Prospects of core–edge integrated no-ELM and small-ELM scenarios for future fusion devices

Viezzer, E.; Austin, M. E.; Bernert, M.; Burrell, K.H.; Cano-Megias, P.; Chen, X.; Cruz-Zabala, D. J.; Coda, S.; Faitsch, M.; Février, O.; Gil, L.; Giroud, C.; Happel, T.; Harrer, G.; Hubbard, A.E.; Hughes, J.W.; Kallenbach, A.; Labit, B.; Merle, A.; Meyer, H.; Paz-Soldan, C.; Oyola, P.; Sauter, O.; Siccinio, M.; Silvagni, D.; Solano, E. R.

doi:10.1016/j.nme.2022.101308

Cited by 27 publications

(16 citation statements)

References 112 publications

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“…Simultaneous control of large ELMs and divertor heat loads in an H-mode plasma are crucial for steady-state operation of a tokamak fusion reactor. 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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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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“…The same problem exists for regimes of operation that are either completely devoid of large ELMs or show only small ELMs that may be tolerable by the plasma facing components. Such no-and small-ELM regimes host some transport mechanism that prevents the edge pressure gradient and current density to grow unconstrained (which constitutes the reason why ELMs become excited) and flushes unwanted impurities out of the confined plasma [5][6][7].…”

Section: Introductionmentioning

confidence: 99%

Probing non-linear MHD stability of the EDA H-mode in ASDEX Upgrade

et al. 2023

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Regimes of operation in tokamaks that are devoid of large ELMs have to be better understood to extrapolate their applicability to reactor-relevant devices. This paper describes non-linear extended MHD simulations that use an experimental equilibrium from an EDA H-mode in ASDEX Upgrade. Linear ideal MHD analysis indicates that the operational point lies slightly inside of the stable region. The non-linear simulations with the visco-resistive extended MHD code, JOREK, sustain non-axisymmetric perturbations that are linearly most unstable with toroidal mode numbers of ${n=\{6\dots9\}}$, but non-linearly higher and lower $n$ become driven and the low-n become dominant. The poloidal mode velocity during the linear phase is found to correspond to the expected velocity for resistive ballooning modes. The perturbations that exist in the simulations have somewhat smaller poloidal wavenumbers (${k_\theta\sim0.1~\mathrm{to}~0.5~\mathrm{cm^{-1}}}$) than the experimental expectations for the quasi-coherent mode in EDA, and cause non-negligible transport in both the heat and particle channels. In the transition from linear to non-linear phase, the mode frequency chirps down from approximately 35~kHz to 13~kHz, which corresponds approximately to the lower end of frequencies that are typically observed in EDA H-modes in ASDEX Upgrade.

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“…In this configuration the H-mode power threshold increases, and an operational window for the I-mode opens up. In this way, the I-mode has been achieved in several devices (Alcator C-Mod [2], ASDEX Upgrade (AUG) [8], DIII-D [9], EAST [10], KSTAR [11]), with different fueling species (deuterium, hydrogen and helium) [12,13] and in a wide range of plasma parameters, such as edge safety factor, triangularity, pedestal top collisionality and plasma beta poloidal [14]. Nonetheless, the I-mode still needs to be proven to be compatible with the strict requirements of a DEMO operational scenario.…”

Section: Introductionmentioning

confidence: 99%

“…This is required to maximize fusion energy production. The I-mode is typically achieved at Greenwald fractions f GW = n/n GW < 0.5 [14] and, as yet, has never been obtained in combination with pellet fueling.…”

Section: Introductionmentioning

confidence: 99%

Pellet-fueled I-mode plasmas in ASDEX Upgrade

et al. 2023

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This letter reports on the efforts carried out at the ASDEX Upgrade tokamak to integrate I-mode plasmas with pellet fueling and to increase the I-mode Greenwald fraction fGW, two important requirements for any DEMO operational scenario. For the first time, stationary I-mode plasmas have been achieved with pellet fueling and the core Greenwald fraction has been increased up to 0.8. Larger fGW were not achieved due to technical constraints rather than to a physics-based limit. Pellet-fueled I-mode plasmas exhibit enhanced core and edge density, while core and edge temperature are reduced. Nonetheless, edge normalized gradients remain I-mode-like, namely shallow for the density and steep for the temperature. The I-mode energy confinement time is found to obey two distinct density dependencies: for fGW < 0.4 it rises with increasing fGW , while for fGW > 0.4 the energy confinement time plateaus with increasing fGW , or even decreases with fGW for the pellet-fueled plasmas. Similarities with Ohmic and L-mode energy confinement time dependency on density are discussed.

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Prospects of core–edge integrated no-ELM and small-ELM scenarios for future fusion devices

Cited by 27 publications

References 112 publications

Turbulence spreading effects on the ELM size and SOL width

Turbulence spreading effects on the ELM size and SOL width

Probing non-linear MHD stability of the EDA H-mode in ASDEX Upgrade

Pellet-fueled I-mode plasmas in ASDEX Upgrade

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