Core-edge Coupling Research Articles

Toward the core-edge coupling of delta-f and total-f gyrokinetic models

The coupling of core reduced delta-f and edge total-f gyrokinetic models could enable a significant speed-up of the whole device modeling simulations. It is thus interesting to compare these models in the core where they will be coupled. The main difference between these models is the presence of the zeroth order term on the right-hand side of the total-f gyrokinetic equation. This term is responsible for driving a radial electric field that is also associated with Geodesic acoustic mode-like oscillations. To investigate the coupling between these models, the subsequent large-scale gyrokinetic 3D turbulence simulations have been performed starting from an already saturated gyrokinetic axisymmetric equilibria (collisions are not included). This work has also been extended to couple different axisymmetric and turbulent models available in XGC. It is shown that the radial electric field and its drive have to be taken care of consistently while coupling different models together.

Confinement improvement during detached phase with RMP application in deuterium plasmas of LHD

In order to explore the compatibility of good core plasma performance with divertor heat load mitigation, the interaction between cold edge plasma and core plasma transport, including the edge transport barrier (ETB), has been analysed in the divertor detachment discharges of deuterium plasmas in LHD with resonant magnetic perturbation (RMP) field application. The RMP application introduces a widened edge stochastic layer and sharp boundary in the magnetic field structure between the confinement region and the edge stochastic layer. The widened edge stochastic layer enhances impurity radiation and provides stable detachment operation as compared with the case without RMP. It is found that ETB is formed at the confinement boundary at the onset of detachment transition. However, as the detachment deepens, the resistive pressure gradient-driven MHD mode is excited, which degrades the ETB. At the same time, however, the core transport decreases to keep global plasma stored energy (W p) unchanged, showing clear core-edge coupling. After a gradual increase of density fluctuation during the MHD activity, a spontaneous increase of W p and the recovery of ETB are observed while the detachment is maintained. Then, the coherent MHD mode ceases and ELM-like bursts appear. In the improved mode, impurity decontamination occurs, and the divertor heat load increases slightly. Key controlling physical processes in the interplay between core and cold edge plasma are discussed. A comparison between deuterium and hydrogen plasmas shows that hydrogen plasmas exhibit similar features to the deuterium ones in terms of density and magnetic fluctuations, impurity decontamination towards higher confinement, etc. But most of the features are modest in the hydrogen plasmas and thus no clear confinement mode transition with clear ETB formation is defined. Better global confinement is obtained in the deuterium plasmas than the hydrogen ones at a higher radiation level.

Spatial core-edge coupling of the particle-in-cell gyrokinetic codes GEM and XGC

Two existing particle-in-cell gyrokinetic codes, GEM for the core region and XGC for the edge region, have been successfully coupled with a spatial coupling scheme at the interface in a toroidal geometry. A mapping technique is developed for transferring data between GEM's structured and XGC's unstructured meshes. Two examples of coupled simulations are presented to demonstrate the coupling scheme. The optimization of GEM for graphics processing unit is also presented.

Evaluation of tritium burnup fraction for CFETR scenarios with core-edge coupling simulations

A key mission for the next-step fusion tokamak device China Fusion Engineering Test Reactor (CFETR) is demonstrating tritium self-sufficiency, which requires a sufficiently high tritium burnup fraction (fburnup) in order to match a practically achievable tritium breeding ratio (TBR) with the blanket design constraints. Core-edge coupling simulations are performed to investigate the dependence of fburnup on different controlling parameters for CFETR scenarios. Core plasma profiles with a range of pedestal densities are simulated by consistent iterative calculations of equilibrium, transport, auxiliary heating and current drives within the OMFIT framework. The core-SOL integrated COREDIV code is then used to evaluate fburnup with the OMFIT modelled core plasma parameters as input. According to the simulations, fburnup can be effectively increased with a higher pedestal density on account of the fusion power increasing faster than the fueling source required to maintain steady-state. Higher can also increase the fburnup due to increase of fuel recycling. Deeper pellet fueling deposition and lower ratio of particle to thermal diffusivities D/χ can both increase the effective particle confinement time and thus fburnup. However, the effect of helium and other impurities (Ar and W) is shown to reduce fburnup for comparable impurity and main ion transport. Based on our analysis, using present pellet fueling technology, achieving fburnup > 3% for CFETR will be very challenging. This is a lower limit for the required TBR (>1) to match the achievable TBR for tritium self-sufficiency. Furthermore, our study suggests that if fueling can penetrate deeper than r/a < 0.8 under optimistic conditions, the required burnup fraction could be attainable. The modelling results thus provide important suggestions and implications for the optimization of CFETR scenarios and development of advanced fueling systems.

Mesoscopic Transport Events and the Breakdown of Fick’s Law for Turbulent Fluxes

This paper presents a pedagogical review of the physics of mesoscopic transport events and their role in the breakdown of Fick’s Law for turbulent transport in magnetically confined plasma. It is now clear that the conventional picture of localized turbulence and quasi-linear calculation of fluxes fails to address and account for the phenomenology of tokamak transport. One key issue is the observed departure from the expected gyro-Bohm transport scaling. The causes of this breakdown of Fickian thinking include turbulent avalanching and pulse propagation (turbulence spreading). Both are mesoscopic transport events, and both tend to de-localize the flux–gradient relation. Turbulence spreading is the process of self-scattering and expansion of a slug or other local exciton of turbulence. Spreading is described by theoretically-motivated, phenomenological reaction–diffusion models for the turbulence activity (intensity) field, much in the spirit of Ginzburg–Landau theory. Such models imply that spreading will occur by propagation of intensity fronts. After discussing the basic theory, this paper presents several critical tests of turbulence spreading models using gyrokinetic simulation. Applications include rho-star scaling, penetration of transport barriers and core-edge coupling. Relevant experiment–theory comparisons are addressed, as well. Avalanching refers to a process whereby correlated topplings of nearby localized cells overturn sequentially and drive a burst of transport. Avalanching is a process intrinsic to systems that support a broad range of scales l between a cell size Δ and system size L, i.e. Δ < l < L. Avalanching is also a natural way to produce transport events on scales that exceed the cell size or correlation length. Therefore, the PDF (probability distribution function) of avalanches as a function of l is a crucial quantity, necessary for predicting confinement in a system like ITER, with a very large-scale separation between L and Δ. Avalanching emerged from the theory of selforganized criticality but is a more general phenomenon. The paper traces the intellectual prehistory of avalanching through the advent of self-organized criticality. Special focus is devoted to reduced continuum models of avalanching. The physics of avalanching in confined plasma is discussed in detail, via several multi-faceted comparisons to flux-driven fluid and gyrokinetic simulations. The dominance of bursty, large transport events in the flux is identified. Evidence for avalanching in basic and confinement experiments is summarized. The paper concludes with sections on selected special topics, a discussion of the relation between turbulence spreading and avalanching, and a list of possible future directions. Throughout the paper, an effort is made to set fusion theory and phenomenology in the context of ideas discussed in the broader scientific community.

Simulation Study of Large Power Handling in the Divertor for CFETR Phase II

The Chinese Fusion Engineering Testing Reactor (CFETR) is a next fusion device proposed in China. The ability to exhaust the plasma power loss is a critical issue for the successful production of a fusion power reactor. For Phase II of CFETR with fusion power ~1 GW, the extremely large divertor heat load would be a severe challenge to achieve steady-state operation. In this paper, we have simulated the divertor performance by using SOLPS5.0 (B2.5-EIRENE) code package for a lower single null divertor configuration based on a core self-consistent steady-state scenario with Ar seeding. The core self-consistent steady-state scenario is calculated by a 1.5-D modeling workflow utilizing the One Modeling Framework for Integrated Tasks (OMFIT) framework. The modeling shows that Ar puffing from the lower divertor region together with D2 puffing from upstream is highly effective in mitigation of the divertor peak heat flux and temperature for the whole outer target and near strike point on the inner divertor target, however, electron and ion temperature ( $T_{e}$ and $T_{i})$ at far scrape-off layer on inner divertor is still higher than the erosion mitigation. In addition, comparison simulation of different divertor geometries has been performed in this paper. Further work on self-consistent core-edge coupling to estimate the effect of impurity seeding on core performance as well as the optimization of divertor geometry and operations will be studied.

Self‐Consistent COREDIV Modelling of WEST Plasma Scenarios

AbstractThe importance of core‐edge coupling in the WEST tokamak via tungsten production and core radiation is investigated with the COREDIV code. We first focus on the idealized situation where the plasma is free of light impurities, and show that for achievable high density discharges tungsten production can be essentially extinguished. For lower densities, core edge coupling is strong, that is tungsten plays a major role in regulating the power flowing through the Scrape‐off layer. The latter is found to saturate with increasing heating power due to increasing radiation losses in the core. Then we investigate the extent to which light impurities change this picture by performing a scan in boron concentration, since Ion Cyclotron Resonance Heating antenna protections may be boron coated. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Comparing 1.5D ONETWO and 2D SOLPS analyses of inter-ELM H-mode plasma in DIII-D

A DIII-D inter-ELM H-mode plasma that is in approximate transport equilibrium is analysed with the 1.5D ONETWO core code and the 2D SOLPS code. In order to investigate the importance of core–edge coupling and 2D effects, including divertor fuelling across the X-point and poloidal asymmetries that are not explicitly included in ONETWO, the domain of SOLPS is extended to very near the magnetic axis. Two principal objectives are (1) to determine whether poloidal asymmetries in the plasma distributions are large enough to vitiate a core-type interpretive plasma transport analysis and (2) to determine whether the interpretive transport coefficients and neutral beam power and particle sources from ONETWO, when used in 2D SOLPS full plasma simulations, yield the same quality fits to the measured upstream density and temperature profiles as obtained with ONETWO. Results show that only a small increase in the separatrix value of the particle diffusion coefficient, and no change in the thermal diffusivities from ONETWO was needed to get excellent agreement of the upstream SOLPS density and temperature profiles and the Thomson scattering and CER data. Good agreement of the ONETWO and SOLPS flux surface averaged distributions of the core electron and D+ densities and temperatures are also obtained. Likewise the C6+ density, with a simple chemical sputtering model based on a constant fraction of the divertor D+ flux, the core heat and particle fluxes and the neutral density reveal no 2D effects in the core/pedestal region that would vitiate a 1.5D treatment of the inter-ELM H-mode plasma.

Whole-volume integrated gyrokinetic simulation of plasma turbulence in realistic diverted-tokamak geometry

Performance prediction for ITER is based upon the ubiquitous experimental observation that the plasma energy confinement in the device core is strongly coupled to the edge confinement for an unknown reason. The coupling time-scale is much shorter than the plasma transport time-scale. In order to understand this critical observation, a multi-scale turbulence-neoclassical simulation of integrated edge-core plasma in a realistic diverted geometry is a necessity, but has been a formidable task. Thanks to the recent development in high performance computing, we have succeeded in the integrated multiscale gyrokinetic simulation of the ion-temperature-gradient driven turbulence in realistic diverted tokamak geometry for the first time. It is found that modification of the self-organized criticality in the core plasma by nonlocal core-edge coupling of ITG turbulence can be responsible for the core-edge confinement coupling.

06/01178 Influence of core-edge coupling and impurities on the operation regimes of a fusion reactor: Stankiewicz, R. and Zagórski, R. Journal of Nuclear Materials, 2005, 337-339, 191-195

06/01178 Influence of core-edge coupling and impurities on the operation regimes of a fusion reactor: Stankiewicz, R. and Zagórski, R. Journal of Nuclear Materials, 2005, 337-339, 191-195

Influence of core–edge coupling and impurities on the operation regimes of a fusion reactor

The extended version of the COREDIV code has been used to analyze the combined effect of different impurity species on the ITER–FEAT operation regimes. The core plasma is treated in the frame of 1D radial transport model whereas in the edge the 2D multifluid code EPIT is used. The steady states of ITER–FEAT reactor have been studied numerically and the influence of the core–edge coupling due to the production of the sputtered impurities has been analyzed for nickel and carbon plasma facing components. The effect of injected impurities on the divertor operation and heat removal is addressed.

Effect of core-edge coupling on operation regimes of ITER-like reactor

A model has been developed which is capable to describe in a self-consistent way plasma dynamics in the center and edge region of fusion reactor. The core plasma is treated in the frame of 1D radial transport model whereas in the edge the 2D multifluid code EPIT has been used. The EPIT code solves in the slab geometry the MHD equations for densities and velocities for all ions species as well as for electron and ion temperatures. In the paper the iteration scheme leading to steady state solution of coupled core and SOL system has been analyzed and tested numerically. The model has been used to investigate operation regimes of the ITER-FEAT tokamak.

Core-edge coupling and the effect of the EDGE on overall plasma performance

Several attempts to model the entire plasma cross section have been reported in the last few years. Two possibilities are to either couple a core code to a scrape-off layer (SOL) code at a specified interface or to extend the computational region of an SOL-code all the way to the plasma centre. The most advanced global code is the code COCONUT which is based on the former principle and comprises the Monte-Carlo code NIMBUS, the 2D scrape-off layer code EDGE2D, the core transport code JETTO and the core impurity transport code SANCO. A main feature of COCONUT is its modular structure which ensures a high degree of flexibility and the capability to cover a large range of time-scales. The influence of the SOL on the core is lllustrated with a range of global simulations carried out with COCONUT. The simulations show that the primary effect of the SOL is the control of the particle sources and sinks with a secondary effect on plasma dilution, radiation and perhaps pedestal temperatures.