Scanning and transmission electron microscopy analyses have been performed for tiles extracted from the toroidal pump limiter of Tore Supra for erosion- and deposition-dominated zones. Deposit ...thicknesses have been estimated for the plasma-facing top and the gap side lateral surfaces. Deposit thickness profiles have been measured inside gaps, showing that deposition mainly occurs in the first millimetre and that both poloidal and toroidal gap deposition is asymmetric. Quantitative information on the deposit volume and on D-retention are thus obtained from these measurements. Carbon probed at the tile top surfaces is mainly amorphous carbon, due either to the amorphization induced by ion bombardment in the erosion dominated zone, or to deposit formation processes in the deposition-dominated zones. Deposits are tip-shaped and are oriented, which should give information on transport processes.
The paper reports on simulation of pellet-fueled plasmas in a fusion reactor. The simulations have been performed by means of the ASTRA transport code. We have studied physical modeling of pellet ...injection as well as the numerical conditions to resolve pellet injection correctly. As a first step the essential mechanisms for density control have been studied based on simplified assumptions with a generic source of additional heating. The experience gained has been used to simulate advanced scenarios including internal transport barriers. It has been confirmed that it is possible to drive the plasma of a next-generation tokamak into a high-Q regime and to maintain it in a steady-state regime. Nevertheless, the pellet injection parameters required are rather demanding and imply a significant technological improvement of pellet injectors. Those investigations represent an improvement of simulations done earlier with a control of the central density at constant profile.
Abstract
WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source ...and turbulent transport up to the core in L-mode, high recycling plasmas, an unavoidable phase of all scenarios. The best core performances correlate with the lowest values of the density at the separatrix,
n
sep
, similarly to ASDEX Upgrade (AUG) tokamak and Joint European Torus (JET) tokamak in H-mode (Verdoolaege
et al
2021
Nucl. Fusion
61
076006). Reflectometry in the midplane provides
n
sep
, while the temperature at the separatrix,
T
sep
is inferred by the ‘two-point model’ using Langmuir probe data on divertor targets. Lower separatrix resistivity does not correlate with better core performances, unlike H-mode observations (Eich
et al
2020
Nucl. Fusion
60
056016). As expected in the presence of an efficient neutral source due to recycling fluxes,
n
sep
correlates with the D recycled particle flux at the divertor measured by visible spectroscopy. Coherently, at a given controlled central line integrated density
n
ˉ
, lower
n
sep
correlates with a larger density gradient around the separatrix as well as a larger global density peaking,
n
ˉ
/
⟨
n
⟩
, measured by interferometry. The latter correlates as well with lower collisionality in the core, similarly to JET and AUG H-modes (Angioni
et al
2007
Nucl. Fusion
47
1326). The correlations reported allow phrasing the subsequent causality question: what is the interplay chain between low neutral recycling at the divertor plates, low density at the separatrix, high density peaking at the separatrix, high global density peaking, higher central temperature and better core energy confinement quality? Understanding the causality chain is essential to prepare ITER operation and design DEMO scenarios where the ratio of the divertor leg to the ionization length will be larger and where the pumped flux with respect to the plasma volume will be lower than presently operating tokamaks.
Numerical studies of the ablation of pellets and shattered pellet injection (SPI) fragments into a runaway electron beam in ITER have been performed using a time-dependent pellet ablation code R. ...Samulyak at el., Nucl Fusion, 61 (4), 046007 (2021). The code resolves detailed ablation physics near pellet fragments and large-scale expansion of ablated clouds. The study of a single fragment ablation quantifies the influence of various factors, in particular the impact ionization by runaway electrons and cross-field transport models, on the dynamics of ablated plasma and its penetration into the runaway beam. Simulations of SPI performed using different numbers of pellet fragments study the formation and evolution of ablation clouds and their large-scale dynamics in ITER. The penetration depth of ablation clouds is found to be of the order of 50 cm.
Pellet injection represents to date the most realistic candidate technology for core fueling of a demonstration fusion power reactor tokamak fusion reactor. Modeling of both pellet penetration and ...fuel deposition profiles, for different injection locations, indicates that effective core fuelling can be achieved launching pellets from the inboard high field side at speeds not less than ~1 km/s. Inboard pellet fueling is commonly achieved in present tokamaks, using curved guide tubes; however, this technology might be hampered at velocities ≥1 km/s. An innovative approach, aimed at identifying suitable inboard "direct line" paths, to inject high-speed pellets (in the 3 to 4 km/s range), has recently been proposed as a potential complementary solution. The fuel deposition profiles achievable by this approach have been explored using the HPI2 simulation code. The results presented here show that there are possible geometrical schemes providing good fueling performance. The problem of neutron flux in a direct line-of-sight injection path is being investigated, though preliminary analyses indicate that, perhaps, this is not a serious problem. The identification and integration of straight injection paths suitably tilted may be a rather difficult task due to the many constraints and to interference with existing structures. The suitability of straight guide tubes to reduce the scatter cone of high-speed pellets is, therefore, of main interest. A preliminary investigation, aimed at addressing these technological issues, has recently been started. A possible implementation plan, using an existing Italian National Agency for New Technologies, Energy and Sustainable Economic Development-Oak Ridge National Laboratory facility is shortly outlined.
Dans ce travail, nous étudions la stabilité thermique et les effets des irradiations par un plasma d'hélium ou de deutérium de films minces de WO3 d’intérêt pour la fusion magnétique (projet ITER). ...L’objectif est de comprendre comment une oxydation du divertor modifie les interactions plasma paroi. Pour cela, nous avons synthétisé des films de WO3 par oxydation thermique de substrats de W à 400°C et caractérisé les effets du type de substrat, de la pression d’oxygène et du temps d’oxydation sur la structure et sur l’épaisseur des oxydes formés. La structure (monoclinique nanocristalline), la morphologie et les défauts des échantillons ont été analysés avant et après traitement, à différentes échelles, en utilisant la microscopie électronique, la microscopie Raman, la diffraction de rayons X, et la microscopie à force atomique.Le chauffage sous vide (400 - 800°C) a conduit à la formation de WO2. Le bombardement aux ions D+ (11 eV) a mené à une diffusion profonde du deutérium à travers le film d’oxyde, engendrant un effet électrochimique, observé ici pour la première fois sous irradiation plasma. Cet effet, réversible, est associé à la formation de bronzes de tungstène (DxWO3) et à une transition de phase vers une structure hexagonale. Des bombardements aux ions He+ (20 eV) ont été réalisés afin de dissocier les effets physiques et chimiques. A température ambiante, le bombardement a causé peu de changements morphologiques et structuraux. Par contre, le autre bombardement à 400°C a causé une érosion du film d’oxyde accompagnée d’un changement de couleur, une amorphisation en surface et la formation de bulles à l’interface W / WO3.
As part of laboratory studies devoted to magnetic fusion we have investigated the thermal stability and the effects of helium and deuterium plasma irradiation on tungsten oxide thin films. The objective is to predict the consequences of the oxidation of the W plasma facing component (divertor) for plasma wall interactions.To this aim, we have synthesized WO3 films by thermal oxidation of W substrates at 400°C and we have characterized the effects of the W substrate, the oxygen pressure and the oxidation duration on the structure and the thickness of the oxide films. The sample crystalline structure (monoclinic nanocrystalline), defects and morphologies were characterized before and after treatment using scanning and transmission electron microscopies, Raman microscopy, X-Ray diffraction and atomic force microscopy. Heating under vacuum up to 800°C leads to changes in the film structure and composition which results in the formation of WO2. D+ bombardment (11 eV) leads to D+ diffusion throughout the oxide film and to an electrochromic effect, here observed for the first time under plasma irradiation. This effect - which turned out to be reversible - is related to the formation of W bronzes (DxWO3) and to a phase transition of the oxide toward a hexagonal structure. Helium bombardments (20 eV) have then been performed to unravel physical and chemical processes at play. He+ bombardment at room temperature causes slight structural and morphological changes. On the contrary, He+ bombardment at 400°C leads to a significant erosion of the oxide film, accompanied by a colour change, the surface amorphisation and the formation of bubbles at the W / WO3 interface.
Les tokamaks visent à réaliser la fusion contrôlée de noyaux de deutérium et de tritium par le confinement magnétique d'un plasma chaud. L'interaction entre le plasma et les parois a été étudiée en ...détail pour le tokamak Tore Supra. Au cours des décharges, le plasma interagit fortement avec le limiteur, formé de milliers de tuiles en composite carbone/carbone. L'érosion de ces tuiles par les flux de particules du plasma mène à la formation de co-dépôts de carbone et de deutérium qu'il est essentiel de limiter. Nous avons effectué une étude multi-échelle, principalement avec les différents outils de la microscopie électronique, sur des tuiles provenant du limiteur de Tore Supra. Une analyse des co-dépôts a permis de mettre en évidence leur topographie en forme de pointes, orientées dans une même direction quelque soit la position de la tuile sur le limiteur. L'étude de la surface de tuiles appartenant à des zones majoritairement érodées a révélé la présence d'une striation périodique de surface. Ces deux phénomènes ont été mis en relation avec la direction des flux et l'effet de la gaine faiblement magnétisée de Tore Supra a été mis en évidence. L'analyse des dépôts présents dans les interstices entre les tuiles a révélé une physique propre à ces interstices permettant la formation de dépôts en profondeur. Des nanoparticules graphitiques sphériques ont été observées, signe d'une croissance homogène locale en phase plasma. Nous avons développé des méthodes de mesure des volumes de dépôt et des volumes érodés, menant à l'établissement d'un bilan carbone et à l'évaluation de la masse de deutérium piégé, en bon accord avec les mesures in-situ réalisées dans Tore Supra.
Tokamaks are devices aiming at achieving controlled fusion of deuterium and tritium by magnetically confining a hot plasma. The interaction between the plasma and the inner walls is a crucial issue and has been studied in detail in Tore Supra. During discharges the plasma strongly interacts with limiter, designed with thousands of carbon tiles (C/C composite). The plasma particle fluxes erode the tiles, leading to co-deposition of carbon and deuterium that should be limited. We have performed a multi-scale study of tiles extracted from the Tore Supra limiter, mainly using electron microscopy. The analysis of the co-deposits has revealed a tip-shaped topography, tips being oriented in the same direction wherever the tile over the limiter. Analyses of tiles extracted from erosion-dominated zones have revealed the presence of a periodic ripple on their surfaces. Both phenomena have been related with the direction of ion fluxes and the effect of the weakly magnetized sheath of Tore Supra has been shown. Analyses of the deposits inside the gaps in-between the tiles have revealed the existence of specific processes leading to the formation of deposits deeply inside the gaps. Graphitic nano-particles have been observed, showing the existence of local homogeneous growth processes. Finally, by measuring the deposit volume and the C/C composite eroded volume we have obtained an inventory of both carbon and deuterium which is consistent with the analyses of Tore Supra in-situ measurements.
The sputtering of inside wall components of tokamaks can lead to the
injection of supersaturated vapour in the plasma edge. The resulting
condensation favours the formation of clusters which can give ...rise to solid
particulates by further accretion. Sputtering discharges are proposed to have
highlight on the formation of carbonaceous dust observed in the tokamaks with
graphite based wall components. The flux of the sputtered carbon atoms is
evaluated in the conditions of our laboratory discharges as well as the
evolution of their energy distribution. It is shown that a cooling mechanism
occurs through collisions with the discharge argon atoms, leading to a
nucleation phase. A comparison between the carbon structure of the resulting
dust particles and a dust sample collected in the Tore Supra tokamak is
proposed. The structural differences are discussed and can be correlated to
specific plasma conditions.
The sputtering of inside wall components of tokamaks can lead to the injection of supersaturated vapour in the plasma edge. The resulting condensation favours the formation of clusters which can give ...rise to solid particulates by further accretion. Sputtering discharges are proposed to have highlight on the formation of carbonaceous dust observed in the tokamaks with graphite based wall components. The flux of the sputtered carbon atoms is evaluated in the conditions of our laboratory discharges as well as the evolution of their energy distribution. It is shown that a cooling mechanism occurs through collisions with the discharge argon atoms, leading to a nucleation phase. A comparison between the carbon structure of the resulting dust particles and a dust sample collected in the Tore Supra tokamak is proposed. The structural differences are discussed and can be correlated to specific plasma conditions.
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