C. Konz

SOLPS modelling of ASDEX upgrade H-mode plasma

A low density H-mode plasma has been selected for detailed inter-ELM modelling by the SOLPS code package, with the coupled treatment of its plasma (fluid code B2) and neutral (Monte-Carlo code Eirene) parts. Good quality measured midplane density and temperature profiles, covering the pedestal region and stretching far into the SOL, as well as several other parameters and profiles measured in the divertor, have enabled testing the consistency of code solutions with experiment. Once the upstream, midplane profiles have been fitted, and the global parameters (e.g. input power into the computational grid, radiated power) matched, the code reproduces experimental profiles and control parameters in the divertor with an accuracy within a factor of 2. Deviations of modelled parameters from the experiment were found around the strike point position where most of the power was deposited on the target. The deviations are consistent among themselves and all point to one common problem with the modelling: the predicted divertor electron temperature is too low and the density too high, compared with the experiment. The largest inconsistency between the code and experiment was in the magnitude of the peak Hα radiation in the outer divertor, which was larger by a factor of 2 in the code simulations. In addition, the code predicts a somewhat higher sub-divertor neutral flux but lower carbon impurity content in the edge plasma than in the experiment, as well as lower CIII emission. The discrepancy between Hα profiles can to a large degree be attributed to profile effects: the simulated Hα emission profiles are narrower than in the experiment, reflecting the tendency of the neutral–plasma mix to congregate excessively around the strike point in the modelling. At the same time, the integrated Hα emission matches very well with the experiment.Extensive sensitivity studies of the influence of variations in input parameters and assumptions of the code on the modelled divertor conditions have been conducted. They have not resulted in an identification of any SOLPS input/control parameters capable of removing the main disagreement between the code output and experiment. A possibility of parallel transport effects related to low collisionality to increase the effective plasma temperature near the strike point position or of increased perpendicular transport by neutrals (due to some missing reactions in Eirene) to widen the target profiles, will be explored in the future.

Disruption studies in ASDEX Upgrade in view of ITER

Experiments on ASDEX Upgrade and other tokamaks have shown that the magnitude of mechanical forces and thermal loads during disruptions can be significantly reduced by raising the plasma density with massive injection of noble gases. This method should be applicable to ITER too. Nevertheless, the suppression of the runaway electron (RE) avalanche requires a much larger (two order of magnitude) density rise. This paper reports on recent experiments aimed at increasing the plasma density towards the critical value, needed for the collisional suppression of REs. An effective electron density equal to 24% of the critical density has been reached after injection of 3.3?bar?l of neon. However, the resultant large plasma density is very poloidally and toroidally asymmetric; this implies that several valves distributed around the plasma periphery become necessary at this level of massive gas injection to ensure a homogeneous density distribution.

A generic data structure for integrated modelling of tokamak physics and subsystems

The European Integrated Tokamak Modelling Task Force (ITM-TF) is developing a new type of fully modular and flexible integrated tokamak simulator, which will allow a large variety of simulation types. This ambitious goal requires new concepts of data structure and workflow organisation, which are described for the first time in this paper. The backbone of the system is a physics- and workflow-oriented data structure which allows for the deployment of a fully modular and flexible workflow organisation. The data structure is designed to be generic for any tokamak device and can be used to address physics simulation results, experimental data (including description of subsystem hardware) and engineering issues.

The European Integrated Tokamak Modelling (ITM) effort: achievements and first physics results

A selection of achievements and first physics results are presented of the European Integrated Tokamak Modelling Task Force (EFDA ITM-TF) simulation framework, which aims to provide a standardized platform and an integrated modelling suite of validated numerical codes for the simulation and prediction of a complete plasma discharge of an arbitrary tokamak. The framework developed by the ITM-TF, based on a generic data structure including both simulated and experimental data, allows for the development of sophisticated integrated simulations (workflows) for physics application.The equilibrium reconstruction and linear magnetohydrodynamic (MHD) stability simulation chain was applied, in particular, to the analysis of the edgeMHDstability of ASDEX Upgrade type-I ELMy H-mode discharges and ITER hybrid scenario, demonstrating the stabilizing effect of an increased Shafranov shift on edge modes. Interpretive simulations of a JET hybrid discharge were performed with two electromagnetic turbulence codes within ITM infrastructure showing the signature of trapped-electron assisted ITG turbulence. A successful benchmark among five EC beam/ray-tracing codes was performed in the ITM framework for an ITER inductive scenario for different launching conditions from the equatorial and upper launcher, showing good agreement of the computed absorbed power and driven current. Selected achievements and scientific workflow applications targeting key modelling topics and physics problems are also presented, showing the current status of the ITM-TF modelling suite.

Discrepancy between modelled and measured radial electric fields in the scrape-off layer of divertor tokamaks: a challenge for 2D fluid codes?

Examination of radial electric field (E,.) profiles in the scrape-off layer (SOL) of ASDEX Upgrade (AUG) and JET revealed large discrepancies between 2D fluid edge modelling and experiment. Experimental profiles of plasma potential (V-p) in the outer (low field) side of the plasma, obtained with reciprocating Langmuir probes, decay radially with electron temperature, T-e, with the -eE(r)/del T-e ratio being > 1.5. In contrast, code simulated E-r are fairly low in most of the SOL (compared with -del T-e/e). Modelling with kinetic treatment of neutrals and drifts was performed using the SOLPS code for AUG cases and EDGE2D-Nimbus for JET cases. Mismatches between modelled and experimental E-r may be caused by the recently established tendency for the SOLPS code to underestimate T-e in the divertor of AUG. It was attributed to non-locality of parallel transport of supra-thermal, heat-carrying electrons originating upstream of the divertor, which are usually only weakly collisional and can penetrate, with few collisions, to the target. Ratios -eE(r)/del T-e obtained from the probe measurements in JET are of order 1.6, while in AUG, JT-60U and TCV they are of order 3. Such high values point to the possibility of fast electrons contributing, apart from target heat fluxes, also to the formation of the Debye sheath. The problem of the underestimation of E-r. in the codes must be closely related with the well-known problem of the underestimation of those parts of parallel ion flows in the SOL that are influenced by the toroidal field direction. It was demonstrated earlier that parallel ion flow at the outer midplane is dominated by the ion Pfirsch-Schluter flow, which in turn is partly driven by the radial electric field. The T-e and E-r discrepancies, as well as discrepancies between simulated and experimental parallel ion flows, raise a question of the validity of fluid codes for the plasma edge modelling and prompt the inclusion of kinetic effects into present-day 2D fluid codes which assume strong collisionality.

Pedestal and core confinement of hybrid scenario in ASDEX Upgrade and DIII-D

Pedestal and core confinement of hybrid discharges in ASDEX Upgrade (AUG) and DIII-D are studied in dedicated power scan experiments. The H98(y,2) confinement factor increases with total ?N in both tokamaks and it is higher in DIII-D with higher ? plasma shape at a given ?N. The pedestal beta, , increases linearly with total beta in AUG hybrid discharges, while it is roughly constant with ?N at fixed shape in the DIII-D power scans. The confinement enhancement with power observed with respect to the IPB98(y,2) scaling is due to an increase in pedestal confinement in AUG hybrid discharges and to an increase in core confinement in the DIII-D hybrid power scans. The increase in pedestal pressure with power in AUG hybrid discharges is primarily due to an increase in the width of the edge transport barrier at constant pressure gradient. In the DIII-D discharges the widths of the Te and ne pedestals, and , are consistent with a scaling. In the AUG hybrid power scans a dependence of on ?pol,PED cannot be excluded, while shows no dependence on ?pol,PED In both machines increases with ?. The maximum pedestal pressure achieved in the experiment prior to the onset of type I ELMs is consistent with predictions from ideal MHD; however, a physics model explaining the increase in the pedestal width with ? is still missing. The increase in with ? in the core of DIII-D is consistent with predictions by linear gyrokinetic simulations. In the plasma core, E ? B shearing rate stabilization of the ITG modes is significant in both machines as beta is increased. Inclusion of electromagnetic effects in the gyrokinetic calculations provides additional stabilization at ?N values achieved in the experiment. In AUG, proximity to the kinetic ballooning threshold and/or a stronger reduction in normalized ion heat flux with increasing input power are possible explanations for the constancy of at mid-radius as beta is increased.

Investigation of inter-ELM pedestal profiles in ASDEX Upgrade

Integrated data analysis and equilibrium reconstruction on a millisecond time base are used to gain edge profiles with high spatial and temporal resolution. The measured radial electric field profiles for a series of discharges with different gas fuelling levels show their minimum at the position of maximal ratio of pressure gradient and density (?p/n). The analysis of electron density and temperature profiles in between edge localized modes (ELMs) reveals a characteristic sequence of phases, starting with a fast recovery phase, a quiet pressure build-up phase and a strongly fluctuating phase before the next ELM breaks out. These phases are described in terms of profile development, the behaviour of maximal gradients as well as their positions.

Lessons learned from jointly using HTC- and HPC-driven e-Science infrastructures in fusion science

The interoperability of e-Science infrastructures like DEISA/PRACE and EGEE/EGI is an increasing demand for a wide variety of cross-Grid applications, but interoperability based on common open standards adopted by Grid middleware is only starting to emerge and is not broadly provided today. In earlier work, we have shown how refined open standards form a reference model, which is based on careful academic analysis of lessons learned obtained from production cross-Grid applications that require access to both, High Throughput Computing (HTC) resources as well as High Performance Computing (HPC) resources. This paper provides insights in several concepts of this reference model with a particular focus on the finding of using HPC and HTC resources with the fusion applications BIT1 and a cross-infrastructure workflow based on the HELENA and ILSA fusion applications. Based on lessons learned over years gained with production interoperability setups and experimental interoperability work between production Grids like EGEE, DEISA, and NorduGrid, we illustrate how open Grid standards (e.g. OGSA-BES, JSDL, GLUE2, etc) can be used to overcome several limitations of the production architecture of the EUFORIA framework paving the way to a more standards-based and thus more maintainable and efficient solution.

Corrigendum: The European Integrated Tokamak Modelling (ITM) effort: achievements and first physics results (2014 Nucl. Fusion 54 043018)

Reference EPFL-ARTICLE-202347doi:10.1088/0029-5515/54/9/099501View record in Web of Science Record created on 2014-10-23, modified on 2017-05-12

Erratum: The European Integrated Tokamak Modelling (ITM) effort: Achievements and first physics results (Nuclear Fusion (2014) 54 (043018))

Reference EPFL-ARTICLE-202347doi:10.1088/0029-5515/54/9/099501View record in Web of Science Record created on 2014-10-23, modified on 2017-05-12