Tesis (Física Atómica, Molecular y Nuclear)

URI permanente para esta colecciónhttps://hdl.handle.net/11441/10867

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Development of advanced numerical Tools for fusion reactor Diagnostics and nonlinear modeling of Plasma Dynamics
(2024-09-18) Oyola Domínguez, Pablo; Viezzer, Eleonora; Birkenmeier, G.; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
A precise control and understanding of the plasma dynamics is crucial for future fusion reactors. The complex dynamics and the harsh environments in magnetic fusion devices, such as tokamaks, require the most advanced modeling techniques both from the theoretical predictive side, to the experiment diagnosis and data analysis. In this PhD thesis, a two-fold approach is presented to advance in the understanding and control of the plasma dynamics. From the experimental side, a novel diagnostic, the imaging Heavy Ion Beam Probe (i-HIBP), has been installed, commissioned and operated at the ASDEX Upgrade tokamak during this thesis. The i-HIBP has been installed in the ASDEX Upgrade tokamak (Germany), in collaboration with the Max-Planck Institute for Plasma Physics. This diagnostic uses heavy alkali as probe ions that, after passing through the plasma, reach a scintillator plate. The footprint on the scintillator encodes information of the plasma density and the electromagnetic perturbations along their trajectories in the plasma. In this work, the modeling tools have been developed and applied to reproduce the signals and extract relevant information on edge plasma density and current density perturbations. In particular, it is shown that even in a low signal-to-noise ratio scenario, the plasma density can be reconstructed at the plasma edge with an excellent radial resolution. On the theoretical and modeling side, this PhD thesis focuses on studying the fast ions(suprathermal particles), as they will play a key role in the fusion power generation: they are envisioned to be the most important source of energy (heating) and momentum. Fast-ion losses can lead to a decrease in the power throughput and, when localized, could potentially damage the first wall components. A renewed interest in an alternative tokamak scenario, based on the negative-triangularity shaped plasma, has arisen the question of the fast-ion confinement and the possible instabilities that they could suffer. In this work, this topic is addressed by using one of the most advanced hybrid kinetic magnetohydrodynamics codes in the community, the MEGA code, to study the behavior and transport of a Toroidal Alfvén Eigenmode (TAE). The results presented here show that the confinement of the fast-ions is more resilient
Low Dimensional Optoelectronic Devices enabled by Vacuum and Plasma Technologies
(2024-07-30) Castillo Seoane, Javier; Sánchez Valencia, Juan Ramón; Borrás Martos, Ana Isabel; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
The need for energy-efficient technologies is critical for confronting urgent environmental challenges. This Ph.D. thesis explores the development of nanostructured optoelectronic devices with enhanced efficiency, durability, and functionality using plasma and vacuum technologies. It focuses on synthesizing and integrating low-dimensional nanostructures, such as 1D nanowires, nanotubes, core@(multi)shell systems, and nanowalls, through multistep soft-template methods and glancing angle deposition (GLAD). These materials were integrated into photovoltaic devices and photodetectors, demonstrating improved stability and multifunctionality. The research not only highlights the transformative potential of vacuum and plasma technologies in creating advanced optoelectronic devices but also paves the way for more sustainable, energy-efficient technologies.
Imaging neutral particle analyzer for fast-ion transport measurements in the ASDEX upgrade tokamak
(2024-03-19) Rueda Rueda, José; García Muñoz, Manuel; Viezzer, Eleonora; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
In future fusion reactors, suprathermal particles (fast ions, FI) will play a key role in the generation of fusion power as they are an important source of energy (heating) and momentum (current drive). A loss of their confinement will lead to a decrease in reactor performance, and, when localized and intense, to damage in the first wall components. Understanding the mechanisms behind the suprathermal particle transport and losses is capital for achieving a future fusion power plant. One of the main observed causes for the FI transport and eventual loss is their interaction with a wide range of electromagnetic fluctuations. An accurate understanding of the fast-ion behavior in the presence of magnetohydrodynamic fluctuations is required for achieving a good fast-ion confinement. To this end, new diagnostics are being developed to measure the fast-ion distribution over a broad region of the phase space with high resolution. In this PhD thesis, an Imaging Neutral Particle Analyzer (INPA) has been installed and operated at the ASDEX Upgrade (AUG) tokamak, located at the Max Planck Institute for Plasma Physics in Garching (Germany). INPA employs the operational principles of both fast-ion loss detectors (FILD) and neutral particle analyzers (NPA) to measure the fast-ion distribution in energy and radius. This diagnostic system analyses fast neutrals that emerge from charge exchange (CX) reactions between fast ions and neutral particles. These fast neutrals are ionized through an ultra-thin carbon foil located within the in-vessel optical head and are deflected towards a scintillator using the local magnetic field of the tokamak. From the impinging location of a particle on the INPA scintillator, its energy and velocity projection along the magnetic field lines can be deduced. The use of an active source of neutrals enables the direct correlation of this velocity projection with the radial position of the fast ion. The FILDSIM code, which facilitates the calculation of synthetic signals for the FILD diagnostic, has undergone a major upgrade to handle the INPA diagnostic. This upgrade includes a model for simulating the scattering and energy loss of fast neutrals within the carbon foil. Additionally, it encompasses a model for estimating the scintillator yield and the capacity to conduct tomographic reconstructions. This updated code has been benchmarked against experimental data during the 2021-2022 campaign, showing an excellent agreement between simulations and measurements. Tomographic inversions also agree with neoclassical calculations during MHD quiescent phases. Fast-ion acceleration during second harmonic ion cyclotron resonance heating has been characterized and compared to simulations. The agreement found serves as validation of these codes for their extrapolation to future machines. Fast-ion flows driven by Alfvén eigenmodes have been measured for the first time at ASDEX Upgrade. The observed flows align well with the theoretical models and with fullorbit simulations.
Compact neutron sources for Nuclear Physics: from accelerator-based to laser-driven neutron beams
(2023-12-14) Millán Callado, María de los Ángeles; Fernández Martínez, Begoña; Guetrrero Sánchez, Carlos; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Pulsed neutron beams are a valuable tool in nuclear physics with applications in a wide variety of fields, including fission and fusion, astrophysics, homeland security, medicine, cultural heritage, avionics, and other industrial or research applications. Despite the potential use of neutron beams, the transfer of knowledge outside large research centers is limited by the huge size and complexity of conventional high-intensity neutron sources and the progressive shutdown of research reactors. Recently, the neutron beams user community is focusing its attention on developing small-scale and compact neutron sources as a complement to major facilities to fully exploit all the possibilities of these techniques. In this context, laser-driven ion sources are garnering the interest of the nuclear physics community due to the fast development of ultra-short (~fs) and ultra-high power (> 1019 W/cm2) lasers and their applications as compact particle accelerators. Laser-driven neutron sources (LDNS) are particularly attractive for nuclear physics applications based on the time-of-flight technique thanks to their short pulse length and high instantaneous flux. There are several recent works about neutron production by laser reaching fluxes per pulse competitive to those of conventional neutron sources, but there is a lack of studies in terms of their application to nuclear physics experiments. Laser-driven neutron applications will have to rely on detection systems that are commonly used in nuclear physics experiments with conventional neutron sources, and whose behavior needs first to be characterized in the environment resulting from the laser-plasma interaction and the particularities of a laser-driven source. In this context, there has been a lot of effort aimed at mitigating the impact of the harsh prompt radiation and the electromagnetic background in sensitive neutron diagnostics, mostly based on single-shot PW-class and TW-class lasers at high repetition rates. However, the typical current-mode operation of neutron detectors in LDNS experiments is not suitable to carry out neutron-induced nuclear reaction experiments, since those require the detection of single signals corresponding to the observables from the individual reactions and processes involved. In this thesis, a study on the feasibility of time-of-flight nuclear reaction measurements in the complex environment of an LDNS has been carried out at the DRACO laser facility of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Dresden, Germany, producing neutron shots at 0,02 Hz in a high-power system in stable conditions. In addition to conventional scintillators and bubble detectors operated in current/integrated mode, multi-shot neutron production made it possible to use a neutron and charged particle detector with low efficiency, i.e. diamond detector, to measure individual signals from fast neutron interactions. This itself is a milestone in the path towards nuclear physics time-of-flight experiments at LDNS and, to achieve it, a dedicated signal analysis routine had to be developed for the diamond detector. The characterization of the neutron source resulting from two different nuclear reactions, Cu(p,n) and LiF(p,n), by means of the individual signals and the time-of-flight technique, has been positively validated against Monte Carlo simulations, confirming the feasibility of measuring single fast neutron interactions at an LDNS. The results obtained at DRACO, the characteristics of the LDNS, and the performance of the detectors are compared and contextualized with the results obtained in the commissioning of a conventional accelerator-based compact neutron source: the HiSPANoS neutron source at Centro Nacional de Aceleradores (CNA) in Sevilla, Spain. In this facility, different fast neutron beams have been obtained by means of Be(d,n) and Li(d,n) reactions in thick targets, which provide white neutron beams up to 10 and 20 MeV respectively, as well as of D(d,n) reactions, covering an energy range between 2 and 6 MeV with quasi-monoenergetic neutron beams. The characterization was carried out with conventional fast organic scintillators and applying the time-of-flight technique again. Based on the results and the comparative analysis of both experiments, the neutron production per pulse at DRACO has been established to be superior. Also of high interest, the main drawbacks and issues faced at DRACO are identified, and possible solutions are proposed as a first step towards experiments on fast neutron-induced reactions at laser-driven neutron sources.
Semi-inclusive neutrino-nucleus reactions at intermediate energies
(2023-11-24) Franco Patiño, Juan Manuel; Bárbaro, María Benedetta; Caballero Carretero, Juan Antonio; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
The discovery of neutrino oscillations in 1998 by the Super-Kamiokande experiment opened a new frontier in particle physics. Past, ongoing, and forthcoming experiments are dedicated to unraveling the mechanisms responsible for neutrino oscillations by measuring the physical parameters governing this phenomenon. As for now, the neutrino mass hierarchy remains unknown, yet it is an important physical information. Additionally, neutrino oscillations have hinted at the existence of charge-parity violation within the lepton sector. To determine the presence of such violation, a fundamental parameter of the PMNS paradigm needs to be measured. Presently, accelerator-based neutrino experiments are actively gathering data, while new experiments are in development, all geared towards quantifying the degree of CP violation present in neutrino oscillations. One of the main limiting systematic errors in neutrino oscillation physics comes from our limited knowledge of neutrino-nucleus interactions. To help to constrain nuclear effects for the modeling of neutrino-nucleus interactions and improve the reconstruction of the neutrino energy for oscillation experiments, in this thesis we have studied semi-inclusive neutrino interactions with complex nuclei at intermediate lepton energies, i.e., neutrino beam energies ranging from 0.5 up to 10 GeV. Due to the kinematics of the particles involved and the complexity of the interaction, we have developed a fully relativistic and quantum mechanical model able to describe not only inclusive but also semi-inclusive electron and neutrino reactions with complex nuclei. Starting with Chapter 1, an introduction to neutrino oscillations, acceleratorbased neutrino experiments and neutrino-nucleus interactions is presented. Chapter 2 is mainly focused in the general definition of a semi-inclusive neutrinonucleus reaction and the analysis, from a theoretical point of view, of semiinclusive results using different models of the nucleus, but neglecting the effects introduced by final state interactions. Chapter 3 is entirely dedicated to the description of the one-proton knockout process, a type of semi-inclusive reaction, using a fully relativistic and quantum mechanical model of the nuclear dynamics and final-state interactions called relativistic distorted-wave impulse approximation. In Chapter 4 we review the current approach used by neutrino event generators to describe semi-inclusive reactions. Different approximations for the description of semi-inclusive reactions are compared with semi-inclusive cross section measurements performed by different international collaborations, like T2K, MINERvA and MicroBooNE, in Chapter 5. Finally, in Chapter 6 a summary and the conclusions of this thesis are presented.
3D hybrid kinetic-MHD modelling of the interaction between Edge Localised Modes and Energetic Particles in the ASDEX Upgrade tokamak
(2023-11-03) Domíngez Palacios Durán, Jesús José; Futatani, S.; Toscano Jiménez, Manuel; Universidad de Sevilla. Departamento de Física Aplicada III; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Nuclear fusion is a clean and virtually unlimited energy source that might meet the large energy demands in the near future. For the successful realization of a future fusion reactor, Edge Localized Modes [ELMs, periodic magnetohydrodynamic (MHD) instabilities that expel particles and energy from the plasma in a similar way to solar flares from the edge of the Sun] must be kept under control to avoid the large heat fluxes onto the plasma facing components, which will reduce the lifetime of the reactor. Although the ELM nature is well understood, its behavior and consequences in a burning plasma with a significant fraction of energetic (supra-thermal) ions is still missing. Energetic ions, which are produced by auxiliary heating systems or by the fusion reaction, are an essential source of momentum and energy that must be kept well confined until they slow down to the plasma bulk through Coulomb collisions. However, energetic ions are prone to a rich variety of wave-particle interactions due to their large velocities and long mean free paths, that can lead to an efficient exchange of energy and momentum with a broad spectrum of MHD fluctuations. This thesis shows the first nonlinear hybrid kinetic-MHD simulations of ELMs, aimed to study the self-consistent interaction between ELMs and fast-ions, applying the nonlinear hybrid kinetic-MHD code MEGA to an ASDEX Upgrade plasma. During this thesis, the numerical set up of MEGA code has been modified to study both the thermal plasma and fast-ion dynamics at the plasma edge. First, simulations without fast-ions are performed to simulate the basic physics of an ELM crash. In the linear phase, it is found that high-n ballooning modes are more unstable. The nonlinear coupling between the modes is observed in the early non-linear phase. The ELM crash is finally simulated, observing the plasma filaments that are ejected from the plasma region, with the consequence of the flattening of the driving sources. Fast-ions are then included in the model and different parameters of the fast-ion distribution have been scanned to understand how fast-ions and ELMs interact with each other. First, single-n simulations, which includes n = 0, 10 modes, are performed. When the fast-ion distribution peaks at the plasma core, both the ELM at the plasma edge and an Energetic Particle Mode (EPM) at the plasma core are simulated. Only when the fast-ion distribution is closer to the plasma edge, a strong interaction between ELMs and fast-ions is observed. In such a case, the simulations indicate that fast-ion kinetic effects have a strong impact on the spatio-temporal structure of the ELM. The interaction mechanism between the ELM and fast-ions has been analyzed studying the nature of the power exchange between ELMs and fast-ions in the phase-space of the energetic particles. Although the ELM is driven by the thermal plasma pressure gradient, a resonant interaction between the drift orbits of the edge fast-ion population and the ELM electromagnetic perturbation leads to a net wave-particle energy and momentum exchange that determines the resulting ELM spatio-temporal structure. An Energetic particle driven Geodesic Acoustic Mode (EGAM) appears after the ELM in the hybrid kinetic-MHD simulations, whose mode structure is strongly impacted by the ELM and that might help to understand the frequency pattern of the n = 0 mode observed in NBI heated plasmas. A hybrid kinetic MHD multi-n simulation of ELM, which includes n = 0, …, 10 modes, has also been performed to account for the wave-wave coupling in the presence of fast-ions. Without fast-ions, n = 9, 10 modes are the most unstable modes. In the presence of fast-ions, the most unstable mode number is n = 8 with an energy almost 5 times larger than the mode energies obtained in the MHD multi-n simulation without fast-ions. The shifting of the most unstable mode is due to a large energy exchange between n = 8 mode and energetic ions. The impact of the fast-ion kinetic effects on the spatio-temporal structure of high-n modes is qualitatively the same in both the multi-n and single-n hybrid kinetic-MHD simulations. A strong power exchange between the ELM and fast-ions is also found in the multi-n simulations. In this simulation, a resonance overlap between the different toroidal modes is probably taking place, given the closeness of the resonances associated to the different modes. Additionally, in the multi-n simulations, a strong impact on the spatio-temporal structure of low-n harmonics has been observed. The simulations presented in this manuscript reproduce some outstanding ELM observations in low collisionality plasmas with large fast-ion contents that feature abrupt and large ELM crashes.
Towards PET range verification in proton therapy: new cross sections for improved accuracy
(2023-04-24) Rodríguez González, María Teresa; Guerrero Sánchez, Carlos; Quesada Molina, José Manuel; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Proton therapy is a form of radiotherapy that allows maximizing the deposited dose inside the tumour while reducing the dose in the healthy tissues, thanks to its superior depth-dose distribution when compared against conventional photon therapy. Uncertainties in the beam range, however, require considering additional safety margins to ensure the tumour coverage and the non-irradiation of surrounding tissues. In this context, a method to validate the range of the beam in-vivo should lead to better treatment designs, minimizing normal tissue complications and hence improving tumour control. Among the different options proposed for this, PET range verification has received a very significant attention in the last 15 years and has even been clinically tested. Correspondingly, there is a worldwide effort to make it feasible and reliable aiming at its eventual clinical implementation. PET range verification requires a comparison of the measured (with a PET scanner) and expected (from Monte Carlo simulations) β + activity distributions produced by the proton field in the patient’s body, which can be, depending on the half-life of the isotope involved, online (ms to seconds) or offline (minutes). The accuracy of the mentioned expected activity distribution is based, among others factors, on the accuracy of production cross sections resulting in β + emitters used as input in the Monte Carlo simulations, which are 11C with t1/2=20.36 min, 13N with t1/2=9.97 min, and 15O with t1/2=122 s, produced in C, N y O, 12N with t1/2=11.0 ms, produced in C, 38mK with t1/2=926 ms, produced in Ca and 29P with t1/2=4.14 s, produced in P. Unfortunately, the situation is such that experimental data are completely missing for some reactions of interest and there are sizable discrepancies between the data sets available in EXFOR. Therefore, both the IAEA nuclear data evaluators and the medical physics community have call for a significant improvement of these nuclear data in order to reduce the uncertainties in the estimation of the activity distributions to a level that allows detecting beam range variations within 1 mm. In this context, the work developed in this thesis consists on the determination of the cross sections up to 200 MeV of the reactions involved in PET range verification to improve the simulations of the expected activity distributions in the patient. The reactions of interest are 11 in total, producing either the long-lived isotopes via 12C(p,x)11C, 12C(p,x)13N, 14N(p,x)11C, 14N(p,x)13N, 14N(p,x)15O, 16O(p,x)11C, 16O(p,x)13N and 16O(p,x)15O, or the short-lived isotopes via 12C(p,x)12N, 40Ca(p,x)38mK and 31P(p,x)29P. In this manuscript, a description of the experiments, analyses and results is presented. The experiments have been performed at the National Center of Accelerators (CNA, Spain), the West German Proton Therapy Center (WPE, Germany) and the Heidelberg Ion-Beam Therapy Center (HIT, Germany), using three different detection systems (PET scanners, NaI and LaBr3 detectors). The data presented herein have been obtained either by the multi-foil activation technique combined with the measurement with a PET scanner or by single foil activation and conventional detectors. A wide variety of strategies have been implemented to validate and ensure the accuracy of the results. In order to assess the impact for PET range verification of these new cross sections, in some cases measured for the first time, they have been used to simulate the β + production and activity profiles (as a function of time) of each isotope in tissue-equivalent phantoms and compared with the ones calculated with the current evaluations. The results illustrate the importance of new data and the need of revised evaluations for a reliable implementation of PET range verification. This is specially relevant for some of the reactions producing long-lived isotopes, but it is of upmost importance for reactions producing the short-lived isotopes needed for online verification, as these are the first cross section data ever. Overall, the new cross sections data base is expected to have an impact on the eventual implementation of both offline and online PET range verification aiming at adaptive proton therapy treatments.
Study and characterisation of semiconductor radiation detectors using the IBIC technique
(2022-12-19) García Osuna, Adrián; García López, Francisco Javier; Jiménez Ramos, María del Carmen; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Universidad de Sevilla. Departamento de Física Aplicada II
The work carried out in this thesis focuses on the study of radiation detectors based on semiconductor materials by applying the technique known as Ion Beam Induced Current (or IBIC technique). Radiation and particle detectors in general are essential in practically any experiment or application in nuclear physics (high energy accelerators, nuclear reactors, medical applications, etc.) and, as experiments and applications grow in complexity and requirements, detectors must also constantly improve in performance and features to be able to meet their needs. It is therefore necessary not only to improve detector manufacturing technology, but also to improve the technology related to the various methods for the study and characterisation of new detectors (IBIC, EBIC, TCT, TPA, etc.). The technique used in this thesis, i.e., the IBIC technique, is a powerful and versatile tool that uses focused ion beams for the characterisation of semiconductor radiation detectors. In this thesis, the IBIC technique has been applied in the context of three different research projects, each with different motivations and objectives and, therefore, each detector has a completely different design to offer specific features. However, thanks to the IBIC technique, it is possible to study and characterise all of them and obtain information that would not be possible with other techniques, getting to understand and deepen the physical mechanisms underlying their operation. On the one hand, the IBIC technique was applied to a commercial silicon detector in order to study the formation and evolution of defects caused by radiation in this material. This work is part of an IAEA Coordinated Research Project and the results will provide experimental data to extend current defect dynamics simulation models beyond existing time limits. On the other hand, the IBIC technique was used to study and characterise innovative silicon carbide (SiC) detectors manufactured at the Instituto de Microelectrónica de Barcelona, which are designed to operate under extreme temperature and radiation conditions in general and in future fusion reactors in particular. These results will be used to study the optimal operating point and its application limits, validating its use for the required application. In addition, this technique allows further study of various physical phenomena occurring within the detector, such as the reduction of the charge carriers mean lifetime, or the reduction of the mean energy required to generate an electron-hole pair in SiC as a function of temperature. Finally, the IBIC technique and also the Time-Resolved IBIC (TRIBIC) technique were used to study the phenomenon of charge density-induced gain suppression in Low Gain Avalanche Detectors (LGADs) fabricated at the Instituto de Microelectrónica de Barcelona. These detectors are aimed to detect minimum ionising particles in the future High-Luminosity Large Hadron Collider (HL-LHC) at CERN. Our results will help to better understand the phenomenon of charge multiplication and the role of the ionization charge density in the gain value. This information is of vital importance in order to correctly interpret the data provided by future HL-LHC experiments.
Characterization of main ion properties for the optimization of future fusion power plants
(2022-12-19) Cano Megías, Pilar; Chacartegui, Ricardo; Viezzer, Eleonora; Universidad de Sevilla. Departamento de Ingeniería Energética; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
In the search for a clean and sustainable energy source for our society, fusion energy emerges as a promising candidate. The realization of a fusion power plant on Earth faces important technological and physical challenges. This thesis is a multidisciplinary project that addresses the optimization of future fusion devices from a plasma physics and engineering perspective. From the plasma physics perspective, the performance of future fusion reactors depends on the properties of the plasma, the fusion fuel. The main ion properties (in present experimental devices, deuterium) are particularly important as they determine the fusion power, which sets the electricity production. Traditionally, the main ions have been rarely diagnosed and their properties have typically been inferred from minority impurity measurements and theoretical models. In this thesis, a novel diagnostic method has been established that enables the direct experimental measurement of the main ions with a focus on the plasma edge. The plasma edge is a critical region, as it prescribes the boundary conditions for the plasma core performance, while it must enable a heat exhaust solution that limits the power loads to the plasma facing components, keeping the integrity of the fusion reactor. Plasma edge diagnostics are demanding in terms of spatial and temporal resolution, as they need to resolve fast transient events and strong spatial gradients. From an engineering perspective, the efficiency of the power conversion cycle coupled to a fusion reactor has been studied and optimized to maximize the electric power output. Cogeneration schemes as opportunities for boosting the efficiency of future fusion power plants have also been investigated. In the framework of this thesis, a new edge main ion diagnostic based on the Charge Exchange Recombination Spectroscopy technique has been installed and exploited at the ASDEX Upgrade experimental reactor, a full metal wall device, to provide main ion temperature and toroidal rotation velocity measurements. A new in-vessel optical head has been installed, which covers the outermost plasma region with a resolution down to 3 mm. A forward model, based on the collisional radiative model implemented in the fidasim code, and data analysis tools have been developed to enable an accurate interpretation of the main ion data. These are state-of-the-art measurements of edge deuterium temperature and toroidal rotation profiles in a tungsten environment, which resembles conditions relevant for future fusion reactors. Several experiments have been carried out at the ASDEX Upgrade tokamak to characterize the main ion temperature and toroidal rotation in a variety of plasma conditions. The role of plasma collisionality and heating scheme on the main ion temperature and toroidal rotation has been addressed. The main ion properties have been compared to minority impurity ion and electron measurements and serve as a testbed for theoretical transport models. In particular, the measurements are compared against neoclassical transport theory. The main ion properties in high and low confinement regimes, such as the high confinement mode (H-mode), low confinement mode (L-mode), improved energy confinement mode (I-mode) and quiescent high confinement mode (QH-mode), have been documented. It has been found that the impurity ion temperature does not always give a good description of the main ion properties, and the thermal equilibration between main and impurity ions is a complex function of heating scheme and collisionality. In H-mode, the main ion toroidal rotation is in remarkably good agreement with neoclassical theory in the steep gradient region of the plasma edge. The detailed diagnosis of the edge plasma properties is essential for understanding plasmas in present experimental devices, and consequently, for the projection towards future fusion power plants and their optimization. The integration of a portfolio of Rankine and Brayton power conversion cycles with a fusion reactor has been studied in a framework that couples the engineering equation solver and the process systems code. The fusion reactor is based on the European DEMO Baseline 2018, which sets the temperature and power boundary conditions. In the intermediate temperature range envisaged for the EU-DEMO Baseline 2018, supercritical carbon dioxide power conversion cycles constitute a very attractive technology. Nuclear fusion cogeneration of heat and electricity has been put forward as a strategy for boosting the efficiency of future fusion devices. The use of district heating networks for the recovery of low-grade heat yields efficiency improvements for all power cycle layouts. The economic viability has been studied by the definition of the levelized cost of hybrid production, which is an indicator that integrates cost estimates from process and production and distribution costs. The cogeneration scheme is feasible from an economic point of view for Rankine and supercritical carbon dioxide power cycles. This work expands potential fusion energy applications and its deployment in the energy market.
Reaction dynamics in clustered nuclear systems
(2022-05-26) Perrotta, Salvatore Simone; Lay Valera, José Antonio; Colonna, M.; Greco, Vincenzo; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
This work is concerned with the theoretical study of nuclear reactions between light charged ions at incident energies around and below the reactants Coulomb barrier, with a focus on the energy range of astrophysical interest for Big Bang and quiescent stellar processes. The main goal of the analysis is to investigate the sensitivity of nuclear reaction dynamics, and more specifically the cross-section predictions, to the description of the structure of each reactant, and in particular to clustering phenomena. The case of the 6Li + p -> 3He + 4He reaction was investigated explicitly, treating the process as a direct deuteron transfer in first- or second-order distorted-wave Born approximation (DWBA) and through a coupled reaction channels (CRC) approach. The penetrability of the Coulomb barrier in the initial state was also analysed phenomenologically from available experimental data, and studied theoretically taking into account the 6Li ground-state deformation. Chapter 1 of this thesis treats some phenomenological aspects of the class of nuclear reactions of interest, both taking place in vacuum (``bare-nuclei'') or immersed in an external medium. The tools developed in this chapter, in spite of (and thanks to) their simplicity, allow to understand the main features of the non-resonant sub-Coulomb reaction cross-section, providing analytical expressions that can be compared to the results of more advanced computations to discuss their qualitative meaning and implications. Section 1.2 includes a quantitative analysis on the electron screening anomalies observed in the 6Li + p -> 3He + 4He reaction. Chapter 2 is a dissertation on some characteristics of the static structure of an isolated nucleus, with a focus on clustered systems. In particular, section 2.2 presents the formalism of overlap functions, which are important for treating direct transfer reactions and encode the required information to represent reactants within a cluster model. Section 2.3 instead is dedicated to the explicit computation of the root-mean-square radius and the electric quadrupole moment of a nucleus described as a bound state of several clusters. Chapter 3 reviews the theory of direct transfer reactions that is later employed in the practical calculations shown in chapter 4. In particular, the discussion is focused on the formalism of first- and second-order DWBA and of CRC approaches. Chapter 4 is concerned with the study of the 6Li + p -> 3He + 4He reaction, treated as the direct transfer of either a structureless deuteron or a generic p+n system, through the whole sub-Coulomb energy range and up to the resonance corresponding to the second 5/2- state of 7Be (namely, at centre-of-mass incident energies between few keV to about 1.5 MeV). The excitation function of the process is explicitly evaluated within a fully quantum framework and without adjusting the calculation parameters on transfer experimental data. Different descriptions of the reactants structure are considered in the evaluation of the cross-section. First- and second-order DWBA calculations are employed to study the role of the system ground state in the initial and final partition, and in particular the model adopted for the transferred particle internal structure, and the corresponding strength of clustered configurations. Virtual excitations in the relative motion between core and transferred systems, which can account for a dynamical deformation of the reactants during the reaction, are investigated in a preliminary calculation using a CRC scheme. The physical ingredients required to perform the calculations are also discussed here. Chapter 5 presents an investigation on the quadrupole deformation of the ground state of 6Li, described within a di-cluster model. Such deformation is required to explain the measured electric quadrupole moment of 6Li, and induces tensor components in the interaction of 6Li with other particles, breaking the angular symmetry of the Coulomb barrier. Previous semi-classical calculations are here improved and expanded, and employed to evaluate and compare the impact of the classical and quantum-mechanical cluster model on the Coulomb-barrier penetrability within a scattering process.
Edge poloidal impurity asymmetry studies using gas puff based charge exchange recombination spectroscopy at the ASDEX Upgrade tokamak
(2022-06-17) Cruz Zabala, Diego José; García López, Francisco Javier; Viezzer, Eleonora; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Nuclear fusion, the energy source of the stars, is expected to be a clean and abundant source of energy on Earth in the near future. However, fusion reactors have to face different challenges due to the high temperatures required to reach ignition. One of the major concerns is the integrity of the first wall of the machine. If enough heat is transported from the fuel, which is in plasma state, to the wall of the reactor, the integrity of the machine could be at stake. The injection of impurities in the plasma has shown to reduce the heat loads in the reactor walls via radiation. Hence, understanding the physics of these impurities and their distribution along the plasma edge is a requirement for future fusion devices. Furthermore, some impurity species have been shown to be beneficial for the plasma confinement and thus, for fusion performance. The work presented in this thesis is based on the Charge eXchange Recombination Spectroscopy (CXRS) technique. This technique exploits the light emitted after the charge transfer between injected neutrals and ionized impurities of the plasma. The emitted light gives information on the temperature, rotation and density of the observed species. Normally, the CXRS measurements are taken at the Low Field Side (LFS). However, this work is focussed on the study of the impurity properties at two different poloidal locations. For this purpose, the High Field Side (HFS) gas puff based CXRS system of the ASDEX Upgrade tokamak has been upgraded. This diagnostic injects thermal neutrals into the plasma to produce the charge exchange reactions. A new piezo driven gas valve provides higher signal to noise ratio and a better characterization of the background light. The upgraded optical heads have 16 lines of sight (LOS) each, covering around 7 cm of the HFS edge region. While impurity temperature and rotation can be obtained directly from the emitted light, the evaluation of the impurity density requires information on the injected neutral density. In order to enable impurity density measurements from gas puff based CXRS diagnostics, a new gas puff module has been included in the FIDASIM code. This module simulates the injection of neutrals by a gas puff system and provides the generated neutral population. Several experiments have been carried out at ASDEX Upgrade to study edge poloidal impurity asymmetries in different scenarios. In H-mode, asymmetries in the toroidal and poloidal impurity rotations have been found. Impurity densities obtained with the new gas puff module show poloidal asymmetries between HFS and LFS. Close to the separatrix, the HFS impurity density exceeds the LFS values. These density asymmetries are in agreement with the poloidal impurity flow structure. The impact of the heating scheme on edge poloidal impurity asymmetries is addressed. The comparison between neutral beam heating and wave heating results in stronger impurity density asymmetries when wave heating is applied. In L-mode, no asymmetries in the toroidal rotation, poloidal rotation, density and temperature of the impurities have been found. In I-mode, asymmetries in the toroidal impurity rotation have been measured. In summary, the existence of impurity density asymmetries is linked to strong poloidal impurity flows at the edge, as it occurs in H-mode plasmas. In general, the edge impurity properties measured at the LFS should not be understood as global parameters. The edge poloidal impurity asymmetries observed in this work should be taken into account in studies where impurities play an important role.
Atmospheric Pressure Plasmas for More Sustainable Chemical Processes and Environmental Applications
(2022-06-01) Navascués Garvín, Paula de; Gómez Ramírez, Ana María; Rodríguez González-Elipe, Agustín; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Nonthermal plasma technology operated at atmospheric pressure is an ideal candidate to induce chemical reactions and overcome the current limitations of conventional catalytic methods carried out at high pressures and temperatures. In nonthermal plasmas, the high energy of the electrons, together with the existence of radicals and excited species, make possible the activation of different chemical processes. Nonthermal plasma reactors are small in size, can be operated in a distributed way, are easy to operate and to scale up, and do not entail significant risks. Furthermore, the induction time is negligible as compared to conventional catalytic techniques, and they can be powered by intermittent renewable energy sources. It could be said that, in terms of the promotion of the green chemical industry, nonthermal plasma technology represents a non-aggressive alternative characterized by low energy costs and reduced carbon dioxide emissions. Nonthermal plasma reactors are straightforward candidates in the search for innovative and more sustainable solutions to replace thermally induced methods. This thesis work has been developed in the Laboratory of Nanotechnology on Surfaces and Plasma of the Materials Science Institute of Seville, which is a join research center between the Spanish National Research Council and the University of Seville. The thesis addresses an experimental study, carried out using an atmospheric-pressure plasma reactor, of various highly impact chemical processes, such as the ammonia synthesis and decomposition reactions, the splitting removal of CO2, as well as its revaluation, using mixtures with methane in this latter case. The nonthermal plasma reactors used in this thesis work are known as packed-bed reactors. Their operating mode is similar to that of Dielectric Barrier Discharges (DBDs), where a self-sustained plasma is generated between two electrodes separated by, at least, one dielectric barrier. The stable ignition of these discharges enables the processing of gases flowing through the free space between the reactor electrodes, thus promoting the realization of specific chemical reactions. In a packed-bed configuration, the space between electrodes is filled with pellets of dielectric materials (note that in classical DBDs the dielectric barrier(s) is/are usually situated onto the electrode(s)) and the plasma is ignited in the voids between the pellets. Therefore, chemical reactions can occur not only at the plasma bulk but also at the barrier material surface, which can contribute to promote new reaction pathways and plasma-catalysis synergies. Accordingly, this research work focuses on the fields known as Plasma Chemistry and Plasma Catalysis. The thesis is structured in three main sections. The first section, encompassing Chapters 1, 2, and 3, summarizes the background frame of the research, as well as a description of the objectives and the materials and methods utilized to accomplish these objectives. The second section contains the discussion of the obtained results, and it is also divided into three parts: Chapter 4, Chapters 5-7, and Chapters 8-9. Firstly, in Chapter 4, the electrical behavior of the packedbed reactor is characterized as a function of the utilized barrier material: classical dielectrics and high dielectric constant ferroelectrics. It is demonstrated the convenience of using the latter and the importance of the material´s Curie temperature. More specifically, PZT (Lead Zirconate Titanate) has been selected as a suitable moderator material providing high reaction yields and energy efficiencies for the ammonia and CO2 reactions studied in following Chapters. Chapters 5, 6, and 7, present a comprehensive study of ammonia reactions. Chapter 5 is focused on the study of the reaction mechanisms for ammonia synthesis and decomposition reactions. By applying a disruptive isotope labeling methodology, commonly used in conventional catalysis but scarcely used in Plasma Catalysis, it has been possible to identify reaction mechanisms and intermediate processes that do not contribute to the ammonia synthesis and, therefore, limit the efficiency of the plasma-assisted process. Furthermore, it has allowed us to experimentally demonstrate the contribution of the surface of the moderator material (PZT) to the overall process. In Chapters 6 and 7, addressing respectively the ammonia synthesis and decomposition reactions, it has been studied the incorporation of a ruthenium-based catalyst into the PZT barrier to analyze how it affects the reaction yield and energy efficiency. A systematic study concludes that the use of a metal catalyst is not an efficient strategy for optimizing the ammonia synthesis reaction, but can increase, to some extent, the reaction yield for hydrogen production through the decomposition of ammonia. Chapters 8 and 9 present the study of CO2 reactions. On the one hand, Chapter 8 deals with the conversion of CO2 into CO and O2 in the packed-bed plasma reactor moderated with PZT, achieving conversion rates and energy efficiencies higher than those obtained with other more commonly used 7 ferroelectric materials. To analyze the CO2 splitting process under more realistic conditions, mixtures of CO2 with O2 and with dry air have been studied, obtaining high efficiencies and without formation of harmful products. On the other hand, CO2 and CH4 mixtures have been studied in Chapter 9, focusing on the valorization of these two greenhouse gasses. The ignition of these plasmas gives rise to a wide range of possible products, particularly in the form of multiple types of hydrocarbons. The application of the isotope labeling methodology has allowed us to conclude that CO2 and CH4 roughly follow independent reaction pathways and that they scarcely interact in the plasma. Finally, the last section of the thesis work contains the general conclusions (Chapter 10) and a summary of the scientific production in relation to the work of the Ph.D candidate.
Avances en espectrometría alfa y gamma para su aplicación en la evaluación del impacto radiactivo de una industria NORM
(2013-09-29) Mantero Cabrera, Juan; García-Tenorio García-Balmaseda, Rafael; Hurtado Bermúdez, Santiago José; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear; Universidad de Sevilla. Departamento de Física Aplicada II
Es bien conocido que la peligrosidad de las radiaciones ionizantes ha hecho necesario el establecimiento de medidas que garanticen la protección tanto de los trabajadores expuestos como del público en general contra los riesgos resultantes de la exposición a las mismas. En esta dirección se han implementado distintas normativas a nivel mundial, y más concretamente en el ámbito europeo la normativa actualmente vigente es la Directiva 96/29/EURATOM, de la que emana la normativa española actual en forma de Reglamento sobre la protección sanitaria contra radiaciones ionizantes (Real Decreto 783/2001). Dicho Reglamento es de aplicación a todas las prácticas que den lugar a un riesgo derivado de las mismas, tanto si la procedencia de las radiaciones es de origen artificial como natural. Desde hace más de veinte años existe una clara conciencia en la comunidad científica sobre la necesidad de evaluar el impacto radiológico ocupacional y ambiental producido por las actividades realizadas en industrias convencionales no nucleares que se caracterizan bien por utilizar en sus procesos de producción materias primas enriquecidas en radionucleidos naturales o bien por producir productos comerciales, sub-productos o residuos enriquecidos en estos radionucleidos (industrias conocidas como industrias NORM, acrónimo de Naturally Occurring Radioactive Material). La regulación de este tipo de actividades se recoge en el reglamento vigente sobre Protección Sanitaria contra las radiaciones ionizantes de un título específico (titulo VII) Pues bien, dentro de los estudios que actualmente realiza el grupo de investigación Física Nuclear aplicada de la Universidad de Sevilla, centrados en la evaluación y control desde el punto de vista radiactivo y radiológico de las actividades realizadas por diversas industrias no nucleares ubicadas en el Sur de España ( proyecto I+D financiado por el Consejo de Seguridad Nuclear) se enmarca la investigación desarrollada en esta tesis. En particular, mostraremos el estudio radiológico que se ha realizado del proceso completo de producción que se aplica en una industria situada en el Polo Químico de Huelva y cuya materia prima está catalogada como material NORM. Más concretamente la práctica industrial se basa en la fabricación de pigmentos de dióxido de titanio (TiO2) a partir de un mineral conocido como ilmenita, que además obviamente de estar enriquecido en Ti, tiene la particularidad de presentar concentraciones elevadas de los radionucleidos pertenecientes a las series naturales del Uranio y el Torio Nuestro objetivo en este trabajo ha consistido en determinar y evaluar las concentraciones de actividades de diversos radionucleidos naturales en muestras representativas de las diversas etapas que conforman el proceso de producción de dicho pigmento, analizando a partir de los resultados obtenidos el comportamiento de estos radionucleidos y las rutas preferentes que éstos siguen dentro del proceso de producción. La información obtenida en este estudio radiométrico, constituye una base esencial e imprescindible para poder posteriormente realizar de forma rigurosa una evaluación del impacto radiológico producido por la actividad industrial analizada. El estudio radiométrico realizado dista mucho de poder ser considerado como un estudio rutinario o trivial. Dada la enorme heterogeneidad (tanto en composición como en estado físico) de los distintos tipos de muestras que se han analizado, ha sido imprescindible optimizar distintos procedimientos ya desarrollados en el seno de nuestro grupo de investigación, e incluso desarrollar nuevos métodos que nos permitan caracterizar (desde el punto de vista radiactivo) perfectamente, y con garantías, las distintas muestras estudiadas a lo largo del proceso industrial. Nuestro estudio se basará en la determinación en las muestras analizadas de las concentraciones de actividad de los isótopos de U, Th y Ra que forman parte de las series naturales del 238U y del 232Th. Para ello, haremos uso de dos técnicas radiométricas como son la espectrometría gamma y la espectrometría alfa. Adicionalmente también utilizaremos una técnica complementaria en momentos puntuales del desarrollo de este trabajo como es la fluorescencia de Rayos-X o TTPIXE con objeto de caracterizar la composición elemental de algunas de las muestras del proceso industrial, al ser imprescindible el conocimiento de esa composición para una apropiada aplicación de la técnica basada en la espectrometría gamma. Atendiendo a los objetivos establecidos, y considerando las técnicas experimentales aplicadas, hemos decidido dividir esta memoria en tres bloques de contenido bien diferenciado. Un primer bloque, que comprende a los capítulos 2 y 3, está dedicado a la espectrometría gamma donde se lleva a cabo una descripción de la técnica, sistemas experimentales y mejoras implementadas en nuestro grupo de investigación. Un segundo bloque, capítulos 4 y 5, se centra en la espectrometría alfa con una estructura de contenidos similar al bloque inicial, en el que se describen en detalle los distintos procesos de optimización llevados a cabo en el ámbito de la radioquímica de la que se ha hecho uso en esta tesis. El tercer y último bloque, que abarca los capítulos 6 y 7, se centra en la descripción del proceso de producción industrial bajo estudio así como en la descripción y análisis de los resultados obtenidos de aplicar tanto la espectrometría alfa y gamma en muestras procedentes de este contexto industrial. Como capítulo final, el capítulo 8, resumirá y recopilará las principales conclusiones obtenidas en nuestro estudio. De una forma algo más detallada, podemos indicar que el capítulo segundo presenta los conceptos teóricos asociados a la espectrometría gamma así como los distintos procesos de calibración que hay que aplicar a un detector de Ge Hiperpuro coaxial como los usados en esta tesis. Se comienza con la interacción gamma-materia, se describen las componentes que forman un espectro gamma y las calibraciones en energía, resolución y eficiencia de fotopico. Relacionada con esta última, se presenta la metodología seguida en procedimientos experimentales y semi-experimentales de calibración en eficiencia así como correcciones en ésta por suma en coincidencias mediante una doble vía: experiemental y teórica mediante un programa basado en la transferencia de eficiencias denominado EFFTRAN. También se describe la sistemática seguida, tanto experimental como semi-teórica, para obtener el factor de autoabsorción relativo en muestras reales partiendo de patrones preparados con diversos cocteles multi-gamma. En el capítulo 3, se ponen en práctica todos los conceptos y metodologías descritos en el capítulo previo que mejoran notablemente los resultados finales a la hora de calcular la concentración de actividad en muestras medioambientales. En una primera parte de este capítulo, se caracterizarán dos detectores coaxiales, un XtRa y un REGe llevando a cabo los procesos de calibración en energía y resolución. La parte de electrónica en este tipo de detectores se ha emplazado en el apéndice 1 para reducir el contenido teórico de este capítulo. Tras esto, se hace un estudio comparativo del fondo que presentan ambos sistemas concluyendo que el XtRa es un sistema de mucho menor fondo y más adecuado para la medida de la gran mayoría de muestras de esta tesis. A continuación se obtienen las curvas de calibración experimental en eficiencia para una batería de patrones en distintas geometrías cilíndricas: cajas petri y duquesas a distintas alturas, en distintas matrices: acuosas y sólidas inorgánicas, tratando de cubrir un amplio espectro de posibilidades en cuanto a geometrías de medida. Indicar que, es genérico en toda la tesis, el uso de incertidumbres tanto en tablas como en gráficos asociadas a valores con criterio 1-¿ salvo que, excepcionalmente se especifique lo contrario. El siguiente paso consiste en corregir por el efecto de suma en coincidencia de cascada aquellos valores de emisores gamma multienergéticos que sufren este efecto, re-obteniendo curvas en eficiencia de fotopico más realistas. Otro proceso de corrección implementado consiste en la corrección por el efecto de autoabsorción que se producen en muestras reales en relación a los patrones experimentales. En este sentido varias técnicas diferentes, tanto experimentales como semi-experimentales, son usadas e intercomparadas en la parte final de este capítulo que concluye con una muestra de los ejercicios de intercomparación en los que nuestro laboratorio ha tomado parte y se han ido aplicando las correcciones aquí implementadas y que corroboran la bondad de la metodología seguida en nuestro laboratorio de espectrometría gamma. Aquí se cierra el primer bloque dedicado a la espectrometría gamma en esta memoria. El capítulo 4 inicia la parte de espectrometría alfa, como técnica complementaria de la gamma, repasando las bases teóricas que subyacen en el uso de este tipo de espectrometría. Se describen brevemente los detectores de implantación iónica así como el sistema Alpha Analist del que dispone nuestro laboratorio y los distintos procesos de calibración a que es sometido este sistema. Posteriormente, en se describe en detalle el sistema de espectrometría alfa con sus ocho cámaras de detección equipadas con detectores de silicio de implantación iónica, para continuar describiendo brevemente el tercer sistema de medida usado, la fluorescencia de rayos X dispersiva en longitud de onda (WDXRF). Por último, en este capítulo se ha incluido un apartado sobre el cálculo de los límites de detección de las distintas técnicas que se han descrito, con el objetivo de facilitar de esta forma la elección de una u otra técnica dependiendo del previsible contenido radiactivo de la muestra sujeta a análisis. El tercer capítulo describe los métodos experimentales desarrollados para el análisis cuantitativo de las muestras en este trabajo con los sistemas descritos en el capítulo anterior. Así, y en el apartado de análisis mediante espectrometría gamma se describen los distintos procesos de calibración aplicados a los sistemas experimentales utilizados, poniendo especial énfasis en su calibración en eficiencias. Estas calibraciones en eficiencia han tenido en consideración obviamente la influencia de la geometría de medida, y, con gran detalle, la influencia de la autoabsorción que se produce en las propias muestras durante las medidas y que afectan de distinto grado a cada una de ellas. Para conseguirlo ha sido necesario recurrir a la preparación de una serie limitada de muestras patrón, y al desarrollo de un procedimiento para corregir por los efectos de autoabsorción que nos permitiera generalizar las medidas en nuestros sistemas gamma a otras matrices distintas a los patrones sin la necesidad de invertir tiempo, esfuerzo y material en la preparación de un patrón por cada muestra analizada. El procedimiento completo de calibración en eficiencias, y consecuentemente el procedimiento completo de determinación de concentraciones de actividad por espectrometría gamma ha sido validado mediante la participación en diferentes ejercicios de intercomparación, y la medida de muestras certificadas de la IAEA (Agencia Internacional de la Energía Atómica). El capítulo tercero finaliza, en relación a la espectrometría alfa, desarrollando los distintos procesos llevados a cabo para calibración del sistema de medida, así como describiendo los procedimientos radioquímicos utilizados en al aislamiento de los radionucleidos emisores alfa de interés para su correcta medida. El cuarto capítulo nos introduce en la descripción como industria NORM del proceso industrial de obtención de pigmentos de TiO2, particularizando obviamente éste al aplicado en la factoría situada en el Polo Químico de Huelva (¿Tioxide Europa S.L.¿). En este capítulo se muestra desde el uso e importancia de la fabricación de estos pigmentos y su presencia en nuestra vida cotidiana, hasta la descripción paso a paso de las transformaciones que experimenta la materia primera a lo largo del proceso industrial para la obtención del producto comercial deseado. La descripción detallada del proceso de producción sirve adicionalmente para indicar y remarcar en este capítulo los diversos puntos a lo largo del proceso en los que se han tomado muestras que serán caracterizadas radiométricamente con posterioridad. Finalmente, en el quinto capítulo se procede a la exposición, discusión y análisis de resultados obtenidos en los distintos muestreos realizados en la fábrica. En particular se discute y razona el comportamiento que presentan los radionucleidos naturales de interés (isótopos de U, isótopos de Th, isótopos de Ra y 40K) en las distintas etapas del proceso industrial, poniendo especial interés en la caracterización radiactiva de los materiales de entrada/salida del proceso, y se delimitan las rutas preferentes seguidas por los radionucleidos analizados a lo largo del proceso de producción. El interés particular en la caracterización de los materiales de salida se enmarca en el hecho de que para los residuos producidos a lo largo del proceso existen actualmente varios proyectos en vías de desarrollo por la propia industria, cuya finalidad es la de darle salida pretendiendo causar el menor impacto radiológico posible. Por último, y asociado a un muestreo centrado en la colección de un número limitado de muestras representativas del proceso, se ha analizado también la uniformidad temporal desde el punto de vista radiactivo del proceso industrial, en el sentido de comprobar que la distribución de los radionucleidos de interés por las distintas rutas que éstos pueden seguir dentro del proceso, se mantiene constante a lo largo del tiempo.
Fast-Ion Transport and Acceleration Induced by Edge Localized Modes in MAST Upgrade and ASDEX Upgrade
(2021-07-16) Rivero Rodríguez, Juan Francisco; García Vallejo, Daniel; García Muñoz, Manuel; Universidad de Sevilla. Departamento de Ingeniería Mecánica y de Fabricación; Universidad de Sevilla. Departamento Física Atómica, Molecular y Nuclear
La fusión nuclear se esboza como una fuente de energía sostenible, de bajas emisiones y que aportará potencia de base al mix eléctrico. Los tokamaks son los dispositivos más avanzados para la obtención de energía mediante fusión nuclear y operan en condiciones cercanas a las de las futuras centrales nucleares. Hoy en día, los tokamaks existentes tienen como objetivo resolver los desafíos científicos y técnicos que plantean las futuras plantas de fusión nuclear. En los tokamaks, los iones rápidos, -- aquellos iones cuya energía es superior a la del resto del volumen de plasma --, se emplean para aumentar la temperatura del plasma hasta energías donde la fusión se hace patente. Los iones rápidos pueden perder el confinamiento debido a interacciones con perturbaciones electromagnéticas de diversa naturaleza, siendo esto un riesgo para el rendimiento del plasma y para la integridad del reactor. En el modo de operación de alto confinamiento, conocido por H-mode, aparecen de forma repetitiva inestabilidades magnetohidrodinámicas (MHD) explosivas localizadas en el borde, conocidas como Edge Localized Modes (ELMs). Los ELMs liberan una gran cantidad de energía y momento hacia las paredes del plasma, cuyo efecto se estima intolerable en futuros tokamaks. Además, investigaciones recientes en el tokamak ASDEX Upgrade han observado un aumento en las pérdidas de iones rápidos a energías por encima de su energía de inyección, que aparecen relacionadas con la actividad de los ELMs. Estas observaciones sugieren que existe una interacción entre los ELMs y los iones rápidos, que resulta en la aceleración y pérdida de estos últimos. Este trabajo tiene como objetivo de estudio las pérdidas de iones rápidos inducidos por ELMs. Para conseguir este objetivo, la tesis ha abarcado el desarrollo de herramientas numéricas y el diseño de diagnósticos experimentales. En el aspecto numérico, se han utilizado códigos de seguimiento de órbitas como ASCOT5 y se han implementado perturbaciones electromagnéticas 3D que evolucionan en el tiempo. Esto ha permitido investigar los principales mecanismos de transporte y aceleración planteados para esclarecer la pérdida de iones rápidos inducida por ELMs. En el ámbito experimental, durante esta tesis se ha diseñado el primer detector de pérdidas de iones rápidos (FILD) en el tokamak esférico MAST-U. El diagnóstico está montado sobre un mecanismo de rotación y traslación que permite adaptar la sonda a diferentes orientaciones [0o, 90o] y posiciones radiales [1.40 m, 1.60 m]. Los primeros datos experimentales del FILD de MAST-U se esperaban obtener a lo largo de esta tesis. Sin embargo, distintos retrasos en el comienzo de la campaña experimental de MAST-U han impedido poner el diagnóstico en funcionamiento. Por este motivo, los planes experimentales y numéricos han tenido que seguir distintos planteamientos. Los experimentos, que se han realizado en ASDEX Upgrade, han tenido como objetivo expandir el alcance de las observaciones de pérdida de iones rápidos inducidas por ELMs, llevando a cabo nuevos rastreos para detectar los parámetros que más afectan en la interacción entre los iones rápidos y los ELMs. Con respecto al modelado, se busca realizar un modelado que permita reproducir las principales observaciones experimentales en ASDEX Upgrade y revele los mecanismos básicos de transporte y aceleración de iones rápidos durante los ELMs. Además, el modelado en MAST-U ha permitido preveer la señal de FILD, que se ha podido comparar con la señal en ASDEX Upgrade.
Development of an external beam line for radiobiology experiments and microdosimetry applications at the 18 MeV proton cyclotron facility at CNA
(2020-11-25) Baratto Roldán, Anna; Cortés Giraldo, Miguel Antonio; Jiménez Ramos, María del Carmen; Espino Navas, José Manuel; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Cancer is one of the leading causes of mortality world-wide, killing more than one million people per year just in Europe. Nowadays, proton therapy is one of the most promising techniques in the fight against cancer, being two the main bases of its success: (1) the physical advantages of protons with respect to conventional radiotherapy with photons, resulting in a more selective energy deposition in depth; (2) the increased biological effectiveness of protons with respect to photons and their denser pattern of energy deposition in matter, usually determining a more lethal damage to the DNA. The biological effect of protons and other ions with respect to photons is described in terms of the Relative Biological Effectiveness (RBE), i.e., the ratio between the doses of the reference and studied radiation determining the same effect. In clinical proton therapy, a RBE value of 1.1 is currently used. However, there is an increasing awareness that proton RBE is not a constant, but seems to increase linearly with the Linear Energy Transfer (LD) of the proton as it slows down in tissues, especially close to the distal region of the Bragg peak, possibly leading to toxicity in healthy tissue beyond the target. In this context, recent studies aim at including dose-averaged LET objective functions in treatment planning optimization to take full advantage of the increased RBE in protons beams. This last problem, and the characterisation of RBE, can be addressed with the formalism of microdosimetry, which, on one hand, permits the calculation of RBE from a microscopic approach by means of the microdosimetric kinetic model (MKM) and, on the other hand, provides physical concepts and computational tools to calculate macroscopic LD distributions. The rationale behind this thesis project is, therefore, given by the necessity of performing studies of proton RBE at low energies, close to the Bragg peak region of clinical proton beams (below 40MeV), which would help reaching a consensus on the variation of proton RBE with LET. To do so, two main objectives were foreseen: (1) the design and mounting of a low energy proton facility at CNA (proton kinetic energy below 18MeV) for the experimental study of RBE in mono-layer cell cultures and (2) the development of a simulation tool to study the patterns of energy deposition of protons in water at a micrometric scale, for the computation of microdosimetric quantities. This thesis is divided in four chapters. In Chapter 1, the physics foundations of proton therapy are presented, followed by a description of the relevant biological parameters. In this context, special attention is given to the formalisms of microdosimetry and its most relevant quantities. Then, an insight into Monte Carlo simulations and the main codes used in this work is presented, together with a description of the radiation dosimeters employed for the experimental measurements performed. Chapter 2 is dedicated to the description of the radiobiology beam line designed and mounted at the 18MeV proton cyclotron facility installed at the National Centre of Accelerators (CNA, Seville, Spain), focusing especially on the overall optimization of the beam parameters to define the best setup for the irradiation of mono-layer cell cultures. In this chapter, a Monte Carlo simulation of the beam line, realised with Geant4 and validated towards experimental measurements, is also presented. In Chapter 3 a Monte Carlo track structure application, which was developed for the computation of microdosimetric distributions of protons in liquid water, is described. This application, based on Geant4-DNA, provides two sampling methods, uniform and weighted, for the scoring of the quantities of interest in spherical sites. Furthermore, it is used to verify the validity range of a formula that links microdosimetric quantities to the macroscopic dose-averaged LET distribution, being a powerful tool for the development of analytical models to be used in treatment planning optimisation. Chapter 4 presents the results of the first irradiation of cell cultures at the radiobiology beam line developed at the cyclotron facility. In this context, an application of the Monte Carlo code for the computation of microdosimetric quantities is shown. With this code, a theoretical derivation of the expected RBE for the experimental irradiation and cells under study could be done, through the use of the microdosimetric kinetic model. Finally, a summary of the results obtained and a brief discussion on the future perspectives of this project conclude this work.
Microdosimetry applied to proton radiotherapy
(2020-09-21) Bertolet Reina, Alejandro; Cortés Giraldo, Miguel Antonio; Carabe Fernández, Alejandro; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
El uso de la radioterapia con protones como tratamiento contra el cáncer es cada vez más común. Sus actuales indicaciones clínicas se basan en las propiedades físicas de los haces de protones, aceptando una efectividad biológica relativa (RBE por sus siglas en inglés) constante e igual a 1.1 con respecto a la radioterapia con rayos X. Sin embargo, hay cada vez más evidencia de una RBE variable y dependiente del lineal energy transfer (LET) del haz de protones, que, a su vez, depende de la posición relativa a lo largo de la traza del haz. Además, la RBE depende del efecto considerado y de las propiedades biológicas de las células o tejidos considerados, así como del esquema de fraccionamiento de dosis. La microdosimetría es la teoría que estudia los patrones de deposición de energía por haces de radiación en volúmenes de tamaño microscópico. Determinando la concentración local de energía impartida en estas dimensiones, es posible calcular el LET y otras magnitudes que caracterizan la calidad del haz. El estudio de las distribuciones de energía impartidas a esta escala puede llevar a una mejor comprensión de la RBE de los haces de protones y a una potencial aplicación clínica. Esta tesis es una colección de siete estudios en esta dirección. Como un todo, pretende establecer la conexión entre la descripción elemental de las interacciones entre protones y materia en términos de microdosimetría y su aplicación clínica. En este sentido, el artículo I estudia las magnitudes microdosimétricas básicas y sus dependencias, y se centra en cómo producir resultados microdosimétricos correctos usando simulaciones Monte Carlo (MC). El artículo II emplea esta metodología para generar distribuciones monoenergéticas de protones de energía cinética de hasta 100 MeV. Estas distribuciones se emplean para generar modelos analíticos de los cuales las magnitudes microdosimétricas de haces de protones polienergéticos pueden ser derivadas en lugar de emplear simulaciones MC para cada caso individual. En el artículo III, los resultados de estos modelos analíticos para haces monoenergéticos se comparan con medidas tomadas con microdosímetros de silicio expuestos a haces de protones monoenergéticos. Para calcular distribuciones espaciales de magnitudes microdosimétricas con el formalismo del artículo II, se necesita determinar el espectro del haz, o más precisamente la fluencia espectral, en cada punto. El artículo IV proporciona un formalismo independiente para calcular fluencias espectrales en haces de protones como función de la profundidad y la posición lateral respecto al eje del haz en agua líquida. Este formalismo, junto a los modelos microdosimétricos del artículo II, permite la determinación tridimensional de magnitudes microdosimétricas. El artículo V ilustra la combinación de los formalismos de los artículos II y IV para producir cálculos de distribuciones de dosis en un sistema de planificación de tratamientos. Estos resultados se comparan con simulaciones MC independientes y cálculos analíticos clínicamente validados, mostrando resultados consistentes. Así, se proporciona una validación parcial de los modelos microdosimétricos presentados en el artículo II. A partir de estos dos formalismos también es posible obtener cálculos microdosimétricos de la dose-mean lineal energy (yD) y el doseaveraged restricted LET, lo que se muestra en el artículo VI. Además, las relaciones de la teoría de microdosimetría entre LET y lineal energy se reevalúan, y se propone y testea una nueva ecuación para calcular restricted LET. Finalmente, en el artículo VII, se usa el Microdosimetric Kinetic Model (MKM) para calcular distribuciones de RBE basadas en las magnitudes físicas obtenidas en los artículos previos. Se evalúa igualmente un nuevo método para la determinación del tamaño del dominio relevante para diferentes líneas celulares de acuerdo con el MKM. Aunque el desarrollo de esta tesis se refiere específicamente a haces terapéuticos de protones, cálculos similares para otras partículas son posibles siguiendo procedimientos y formalismos análogos, lo cual representa una de las líneas de investigación más interesantes a partir de este trabajo.
Algoritmos de simulación para equilibrio de fases en fluidos
(1992-07) Vega Fernández, María Lourdes; Rull Fernández, Luis Felipe; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Análisis, construcción y verificación experimental de un sistema de radiofrecuencia para la detección no destructiva de fluctuaciones de fase en haces iónicos pulsados emergentes de un acelerador electrostático
(1999-05) Barbadillo Rank, Manuel; Arias Carrasco, José Miguel; Quesada Molina, José Manuel; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Esta tesis doctoral abarca el análisis previo, los fundamentos de proyecto, los procedimientos para la construcción y la verificación de funcionamiento de un sistema basado en tecnologías de radiofrecuencia, cuya aplicación principal (no la ´7unica) es la detección de las fluctuaciones de fase en los haces pulsados de iones, provocadas por las oscilaciones en la tensión del terminal en un acelerador electrostático. El sistema esta compuesto principalmente por tres dispositivos: una cavidad resonante en la banda de radiofrecuencias, un modulo electrónico para el procesamiento de las señales de radiofrecuencia y un mezclador de señales. La cavidad (con resonancia de banda estrecha) actúa a modo de antena/sensor del haz generando una señal FM cuya fase evoluciona en el tiempo de igual forma que las oscilaciones en la tensión del terminal. El módulo electrónico amplifica y estabiliza la señal (atenuando el efecto de ruidos) independizándola de la intensidad del haz. Finalmente, siguiendo el procedimiento heterodino, el mezclador de señales extrae de forma sincrónica la información de los desfases.
Fluctuaciones críticas en medios granulares
(2006) García de Soria Lucena, María Isabel; Brey Abalo, José Javier; Ruiz Montero, María José; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Simulación por ordenador de las transiciones de fase en un modelo de cristal líquido
(1991-01) Miguel Agustino, Enrique de; Rull Fernández, Luis Felipe; Universidad de Sevilla. Departamento de Física Atómica, Molecular y Nuclear
Se presentan resultados de simulación por ordenador de fluidos gay-berne con parámetros de anisotropía k=3 y k'=5. Usando dinámica molecular en el colectivo nvt, se han identificado las fases fluidas isótropa, neumática y esméctica-b. Se observa que la fase neumática es estable para t* 0.80. A temperaturas inferiores, la fase isótropa evoluciona directamente a la fase esméctica-b vía una transición de primer orden. Se dan evidencias de la existencia de una transición que implica una inclinación de las orientaciones moleculares respecto a las capas esmécticas. Se estudia el efecto de las fuerzas atractivas en la estabilización de fases ordenadas. Junto con resultados de la transición isótropo-neumático por integración termodinámica y de la transición liquido-vapor por el método de Gibbs los resultados anteriores proporcionan una visión bastante completa del diagrama de fases. Por último, se analiza la dinámica de translación y rotación con funciones de autocorrelación.

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