Bobine (électricité) | EPFL Graph Search

Une bobine, solénoïde, auto-inductance ou quelquefois self (par anglicisme), est un composant courant en électrotechnique et électronique. Une bobine est constituée d'un enroulement de fil conducteur éventuellement autour d'un noyau en matériau ferromagnétique qui peut être un assemblage de feuilles de tôle ou un bloc de ferrite. Les physiciens et ingénieurs français l'appellent souvent par synecdoque « inductance », ce terme désignant la propriété caractéristique de la bobine, qui est son opposition à la variation du courant dans ses spires. vignette|Une bobine d'arrêt de avec noyau en ferrite. Description La partie la plus visible est un enroulement de fils conducteurs. L'espace au milieu de ces spires s'appelle le noyau. Il peut être vide ou inclure une pièce en matériau ferromagnétique favorisant l'induction électromagnétique, afin d'augmenter la valeur de l'inductance. Le noyau peut être un circuit magnétique complètement ou partiellement fermé pour améliorer la li

Sub-Micron Nano-Composite CoC Hall Sensors by Focused Electron-Beam Induced Deposition for Microbead Detection

Laurent Bernau

In order to track single superparamagnetic microbeads serving as markers in biomolecular assays, high-sensitivity, sub-micron magnetic sensors which can be integrated onto Lab-on-a-chip platforms are needed. This thesis studies the realization and characterization of such magnetic sensors using Focused Electron Beam Induced Deposition (FEBID) of Cobalt in a High Vacuum (HV) Scanning Electron Microscope (SEM). In FEBID, a finely focused electron beam is used to selectively irradiate a surface where functional precursor molecules are adsorbed. Electron-impact dissociation of the adsorbates lead to non-volatile fragments being deposited at the point of irradiation, while volatile fragments desorb and are pumped away. We have investigated FEBID of Dicobalt octacarbonyl [Co2(CO)8]. The deposit consists of a nanocomposite CoC material, where Co nanocrystals 2-3 nm in size are embedded in a carbonaceous matrix. This material exhibits a scattering enhanced Extraordinary Hall Effect (EHE) which allows for high magnetic sensitivities. Combined with the inherent nanometric resolution of the deposition process, this makes this nanocomposite CoC material the material of choice for the deposition of small (sub-micron), high-sensitivity magnetic sensors. However, the magnetic field resolution is found to be highly dependent on the exact composition of the deposition process. We report for the first time the controlled tuning of the composition of deposits from Co2(CO)8 using the electron beam pulse time as the process parameter. We explain the tunability in terms of co-deposition of chamber background hydrocarbons. A novel, general model describing the electron-induced deposition in terms of surface adsorbate densities in the presence of two adsorbate species is presented. The analytical model allows to describe the conditions for a broad tunable composition window for any two-adsorbate system. The model is applied to the two-adsorbate system consisting of Co2(CO)8 and HV chamber background hydrocarbons and we show that it allows for a quantitative description of the compositional variations. The CoC nanocomposite material is studied and characterized using a Langevin fit of the Hall signal. We show that this approach yields insight into the nanocrystal mean size, providing a nanocharacterization method of the material. A linear dependency between the electrical resistivity and the Hall resistivity at saturation is found. Empirically, the material is found to exhibit an optimum in terms of magnetic field resolution at around 65 at.% Co, corresponding to a trade-off between magnetic field sensitivity and electrical resistivity. Finally, we demonstrate single superparamagnetic bead detection using a sub-micron, CoC Hall sensor. Using a novel nanomanipulation setup providing a magnetic coil integrated into a SEM, we show the bead tracking capacities of these sensors.

EPFL2011

Modelling of non-linear edge harmonic oscillations and the effect of non-axisymmetric magnetic coils

Daniele Brunetti, Guillermo Bustos Ramirez, Wilfred Anthony Cooper, Jonathan Graves, Andreas Kleiner

Quiescent H-mode (QH-mode) operation is highly desirable relative to the well known H- mode operation because it allows for high energy density in the core as well as potentially im- proved confinement of energy and particles. A feature of QH-mode is that the Edge Localized Mode (ELM) instabilities are replaced with continuous Edge Harmonic Oscillations (EHO) [1]. Control and reproduction of robust QH-mode regimes could be crucial for future fusion opera- tion. In some cases it may be useful to modify the main characteristics of QH-mode discharges while keeping constant the key elements needed to achieve QH-mode (such as applied torque, edge current density, pedestal pressure, etc.). With that in mind, the effect of global toroidal mode seeding in the vacuum magnetic field on the amplitude of EHO is investigated. Previ- ous numerical simulations have shown that free boundary equilibrium calculations are able to recover non-linear saturated 3D equilibria with edge corrugations associated with EHO in QH- mode [2, 3]. Here we use the VMEC free boundary code to do a proof of principle investigation of the coupling between EHO and global toroidal mode seeding. This is done by first producing a QH-mode plasma using the vacuum magnetic field modeled by JET-like toroidal and poloidal field coils. Then, a global toroidal mode n=1,2 is seeded numerically by perturbing the vacuum field using a set of non-axisymmetric external coils associated with the Error Field Correction Coils (EFCC’s) in JET and the plasma response is studied. Due to the EFCC’s geometry, it is crucial to account for up-down asymmetry in the VMEC code. Spectral decomposition of the 3D plasma displacement with respect to the equivalent 2D axisymmetric equilibrium is per- formed and compared with linear numerical simulations using the KINX code. A study that effectively isolates the coupling of global n=1,2 toroidal modes with current driven and pres- sure driven EHO is presented, and the impact of external coils and associated plasma response clearly determined.

European Physical Society2019

A Far-Infrared Polarimeter Diagnostic for the Measurement of the Poloidal Magnetic Field in the TCV Tokamak

Information about the poloidal field inside the plasma and about the current density profile is essential for the understanding of the behavior of tokamak plasmas. The poloidal field outside the plasma edge is easily measured by a set of magnetic coils, whereas measurements of the magnetic field distribution inside the plasma requires dedicated and often complex diagnostic systems. One of the methods is polarimetry using beams of electromagnetic waves traversing the plasma. It makes use of the fact that the magnetic field modifies the refractive index of the plasma and as a consequence affects the state of polarization of the passing waves. Specifically, Faraday rotation refers to the case of a linearly polarized wave propagating parallel to a magnetic field. In this case, the rotation of the polarization vector is proportional to the product of electron density and parallel magnetic field component. This thesis is devoted to the design of a specific polarimeter system and its implementation on the TCV tokamak. The conceptual design took into account the particular requirements of the TCV experiments and its experimental program with particular interest in measurements of current density profiles in low-density, EC-heated plasmas. Aiming at optimum performance of the polarimeter in this parameter range, the FIR laser wavelength of 432.5 µm was chosen. Since TCV is already equipped with an interferometer (at 214 µm) for measurements of the line-integrated density along 14 chords, the polarimeter was built as a separate instrument. The number of 10 spatial channels to cover the plasma diameter was found as a compromise between desired spatial resolution, access constraints and cost. The measurement of the Faraday rotation angle is based on a method suggested by Dodel and Kunz [1], which uses optical beams with rotating linear polarization. In this case, the Faraday rotation angle is retrieved from a phase measurement comparing the modulated signals from the probe detectors with that from a reference detector and to first order is insensitive to amplitude variations of the signals. Waveguide detectors based on Schottky barrier diodes are used as detectors for their sensitivity and large electrical bandwidth. Signal processing and analysis is performed in two branches : a) using specifically built analog electronic phase detectors followed by slow (250 kHz) ADCs b) using fast (5 MHz) ADCs and numeric signal processing on a mainframe computer. The thesis covers the description of the design and the various components of the polarimeter, presents initial tests and an evaluation of its performance based on simulations using real plasma configurations of TCV. The analysis of specific tests with the system installed on TCV revealed the presence of perturbations leading to parasitic contributions to the measured phase angles. The results of first measurements of the Faraday rotation angle for different plasma conditions on TCV are presented. Comparing the measured Faraday rotation angles with the results of calculations showed qualitative agreement, in particular the effects of an increase in electron density and of the reversal of the current direction were clearly seen. The system is also capable to detect a radial displacement of the magnetic axis of the plasma, as was demonstrated by comparing measurements from two specific plasma configurations. However, the absolute values still show significant deviations from the expected ones based on calculations. The discrepancies increase with increasing Faraday angle and depend on the direction of the plasma current. At high electron densities beam refraction becomes a problem and may lead to significant errors in the measurements and eventually to complete loss of the signal in several channels. In its present status, the polarimeter cannot yet provide results that are suitable to reconstruct the poloidal field or the current density profile of the plasma. Tentative explanations for this problem are given, but further specific tests are necessary to confirm them. Even in its present state, the polarimeter may be used to detect transient relative changes in the profiles of electron density and current. This was demonstrated by a series of experiments in plasmas with sawtooth activity. Comparing the signals from the polarimeter with those from other multi-chord diagnostics (interferometer and soft X-ray detectors) clearly revealed correlations but also specific differences. These experimental studies showed that the Far-Infrared Polarimetry still requires further improvements, but has potential to become a valuable diagnostic capable of directly measuring the current density profile in the TCV tokamak.

EPFL2011

A Far-Infrared Polarimeter Diagnostic for the Measurement of the Poloidal Magnetic Field in the TCV Tokamak

EPFL2011

Modelling of non-linear edge harmonic oscillations and the effect of non-axisymmetric magnetic coils

Daniele Brunetti, Guillermo Bustos Ramirez, Wilfred Anthony Cooper, Jonathan Graves, Andreas Kleiner

European Physical Society2019

Sub-Micron Nano-Composite CoC Hall Sensors by Focused Electron-Beam Induced Deposition for Microbead Detection

Laurent Bernau

EPFL2011