Analysis of Electrostatic Turbulence Drive in Texas Helimak.

Plasma turbulence and particle transport in Texas Helimak change with the radial electric field profile modified by an external voltage bias. When the bias is positive, the turbulence shows enhanced level and broadband spectra with extreme events, similar to the turbulence in tokamak scrape-‐off layer. However, negative bias reduces the turbulence level and decreases the spectrum widths. Moreover, for negative biased shots, the particle transport is strongly affected by a wave particle resonant interaction. On the other hand, for positive bias values, the plasma presents a transport barrier in the reversed shear flow region.

Entanglement of two-qubit photon beam by magnetic field.

We have studied the possibility of affecting the entanglement measure of 2-qubit system consisting of two photons with different fi xed frequencies but with two arbitrary linear polarizations, moving in the same direction, by the help of an applied external magnetic field. The interaction between the magnetic fi eld and the photons in our model is achieved through intermediate electrons that interact with both the photons and the magnetic fi eld. The possibility of exact theoretical analysis of this scheme is based on known exact solutions that describe the interaction of an electron subjected to an external magnetic fi eld (or a medium of electrons not interacting with each other) with a quantized field of two photons. We adapt these exact solutions to the case under consideration. Using explicit wave functions for the resulting electromagnetic fi eld, we calculate the entanglement measure of the photon beam as a function of the applied magnetic field and parameters of the electron medium.

Diagnostic techniques for EHD and MHD interaction

The impact of plasma technologies is growing both in the academic and in the industrial fields. Nowadays, a great interest is focused in plasma applications in aeronautics and astronautics domains. Plasma actuators based on the Magneto-Hydro-Dynamic (MHD) and Electro- Hydro-Dynamic (EHD) interactions are potentially able to suitably modify the fluid-dynamics characteristics around a flying body without utilizing moving parts. This could lead to the control of an aircraft with negligible response time, more reliability and improvements of the performance. In order to study the aforementioned interactions, a series of experiments and a wide number of diagnostic techniques have been utilized. The EHD interaction, realized by means of a Dielectric Barrier Discharge (DBD) actuator, and its impact on the boundary layer have been evaluated by means of two different experiments. In the first one a three phase multi-electrode flat panel actuator is used. Different external flow velocities (from 1 to 20m/s) and different values of the supplied voltage and frequency have been considered. Moreover a change of the phase sequence has been done to verify the influence of the electric field existing between successive phases. Measurements of the induced speed had shown the effect of the supply voltage and the frequency, and the phase order in the momentum transfer phenomenon. Gains in velocity, inside the boundary layer, of about 5m/s have been obtained. Spectroscopic measurements allowed to determine the rotational and the vibrational temperature of the plasma which lie in the range of 320 ÷ 440°K and of 3000 ÷ 3900°K respectively. A deviation from thermodynamic equilibrium had been found. The second EHD experiment is realized on a single electrode pair DBD actuator driven by nano-pulses superimposed to a DC or an AC bias. This new supply system separates the plasma formation mechanism from the acceleration action on the fluid, leading to an higher degree of the control of the process. Both the voltage and the frequency of the nano-pulses and the amplitude and the waveform of the bias have been varied during the experiment. Plasma jets and vortex behavior had been observed by means of fast Schlieren imaging. This allowed a deeper understanding of the EHD interaction process. A velocity increase in the boundary layer of about 2m/s had been measured. Thrust measurements have been performed by means of a scales and compared with experimental data reported in the literature. For similar voltage amplitudes thrust larger than those of the literature, had been observed. Surface charge measurements led to realize a modified DBD actuator able to obtain similar performances when compared with that of other experiments. However in this case a DC bias replacing the AC bias had been used. MHD interaction experiments had been carried out in a hypersonic wind tunnel in argon with a flow of Mach 6. Before the MHD experiments a thermal, fluid-dynamic and plasma characterization of the hypersonic argon plasma flow have been done. The electron temperature and the electron number density had been determined by means of emission spectroscopy and microwave absorption measurements. A deviation from thermodynamic equilibrium had been observed. The electron number density showed to be frozen at the stagnation region condition in the expansion through the nozzle. MHD experiments have been performed using two axial symmetric test bodies. Similar magnetic configurations were used. Permanent magnets inserted into the test body allowed to generate inside the plasma azimuthal currents around the conical shape of the body. These Faraday currents are responsible of the MHD body force which acts against the flow. The MHD interaction process has been observed by means of fast imaging, pressure and electrical measurements. Images showed bright rings due to the Faraday currents heating and exciting the plasma particles. Pressure measurements showed increases of the pressure in the regions where the MHD interaction is large. The pressure is 10 to 15% larger than when the MHD interaction process is silent. Finally by means of electrostatic probes mounted flush on the test body lateral surface Hall fields of about 500V/m had been measured. These results have been used for the validation of a numerical MHD code.

Investigation of the dipole response of nickel isotopes in the presence of a high-frequency electromagnetic field

The electric dipole response of neutron-rich nickel isotopes has been investigated using the LAND setup at GSI in Darmstadt (Germany). Relativistic secondary beams of 56−57Ni and 67−72Ni at approximately 500 AMeV have been generated using projectile fragmentation of stable ions on a 4 g/cm2 Be target and subsequent separation in the magnetic dipole fields of the FRagment Separator (FRS). After reaching the LAND setup in Cave C, the radioactive ions were excited electromagnetically in the electric field of a Pb target. The decay products have been measured in inverse kinematics using various detectors. Neutron-rich 67−69Ni isotopes decay by the emission of neutrons, which are detected in the LAND detector. The present analysis concentrates on the (gamma,n) and (gamma,2n) channels in these nuclei, since the proton and three-neutron thresholds are unlikely to be reached considering the virtual photon spectrum for nickel ions at 500 AMeV. A measurement of the stable 58Ni isotope is used as a benchmark to check the accuracy of the present results with previously published data. The measured (gamma,n) and (gamma,np) channels are compared with an inclusive photoneutron measurement by Fultz and coworkers, which are consistent within the respective errors. The measured excitation energy distributions of 67−69Ni contain a large portion of the Giant Dipole Resonance (GDR) strength predicted by the Thomas-Reiche-Kuhn energy-weighted sum rule, as well as a significant amount of low-lying E1 strength, that cannot be attributed to the GDR alone. The GDR distribution parameters are calculated using well-established semi-empirical systematic models, providing the peak energies and widths. The GDR strength is extracted from the chi-square minimization of the model GDR to the measured data of the (gamma,2n) channel, thereby excluding any influence of eventual low-lying strength. The subtraction of the obtained GDR distribution from the total measured E1 strength provides the low-lying E1 strength distribution, which is attributed to the Pygmy Dipole Resonance (PDR). The extraction of the peak energy, width and strength is performed using a Gaussian function. The minimization of trial Gaussian distributions to the data does not converge towards a sharp minimum. Therefore, the results are presented by a chi-square distribution as a function of all three Gaussian parameters. Various predictions of PDR distributions exist, as well as a recent measurement of the 68Ni pygmy dipole-resonance obtained by virtual photon scattering, to which the present pygmy dipole-resonance distribution is also compared.

Studies of degradation of organic solar cell materials using electrical scanning probe microscopy

Intense research is being done in the field of organic photovoltaics in order to synthesize low band-gap organic molecules. These molecules are electron donors which feature in combination with acceptor molecules, typically fullerene derivarntives, forming an active blend. This active blend has phase separated bicontinuous morphology on a nanometer scale. The highest recorded power conversionrnefficiencies for such cells have been 10.6%. Organic semiconductors differ from inorganic ones due to the presence of tightly bonded excitons (electron-hole pairs)resulting from their low dielectric constant (εr ≈2-4). An additional driving force is required to separate such Frenkel excitons since their binding energy (0.3-1 eV) is too large to be dissociated by an electric field alone. This additional driving force arises from the energy difference between the lowest unoccupied molecular orbital (LUMO) of the donor and the acceptor materials. Moreover, the efficiency of the cells also depends on the difference between the highest occupied molecular orbital (HOMO) of the donor and LUMO of the acceptor. Therefore, a precise control and estimation of these energy levels are required. Furthermore any external influences that change the energy levels will cause a degradation of the power conversion efficiency of organic solar cell materials. In particular, the role of photo-induced degradation on the morphology and electrical performance is a major contribution to degradation and needs to be understood on a nanometer scale. Scanning Probe Microscopy (SPM) offers the resolution to image the nanometer scale bicontinuous morphology. In addition SPM can be operated to measure the local contact potential difference (CPD) of materials from which energy levels in the materials can be derived. Thus SPM is an unique method for the characterization of surface morphology, potential changes and conductivity changes under operating conditions. In the present work, I describe investigations of organic photovoltaic materials upon photo-oxidation which is one of the major causes of degradation of these solar cell materials. SPM, Nuclear Magnetic Resonance (NMR) and UV-Vis spectroscopy studies allowed me to identify the chemical reactions occurring inside the active layer upon photo-oxidation. From the measured data, it was possible to deduce the energy levels and explain the various shifts which gave a better understanding of the physics of the device. In addition, I was able to quantify the degradation by correlating the local changes in the CPD and conductivity to the device characteristics, i.e., open circuit voltage and short circuit current. Furthermore, time-resolved electrostatic force microscopy (tr-EFM) allowed us to probe dynamic processes like the charging rate of the individual donor and acceptor domains within the active blend. Upon photo-oxidation, it was observed, that the acceptor molecules got oxidized first preventing the donor polymer from degrading. Work functions of electrodes can be tailored by modifying the interface with monomolecular thin layers of molecules which are made by a chemical reaction in liquids. These modifications in the work function are particularly attractive for opto-electronic devices whose performance depends on the band alignment between the electrodes and the active material. In order to measure the shift in work function on a nanometer scale, I used KPFM in situ, which means in liquids, to follow changes in the work function of Au upon hexadecanethiol adsorption from decane. All the above investigations give us a better understanding of the photo-degradation processes of the active material at the nanoscale. Also, a method to compare various new materials used for organic solar cells for stability is proposed which eliminates the requirement to make fully functional devices saving time and additional engineering efforts.

Field-free control of magnetism in nanostructured materials probed with high resolution X-ray microscopy

Magnetic memories are a backbone of today's digital data storage technology, where the digital information is stored as the magnetic configuration of nanostructured ferromagnetic bits. Currently, the writing of the digital information on the magnetic memory is carried out with the help of magnetic fields. This approach, while viable, is not optimal due to its intrinsically high energy consumption and relatively poor scalability. For this reason, the research for different mechanisms that can be used to manipulate the magnetic configuration of a material is of interest. In this thesis, the control of the magnetization of different nanostructured materials with field-free mechanisms is investigated. The magnetic configuration of these nanostructured materials was imaged directly with high resolution x-ray magnetic microscopy. rnFirst of all, the control of the magnetic configuration of nanostructured ferromagnetic Heusler compounds by fabricating nanostructures with different geometries was analyzed. Here, it was observed that the magnetic configuration of the nanostructured elements is given by the competition of magneto-crystalline and shape anisotropy. By fabricating elements with different geometries, we could alter the point where these two effects equilibrate, allowing for the possibility to tailor the magnetic configuration of these nanostructured elements to the required necessities.rnThen, the control of the magnetic configuration of Ni nanostructures fabricated on top of a piezoelectric material with the magneto-elastic effect (i.e. by applying a piezoelectric strain to the Ni nanostructures) was investigated. Here, the magneto-elastic coupling effect gives rise to an additional anisotropy contribution, proportional to the strain applied to the magnetic material. For this system, a reproducible and reversible control of the magnetic configuration of the nanostructured Ni elements with the application of an electric field across the piezoelectric material was achieved.rnFinally, the control of the magnetic configuration of La0.7Sr0.3MnO3 (LSMO) nanostructures with spin-polarized currents was studied. Here, the spin-transfer torque effect was employed to achieve the displacement of magnetic domain walls in the LSMO nanostructures. A high spin-transfer torque efficiency was observed for LSMO at low temperatures, and a Joule-heating induced hopping of the magnetic domain walls was observed at room temperatures, allowing for the analysis of the energetics of the domain walls in LSMO.rnThe results presented in this thesis give thus an overview on the different field-free approaches that can be used to manipulate and tailor the magnetization configuration of a nanostructured material to the various technological requirements, opening up novel interesting possibilities for these materials.

From the chiral magnetic wave to the charge dependence of elliptic flow

The quark-gluon plasma formed in heavy ion collisions contains charged chiral fermions evolving in an external magnetic field. At finite density of electric charge or baryon number (resulting either from nuclear stopping or from fluctuations), the triangle anomaly induces in the plasma the Chiral Magnetic Wave (CMW). The CMW first induces a separation of the right and left chiral charges along the magnetic field; the resulting dipolar axial charge density in turn induces the oppositely directed vector charge currents leading to an electric quadrupole moment of the quark-gluon plasma. Boosted by the strong collective flow, the electric quadrupole moment translates into the charge dependence of the elliptic flow coefficients, so that $v_2(\pi^+) < v_2(\pi^-)$ (at positive net charge). Using the latest quantitative simulations of the produced magnetic field and solving the CMW equation, we make further quantitative estimates of the produced $v_2$ splitting and its centrality dependence. We compare the results with the available experimental data.

Statistical analysis of multichannel scalp field data

High density spatial and temporal sampling of EEG data enhances the quality of results of electrophysiological experiments. Because EEG sources typically produce widespread electric fields (see Chapter 3) and operate at frequencies well below the sampling rate, increasing the number of electrodes and time samples will not necessarily increase the number of observed processes, but mainly increase the accuracy of the representation of these processes. This is namely the case when inverse solutions are computed. As a consequence, increasing the sampling in space and time increases the redundancy of the data (in space, because electrodes are correlated due to volume conduction, and time, because neighboring time points are correlated), while the degrees of freedom of the data change only little. This has to be taken into account when statistical inferences are to be made from the data. However, in many ERP studies, the intrinsic correlation structure of the data has been disregarded. Often, some electrodes or groups of electrodes are a priori selected as the analysis entity and considered as repeated (within subject) measures that are analyzed using standard univariate statistics. The increased spatial resolution obtained with more electrodes is thus poorly represented by the resulting statistics. In addition, the assumptions made (e.g. in terms of what constitutes a repeated measure) are not supported by what we know about the properties of EEG data. From the point of view of physics (see Chapter 3), the natural “atomic” analysis entity of EEG and ERP data is the scalp electric field

Brain electric microstates and momentary conscious mind states as building blocks of spontaneous thinking: I. Visual imagery and abstract thoughts

Prompted reports of recall of spontaneous, conscious experiences were collected in a no-input, no-task, no-response paradigm (30 random prompts to each of 13 healthy volunteers). The mentation reports were classified into visual imagery and abstract thought. Spontaneous 19-channel brain electric activity (EEG) was continuously recorded, viewed as series of momentary spatial distributions (maps) of the brain electric field and segmented into microstates, i.e. into time segments characterized by quasi-stable landscapes of potential distribution maps which showed varying durations in the sub-second range. Microstate segmentation used a data-driven strategy. Different microstates, i.e. different brain electric landscapes must have been generated by activity of different neural assemblies and therefore are hypothesized to constitute different functions. The two types of reported experiences were associated with significantly different microstates (mean duration 121 ms) immediately preceding the prompts; these microstates showed, across subjects, for abstract thought (compared to visual imagery) a shift of the electric gravity center to the left and a clockwise rotation of the field axis. Contrariwise, the microstates 2 s before the prompt did not differ between the two types of experiences. The results support the hypothesis that different microstates of the brain as recognized in its electric field implement different conscious, reportable mind states, i.e. different classes (types) of thoughts (mentations); thus, the microstates might be candidates for the `atoms of thought'.

Proton entry into the near-lunar plasma wake for magnetic field aligned flow

We report the first observation of protons in the near-lunar (100-200 km from the surface) and deeper (near anti-subsolar point) plasma wake when the interplanetary magnetic field (IMF) and solar wind velocity (vsw) are parallel (aligned flow; angle between IMF and vsw≤10°). More than 98% of the observations during aligned flow condition showed the presence of protons in the wake. These observations are obtained by the Solar Wind Monitor sensor of the Sub-keV Atom Reflecting Analyser experiment on Chandrayaan-1. The observation cannot be explained by the conventional fluid models for aligned flow. Back tracing of the observed protons suggests that their source is the solar wind. The larger gyroradii of the wake protons compared to that of solar wind suggest that they were part of the tail of the solar wind velocity distribution function. Such protons could enter the wake due to their large gyroradii even when the flow is aligned to IMF. However, the wake boundary electric field may also play a role in the entry of the protons into the wake.

On vertical electric fields at lunar magnetic anomalies

We study the interaction between a magnetic dipole mimicking the Gerasimovich magnetic anomaly on the lunar surface and the solar wind in a self-consistent 3-D quasi-neutral hybrid simulation where ions are modeled as particles and electrons as a charge-neutralizing fluid. Especially, we consider the origin of the recently observed electric potentials at lunar magnetic anomalies. An antimoonward Hall electric field forms in our simulation resulting in a potential difference of <300V on the lunar surface, in which the value is similar to observations. Since the hybrid model assumes charge neutrality, our results suggest that the electric potentials at lunar magnetic anomalies can be formed by decoupling of ion and electron motion even without charge separation.

Experimental study of electric breakdowns in liquid argon at centimeter scale

In this paper we present results on measurements of the dielectric strength of liquid argon near its boiling point and cathode-anode distances in the range of 0.1 mm to 40 mm with spherical cathode and plane anode. We show that at such distances the applied electric field at which breakdowns occur is as low as 40 kV/cm. Flash-overs across the ribbed dielectric of the high voltage feed-through are observed for a length of 300 mm starting from a voltage of 55 kV. These results contribute to set reference for the breakdown-free design of ionization detectors, such as Liquid Argon Time Projection Chambers (LAr TPC).

(Table 2) Atmospheric electric measurements during the Atlantic Expedition 1965 of METEOR

During the Atlantic expedition potential gradient, small ion density and space charge density have been recorded. Laborious efforts have been taken for receiving an exact estimation of the reduction factor for the field measurements. The mean value of the potential gradient on the free Atlantic Ocean was 105 V/m. The mean daily course is in very good agreement with the results of the Carnegie Institution. Even records taken on individual days near the quator show this course. For the first time it has been attempted to correlate the potential gradient at sea and the voltage between ionosphere and earth measured over land. A narrow relation has been found in 10 cases of balloon ascents with radiosondes. A further remarkable result is, that the short periodical fluctuations of the air electric field at sea with periods of 2 to 20 minutes have amplitudes of the magnitude of the mean field strength and exist all over the oceans. Recordings of the space charge density show, that positively charged air parcels drift in the first hectometer of the air near the sea surface and produce the fluctuation of the potential gradient. A period analysis did not indicate a recognizable relation to the wind velocity up to now, although an effect of air turbulence must be involved. The concentration of small ions also has been measured occasionally. With this and mean values of the potential gradient the air earth curent density has been computed. With n+ = 310 cm**-3, n- = 220 cm**-3 the air conductivity would be Lambda = 1,14 * 10**-14 Ohm**-1 m**-1. These values are smaller than values of other authors by a factor of 2 or 3. Therefore the computed air earth current density is also smaller. The discrepancy could not be explained yet.

Desarrollo de estructuras nanofotonicas de campo cercano para intensificacion localizada de luz en celulas solares de banda intermedia - Near-field nanophotonic structures for localized light enhancement in intermediate band solar cells

This doctoral thesis explores some of the possibilities that near-field optics can bring to photovoltaics, and in particular to quantum-dot intermediate band solar cells (QD-IBSCs). Our main focus is the analytical optimization of the electric field distribution produced in the vicinity of single scattering particles, in order to produce the highest possible absorption enhancement in the photovoltaic medium in their surroundings. Near-field scattering structures have also been fabricated in laboratory, allowing the application of the previously studied theoretical concepts to real devices. We start by looking into the electrostatic scattering regime, which is only applicable to sub-wavelength sized particles. In this regime it was found that metallic nano-spheroids can produce absorption enhancements of about two orders of magnitude on the material in their vicinity, due to their strong plasmonic resonance. The frequency of such resonance can be tuned with the shape of the particles, allowing us to match it with the optimal transition energies of the intermediate band material. Since these metallic nanoparticles (MNPs) are to be inserted inside the cell photovoltaic medium, they should be coated by a thin insulating layer to prevent electron-hole recombination at their surface. This analysis is then generalized, using an analytical separation-of-variables method implemented in Mathematica7.0, to compute scattering by spheroids of any size and material. This code allowed the study of the scattering properties of wavelengthsized particles (mesoscopic regime), and it was verified that in this regime dielectric spheroids perform better than metallic. The light intensity scattered from such dielectric spheroids can have more than two orders of magnitude than the incident intensity, and the focal region in front of the particle can be shaped in several ways by changing the particle geometry and/or material. Experimental work was also performed in this PhD to implement in practice the concepts studied in the analysis of sub-wavelength MNPs. A wet-coating method was developed to self-assemble regular arrays of colloidal MNPs on the surface of several materials, such as silicon wafers, amorphous silicon films, gallium arsenide and glass. A series of thermal and chemical tests have been performed showing what treatments the nanoparticles can withstand for their embedment in a photovoltaic medium. MNPs arrays are then inserted in an amorphous silicon medium to study the effect of their plasmonic near-field enhancement on the absorption spectrum of the material. The self-assembled arrays of MNPs constructed in these experiments inspired a new strategy for fabricating IBSCs using colloidal quantum dots (CQDs). Such CQDs can be deposited in self-assembled monolayers, using procedures similar to those developed for the patterning of colloidal MNPs. The use of CQDs to form the intermediate band presents several important practical and physical advantages relative to the conventional dots epitaxially grown by the Stranski-Krastanov method. Besides, this provides a fast and inexpensive method for patterning binary arrays of QDs and MNPs, envisioned in the theoretical part of this thesis, in which the MNPs act as antennas focusing the light in the QDs and therefore boosting their absorption

Electric current of an electrified jet issuing from a long metallic tube.

This paper presents an analysis of the transport of electric current in a jet of an electrically conducting liquid discharging from a metallic tube into a gas or a vacuum, and subject to an electric field due to a high voltage applied between the tube and a far electrode. The flow, the surface charge and the electric field are computed in the current transfer region of the jet, where conduction current in the liquid becomes surface current due to the convection of electric charge accumulated at its surface. The electric current computed as a function of the flow rate of the liquid injected through the tube increases first as the square root of this flow rate, levels to a nearly constant value when the flow rate is increased and finally sets to a linear increase when the flow rate is further increased. The current increases linearly with the applied voltage at small and moderate values of this variable, and faster than linearly at high voltages. The characteristic length and structure of the current transfer region are determined. Order-of-magnitude estimates for jets which are only weakly stretched by the electric stresses are worked out that qualitatively account for some of the numerical results.