evolution of the plasma current is found to be dominantly set by its electrical circuit. The current response to the applied electric field is mainly inductive, which in turn strongly influences the reconnection rate. The three-dimensional distribution of the current sheet is determined by the magnetic mapping of the plasma gun along the sheared magnetic field lines, as well as by radial cross-field expansion. This expansion is due to a lack of equilibrium in the in-plane force balance. Resistive diffusion of the magnetic field by E=η j is found to be by far insufficient to account for the high reconnection rate E=-dΨ/dt at the X-line, indicating the presence of large electrostatic fields which do not contribute to dissipative reconnection. High-frequency magnetic fluctuations are observed throughout the current sheet which are compared to qualitatively similar observations in the Magnetic Reconnection Experiment (MRX, Princeton). The turbulent fluctuation spectra in both experiments display a spectral kink near the lower hybrid frequency, indicating the presence of lower hybrid type instabilities. In contrast to the expected perpendicular propagation of mainly electrostatic waves, an electromagnetic wave is found in VINETA.II that propagates along the guide field and matches the whistler wave dispersion. Good correlation is observed between the local axial current density and the fluctuation amplitude across the azimuthal plane. Instabilities driven by parallel drifts can be excluded due to the large required drift velocities or low resulting phase velocities that are not observed. It is instead suggested that a perpendicular, electrostatic lower hybrid mode indeed exists that resonantly excites a parallel, electromagnetic whistler wave through linear mode conversion. The resulting fluctuations are found to be intrinsic to the localized current sheet and are independent of the slower reconnection dynamics. Their amplitude is small compared to the in-plane fields, and have a negligible contribution to anomalous resistivity through momentum transport in the present parameter regime.
The collisionless tearing mode is investigated by means of the delta-f PIC code EUTERPE solving the gyrokinetic equation. In this thesis the first simulations of electromagnetic non-ideal MHD modes in a slab geometry with EUTERPE are presented. Linear simulations are carried out in the cases of vanishing and finite temperature gradients. Both cases are benchmarked using a shooting method showing that EUTERPE simulates the linearly unstable tearing mode to a very high accuracy. In the case of finite diamagnetic effects and values of the linear stability parameter Delta of order unity analytic predictions of the linear dispersion relation are compared with simulation results. The comparison validates the analytic results in this parameter range. Nonlinear single-mode simulations are performed in the small- to medium-Delta range measuring the dependency of the saturated island half width on the equilibrium current width. The results are compared with an analytic prediction obtained with a kinetic electromagnetic model. In this thesis the first simulation results in the regime of fast nonlinear reconnection~(medium- to high-Delta range) are presented using the standard gyrokinetic equation. In this regime a nonlinear critical threshold has been found dividing the saturated mode from the super-exponential phase for medium-Delta values. This critical threshold has been proven to occur in two slab equilibria frequently used for reconnection scenarios. Either changing the width of the equilibrium current or the wave number of the most unstable mode makes the threshold apparent. Extensive parameter studies including the variation of the domain extensions as well as the equilibrium current width are dedicated to a comprehensive overview of the critical threshold in a wide range of parameters. Additionally, a second critical threshold for high-Delta equilibria has been observed. A detailed comparison between a compressible gyrofluid code and EUTERPE is carried out. The two models are compared with each other in the linear regime by measuring growth rates over wave numbers of the most unstable mode for two setups of parameters. Analytical scaling predictions of the dispersion relation relevant to the low-Delta regime are discussed. Employing nonlinear simulations of both codes the saturated island half width and oscillation frequency of the magnetic islands are compared in the small-Delta range. Both models agree very well in the limit of marginal instability and differ slightly with decreasing wave vector. Recently, the full polarisation response in the quasi-neutrality equation was implemented in EUTERPE using the Padé approximation of the full gyrokinetic polarisation term. Linear simulation results including finite ratios of ion to electron temperature are benchmarked with the dispersion relation obtained from a hybrid model. Finite temperature effects influence the saturated island width slightly with increasing ion to electron temperature ratio which has been verified by both models.
This thesis investigated dielectric barrier discharges (DBDs) in N2-O2 gas mixtures at atmospheric pressure, with a focus on the gas discharge physics. The main goal was to evaluate whether possible control mechanisms exist that can manipulate the breakdown and the development of DBDs, especially for pulsed operation. To examine the pre-breakdown phase, the actual breakdown and the main DBD development, DBDs in a double-sided, single filament arrangement with a 1 mm discharge gap were investigated by means of electrical and optical diagnostics with high resolutions. Spectrally- and temporally-resolved iCCD pictures (2D in space), spectrally- and spatio-temporally-resolved streak camera and CCS images (1D in space) were simultaneously recorded accompanied by a full electrical characterisation with fast voltage and current probes. Sinusoidal- and pulsed-driven DBDs were found to have a qualitatively similar spatio-temporal development, i.e. a cathode-directed ionisation front (v ~ 10^6 m/s, positive streamer mechanism), followed by a transient glow-like phase in the gap. For sinusoidal operation, the slope of the applied voltage is flat (dU/dt ~ 1 V/ns) compared to pulsed operation (dU/dt ~ 100 V/ns). Thus, during the longer pre-phase of the sine-driven DBD, many more charge carriers were generated, in contrast to the pulsed-driven DBDs, where the pre-phase is limited by the short voltage rise time. Consequently, just before the breakdown occurs, the charge carrier density is higher for sine-driven DBDs, i.e. the positive streamer starts in a highly pre-ionised environment, which leads to a lower propagation velocity. In addition to limiting the pre-phase (lower pre-ionisation), the steep voltage slope of the pulsed DBD amplifies the streamer breakdown because the applied voltage rises significantly during its propagation. Therefore, the transferred electrical charge and the electrical power of a single DBD can be controlled by the applied voltage amplitude, but only in pulsed operation. In addition to the effects of different voltage slope steepness, the pulse width is an excellent parameter in the pulsed operation to set the pre-ionisation, by shifting the DBDs into the after-glow of the previous discharge using asymmetrical HV pulse waveforms. The subsequent DBDs ignite in different pre-ionised conditions, defined by the residual charge carrier densities in the gap that originated from the previous DBD. The breakdown characteristics of these DBDs could be controlled down to the fundamental level. This thesis has described for the first time four different breakdown regimes in single filament DBDs for 0.1 vol% N2 in O2 and connected them to the processes during their pre-phases. The “classic” DBD development (a cathode-directed streamer followed by a transient glow discharge) could be controlled in a certain range, followed by a transition first to a breakdown regime featuring a simultaneous propagation of a cathode- and an anode-directed streamer, and finally to a reignition of the previous DBDs without any propagation, just by reducing the pulse width (time between two subsequent DBDs), i.e. increasing the pre-ionisation level. All differences between the DBDs at rising and falling slopes could be explained by the different pre-conditions in the gap. The O2 concentration in the N2-O2 gas mixtures offers another way of controlling the pre-ionisation. Due to the electron attachment as a consequence of the electronegativity of oxygen, the electron density decreases for higher O2 admixtures. Furthermore, the differences in the first Townsend ionisation coefficient and in the photo-ionisation between N2 and O2 influence the DBD behaviour as well. To some extent, some of the reported effects achieved by varying the pulse width at a fixed O2/N2 ratio were also observed for a fixed pulse width and changing O2 concentration. Hence, the response of the DBD properties to changing pre-ionisation levels seems to be a general principle of DBD control. Additional effects of the O2/N2 ratio, such as an increasing DBD inception jitter or higher streamer velocities, were also reported. Finally, a reverse of the effects induced by the O2 admixture such as DBD emission duration or DBD inception delay, was observed for O2 concentrations below 0.01 vol%, and were especially pronounced at a pressure of 0.5 bar. For 0.1 vol% O2 in N2, a minimal electron recombination rate was found, which can be explained by the different decay and recombination rates of positive nitrogen and oxygen ions. These different rates effect the charge carrier dynamics and consequently, the pre-ionisation in the gap. In conclusion, this investigation has highlighted the importance of volume memory processes on the breakdown and development of single filament DBDs at elevated pressures.
The present thesis deals with dynamic structures that form during the expansion of plasma into an environment of much lower plasma density. The electron expansion, driven by their pressure, occurs on a much faster time scale than the ion expansion, owed to their mobility. The high inertia of the ions causes the generation of an ambipolar electric field that decelerates the escaping electrons while accelerating the ions. The ambipolar boundary propagates outwards and forms a plasma density front. For a small density differences, the propagation of the front can be described with the linear ansatz for ion acoustic waves. For a large density differences, experiments have shown that the propagation velocity of such a front is still related to the ion sound velocity. However, the reported proportionality factors are scattered over a wide range of values, depending on the considered initial and boundary conditions. In this thesis, the dynamics during plasma expansion are studied with the use of experiments and a versatile particle-in-cell simulation. The experimental investigations are performed in the linear helicon device Piglet. The experiment features a fast valve, which is used to shape the neutral gas density profile. During the pulsed rf-discharges, plasma is generated in the source region and expands collisionless into the expansion chamber. The computer simulation is tailored very close to the experiment and provides a deeper insight in the particle kinetics. The experimental results show the existence of a propagating ion front. Its velocity is typically supersonic and depends on the density ratio of the two plasmas. The ion front features a strong electric field. The front can have similar properties to a double layer is not necessarily a double layer by definition. The computer simulation reveals that the propagating electric field repels the downstream ambient ions. These ions form a stream with velocities up to twice as high as the front velocity. The observed ion density peak is due to the accumulation of the repelled ions and is located at their turning point. The ion front formation depends strongly on the initial ion density profile and is part of a wave-breaking phenomenon. The observed front is followed by a plateau of little plasma density variation. This could be confirmed for the expansion experiment by a comparison with virtual diagnostics in the computer simulation. The plateau has a plasma density determined by the ratio between the high and low plasma density. It consists of streaming ions that have been accelerated in the edge of the main plasma. The presented results confirm and extend findings obtained by independent numerical models and simulations.
The central aim of this thesis was the investigation of protein/polyanion interaction using circular dichroism (CD) spectroscopy, enzyme immune assay (EIA), isothermal titration calorimetry (ITC) and flow cytometry (FC). A further aim was to understand why an endogenous protein becomes immuno-genic when forming a complex. The focus was on the protein platelet factor (PF4), which gained wide interest in the clinical field, due to its role in the life-threatening, immune-driven, adverse drug effect heparin-induced thrombocytopenia (HIT). PF4 is a small homotetrameric chemokine with several basic amino acids on its surface, forming a positively charged ring. The antibodies that are formed during HIT recognize an epitope exposed on PF4, when it is in a complex with heparin at a certain molar ratio at which, PF4 tetramers are aligned on the heparin and forced into close approximation. The main results and conclusions of the thesis are summarized below: 5.1 Evolutionary Conservation of PF4 (Paper I – PF4/Evolution) By carrying out an amino acid sequence survey we found that the positively charged amino acids contributing to the heparin binding site on the surface of PF4 and related proteins are highly conserved in all vertebrates, including fish species. PF4 interacts with the phospholipid lipid A, the innermost part of the lipopolysaccharide (LPS) of Gram negative bacteria. We showed that the shorter the sugar chain of the O antigen, outer and inner core of the LPS were the more PF4 was binding. The interaction of PF4 with lipid A is inhibited by heparin, suggesting that the amino acids known to contribute to heparin binding are also involved in binding to lipid A. 5.2 PF4 Interaction with Polyanions (PA) of varying Length and Degree of Sulfation (Paper II – PF4/PA) CD spectroscopy was found to be a powerful technique to monitor structural changes of PF4 caused by binding to various clinically relevant polyanions. Therefore PF4 was titrated with different PA to investigate the dependencies: i. impact of the PF4:PA molar ratio, ii. degree of polymerization of the PA and iii. degree of sulfation of the PA. In all cases, exposure of HIT-relevant epitope(s) was only observed for PA that also induced changes in secondary structure of PF4. A comparison of results of an immune ¬assay with CD spectroscopic data showed that the extent of complex anti¬genicity correlates well with the magnitude of changes in PF4 secondary structure, and that the structural changes of PF4 have to exceed a certain threshold to achieve PF4/PA complex antigenicity. These findings allowed us to calculate expectation intervals for complex antigenicity solely using CD spectroscopic data. To our knowledge, this was the first demonstration that the capability of drugs to induce antigenicity of PF4 can be assessed without the necessity of in vivo studies or the use of antibodies obtained from immunized patients specific for the antigens. The antigenicity of PF4 in complex is not restricted to negative charges originating from sulfate groups, PA with phosphate groups are also capable (binding to phospholipids). We investigated inorganic polyphosphates (polyP) with a chain length of 75 Pi and showed that the induced secondary structural changes are even higher compared to the changes induced by the different heparins and that the PF4/P75 complexes are antigenic as well. 5.3 PF4 Interaction with defined oligomeric Heparins (Paper III – PF4/defined Heparins) We tested highly purified, monodisperse heparins. In contrast to the clinically relevant but relatively undefined (high polydispersity index) glycosamino glycans reported in paper II (PF4/PA). The defined heparins induced higher secondary structural changes. Here we showed for the first time that strong conformational changes during PF4/PA complex formation are necessary but not sufficient for to the expression of the anti-PF4/heparin antibody binding site. Also, the size of the complexes is not the only prerequisite for anti-PF4/heparin antibody binding (tested by atomic force microscopy). By ITC we found that antigenicity is only induced if the PF4/PA complex has a high binding enthalpy and the complex formation leads to a negative change in entropy. 5.4 PF4/Polyphosphates (polyP) Complex Antigenicity and Interaction with Escherichia coli (E. coli, Paper IV – PF4/polyP) PolyP with chain lengths of 45 Pi and 75 Pi induced remarkable secondary structural changes in the PF4 molecule, thereby exposing the epitope recognized by anti-PF4/heparin antibodies. The induced conformational changes were similar to the changes induced by the defined heparins. Again a high binding enthalpy was observed but here in connection with a positive change in entropy. Further we showed that polyP (≥45 Pi) enhance PF4 binding to the surface of Gram negative E. coli at intermediate concentration and disrupt the binding at elevated polyP concentrations. The increased amounts of PF4 on the bacterial surface also improved the binding of anti-PF4/heparin antibodies and thereby the phagocytosis of the bacteria by poly¬morpho¬nuclear leucocytes. 5.5 Nucleic acid based Aptamers induce structural Changes in the PF4 Molecule (Paper V – PF4/Aptamer) Nucleic acids are another class of molecules containing phosphate groups. Especially after cell damage their extra¬cellular concentration can be locally quite high (>2 mg/ml). We found that certain aptamers form complexes with PF4 and thereby inducing anti-PF4/aptamer antibodies which cross-react with PF4/heparin complexes. Moreover by CD spectroscopy we showed that the protein C-aptamer caused similar secondary structural changes of PF4 like heparin, but already at much lower concentration. The maximally induced changes by the protein-C aptamer were even higher and persisted over a broader concentration range. 5.6 Protamine Interaction with Heparin (Paper VI – PS/Heparin) After the intensive investigation of the complex formation between PF4 and many different classes of PA we assessed another protein for structural changes upon complex formation with heparin. Protamine (PS) a protein in routinely used in post-cardiac surgery to reverse the anticoagulant effects of heparin was found to unfold but not to refold with increasing concentration of PA in solution. 5.7 Conclusion and Outlook When starting this thesis, it was believed that repetitive structures formed by PF4 on a heparin chain mold the epitope recognized by antibodies inducing HIT. These repetitive structures might exhibit similarities with viral capsids and are therefore recognized by the immune system of some patients. We found that induced by the close approximation PF4 changes its conformation, thereby exposing a neoepitope. The conserved positively charged amino acids of the heparin binding site and the involvement of these amino acids in the binding to lipid A confirm our hypothesis of PF4 as part of an ancient immune-mediated host defense mechanism. As possible consequence of the “primitive mechanism of defense” the highly variable O-antigens of LPS might have significantly contributed to an efficient escape mechanism by hiding the structures that made the bacteria vulnerable. In turn polyP might be an adaption of the host improve pathogen recognition by PF4 and further by antibodies inducing phagocytosis of the PF4-marked objects. Although shown only for PF4 and PS, our findings might be applicable to other proteins that also express epitopes upon changes in their secondary structure. Our physicochemical methods may further be applied: i. to drug development for the prediction of antigenicity induced by polyanionic drugs, ii. to guide the development of synthetic heparins and other polyanion based drugs, e.g. aptamers, that do not lead to HIT and iii. to provide relevant aspects for other biological functions of heparins.
Die Forschung an mikrowelleninduzierten Atmosphärendruckplasmen am INP führte zu verschiedenen potentiellen Applikationen. Dabei besitzt die mikrobiologische Dekontamination sowohl von thermolabilen Medizinprodukten als auch von Lebensmitteln schon zum jetzigen Zeitpunkt ein hohes industrielles Anwendungspotential. Den aufgeführten Anwendungen gemeinsam ist, dass für eine erfolgreiche Weiterentwicklung der Prozesse, sowie der Plasmaquelle, ein grundlegendes Verständnis der vorliegenden dynamischen Mikrowellenplasmawechselwirkung notwendig ist. Durch den begrenzten diagnostischen Zugang der zu untersuchenden Plasmaquelle wird ein kombinierter Ansatz aus diagnostischen Methoden und Modellierung gewählt. Die Entladung wird in Argon bei reduziertem Druck (ab 10 mbar) zur Vereinfachung des Modells betrieben. Daher musste die Plasmaquelle für diesen Einsatz weiterentwickelt werden. Dieses beinhaltet die Neuauslegung der Prozesswärmeabfuhr, auf Grund der nicht oder nur teilweisen Anwendbarkeit von etablierten Verfahren im Atmosphärendruck (hohe Gasflüsse, Wasserkühlung). Die Plasmamikrowellenwechselwirkung dieser Quelle ist anschließend mit Methoden zur Charakterisierung des Plasmas und des Mikrowellenfeldes für unterschiedliche Arbeitspunkte in Druck und Leistung untersucht worden. Zur Bestimmung der Elektronendichte des Plasmas wurde ein frequenzvariables Mikrowelleninterferometer auf Basis eines Vektornetzwerkanalysators erstmalig etabliert. Dieses neue Messsystem wurde im Vorfeld detailliert untersucht, um das korrekte Zusammenspiel aller Komponenten zu überprüfen. In diesem Zusammenhang wurde ein frequenzaufgelöstes Mikrowelleninterferometer zur Messung der Elektronendichte in einer Fluoreszenzlampe aufgebaut. Durch diesen neuartigen Ansatz konnte der Einfluss der dielektrischen Umhüllung (Glasrohr der Lampe) auf die Mikrowelleninterferometrie untersucht werden. In einer weiteren Untersuchung an einem Induktiv Gekoppelten Plasma wurden die Resultate dieses Messsystems mit denen von Langmuir-Sondenmessungen. Auf Grund der konstruktiven Gegebenheiten des Reaktors ist das Plasma nur über ein Fenster für das Mikrowelleninterferometer zugänglich. Der Vergleich der ermittelten Elektronendichten ergab einen Unterschied von Faktor zwei zwischen Interferometer und Langmuir-Sonde. Die Untersuchungen an der Fluoreszenzlampe und dem Induktiv Gekoppelten Plasma zeigten zum einen die korrekte Funktion des neu etablierten frequenzvariablen Mikrowelleninterferometers mit erreichbaren Phasenauflösungen unterhalb von 0,1 mrad. Zum anderen wurde festgestellt, dass die dielektrische Umhüllung des Plasmas zu einem systematischen Fehler von bis 53 % bei der Elektronendichtebestimmung führen kann. Diese gewonnenen Erkenntnisse hatten bei der Konzipierung des Mikrowelleninterferometers zur Untersuchung der Plasmamikrowellenwechselwirkung einen entscheidenden Einfluss. Neben der Untersuchung des Plasmas ist ebenfalls eine Diagnostik des Mikrowellenfeldes nötig, um die Plasmamikrowellenwechselwirkung dieser Entladung experimentell zu charakterisieren. Auf Grundlage dieser Daten können die Resultate des Modells bewertet werden, die einen Einblick in die Plasmaquelle und ihrer dynamischen Vorgänge erlaubt, was für die Weiterentwicklung der Applikationen essentiell ist. Aus diesem Grund ist ein heterodynes Reflektometer entwickelt worden. Dieses Messsystem wurde umfangreich getestet und kann mit einer maximalen Zeitauflösung von 100 ns den komplexen Reflektionsfaktor mit einer Phasengenauigkeit von 10 mrad bestimmen. Das Reflektometer erlaubt einen experimentellen Zugang zur aktiven Zone schon in der Frühphase der Entladung. Mit Hilfe der Diagnostiken zur Untersuchung des Plasmas und des Mikrowellenfeldes wurde die Entladung von der Zündung bis zur stationären Phase charakterisiert und mit den Ergebnissen des Modells verglichen. Es zeigte sich eine gute Übereinstimmung im Millisekundenzeitbereich, sowie eine starke Dynamik im Reflektionsfaktor in der ersten Millisekunde, hervorgerufen durch die Plasmamikrowellenwechselwirkung. Durch die hohe Zeitauflösung des Reflektometers konnten diese Vorgänge im Mikrosekundenzeitbereich erstmalig experimentell aufgelöst werden, was die Interpretation mittels des Modells möglich macht. Es konnten die Vorgänge während der Zündung des Plasmas detailliert untersucht werden und damit die Richtigkeit von Annahmen, die bei der Entwicklung der Zündtechnologie getroffen wurden, überprüft werden. Dieses erworbene grundlegende Verständnis ermöglicht eine Weiterentwicklung dieser Technologie. Mit Hilfe der erzielten Ergebnisse wurde eine neue Optimierungsstrategie für die Abstimmung der Mikrowellenplasmaquelle entwickelt. Dies führte zu einer wesentlichen Verbesserung der Reproduzierbarkeit der mikrobiologischen Ergebnisse. Darüber hinaus bilden die erzielten Ergebnisse eine solide Grundlage für weitere experimentelle und theoretische Untersuchungen dieser Entladung in beispielsweise anderen Arbeitsgasen.
In der vorliegenden Arbeit wurde die Wechselwirkung reaktiver Sauerstoffspezies (ROS) mit organischen Molekülen anhand zweier unterschiedlicher Systeme analysiert. Während einerseits der Einfluss von ROS auf eine organische Monoschicht am Beispiel des synthetischen, kationischen Polyelektrolyten Polyethylenimin (PEI) untersucht wurde, stand andererseits die Wechselwirkung von ROS mit einem DNS-Molekül, dem Biopolyelektrolyten pBR322 im Fokus des Interesses. Für die Untersuchungen der ROS-PEI-Wechselwirkung wurde zunächst verzweigtes PEI flach (RMS-Rauigkeit < 1 nm) auf einem Silizium-Substrat adsorbiert. Mit Hilfe der Fenton-Reaktion wurde die PEI-Monoschicht dem Einfluss von ROS ausgesetzt. Anhand von Kraft-Abstands-Kurven (KAK) konnte gezeigt werden, dass die flache Konformation der PEI-Monoschicht nach dem ROS-Einfluss erhalten bleibt. Des Weiteren konnte mittels Adsorption negativ geladener Gold-Nanopartikel (AuNP) demonstriert werden, dass die PEI-Oberfläche auch nach der Wechselwirkung mit ROS positiv geladene Gruppen enthält. Darüber hinaus konnte mit Hilfe der KAK gezeigt werden, dass sowohl die Oberflächenladungsdichte (OFL) als auch das Oberflächenpotential (OFP) unter ROS-Einfluss um einen Faktor 0,5 reduziert wurden. Es wurde gezeigt, dass die Reduzierung von OFL bzw. OFP auf die Abspaltung positiv geladener Gruppen der PEI-Monoschicht zurückgeführt werden kann. Mit Hilfe der dreidimensionalen Kraftspektroskopie wurde gezeigt, dass OFL und OFP auf einer Längenskala von 1,8 bis 30 µm lateral homogen sind. Darüber hinaus wurde anhand der AuNP-Belegungsdichte demonstriert, dass die Ladungsträger innerhalb der PEI-Oberfläche auf einer Längenskala oberhalb von 36 nm homogen verteilt sind. Hinsichtlich kleiner Längenskalen (< 36 nm) kann konstatiert werden, dass aufgrund einer verzögerten Adsorptionskinetik der AuNP nach der ROS-PEI-Wechselwirkung mit einer partiell reduzierten Bindungswahrscheinlichkeit zu rechnen ist. Vermutlich bewirkt der ROS-Einfluss eine inhomogene Verteilung der positiven Ladungsträger innerhalb der PEI-Monoschicht auf einer Längenskala von einigen nm. Experimentell ergibt sich darüber hinaus, dass eine 50 %ige Reduzierung des PEI-Oberflächenpotentials einer Abnahme von etwa 10 % der maximalen, anteiligen AuNP-Belegungsdichte entspricht. Diese experimentell bestimmten Parameter konnten unter Einbeziehung eines erweiterten drei-Körper RSA-Modelles erklärt werden. Im zweiten Teil der vorliegenden Arbeit wurde eine neue Methode der Quantifizierung ROS-induzierter DNS-Schäden eingeführt. Dazu wurden die DNS-Moleküle zunächst mittels Fenton-Reaktion dem Einfluss von ROS ausgesetzt, auf Polyallylamin-Hydrochlorid-funktionalisiertem Glimmer adsorbiert und mittels des RKM im intermittierenden Modus (RKM-IM) abgebildet. Die Klassifizierung der DNS-Moleküle erfolgt unter Berücksichtigung des Kettenhöhenunterschiedes von doppelsträngiger- (dsDNS) und einzelsträngiger (esDNS) DNS. Als ROS-induzierter DNS-Schaden wird hierbei der Konformationsübergang von dsDNS (intakt) in esDNS (defekt) identifiziert. Die zentrale Messgröße der vorgestellten Methode ist demnach die DNS-Kettenhöhe, welche sich im Falle immobilisierter DNS-Moleküle mit einer Genauigkeit im Sub-Ångström-Bereich mit Hilfe des RKM-IM bestimmen lässt. Für die automatisierte Quantifizierung der Flächen, welche mit dsDNS respektive esDNS belegt sind, wurde ein Höhengrenzwert-basierter Auswertungs-Algorithmus konstruiert. Neben der Variation der Stärke der ROS-DNS-Wechselwirkung mittels verschiedener H2O2-Konzentrationen innerhalb der Fenton-Reaktion, wurde der Einfluss eines Radikalfängers am Beispiel des Natriumacetats (NaOAc) auf diese Wechselwirkung untersucht. Mit der Quantifizierung der ROS-DNS-NaOAc-Wechselwirkung wurde gezeigt, dass der anteilige DNS-Schaden mit wachsender H2O2-Konzentration zunimmt und mit steigender NaOAc-Konzentration abnimmt. Darüber hinaus wurde die Anwendbarkeit der in dieser Arbeit eingeführten Quantifizierung ROS-induzierter DNS-Schäden mit Hilfe eines reaktionskinetischen Ansatzes unter Verwendung des Modelles der kompetitiven Hemmung analysiert. Auf diese Weise wurden Ratenkonstanten der Wechselwirkung zwischen NaOAc mit Hydroxylradikalen verifiziert und somit die Validität des eingeführten Konzeptes demonstriert. Des Weiteren ermöglicht die automatisierte Datenanalyse einen vergleichsweise hohen Datendurchsatz und eignet sich daher gut für die Untersuchung der komplexen Wechselwirkung zwischen ROS, Radikalfänger und DNS. Anhand eines Vergleiches mit den etablierten Methoden zur Quantifizierung ROS-induzierter DNS-Schäden ergibt sich unter Einbeziehung des, in dieser Arbeit eingeführten Analyseverfahrens, ein komplementäres Verständnis der ROS-DNS-Wechselwirkung über einen großen Längenskalenbereich.
Ausgangspunkt aller Untersuchungen sind Langmuir-Monoschichten an der Wasser/Luftgrenzfläche. Denn mit diesen Monoschichten kann die Oberflächeladungsdichte eingestellt werden. Sie werden durch amphiphile Moleküle gebildet.Die hydrophoben Alkylketten sind zur Luftseite und die hydrophile Kopfgruppe zur Wasserseite orientiert.Die Phasen der Lipid- Monoschicht und die Belegungsdichte werden extern eingestellt. Die Lipid- Monoschicht kann je nach Anordnung der Alkylketten verschiedene unter- schiedliche Phasen zeigen. Um die adsorbierten Polyelekrolyte zu beschreiben zu können, benötigt man experimentelle Methoden, die in der Lage sind, Konformation und Be- legungsdichte von adsorbierten Polyelektrolyten an Oberflächen aufzulösen. Diese Strukturen, auf der Nanometerskala, werden mit Röntgenreflektion und Röntgendiffraktion unter streifendem Einfall untersucht.Die Strukturda- ten werden durch thermodynamische Untersuchungen ergänzt. Die Untersuchungen werden für elektrostatische Kräfte mit hoher Ampli- tude und langer Reichweite durchgeführt. Die Lösungen der Polyelektrolyte (c PSS = 0.01 mmol/L bezogen auf die Monomerkonzentration) sind so einge- stellt, dass eine fast vollständige Ladungskompensation (70-90%) stattfindet. Unter diesen Bedingungen adsorbieren PE entweder flach als 2-dimensionales Knäuel oder geordnet in einer 2-dimensionalen lamellaren Phase. Die Untersuchungen sollen herausfinden, welchen Einfluss die elektrostatische Wechselwirkung auf die Kettensteifigkeit von adsorbierten Polyelektrolyten,die durch die Persistenzlänge LP charakterisiert werden.So kann geklärt werden,ab welcher Konturlänge LK Polyelektrolyte nicht mehr stäbchenförmig sondern als 2-dimensionales Knäuel adsorbieren. Untersucht werden negativ geladene PSS (Polystyron Sulfonat) mit un- terschiedlichen Konturlängen LK.Diese adsorbieren an die positiv geladene DODA Lipid-Monoschicht (Dioctadeyldimethylammonium). Die Persistenzlänge LP für adsorbiertes PSS an DODA ist direkt aus den thermodynamischen Daten ermittelt worden. Als Funktion der Konturlänge LK wurde der Oberflächendruck π c sowie die erste Ableitung dπc /dT unter- sucht. Daraus läßt sich die Persistenzlänge des adsorbierten PSS LP ≈ 210 Å bestimmen. Die Oberflächenladungsdichte der Monoschicht wird durch die Kompression verdoppelt.Simultan wird der Abstand der Polyelektrolytketten dPE halbiert, so dass immer eine 70-90% Ladungskompensation erreicht wird. Es wird mit Röntgendiffraktion immer eine fache 2-dimensionale lamellare Phase der adsorbierten PSS Ketten detektiert.Hierbei bilden Konturlängen LK < 110 Å eine Ausnahme. Es wird keine 2-dimensionale lamellare Phase für geringe Oberflächenladungsdichte (LE Phase der Lipide) gefunden. Die Röntgenreflektion kann aber zeigen, dass PSS Ketten, unabhängig von der Konturlänge LK, immer flach an der Lipidmonoschicht adsorbieren. Wenn steife kurze Ketten (LK≤110Å<LP ) an der LC Phase der Lipid- Monoschicht adsorbieren, liegen sie aber in der 2-dimensionalen lamellare Phase. Der Übergang von der LE in die LC Phase der Lipid-Monoschichten erfolgt gleichzeitig mit dem Übergang von der ungeordneten flachen zur 2- dimensionalen lamellare Phase der adsorbierten kurzen PSS Ketten. Daher zeigt die Enthalpie ∆H des LE/LC Phasenübergangs ein Maximum bei L K =110 Å. Im nächsten Schritt wird die Elektrostatik zwischen der Lipid-Monoschicht und den Polyelektrolyte untersucht.Die maximale Oberflächenladungsdichte der Lipid-Monoschicht wird durch eine Mischung von geladenen DODA und ungeladenen DPPC (Dipalmitoylphosphatidylcholine) eingestellt. Röntgendiffraktionsmessungen zeigen bis zu einem DODA-Anteil von 75% die Ausbildung einer 2-dimensionalen lamellaren Phase an der LC Phase der Lipid-Monoschicht. Nimmt die maximale Oberflächenladungsdichte weiter ab, so wird keine Bildung der 2-dimensionalen lamellaren Phase beobachtet. Die Abnahme der maximalen Oberflächenladung führt zu einer niedrigen Belegungsdichte und so zu einem größerem Kettenabstand dPE. Die Linienladungsdichte wird durch unterschiedliches P−TrisAAx−rand−AMPS1−x auf 90% oder 50% verringert. Röntgendiffraktionsmessungen an PE mit unterschiedlichen Linienladungsdichten zeigen auch hier eine 2-dimensionale lamellare Phase. Die verringerte Linienladungsdichte erzwingt eine höhere Belegungsdichte um eine 70-90% Ladungskompensation zu erreichen. Dieses führt zu einem kleineren Kettenabstand dPE. Ein Grenzfall ist bei einer 50%-igen Linienladungsdichte zu beobachten. Dort konnte an der LC Phase der Lipid-Monoschicht keine 2-dimensionale lamellare Phase der PE beobachtet werden. Der Kettenabstand dPE ist so gering, dass er mit der Röntgendiffraktion nicht mehr aufgelöst werden kann.
This work describes the recent scientific and technical achievements obtained at the high-precision Penning trap mass spectrometer SHIPTRAP. The scientific focus of the SHIPTRAP experiment are mass measurements of short-lived nuclides with proton number larger than 100. The masses of these isotopes are usually determined via extrapolations, systematic trends, predictions based on theoretical models or alpha-decay spectroscopy. In several experiments the masses of the isotopes 252-255No and 255,256Lr have been measured directly. With the obtained results the region of enhanced nuclear stability at the deformed shell closure at the neutron number 152 was investigated. Furthermore, the masses have been used to benchmark theoretical mass models. The measured masses were compared selected mass models which revealed differences between few keV/c² up to several MeV/c² depending on the investigated nuclide and model. In order to perform mass measurements on superheavy nuclei with lower production rates, the efficiency of the SHIPTRAP setup needs to be increased. Currently, the efficiency is 2% and mainly limited by the stopping- and extraction efficiency of the buffer gas cell. The stopping and extraction efficiency of the current buffer gas cell is 12%. To this end, a modified version of the buffer gas cell was developed and characterized with 223Ra ion source. Besides a larger stopping volume and a coaxial injection the new buffer gas cell is operated at a temperature of 40K. The operation at cryogenic temperatures increases the cleanliness of the buffer gas. From extraction measurements and simulations an overall efficiency of 62(3)% was determined which results in an increase by a factor of 5 in comparison to the current buffer gas cell. Aside from high-precision mass measurements of heavy radionuclides the mass differences of metastable isobars was measured to identify candidates for the neutrinoless double-electron capture. Neutrinoless double-electron capture can only occur if the neutrino is its own antiparticle and a physics beyond the standard model exists since the neutrinoless double-electron capture violates the conservation of the lepton number. Due to its expected long half-life this decay has not yet been observed. However, the decay rate is resonantly enhanced if mother and daughter nuclide are degenerate in energy. Suitable candidates for the search of the neutrinoless double-electron capture have been identified with mass difference measurements uncertainties of about 100eV/c². In this work the results of the mass difference measurements of 12 possible candidates are presented.
The Atomic Force Microscope (AFM) has become an important tool for probing the mechanical properties of cells and microparticles by force-indentation experiments. In this thesis optimized AFM approaches for these experiments are developed and applied to three types of living human cells in order to answer biologically relevant questions about their mechanics. These microscopic investigations are then interpreted with respect to nanoscopic and macroscopic biologic parameters, such as the function of cell surface receptors or the size of human heart ventricles. This thesis comprises two physical/technical chapters and three medical/biological chapters. The physical/technical chapters discuss the measurement process itself, aiming for its improvement with respect to a proper data analysis and contact model (for spherical cells). The medical/biological chapters investigate the elasticity of cells by the use of optimized AFM approaches, with respect to the used data analysis.
Magnetic reconnection is a fundamental plasma process where a change in field line connectivity occurs in a current sheet at the boundary between regions of opposing magnetic fields. In this process, energy stored in the magnetic field is converted into kinetic and thermal energy, which provides a source of plasma heating and energetic particles. Magnetic reconnection plays a key role in many space and laboratory plasma phenomena, e.g. solar flares, Earth’s magnetopause dynamics and instabilities in tokamaks. A new linear device (VINETAII) has been designed for the study of the fundamental physical processes involved in magnetic reconnection. The plasma parameters are such that magnetic reconnection occurs in a collision-dominated regime. A plasma gun creates a localized current sheet, and magnetic reconnection is driven by modulating the plasma current and the magnetic field structure. The plasma current is shown to flow in response to a combination of an externally induced electric field and electrostatic fields in the plasma, and is highly affected by axial sheath boundary conditions. Further, the current is changed by an additional axial magnetic field (guide field), and the current sheet geometry was demonstrated to be set by a combination of magnetic mapping and cross-field plasma diffusion. With increasing distance from the plasma gun, magnetic mapping results in an increase of the current sheet length and a decrease of the width. The control parameter is the ratio of the guide field to the reconnection magnetic field strength. Cross-field plasma diffusion leads to a radial expansion of the current sheet at low guide fields. Plasma currents are also observed in the azimuthal plane and were found to originate from a combination of the field-aligned current component and the diamagnetic current generated by steep in-plane pressure gradients in combination with the guide field. The reconnection rate, defined via the inductive electric field, is shown to be directly linked to the time-derivative of the plasma current. The reconnection rate decreases with increasing ratio of the guide field to the reconnection magnetic field strength, which is attributed to the plasma current dependency on axial boundary conditions and the plasma gun discharge. The above outlined results offer insights into the complex interaction between magnetic fields, electric fields, and the localized current flows during reconnection.
Modern cavity QED and cavity optomechanical systems realize the interaction of light with mesoscopic devices, which exhibit discrete (atom-like) energy spectra or perform micromechanical motion. In this thesis we have studied the crossover from the quantum regime to the classical limit of two prototypical models, the Dicke model and the generic optomechanical model. The physical problems considered in this approach range from a ground state phase transition, its dynamical response to general nonequilibrium dynamics including Hamiltonian and driven dissipative chaotic motion. The classical limit of these models follows from the classical limit of at least one of its subsystems. The classical equations of motion result from the respective quantum equations through the application of the semiclassical approximation, i.e., the neglect of quantum correlations. The approach of the results from quantum mechanics to the prediction of the classical equations can be obtained by subsequently decreasing the respective scaling parameter. In order to obtain exact results we have utilized advanced numerical methods, e.g., the Lanczos diagonalization method for ground state calculations, the Kernel Polynomial Method for dynamical response functions, Chebyshev recursion for time propagation, and quantum state diffusion for open system dynamics. We have studied the quantum phase transition of the Dicke model in the classical oscillator limit. Our work shows that in this limit the transition occurs already for finite spin length but with the same critical behavior as in the classical spin limit. We have derived an effective model for the oscillator degrees of freedom and have discussed the differences of both classical limits with respect to quantum fluctuations around the mean-field ground state and spin-oscillator entanglement. In this thesis we have proposed a variational ansatz for the Dicke model which extends the mean-field description through the inclusion of spin-oscillator correlations. The ansatz becomes correct in the limit of large oscillator frequency and in the limit of a large spin. For the latter it captures the leading quantum corrections to the classical limit exactly including the spin-oscillator entanglement entropy. We have studied the dynamics of spin and oscillator coherent states in the nonresonant Dicke model at weak coupling. In this regime periodic collapses and revivals of Rabi oscillations occur, which are accompanied by the buildup and decay of atom-field entanglement. The spin-oscillator wave function evolves into a superposition of multiple field coherent states that are correlated with the spin configuration. In our work we provide a description of the underlying dynamical mechanism based on perturbation theory. Our analysis shows that collapse and revival at nonresonance is distinguished from the resonant case treated within the rotating wave approximation by the appearance of two time scales instead of one. We have extended our study of the Dicke dynamics to the case of increasing spin length, as the system approaches the classical spin limit. We described the emergence of collective excitations above the ground state that converge to the coupled spin-oscillator oscillations observed in the classical limit. With increased spin length the corresponding Green functions thus reveal quantum dynamical signatures of the quantum phase transition. For the dynamics at larger coupling and energy, classical phase space drift and quantum diffusion hinders the direct comparison of quantum and classical observables. As we show in our work, signatures of classical quasiperiodic orbits can be identified in the Husimi phase-space functions of the propagated wave function and individual eigenstates with energies close to that of the quasiperiodic orbits. The analysis of the generic optomechanical system complements our study of cavity QED systems by a quantum dissipative system. In this thesis we have shown for the first time, how the route to chaos in the classical optomechanical system takes place, given as a sequence of consecutive period doubling bifurcations of self-induced cantilever oscillations. In addition to the semiclassical dynamics we have analyzed the possibility of chaotic motion in the quantum regime. Our results showed that quantum mechanics protects the optomechanical system against irregular dynamics. In sufficient distance to the semiclassical limit simple periodic orbits reappear and replace the classically chaotic motion. In this way direct observation of the dynamical properties of an optomechanical system makes it possible to pin down the crossover from quantum to classical mechanics.
The laser-matter interaction is a topic of current research. In this context, the interaction of intensive laser radiation with atomic clusters is of special interest. Du to the small cluster size, the laser field can penetrate the whole cluster volume, which leads to a high absorption of energy in the cluster. As a result, plasmas with high density and high temperature are produced. In the early phase of the laser-cluster interaction, free electrons are initially created in the cluster due to tunnel ionization or photoionization. Via collisions of these electrons with the cluster atoms, the ionization is increased and thus a dense nanoplasma is produced, which is heated by the laser. If free electrons leave the cluster during the laser-cluster interaction (outer ionization), a positive charge buildup is created. The associated charge repulsion finally can lead to the fragmentation of the cluster due to Coulomb explosion. Experimentally, interesting phenomena emerging from laser-excited clusters are observed, e.g., the creation of fast electrons, the production of highly charged ions, and X-ray emission. In this dissertation, the interaction of Gaussian laser pulses in the infrared regime with argon and xenon clusters is simulated by means of a nanoplasma model. Considering laser intensities in the non-relativistic regime, the relevant processes such as ionization, heating and expansion are theoretically described in this model with a set of coupled rate equations and hydrodynamic equations. One focus of the thesis is on the heating of the nanoplasma via inverse bremsstrahlung (IB), which is due to the absorption of laser photons in electron-ion collisions. In particular, the important question is investigated whether the consideration of the ionic structure – that means, the nuclear charge and the bound electrons – modifies the electron-ion collisions and thus the IB heating rate. Starting from a quantum statistical description, effective electron-ion potentials are used which account for both the screening due to the dense plasma and the inner ionic structure. Within the quantum mechanical first Born approximation, the consideration of the ionic structure leads to a drastic increase of the IB heating rate, in particular for high nuclear charges and low ionic charge states. However, for the parameters relevant in experiments, the applicability of the first Born approximation is questionable. Therefore, quantum mechanical calculations going beyond the first-order perturbation theory are performed. In addition, the IB heating rate is investigated with different classical methods. These are based either on transport cross sections for elastic electron-ion scattering or on classical simulations of inelastic scattering processes. Also within the classical approaches, the consideration of the ionic structure leads to an increase of the heating rate. However, this increase is shown to be only moderate. In a further part, the thesis focuses on the question how the dynamics of the laser-cluster interaction is influenced by the consideration of excited states. This is explored exemplarily for argon clusters excited by single or double laser pulses. The consideration of excitation processes in the nanoplasma leads to a decrease of the electron temperature and to an increase of the density of free electrons. Moreover, it is shown that the consideration of excitation processes results in an essential acceleration of the ionization dynamics. As a consequence, the mean ionic charge state in the plasma as well as the number of highly charged ions is significantly increased. For the population of ground states and excited states within an ionic charge state Z, collisional deexcitation processes play an important role. By means of an analytical relation between excitation and deexcitation cross sections, the rates for the respective processes in the presence of the laser field are calculated. The role of deexcitation processes is studied in detail, showing that the inclusion of these processes is essential for the correct theoretical description of the photon emission from laser-excited clusters. Based on these results, the photon yield is calculated for selected radiative transitions resulting from highly charged argon ions in the UV and X-ray regime.
Energetic ions are made to collide with atmospheric molecules. Positively charged ions of argon (Ar^+), helium (He^+), hydrogen (H_2^+ ), and protons (H^+) with energies of 50 keV to 350 keV are used as the bombarding ion. The ion beam of desired energy is produced using a linear ion accelerator at the University of Greifswald. The mass and energy distribution of sputtered particles were analysed using an Electrostatic Quadrupole SIMS (EQS) analyser. The target gases used are oxygen (O_2), sulfur hexafluoride (SF_6), and nitrogen (N_2). The ionized and fragmented particles due to collisions have been investigated. We have discovered a new process for negative ion formation in energetic ion collision with O_2 and SF_6 molecules. The process is a two body reaction between the projectile and the molecule without the need for a third particle (such as an external electron). It requires a direct charge transfer from the projectile to the molecule leaving it intact as O_2^- or SF_6^- . The process is experimentally confirmed by using a proton as projectile which does not have an electron to transfer. In comparison with positive ion fractions (O_2^+ , SF_5^+ ), the negative ions fraction is smaller by 2 orders of magnitude. This shows that the two body charge exchange process is weak due to the larger energy transfer required compared to the positive ion forming mechanisms. The two body charge exchange mechanism is not observed for ion collisions with N_2 molecule. No stable negative ion exist for N_2 molecule. The collision cross section for the ion formation during energetic ion – O_2 collision has been determined within the investigated impact energy. For SF_6 molecule the partial ion fraction of the secondary ions are determined for different projectiles involved. This kind of investigation is of great importance mainly in atmospheric physics. Energetic ions are constantly emitted from mass of the energy sources in the universe (e.g. sun). They interact with planetary objects or atmosphere on their way. A deep knowledge about the interaction processes is necessary to understand the ionospheric physics and space exploration. As second part of my thesis, a GaAs(100) surface is bombarded with 150 keV Ar^+ ion beam. From etching the surface to thin film coating, ion bombardment on solid surface found great role in the fabrication process of modern electronic and optical devices. In order to increase the knowledge on sputtering materials and because of profound importance in modern electronics, we choose GaAs(100) as our target. Among the sputtered atoms and ions, small sized cluster ions having more than 6 atoms have been identified. GaAs is a heteroatomic semiconductor containing gallium and arsenic in equal ratio. A preferential phenomenon of ’abundant sputtering’ of gallium compared to little arsenic (GaAs) has been investigated from their mass intensity. The experimental ion counts are compared with theoretically predicted relative abundance. This phenomenon of preferential sputtering is known for atomic species of sputtered GaAs but not for the sputtered cluster ions. The main reasons for this abundant sputtering of one element is attributed to the difference in ion formation energies and surface compositional change taking place during the sputtering process. Another notable characteristics is the preference in charge state among the sputtered ions. For instance, among sputtered atomic ions the ion counts of Ga^+ is 3 orders larger than As^+ ion and As^- is 2 orders larger than Ga^- ion. To get a clue for this behavior, we have investigated the energy distribution of both negatively and positively charged clusters. Different ion formation mechanisms were discussed. The energy distribution of atomic ion is partially explained by using a modified theory given by M. W. Thompson.