Publication: Estudio de las impurezas y la radiación continua en el rango del espectro visible mediante técnicas espectroscópicas y láseres en el stellarator TJ-II
Loading...
Official URL
Full text at PDC
Publication Date
2021-07-02
Authors
Advisors (or tutors)
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
Universidad Complutense de Madrid
Citations
Exportar
Abstract
Disponer de soluciones energéticas abundantes y sostenibles es uno de los retos a los que ha de enfrentarse la sociedad actual. La obtención de energía por medio de la fusión de isótopos de hidrógeno en dispositivos de fusión confinados magnéticamente se erige como una alternativa prometedora. En las máquinas de tipo stellarator, uno de los principales retos que hay que superar es la acumulación de impurezas, debido a que su presencia provoca bien, la dilución del combustible y, en consecuencia, su enfriamiento o bien, compromete la integridad de la cámara de vacío al redepositarse en las paredes de la misma. En la actualidad, se ha hecho un enorme esfuerzo para entender los mecanismos involucrados en la estimación de los niveles de impurezas potenciales y de los requisitos de control de las mismas en futuros reactores de fusión. A pesar de ello, algunos resultados experimentales no se comprenden completamente o bien, las simulaciones teóricas han de ser validadas experimentalmente. En este sentido, el control de las impurezas sigue siendo un desafío y el TJ-II, un excelente banco de pruebas donde estudiar estos problemas.
En este trabajo, se llevan a cabo experimentos de espectroscopia óptica en plasmas del TJ-II con hidrógeno calentado por emisión ciclotrónica electrónica (ECRH) y con densidades electrónicas típicas alrededor de 0.5×1019 m-3. Por un lado, se emplea un espectrómetro de baja resolución, en el rango comprendido entre 200 y 950 nm, para identificar líneas espectrales de emisión y espectros continuos en regiones libres de emisión de líneas. Identificadas las regiones del espectro de emisión, libres de emisión de líneas (221.5, 523.5, 715.5 nm), se procede a la medida de la radiación continua o bremmstrahlung, que permite, estimar la carga efectiva (Zeff) del plasma. Hasta la fecha, la Zeff en el TJ-II se había estimado empleando técnicas de rayos X. Los intentos previos de emplear la radiación de frenado para su determinación habían resultado en valores de la Zeff muy superiores a los obtenidos con la primera técnica. Aquí, se calibra absolutamente el sistema espectral empleado, obteniéndose una buena correlación entre la señal obtenida y la señal esperada. En segundo lugar, con objeto de monitorizar simultáneamente las impurezas y la composición isotópica de la pared de la cámara de vacío, se emplea la técnica espectroscópica de disociación inducida por láser (LIBS) identificándose la presencia de líneas de oxígeno. Así mismo, los experimentos de tiempo de vuelo de impurezas de litio desorbidas de la pared por láser permiten determinar la temperatura iónica en la periferia del plasma vertiendo resultados similares a los obtenidos mediante otro sistema empleado en el TJ-II, el Reverse Field Analyzer (RFA). La implementación de la técnica LIBS en el TJ-II resulta novedosa, abriendo nuevas líneas de investigación...
Having abundant and sustainable energy sources is one of the challenges facing today's society. Obtaining energy by means of the nuclear fusion of hydrogen isotopes in magnetic confinement plasma devices stands a promising option. The study of impurities is a key parameter to the development of steady state plasmas in future magnetically confined fusion devices. In stellarator-type machines, one of the main challenges is to overcome the accumulation of impurities in the plasma core, because the presence of impurities can result in the dilution of the fuel and, consequently, its cooling and collapse, while also compromising the integrity of the vacuum chamber when impurities are redeposited in the walls. At present, an enormous effort has been made to understand the mechanisms involved in estimating the levels of potential impurities and their control requirements for future fusion reactors. Despite this, some experimental results are not fully understood or, theoretical simulations have to be validated experimentally. For this reason, impurity control remains a challenge and the TJ-II, an excellent workbench to test these problems. In this work, optical spectroscopy experiments in TJ-II plasmas are carried out with hydrogen heated by electronic cyclotronic emission (ECRH) and using hydrogen, deuterium or helium as the working gas that typically achieve electronic densities around 0.5×1019 m-3. On the one hand, first, a low-resolution spectrometer is used, in the range between 200 and 950 nm, to identify spectral lines emitted by impurities and regions free of emission lines. Once the impurities present in the TJ-II plasmas are identified, the continuous radiation is studied, which allows to estimate the effective charge (Zeff) of the plasma from the visible bremmstrahlung radiation. To date, the Zeff in the TJ-II had been estimated using X-ray techniques. Previous attempts to use visible bremmstrahlung radiation for its determination had resulted in Zeff values much higher than those obtained with the X-ray technique. Here, an optical spectral system that repeatedly scans the TJ-II plasma is absolutely calibrated, obtaining a good correlation between the obtained and the expected signal. Three spectral regions are identified to be free of spectral lines, i.e. narrow bands about 221.5, 523.5 and 715.5 nm where to estimate the Zeff values close to those obtained with the X-ray technique, although slightly higher. It is proposed to exchange the original lens of this optical system for a panchromatic doublet and, with the Zemax® design program, the optimal positions of it for each of the spectral ranges are estimated. With this configuration, the Zeff is again estimated for the 523.5 nm wavelength, observing a reduction in its value and a reduction in the observed spatial structure. In order to simultaneously monitor impurities and the isotopic composition of the wall chamber, the laser induced breakdown spectroscopy (LIBS) technique has allowed the determination of the molecular ionic temperature at the periphery of the plasma that are comparable with values obtained by the RFA technique. The implementation of the LIBS technique in the TJ-II is novel, opening new lines of research...
Having abundant and sustainable energy sources is one of the challenges facing today's society. Obtaining energy by means of the nuclear fusion of hydrogen isotopes in magnetic confinement plasma devices stands a promising option. The study of impurities is a key parameter to the development of steady state plasmas in future magnetically confined fusion devices. In stellarator-type machines, one of the main challenges is to overcome the accumulation of impurities in the plasma core, because the presence of impurities can result in the dilution of the fuel and, consequently, its cooling and collapse, while also compromising the integrity of the vacuum chamber when impurities are redeposited in the walls. At present, an enormous effort has been made to understand the mechanisms involved in estimating the levels of potential impurities and their control requirements for future fusion reactors. Despite this, some experimental results are not fully understood or, theoretical simulations have to be validated experimentally. For this reason, impurity control remains a challenge and the TJ-II, an excellent workbench to test these problems. In this work, optical spectroscopy experiments in TJ-II plasmas are carried out with hydrogen heated by electronic cyclotronic emission (ECRH) and using hydrogen, deuterium or helium as the working gas that typically achieve electronic densities around 0.5×1019 m-3. On the one hand, first, a low-resolution spectrometer is used, in the range between 200 and 950 nm, to identify spectral lines emitted by impurities and regions free of emission lines. Once the impurities present in the TJ-II plasmas are identified, the continuous radiation is studied, which allows to estimate the effective charge (Zeff) of the plasma from the visible bremmstrahlung radiation. To date, the Zeff in the TJ-II had been estimated using X-ray techniques. Previous attempts to use visible bremmstrahlung radiation for its determination had resulted in Zeff values much higher than those obtained with the X-ray technique. Here, an optical spectral system that repeatedly scans the TJ-II plasma is absolutely calibrated, obtaining a good correlation between the obtained and the expected signal. Three spectral regions are identified to be free of spectral lines, i.e. narrow bands about 221.5, 523.5 and 715.5 nm where to estimate the Zeff values close to those obtained with the X-ray technique, although slightly higher. It is proposed to exchange the original lens of this optical system for a panchromatic doublet and, with the Zemax® design program, the optimal positions of it for each of the spectral ranges are estimated. With this configuration, the Zeff is again estimated for the 523.5 nm wavelength, observing a reduction in its value and a reduction in the observed spatial structure. In order to simultaneously monitor impurities and the isotopic composition of the wall chamber, the laser induced breakdown spectroscopy (LIBS) technique has allowed the determination of the molecular ionic temperature at the periphery of the plasma that are comparable with values obtained by the RFA technique. The implementation of the LIBS technique in the TJ-II is novel, opening new lines of research...
Description
Tesis inédita de la Universidad Complutense de Madrid, Facultad de Óptica y Optometría, Departamento de Óptica, leída el 28/01/2021