Broadening of the power fall-off length in a high density, high confinement H-mode regime in ASDEX Upgrade

Faitsch, M.; Eich, T.; Harrer, G.; Wolfrum, E.; Brida, D.; David, P.; Griener, M.; Stroth, U.

doi:10.1016/j.nme.2020.100890

Cited by 29 publications

(62 citation statements)

References 43 publications

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“…As it will be shown, the NBI voltage scan provides a stringent test on the consistency among the different components of the integrated model, addressing in particular the reliability of the transport assumptions adopted to describe the pedestal, and their consistency with the MHD stability. We also explore the validity of the model on the QCE regime (quasi continuous exhaust, also referred as type-II and small ELMs) [36][37][38][39] and ITER baseline scenarios [40]. It is important to understand whether the model can reproduce the experimental behavior of these operational parameters, since these are potentially relevant for future fusion reactors.…”

Section: Introductionmentioning

confidence: 99%

Validation of a full-plasma integrated modeling approach on ASDEX Upgrade

et al. 2021

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In this work we present the extensive validation of a refined version of the integrated model based on engineering parameters (IMEP) introduced in reference (Luda et al 2020 Nucl. Fusion 60 036023). The modeling workflow is now fully automated, computationally faster thanks to the reduced radial resolution of the TGLF calculation, and it includes the modeling of the toroidal rotation, which was still taken from experimental measurements in our previous work. The updated model maintains the same accuracy as its previous version when tested on the cases presented in the initial publication. The confined plasma, from the magnetic axis to the separatrix, is simulated without using any experimental information from profiles measurements, and the inputs of IMEP are the same engineering parameters used when programming a plasma discharge. The model validation database consists of 50 ASDEX Upgrade (AUG) stationary (over a few energy confinement time) H-mode phases, which largely cover the entire AUG operational domain. The prediction of IMEP is compared with experimental measurements and with scaling laws, such as the IPB98(y,2), the ITPA20-IL, and AUG specific regressions. This modeling framework has proven to be very accurate over the entire set of 50 cases, with a significantly lower mean relative error with respect to each of the scaling laws considered, accurately reproducing the change in pedestal and core confinement caused by a change in plasma current, heating power, fueling rate, triangularity, magnetic field, NBI voltage (i.e. the effect of a change in the core particle source), and heating mix (e.g. correctly predicting the effect on confinement caused by a change in T e/T i). Plasma confinement is correctly described by IMEP also for two particular operating regimes, such as the ITER baseline scenario, and the QCE regime (quasi continuous exhaust, also referred as type-II and small ELMs). This work clearly demonstrates the power of this approach in pulling out physics mechanisms to interpret subtle interdependencies and that a 1D integrated model can reproduce experimental results over very large parameter variations with a higher accuracy than any statistical regression. This approach has therefore the potential to improve the prediction of the fusion performance in future tokamak reactors.

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Section: Introductionmentioning

confidence: 99%

Validation of a full-plasma integrated modeling approach on ASDEX Upgrade

et al. 2021

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“…These modes give rise to a quasicontinuous exhaust of heat and particles which impinge onto the divertor. It has been found that the power fall-off length is larger during the QCE regime than expected from the empirical Eich scaling [8]. Other small ELM regimes that can be free of type-I ELMs include grassy ELMs (observed in JT-60U [9], JET [10], EAST and it is foreseen as a potential operational regime for CFETR [11]), type-III ELMs (not reactor-relevant because they cause confinement degradation [12]), and a low density small ELM regime in JET [13].…”

Section: Introductionmentioning

confidence: 87%

“…The QCE regime is posited to be an attractive scenario for ITER because it completely avoids type-I ELMs while maintaining good confinement properties. Another favourable feature of this regime is that it deposits the expelled energy in a quasi-continuous manner and in a broader area than observed in the inter type-I ELM phase [8]. Small ELMs act as a transport mechanism that expels heat and particles such that the pedestal cannot build-up to a point where type-I ELMs are excited; it is presently hypothesised that small ELMs are ballooning modes and/or high-n peeling-ballooning modes that are located at, or very near, the magnetic separatrix.…”

Section: Small Elms and Eda Hmode At Asdex Upgradementioning

confidence: 93%

MHD simulations of small ELMs at low triangularity in ASDEX Upgrade

Cathey,

Hoelzl,

Harrer

et al. 2021

Preprint

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The development of small-and no-ELM regimes for ITER is a high priority topic due to the risks associated to type-I ELMs. By considering non-linear extended MHD simulations of the ASDEX Upgrade tokamak with the JOREK code, we probe a regime that avoids type-I ELMs completely provided that the separatrix density is high enough. The dynamics of the pedestal in this regime are observed to be qualitatively similar to the so-called quasi-continuous exhaust (QCE) regime in several ways. Repetitive type-I ELMs are substituted by roughly constant levels of outwards transport caused by peelingballooning modes (with dominant ballooning characteristics) which are localised in the last 5% of the confined region (in normalised poloidal flux). The simulated low triangularity plasma transitions to a type-I ELMy H-mode if the separatrix density is sufficiently reduced or if the input heating power is sufficiently increased. The stabilising factors that play a role in the suppression of the small ELMs are also investigated by analysing the simulations, and the importance of including diamagnetic effects in the simulations is highlighted. By considering a scan in the pedestal resistivity and by measuring the poloidal velocity of the modes (and comparing to theoretical estimates for ideal and resistive modes), we identify the underlying instabilities as resistive peeling-ballooning modes. Decreasing the resistivity below experimentally-relevant conditions (i.e., going towards ideal MHD), the peeling-ballooning modes that constrain the pedestal below the type-I ELM stability boundary display sharply decreasing growth rates.

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“…The quasi-continuous exhaust (QCE) regime was observed at the pedestal foot in the case with high separatrix density by gas fuelling (Harrer et al 2018(Harrer et al , 2022. The power fall-off length in the QCE regime measured with the infrared diagnostic in the divertor is increased by a factor of 4, when compared with the scaling for the inter-ELM power exhaust profile width (Faitsch et al 2021). In JET, a Baseline Small ELM (BSE) regime has been found, where simultaneous access to small ELMs, high confinement and high D-D fusion rate were achieved without core impurity accumulation (Garcia et al 2022).…”

Section: Introductionmentioning

confidence: 97%

“…Recently, experiments showed that H-mode plasma regimes with small/grassy ELMs offer a potential solution for core-edge-integration to future tokamak reactors (Viezzer et al 2023). In addition, DIII-D and ASDEX Upgrade experiments showed that the heat flux width is broadened with quasi-continuous particle and power exhaust to the divertors, and the peak divertor heat flux is much smaller for small/grassy ELMs (Nazikian et al 2018;Xu 2020;Faitsch et al 2021).…”

Section: Introductionmentioning

confidence: 99%

Turbulence spreading effects on the ELM size and SOL width

Li,

Xu,

Diamond

et al. 2024

J. Plasma Phys.

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BOUT++ turbulence simulations were performed to investigate the impact of turbulence spreading on the edge localized mode (ELM) size and divertor heat flux width $({\lambda _q})$ broadening in small ELM regimes. This study is motivated by EAST experiments. BOUT++ linear simulations of a pedestal radial electric field (Er) scan show that the dominant toroidal number mode (n) shifts from high-n to low-n, with a narrow mode spectrum, and the maximum linear growth rate increases as the pedestal Er well deepens. The nonlinear simulations show that as the net E × B pedestal flow increases, the pressure fluctuation level and its inward penetration beyond the top of the pedestal both increase. This leads to a transition from small ELMs to large ELMs. Both inward and outward turbulence spreading are sensitive to the scrape-off-layer (SOL) plasma profiles. The inward turbulence spreading increases for the steep SOL profiles, leading to increasing pedestal energy loss in the small ELM regime. The SOL width $({\lambda _q})$ is significantly broadened progressing from the ELM-free to small ELM regime, due to the onset of strong radial turbulent transport. The extent of the SOL width $({\lambda _q})$ broadening depends strongly on outward turbulence spreading. The fluctuation energy intensity flux ${\varGamma _\varepsilon }$ at the separatrix can be enhanced by increasing either pedestal Er flow shear or local SOL pressure gradient. The ${\lambda _q}$ is broadened as the fluctuation energy intensity flux ${\varGamma _\varepsilon }$ at the last close flux surface (LCFS) increases. Local SOL E × B flow shear will restrain outward turbulence spreading and the associated heat flux width broadening. Operating in H-mode with small ELMs has the potential to solve two critical problems: reducing the ELM size and broadening the SOL width.

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Broadening of the power fall-off length in a high density, high confinement H-mode regime in ASDEX Upgrade

Cited by 29 publications

References 43 publications

Validation of a full-plasma integrated modeling approach on ASDEX Upgrade

Validation of a full-plasma integrated modeling approach on ASDEX Upgrade

MHD simulations of small ELMs at low triangularity in ASDEX Upgrade

Turbulence spreading effects on the ELM size and SOL width

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