ITER relevant runaway electron studies in the FTU Tokamak

Mención Internacional en el título de doctorDisruptions represent a serious danger for the operation of future magnetic confinement fusion devices based on the tokamak concept, as the international ITER (International Thermonuclear Experimental Reactor) project, currently under construction in Cadarache (France). During a disruption, a violent instability occurs that causes a sudden loss (in characteristic times of milliseconds) of the plasma energy and current. As a consequence, large heat loads and electromagnetic forces take place on the first wall components and device structures that can seriously limit its lifetime. Besides, during disruptions, large electric fields are produced that can lead to the generation and acceleration of a fraction of the plasma electrons to very high energies (up to tens or hundreds of MeV): the runaway electrons. The interaction of these energetic electrons with the first wall components can cause severe damage and even oblige to a temporary stop of the tokamak operation. Thereby, controlling and mitigating the effects associated with the disruptions and, in particular, with the runaway electrons constitutes currently one of the critical areas of research in magnetic confinement fusion with views to the future ITER project. This doctoral thesis includes a collection of studies on runaway electrons carried out in the FTU (Frascati Tokamak Upgrade) tokamak in Frascati (Rome), a medium sized tokamak whose high magnetic field and density make it especially adequate to make extrapolations for ITER. The work is the result of collaborations with the FTU tokamak team within the framework of the european EUROFUSION projects, WP14-MST2-9, WP15-MST2-15 (“Runaway Electron Studies in FTU”; 2014 - 2016), WP18-MST2-15 (“REIS activities”; 2018), MST1-2017 and MST1-2018 (“Medium Sized Tokamaks”; 2017 and 2018), as well as the national projects ENE2012-31753 (“ITER-relevant disruption and runaway electron studies”; 2013 - 2016) and ENE2015-66444-R (“Runaway electron generation, control and dissipation during disruptions: implications for ITER”; 2016 - 2019), all of them oriented to ITER. The thesis has been divided into three parts. The first part consists of two chapters introducing the basic concepts: the controlled thermonuclear fusion, the tokamak scheme, the ITER project, and the FTU tokamak on which the work has been performed (Chapter 1), and the Chapter 2 which introduces the basic ideas regarding the runaway electrons in tokamaks, including basic elements of runaway physics, in particular the critical electric field for runaway electron generation, to which a chapter on FTU results is devoted, the generation and energy dynamics of the runaway electrons, as well as the runaway electrons during disruptions. The second part of the manuscript consists of three chapters in which the results related to the runaway electrons in the FTU tokamak are presented. The Chapter 3 introduces new diagnostic systems, recently installed and validated, for the detection of runaway electrons in FTU: the Gamma camera, for the detection of hard X-rays, that allows to obtain spatial and temporal information on the runaway electrons and their energy, and the cameras in visible and infrared spectrum (REIS system: Runaway Elec- tron Imaging and Spectrometry System) that make possible to acquire images of the runaway beam in flight as well as information about their energy. Both systems are of great importance for an adequate description of the runaway electron dynamics in the experiment (as in Chapters 4 and 5). In this chapter the main characteristics of both systems are described, together with examples of their application in the FTU tokamak. The Chapter 4 is dedicated to the experiments performed in the FTU tokamak on one of the basic parameters of the runaway physics: the criti- cal (threshold) electric field for runaway electron generation. This critical field is the minimum electric field necessary to generate runaway electrons and is of great practical importance because it determines the minimum amount of impurities required to inject into the plasma to suppress and control the runaway electrons. The experiments carried out for the determination of the value of the critical electric field are described in this chapter and the measured values are compared with the theory. The results indicate that the measured values are always substantially larger than the predicted by the classical theory, which assumes that the runaway energy looses are dominated by the collisions, and that the increase in the observed value of the critical electric field is consistent with the losses of energy associated with electron synchrotron radiation. This would mean that the amount of impurities that would be necessary to inject into the plasma in order to suppress the runaway electrons could be significantly lower than initially predicted based on the classical collisional theory. Finally, the Chapter 5 presents a summary of the experiments performed in the FTU tokamak, in which we have collaborated, on the active control of the runaway beam currents formed during disruptions. The control of the runaway beam position, in order to avoid the interaction of runaway electrons with the tokamak structures, together with the current dissipation through a slow current ramp-down, currently represents one of the main schemes proposed for the control of the runaway current, alternative to the plasma impurity injection. All the experimental work described in these three chapters has been done in collaboration with the staff of the FTU tokamak and included in all the cases: my participation in the design and planning of the experiments, set-up and validation of the diagnostic systems and data acquisition, the analyis and interpretation of the measurements, as well as the elaboration of databases. Lastly, in the third part of the manuscript (Chapter 6), the conclusions and future lines of work are presented.Las disrupciones constituyen un serio peligro para la operación de dispositivos de fusión por confinamiento magnético tipo tokamak como el proyecto internacional ITER (International Thermonuclear Experimental Reactor) actualmente en construcciónn en Cadarache (Francia). Durante una disrupción se produce una violenta inestabilidad que da lugar a la pérdida súbita (en tiempos característicos de milisegundos) de la energía y la corriente del plasma. Como consecuencia, se crean grandes cargas de calor y fuerzas electromecánicas sobre las componentes de la primera pared y las estructuras del dispositivo que pueden limitar seriamente la integridad del mismo. Además, durante disrupciones, se producen grandes campos eléctricos que pueden dar lugar a la generación y aceleración a muy altas energías (hasta decenas o cientos de MeV) de una fracción de los electrones del plasma: los electrones runaway. La interacción de estos electrones energéticos con los elementos de la primera pared puede dañarlos severamente e incluso obligar a parar temporalmente la operación del tokamak. De este modo, controlar y mitigar los efectos asociados a las disrupciones y, en particular, a los electrones runaway constituyen en la actualidad una las áreas críticas de la investigación en fusión por confinamiento magnético con vistas al futuro proyecto ITER. Esta tesis doctoral incluye un conjunto de estudios sobre electrones runaway realizados en el tokamak FTU (Frascati Tokamak Upgrade) en Frascati (Roma), una máquina de tamaño medio cuyo alto campo magnético y alta densidad la hacen especialmente adecuada para realizar extrapolaciones a ITER. El trabajo es el resultado de colaboraciones realizadas con el equipo del tokamak FTU dentro del marco de los proyectos europeos de EUROFUSION WP14-MST2-9, WP15-MST2-15 (“Runaway Electron Studies in FTU”; 2014 - 2016), WP18-MST2-15 (“REIS activities”; 2018), MST1-2017 y MST1-2018 (Medium Sized Tokamaks”; 2017 y 2018), así como de los proyectos nacionales ENE2012-31753 (“Estudios sobre electrones runaway y disrupciones relevantes para ITER”; 2013 - 2016) y ENE2015-66444-R (Generación, control y disipación de electrones runaway durante disrupciones: implicaciones para ITER”; 2016 - 2019), todos ellos orientados a ITER. La tesis se ha dividido en tres partes. La primera parte consta de dos capítulos de introducción a los conceptos básicos: la fusión termonuclear controlada, el esquema tokamak, el proyecto ITER, y el tokamak FTU sobre el que se desarrolla este trabajo (Capítulo 1), y el Capítulo 2 que introduce ideas básicas relativas a los electrones runaway en tokamaks, incluyendo elementos básicos de la física runaway, en particular el campo eléctrico crítico para la generación de electrones runaway, al que se dedicará posteriormente un capítulo de resultados en el tokamak FTU, la generación y dinámica en energía de los electrones runaway, así como los electrones runaway durante disrupciones. La segunda parte de la memoria consta de tres capítulos en los que se presentan los resultados relativos a los electrones runaway en el tokamak FTU. El Capítulo 3 introduce nuevos sistemas de diagnóstico, recientemente instalados y validados, para la detección de electrones runaway en FTU: la Cámara gamma, para la detección de rayos X duros, que permite obtener información espacial y temporal sobre los electrones runaway y su energía, y las cámaras en la región del espectro visible e infrarrojo (sistema REIS: Runaway Electron Imaging and Spectrometry System) que hacen posible obtener imágenes del haz runaway así como información sobre su energía. Ambos sistemas son de gran importancia para una descripción adecuada de la dinámica de los electrones runaway en el experimento (como en los Capítulos 4 y 5). En este capítulo se describen las características principales de ambos sistemas así como ejemplos de aplicación en el tokamak FTU. El Capítulo 4 está dedicado a los experimentos realizados en el tokamak FTU sobre uno de los parámetros básicos de la física runaway: el campo eléctrico crítico para la generación de electrones runaway. Dicho campo crítico es el campo eléctrico mínimo necesario para que se puedan generar electrones runaway y es de gran importancia práctica porque determina la cantidad mínima de impurezas que es necesario inyectar en el plasma para poder suprimir y controlar los electrones runaway. En este capítulo se describen los experimentos realizados para determinar el valor del campo eléctrico crítico y se comparan los valores medidos con la teoría. Los resultados indican que los valores medidos son siempre sustancialmente mayores que los predichos por la teoría clásica, que supone que las pérdidas de energía de los electrones runaway están dominadas por las colisiones, y que dicho aumento en el valor del campo crítico es consistente con las pérdidas de energía de los electrones runaway asociadas a la radiación sincrotrón del electrón. Esto significaría que la cantidad de impurezas que sería necesario inyectar en el plasma para suprimir los electrones runaway podría ser significativamente menor que la predicha inicialmente sobre la base de la teoría clásica colisional. Finalmente, el Capítulo 5 presenta un resumen de los experimentos realizados en el tokamak FTU, en los que hemos colaborado, sobre el control activo de haces de corriente runaway formados durante disrupciones. El control de la posición del haz runaway, para evitar la interacción de los electrones runaway con las estructuras, junto con una “suave caída” de la corriente para disiparla, constituye en la actualidad uno de los principales esquemas propuestos para el control de la corriente runaway alternativos a la inyección de impurezas en el plasma. Todo el trabajo experimental descrito en estos tres capítulos ha sido realizado en colaboración con personal del tokamak FTU y ha incluido en todos los casos: mi participación en el diseño y planificación de los experimentos, la puesta a punto y validación de los sistemas de medida y adquisición de datos, el tratamiento e interpretación de las medidas realizadas, así como la elaboración de bases de datos. Por último, en la tercera parte de la memoria (Capítulo 6) se presentan las conclusiones y líneas futuras de trabajo.The work is the result of collaborations with the FTU tokamak team within the framework of the european EUROFUSION projects, WP14-MST2-9, WP15-MST2-15 (”Runaway Electron Studies in FTU”; 2014 - 2016), WP18-MST2-15 (”REIS activities”; 2018), MST1-2017 and MST1-2018 (”Medium Sized Tokamaks”; 2017 and 2018), as well as the national projects ENE2012-31753 (”ITER-relevant disruption and runaway electron studies”; 2013 - 2016) and ENE2015-66444- R (”Runaway electron generation, control and dissipation during disruptions: ix implications for ITER”; 2016 - 2019), all of them oriented to ITER.Programa de Doctorado en Plasmas y Fusión Nuclear por la Universidad Carlos III de MadridPresidente: Luis Raúl Sánchez Fernández.- Secretario: José Ángel Mier Maza.- Vocal: Peter Christiaan de Vrie

THE ROLE OF CANDIDA ALBICANS ON THE DEVELOPMENT OF STOMATITIS IN PATIENTS WEARING DENTURES

Denture stomatitis is the most common inflammatory reaction that occurs in people who wear dentures. It is believed that in 60-65% of cases the cause of this inflammation is infections by yeasts from the genus Candida (C.), primarily Candida albicans infection. C.albicans is a part of the normal microflora of the respiratory and digestive tract. This yeast has the ability to adhere to the oral mucosa and to the base of the denture, as well as to form a biofilm. Its virulence is especially supported by the state of weakened resistance of the organism, when C.albicans expresses its pathological effect. This paper presents the pathogenesis of C. albicans-associated denture stomatitis, as well as the most common diagnostic and therapeutic procedures used to diagnose and successful therapy

On the measurement of the threshold electric field for runaway electron generation in the Frascati Tokamak Upgrade

Experiments have been carried out to evaluate the threshold electric field for runaway generation during the flat-top phase of ohmic discharges in the Frascati Tokamak Upgrade tokamak. An investigation of the conditions for runaway electron generation and suppression has been performed for a wide range of plasma parameter values. The measured threshold electric field is found to be significantly larger (similar to 2 - 5 times) than predicted by the relativistic collissional theory of runaway generation, E-R = n(e) e(3) ln Lambda/4 pi e(0)(2) m(e) c(2), and can be explained to a great extent by an increase of the critical electric field due to the effect of the electron synchrotron radiation losses. These findings are consistent with the results of an ITPA joint experiment to study the onset, growth, and decay of relativistic runaway electrons [Granetz et al., Phys. Plasmas 21, 072506 (2014)]. Confirmation of these results for disruptions with high electric field might imply significantly lower requirements on electron densities for suppression and prevention of runaway formation in ITER.This work was carried out with financial support from Dirección General de Investigación, Científica y Técnica, Project No. ENE2012-31753 (MINECO; Spain).Publicad

Runaway electron imaging spectrometry (REIS) system

A portable Runaway Electron Imaging and Spectrometry System (REIS) was developed in ENEA-Frascati to measure synchrotron radiationspectra from in-flight runaway electrons in tokamaks. The REIS is a wide-angle optical system collecting simultaneously visible and infraredemission spectra using an incoherent bundle of fibers, in a spectral range that spans from 500 nm to 2500 nm, and visible images using a CCDcolor microcamera at a rate of 25 frames/s. The REIS system is supervised and managed using a dedicated LabVIEW program to acquire datasimultaneously from three spectrometers every 20 ms (configurable down to 10 ms). An overview of the REIS architecture and acquisitionsystem and resulting experimental data obtained in FTU are presented and discussed in this paper.This work was carried out within the framework of the EUROfusion Consortium (Project No. MST2-15: Runaway Electron Imaging) and received funding from the Euratom research and training programme 2014–2018 under Grant Agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The authors would like to thank M. Turnyanskiy, responsible officer for this EUROfusion project, for his continuous support and encouragement throughout this work.Publicad

Runaway electron generation and control

Special issue featuring the invited talks from the 43rd EPS Conference on Plasma Physics, Leuven, 4-8 July, 2016We present an overview of FTU experiments on runaway electron (RE) generation and control carried out through a comprehensive set of real-time (RT) diagnostics/control systems and newly installed RE diagnostics. An RE imaging spectrometer system detects visible and infrared synchrotron radiation. A Cherenkov probe measures RE escaping the plasma. A gamma camera provides hard x-ray radial profiles from RE bremsstrahlung interactions in the plasma. Experiments on the onset and suppression of RE show that the threshold electric field for RE generation is larger than that expected according to a purely collisional theory, but consistent with an increase due to synchrotron radiation losses. This might imply a lower density to be targeted with massive gas injection for RE suppression in ITER. Experiments on active control of disruption-generated RE have been performed through feedback on poloidal coils by implementing an RT boundary-reconstruction algorithm evaluated on magnetic moments.This work was carried out within the framework of the EUROfusion Consortium and received funding from the Euratom research and training programme 2014–2018 under grant agreement No 633053 (Projects MST2-9 and MST2-15). The views and opinions expressed herein do not necessarily reflect those of the European Commission. Additional financial support was received from MINECO (Spain), Projects No. ENE2012-31753 and ENE2015-66444-R.Publicad