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Habilitation Thesis
DOI
10.11606/T.43.2015.tde-26102015-131414
Document
Author
Full name
Alvaro Vannucci
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 1998
Committee
Lepine, Jacques Raymond Daniel (President)
Caldas, Ibere Luiz
Galvao, Ricardo Magnus Osorio
Ludwig, Gerson Otto
Maciel, Homero Santiago
Title in Portuguese
Instabilidades de disruptura em tokamaks
Keywords in Portuguese
Tokamaks
Abstract in Portuguese
A origem das instabilidades de disruptura foi investigada. Trabalhos realizados em três tokamaks diferentes, um pequeno (TBR-1), outro médio (TEXT-U) e outro de grande porte (JET), mostraram que o acoplamento entre os modos MHD m = 2 e m = 1 constituiria o principal mecanismo de desencadeamento das disrupturas maiores. Os resultados obtidos indicam que este tipo de processo de desencadeamento das disrupturas possuiria um caráter universal, isto é, valeria para todos os tokamaks com baixo e alta razão de aspecto. Neste trabalho são discutidos também alguns resultados envolvendo mecanismos de controle das instabilidades de disruptura. No TBR-1, como consequência da aplicação de campos magnéticos externos criados por um conjunto de malhas helicoidais e por um sistema de limitador ergódico magnético, uma forte atenuação da atividade MHD pôde ser obtida. Finalmente, em um trabalho bastante inédito na área de fusão, redes neurais foram utilizadas para se tentar prever, com a maior antecedência possível, o instante de ocorrência de uma disruptura nas descargas de plasma do tokamak TEXT. Utilizando-se sinais de oscilações magnéticas de Mirnov na rede neural, pôde-se realizar a previsão da ocorrência de uma disruptura com mais de 1 ms de antecedência. Mais recentemente, este tempo de previsão pôde ser quase triplicado, através da utilização de sinais de raios-X de baixas energias provenientes da região central da coluna de plasma em lugar dos sinais de Mimov. Comparações realizadas entre os sinais experimentais destes dois sistemas de diagnóstico, nos instantes finais do pulso de plasma, indicaram que o processo disruptivo inicia-se em tomo da região central da coluna de plasma e somente depois afeta a parte mais externa do plasma, onde localiza-se a superfície magnética correspondente ao fator de segurança q = 2.
Title in English
Instabilities disruption in tokamaks
Keywords in English
Tokamaks
Abstract in English
The origin of the disruptive instabilities was experimentally investigated. The research was carried out in three tokamaks of different sizes and characteristics: the small TBR-1, the medium size TEXT-U, and JET, which is one of the largest devices that have been constructed and put in operation. For ali of them, the m = 2 and m = 1 mode coupling was found to be the main triggering mechanism for the major disruptions analyzed. These results indicate, therefore, that this triggering mechanism may be considered rather universal in a sense that it would be valid for tokamaks with low-f3 and low aspect ratio. Also, some controlling mechanism for disruptions were investigated. In the TBR-1 tokamak a strong attenuation of the MHD activity was obtained when external magnetic fields were applied. These external perturbing fields were produced by a set of helical windings and also by an ergodic magnetic limiter. Finally, the results obtained with neural networks to forecast the onset of disruptions in TEXT discharges are discussed. This original work was done by using Mimov magnetic signals to feed the neural net and predictions greater than 1 ms could be obtained. More recently, it was possible to increase the forecasting time (for almost three times longer) by using the central soft X-rays signal instead of the Mimov magnetic signal in the neural networks. A comparison between the experimental data of both diagnostic systems, close to the disruption time, was made and indications were found that the disruptive process initiates around the central part of the plasma column and, only afterwards, it disturbs the plasma around the region where the q = 1 magnetic surface is located.
 
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1998Vannucci.pdf (30.75 Mbytes)
Publishing Date
2015-10-26
 
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