1,721,007 research outputs found
Commissioning of a Calibration Device for Second Sound Based Quench Detection and Reconstruction Analysis
No full textAn important part of research and development (R&D) in the field of superconducting radiofrequency cavities is quench detection since these local breakdowns of superconductivity primarilylimit the cavity performance. Due to the detection of second sound waves propagating inliquid helium, the localization of quench spots on superconducting radio frequency cavities no longer requires the laborious attachment of many temperature sensors. Although second sound detection is widely used, estimating its accuracy is difficult and only few studies dealing with systematic uncertainties exist. Particularly helpful for this task is the artificial generation of second sound signals using a calibration device. Therefore, the already existing second sound system at the cavity test facility of Deutsches Elektronen-Synchrotron DESY is extended by calibration device prototypes in the framework of this thesis.It will be shown that ohmic resistors play a crucial role in the construction of such a tool. Heated by means of short electrical pulses, these resistors generate a second sound wave, which can be detected even within larger cryostats using noise cancelling algorithms and the existing reconstruction software. With the calibration device at hand, the influences of sensor positioning and quench location are thoroughly investigated
Systematic Studies of a Cavity Quench Localization System
No full textSeveral tools for quench localization at Superconducting Radio Frequency cavities exist. One of these techniques uses the excitation of temperature waves in liquid Helium below the lambda point. These waves are created by a quench of the cavity surface and are called second sound waves. Special sensors, such as Oscillating Superleak Transducers (OST), are able to detect these waves. With the help of the OSTs the propagation times of the signals are determined. This method allows a parallel measurement of the quench-spot during a standard cavity performance test, whereas other methods require their own dedicated cold test or bear the risk of a contamination of the cavity surface, which requires a re-treatment of the cavity surface afterwards.Using a set of algorithms a most probable spot for the quench location can be calculated via the reconstruction of the second sound wave propagation path. Two such methods are used at DESY: Multilateration and Raytracing. New algorithms are implemented for these two basic methods in the scope of this thesis. They are including a method to introduce a propagation time offset and a second sound velocity dependent reconstruction. The precision, constraints and limits of these algorithms are studied in this thesis. A data set of nearly 200 quench events measured at 18 different tested single-cell cavities is used for a stochastic evaluation. To analyze the large number of events two automation processes are introduced.In the data set an angular and height dependency of the reconstructed quench-spot is observed and attributed to specific properties of the experimental setup. A common anomaly that the reconstructed quench-spot appears several millimeters above the cavity surface could be observed with all used algorithms in different manifestations. For the generation of an uncertainty propagation a Monte Carlo simulation is written. Using the improved algorithms developed in the scope of the thesis a spatial resolution of and \,mm respectively can be achieved with the two best reconstruction methods. A rating of the usability for quench-detection of the different reconstruction algorithms is given as well. In order to enable an experimental based uncertainty estimation of the quench localization a tool for calibration via simulation of a quench generated second sound wave is developed. The simulation is realized by injection of short heat pulses at exact known time and space coordinates. First measurements with the calibration tool are presented in the end
Magnetometric Studies on Flux Trapping Sensitivity of Superconducting Radiofrequency Cavities
No full textThe increasing demand for high-performance superconducting radiofrequency (SRF) cavities in particle accelerators,especially in free electron lasers, has led to intensified research on optimizing cavity treatments. One promisingrecent development is the application of medium temperature heat treatments, which are typically performed at250 ◦C - 350 ◦C in a vacuum furnace for several hours. It has been shown that these mid-T heat treatmentsenhance cavity performance by increased quality factors. However, the improved performance comes with thetrade-off of an increased sensitivity to trapped magnetic flux of the cavity, which leads to lower quality factors. Forthe first time at DESY, the sensitivity to magnetic flux trapping is deduced from parasitic cavity measurements.The data retrieval and analysis in the scope of this project was only accessible due to a new systematic approachto both experimental test conditions and data acquisition systems. The sensitivity to trapped magnetic flux issystematically investigated on four cavities with very different heat treatments. Magnetic field measurements wereconducted using three magnetic sensors mounted on the cavity equator. Additionally, the temperature gradientsduring the cooldown procedures are analyzed and showed consistencies in regard to the according cavity qualityfactors. Finally, results are compared to data of previous studies. These findings display the complex interplaybetween chemical and heat treatments, material properties and thermal dynamics on SRF niobium cavities. It canbe confirmed that mid-T heat treated cavities show significantly increased sensitivity values 3-5 times higher thanfor cavities without this heat treatment. Moreover, differences in sensor signals after applying an external magneticfield highlight the influence of cavity characteristics, such as grain structure and defects. These findings provide astrong foundation for future optimization of cavity treatments for a second generation of EuXFEL cavities, whichare required for the planned upgrade of the European XFEL accelerator
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
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