Fluctuation characteristics of the TCV snowflake divertor measured with high speed visible imaging

Tangentially viewing fast camera footage of the low-field side snowflake minus divertor in TCV is analysed across a four point scan in which the proximity of the two X-points is varied systematically. The motion of structures observed in the post- processed movie shows two distinct regions of the camera frame exhibiting differing patterns. One type of motion in the outer scrape-off layer remains present throughout the scan whilst the other, apparent in the inner scrape-off layer between the two nulls, becomes increasingly significant as the X-points contract towards one another. The spatial structure of the fluctuations in both regions is shown to conform to the equilibrium magnetic field. When the X-point gap is wide the fluctuations measured in the region between the X-points show a similar structure to the fluctuations observed above the null region, remaining coherent for multiple toroidal turns of the magnetic field and indicating a physical connectivity of the fluctuations between the upstream and downstream regions. When the X-point gap is small the fluctuations in the inner scrape-off layer between the nulls are decorrelated from fluctuations upstream, indicating local production of filamentary structures. The motion of filaments in the inter-null region differs, with filaments showing a dominantly poloidal motion along magnetic flux surfaces when the X-point gap is large, compared to a dominantly radial motion across flux-surfaces when the gap is small. This demonstrates an enhancement to cross-field tranport between the nulls of the TCV low-field-side snowflake minus when the gap between the nulls is small.Comment: Accepted for publication in Plasma Physics and Controlled Fusio

Eddy heat fluxes from direct current measurements of the Antarctic Polar Front in Shag Rocks Passage

Determining meridional heat flux in the Southern Ocean is critical to the accurate understanding and model simulation of the global ocean. Mesoscale eddies provide a significant but poorly-defined contribution to this transport. An eighteen-month deep-water current meter array deployment in Shag Rocks Passage (53°S, 48°W) between May 2003 and November 2004 provides estimates of the eddy flux of heat across the Polar Front. We calculate a statistically nonzero (99% level), vertically coherent local poleward heat flux of 12.0 ± 5.8 kW m-2 within the eddy frequency band at ~2750 m depth. Exceeding previous deep-water estimates by up to an order of magnitude, this highlights the large spatial variation in flux estimates and illustrates that constriction of circumpolar fronts facilitates large eddy transfers of heat southwards

A novel flexible field-aligned coordinate system for tokamak edge plasma simulation

Tokamak plasmas are confined by a magnetic field that limits the particle and heat transport perpendicular to the field. Parallel to the field the ionised particles can move freely, so to obtain confinement the field lines are “closed” (ie.form closed surfaces of constant poloidal flux) in the core of a tokamak. Towards, the edge, however, the field lines intersect physical surfaces, leading to interaction between neutral and ionised particles, and the potential melting of the material surface. Simulation of this interaction is important for predicting the performance and lifetime of future tokamak devices such as ITER. Field-aligned coordinates are commonly used in the simulation of tokamak plasmas due to the geometry and magnetic topology of the system. However, these coordinates are limited in the geometry they allow in the poloidal plane due to orthogonality requirements. A novel 3D coordinate system is proposed herein that relaxes this constraint so that any arbitrary, smoothly varying geometry can be matched in the poloidal plane while maintaining a field-aligned coordinate. This system is implemented in BOUT++ and tested for accuracy using the method of manufactured solutions. A MAST edge cross-section is simulated using a fluid plasma model and the results show expected behaviour for density, temperature, and velocity. Finally, simulations of an isolated divertor leg are conducted with and without neutrals to demonstrate the ion-neutral interaction near the divertor plate and the corresponding beneficial decrease in plasma temperature

The flow of the Antarctic circumpolar current over the North Scotia Ridge

The transports associated with the Subantarctic Front (SAF) and the Polar Front (PF) account for the majority of the volume transport of the Antarctic Circumpolar Current (ACC). After passing through Drake Passage, the SAF and the PF veer northward over the steep topography of the North Scotia Ridge. Interaction of the ACC with the North Scotia Ridge influences the sources of the Malvinas Current. This ridge is a major obstacle to the flow of deep water, with the majority of the deep water passing through the 3100 m deep gap in the ridge known as Shag Rocks Passage. Volume transports associated with these fronts were measured during the North Scotia Ridge Overflow Project, which included the first extensive hydrographic survey of the ridge, carried out in April and May 2003. The total net volume transport northward over the ridge was found to be . The total net transport associated with the SAF was approximately , and the total transport associated with the PF was approximately . Weddell Sea Deep Water was not detected passing through Shag Rocks Passage, contrary to some previous inferences

Un modelo predictivo de la evolución en costas rocosas.

El retroceso de las costas acantiladas es un fenómeno muy extendido sobre los litorales rocosos expuestos a la incidencia combinada de los procesos marinos y meteorológicos que se dan en la franja costera. Este fenómeno se revela violentamente como movimientos gravitacionales del terreno esporádicos, pudiendo causar pérdidas materiales y/o humanas. Aunque el conocimiento de estos riesgos de erosión resulta de vital importancia para la correcta gestión de la costa, su predicción es complicada. Los modelos de predicción publicados son escasos y con importantes inconvenientes: extrapolación, extienden información registros históricos; empíricos, sobre registros históricos estudian respuesta al cambio de un parámetro; estocásticos, determinan la cadencia y magnitud de los eventos futuros extrapolando las distribuciones de probabilidad extraídas de catálogos históricos; proceso-respuesta, de estabilidad y propagación del error inexplorada; en EDPs, computacionalmente costosos y poco exactos. En este trabajo se desarrolla un modelo combinado de proceso-respuesta basado en incorporar un balance de fuerzas de los mecanismos que actúan sobre el proceso erosivo en el frente del acantilado. El modelo simula la evolución espacio-temporal de un perfil-2D del acantilado, formado por materiales heterogéneos, acoplando la dinámica marina con la evolución del terreno en cada periodo de marea. Integra en cada periodo una función de erosión, dependiente de la pendiente de la zona afectada, que se desplaza sobre la onda de marea. Se ha estudiado el error de discretización del modelo y su propagación en el tiempo a partir de las soluciones exactas para los dos primeros periodos de marea para diferentes aproximaciones numéricas en la integración y de la pendiente. Los resultados obtenidos han permitido justificar las discretizaciones que minimizan el error y los métodos de aproximación más adecuados