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Real Time Plasma State Monitoring
Get PDFThe thesis describes several methods of plasma state monitoring for feedback control. For a tokamak device operation, one needs to gain in real time some information about the plasma state. The amount of needed information increases with the size of the device. In small machines, such as ISTTOK and Golem, the plasma current centroid position control is sufficient, as the heat fluxes are low and the plasma is in limiter regime. In larger devices, like RFX-mod, TCV or ASDEX-Upgrade with more complex plasma shapes and higher heat fluxes on the first wall, it is necessary to measure and control the plasma shape and the heat flux deposition location on the first wall (strike points in diverted regime and plasma-wall contact point in limited regime). The plasma performance is also strongly affected by internal current and pressure profile shape. Additionally, the bootstrap current fraction, that can not be directly measured, plays significant role in more advanced tokamak operation regimes. These quantities related to internal profiles have to be well diagnosed in order to avoid disruptions and mitigate the MHD instabilities. The thesis deals with all the above mentioned layers of control complexity, starting from the simplest ones focused on plasma current centroid position measurement on ISTTOK, over more complicated real time shape and equilibrium monitoring to the most advanced tools useful for plasma internal profiles monitoring, control, instabilities mitigation and disruption avoidance. The work described in this thesis provides original contribution to each category of these methods.
The first part of the thesis deals with real time applicable plasma current centroid position measurement on ISTTOK. In the frame of this work, two major issues had to be solved. The first, which is common for most small devices with highly conductive shell, is related to the impact of the eddy currents induced in the conductive structures that corrupt the magnetic diagnostic signal and consequently disable the plasma current centroid position measurement. This was solved by a simple, real time applicable state space model that takes into account both the vacuum vessel and the shell and is capable to clean all 12 magnetic sensors installed on ISTTOK from the eddy currents effect. The method has also been tested on RFX-mod and on data from tokamak Golem located in Prague, Czech Republic. The second problem is that the present magnetic diagnostics on ISTTOK is not well calibrated. Therefore a simple method less sensitive to calibration errors was used and validated against Heavy Ion Beam vertical position estimate. The resulting algorithm is reliable enough and capable of providing information about the plasma current centroid position both for real time plasma control and offline post-discharge analysis. In addition, a more advanced method of the plasma current centroid position measurement was proposed to be used after the magnetic diagnostic upgrade planned in near future.
The next part of the work is dedicated to the tools for plasma shape and global macroscopic parameters measurement. It was performed on RFX-mod, a medium sized reversed field pinch operated also in tokamak regime. It is important especially because it started a diverted discharges program last years with the aim of performing MHD active control experiments in H-mode plasma. This part of the research activity focuses on the development of an algorithm for reliable plasma boundary reconstruction, both real time and offline that is applicable on RFX-mod. A real time plasma boundary reconstruction algorithm based on magnetic flux extrapolation in the vacuum suited to RFX-mod was developed and validated against Grad-Shafranov solver MAXFEA. This algorithm provides very good results (error below 2 % of the plasma minor radius) even in the presence of the limited poloidal number of sensors (8 pick-up coils and 8 poloidal flux loops) in RFX-mod with respect to standard tokamaks. As the algorithm does not provide just the knowledge of the plasma boundary, but it is also capable to compute the magnetic field and flux everywhere in the vacuum, it can be used to calculate several equilibrium related parameters such as βp + li/2, q95 in post-discharge analysis. For elongated discharges, the βp and li can be also separated. The value of βp provided by this algorithm is almost identical with an independent diamagnetic measurement. It was also used to provide feedback signals to the plasma shape control system designed for RFX-mod diverted discharges. Some shape control operation results are also reported in the thesis. The initial work on the real time plasma shape measurement and control opened the way to LH transition experiments. These experiments have shown H-mode evidence neither in steady state nor in transition phases of the ohmic discharges (mainly current ramp-up and ramp-down). However, the possibility of achieving H-mode in ohmic discharges is not excluded in future campaigns, since the feedback control on plasma density has been significantly improved. As a second attempt, a biasing electrode to induce LH transition was built and another campaign was carried out. These experiments were successful, since several clear signs of LH transition were observed: the steep drop of H alpha signal, increase of plasma density and pedestal formation at the edge. The description of these experiments is given within the thesis.
The last part of the thesis was dedicated to a tool that is being developed to improve present methods of control of plasma current, temperature, pressure profile, and disruption avoidance, RApid Plasma Transport simulatOR (RAPTOR). This code complements a simple set of real time diagnostics and computes the missing information using a lightweight set of transport equations. The thesis contains description of the code, especially the physical simplifications made to cope the computational speed requirements of a real time code, and reports possible applications in the field of disruption prediction and avoidance. Dedicated experiments to test the prediction method have been performed on RFX-mod for Resistive Wall Mode disruptions in discharges with q(a)< 2 and on TCV for density limit disruptions. In both disruption classes, the sawteeth behavior is significantly modified before the disruption, but the modification is not reflected in the RAPTOR prediction. Therefore the sawteeth period has been used as a feedback variable: huge discrepancy between RAPTOR and experimental sawteeth period activates the disruption alarm. The experiments showed that RAPTOR in its present state of development is a valid tool for disruption prediction in ohmic L-mode plasma without significant shape changes: in RFX-mod, the disruption alarm was activated several tens of milliseconds before the disruption and in TCV hundreds of milliseconds before. The main benefit is that this method is physics based, hence it does not require huge discharge database to train neural networks as the present algorithms. It is also transferable from one device to another, which is not the case of present days disruption prediction tools. However, the future disruption avoidance tools must be capable to deal with much more complex plasmas than the studied L-mode ohmic cases. The last section of this thesis gives examples of disruption prediction on ASDEX-Upgrade and identifies the gaps that must be filled before RAPTOR becomes a full-blown disruption prediction tool. First of all, inclusion of all heating and current drive sources will be needed. Second important issue is the inclusion of a model of a fast ion pressure, that significantly contributes to the total plasma energy content and affects MHD modes dynamics, for example stabilizes sawteeth. Another important piece of the puzzle is the coupling of a real time Grad-Shafranov solver with RAPTOR, that can improve the equilibrium reconstruction quality, which is closely related for example to the control of the NTM instabilities. Last, not least, huge benefit can be obtained from a better real time estimate of the electron temperature profile by plasma diagnostic
Cleaning of the eddy current effects from magnetic diagnostics
No full textThe plasma configuration in tokamaks is created and controlled by magnetic fields generated by the external coils. Similarly, plasma macroscopic parameters are determined using sensors measuring magnetic field or magnetic flux. Some algorithms require separation of the plasma contribution from the contribution of poloidal field coils and currents induced in passive structures. In this paper, an efficient, real-time applicable method is introduced. The method is based on the identification of a state-space model in vacuum shots using both experimental and finite-element model data. The tests were performed for ISTTOK and RFX-mod. In Section II, a simple method for plasma position determination for ISTTOK is proposed. We show that the centroid position measurement by magnetic diagnostics is feasible using our algorithm for separation of the plasma signal. The correctness of the resulting method is proven by comparison with the results of the heavy-ion beam
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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