D. Kalupin

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.

Modelling of pedestal transport during ELM suppression by external magnetic field perturbations

Particle and energy transport in the edge transport barrier is analysed in the presence of magnetic field perturbations from external resonant coils successfully used recently for the mitigation of type I edge localized modes (ELMs). The modification of transport due to charged particle and heat flows along perturbed field lines in a small region near the separatrix, spanning from 2% to 4% of the total poloidal flux, where complete stochastization is provided by the overlap of the main magnetic islands, is taken into account. The observed reduction of the density in plasmas of low collisionality is explained by the generation of charged particle flows along perturbed field lines, the increase in the electron and ion temperatures in the barrier—by the reduction of the perpendicular neoclassical transport with decreasing density and non-locality of parallel heat transport. On the basis of the heat flux limit concept in a deeply collisionless regime, the parallel thermal conductivities are taken to be 17 times smaller for electrons and 7 times smaller for ions than from a standard free-streaming estimate. The model elaborated before is developed further by taking into account the radial variation of the inclination angle of stochastic field lines and convective energy losses including the acceleration of ions by the pressure gradient and ambipolar electric field. It is demonstrated that convection of parallel kinetic energy of ions gives greater losses than parallel thermal conduction in the outer 50% of the stochastic layer and its inclusion improves the agreement with experimental results. This modelling is performed by assuming in agreement with observations that the influx of recycling neutrals through the separatrix is not reduced with I-coils compared with its level between ELMs before the mitigation stage. By trying to match experimental profiles with this influx decrease, some enhanced thermal losses of another nature than that considered here are needed in order to mitigate the drop in the perpendicular thermal conductivities for the assumed density scaling. The impact of the neutral particle influx increase by gas puffing applied in order to restore the plasma density is investigated.

Numerical analysis of JET discharges with the European Transport Simulator

The European Transport Simulator (ETS) (Coster et al 2010 IEEE Trans. Plasma Sci. 38 2085-92, Kalupin et al 2011 Proc. 38th EPS Conf. on Plasma Physics (Strasbourg, France, 2011) vol 35G (ECA) P. 4.111) is the new modular package for 1D discharge evolution developed within the EFDA Integrated Tokamak Modelling (ITM) Task Force. It consists of precompiled physics modules combined into a workflow through standardized input/output data structures. Ultimately, the ETS will allow for an entire discharge simulation from the start up until the current termination phase, including controllers and sub-systems. The paper presents the current status of the ETS towards this ultimate goal. It discusses the design of the workflow, the validation and verification of its components on the example of impurity solver and demonstrates a proof-of-principles coupling of a local gyrofluid model for turbulent transport to the ETS. It also presents the first results on the application of the ETS to JET tokamak discharges with the ITER like wall. It studies the correlations of the radiation from impurity to the choice of the sources and transport coefficients.

Predictive modelling of L and H confinement modes and edge pedestal characteristics

The results of predictive self-consistent modelling of plasma parameters in low (L) and high (H) confinement modes by the one-dimensional transport code RITM, with particular emphasis on the properties of the edge transport barrier, are presented and discussed. The same transport model is used under both L- and H-mode conditions and includes contributions from ion temperature gradient (ITG), trapped electron, drift Alfven (DA) and drift resistive ballooning instabilities described in the fluid approximation. The computations predict the formation of the edge transport barrier at a high enough heating power due to stabilization of ITG and DA modes, dominating the edge transport in the L-mode, through the effects of the density gradient and the pressure gradient at low collisionality, respectively. The calculated radial profiles and scalings for pedestal and confinement characteristics are compared with measurements on JET, DIII-D and JT-60U tokamaks.

The European Transport Solver

The status of the European Transport Solver, a new 1-D core transport code that is being developed by the members of Integrated Modelling Project 3 (“Transport Code and Discharge Evolution”) of the EFDA Task Force on Integrated Tokamak Modelling (ITM), is described. The approach taken by the ITM is to couple codes so that the only exchange is via well-specified data structures (Consistent Physical Objects), with the aim of having the workflow managed by Kepler, a scientific workflow engine.

Numerical solution of transport equations for plasmas with transport barriers

An approach to solve numerically transport equations for plasmas with spontaneously arising and arbitrarily located transport barriers, regions with a strongly reduced transfer of energy, is proposed. The transport equations are written in a form conserving heat flux and solved numerically by using piecewisely exact analytical solutions of linear differential equations. Compared to standard methods, this approach allows to reduce significantly the number of operations required to obtain a converged solution with a heat conductivity changing abruptly at a critical temperature gradient and to use large time steps in modeling the formation and dynamics of transport barriers. Computations for the tokamak JET are done.

Influence of the boundary conditions on the H-mode power threshold

The effect of boundary conditions at the last closed magnetic surface (LCMS) on the formation of the edge transport barrier (ETB) in tokamaks is investigated by one-dimensional transport calculations for the radial profiles of plasma parameters. For a given heating power the transition from the low confinement mode (L mode) to the high confinement mode (H mode) can be triggered by increasing the density e-folding length, δn, or reducing the temperature e-folding length, δT, at the LCMS. This is explained by the decrease of heat losses from the confined plasma with the convection of charged particles and changeover to the case where losses are mostly due to heat conduction. In such a case, corresponding to a divertor configuration, the computed power threshold for the L- to H-mode transition (L-H transition) is in a good agreement with the experimental multimachine scaling.

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

Modelling of radiative power exhaust by sputtered and seeded impurities in fusion reactors with carbon and molybdenum target plates

Argon and neon seeded ITER discharges are numerically simulated by coupling a 1D multifluid model for the plasma core with a 2D model for the SOL-divertor region. The model is fully self-consistent with respect to both the effects of impurities on the α-power level and the interaction between seeded and intrinsic impurities. This interaction leads to a significant change in the intrinsic impurity fluxes, and it is found to be essential for a correct evaluation of the average power to the target plates. Even though carbon and tungsten are the real candidates for the ITER target plates, we have compared carbon and molybdenum plates for two ITER inductive scenarios, due to the uncertainties in the tungsten atomic data. In general, the integrated edge-core scenarios with impurity seeding are more favourable in the case of carbon than of molybdenum plates. However, a high density/low confinement scenario with Ne seeding is found for which the requirements in terms of power to the plate and of power through the separatrix are also fulfilled in the case of Mo. The interplay mechanism among different impurities also holds for He ash resulting in a rather weak dependence of the power amplification factor on He confinement.