10 research outputs found
DTT - Divertor Tokamak Test facility - Interim Design Report
The âDivertor Tokamak Test facility, DTTâ is a milestone along the international program aimed at demonstrating â in the second half of this century â the feasibility of obtaining to commercial electricity from controlled thermonuclear fusion. DTT is a Tokamak conceived and designed in Italy with a broad international vision. The construction will be carried out in the ENEA Frascati site, mainly supported by national funds, complemented by EUROfusion and European incentive schemes for innovative investments. The project team includes more than 180 high-standard researchers from ENEA, CREATE, CNR, INFN, RFX and various universities.
The volume, entitled DTT Interim Design Report (âGreen Bookâ from the colour of the cover), briefly describes the status of the project, the planning of the design future activities and its organizational structure. The publication of the Green Book also provides an occasion for thorough discussions in the fusion community and a broad international collaboration on the DTT challenge
Modelling for JET Vertical Stabilization System
Nuclear fusion is, in a sense, the opposite of nuclear fission. Fission, which is a mature technology, produces energy through the splitting of heavy atoms like uranium in controlled chain reactions. Unfortunately, the by-products of fission are highly radioactive and long lasting. On the other hand, fusion is the process by which the nuclei of two light atoms such as hydrogen are fused together to form a heavier (helium) nucleus, with energy produced as a by-product.
Although controlled fusion is extremely technologically challenging, a fusion-power reactor would offer significant advantages over existing energy sources.
This thesis is devoted to the control of tokamaks, magnetic confinement devices constructed in the shape of a torus (or doughnut). Tokamaks are the most promising of several proposed magnetic confinement devices.
The need to improve the performance of modern tokamak operations has led to a further development of the plasma shape and position control systems. In particular, extremely elongated plasmas, with high vertical-instability growth rate, are envisaged to reach the required performance for ignition. This request for better performance from the experimentalistsâ side has motivated the development of the new vertical-stabilization (VS) system at the JET tokamak, which has been proposed within the Plasma Control Upgrade project.
This thesis presents the activity carried out to increase the capability of the VS system and to understand the operational limits in order to assess what can be done to improve the overall performance with the existing hardware and control system so as to minimize the impact on JET operation.
The first objective of this work is the analysis of the new diagnostic system and the influence of the mechanical structure on the magnetic measurements used as diagnostics by the VS controller; the main focus is on the influence on the controller performance in the presence of large perturbations. The second objective is to design a new controlled variable to increase the performance of the VS system. The third objective is to provide an equivalent model of an ELM (Edge Localized Mode), in terms of internal plasma profile parameters via best fit of the vertical velocity estimation. The last objective is to obtain a reliable and accurate model of the overall system, based on the new platform MARTe, developed at JET and useful also for
other devices
Design and Validation of a 3D Magnetic Nozzle for Thrust Steering
Electric space propulsion is one of the main fields of study in space engineering
nowadays. It is considered to be the future of space propulsion, already being
implemented in one third of existent satellites. However, operating such motors
requires expensive, heavy and complex steering platforms in order to orient the
direction of the plasma, and thus the direction of the thrust vector.
The project presented aims to explain the development of a prototype of a 3-
dimensional magnetic nozzle, which has been recently patented by the research group
of EP2 (Equipo de PropulsiÂŽon Espacial y Plasmas). This magnetic nozzle will allow
to steer the thrust vector freely and without moving parts, and involves only slight
modifications in thrusters which already have a magnetic nozzle.
In order to do so, the first prototype showing this technology presented has been
designed in such a way that it is compatible with the plasma source of the EP2
laboratory. This model will be tested in the vacuum chamber simulating space
conditions. The process of validating such a prototype is also presented.
The work presented shows the complete design process of such an innovative
technology and the different achieved characteristics. This is followed by the development
and verification of the validation process and the proposed experiments
to completely validate the prototype. Since the model could not be manufactured
for validation, different experiment proposals are given to prove the validity of the
thrust-steering device.IngenierĂa Aeroespacia
Apport de nouvelles techniques dans lâĂ©valuation de patients candidats Ă une chirurgie dâĂ©pilepsie : rĂ©sonance magnĂ©tique Ă haut champ, spectroscopie proche infrarouge et magnĂ©toencĂ©phalographie
L'Ă©pilepsie constitue le dĂ©sordre neurologique le plus frĂ©quent aprĂšs les maladies cĂ©rĂ©brovasculaires. Bien que le contrĂŽle des crises se fasse gĂ©nĂ©ralement au moyen d'anticonvulsivants, environ 30 % des patients y sont rĂ©fractaires. Pour ceux-ci, la chirurgie de l'Ă©pilepsie s'avĂšre une option intĂ©ressante, surtout si lâimagerie par rĂ©sonance magnĂ©tique (IRM) cĂ©rĂ©brale rĂ©vĂšle une lĂ©sion Ă©pileptogĂšne bien dĂ©limitĂ©e. Malheureusement, prĂšs du quart des Ă©pilepsies partielles rĂ©fractaires sont dites « non lĂ©sionnelles ». Chez ces patients avec une IRM nĂ©gative, la dĂ©limitation de la zone Ă©pileptogĂšne doit alors reposer sur la mise en commun des donnĂ©es cliniques, Ă©lectrophysiologiques (EEG de surface ou intracrĂąnien) et fonctionnelles (tomographie Ă Ă©mission monophotonique ou de positrons). La faible rĂ©solution spatiale et/ou temporelle de ces outils de localisation se traduit par un taux de succĂšs chirurgical dĂ©cevant. Dans le cadre de cette thĂšse, nous avons explorĂ© le potentiel de trois nouvelles techniques pouvant amĂ©liorer la localisation du foyer Ă©pileptique chez les patients avec Ă©pilepsie focale rĂ©fractaire considĂ©rĂ©s candidats potentiels Ă une chirurgie dâĂ©pilepsie : lâIRM Ă haut champ, la spectroscopie proche infrarouge (SPIR) et la magnĂ©toencĂ©phalographie (MEG).
Dans une premiĂšre Ă©tude, nous avons Ă©valuĂ© si lâIRM de haut champ Ă 3 Tesla (T), prĂ©sentant thĂ©oriquement un rapport signal sur bruit plus Ă©levĂ© que lâIRM conventionnelle Ă 1,5 T, pouvait permettre la dĂ©tection des lĂ©sions Ă©pileptogĂšnes subtiles qui auraient Ă©tĂ© manquĂ©es par cette derniĂšre. Malheureusement, lâIRM 3 T nâa permis de dĂ©tecter quâun faible nombre de lĂ©sions Ă©pileptogĂšnes supplĂ©mentaires (5,6 %) dâoĂč la nĂ©cessitĂ© dâexplorer dâautres techniques.
Dans les seconde et troisiĂšme Ă©tudes, nous avons examinĂ© le potentiel de la SPIR pour localiser le foyer Ă©pileptique en analysant le comportement hĂ©modynamique au cours de crises temporales et frontales. Ces Ă©tudes ont montrĂ© que les crises sont associĂ©es Ă une augmentation significative de lâhĂ©moglobine oxygĂ©nĂ©e (HbO) et lâhĂ©moglobine totale au niveau de la rĂ©gion Ă©pileptique. Bien quâune activation contralatĂ©rale en image miroir puisse ĂȘtre observĂ©e sur la majoritĂ© des crises, la latĂ©ralisation du foyer Ă©tait possible dans la plupart des cas. Une augmentation surprenante de lâhĂ©moglobine dĂ©soxygĂ©nĂ©e a parfois pu ĂȘtre observĂ©e suggĂ©rant quâune hypoxie puisse survenir mĂȘme lors de courtes crises focales.
Dans la quatriĂšme et derniĂšre Ă©tude, nous avons Ă©valuĂ© lâapport de la MEG dans lâĂ©valuation des patients avec Ă©pilepsie focale rĂ©fractaire considĂ©rĂ©s candidats potentiels Ă une chirurgie. Il sâest avĂ©rĂ© que les localisations de sources des pointes Ă©pileptiques interictales par la MEG ont eu un impact majeur sur le plan de traitement chez plus des deux tiers des sujets ainsi que sur le devenir postchirurgical au niveau du contrĂŽle des crises.Epilepsy is the most common chronic neurological disorder after stroke. The major form of treatment is long-term drug therapy to which approximately 30% of patients are unfortunately refractory to. Brain surgery is recommended when medication fails, especially if magnetic resonance imaging (MRI) can identify a well-defined epileptogenic lesion. Unfortunately, close to a quarter of patients have nonlesional refractory focal epilepsy. For these MRI-negative cases, identification of the epileptogenic zone rely heavily on remaining tools: clinical history, video-electroencephalography (EEG) monitoring, ictal single-photon emission computed tomography (SPECT), and a positron emission tomography (PET). Unfortunately, the limited spatial and/or temporal resolution of these localization techniques translates into poor surgical outcome rates.
In this thesis, we explore three relatively novel techniques to improve the localization of the epileptic focus for patients with drug-resistant focal epilepsy who are potential candidates for epilepsy surgery: high-field 3 Tesla (T) MRI, near-infrared spectroscopy (NIRS) and magnetoencephalography (MEG).
In the first study, we evaluated if high-field 3T MRI, providing a higher signal to noise ratio, could help detect subtle epileptogenic lesions missed by conventional 1.5T MRIs. Unfortunately, we show that the former was able to detect an epileptogenic lesion in only 5.6% of cases of 1.5T MRI-negative epileptic patients, emphasizing the need for additional techniques.
In the second and third studies, we evaluated the potential of NIRS in localizing the epileptic focus by analyzing the hemodynamic behavior of temporal and frontal lobe seizures respectively. We show that focal seizures are associated with significant increases in oxygenated haemoglobin (HbO) and total haemoglobin (HbT) over the epileptic area. While a contralateral mirror-like activation was seen in the majority of seizures, lateralization of the epileptic focus was possible most of the time. In addition, an unexpected increase in deoxygenated haemoglobin (HbR) was noted in some seizures, suggesting possible hypoxia even during relatively brief focal seizures.
In the fourth and last study, the utility of MEG in the evaluation of nonlesional drug-refractory focal epileptic patients was studied. It was found that MEG source localization of interictal epileptic spikes had an impact both on patient management for over two thirds of patients and their surgical outcome
Turbulent ion heating in TCV tokamak plasmas
The Tokamak Ă configuration variable (TCV) features the highest electron cyclotron wave power density available to resonantly heat (ECRH) the electrons and to drive noninductive currents in a fusion grade plasma (ECCD). In more than 15 years of exploitation, much effort has been expended on real and velocity space engineering of the plasma electron energy distribution function and thus making electron physics a major research contribution of TCV. When a plasma was first subjected to ECCD, a surprising energisation of the ions, perpendicular to the confining magnetic field, was observed on the charge exchange spectrum measured with the vertical neutral particle analyser (VNPA). It was soon concluded that the ion acceleration was not due to power equipartition between electrons and ions, which, due to the absence of direct ion heating on TCV, has thus far been considered as the only mechanism heating the ions. However, although observed for more than ten years, little attention was paid to this phenomenon, whose cause has remained unexplained to date. The key subject of this thesis is the experimental study of this anomalous ion acceleration, the characterisation in terms of relevant parameters and the presentation of a model simulation of the potential process responsible for the appearance of fast ions. The installation of a new compact neutral particle analyser (CNPA) with an extended high energy range (†50 keV) greatly improved the fast ion properties diagnosis. The CNPA was commissioned and the information derived from its measurement (ion temperature and density, isotopic plasma composition) was validated against other ion diagnostics, namely the active carbon charge exchange recombination spectroscopy system (CXRS) and a neutron counter. In ohmic plasmas, where the ion heating agrees with classical theory, the radial ion temperature profile was successfully reconstructed by vertically displacing the plasma across the horizontal CNPA line of sight. Active charge exchange measurements, by doping the plasma with ion neutralisation targets injected with the diagnostic neutral beam (DNBI), were used to absolutely calibrate the NPA. Advanced modelling of the measured hydrogenic charge exchange spectra with the neutralisation and neutral transport codes KN1D and DOUBLE-TCV permitted a calculation of the absolute neutral density profiles of the plasma species. The energisation and the properties of fast ions were studied in dedicated, low density, cold ion, hot electron plasmas, resonantly heated at the second harmonic of the electron cyclotron frequency. The ion acceleration occurs on a characteristic timescale in the sub-millisecond range and comprises up to 20 % of the plasma ions. The number of fast ions nis and their effective temperature Tis are found to depend strongly on the bulk and suprathermal electron parameters, in particular Tis †Teb (electron bulk) and nis ⌠Vde (toroidal electron drift speed). The suprathermal electrons, abundantly generated in plasmas subjected to ECCD, are diagnosed with perpendicular and oblique viewing electron cyclotron emission (ECE) antennas and the measured frequency spectra are reconstructed with the relativistic ECE radiation balance code NOTEC-TCV. With steady-state ECRH and ECCD, the fast ion population reaches an equilibrium state. The spatial fast ion temperature profile is broad, of similar shape compared to the bulk ion temperature profile. The hottest suprathermal temperature observed is Tis †6 keV. Various potential ion acceleration mechanisms were examined for relevance in the TCV parameter range. The simultaneous waveâelectron and waveâion resonances of ion acoustic turbulence (IAT) show the best correlation with the available experimental knowledge. Ion acoustic waves are emitted by the weakly relativistic circulating electrons and are mainly Landau damped onto the ions. Destabilisation of IAT is markedly facilitated by the important degree of nonisothermicity Te/Ti â„ 40 of X2 EC heated TCV plasmas. Efforts were undertaken to consistently model the experimental observations using a numerical experiment. The relevant physics describing IAT was implemented in a finite difference code solving the quasilinear diffusion equation describing the time evolution of the electron and ion distribution functions. The simulations, fed as far as possible with experimentally available information, confirm the growth and saturation of IAT. Electrons and ions are initially preferentially heated in the toroidal direction. As the ions gain energy, the ion waves are damped more efficiently and only modes propagating at oblique angles can still grow, thus accelerating ions into the radial perpendicular direction. The simulation shows that turbulence reaches a steady-state when the ions are sufficiently hot to permanently stabilise IAT. The parameters describing the tail of the modelled equilibrium ion distribution agree quantitatively well with the CNPA measurement. Preliminary studies investigated on the interaction of fast ions with the sawtooth instability. It is found that the fast ion population in sawtoothing plasmas is transiently enforced with each sawtooth collapse. It is presently thought that the toroidal electric reconnection field lowers the IAT stability threshold thus producing more suprathermal ions
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Research Needs for Magnetic Fusion Energy Sciences. Report of the Research Needs Workshop (ReNeW) Bethesda, Maryland, June 8-12, 2009
Nuclear fusion - the process that powers the sun - offers an environmentally benign, intrinsically safe energy source with an abundant supply of low-cost fuel. It is the focus of an international research program, including the ITE R fusion collaboration, which involves seven parties representing half the world's population. The realization of fusion power would change the economics and ecology of energy production as profoundly as petroleum exploitation did two centuries ago. The 21st century finds fusion research in a transformed landscape. The worldwide fusion community broadly agrees that the science has advanced to the point where an aggressive action plan, aimed at the remaining barriers to practical fusion energy, is warranted. At the same time, and largely because of its scientific advance, the program faces new challenges; above all it is challenged to demonstrate the timeliness of its promised benefits. In response to this changed landscape, the Office of Fusion Energy Sciences (OFES ) in the US Department of Energy commissioned a number of community-based studies of the key scientific and technical foci of magnetic fusion research. The Research Needs Workshop (ReNeW) for Magnetic Fusion Energy Sciences is a capstone to these studies. In the context of magnetic fusion energy, ReNeW surveyed the issues identified in previous studies, and used them as a starting point to define and characterize the research activities that the advance of fusion as a practical energy source will require. Thus, ReNeW's task was to identify (1) the scientific and technological research frontiers of the fusion program, and, especially, (2) a set of activities that will most effectively advance those frontiers. (Note that ReNeW was not charged with developing a strategic plan or timeline for the implementation of fusion power.) This Report presents a portfolio of research activities for US research in magnetic fusion for the next two decades. It is intended to provide a strategic framework for realizing practical fusion energy. The portfolio is the product of ten months of fusion-community study and discussion, culminating in a Workshop held in Bethesda, Maryland, from June 8 to June 12, 2009. The Workshop involved some 200 scientists from Universities, National Laboratories and private industry, including several scientists from outside the US. Largely following the Basic Research Needs model established by the Office of Basic Energy Sciences (BES ), the Report presents a collection of discrete research activities, here called 'thrusts.' Each thrust is based on an explicitly identified question, or coherent set of questions, on the frontier of fusion science. It presents a strategy to find the needed answers, combining the necessary intellectual and hardware tools, experimental facilities, and computational resources into an integrated, focused program. The thrusts should be viewed as building blocks for a fusion program plan whose overall structure will be developed by OFES , using whatever additional community input it requests. Part I of the Report reviews the issues identified in previous fusion-community studies, which systematically identified the key research issues and described them in considerable detail. It then considers in some detail the scientific and technical means that can be used to address these is sues. It ends by showing how these various research requirements are organized into a set of eighteen thrusts. Part II presents a detailed and self-contained discussion of each thrust, including the goals, required facilities and tools for each. This Executive Summary focuses on a survey of the ReNeW thrusts. The following brief review of fusion science is intended to provide context for that survey. A more detailed discussion of fusion science can be found in an Appendix to this Summary, entitled 'A Fusion Primer.