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The main objective of this work is to contribute to the understanding of the grafting of nitrogen and amino surface functional groups on polymers by means of plasmas containing nitrogen and hydrogen. For this purpose, many aspects of plasma surface modification were studied. In the frame of this work, a new, UHV-sealed plasma reactor system was put into operation. The system is special for its clean reaction environment and the possibility to perform quasi in situ XPS measurements. A comparison of the UHV system to a fine vacuum reactor showed that a clean reaction environment is mandatory for reproducible plasma processing and efficient nitrogen and amino functionalisation. A key motivation for the present work was the observation that the non-coating plasma processes reported in literature fail to graft primary amino groups on polymer surfaces with densities that significantly exceed 3 - 4% NH2/C. In order to investigate this phenomenon in detail, this work followed two experimental tracks: On the one hand, a broad systematic study of plasma processing parameters was performed. On the other, the surface diagnostics methods used for the quantification of amino groups were critically reviewed. For this, a numerical algorithm was developed to reconstruct the element depth profile from angle-resolved XPS data. In the scope of the process parameter study, cw and pulsed microwave (MW) plasma excitation was compared to radio-frequency (RF) excitation. The home-built MW source was studied and optimised with respect to ignition behaviour and power efficiency. The performance of the MW and RF plasmas in polymer surface modifications was studied in various gas mixtures containing NH3 and H,, or N2 and H,. Also the differences of glow and afterglow processing of polymers were investigated. Large variations of the nitrogen and primary amino grafting efficiencies were obtained. They triggered a number of new ideas for the underlying reaction mechanisms. Special attendance was devoted to the selectivity of the functionalisation processes for primary amino groups. Nitrogen-containing discharges that were rich in hydrogen achieved selectivities up to 100%. The upper limit of 3 - 4% amino groups on the surface, however, was not passed. Angle-resolved XPS measurements revealed a systematic problem for the definition of a surface density, which is capable of explaining the upper limit for amino groups. It is either due to a limited labelling depth of amino groups by the applied TFBA derivatisation reaction, or to a limited functionalisation depth of the plasma process. One very efficient nitrogen-grafting plasma process that was developed on polystyrene was applied to seven other unfluorinated polymers. The similarity of the resulting functionalisation demonstrated a good transfer-ability of plasma surface functionalisation processes. Plasma treatments of polymer surfaces, especially in hydrogen-containing gases, are known to be generally followed by uncontrollable oxidation phenomena. The properties of plasma-functionalised polymer surfaces were therefore studied in conjunction with ageing effects. Quasi in situ XPS analysis allowed to distinguish the influence of oxygen contamination during the plasma process from post-process oxidation due to contact of plasma-treated samples to atmospheric oxygen. The surface modification experiments were accompanied by several gas phase diagnostic techniques. In the scope of this work, the UHV reactor system was equipped with optical emission spectroscopy (OES), two-photon absorption laser-induced fluorescence (TALIF), and tunable diode laser absorption spectroscopy (TDLAS). A separate plasma source was setup to perform an absolute quantification of the vacuum-ultra-violet (VUV) emission intensity of hydrogen-containing MW-excited plasmas. The techniques were evaluated with respect to their contribution to an understanding of the plasma processing of polymers. The rich experimental data allowed to suggest new reaction mechanisms for the grafting of nitrogen- and amino functional groups. Surface passivation experiments in H, plasmas of nitrogen-functionalised surfaces initiated a re-evaluation and an extension of the mechanism of selective etching [1]. Together with two other new reaction mechanisms, a hypothetical reaction scheme was suggested. It was studied by the help of two numerical models for heterogenous reactions of radicals with the surface. In order to avoid the complexity of the fragmentation process of NH,, the models were restricted to discharges in N, and H9. Despite the sparse information on the composition of the gas phase, the data of two experimental series showed a very particular phenomenology that allowed a first test of the model. The test supports the newly-suggested reaction mechanisms. Especially the role of NH2 attachment to open reaction sites for the grafting of amino groups was emphasised. A more stringent test of the model is left to future experiments with extended gas phase diagnostic means.
The present experimental work investigates plasma turbulence in the edge region of magnetized high-temperature plasmas. A main topic is the turbulent dynamics parallel to the magnetic field, where hitherto only a small data basis existed, especially for very long scale lengths in the order of ten of meters. A second point of special interest is the coupling of the dynamics parallel and perpendicular to the magnetic field. This anisotropic turbulent dynamics is investigated by two different approaches. Firstly, spatially and temporally high-resolution measurements of fluctuating plasma parameters are investigated by means of two-point correlation analysis. Secondly, the propagation of signals externally imposed into the turbulent plasma background is studied. For both approaches, Langmuir probe arrays were utilized for diagnostic purposes. The main findings can be summarized as follows: Greatly elongated fluctuation structures exist in plasma edge turbulence. The structures are aligned along the confining magnetic field (k|| = 0). The correlation degree of fluctuations for a short connection length of 0.75m is greater than 80%. For much longer connection lengths of 23m and 66m, the correlation degree is reduced to approximately 40%. A conceptual interpretation of these observations is the coexistence of two different fluctuation components. One component has a correlation length parallel to the magnetic field below 20m and the other component a correlation length greater than 70m. Sine signals in the frequency range 1-100 kHz were injected into the turbulent plasma background. The propagation parallel and perpendicular to the magnetic field of the signals was studied. In poloidal direction, an asymmetry is observed, that can be explained by a copropagation of the signal with the background E × B-rotation of the plasma. The signal propagation parallel to the magnetic field shows no such asymmetry. As an advanced approach, spatio-temporal wave patters were injected into the edge plasma. The waves launched that way can be seen as test waves' in a turbulent background. The coupling strength of the imposed wave patterns to the background turbulence relies on the match of the imposed waves to the dynamics of turbulent structures. If the propagation direction of the imposed waves is parallel to the propagation direction of the background plasma, improved coupling is observed. This finding underlines the importance of the background plasma rotation for future attempts of controlling the plasma edge turbulence. Further optimization of frequency and wave vector of the imposed waves is probably a promising approach for achieving a significant and systematic influence of turbulence. Taking into account the present experimental state-of-the-art, for a deeper insight into the mechanism of the plasma edge turbulence of magnetized high-temperature plasmas a joint effort of numerical modeling and experimental results is a valuable approach. Such a cooperation should cover the explanation of the correlation observations as well as the experiments on signal injection into background turbulence. A quantitative comparison between the results presented in this work and a dedicated numerical drift wave simulation would be a significant step forward to a better understanding of plasma edge turbulence.
The role of large-scale fluctuation structures in electrostatic
drift-wave-type plasma turbulence is highlighted. In particular,
well-defined laboratory experiments allow one to study the
dynamics of drift wave mode structures as well as `eddies' in
drift wave turbulence. In the present paper we discuss the
mutual relationships between observations made in linear
magnetic geometry, purely toroidal geometry and magnetic
confinement. The simplest structure, a saturated, nonlinear
drift mode, is the starting point for a Ruelle-Takens-Newhouse
transition route to chaos and weakly developed turbulence. Both
spectral and phase space analysis are applied to characterize in
detail the transition scenario, which is enforced due to an
increased drive by the plasma equilibrium state. In addition to
direct multi-probe observation, statistical approaches are most
revealing for the systematic study of the spatiotemporal
dynamics in fully developed drift wave turbulence. In
particular, the propagation of large-scale `eddy' structures is
traced by conditional statistics methods. Finally, the control
of drift wave turbulence by spatiotemporal synchronization is
discussed.
Within the scope of this work, a versatile large linear magnetised plasma experiment was designed, constructed, and subsequently put into operation. The magnetised plasma was used to investigate the dispersion of whistler waves (circular polarised electromagnetic waves) with regard to the influence of the plasma boundaries. After a brief review over electromagnetic plasma waves and the three discharge modes of a helicon source, the experimental device and the diagnostic tools are explained in detail. Great attention is devoted to the identification of a reliable, calibrated magnetic fluctuation probe design. To the understanding of dynamical phenomena in ionospheric plasmas, whistler wave measurements in laboratory experiments may contribute significantly because of the ability to vary plasma parameters and to do measurements with high spatial and temporal resolution. However, the boundaries of laboratory experiments change the dispersion behaviour of whistler waves significantly if compared to the unbounded ionospheric situation. The influence of the plasma boundary is studied in the present work on three different levels of increasing complexity. First, a high density, small wavelength regime is established to make the effect of the boundary negligible. Measurements are in full agreement with whistler wave theory for unbounded plasma geometry. Measurements below the ion cyclotron frequency reveal the strong influence of the ion dynamics on whistler wave propagation, but are not straightforward to interpret in terms of dispersion theory. Second, the other limit case is examined: bounded plasma helicon modes. These waves are, mathematically speaking, eigenfunctions of the plasma-boundary system and are of great practical importance for high density plasma discharges, the helicon source. Careful measurements of the equilibrium plasma parameters as well as the magnetic fluctuation profiles of the helicon source are done in all three modes of operation, the capacitive, inductive, and helicon wave sustained mode. The first two modes are fairly well understood and the measurements are consistent with existing models. The high density helicon mode, however, is still a scientific case. The measurements partially confirm existing assumptions. It is demonstrated that the plasma production is detached from the antenna edge region. Moreover, it is shown that the plasma parameters are self-consistently determined by the antenna geometry and the discharge parameters according to basic helicon wave theory. Finally, it is ruled out that the plasma density is the control parameter determining the transition point into the high density helicon mode. The measurements rather suggest that the rf power density is the important value. As a third aspect, whistler waves in an intermediate wavelength regime are studied and the transition from unbounded to bounded plasma wave dispersion is systematically investigated. It is shown both experimentally and numerically that the wave dispersion in a plasma filled metal waveguide cannot be determined solely from wave vector measurements parallel to the magnetic field. For a correct description, the perpendicular mode profile has to be correctly taken into account. In contrast to simple helicon wave theory, it is demonstrated that the perpendicular mode profile is not only determined by the conducting vessel boundaries alone but the entire plasma-boundary system has to be considered as a unity. To summarise, this work has contributed to a better understanding of the physics of the propagation of whistler waves, where the particular role of metal boundaries acting as wave guides was highlighted. This basic science approach to the waves' dynamics is believed to be of significance in the course of the scientific debate on the physics principles of helicon discharges.
Two main aspects concerning drift wave dynamics in linear, magnetized plasma devices are addressed in the work: In part I of the thesis, drift waves are studied in a helicon plasma. The plasma parameter regime is characterized by comparably high collision frequencies and comparably high plasma-p exceeding the electron-ion mass ratio. Single Langmuir probes and a poloidal probe array are used for spatiotemporal studies of drift waves as well as for characterization of background plasma parameters. The main goals are the identification of a low-frequency instability and its major destabilization mechanisms. All experimentally observed features of the instability were found to be consistent with drift waves. A new code, based on a non-local cylindrical linear model for the drift wave dispersion, was used to gain more insight into the dominating destabilzation mechanisms, and also into dependencies of mode frequencies and growth rates on different parameters. In the experiment and in the numerical model, poloidal mode structures were found to be sheared. Part II of the thesis reports about mode-selective spatiotemporal synchronization of drift wave dynamics in a low-P plasma. Active control of the fluctuations is achieved by driving a preselected drift mode to the expense of other modes and broadband turbulence. It is demonstrated that only if a resonance between the driver signal and the drift waves in both space and time is reached, the driver has a strong influence on the drift wave dynamics. The synchronization effect is qualitatively well reproduced in a numerical simulation based on a Hasegawa-Wakatani model.
Low-pressure plasmas offer a unique possibility of confinement, control and
fine tailoring of particle properties. Hence, dusty plasmas have grown
into a vast field and new applications of plasma-processed dust particles
are emerging. There is demand for particles with special properties and
for particle-seeded composite materials. For example, the stability of
luminophore particles could be improved by coating with protective Al2O3
films which are deposited by a PECVD process using a metal-organic precursor gas.
Alternatively, the interaction between plasma and injected micro-disperse powder
particles can also be used as a diagnostic tool for the study of plasma surface
processes. Two examples will be provided: the interaction of micro-sized (SiO2)
grains confined in a radiofrequency plasma with an external ion beam as well as
the effect of a dc-magnetron discharge on confined particles during deposition
have been investigated.
Colossal magneto-resistance manganites are characterized by a complex interplay of charge, spin, orbital and lattice degrees of freedom. Formulating microscopic models for these compounds aims at meeting two conflicting objectives: sufficient simplification without excessive restrictions on the phase space. We give a detailed introduction to the electronic structure of manganites and derive a microscopic model for their low-energy physics. Focusing on short-range electron–lattice and spin–orbital correlations we supplement the modelling with numerical simulations.
This work studies different alternatives for parallelization of ground-state DMRG, with a focus on shared memory multiprocessor systems. Exploiting the parallelism in the dominant part of a DMRG calculation (diagonalization of the superblock Hamiltonian), speedups of 5 to 6 on 8-CPU machines can be achieved. A performance analysis gives hints as to which machine is best siuted for the task. The parallelized DMRG code is then applied to current problems in theoretical solid state physics with electronics, bosonic and spin degrees of freedom. Stripe-like modulations of the hole density in the ground state of doped Hubbard with cylindrical boundary conditions are idenficied in the thermodynamic limit using extrapolation techniques. In the 1D Holstein model of spinless fermions at half filling, Luttinger parameters and the charge structure factor are determinde in order to derive the phase diagram that had previously been established only on small lattices. For the 1D half-filled Holstein-Hubbard model, a finite size analysisof spine and charge excitation gaps in the relevant sectors (Mott insulator, Peierls band insulator and bipolaronic Peierls insulator) is able to yield the phase diagram as well. Finally, is the Heisenberg spin chain with dynamical phonons is considered as a relevant model for a spin-Peierls transition in Copper Germanate. Using DMRG, the relation between singlet-triplet excitation gap and dynamical dimeriaztion is calculated for the first time.
First-principle path integral Monte Carlo simulations were performed in order to analyze correlation effects in complex electron-hole plasmas, particularly with regard to the appearance of excitonic bound states. Results are discussed in relation to exciton formation in unconventional semiconductors with large electron hole mass asymmetry.
Collisional absorption of dense fully ionized plasmas in strong high-frequency laser fields is investigated in the non-relativistic case. Quantum statistical methods are used as well as molecular dynamics simulations. In the quantum statistical expressions for the electrical current density and the electron-ion collision frequency–valid for arbitrary field strength–strong correlations are taken into account. In addition, molecular dynamic simulations were performed to calculate the heating of dense plasmas in laser fields. Comparisons with the analytic results for different plasma parameters are given. Isothermal plasmas as well as two-temperature plasmas are considered.