Improvements of the particle-in-cell code EUTERPE for petascaling machines

In the present work we report some performance measures and computational improvements recently carried out using the gyrokinetic code EUTERPE (Jost, 2000 [1] and Jost et al., 1999 [2]), which is based on the general particle-in-cell (PIC) method. The scalability of the code has been studied for up to sixty thousand processing elements and some steps towards a complete hybridization of the code were made. As a numerical example, non-linear simulations of Ion Temperature Gradient (ITG) instabilities have been carried out in screw-pinch geometry and the results are compared with earlier works. A parametric study of the influence of variables (step size of the time integrator, number of markers, grid size) on the quality of the simulation is presented.Fil: Sáez, Xavier. Barcelona Supercomputing Center - Centro Nacional de Supercomputacion; EspañaFil: Soba, Alejandro. Barcelona Supercomputing Center - Centro Nacional de Supercomputacion; España. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sánchez, Edilberto. No especifíca;Fil: Kleiber, Ralf. No especifíca;Fil: Castejón, Francisco. No especifíca;Fil: Cela, José M.. Barcelona Supercomputing Center - Centro Nacional de Supercomputacion; Españ

Simulations of Alfvénic modes in TJ-II Stellarator

Alfvénic modes are one of the subclass of instabilities prevalent in burning plasmas due to interaction of energetic particles with background plasma. In this paper we investigate the properties of these modes with 3D simulations using modeling tools STELLGAP [1] and AE3D [2] of Neutral Beam Injection (NBI) heated H-plasmas in TJ-II low-magnetic-shear flexible heliac (B0 = 0.95 T, = 1.5 m, = 0.22 m). These simulations focus on modelling the experimental observations [3] for prominent modes in TJ-II plasmas. Our simulations show consistency in frequency and radial location with the measured Alfvén Eigenmodes [3]. These simulations are performed for chirping and steady modes in TJ-II discharge # 29839 at t = 1150 and 1160ms respectively

Asymptotic Behaviour of Inhomogeneous String Cosmologies

The asymptotic behaviour at late times of inhomogeneous axion-dilaton cosmologies is investigated. The space-times considered here admit two abelian space-like Killing vectors. These space-times evolve towards an anisotropic universe containing gravitational radiation. Furthermore, a peeling-off behaviour of the Weyl tensor and the antisymmetric tensor field strength is found. The relation to the pre-big-bang scenario is briefly discussed.Comment: 15 pages, Late

Zonal flows and long-distance correlations during the formation of the edge shear layer in the TJ-II stellarator

A theoretical interpretation is given for the observed long-distance correlations in potential fluctuations in TJ-II. The value of the correlation increases above the critical point of the transition for the emergence of the plasma edge shear flow layer. Mean (i.e. surface averaged, zero-frequency) sheared flows cannot account for the experimental results. A model consisting of four envelope equations for the fluctuation level, the mean flow shear, the zonal flow amplitude shear, and the averaged pressure gradient is proposed. It is shown that the presence of zonal flows is essential to reproduce the main features of the experimental observations.Comment: 19 pages, 7 figure

Discrimination between magnetic shear and toroidal electric field effects in TJ-II plasmas

The response of TJ-II plasmas to an induced toroidal electric field, Eǿ , was studied in past experiments with the aid of Ohmic inducted current. It was found that positive induced plasma current, Ip, degrades the confinement while negative Ip improves it. Candidates to provide a physical explanation are Eǿ itself and the plasma current via magnetic shear modification. A movable mirror in an electron Cyclotron Heating line of TJ-II permits varying the refraction index of the heating wave, N||, thus allowing for non inductive electron cyclotron current drive (ECCD) up to Ip ≈ ±1 kA. Comparing discharges without and with ECCD (changing shear but not Eǿ ) but sharing the same transformer action (changing both shear and Eǿ ), magnetic shear and electric field effects can be discriminated in time. It has been found that plasma response with ECCD is delayed respect to the transformer switch-on time, clearly pointing to shear effects. Effects related to Eǿ alone, if they exist, are weaker

Space-times admitting a three-dimensional conformal group

Perfect fluid space-times admitting a three-dimensional Lie group of conformal motions containing a two-dimensional Abelian Lie subgroup of isometries are studied. Demanding that the conformal Killing vector be proper (i.e., not homothetic nor Killing), all such space-times are classified according to the structure of their corresponding three-dimensional conformal Lie group and the nature of their corresponding orbits (that are assumed to be non-null). Each metric is then explicitly displayed in coordinates adapted to the symmetry vectors. Attention is then restricted to the diagonal case, and exact perfect fluid solutions are obtained in both the cases in which the fluid four-velocity is tangential or orthogonal to the conformal orbits, as well as in the more general "tilting" case.Comment: Latex 34 page

Posidonia oceanica restoration, a relevant strategy after boat anchoring degradation?

Trabajo presentado al 7th Mediterranean Symposium on Marine Vegetation, celebrado en Génova (Italia), los días 19 y 20 de septiembre de 2022.The anchoring of large pleasure boats constitutes one of the main threats in shallow marine habitats and particularly for seagrass beds. In the Mediterranean, this activity has seen constant development during the last decades, causing major physical disturbances in Posidonia oceanica meadows and associated ecosystem services, notably in terms of climate change mitigation (i.e. carbon fixation and sequestration capacities). In this context, the aims of the present study are to estimate the impact of these anchoring activities on P. oceanica meadows in a particularly highlyfrequented area (Sant’Amanza gulf, SE Corsica Island) and to set up a strategy to restore this major carbon sink. Since the last decade, time-series of marine habitat maps revealed an important regression of P. oceanica meadows, with a loss of 72.9 ha, corresponding to 11% of the meadow surface and 9% decline in the total carbon fixation and sequestration performed each year. Moreover, in the most impacted part of the bay (Balistra bay), a loss of 16.6 ha (28%) has been recorded between 2011 and 2022. Following recent enforcement of anchoring regulation, prohibiting the anchoring of large units (greater than 24 m), and the lockdown linked to the COVID-19 pandemic, anchorages in the seagrass reduced by 92%, between 2018 and 2022. Natural recovery was observed at the edge of the meadow (plagiotropic rhizomes) but this growth is slow and the areas to recover are large. As a result, a transplant experiment, from cuttings harvested from the adjacent meadows, was initiated in the spring 2021. Four restoration techniques are being tested in the some pilot sites and the development of these transplants will be monitored and compared to the natural recovery.This research has been co-financed by INTERREG program (GIREPAM), Collectivity of Corsica Corse (PADDUC-CHANGE), SETEC Foundation, Environment Office of Corsica and the French Office of Biodiversity (RenforC program)

Variation in Fish Abundance, Diversity and Assemblage Structure in Seagrass Meadows across the Atlanto-Mediterranean Province

Seagrasses worldwide provide key habitats for fish assemblages. Biogeographical disparities in ocean climate conditions and seasonal regimes are well-known drivers of the spatial and temporal variation in seagrass structure, with potential effects on associated fish assemblages. Whether taxonomically disparate fish assemblages support a similar range of ecological functions remains poorly tested in seagrass ecosystems. In this study, we examined variation in the abundance, diversity (from a taxonomic and functional perspective), and assemblage structure of fish community inhabiting nine meadows of the seagrass Cymodocea nodosa across three regions in the Mediterranean (Mallorca and Alicante) and the adjacent Atlantic (Gran Canaria), and identified which attributes typifying the structure of meadows, and large-scale variability in ocean climate, contributed most to explaining such ecological variation. Despite a similar total number of species between Mallorca and Gran Canaria, the latter region had more taxonomically and functionally diverse fish assemblages relative to the western Mediterranean regions, which translated into differences in multivariate assemblage structure. While variation in the abundance of the most conspicuous fish species was largely explained by variation in seagrass structural descriptors, most variation in diversity was accounted for by a descriptor of ocean climate (mean seasonal SST), operating at regional scales. Variation in fish assemblage structure was, to a lesser extent, also explained by local variability in seagrass structure. Beyond climatic drivers, our results suggest that lower temporal variability in the canopy structure of C. nodosa meadows in Gran Canaria provides a more consistent source of food and protection for associated fish assemblages, which likely enhances the more abundant and diverse fish assemblages thereEn prens