Plasma Diagnostics and Modelling of Nanosecond Pulsed Surface Dielectric Barrier Discharge Actuators

During the past years, an increasing number of studies have been conducted on the use of electrical discharges for the stabilization of airflows (plasma flow control). Electrical gas discharges transfer energy and momentum to the gas through collisions of free electrons with atoms and molecules. Chemically active species such as ions, radicals and excited species are produced due to these collisions. The use of plasma actuators, notably surface dielectric barrier discharges (SDBD), for flow control applications has been largely investigated, and it has been demonstrated to effectively control the flow at low flow speed (below 30 m/s). Nowadays, the research in this area focuses on ways to improve the plasma actuators for flow speeds relevant to real flight conditions. One promising device for plasma flow control at high flow speed is the nanosecond pulsed surface dielectric barrier discharge. Nanosecond pulsed plasmas in general have also drawn attention from other fields, such as plasma assisted combustion, due to their ability to produce a large amount of active species and producing substantial overheating in a very short time. In this thesis an investigation of nanosecond pulsed SDBD is presented, with a focus on flow-control applications, but also on the production of active species, which are of great interest for plasma-assisted combustion and for other fields. The experimental characterization of the plasma created for different conditions, such as the operating pressure, the polarity and the amplitude of the applied voltage, is conducted. Important plasma parameters such as the gas temperature, the excited species produced, the reduced electric field or the electron density are either directly measured or inferred from emission spectroscopy using existing and novel diagnostic methods. The validity of the diagnostic methods is demonstrated using a numerical model of the plasma. The numerical modelling of the plasma also allows determining the influence of the plasma on the flow for several conditions. The experimentally studied conditions are simulated and compared with experimental results to show the strengths and limitations of the numerical model.

Aero Engine Flow Control by Surface Discharge Plasma

Sami Goekce, Christoph Hollenstein, Pénélope Leyland, Peter Ott, Philip Peschke

Experimental investigations and numerical modeling with the objective to study the effects that surface discharge plasmas can provoke on aerodynamic flow situations, including high speed flows present in aero-engines, are undertaken at EPFL. The actuator’s potential to influence the flow under realistic flight and engine operating conditions, largely depends on its geometric configuration and the power supply. Initially, a long-life (more than 12 hours) surface discharge actuator was designed at EPFL and proven to survive transonic flow speeds on typical compressor blades. This was the first time that such a long-term, continuously operating actuator was applied successfully. However, with AC voltage applied to the electrodes, the effects on the shock waves did not appear to be significant [1, 2]. Recent experiments conducted at EPFL focused on fast rise voltage pulse driven dielectric barrier discharge (DBD) actuators and the development of a power supply that could provide the required voltage pulses. A generic DBD actuator with two electrodes in asymmetric configuration, separated by a dielectric was chosen. High speed imaging was applied to investigate the spatial and temporal discharge development. In order to study the influence of voltage and current on the plasma, several power supply configurations were examined by varying voltage rise and fall time, maximum voltage, peak current and pulse width. Two discharge periods were observed on the positive and the negative edge of the voltage pulse. It was observed that the characteristics of the plasma differ considerably between these two periods. Furthermore, the light emission intensity and the propagation speed of the plasma along the surface of the dielectric increased with the maximum voltage and peak current. A similar effect was observed when the voltage rise time was decreased. In a next step, a technique similar to synthetic schlieren was applied to examine the interaction of the DBD actuator with air under atmospheric pressure. A micro shock wave that propagates from the electrode edge normal to the surface was observed, which agrees well with numerical results gained during a parallel investigation. In addition, the impact on the flow structure of maximum voltage and frequency is currently examined. First results suggest that the intensity of the micro shock wave increases with the voltage whereas the frequency primarily provokes electrode heating. With the objective to investigate the modelling of the fast rise pulse driven DBD discharge, a numerical tool was developed that applies emission spectroscopy to determine the electron energy distribution function (EEDF) by comparing the relative intensity of bands, as described by several authors [3-5]. The EEDF is then used to infer populations present in the gas through electron impact ionization, excitation or dissociation cross sections data [6]. Contrary to previous studies, this approach employs two naturally occurring transitions in atmospheric plasmas in conjunction with recent data on collisional quenching [7], which makes the method more reliable and does not require the adjunction of other species such as Helium. References: 1. Pavon S., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Effects of high-speed airflows on a surface dielectric barrier discharge”, J. Phys. D: Appl. Phys. 40. No 6.2 1733-1741, 2007. 2. Pavon S., Sublet A., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Long Lifetime system for the generation of Surface Plasmas”, International Patent no: P1883PC00/13-71, PCT/IB2009/050489, 2008. 3. Bibinov N. K., Kokh D.B., Kolokolov N.B., Kostenko V.A., Meyer D., Vinogradov I.P., Wiesemann K., “A comparative study of the electron dis-tribution function in the positive columns in N2 and N2/ He dc glow dis-charges by optical spectroscopy and probes”, Plasma Sources Science and Technology, 298-309,1998. 4. Behringer K., Fantz U., “Spectroscopic diagnostics of glow discharge plasmas with non-maxwellian electron energy distributions”, Journal of Applied Physics D, 2128-2135, 1994. 5. Isola L.M., Gómez B.J., Guerra V., “Determination of the electron tem-perature and density in the negative glow of a nitrogen pulsed discharge using optical emission spectroscopy”, Journal of Applied Physics D, 015202, 2010. 6. Itikawa Y., “Cross sections for electron collisions with nitrogen mole-cules”, Journal of Physical and Chemical Reference Data, 31-53, 2006. 7. Dilecce G., Ambrico P.F., Benedictis S., “On the collision quenching of N2plus by N2 and O2 and its influence on the measurement of E over N by intensity ratio of nitrogen spectral bands”, Journal of Applied Physics D, 195201, 2010.

2010

Single molecule detection and fluorescence correlation spectroscopy on surfaces

Kai Hassler

In this thesis a new approach for single molecule detection and analysis is explored. This approach is based on the combination of two well established methods, fluorescence correlation spectroscopy (FCS) and total internal reflection fluorescence microscopy (TIRFM). In contrast to most existing fluorescence spectroscopy techniques, the subject of primary interest in FCS is not the fluorescence intensity itself but the random intensity fluctuation around the mean value. Intensity fluctuations are induced by thermal noise in a minute observation volume, which is in classical FCS the confocal volume of a confocal microscope. E.g. FCS is commonly utilized to investigate diffusion. In this case, diffusing fluorescent molecules entering or leaving the observation volume cause intensity fluctuations, which are analyzed by calculating the temporal autocorrelation of the observed signal. The autocorrelation is a measure for the self-similarity of a signal and contains information about the average fluctuation strength and duration. The confocal observation volume, i.e. the measurement volume that is actually seen by the detector is approximately given by the product of the optical transfer function with the fluorescence exciting intensity distribution of a focused laser beam. To achieve a high signal-to-background ratio a small observation volume is absolutely essential, first of all because the background from e.g. scattered light increases with the size of the observation volume. Second, a small volume assures for a small average number of fluorophores inside the observation volume and therefore for a high fluctuation amplitude i.e. FCS signal. This thesis proposes and discusses an alternative to confocal FCS specially conceived for measurements on surface-bound molecular systems, such as biological receptors or immobilized enzymes. In contrast to confocal FCS, fluorescence is excited within an evanescent field generated by total internal reflection (TIR) of a laser beam at the interface between a microscope coverslip and the sample. This is achieved by focusing the laser beam off-axis at the back focal plane of a high NA oil-immersion objective. The collimated beam that emerges from the objective is incident at an oblique angle at the coverslip-sample interface and totally internal reflected. In contrast to confocal FCS, the generated observation volume is completely confined to the surface and background fluorescence as well as scattered light from the bulk is efficiently suppressed. Our method, called objective-type TIR-FCS in the following, features an increased collection efficiency compared to existing techniques that combine evanescent wave excitation and FCS. Existing techniques use total internal reflection on the surface of a prism to generate an evanescent field. This leads to a configuration where the choice of objectives is limited to air or water-immersion objectives. In our system we use a high NA oil-immersion objective, specially conceived for TIR applications, which collects light efficiently. The collection efficiency is further enhanced by a naturally occurring change of the emission properties of fluorophores close to interfaces between dielectric media. The presence of the interface favors emission into the optically denser medium so that about 60% of the emitted light can be collected. These factors, together with a reduced observation volume lead to a very sensitive method with a high potential for applications in single molecule detection and analysis. The performance of the proposed method was experimentally shown for measurements on molecules subject to Brownian motion and binding to modified coverslips. In particular, it was experimentally shown that objective-type TIR-FCS features high signal-to-background ratio on a single molecule level. In this thesis, concise derivations of analytical expressions for autocorrelation functions for diffusion and most important, the case of ligands reversibly binding to a single and localized binding site are presented. The derived model allows for the quantitative determination of binding rates for a single receptor. We strongly believe that the application of these results in the context of investigations of receptor-ligand binding kinetics will allow for deeper understanding of cellular signaling. Moreover, this thesis discusses the applicability of the proposed method in enzymology. Enzymes, as most proteins are subject to continuous changes of their structure or conformation. These conformational changes are correlated with the function of the enzyme. In the discussed example the enzyme catalyzes an oxidation where the product is fluorescent but the substrate is not. The function of the enzyme i.e. the recurring product formation leads to observable intensity fluctuations. Since function and conformational states are correlated, conformational fluctuations can be investigated by means of FCS as was already shown for confocal FCS. A technique closely related to FCS is fluorescence lifetime spectroscopy (where lifetime refers to the mean lifetime of the electronic excited state). Whereas in FCS the relaxation after random deviations from thermal equilibrium is investigated, relaxation of excited fluorophores towards their electronic ground-state is investigated in lifetime spectroscopy. The technique is used for e.g. discrimination between fluorophores with different lifetimes. Lifetime spectroscopy combined with imaging is used in many domains of life-science, including microarray reading and medical diagnostics. In preliminary work we developed a novel approach to perform lifetime imaging that is based on a multiplexing technique. The proposed method requires no mechanical scanning stage and only a single-point detector. Furthermore, noise is reduced under certain circumstances if the signal is low. Characteristics of this technique as well as advantages and disadvantages are shortly discussed.

EPFL2006

On the Powder Formation in the Plasma Enhanced Chemical Vapor Deposition Process for the Deposition of SiOx Barrier Coatings

Marina Ricci

Nowadays thin films play an important role in everyday life and in industries. Thin film technology has been developed primarily for the growing demand for development of smaller and smaller devices that require advanced materials and new processing techniques suitable for micro and nano technology. One of the methods of producing thin films is by means of plasma technology. This is a technique by which plasma, also known as 'the fourth state of matter', is used to generate ions, electrons and radicals which deposit at a surface where they will form a thin film. The source of these depositing species could be either gaseous, liquid or solid in origin. Most of the time a plasma is generated by means of an electrical discharge. Plasma technologies are used for various applications. One of these is Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD systems have widespread applications in the electronic sector, because of their exibility in depositing different films such as silicon oxide thin films (SiOx). These films, currently deposited by organosilicon compounds, such as hexamethyldisiloxane (HMDSO), have dielectric and good barrier properties. The aim of the work at CRPP consists in optimizing PECVD processes for the deposition of these films on polymers. In this study, the species in gaseous phase and the powder produced in oxygen-diluted HMDSO plasmas were experimentally characterized in a radiofrequency (RF) capacitively-coupled reactor at 13.56 MHz. The gaseous phase of these plasmas and the particle formation were studied by in-situ infrared absorption spectroscopy and optical emission spectroscopy. The study of the powder formation is important because the particle deposition imposes an upper limit on the deposition rate. Particle formation was firstly studied as a function of HMDSO/O2 ratio. Analysis of the time evolution of the infrared absorption spectra gives the development of the size, the number density, the structure and the particle composition. At high oxygen dilution the formation of large particles is observed. The analysis of HMDSO/O2 plasma at low O2 content suggests that at the beginning of the process very small particles are formed by polymerization; the same particles do not grow much and keep sizes around 50 nm. The admixture of inert or non-reactant gases, such as Ar or N2, into the plasma promotes polymerization of the monomer; a high value of one of these inert or non-reactant gases leads to small powder particle size, since only polymerization takes place in this case. Particle size around 50 nm is also found in this case. At the same time we see a decrease of the carbon content and a weak reduction of the stoichiometry in SiOx indicated by the shift of the SiOSi asymmetric stretching vibration. This reduction also influences the deposited layers, less oxidized and, thus, quite far from a stoichiometric character. Hexamethyldisilazane (HMDSN) and tetramethylsilane (TMS) are other reagents that have been identified as alternatives to HMDSO. The in-situ infrared absorption spectra show that HMDSO/O2 and HMDSN/O2 plasmas are quite similar, but in the second case a lower SiOSi stretching peak intensity, due to NH and SiN peaks, is seen. TMS/O2 and HMDSN/O2 plasma spectra show a low SiOSi bending and stretching peak intensity, due also to the oxygen atom absence in their chemical structure. Two separate chapters are respectively devoted to the silicon nitride thin films (SiNx) and plasma crystal formation. SiNx layers are deposited by three different organosilicon compounds, such as HMDSN, TMS and bis(dimethylamino)dimethylsilane (BDMADMS). No powder formation is visualized in this contest. The ex-situ infrared absorption spectra show the difficulty to produce stable SiNx films; they present a low SiN peak intensity and are quite hygroscopic. Furthermore, in HMDSO/O2 plasmas at high oxygen dilution and high pressure, later in time into the discharge, accretion leads to larger and larger particles which can finish up in particles forming a plasma crystal. The study of RF harmonics shows the presence of instabilities, clear indications of the passage from small particulate to plasma crystal structure.

EPFL2011