1,721,015 research outputs found
THERMAL, ELECTROMAGNETIC AND STRUCTURAL ANALYSIS OF PLASMA FACING COMPONENTS OF TCV TOKAMAK BY FINITE ELEMENT METHODS
No full textInvestigation of scrape-off layer and divertor transport using infrared thermography and SOLPS-ITER simulations
No full textNuclear fusion, with its potential to provide a nearly unlimited energy source, has inspired scientists to work on the development of fusion power plants. Among the various devices designed to harness nuclear fusion, the tokamak confines the fusion fuel, which has to reach high temperatures for fusion reactions to occur and enters the plasma state, using strong magnetic fields. However, the magnetic confinement is not perfect and plasma particles and energy reach the outermost chamber region, the Scrape-Off-Layer (SOL), where magnetic field lines intersect the material surface, and ultimately reach the wall. In current reactor designs, unmitigated peak heat fluxes would greatly exceed the material limit and a safe power removal from the plasma must be attained for successful operations of fusion reactors. The entire plasma dynamics of the SOL plays a fundamental role in determining the plasma exhaust. To understand its critical aspects and extrapolate from today's experiments to future fusion reactors, several numerical codes modelling the plasma edge have been developed.
This doctoral thesis seeks to improve the understanding of divertor power exhaust by studying the effect of particle drifts on the SOL transport. The investigation of particle drift-related transport is carried out using the SOLPS-ITER code for the plasma edge and experiments at the TCV tokamak. The thesis thus contributes to the validation of the SOLPS-ITER drift model. Simulations and experiments are carried reversing the magnetic field direction, as the particle drift direction depends on it, and are compared employing numerous diagnostics, with particular focus on the Infrared thermography system. Part of this work is devoted to the improvement of the IR system, whose analysis was improved as part of this thesis.
Particle drifts are found to significantly affect plasma conditions in the divertor, contributing significantly to the particle and convective heat fluxes. The importance of drifts is found to increase with increasing divertor plasma temperature. The diamagnetic drift is responsible for the dominant cross-field current in the SOL. The E×B drift is responsible for the in-out divertor redistribution of plasma density and power. Within the experimentally achieved density range, the radial E×B drift is identified as the dominant drift. Most of the experimental observations found with the field reversal are confirmed by drift simulations. However, a quantitative agreement between measurements and simulations cannot be achieved, with simulations generally predicting higher densities and lower temperatures in the divertor than experimentally observed. The comparison thereby guides future work to improve the experiments as well as the divertor model.SPC-TC
Impurity-seeded detachment in conventional and optimized TCV divertor geometry
No full textNuclear fusion provides a potentially unlimited and clean energy source which motivates the development of fusion power plants. The tokamak is one of the most promising concepts for a future reactor, confining the fusion plasmas in a torus using strong magnetic fields. A significant fraction of the heating power required to maintain fusion reaction is persistently transported by plasma across the magnetic flux surface into a thin, open-field-line region called scrape-off layer (SOL), where the power is driven towards the divertor target following the magnetic field lines. Unmitigated heat fluxes at the targets would far exceed the acceptable material limits, causing serious plasma exhaust issue that could possibly be solved by operating the tokamak in a detached regime, characterized by low plasma temperatures in front of the targets and reduced particle fluxes.
This thesis contributes to the development of fusion energy with the SOLPS-ITER code and experiments in the TCV tokamak, focusing on the reduction of plasma exhaust on the divertor target using impurity-seeded detachment in single-null (SN) and other divertor geometries. Injecting impurity neutrals such as nitrogen, neon, and argon could transfer a portion of the SOL heat flux into radiation, which distributes power more evenly across the vessel wall rather than concentrating it on the target plates. However, excessive impurity penetration into the core region in turn degrades the confinement or even leads to a radiative collapse. The impurity transport to the core could be controlled by introducing gas baffles which increases both the deuterium and the impurity neutral concentration in the divertor and enhance the momentum and power loss. The proposed tightly baffled, long-legged divertor configuration promises an even better performance and a passively stable, fully detached regimes.
The first issue, addressed in this thesis, is the nitrogen-seeded detachment in both L-mode and H-mode.. The simulations in the first year agree well with the trends observed in TCV experiments in L-mode discharges, , though the simulation predicts a colder and denser divertor. Stringent comparison between SOLPS simulation and TCV experiments in H-mode will be made, in additional to the X-point radiator in H-mode discharges. Alternative divertor geometries including baffles and long-legged divertor will also be included in simulation to test their influences on the impurity seeding.
The comparison with experiment requires the development of ASDEX-type pressure gauge (APG) which measures the neutral pressures in a variety of locations in the TCV chamber, from which the neutral compression is calculated. The APGs have been tested in vacuum chamber without fusion plasma in the first year, and are currently being installed in TCV. The data acquisition and storage, gauge calibration, operation in impurity-seeded discharges will be further developed during the thesis research.
The results obtained in the first year and the planned thesis research are expected to provide improved understanding of achieving divertor detachment of a fusion reactor using impurity seeding and alternative divertor configurations for future fusion reactors.SPC-TC
Plasma breakdown and current formation in single core and doublet configurations on TCV
Get PDFThis thesis presents an experimental investigation of the plasma formation in the TCV tokamak. The primary goal of this work was to program a reliable and smooth plasma formation at several positions within the TCV vessel and then use the gained understanding to revisit the creation of doublet plasma formation. The first part of this thesis is dedicated to understanding and improving the single-axis plasma formation scenario in TCV. A database for the single-axis TCV plasma formation scenario was created for discharges spanning several years of operation to understand the physics of the plasma formation dynamics. The database shows that most of the failed plasma formation in TCV were during the burn-through and ramp-up phase with only 0.5% of the discharges failing at breakdown. The failed plasma breakdowns are mainly attributed to technical issues, such as no injection of neutral gas into the vacuum vessel, absence of the toroidal field or the Ohmic coil current, and issues with the plasma control system. The improvement of the single-axis plasma formation was separated into two parts: improvement of the breakdown scenario and improvement of the plasma burn-through and ramp-up scenario. During the breakdown phase, a large mismatch was exposed between the intended and experimentally obtained vertical breakdown position, for both Z=0.05m and Z=+0.23m standard vertical breakdown positions and for both I_P and B_phi directions. This mismatch was caused by an additional poloidal field mainly due to errors in the back-off of the stray field generated by vessel currents. The use of a vessel resistivity assuming axisymmetry in the TCV discharge preparation procedure to model the vessel currents was identified as the main reason for the mismatch in the breakdown positions. The analysis of the plasma formation database revealed that most of the failed plasma formation during the burn-through and ramp-up phase occurred due to insufficient, albeit often temporary, Ohmic heating to sustain the plasma. The insufficient Ohmic heating was either due an insufficient initial I_P ramp rate, or a combined effect of strong I_P and/or radial position oscillations caused by the feedback control system due to too high initial I_P ramp rate. A bump-less transfer control technique was implemented to improve the reliability of plasma formation by avoiding the strong oscillations in I_P and radial position that resulted in reliable and sufficient Ohmic heating. The second part of this thesis focuses on developing a doublet shaped plasma configuration, which is a highly unconventional plasma configuration, and TCV's modern and unique shaping capabilities warrant an effort to revisit the configuration. Successful simultaneous breakdown at two locations in TCV was achieved by using the improved inductive breakdown scenario. The similar magnetic properties of the two magnetic null points ensured that the plasma current ramp rate in the two droplets were close and the plasma current in both droplets was ramped up to 50 kA each with Ohmic heating alone. A highly reproducible doublet formation scenario was finally achieved and was verified by several diagnostics. A highly reproducible doublet formation scenario was finally achieved and was verified by several diagnostics. A highest plasma current of 130 kA was achieved in each droplet, with a core electron temperature at 1.3keV, core electron density at 1.3e19m^-3, and 30ms duration with ECRH heating.SP
Investigating Scrape-Off Layer transport in alternative divertor geometries on the TCV tokamak
No full textThermonuclear fusion is a potentially clean and limitless energy source that can substantially change the current global electricity generation mix, which is highly dependent on limited fossil fuels.
This thesis contributes to the development of fusion energy with experiments on the TCV tokamak, addressing power exhaust in the divertor, which remains a major issue for a fusion tokamak reactor. A large fraction of the heating power needed to keep the plasma at the parameters required for fusion reactions, is continuously transported by the plasma across closed magnetic surfaces into a thin layer with open field lines surrounding the plasma, the Scrape-Off Layer (SOL), where it follows magnetic field lines towards the divertor targets.
Unmitigated target peak heat fluxes in a fusion reactor are projected to greatly exceed available material limits. While acceptable target conditions may be possible by operating the divertor in a detached regime, a degradation of core plasma performance may ensue. A unification of a performant core and a detached divertor may be possible using alternative magnetic configurations to the standard Single-Null (SN).
This thesis assesses the power exhaust properties over an unprecedented range of alternative magnetic configurations with the goals of both reducing the power exhaust challenge through divertor geometry modifications and improving the current understanding of SOL transport physics. TCV's infrared thermography systems are extensively used to measure divertor target heat fluxes, fitted to extract the SOL heat power width and the spreading factor .
Power sharing between divertor targets significantly varies with the plasma and/or divertor geometry, mostly interpreted as the effect of parallel electron heat conduction.
The is found sensitive to properties of the plasma core, shape of the plasma and the divertor, and is consistent with empirical cross-machine scalings. The dependence on plasma current can be mostly understood with a model based on the competition between parallel and perpendicular diffusive transport. Poloidal asymmetries and field direction effects indicate that more physics is needed.
The scales with the inverse of the target flux expansion.
In the Low-Field-Side Snowflake Minus configuration, the secondary x-point enhances cross-field transport in the divertor, consistently with turbulence simulations.
At the L- to H-mode transition, cross-field transport reduces in the main and divertor-SOL, suggesting that the edge transport barrier propagates into the SOL. The upstream between Type-I ELMs agrees with cross-machine scaling predictions only with the toroidal magnetic field as regression parameter, revealing that this field dependence is vital.
A comparison of the ELM power deposition duration in TCV and JET supports the hypothesis of it increasing with the parallel connection length, and the ELM peak parallel energy fluence is consistent with a cross-machine scaling, supporting current extrapolations to ITER. Divertor geometry variations reveal that the ELM power width scales with the inverse of the target flux expansion.
These results provide unique input for testing models required to reliably extrapolate to the divertor of a fusion reactor, and may have significant implications in the optimization of its magnetic configuration.SP
Drift-related transport and plasma-neutral interaction in the TCV divertor
No full textThermonuclear fusion of light atoms is the primary energy source of stars, such as our Sun, that led to the emergence of life on Earth. However, its economic exploitation as a virtually unlimited and clean energy source is yet to be developed. One of the most promising designs for a nuclear fusion reactor is the tokamak, a toroidal device that uses strong magnetic fields to contain matter so it may be heated to fusion-relevant temperatures of C. A substantial fraction of the heating power, required to maintain fusion-relevant plasma conditions, is transported across magnetic flux surfaces and channeled through a thin region of open magnetic field lines that surround the confined zone, the scrape-off layer, towards the plasma wall interface at the divertor targets. If left unmitigated, anticipated target heat fluxes in current reactor designs exceed the material limit and raise the need for an accurate predictive model of heat transport and power dissipation in the plasma edge to achieve a viable reactor configuration.
This doctoral thesis is devoted to further understanding of divertor power exhaust, a key challenge on the way towards fusion energy, and contributes to the validation of such a predictive model, the SOLPS-ITER code. This work employs stringent comparisons between simulation results and experiments on the TCV tokamak to validate this model, where possible, and/or indicate remaining problems. This thesis presents the first scrape-off layer simulations that fully account for drifts, currents, carbon impurities and kinetic neutrals, for the TCV tokamak, and thereby provides unprecedented insight into drift-driven transport and plasma-neutral interaction within its divertor.
In an initial stage, the SOLPS-ITER code was used to predict the effectiveness of divertor gas baffles to guide the first baffled TCV campaign. The experimental assessment in 2019 confirmed the essential simulation predictions. Drift simulations identify the diamagnetic current as the dominant cross-field contribution to the divertor charge balance. The resulting characteristics of target current profiles are tested and confirmed in TCV measurements. It is demonstrated, for the first time, that such electric currents give rise to a potential well below the magnetic X-point for the unfavorable magnetic field direction for H-mode access in highly resistive, i.e. low temperature, divertors. The simulation result is supported by reduced analytic models that highlight the underlying physics. The prediction is tested and confirmed in TCV experiments following novel diagnostic capabilities. The presence of such a potential well is shown to substantially reshape the divertor transport in detached divertor conditions. The simulations identify the -drift as the dominant radial transport channel for particles, heat and momentum in the divertor. A stringent comparison of simulation results to a wide set of edge-relevant diagnostics yields typically excellent qualitative agreement, but quantitative differences remain: the simulations conclude a colder and denser divertor plasma state with an overestimated neutral pressure. The identified shortcomings in this approach will contribute to further development towards a predictive divertor model of present day, and future fusion devices.SPC-TC
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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