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Abstract Atmospheric Pressure Discharges have attracted much interest in recent years. The development of a new processes based on this discharge needs a clear understanding of plasma and discharge physics and chemistry. At the present time much attention is paid to the chemical processes in barrier discharge plasma in various gas mixtures, since the understanding of these processes is necessary for the development of industrial reactors. Besides these, hydrocarbons are being used for the formation of diamond like or amorphous carbon (DLC) films. Specially, hydrogenated amorphous carbon (a-C: H) and plasma polymerization. In this work we have used Dielectric Barrier Discharge (DBD) a plasma device used to investigate simple hydrocarbon reactions in a plasma phase. Our aim of plasma phase chemical reaction studies is to form molecular hydrogen, higher order hydrocarbons CnHm up to n ≥ 12 series and nitrogen - containing organic complexes using simple hydrocarbons. Deposition of thin organic films or DLC films were carried out using the DBD. In this study we have chosen certain combination of gases such as C2Hm/N2 (m = 2, 4, 6) and C2Hm/Ar (m = 2, 4, 6); the purpose of using N2 and Ar gases are to dilute and stabilize the hydrocarbon plasma and to investigate plasma chemical reactions with nitrogen gas. All reactions were carried out under an atmospheric pressure (300 mbar) with gas ratio 1:2; Experiments were performed by applying high voltage with a frequency 5.5 kHz. The plasma phase diagnostics have been investigated using mass spectrometry and FTIR spectroscopy. Formation of molecular hydrogen, N-containing organic complexes and higher order hydrocarbons with C ≥ 12, have been investigated with mass spectrometry. FTIR spectroscopy reveals the formation of substituted alkanes (sp3), alkenes (sp2) and alkynes (sp) and nitrogen containing functional groups from the individual gases which are used in this work. Abundant formation of acetylene occurs with C2H6 and C2H4 as precursor gases. Amorphous hydrogenated carbon nitride (a-CNx:H) films have been deposited on Si (100) and glass substrates using gas mixtures C2Hm/N2 (m = 2, 4, 6). Surface chemical compositions have been derived from Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) and X-ray Photo electron Spectroscopy (XPS). FT-IRRAS and XPS show the presence of sp, sp2 and sp3 bonds of carbon and nitrogen for C2Hm/N2 thin films. Various functional groups such as amines, saturated and unsaturated alkyl groups have been identified. Thin films obtained from C2H2/N2 and C2H4/N2 gas mixture had a larger N/C ratio when compared to the film obtained from C2H6/N2. Thickness, refractive index and extinction co-efficient were investigated by ellipsometry. Rate of deposition have been investigated. Different surface morphology has been derived using Scanning Electron Microscopy. Amorphous hydrogenated carbon (a-C:H) films or diamond like carbon (DLC) films have been deposited on Si (100) and glass substrates using gas mixtures C2Hm/Ar (m = 2, 4, 6). Diagnostics for the deposited films have been done using different spectroscopic techniques. Surface chemical compositions have been derived from Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) and X-ray Photo electron Spectroscopy (XPS). FT-IRRAS show the presence of sp, sp2 and sp3 bonds of carbon and hydrogen for C2Hm/Ar (m = 2, 4, 6) thin films. The characteristic peak for C1s has been observed from XPS. Thickness, refractive index and extinction co-efficient were investigated by ellipsometry. Rate of deposition have been investigated.
In the framework of the current work has been the plasma initiated and surface catalysed species conversion studied in low pressure and atmospheric plasmas. The aim of the work is to improve the understanding of the internal processes in order to increase the energy efficiency as well as the selectivity of the reaction products of future plasma devices. Beside many technical applications of plasmas, air purification shows great potential. Over the last decades, plasma based pollution control has proofed its ability to remove harmful contaminants or annoying odours from an air stream. However, the energy efficiency and the selectivity of the products are a remaining challenge.
Motivated by these issues, a multi stage packed-bed reactor has been used to remove admixed ethylene and toluene from an air stream. It has been found that the maximum toluene destruction has been 60%, whereas ethylene has been nearly completely removed. The specific energy β has been between 120 and 1600 JL-1. Fourier Transform Infrared spectroscopy, FTIR spectroscopy, has been used to identify and quantify the species H2O, CO2, CO, O3, HNO3, HCN, CH2O, CH2O2, N2O and NO2. However, none of these experiments led to the detection of NO.
The embedment of packing material into a plasma volume leads to increased surface effects. In order to study them, the inner side of a tube reactor, made of Pyrex, served as the surface under study and has been exposed to a rf plasma for 1h. The surface effects of the plasma treatment have been investigated indirectly by studying the oxidation of NO into NO2. After the plasma exposure, the reactor has been evacuated and filled with a gas mixture of 1% NO in N2 / Ar. Both species have been measured using quantum cascade laser absorption spectroscopy, QCLAS. It has been found that, using oxygen containing plasmas, the NO concentration decreased whereas the NO2 concentration increased. Therefore, oxygen containing plasmas are able to deposit oxygen on the surface. The filling with NO leads to the oxidation via the Eley-Rideal mechanism. A simplified model calculation supports these assumptions.
For a more comfortable application of the QCLAS, a compact multi channel spectrometer has been developed, TRIPLE Q. It combines the high time resolution with the possibility to measure the concentration of at least three infrared active species simultaneously. Due to the high time resolution, a huge number of spectra have to be analysed. In order to calculate absolute number densities, an algorithm has been developed which automatically treats typical phenomena like pulse jitter, rapid passage effect or variations of the intensity of the laser pulses.
The gas temperature is an important parameter in plasma physics. Using the TRIPLE Q system, the gas temperature has been determined for pulsed dc plasmas. For this case, NO has been used as a probe gas. From the spectra, the temperature has been calculated using the line ratio method. The relative intensity of the absorption structures of NO at 1900.5cm-1 and 1900.08cm-1 depend on the temperature. Therefore, the ratio has been used to calculate the gas temperature with a time resolution in the μs range.
Vibrationally excited nitrogen can be an energy reservoir that plays an important role in plasma chemistry. In N2 / N2O plasmas, vibrationally excited N2 can undergo relaxation via a resonant vibration vibration coupling between vibrationally excited N2 and N2O. Due to such an efficient energy transfer, the method allows one to study the relaxation of vibrationally excited N2. Using this method, molecules, which are not infrared active, can be monitored. This approach has extended the field of scientific and commercial applications of the QCLAS.
Particle and heat transport in fusion devices often exceed the neoclassical prediction. This anomalous transport is thought to be produced by turbulence caused by microinstabilities such as ion and electron-temperature-gradient (ITG/ETG) and trapped-electron-mode (TEM) instabilities, the latter ones known for being strongly influenced by collisions. Additionally, in stellarators, the neoclassical transport can be important in the core, and therefore investigation of the effects of collisions is an important field of study. Prior to this thesis, however, no gyrokinetic simulations retaining collisions had been performed in stellarator geometry. In this work, collisional effects were added to EUTERPE, a previously collisionless gyrokinetic code which utilizes the δ f method. To simulate the collisions, a pitch-angle scattering operator was employed, and its implementation was carried out following the methods proposed in [Takizuka & Abe 1977, Vernay Master's thesis 2008]. To test this implementation, the evolution of the distribution function in a homogeneous plasma was first simulated, where Legendre polynomials constitute eigenfunctions of the collision operator. Also, the solution of the Spitzer problem was reproduced for a cylinder and a tokamak. Both these tests showed that collisions were correctly implemented and that the code is suited for more complex simulations. As a next step, the code was used to calculate the neoclassical radial particle flux by neglecting any turbulent fluctuations in the distribution function and the electric field. Particle fluxes in the neoclassical analytical regimes were simulated for tokamak and stellarator (LHD) configurations. In addition to the comparison with analytical fluxes, a successful benchmark with the DKES code was presented for the tokamak case, which further validates the code for neoclassical simulations. In the final part of the work, the effects of collisions were investigated for slab and toroidal ITGs and TEMs in a tokamak configuration. The results show that collisions reduce the growth rate of slab ITGs in cylinder geometry, whereas they do not affect ITGs in a tokamak, which are mainly curvature-driven. However it is important to note that the pitch-angle scattering operator does not conserve momentum, which is most critical in the parallel direction. Therefore, the damping found in a cylinder could be the consequence of this missing feature and not a physical result [Dimits & Cohen 1994]. Nonetheless, the results are useful to determine whether the instability is mainly being driven by a slab or toroidal ITG mode. EUTERPE also has the feature of including kinetic electrons, which made simulations of TEMs with collisions possible. The combination of collisions and kinetic electrons made the numerical calculations extremely time-consuming, since the time step had to be small enough to resolve the fast electron motion. In contrast to the ITG results, it was observed that collisions are extremely important for TEMs in a tokamak, and in some special cases, depending on whether they were mainly driven by density or temperature gradients, collisions could even suppress the mode (in agreement with [Angioni et al. 2005, Connor et al. 2006]). In the case of stellarators it was found that ITGs are highly dependent on the device configuration. For LHD it was shown that collisions slightly reduce the growth rate of the instability, but for Wendelstein 7-X they do not affect it and the growth rate showed a similar trend with collisionality to that of the tokamak case. Collisions also tend to make the ballooning structure of the modes less pronounced.
Quantum-Kinetic Modeling of Electron Release in Low-Energy Surface Collisions of Atoms and Molecules
(2012)
In this work we present a theoretical description of electron release in the collision of atomic and molecular projectiles with metallic and especially dielectric surfaces. The associated electron yield, the secondary electron emission coefficient, is an important input parameter for numerical simulations of dielectric barrier discharges and other bounded low-temperature gas discharges. The available reference data for emission coefficients is, however, very sparse and often uncertain, especially for molecular projectiles. With the present work we aim to contribute to the filling of these gaps by providing a flexible and easy-to-use model that allows for a convenient calculation of the emission coefficient and related quantities for a wide range of projectile-surface systems and the most dominant reaction channels.
This thesis describes investigations of metal clusters stored in an ion-cyclotron resonance (ICR) trap, as well as corresponding trap research and development. Charged clusters are produced and investigated in the experimental setup Cluster-Trap, comprising a cluster-ion source, an ICR trap and a time-of-flight (ToF) mass spectrometer. In the framework of its move to the new building of the Institute of Physics, new components have been added to the ClusterTrap setup. A radio-frequency ion trap is now used for cluster ion preparation prior to the performance of cluster experiments in the ICR trap. A quadrupole ion deflector allows an optimized usage of the ICR trap, as well as simultaneous use of several ion sources and detectors. The implementation of a potential lift at the ToF mass spectrometer enables a more flexible operation of the setup with ion energies up to several hundreds of electron volts. The new components have been tested and characterized, and the experimental procedures have been adapted. An important aspect of cluster investigations is the manipulation of trapped ions by application of appropriate excitation fields. For the ICR trap, a vector representation model has been developed for quick analysis of radial excitation fields, applied to the quarter-segmented ring electrode of an ICR trap. Its application has been demonstrated for asymmetric radial quadrupolar excitation of stored cluster ions, confirming the observation of unintended ion ejection from the trap. Investigation of multiply negatively charged metal clusters at ClusterTrap has been continued. By the "electron-bath" technique, i.e. simultaneous storage of cluster mono-anions and electrons in the ICR trap, high charge states are produced up to a limit which arises from restrictions for ion trapping. A modification of the electron bath, which bypasses this limit, has been introduced and demonstrated by the first-time production and detection of aluminum cluster anions carrying five excess electrons (penta-anions). Results of the penta-anion production as a function of the trapping voltage relate to the Coulomb potentials of the cluster anions involved, in agreement with previous findings. The observed poly-anionic clusters are meta-stable and their abundance as a function of the cluster size is determined by their lifetimes. Observed poly-anion abundances are described by a thermionic-emission approach, by means of the Richardson-Dushman formula. The height of the Coulomb potential in the formula is decreased to match experimental data, thus accounting for electron tunneling. Poly-anions are observed only above a minimum cluster size, the appearance size. To determine this limit from experimental results, a new data evaluation method has been introduced, which considers the poly-anion lifetimes and respective abundances of a range of cluster sizes. As a result, the experimental appearance size is larger than the smallest poly-anionic cluster observed, in contrast to previous approaches.
Beams of ions and electrons are a source of free energy which can be transferred to waves via an instability. Beams exist in almost all plasma environments, but their instabilities are particularly important for the dynamics of space plasmas. In the absence of collisions, the instability drives waves to large amplitudes and forms nonlinear structures such as solitary waves. The electric fields in these waves can scatter particles in the background plasma, or disrupt currents. Both of these effects are important for the overall dynamics of the plasma. In this thesis, both electron and ion beam plasma instabilities have been investigated in the linear plasma device VINETA and using a Particle-in-Cell simulation. The electron beam instability has been demonstrated by previous authors to be a useful diagnostic for the plasma density. The spatial resolution of previous results was confirmed at a few millimetres, and a temporal resolution of 1ms was shown for the first time. An ion beam was generated with a double plasma discharge. Compared to space, this environment and indeed most laboratory plasmas have considerably higher collisionality and a limited spatial extent which introduces gradients in the plasma. Gradients perpendicular to the beam propagation direction are linked to a decrease of both the wavelength and amplitude of the instability. It was observed in both experiment and simulation that gradients in sheaths at the boundaries of the plasma not only affect the time averaged plasma parameters, but also excite instabilities. Fluctuations within the sheath spread the beam in velocity space, effectively increasing its temperature. Warmer beams require a higher drift velocity to excite an instability. This was also confirmed by experimental and numerical results. Collisions are shown to be the dominant damping force for the electron beam instability. For ions, collisions play an important role in the simulation, but appear to be overshadowed by Landau damping from impurities in the experiment. When boundary conditions are removed from the simulation, wave amplitudes increase and nonlinear effects become important. Saturation by particle trapping and coalescence of phase space holes is observed, which could eventually lead to the solitary waves as they are observed in space plasmas.