Refine
Year of publication
- 2013 (16) (remove)
Document Type
- Doctoral Thesis (11)
- Article (4)
- Conference Proceeding (1)
Has Fulltext
- yes (16)
Is part of the Bibliography
- no (16)
Keywords
- - (5)
- Plasmaphysik (4)
- Plasma (3)
- Magnetron (2)
- Plasmadiagnostik (2)
- Stellarator (2)
- plasma diagnostics (2)
- 52.50.Dg (1)
- 52.70.Ds (1)
- 52.75.Hn (1)
Institute
- Institut für Physik (16) (remove)
Publisher
- IOP Publishing (5)
This thesis describes the implementation and first on-line application of a multi-reflection time-of-flight (MR-ToF) mass analyzer for high-resolution mass separation at the ISOLTRAP mass spectrometer at ISOLDE/CERN. On the one hand, the major objective was to improve ISOLTRAPs mass-measurement capabilities with respect to the ratio of delivered contaminating ions to ions of interest. On the other hand, the time necessary to purify wanted from unwanted species should be reduced as much as possible to enable access to even more exotic nuclei. The device has been set up, optimized and tested at the University of Greifswald before its move to ISOLTRAP. The achieved performance comprises mass resolving powers of up to 200000 reached at observation times of 30ms and a contamination suppression of about four orders of magnitude by use of a Bradbury-Nielsen gate. With the characteristics, it outperforms clearly the so far state-of-the-art purification method of a gas-filled Penning trap. To improve the utilization of the MR-ToF mass analyzer, the in-trap lift method has been developed. It simplifies the application and optimization of the device, which is a crucial time factor in an on-line experiment. The device was the first of its kind successfully applied to radioactive ion beams for a mass analysis, for a mass separation (in combination with the Bradbury-Nielsen gate) as a preparatory step for a subsequent Penning-trap mass measurement and as a high-precision mass spectrometer of its own. The later was recently used for the first mass measurement of the neutron-rich calcium isotopes 53Ca and 54Ca. The so-far achieved mass-resolving power of 200000 belongs to the highest reported for time-of-flight mass analyzers at all. The first successful application of the MR-ToF system as the only mass separator at ISOLTRAP resulted in the mass measurement of 82Zn. The new mass value has been compared to mass extrapolations of the most recent Hartree-Fock-Bogoliubov (HFB) mass models, HFB-19 to HFB-21, of the BRUSLIB collaboration. The mass of the nuclide is of high interest for the compositions and depth profile of the outer crust of neutron stars. In the classical model of the outer crust of a cold, non-accrediting and non-rotating neutron star, the sequence of nuclides found within this parts is determined mainly by the binding energy of exotic nuclides. The crustal compositions determined with the three HFB mass models differed with respect to the appearance of a layer of 82Zn, originating from different mass extrapolations of this mass. With the new experimental data, the extrapolations could be evaluated. It was found that the HFB-21 mass value differs less from the experimental data than the ones from HFB-19 and 20. Therefore, in the classical model, 82Zn does not appear anymore in the outer crust. Due to its high resolution and very fast measurement time, the MR-ToF mass analyzer will be an important instruments for future activities at ISOLTRAP, at the ISOLDE facility in general, and at other radioactive ion-beam facilities.
Abstract
The surface charge distribution deposited by the effluent of a dielectric barrier discharge driven atmospheric pressure plasma jet on a dielectric surface has been studied. For the first time, the deposition of charge was observed phase resolved. It takes place in either one or two events in each half cycle of the driving voltage. The charge transfer could also be detected in the electrode current of the jet. The periodic change of surface charge polarity has been found to correspond well with the appearance of ionized channels left behind by guided streamers (bullets) that have been identified in similar experimental situations. The distribution of negative surface charge turned out to be significantly broader than for positive charge. With increasing distance of the jet nozzle from the target surface, the charge transfer decreases until finally the effluent loses contact and the charge transfer stops.
Diese Dissertation beschäftigt sich mit der Erzeugung von edelmetallfreien Katalysatoren für die Sauerstoffreduktion in Brennstoffzellen. Dabei wird ein neuartiger, dualer Plasmaprozess entwickelt, aufgebaut und die so-erzeugten Schichten mit verschiedenen elektrochemischen (CV, RDE und RRDE) und strukturanalytischen Methoden (SEM, EDX, IR, XPS, Leitfähigkeit, XRD, NEXAFS, EXAFS und TEM) untersucht. Auf diese Weise ist es erstmalig gelungen edelmetallfreie Katalysatoren mit einem Plasmaprozess herzustellen, ohne dass eine zusätzliche Pyrolyse benötigt wird. Die katalytische Aktivität der Schichten ist außerdem deutlich höher als die von rein chemisch hergestellten Metall–Polypyrrol-Schichten.
The thesis describes experimental results based on optical diagnostics of low- pressure discharges. The models, which are necessary for the interpretation of the experimental data, are developed and simulations are done. The contents can be categorized into the following topics: 1) the time-resolved tunable diode laser absorption spectroscopy of excited states of argon in pulsed magnetron discharge and modeling the plasma afterglow; 2) optical emission- and laser absorption spectroscopy of excited states of argon in radio-frequency (rf) discharge and calculation of the escape factor for self-absorption; 3) fast video recording of the oscillatory motion of a dust particle in rf discharge and analysis of the data.
Abstract
The spectral properties of three-dimensional dust clusters confined in gaseous discharges are investigated using both a fluid mode description and the normal mode analysis (NMA). The modes are analysed for crystalline clusters as well as for laser-heated fluid-like clusters. It is shown that even for clusters with low particle numbers and under presence of damping fluid modes can be identified. Laser-heating leads to the excitation of several, mainly transverse, modes. The mode frequencies are found to be nearly independent of the coupling parameter and support the predictions of the underlying theory. The NMA and the fluid mode spectra demonstrate that the wakefield attraction is present for the experimentally observed Yukawa balls at low pressure. Both methods complement each other, since NMA is more suitable for crystalline clusters, whereas the fluid modes allow to explore even fluid-like dust clouds.
The confinement of energy has always been a challenge in magnetic confinement fusion devices. Due to their toroidal shape there exist regions of high and low magnetic field, so that the particles are divided into two classes - trapped ones that are periodically reflected in regions of high magnetic field with a characteristic frequency, and passing particles, whose parallel velocity is high enough that they largely follow a magnetic field line around the torus without being reflected. The radial drift that a particle experiences due to the field inhomogeneity depends strongly on its position, and the net drift therefore depends on the path taken by the particle. While the radial drift is close to zero for passing particles, trapped particles experience a finite radial net drift and are therefore lost in classical stellarators. These losses are described by the so-called neoclassical transport theory. Recent optimised stellarator geometries, however, in which the trapped particles precess around the torus poloidally and do not experience any net drift, promise to reduce the neoclassical transport down to the level of tokamaks. In these optimised stellarators, the neoclassical transport becomes small enough so that turbulent transport may limit the confinement instead. The turbulence is driven by small-scale-instabilities, which tap the free energy of density or temperature gradients in the plasma. Some of these instabilities are driven by the trapped particles and therefore depend strongly on the magnetic geometry, so the question arises how the optimisation affects the stability. In this thesis, collisionless electrostatic microinstabilities are studied both analytically and numerically. Magnetic configurations where the action integral of trapped-particle bounce motion, J, only depends on the radial position in the plasma and where its maximum is in the plasma centre, so-called maximum-J configurations, are of special interest. This condition can be achieved approximately in quasi-isodynamic stellarators, for example Wendelstein 7-X. In such configurations the precessional drift of the trapped particles is in the opposite direction from the direction of propagation of drift waves. Instabilities that are driven by the trapped particles usually rely on a resonance between these two frequencies. Here it is shown analytically by analysing the electrostatic energy transfer between the particles and the instability that, thanks to the absence of the resonance, a particle species draws energy from the mode if the frequency of the mode is well below the charateristic bounce frequency. Due to the low electron mass and the fast bounce motion, electrons are almost always found to be stabilising. Most of the trapped-particle instabilities are therefore predicted to be absent in maximum- J configurations in large parts of parameter space. Analytical theory thus predicts enhanced linear stability of trapped-particle modes in quasi-isodynamic stellarators compared with tokamaks. Moreover, since the electrons are expected to be stabilising, or at least less destabilising, for all instabilities whose frequency lies below the trapped-electron bounce frequency, other modes might benefit from the enhanced stability as well. In reality, however, stellarators are never perfectly quasi-isodynamic, and the question thus arises whether they still benefit from enhanced stability. Here the stability properties of Wendelstein 7-X and a more quasi-isodynamic configuration, QIPC, are investigated numerically and compared with another, non-quasiisodynamic stellarator, the National Compact Stellarator Experiment (NCSX) and a typical tokamak. In gyrokinetic simulations, performed with the gyrokinetic code GENE in the electrostatic and collisionless approximation, several microinstabilities, driven by the density as well as both ion and electron temperature gradients, are studied. Wendelstein 7-X and QIPC exhibit significantly reduced growth rates for all simulations that include kinetic electrons, and the latter are indeed found to be stabilising when the electrostatic energy transfer is analysed. In contrast, if only the ions are treated kinetically but the electrons are taken to be in thermodynamic equilibrium, no such stabilising effect is observed. These results suggest that imperfectly optimised stellarators can retain most of the stabilising properties predicted for perfect maximum-J configurations. Quasi-isodynamic stellarators, in addition to having reduced neoclassical transport, might therefore also show reduced turbulent transport, at least in certain regions of parameter space.
Abstract
We formulate exact generalized nonequilibrium fluctuation relations for the quantum mechanical harmonic oscillator coupled to multiple harmonic baths. Each of the different baths is prepared in its own individual (in general nonthermal) state. Starting from the exact solution for the oscillator dynamics we study fluctuations of the oscillator position as well as of the energy current through the oscillator under general nonequilibrium conditions. In particular, we formulate a fluctuation–dissipation relation for the oscillator position autocorrelation function that generalizes the standard result for the case of a single bath at thermal equilibrium. Moreover, we show that the generating function for the position operator fulfils a generalized Gallavotti–Cohen-like relation. For the energy transfer through the oscillator, we determine the average energy current together with the current fluctuations. Finally, we discuss the generalization of the cumulant generating function for the energy transfer to nonthermal bath preparations.
The absolute density of the metastable N2(A,v=0) molecule was extensively studied in nitrogen barrier discharges at 500 mbar. For the detection of the metastables laser-induced fluorescence spectroscopy (LIF) was used, at which for the calibration of the absoute metastables density a comparison with Rayleigh scattering was performed. To get the ratio of the LIF signal to the Rayleigh signal it is shown that the LIF signal is the convolution of the Rayleigh signal with an exponential decay. Besides, the different cross sections are calculated and the ratio of the detection sensitivities at the laser and fluorescence wavelength is determined. As a first step on the way to atmospheric pressure barrier discharges, the laser-induced fluorescence spectroscopy was implemented in low pressure capacitively coupled radio-frequency discharges. The determined metastables density in the capacitively coupled radio-frequency discharge is somewhat below 10^12 cm^(-3) at 40 Pa and somewhat below 10^13 cm^(-3) at 1000 Pa. The axial density profiles show a nearly symmetric shape due to the long lifetime of the metastable state. At a pressure of 500 mbar the two discharge modes of the barrier discharge, the filamentary and the diffuse mode, were analysed. The filamentary mode was mainly investigated in an asymmetric discharge configuration. Typical densities in the detection volume are in the range of 10^13 cm^(-3), resulting in maximal densities of up to 10^15 cm^(-3) in the microdischarge channel. Such large densities are in agreement with the fast decay by the pooling reaction after the maximum of the metastables density in the afterglow of the discharge pulse. The time dependent measurements in the afterglow of single microdischarges offer a delay of the metastables production with respect to the discharge current. This delay indicates that the metastables production takes place mostly by cascades from higher triplet states, which are in turn excited by electron impact. The axial density profiles show a maximum in metastables density in front of the anode in agreement with optical emission spectroscopy, but which cannot be clearly identified because of the asymmetric discharge configuration. The measurements for the diffuse discharge mode were performed in a symmetric discharge configuration. The metastables density is in the range of 10^13 cm^(-3). It increases during the current pulse of the discharge and decays afterwards. The maximum of the metastables density is delayed with respect to the maximum of the discharge current. The depletion of metastables in the early discharge afterglow is dominated by the pooling reaction, afterwards quenching by nitrogen atoms becomes important assuming a nitrogen atom density in the order of 10^14 cm^(-3). As for the filamentary mode, the losses by diffusion are negligible for the measurement positions. The measured axial density profiles show an accumulation of metastables in front of the anode, whereas the density in front of the cathode is below the detection limit. To calculate the metastables current density to the dielectrics after the discharge pulse a simulation is developed including the dominant volume processes for the depletion of metastables and the axial diffusion. Starting point for the simulation is the axial metastables density distribution at the end of the discharge pulse. The calculated metastables current density at the dielectrics is in the range of 10^14 cm^(-2)s^(-1). With the use of recently calculated secondary electron emission coefficients a comparison of the secondary electron emission by metastables with the discharge current is done. It is figured out that the secondary electron emission current is large enough to be important during the discharge ignition. To expand the simulation to the whole voltage cycle, the excitation of metastables is assumed to be proportional to the discharge current and electron density. Using this model, the measured time dependences of the metastables density are well reproduced for the investigated parameter variations. This is not the case for the axial profiles, where a metastables loss process is missed to explain the formation of a density plateau in front of the anode during the discharge pulse. The intended calculation of the metastables current density shows that the delay of the metastables production with respect to the discharge current might be responsible for the ignition of microdischarges at the beginning of the discharge pulse.
In this work, various aspects of fundamental physics and chemistry of molecular gas discharges are presented with emphasis on the interaction between species, activated by low-pressure plasmas, and surfaces. As already known, synergistic effects of multiple plasma-generated species are responsible for surface modification. However, due to the large number of internal parameters of a discharge and the complex plasma processes the identification of correlations between plasma characteristics and their effects on surfaces are complicated. Therefore, the aim of this thesis is to improve the understanding of several phenomena associated with plasma–surface interactions by measuring or calculating fundamental kinetic, transport or spectroscopic data needed to interpret measurements and hereby, to support some future applications of plasmas.
The electron and negative ion densities in an asymmetric capacitively coupled low-pressure RF plasma in oxygen were systematically studied and compared to the electropositive argon RF plasma during continuous and pulsed power input. This work presents the careful design and realization of a non-invasive 160.28 GHz Gaussian beam microwave interferometry (MWI) as an innovative diagnostic tool. MWI directly provides the line integrated electron density without any model assumption. The high microwave frequency enables one to accurately describe the microwave free space propagation by means of Gaussian beam theory. The microwave interferometer is simultaneously coupled with laser photodetachment to experimentally determine the negative ion density in the CCRF oxygen discharge. This is the first time that both diagnostics were combined in low-pressure capacitively coupled RF oxygen plasmas. This thesis first presents comprehensive measurements of the steady state line integrated electron density in dependence on RF power and pressure for an argon and oxygen plasma. For both gases the electron density increases with RF power. However, the line integrated electron density in oxygen is about a factor 3 to 10 smaller than in argon. The reduced electron density is accompanied by a high number of negative ions, which exceeded the electron density and resulted in a high electronegative mode. With increasing RF power, the plasma switches into a low electronegative mode. Consequently, the discharge operates in two different modes, which are distinguished by their degree of electronegativity. The transition between the high and low electronegative modes is step-like and it was concluded that one can here directly see the discharge switches from the &alpha-mode to the &gamma-mode. The &gamma-mode (low electronegative mode, high RF power) is characterized by a strong increase of the electron density and a simultaneous decrease of the negative ion density. The increase may be connected to the production of secondary electrons by collision detachment of negative ions within the RF sheath (“pseudo-secondary electron”), in addition to the classical &gamma process due to positive ion bombardment of the powered electrode. In comparison to the &gamma-mode the &alpha-mode (high electronegative mode, low RF power) reveals more negative ions than electrons. Furthermore, a simple 0d attachment-detachment model was applied to calculate the effective rate coefficients for dissociative electron attachment and collisional detachment from the experimentally determined values of steady state electron and negative ion density, as well as the detachment decay time constant. Hence, the attachment rate coefficient of the molecular ground and the excited metastable state in dependence on RF power were determined. Moreover, the density of metastable molecular oxygen was estimated to 10% of the molecular ground state oxygen. The influence of each electronegative mode to the entire temporal behavior of the oxygen discharge was intensively investigated by pulsing the discharge. Here it was shown that for the low electronegative mode the afterglow behavior is similar to that of an electropositive argon plasma. In the high electronegative mode an electron density peak in the early afterglow was observed. It was concluded that the electron production originates from the collisional detachment of negative ions. The negative ion loss and the electron production in the early afterglow were modeled numerically with a 0d rate equation system. The model accurately describes the afterglow behavior of both electronegative modes and the additional electron density peak in the early of the high electronegative mode. For the high electronegative mode the molecular oxygen plays an important role as a detachment partner for the production of electrons in the early afterglow. Furthermore, the presence of the negative ions causes fluctuations of plasma parameters. 2d spatial and temporal fluctuations of the ion saturation current are measured during the instability. The temporal and phase resolved optical emission spectroscopy shows a strong change in emission pattern during the instability, which becomes more obvious for one RF cycle at characteristic instability phases. Here, the excitation patterns reveal significant changes in the electron heating mechanisms.