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Development and applications of near-field imaging methods in the terahertz spectral domain
Berta, Milan ; Kadlec, Filip (advisor) ; Fejfar, Antonín (referee) ; Adam, Auréle J.L. (referee)
We are reporting on a study of the near-field sensitivity and resolution of a metal-dielectric probe (MDP). The propagation of the electromagnetic field across the probe was studied experimentally by means of time-domain terahertz spectroscopy and numerically simulated by CST MicroWave Studio 2008. Several localised areas at the probe end facet were distinguished and showed to be sensitive to the local dielectric properties and local anisotropy of the sample. Contrast and sensitivity measurements were conducted in several configurations of a MDP; the results were confirmed by simulations. The acquired data were analysed by using singular value decomposition that enabled separating independent physical phenomena in the measured datasets and filtering external disturbances out of the signal. Independent components corresponding to the changes in the output terahertz pulse upon varying the probe-sample distance and reflecting the local anisotropy in a ferroelectric barium titanate (BaTiO3) crystal were extracted and identified. The domain structure with characteristic dimensions of about 5 um was resolved during imaging experiments on the ferroelectric BaTiO3 sample, i.e. the resolved structures were ten times smaller than the characteristic dimensions of the end facet of the probe and forty times smaller than...
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Additional plasma heating of tokamak plasma by powerful beam of deuterium atoms on the COMPASS tokamak
Bogár, Klára ; Stöckel, Jan (advisor) ; Berta, Miklós (referee) ; Kappatou, Athina (referee)
Two neutral beam injectors (NBIs) were installed on the COMPASS tokamak as a main upgrade after its move from the UK to Prague. The thesis deals with general experimental specifications of additionally heated plasmas by the NBI under the wide range of conditions on the COMPASS tokamak. The geometrical parameters of the beams are determined from the spectro- scopic measurements. Consequently, the NBI power passing through the narrow duct connecting NBI and tokamak chamber is investigated. The passing fraction of the NBI power is confirmed by the power balance analysis, which provides information about the delivered power into the plasma based on the plasma parameters. Once the delivered power is well known a description of beams interaction with plasma via global parameters such as the plasma temperatures, the plasma density, the energy confinement time and neutron yield is given. It is shown, that the NBIs heat mostly ions, which can reach ∼ 1 keV in the plasma core. Moreover, the impact of the fast ions presence on the commonly observed MHD activities, the sawtooth instability and the edge localized modes (ELMs), is discussed and compared with results from other tokamaks. The sawtooth period is prolonged by the influence of the NBI from 2 ms up to 4.5 ms. The knowledge of the NBI passing power into...
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Development and applications of near-field imaging methods in the terahertz spectral domain
Berta, Milan ; Kadlec, Filip (advisor) ; Fejfar, Antonín (referee) ; Adam, Auréle J.L. (referee)
We are reporting on a study of the near-field sensitivity and resolution of a metal-dielectric probe (MDP). The propagation of the electromagnetic field across the probe was studied experimentally by means of time-domain terahertz spectroscopy and numerically simulated by CST MicroWave Studio 2008. Several localised areas at the probe end facet were distinguished and showed to be sensitive to the local dielectric properties and local anisotropy of the sample. Contrast and sensitivity measurements were conducted in several configurations of a MDP; the results were confirmed by simulations. The acquired data were analysed by using singular value decomposition that enabled separating independent physical phenomena in the measured datasets and filtering external disturbances out of the signal. Independent components corresponding to the changes in the output terahertz pulse upon varying the probe-sample distance and reflecting the local anisotropy in a ferroelectric barium titanate (BaTiO3) crystal were extracted and identified. The domain structure with characteristic dimensions of about 5 um was resolved during imaging experiments on the ferroelectric BaTiO3 sample, i.e. the resolved structures were ten times smaller than the characteristic dimensions of the end facet of the probe and forty times smaller than...
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Plasma Boundary Reconstruction using Fast Camera on the COMPASS Tokamak
Háček, Pavel ; Berta, Miklós ; Stöckel, Jan ; Weinzettl, Vladimír ; Budai, C. ; Szabolics, T. ; Bencze, A.
Determination of the plasma boundary is an important task for safe operation of the tokamak and diagnostic systems as well as for correct interpretation of the measured data. Magnetic reconstruction codes routinely used to determine the shape of the plasma have a number of limitations which can make the reconstruction problematic. Recently, it has been demonstrated on several devices that it is possible to provide independent measurement of the plasma boundary by observation of the visiblelight emission using fast framing cameras. In the presented work, a single fast camera on the COMPASS tokamak was used for reconstruction of the optical plasma boundary, assuming a toroidally symmetric visible-light emission profile located in the edge of plasma. As a first result, application of the method on D-shaped COMPASS shot 7145 and its comparison with magnetic reconstruction from the EFIT code is given. Both methods show good agreement with average difference 0.5 cm.
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Analysis of enterprise's complex system
Malackanič, Roman ; Sigmund, Tomáš (advisor) ; Berta, Michal (referee)
This diploma thesis focuses on the analysis of a particular soft complex system in the company. This system is a running car service with a determined problem situation. This system and its problem is then evaluated in terms of soft systems methodology, system dynamics and critical systems heuristics. The theoretical section provides concepts to understand the complexity and also individual system approaches. As beneficial of work I consider the practical part, which is aimed to the analysis of complex real-world system using the selected approaches as well as an evaluation of these approaches. Main purpose is to bring comparisons and suggestions for improvement and also a broader view to the system. All of this increases information value of approaches and we are able to deliver more relevant information. Thesis is therefore focused not only to deal with examples from practice but also to extend the selected theories within the system approaches.
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New Detection System for Fast Density Measurements Using the Lithium Beam on the COMPASS Tokamak
Háček, Pavel ; Berta, M. ; Stöckel, Jan ; Weinzettl, Vladimír ; Anda, G. ; Bencze, A. ; Zoletnik, S.
The lithium beam diagnostic at COMPASS is designed for measurements of the edge plasma density profile and fluctuations and edge plasma current fluctuations. The principle of the diagnostic is detection of light coming from collisionally excited Li atoms — beam emission spectroscopy (BES) - and a direct detection of the ionized part of the beam (atomic beam probe — ABP). For slow density measurements a charged coupled device (CCD) camera has been installed and is already working. For fast twodimensional density profile and density fluctuation measurements an array of avalanche photodiode detectors (APDs) will be used. The two-dimensional resolution of the measurement will be possible using fast poloidal deflection and chopping of the beam. Apart from routine density profile measurement, the diagnostic will be capable of investigating the turbulent structures in the edge plasma by cross-correlating the signals coming from poloidally deflected virtual beams.
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