1,721,020 research outputs found
Sawtooth Generated Magnetic Islands and the Properties of the Snowflake Divertor
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Confinement ionique dans le tokamak TCV mesuré par spectroscopie d'échange de charge
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Study of impurity ion transport using charge exchange spectroscopy on TCV
No full textThe study of impurity transport in fusion plasmas has gained interest over the years due to its implication with tokamak performances. In TCV (Tokamak à Configuration Variable), as in many other facilities, impurity ions can be studied through the CXRS (Charge Exchange Recombination Spectroscopy) diagnostic.
The following research plan first presents the principles of CXRS. The main components of the diagnostic are described, as well as the data analysis methods used to infer ion temperature, density and velocity radial profiles.
One of the main goals of the thesis is to measure accurate density profiles. Many transport models, in fact, are still developed without a consistent prediction of the density profile, which in many cases is assumed flat. In recent years, however, experiments have been performed to study ion transport in various plasma scenarios with the aim of providing a conncetion between theoretical predictions and actual measurements.
This works present a study of impurity transport in negative triangularity. A large part of the thesis is dedicated to the investigation of the LOC/SOC transition and the rotation reversal, two phenomena often observed during density ramps that, in specific cases, appear to occur at the same critical density. This work shows a clear separation of the two phenomena, with an extensive study performed in discharges of different majority species (D, H, He), plasma configurations and shapes (limited, diverted, positive and negative triangularity).SPC-TC
Turbulent ion heating in TCV tokamak plasmas
Full text linkThe Tokamak à 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 wave–electron and wave–ion 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 ≥ 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.CRPPSP
Plasma rotation and momentum transport studies in the TCV tokamak based on charge exchange spectroscopy measurements
Full text linkThermonuclear controlled fusion research is a highly active branch of plasma physics. The main goal is the production of energy from the fusion reaction of hydrogen isotope nuclei, the same reaction that powers stars. The most promising present approach are Tokamaks, toroidal devices where high temperature plasmas are confined by means of magnetic fields. This thesis is devoted to a detailed and systematic study of plasma rotation nthe Tokamak à Configuration Variable (TCV), at the Centre de Recherches en Physique des Plasmas (CRPP) in Lausanne Switzerland. In a tokamak, confinement is limited by particle and energy transport from the hot core to the cold edge and by macroscopic perturbations of magnetic equilibrium. Recently, plasma rotation has been demonstrated to beneficially affect both confinement and stability, explaining the great recent interest in plasma rotation studies Relatively little is understood about plasma rotation physics and, in particular, the so called "intrinsic" rotation that will constitute the main component of plasma rotation in next generation machines. That is why a great theoretical and experimental effort is being deployed in studying intrinsic rotation and this work is part of this context. In TCV, plasma rotation is measured by the Charge eXchange Recombination Spectroscopy diagnostic (CXRS). The spectroscopic signal comes from the perpendicular observation of a low power Diagnostic Neutral Beam Injector (DNBI), which applies a negligible torque to the plasma. Hence, the DNBI/CXRS pair is an effective tool for the experimental study of intrinsic tokamak plasma rotation. During this work, the pre existing toroidal observation view was complemented with two new systems, permitting the measurement of toroidal rotation, on inboard plasma radius, and poloidal rotation in the plasma periphery. The implementation of an automated wavelength calibration procedure, based on reference Neon spectra, permitted the first viable (toroidal and poloidal) rotation measurements of TCV, with uncertainties down to 1km/s. Using upgraded light collection optics and fiber optic transmission lines, simultaneous measurement of core and edge plasma was achieved, with a doubling of of the radial resolution of the toroidal rotation measurements. The measurable range of plasma parameters was also extended to higher densities by the installation of back illuminated CCD detectors. In the present configuration (CXRS09), the diagnostic is capable of routinely measuring toroidal and poloidal plasma rotations with a radial resolution of ≲ 1 cm and a sample frequency of 10 ÷ 20 Hz, for plasma densities of 0.8 ≲ ne,av ≲ 8 × 1019 m-3. The basic scenario of Ohmically heated discharges in limiter L-mode configuration was initially addressed. A large core toroidal rotation up to uφ ≈ 50 km/s in the counter current direction is measured, reversing nearly exactly upon reversal of plasma current Ip. The toroidal rotation profile may be schematically divided into a core, a peripheral and an intermediate region. For qe ≈ 3 (magnetic safety factor at the plasma edge) the core region velocity is relatively flat or slightly "bulged" in the co current direction inside the sawtooth inversion radius. In the peripheral region, the toroidal rotation is small with a monotonic intermediate region. The central rotation appears to be limited to approximately its value at the sawtooth inversion radius. Poloidal rotation uθ ≲ 3 km/s, measured in Ohmic discharges, is only weakly dependent on plasma parameters but reverses with reversed magnetic field, with values and direction coherent with neoclassical predictions. Combining uφ and uθ measurements, the profile of the radial electric field Er was estimated through the radial force balance equation. Er down to 8 kVm (inward directed) is found in the plasma bulk, and close to zero at the plasma edge. A spontaneous reversal of the toroidal rotation profile is observed when the average density exceeds ne,av ≈ 4 × 1019 m-3 at low qe ≈ 3, with the plasma now rotating in the co current direction. The transition between the co and counter rotation regimes is studied dynamically using ne and Ip ramps and with the application of Electro Cyclotron Heating (ECH). The rotation reversal is weakly sensitive to impurity concentration and positive plasma triangularity appears to be a key ingredient. Whilst a physical explanation has not been identified yet, dynamic uφ reversal observations indicate that it results from a changed balance of radial non-diffusive fluxes of toroidal momentum. The study was extended to the divertor magnetic configuration in which the plasma column rotates in the co current direction at low ne, and uφ reverses at high ne opposite from the limited configuration. The rotation profile may again be divided into three regions, although the rotation at the plasma periphery does not always remain close to zero but evolves with the plasma parameters. In particular, independently on the core rotation regime, peripheral uφ decreases with ne and Ip and is strongly sensitive to the ion B→ × ∇B direction, suggesting a link with parallel fluxes in the Scrape-Off Layer (SOL). Combined measurements of of CXRS and Mach probe indicate that in the plasma edge toroidal rotation matches the toroidal component of SOL flows. From the analysis of toroidal rotation in stationary and transient phases, a characterization of the momentum transport is presented. The resulting radial momentum diffusivity, of the order of Χφ ∼ 0.1 – 0.3 m2/s, exceeds by 2 orders of magnitude the neoclassical estimation. A remarkable result is the existence of a "residual stress" component, which sustains a substantial stationary rotation gradient for null background rotation. Conversely, the analysis suggests a minor role for the convective (pinch) component, that is preliminarily confirmed by gyro-kinetic simulations including turbulent Coriolis convective pinch. Neoclassical predictions are in quantitative and qualitative disagreement with the experimental observations. The effect of the sawtooth instability on the core rotation was addressed in a specific experimental scenario where the inter crash evolution of the core uφ could be measured. The measurement required the development of a fast CXRS acquisition scheme based on a trigger constructed in real time from a Soft X-ray measurement. At the sawtooth crash the plasma core undergoes a strong acceleration in the co current direction (∆uφ ≈ 9 km/s in the experimental scenario), possibly related to a strong transient toroidal electric field. The systematic and varied observations reported in this work extend the experimental knowledge of bulk plasma rotation in low confinement regimes. In particular, the rotation reversal phenomena constitute an important test for momentum transport models of tokamak plasmas.CRPPSP
Infrared Thermography of Divertor ELM Heat Loads on TCV
No full textEdge Localised Mode (ELM)-induced heat loads, in particular divertor heat loads present one of the main challenges in adopting nuclear fusion as an industrial-scale energy source. At present, the type-I ELMy plasma regime is chosen as the most suitable operational scenario because of its energy confinement and steady-state capabilities, however, with the understanding that the enormous quasi-periodic heat loads deposited by the ELMs will need to be controlled in order to avoid intolerable damage to heat- bearing elements. At present, a detailed theoretical understanding, and in particular, a sufficiently reliable way of predicting ELM behaviour based on basic discharge parameters is missing. The fusion community has been expending a great deal of effort in order to improve this situation by gathering as much experimental data as possible from a range of different-sized tokamaks, in order to develop empirical scalings and refine theoretical models, with the ultimate aim of mitigating their destructive power below thresholds where they won’t affect continuous scientific exploitation of the reactors. This experimental thesis joins into this effort via the installation and commissioning of a fast infrared camera on TCV viewing the outer divertor. Near the end of the thesis, an effort of similar magnitude has been carried out with a camera on temporary loan from the MAST group, in order to image the inner divertor region as well. The cameras possess a sub-array recording capability, enabling acquisition frequencies up to 25 kHz. Both cameras were calibrated with the combination of a low- and high-temperature blackbody source with large and small radiation surfaces, respectively. By estimating the surface emissivity as 0.85 and employing a gray-body model, target surface temperatures could be inferred. The resulting images were processed to produce 1D poloidal temperature profiles serving as an input to the THEODOR (THermal Energy Onto DivertOR) 2D finite-element code, with the goal of calculating the heat fluxes impinging on plasma facing components. The VIR (Vertical InfraRed) outer divertor system images a flat, horizontal surface covered by relatively large tiles from a perpendicular viewing angle. Deposited layers, microscopic surface changes and very small dust particles cause the appearance of microscopic hot-spots in the field-of-view (FOV), leading to a substantial deviation from the assumed model of a single-temperature (the tile bulk graphite) gray body radiator. Near the middle of the thesis, nearly all tiles inside TCV have been removed for cleaning via sandblasting. With 3 of the tiles having been left untouched, this presented a good opportunity to contrast the behaviour of the cleaned and uncleaned (~12 years of operation) surfaces. It has been found that already after 10 days of operation, the temperature signal significantly exceeds what could be expected from a pure graphite surface. This, though increasing error bars of the final heat flux figures, is also beneficial in the sense that it serves as a signal amplifier – e.g. most observed ELM filaments near the main divertors carry very little energy, and were it not for this effect, they would not be visible at all. In contrast to the outer divertor, the inner divertor geometry is more complicated, as the tiles’ shape causes a gradual toroidal change in the magnetic field line attack angle from ~20 to 0 degrees, excluding toroidal averaging. Sequences recorded by this system also require a correction for mechanical vibrations, which, though also present for the vertical system, are much more pronounced, affecting the outcome of heat flux calculations to a great degree. Best results have been achieved with a method based on cross-correlations. Apart from eliminating the effects of the horizontal shaking, as the global vertical movement of the strike point leg on the target needed to be retained, the vertical shaking was either left uncorrected or parametrized in a way to remove the oscillations about the obtained vertical displacement trendline. In the process of calculating the heat flux from the temperature spatio-temporal evolution, the surfaces’ deviation from a one-temperature blackbody radiator manifests as an overshooting during transients, in both the positive and negative directions. This leads to the appearance of unphysical results (heat fluxes conducted outwards from the tiles, whilst plasma is still depositing heat on it). By introducing a simple boundary condition in THEODOR, a crude correction for this effect could be applied. The resulting tile deposited energies were cross-checked with rudimentary heat capacity calculations based on tile-installed thermocouples, yielding ~25% agreement. Two discharges, one in FWD-, the other in REV-B configuration, were selected for detailed analysis of ELM divertor heat loads. Both were heated by 3rd harmonic ECRH heating, the former exhibiting large- and the latter type-III ELMs. A dataset of various parameters such as maximum heat flux, profile width, total divertor deposited powers, ELM deposited energies and in-out power balance during and in between the ELMs, as well as the fraction of ELM energy recovered at the divertors, the rise-to-peak times and integral energy-to-peak was built up and evaluated in detail. The TCV outer divertor power loads dominate those of the inner under all circumstances – a finding that contradicts empirically established ELM characteristics from other tokamaks. The observed power imbalance is larger during the ELMs, and smaller in between. The ratio of ELM deposited energies at the inner- and outer divertors is 1:3, always in favour of the outer divertor. ELM profile broadening of a factor in the range ~1-2 has been found to occur consistently in both field directions, at both divertors. The total fraction of ELM energy recovered at the divertors (EIR/WELM) has been found to decrease with growing ELM size. The ELM pedestal energy fraction versus collisionality, the rise-to-peak time versus parallel convection time and integral rise-to- peak energy versus collisionality were compared to values from other tokamaks. With the exception of the ELM pedestal energy fraction of the “giant” ELMs in FWD-B discharge #38014, all values are in agreement with what would be extrapolated from other devices. In line with the target of predicting ELM heat loads, comparisons with 1D kinetic PIC simulations and the so-called free-streaming particle (FSP) approach (an analytical expression for the power density based on a relatively simple model) were performed. With the exception of the early rise phase, PIC simulations of type-III ELMs reproduce the experimental time evolution well. They give insight into finer details of ELM heat transport: a very fast electron heat pulse (carrying about 5% of total ELM energy), is followed by the main pulse on a slower time-scale. About 3⁄4 of the total energy is carried by the ions, of which 0.3-0.4 is deposited in the rise phase. The FSP expression, based on a collisionless approximation, provides a reasonable fit to the temporal evolution of divertor-deposited power for TCV ELMs of various sizes. In most cases, both the rise- and decay phases can be described by the single time parameter within the expression. This parameter is at least twice as large as the parallel transport time based on pedestal parameters, implying a delaying of particles near the X-point region. In contrast to other tokamaks, filamentary ELM heat deposition patterns are only occasionally recorded at the outer divertor. The observed spatial structure is consistent with a plasma release at discrete toroidal locations in the outer midplane vicinity, but as very few are visible on the IR (when compared to the Langmuir probes), the determination of quasi-mode numbers was deemed unfeasible. A crude estimation of the energy content of individual filament spirals yielded 1% of the total ELM energy in each, consistent with existing non-divertor ELM heat load predictions. Finally, a new divertor configuration, the snowflake, was recently demonstrated in TCV experimentally, and, subsequently, the creation of H-modes in it. This configuration presents many advantages, most prominently the possibility of distributing the divertor heat load to 4 (as opposed to 2 for standard single-null divertors) regions. Infrared measurements at one of these additional legs have confirmed substantial heat loading and a lack of ELM profile broadening.CRPPSP
Variations on the Author
Full text link“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
Full text linkWe 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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