223 research outputs found
Coupling JOREK and STARWALL for Non-linear Resistive-wall Simulations
The implementation of a resistive-wall extension to the non-linear MHD-code
JOREK via a coupling to the vacuum-field code STARWALL is presented along with
first applications and benchmark results. Also, non-linear saturation in the
presence of a resistive wall is demonstrated. After completion of the ongoing
verification process, this code extension will allow to perform non-linear
simulations of MHD instabilities in the presence of three-dimensional resistive
walls with holes for limited and X-point plasmas.Comment: Contribution for "Theory Of Fusion Plasmas, Joint Varenna - Lausanne
International Workshop, Villa Monastero, Varenna, Italy (27.-31.8.2012)",
accepted for publication in Journal of Physics Conference Serie
Observing Galaxy Clusters with eROSITA: Simulations
The eROSITA instrument on board the Russian Spectrum Roentgen Gamma spacecraft, which will be launched in 2013,will conduct an all sky survey in X-rays. A main objective of the survey is to observe galaxy clusters in order to constrain cosmological parameters and to obtain further knowledge about dark matter and dark energy. For the simulation of the eROSITA survey we present a Monte-Carlo code generating a mock catalogue of galaxy clusters distributed accordingto the mass function of [1]. The simulation generates the celestial coordinates as well as the cluster mass and redshift. From these parameters, the observed intensity and angular diameter are derived. These are used to scale Chandra cluster images as input for the survey-simulation
Exploiting the Hierarchical Structure of Rule-Based Specifications for Decision Planning
Rule-based specifications have been very successful as a declarative approach in many domains, due to the handy yet solid foundations offered by rule-based machineries like term and graph rewriting. Realistic problems, however, call for suitable techniques to guarantee scalability. For instance, many domains exhibit a hierarchical structure that can be exploited conveniently. This is particularly evident for composition associations of models. We propose an explicit representation of such structured models and a methodology that exploits it for the description and analysis of model- and rule-based systems. The approach is presented in the framework of rewriting logic and its efficient implementation in the rewrite engine Maude and is illustrated with a case study.
Solitary magnetic perturbations at the ELM onset
Edge localised modes (ELMs) allow maintaining sufficient purity of tokamak
H-mode plasmas and thus enable stationary H-mode. On the other hand in a future
device ELMs may cause divertor power flux densities far in excess of tolerable
material limits. The size of the energy loss per ELM is determined by
saturation effects in the non-linear phase of the ELM, which at present is
hardly understood. Solitary magnetic perturbations (SMPs) are identified as
dominant features in the radial magnetic fluctuations below 100kHz. They are
typically observed close (+-0.1ms) to the onset of pedestal erosion. SMPs are
field aligned structures rotating in the electron diamagnetic drift direction
with perpendicular velocities of about 10km/s. A comparison of perpendicular
velocities suggests that the perturbation evoking SMPs is located at or inside
the separatrix. Analysis of very pronounced examples showed that the number of
peaks per toroidal turn is 1 or 2, which is clearly lower than corresponding
numbers in linear stability calculations. In combination with strong peaking of
the magnetic signals this results in a solitary appearance resembling modes
like palm tree modes, edge snakes or outer modes. This behavior has been
quantified as solitariness and correlated to main plasma parameters. SMPs may
be considered as a signature of the non-linear ELM-phase originating at the
separatrix or further inside. Thus they provide a handle to investigate the
transition from linear to non-linear ELM phase. By comparison with data from
gas puff imaging processes in the non-linear phase at or inside the separatrix
and in the scrape-off-layer (SOL) can be correlated. A connection between the
passing of an SMP and the onset of radial filament propagation has been found.
Eventually the findings related to SMPs may contribute to a future quantitative
understanding of the non-linear ELM evolution.Comment: submitted to Nuclear Fusio
Experimental evidence of magnetic flux pumping in ASDEX upgrade
In high-β scenarios with on-axis co-current electron cyclotron current drive, which normally lowers q 0 below unity, the absence of sawteeth suggests the involvement of an additional current redistribution mechanism beyond neoclassical current diffusion. This is supported by imaging motional Stark effect diagnostic measurements, which indicate that q 0 remains consistently around 1. This phenomenon is observed in the presence of a 1/1 mode, indicating its potential role in the current redistribution. It is shown that the mode’s ability to modify the central current and suppress sawteeth increases with plasma pressure. These findings align with a recent theoretical model, which predicts a pressure threshold for sawtooth avoidance by a 1/1 quasi-interchange mode and where this threshold increases with the strength of inward current diffusion. Moreover, the advantages of the flux pumping scenario for future machines are highlighted.</p
Experimental evidence of magnetic flux pumping in ASDEX upgrade
In high-β scenarios with on-axis co-current electron cyclotron current drive, which normally lowers q 0 below unity, the absence of sawteeth suggests the involvement of an additional current redistribution mechanism beyond neoclassical current diffusion. This is supported by imaging motional Stark effect diagnostic measurements, which indicate that q 0 remains consistently around 1. This phenomenon is observed in the presence of a 1/1 mode, indicating its potential role in the current redistribution. It is shown that the mode’s ability to modify the central current and suppress sawteeth increases with plasma pressure. These findings align with a recent theoretical model, which predicts a pressure threshold for sawtooth avoidance by a 1/1 quasi-interchange mode and where this threshold increases with the strength of inward current diffusion. Moreover, the advantages of the flux pumping scenario for future machines are highlighted.</p
Non-linear magnetohydrodynamic modeling of plasma response to resonant magnetic perturbations
The interaction of static Resonant Magnetic Perturbations (RMPs) with
the plasma flows is modeled in toroidal geometry, using the non-linear
resistive MHD code JOREK, which includes the X-point and the
scrape-off-layer. Two-fluid diamagnetic effects, the neoclassical
poloidal friction and a source of toroidal rotation are introduced in
the model to describe realistic plasma flows. RMP penetration is studied
taking self-consistently into account the effects of these flows and the
radial electric field evolution. JET-like, MAST, and ITER parameters are
used in modeling. For JET-like parameters, three regimes of plasma
response are found depending on the plasma resistivity and the
diamagnetic rotation: at high resistivity and slow rotation, the islands
generated by the RMPs at the edge resonant surfaces rotate in the ion
diamagnetic direction and their size oscillates. At faster rotation, the
generated islands are static and are more screened by the plasma. An
intermediate regime with static islands which slightly oscillate is
found at lower resistivity. In ITER simulations, the RMPs generate
static islands, which forms an ergodic layer at the very edge (ψ
≥0.96) characterized by lobe structures near the X-point and results
in a small strike point splitting on the divertor targets. In MAST
Double Null Divertor geometry, lobes are also found near the X-point and
the 3D-deformation of the density and temperature profiles is observed
Many-Valued Institutions for Constraint Specification
We advance a general technique for enriching logical systems with soft constraints, making them suitable for specifying complex software systems where parts are put together not just based on how they meet certain functional requirements but also on how they optimise certain constraints. This added expressive power is required, for example, for capturing quality attributes that need to be optimised or, more generally, for formalising what are usually called service-level agreements. More specifically, we show how institutions endowed with a graded semantic consequence can accommodate soft-constraint satisfaction problems. We illustrate our approach by showing how, in the context of service discovery, one can quantify the compatibility of two specifications and thus formalise the selection of the most
promising provider of a required resource.Peer Reviewe
Recent EUROfusion Achievements in Support of Computationally Demanding Multiscale Fusion Physics Simulations and Integrated Modeling
Integrated modeling (IM) of present experiments and future tokamak reactors requires the provision of computational resources and numerical tools capable of simulating multiscale spatial phenomena as well as fast transient events and relatively slow plasma evolution within a reasonably short computational time. Recent progress in the implementation of the new computational resources for fusion applications in Europe based on modern supercomputer technologies (supercomputer MARCONI-FUSION), in the optimization and speedup of the EU fusion-related first-principle codes, and in the development of a basis for physics codes/modules integration into a centrally maintained suite of IM tools achieved within the EUROfusion Consortium is presented. Physics phenomena that can now be reasonably modelled in various areas (core turbulence and magnetic reconnection, edge and scrape-off layer physics, radio-frequency heating and current drive, magnetohydrodynamic model, reflectometry simulations) following successful code optimizations and parallelization are briefly described. Development activities in support to IM are summarized. They include support to (1) the local deployment of the IM infrastructure and access to experimental data at various host sites, (2) the management of releases for sophisticated IM workflows involving a large number of components, and (3) the performance optimization of complex IM workflows.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014 to 2018 under grant agreement 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission or ITER.Peer ReviewedPostprint (published version
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