Gyrokinetic investigation of the ASDEX Upgrade I-mode pedestal

Stimmel, K.; Navarro, A. Bañón; Happel, T.; Told, D.; Görler, T.; Wolfrum, E.; Collar, J. P. Martin; Fischer, R.; Schneider, P. A.; Jenko, F.

doi:10.1063/1.5124986

Cited by 12 publications

(13 citation statements)

References 30 publications

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“…As the I-mode develops it will transit to more drift-wave characteristics with increasing temperature (decreasing collisionality). By further increasing the temperature (increasing plasma beta) the WCM may feature micro-tearing turbulence (as observed in linear gyrokinetic simulations [38]). The reduction of coherency at larger scales let the WCM appear to be more coherent than the ambient turbulence.…”

Section: Discussionmentioning

confidence: 81%

Physical mechanism behind and access to the I-mode confinement regime in tokamaks

et al. 2020

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The I-mode is an attractive confinement regime for future tokamak based fusion reactors. A model is presented which explains the I-mode regime by the reduction of ITG turbulence near the separatrix at low collisionality, where the separatrix ion temperature can exceed the electron temperature. Drift-Alfvénturbulence develops, with large and small-scale fluctuations being suppressed by phase randomization and finite-Larmor-radius effects, respectively. The intermediate scales form a broad peak in the frequency spectrum, which features the same properties as the characteristic weakly coherent mode. The model, which is studied by means of gyrofluid simulations, reproduces a number of other experimental I-mode observations, such as decoupled energy and particle transport, intermittent turbulent bursts with precursors, an operational window widening with magnetic field strength, and the challenges met when detaching the plasma.

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Section: Discussionmentioning

confidence: 81%

Physical mechanism behind and access to the I-mode confinement regime in tokamaks

et al. 2020

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“…2019; Stimmel et al. 2019), scanning along increasing with fixed geometry does not reveal instabilities which are bordering a highly unstable kinetic ballooning mode (KBM)-like range at as shown in figure 5. While a critical threshold is observed at higher at , the proximity of the nominal is at least a factor of two away from increased linear growth rates which are indicative of a KBM in both pedestal top scenarios in figure 5.…”

Section: Linear Instability Studymentioning

confidence: 87%

“…2019; Stimmel et al. 2019). However, in this case even setting to zero together with a high estimate of does not recover the experimental heat flux, suggesting the strong relevance of finite-size effects.…”

Section: Nonlinear Gene Simulationsmentioning

confidence: 99%

“…Neoclassical calculations in GENE are based on a linearised solver described previously in Doerk (2012), Oberparleiter (2015) and Stimmel et al. (2019) and are here employing the Landau collision operator.…”

Section: Set-up Of Gyrokinetic Modellingmentioning

confidence: 99%

“…2019; Stimmel et al. 2019) such as L- or I-mode, and even to some degree in an H-mode inter-ELM pedestal scenario (Hatch et al. 2015).…”

Section: Nonlinear Gene Simulationsmentioning

confidence: 99%

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Gyrokinetic analysis of an argon-seeded EDA H-mode in ASDEX Upgrade

Stimmel

Gil²,

Görler³

et al. 2022

J. Plasma Phys.

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Understanding edge-localised-mode (ELM)-free high-confinement (H-)mode scenarios is vital for developing practical future demonstration reactor scenarios. An argon-seeded EDA H-mode discharge performed in ASDEX Upgrade is computationally studied in detail for the first time with the gyrokinetic GENE code using experimental profiles and magnetic equilibrium as direct code inputs. Linear scans outline dominant instabilities in the regime and reveal distinct ion- and electron-scale wavenumber growth-rate peaks for two local core and two local pedestal top scenarios. Linear ion-scale growth rates are found to be relatively insensitive to the addition of argon, and collisionality scans demonstrate increased sensitivity in the pedestal top. The addition of an argon impurity profile while keeping the input main ion temperature gradient (ITG) largely unchanged is found to reduce ITG-driven turbulence in the outer core. Nonlinear electromagnetic simulations reveal close agreement with experimentally predicted heat fluxes in the core, outline key sensitivities to electron $\beta$ and background $\boldsymbol{E\times B}$ shearing, and reveal gyrokinetic challenges in analysing the quasicoherent mode. Global electrostatic nonlinear simulations reduce local simulated heat transport overpredictions at the pedestal top. A quasilinear analysis finds that there is good core agreement but poor agreement in the pedestal between linear and nonlinear temperature and density fluctuation cross-phases. Local simulation limitations are elucidated and paths forward for future computation are suggested.

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Edge turbulence measurements in L-mode and I-mode at ASDEX Upgrade

et al. 2022

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The I-mode confinement regime is promising for future reactor operation due to high energy confinement without high particle confinement. However, the role of edge turbulence in creating I-mode's beneficial transport properties is still unknown. New measurements of edge turbulence ([Formula: see text]) in L-modes and I-modes at low and high densities at ASDEX Upgrade are presented in this paper. A high radial resolution correlation electron cyclotron emission radiometer measures the broadband turbulence throughout the L-mode and I-mode edge and pedestal. The weakly coherent mode (WCM) is measured in both L-mode and I-mode near the last closed flux surface with Te fluctuation levels of 2.3%–4.2%, with a frequency shift between the two phases related to a deeper Er well in I-mode. An [Formula: see text] phase diagnostic captures a change of the WCM [Formula: see text] phase between L-mode and I-mode from [Formula: see text] to [Formula: see text]. The thermal He beam diagnostic measures a WCM wavenumber range of −0.5 to −1.0 cm−1. A low-frequency edge oscillation (LFEO) appears in the I-mode phase of these discharges and displays coupling to the WCM, but the LFEO does not appear in the L-mode phase. Linear gyrokinetic simulations of the outer core and pedestal top turbulence indicate that while the dominant turbulent modes in the outer core are ion directed and electrostatic, the turbulence becomes increasingly electron directed and electromagnetic with increasing radius. Collisionality is not found to impact characteristics of the L-mode and I-mode edge turbulence with respect to the presence of the WCM; however, the quality of global confinement decreases with collisionality.

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Gyrokinetic investigation of the ASDEX Upgrade I-mode pedestal

Cited by 12 publications

References 30 publications

Physical mechanism behind and access to the I-mode confinement regime in tokamaks

Physical mechanism behind and access to the I-mode confinement regime in tokamaks

Gyrokinetic analysis of an argon-seeded EDA H-mode in ASDEX Upgrade

Edge turbulence measurements in L-mode and I-mode at ASDEX Upgrade

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