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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.
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.
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.
In the present thesis, a systematic study of beam driven Alfvén eigenmodes in high-density and low-temperature plasmas of the W7-AS stellarator is performed. The device went out of operation in 2002 and the study is based on stored experimental data. Alfvén instabilities can roughly be divided into ideal MHD Alfvén eigenmodes and those existing due to kinetic effects. The spectrum of ideal MHD Alfvén waves in toroidal fusion devices consists of a continuum of stable waves that are strongly localized. Weakly damped, discrete eigenmodes can exist in gaps of the continuous spectrum which are formed by plasma inhomogeneities and the coupling of Alfvén continua. This allows an identification of ideal MHD Alfvén eigenmodes in terms of their frequency and mode numbers. Kinetic effects can modify this spectrum and cause additional types of eigenmodes, the kinetic Alfvén eigenmodes (KAE) and energetic particle modes (EPM). The goal of this thesis is twofold: (I) identification and description of fast particle driven Alfvén instabilities in W7-AS, and (II) study of energetic particle losses induced by Alfvén instabilities. The reconstruction of the ideal MHD plasma equilibrium for each discharge with sufficient accuracy is the very foundation of all subsequent steps. This is achieved, based on measured plasma parameter profiles that are further refined by validating them to the measurements of other, independent plasma diagnostics. The applied scheme is inspired by an approach of Integrated Data Analysis (IDA) to combine different diagnostic data and provide combined uncertainties. After mode number analysis and eigenmode identification, the theoretically expected, linear growth rate of the instability is calculated where possible, and the various contributions of the fast particle drive to the instability of the mode are identified. Alfvénic activity recorded by the Mirnov diagnostic is analyzed, which consists of a set of spatially distributed coils that measure magnetic fluctuations. On W7-AS, the probes are arranged in three poloidal arrays at different toroidal positions. The spacing between the probes is non-equidistant. In addition, the signals of one probe array are digitized with a different sample rate. These characteristics prohibit the straight-forward use of standard tools available for harmonic analysis. Instead, a new tool has been developed and thoroughly tested. It is a multi-dimensional extension of the Lomb periodogram, able to provide reliable time-resolved frequency and mode number spectra in the case of uneven datapoint spacing. Numerical studies of this periodogram show a good performance with respect to mode number resolution given the low number of available probes, and robustness against perturbations of the signal. Only two of the probe arrays can be used for the analysis of eigenmodes with frequencies >70 kHz, such that for high-frequency phenomena insufficient information about the mode numbers is available. A total of 133 different Alfvén eigenmodes is studied in discharges from different experimental campaigns. A restriction to discharges from various high-beta campaigns with neutral beam heating is required to allow for a realistic reconstruction of plasma equilibrium and velocity distribution functions of energetic particles. The discharges are characterized by high density, ne = 5 x 1019 m-3 to 2.5 x 1020 m-3 at relatively low temperatures of Te = Ti = 150 ... 600 eV. Alfvén eigenmodes often appear transiently in the startup phase of these discharges, where density and heating power are being ramped up. Occasionally, Alfvén eigenmodes are seen in the stationary, high-beta phase in the presence of considerable neutral beam heating. Most of the Alfvén eigenmodes are successfully classified as ideal MHD eigenmodes. 19 global, 47 toroidicity-induced and 8 ellipticity-induced Alfvén eigenmodes (GAEs, TAEs, and EAEs, respectively) are unambiguously identified by their mode numbers and frequencies. Excellent agreement between experimentally observed mode number spectra and theoretically calculated eigenmode structure is shown for a TAE example. Additional 13 events are found to have frequencies inside the EAE gap and could possibly be EAEs. Evidence for high-frequency Alfvén eigenmodes (mirror- and helicity-induced Alfvén eigenmodes) is seen, but can not be proven rigorously due to uncertain mode numbers and the complexity of the Alfvén continuum. The remaining 41 Alfvén eigenmodes can not be classified to be one of the above cases. Reasons are either high frequencies, mode numbers obscured by far-field effects, or mode numbers that could not be related to ideal MHD Alfvén eigenmodes. A selection of these shows indications of strong non-linear wave-particle interactions and are assumed to be EPMs. Kinetic Alfvén eigenmodes are not expected to exist in the experimental conditions that were studied. The radially resolved velocity distribution function is used to describe the parameter regimes in which the modes are observed in terms of the dimensionless parameters vb/vA (beam velocity normalized to the Alfvén velocity) and ßfast/ßth, where beta is the ratio of plasma pressure to magnetic pressure. The first parameter describes through which of the possible resonance velocities particles can interact with the eigenmode. A peculiarity of the fast particle dynamics in fusion devices is that they can resonantly interact with Alfvén eigenmodes through sideband resonances even if v < vA. The second parameter describes the energy content of the destabilizing fast particle population compared to the potentially stabilizing thermal plasma component. These parameters contain relevant information about the instability of an eigenmode and such diagrams are given for all observed modes. In addition to that, the expected linear growth rate of gap modes is calculated based on a theoretical model that extends the ideal MHD by a perturbative, drift-kinetic description of the energy exchange between waves and circulating particles, neglecting the effects of trapped particles. For the discharges under consideration the thermal electron speed is comparable to vA and the electrons provide a significant Landau damping contribution. Due to strong density gradients near the plasma boundary in most of the discharges, the thermal ions can provide a small drive via the spatial inhomogeneity which does not overcome the electron damping, however. The drive by spatial inhomogeneity of thermal ions requires a certain propagation direction of the mode and is equally stabilizing for opposite mode numbers. The fast particles also contribute to the growth rate via spatial inhomogeneity, velocity gradients and velocity anisotropy terms are negligible in W7-AS. Most of the observed GAE or EAE modes have negative mode numbers, which correspond to a propagation direction for which the spatial inhomogeneity of thermal and beam ions is predicted to be stabilizing. A fast particle drive of these modes is not confirmed, whereas the TAEs are found to be strongly destabilized by neutral beam injection. The distribution of plasma parameters for discharges showing TAEs in terms of the dimensionless stability parameters suggests an instability threshold that is qualitatively confirmed by an exploration of the parameter space with the theoretical model. Wave-induced, resonant losses of energetic ions scale linearly with the wave amplitude. To identify them, correlations between ion loss probe signals and wave amplitudes are searched, where correlation times in the order of the slowing-down time of energetic particles are expected. Significant correlations can be established only exceptionally for 3 of the identified ideal MHD Alfvén eigenmodes. Those Alfvén eigenmodes, however, which are assumed to be EPMs frequently show severe losses of energetic ions that are visible in the time traces of the plasma energy as well.
A fluorescent lamp driven with an 'instant start electronic control gear' starts in a glow mode. In the glow mode, which lasts typically for tens of milliseconds, the cathode fall exceeds hundreds of volts. This causes high energy ion bombardment of the electrode which heats the electrode, and induces a transition from glow to arc mode. In the arc mode the electrode emits thermionically and the cathode fall drops to the 12 – 15 V range. Unfortunately, the high energy ion bombardment during the glow mode leads also to intense sputtering of electrode material, including tungsten as well as emitter. Thus, instant started fluorescent lamps often suffer from early failures due to coil fracture. Therefore, the investigation of tungsten erosion during instant start is necessary and was the main goal of this work.
The density of neutral atomic tungsten is determined by laser-induced fluorescence (LIF) and optical emission spectroscopy measurements (OES). Investigations are performed on a low-pressure argon dc discharge and on commercial fluorescent lamps. To include the entire temperature profile along the electrode the diffuse and spot operation modes of the dc lamp are studied experimentally and theoretically. The measured dependencies of the cathode temperature along the coil on the discharge and heating parameters are compared with the calculated results. For the first time the tungsten erosion during instant start of commercial fluorescent lamps was experimentally investigated in this work. The erosion process could be related to sputtering. A reconstruction of the temporal evolution of the absolute tungsten population density of the ground state during the glow mode was presented. The sputtered tungsten density increases immediately with the ignition, reaches a maximum where the discharge contracts at the end of the glow mode, and decreases some milliseconds before the glow-to-arc transition takes place. The maximum tungsten density was observed within a region of a few hundred micrometers only located at the discharge attachment point. The main result achieved in this work is that during the whole glow mode tungsten is sputtered. Therefore, the lifetime of instant started fluorescent lamps can be enhanced by reducing the duration of the glow mode. Additionally, the need for the application of different types of diagnostics for the observation of lamp ignition was shown due to different results of LIF, AAS and OES: The observation of excited tungsten atoms by OES shows the maximum emission signal at the glow-to-arc transition whereas by LIF and AAS measurements of tungsten atoms in the ground state the maximum density is found during the whole glow mode. This can be explained by the fact that the intensity of the spontaneous emitted light is related not only to the density but also to the degree of excitation.
The WEGA stellarator is used to confine low temperature, overdense (densities exceeding the cut-off density of the heating wave) plasmas by magnetic fields in the range of B=50-500 mT. Microwave heating systems are used to ignite gas discharges using hydrogen, helium, neon or argon as working gases. The produced plasmas have been analyzed using Langmuir and emissive probes, a single-channel interferometer and ultra-high resolution Doppler spectroscopy. For a typical argon discharge in the low field operation, B=56 mT, the maximum electron density is n_e~10^18m^{-3} with temperatures in the range of T=4-12 eV. The plasma parameters are determined by using Langmuir probes and are cross-checked with interferometry. It is demonstrated within this work that the joint use of emissive probes and ultra-high resolution Doppler spectroscopy allows a precise measurement of the radial electric field. Here the floating potential measurements using emissive probes have been compared to measurements of the poloidal rotation of the plasma which is also linked to the radial electric field. In order to alter the plasma parameters a biasing probe setup has been used during this work. The focus of this work is on demonstrating the ability to modify the existing radial electric field in a plasma by using the biasing probe. This technique is in principle not new, as it has been around for decades. Looking at details, it turns out that describing low field operation WEGA argon plasmas in connection with biasing is not covered by the present set of theoretical approaches and experimental cognition. This work will commence with a basic approach and first establishes the diagnostic tools in a well-known discharge. Then the perturbation caused by the biasing probe is assessed. Following the characterization of the unperturbed plasmas, plasma states altered by the operation of the energized biasing probe will be characterized. It is demonstrated that modifying the existing radial electric field can be achieved and reliably diagnosed using spectroscopy and probe measurements. In order to verify the different approaches for determining the radial electric field the diagnostics are cross-checked against another whenever possible. During biasing the plasma two different stable plasma states have been found. Stable here refers to the state existing much longer than the confinement time for WEGA. The presence of a calorimetric limiter placed in the scrape-off layer has an impact on the type of the plasma state. The two observed plasma states differ in plasma parameter profiles, such as density, temperature, electric field and confined energy. The results are compared to two simple models. One model relies on the relevant atomic processes and a second one is based on neoclassical theory. Both models can be used to derive the particle and power flux from the plasma. The losses predicted by the atomic models can be tested using bolometry. It can be shown that both models agree well in the description of the particle balance of the electrons for large regions of the plasma. By comparing the models the neoclassical heat flux turns out to be small compared to the energy fluxes caused by atomic processes. For the reference discharge taking the energy flux due to the atomic processes and balancing it by the input microwave power is satisfying the energy balance, without the need for transport. For the biased discharges it turns out that neoclassical transport can be neglected as well, but the additional biasing power has to be taken into account. A simple model for the biasing power is motivated and tested. An agreement in the energy balance can be reached in this way as far as the models are applicable. The models also allow drawing conclusions on the amount of absorbed microwave power.
The present work is the first work dealing with turbulence in the WEGA stellarator. The main object of this work is to provide a detailed characterisation of electrostatic turbulence in WEGA and to identify the underlying instability mechanism driving turbulence. The spatio-temporal structure of turbulence is studied using multiple Langmuir probes providing a sufficiently high spatial and temporal resolution. Turbulence in WEGA is dominated by drift wave dynamics. Evidence for this finding is given by several individual indicators which are typical features of drift waves. The phase shift between density and potential fluctuations is close to zero, fluctuations are mainly driven by the density gradient, and the phase velocity of turbulent structures points in the direction of the electron diamagnetic drift. The structure of turbulence is studied mainly in the plasma edge region inside the last closed flux surface. WEGA can be operated in two regimes differing in the magnetic field strength by almost one order of magnitude (57mT and 500mT, respectively). The two regimes turned out to show a strong difference in the turbulence dynamics. At 57mT large structures with a poloidal extent comparable to the machine dimensions are observed, whereas at 500mT turbulent structures are much smaller. The poloidal structure size scales nearly linearly with the inverse magnetic field strength. This scaling may be argued to be related to the drift wave dispersion scale. However, the structure size remains unchanged when the ion mass is changed by using different discharge gases. Inside the last closed flux surface the poloidal ExB drift in WEGA is negligible. The observed phase velocity is in good agreement with the electron diamagnetic drift velocity. The energy in the wavenumber-frequency spectrum is distributed in the vicinity of the drift wave dispersion relation. The three-dimensional structure is studied in detail using probes which are toroidally separated but aligned along connecting magnetic field lines. As expected for drift waves a small but finite parallel wavenumber is found. The ratio between the average parallel and perpendicular wavenumber is in the order of 10^-2. The parallel phase velocity of turbulent structures is in-between the ion sound velocity and the Alfvènvelocity. In the parallel dynamics a fundamental difference between the two operational regimes at different magnetic field strength is found. At 500mT turbulent structures can be described as an interaction of wave contributions with parallel wavefronts. At 57mT the energy in the parallel wavenumber spectrum is distributed among wavenumber components pointing both parallel and antiparallel to the magnetic field vector. In both cases turbulent structures arise preferable on the low field side of the torus. Some results on a novel field in plasma turbulence are given, i.e. the study of turbulence as a function of resonant magnetic field perturbations leading to the formation of magnetic islands. Magnetic islands in WEGA can be manipulated by external perturbation coils. A significant influence of field perturbations on the turbulence dynamics is found. A distinct local increase of the fluctuation amplitude and the associated turbulent particle flux is found in the region of magnetic islands.