Stationary ELM-free H-mode in ASDEX Upgrade

Gil, L.; Silva, C.; Happel, T.; Birkenmeier, G.; Conway, G. D.; Guimarãis, L.; Kallenbach, A.; Pütterich, T.; Santos, J.; Schneider, P. A.; Schubert, M.; Seliunin, E.; Silva, Arlindo; Stöber, J.; Stroth, U.; Trier, E.; Wolfrum, E.; Asdex,; EUROfusion,

doi:10.1088/1741-4326/ab7d1b

Cited by 42 publications

(41 citation statements)

References 19 publications

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“…Notable are L-mode discharges operating with q 95 just above 2 264,265 . The less explored regimes of EDA-H, Neg-D, and I-mode have as yet only been observed in a subset of I & B operational space, though DIII-D findings with EDA-H and I-mode are consistent with those reported in other devices 184,185,218 .…”

Section: Operational Spacesupporting

confidence: 72%

Plasma Performance and Operational Space without ELMs in DIII-D

Paz-Soldan

2020

Preprint

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A database of DIII-D plasmas without edge-localized modes (ELMs) compares the operating space and plasma performance of stationary no-ELM regimes found in conventional tokamaks: ELM suppression with resonant magnetic perturbations (RMPs), quiescent H-mode (QH, including wide-pedestal variant), improved confinement mode (I-mode), enhanced D-alpha H-mode (EDA-H), conventional L-mode, and negative triangularity L-mode (Neg-D). Operational space is documented in terms of engineering and physics parameters, revealing divergent constraints for each regime. Some operational space discriminants (such as pedestal collisionality) are well known, while others, such as low torque & safety factor, or high power & density, are less commonly emphasized. Normalized performance (confinement quality and normalized pressure) also discriminate the no-ELM regimes and favor the regimes tolerant to power in DIII-D: RMP, QH, and Neg-D. Absolute performance (volume-averaged pressure p , confinement time τ , and triple product p τ ) also discriminates no-ELM regimes and is found to rise linearly with IaB (a metric for the magnetic configuration strength, the product of current I, minor radius a, and field B that has units of force), and also benefits from tolerance to power. The highest normalized performance using the metric p τ /IaB is found in QH and RMP regimes. Focusing on ITER-shaped no-ELM plasmas, Q=10 at 15 MA scaled global performance is met with two out of four considered metrics, but only thus far at high torque. Though comparable QH and RMP performance is found, the pedestal pressure (p ped ) is very different. p ped in RMP plasmas is relatively low, and the best performance is found with a high core p fraction alongside high core rotation, consistent with an ExB shear confinement enhancement. p ped of QH plasmas is significantly higher than RMP, and QH performance does not correlate with core rotation. However, the highest QH p ped are found with high carbon fraction. While normalized performance of Neg-D plasmas is comparable to QH and RMP plasmas, the absolute confinement of Neg-D is lower, owing to low elongation and low p ped achieved thus far. Considering integration with electron cyclotron heating (ECH), the operational space for RMP and QH plasmas narrow, while that for EDA-H plasmas open, and the high p ped / p regimes (EDA-H and QH) preserve the highest performance. Only the EDA-H, Neg-D, and L-mode scenarios have approached divertor-compatible high separatrix density conditions, with Neg-D preserving the highest performance owing to its compatibility with both high power and density. Comparison to highlighted ELMing plasmas finds a clearer pedestal correlation to plasma performance, but also reveals that the peak performance of plasmas without ELMs is significantly lower in DIII-D, owing to limits in operational space accessed so far without ELMs.

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Section: Operational Spacesupporting

confidence: 72%

Plasma Performance and Operational Space without ELMs in DIII-D

Paz-Soldan

2020

Preprint

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“…A successful example is shown in the left part of figure 2, where an ELM free H-mode phase is maintained at 5 MW mixed NBI/ECRH heating power. This ELM-free H-mode regime recently discovered in ASDEX Upgrade [17] exhibits similarities to the Alcator C-mod EDA H-mode [18]. ELM suppression is in this case likely related to an electromagnetic quasicoherent mode causing enhanced transport losses and thus leading to a stable pedestal.…”

Section: Ar Injection Into Intrinsically Elm-free and Close-to Elm Fr...mentioning

confidence: 72%

“…The combination of Ar seeding with a naturally ELM-free H-mode [17] can preserve the MHD modes responsible for ELM suppression and widens the operation window towards higher heating power and beta. Integration of Ar seeding with RMP ELMsupressed H-modes appeared promising, with no unfavourable Ar accumulation at the pedestal top.…”

Section: Discussionmentioning

confidence: 99%

Developments towards an ELM-free pedestal radiative cooling scenario using noble gas seeding in ASDEX Upgrade

et al. 2020

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Integrated edge-localized-mode (ELM)-free or small ELM scenarios for the demonstration fusion power plant (DEMO) are investigated in ASDEX Upgrade using argon seeding for radiative power removal mainly in the pedestal region. An important aspect is the modification of the electron pressure in the pedestal by the additional radiative power losses. Full ELM suppression could be achieved in a no-ELM H-mode scenario featuring an edge electromagnetic quasicoherent mode up to a heating power of 5 MW, where argon radiation allowed the extension of the heating power operational space. At higher powers up to 12 MW (reaching the beta limit), ELMs of reduced size prevail and detachment is obtained by the argon seeding. Control of the position of a radiating zone localized inside the X-point was found to be favorable compared to the control of the separatrix power for low P s e p / P h e a t . Integration of pedestal cooling with Ar and ELM suppression by resonant magnetic perturbations (RMP) allowed an increase of the core radiation and a partial recovery of normalized confinement to H98 = 1. This favorable behavior is finally limited by the loss of the RMP density pump-out effect, followed by an ELM-free H-mode phase and re-occurrence of ELMs. For an active tailoring of the pedestal pressure profile, precise knowledge of the radiation profile is required. Modeling of the argon radiation profile with the STRAHL code showed good agreement with bolometry only if charge exchange of neutral deuterium with argon ions was taken into account, highlighting the importance of the neutral density in the pedestal region. In view of best integration of a no-ELM scenario with divertor detachment and high heating power, the divertor compression and enrichment of argon and neon were compared using dynamic gas puff experiments. Argon shows more than a factor of 3 higher divertor enrichment compared to neon, but the absolute values decrease with higher neutral deuterium pressure.

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“…Nonetheless, a stationary EDA H-mode inevitably transitions to a type-I ELMy H-mode upon a sufficient increase of the heating power. In AUG, the QCM moves in the electron diamagnetic direction and has fluctuation frequency in the range f ≈ 20 − 80 kHz [19]. Similarly, type-II ELMs are accompanied by broadband fluctuations (in magnetic pick-up coils and ECE signals) with frequencies in the range f ≈ 30 − 50 kHz [38].…”

Section: Small Elms and Eda Hmode At Asdex Upgradementioning

confidence: 99%

“…Naturally ELM-free operation includes regimes like QH-mode (in DIII-D [14], AUG and JET with carbon wall [15,16], and JT-60U [17]), I-mode (features a pedestal in the temperature but not in the density profile [4]), and EDA H-mode (found in Alcator C-mod [18]). The latter, i.e., the enhanced D-alpha H-mode, was recently achieved in AUG [19]. First experiments of EDA H-mode in AUG were performed at low triangularity with pure electron heating and observed a narrow operational window in terms of the applied heating power.…”

Section: Introductionmentioning

confidence: 98%

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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Stationary ELM-free H-mode in ASDEX Upgrade

Cited by 42 publications

References 19 publications

Plasma Performance and Operational Space without ELMs in DIII-D

Plasma Performance and Operational Space without ELMs in DIII-D

Developments towards an ELM-free pedestal radiative cooling scenario using noble gas seeding in ASDEX Upgrade

MHD simulations of small ELMs at low triangularity in ASDEX Upgrade

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