The m: 1 motion of a screw pinch measured by external magnetic probes

Measurements of the m = 1 growing mode of a linear screw pinch, by external dipole probes, are compared with the recorded plasma motion on streak photographs. Good agreement is found up to a maximum amplitude which depends on plasma density and growth rat

Plasma shape stabilization of current rise MHD instabilities in TCV

The well-known and potentially disruptive plasma 'current rise' instabilities are studied as a function of the plasma shape in the Tokamak a a Configuration Variable (TCV). Disruptions typically occur in quasi-circular plasmas at q(a) - 3 in both non-sawtoothing and sawtoothing discharges with peaked current profiles. The perturbations in the plasma parameters before disruption are characterized, and the main unstable modes identified as coupled m/n = 2/1 and 3/2 rotating tearing modes. In the early phase, coupling between 3/1 and 2/1 modes is found to play a major role in determining whether or not the disruption will occur. Plasma cross section shaping is observed to reduce or to completely stabilize the disruptive mode and is regularly used in TCV operation as a tool for safe initial current ramp-up. Plasma elongation, positive and negative triangularity prevent the growth of a large 2/1 mode at q(a) - 3, thus reducing or even suppressing the disruptions. We also attempt an interpretation of the experimental results. Calculations of the tearing-mode stability parameter triangle' using the experimental plasma equilibria suggest the dominant role of toroidal mode coupling in the destabilization of the m/n = 2/1 mode in quasi-circular TCV plasmas. The effect of shaping on the reconstructed current profile and tearing stability is then considered. The analysis shows a destabilising trend with elongation and triangularity in contrast with the experiment. Other stabilizing mechanisms are discussed and shown to potentially contribute to the safe crossing of q(a) = 3 in shaped plasmas

Distribución vertical y horizontal de larvas de Monodaeus couchii frente a la costa sur de Portugal

A strong understanding of larval distribution and abundance is of major value in delineating the location and size of a breeding population of deep-sea species such as Monodaeus couchii. In this study, vertical distribution of the larvae of a brachyuran crab, M. couchii, was assessed during two week-long cruises conducted at the end of January 2006 and 2007 off the South Coast of Portugal. Larvae were collected by oblique plankton hauls with a Longhurst-Hardy Plankton Recorder from the surface to 300 m. Abundance and distribution of zoeae I and II were correlated during both years. For all stages, abundance decreased with depth during the day while it increased with depth at night; the larvae thus displayed reverse diel vertical migration. Abundance of zoeae I and II was correlated with chlorophyll a levels, whereas those of later stages were correlated with neither physical parameters (chlorophyll a, temperature or salinity) nor each other. An ontogenic shift in vertical distribution explained the results; earlier zoeal stages remain in the food-rich upper water column while later stages were not correlated with any physical parameters (i.e. chlorophyll a, salinity or temperature) and migrated to the bottom for settlement.El conocimiento de la distribución y abundancia larval es de vital importancia para comprender la localización y tamaño de la población reproductora adulta en especies abisales como Monodaeus couchii. En este estudio, la distribución vertical de las larvas de un cangrejo braquiuro, M. couchii, se evaluó durante dos campañas de una semana de duración al final de enero de 2006 y 2007, frente a la costa sur de Portugal. Las larvas se recogieron por pescas oblicuas de plancton con un “Longhurst-Hardy Plankton Recorder” (LHPR) desde la superficie hasta 300 m de profundidad. La abundancia y distribución de las zoeas I y II fueron correlacionadas para cada uno de los años de estudio. Para todos los estadios larvares la abundancia disminuyó con la profundidad durante el día, mientras que se incrementó con la profundidad durante la noche, mostrando por tanto un patrón de migración vertical inverso. La abundancia de zoeae I y II se correlacionó con los niveles de clorofila a, mientras que la abundancia de estadios posteriores no mostró correlación alguna con los parámetros físicos (clorofila a, temperatura o salinidad), ni entre sí. Estos resultados indican un cambio ontogénico en la distribución vertical de las larvas; los primeros estadios de zoea permanecen en la zona superficial de la columna de agua rica en alimento, mientras que las ultimas etapas que no mostraron correlación con los parámetros físicos (i.e. clorofila a, salinidad o temperatura) migran a zonas más profundas para el recrutamento