Physical mechanisms for the transition from type-III to large ELMs induced by impurity injection on EAST

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It is necessary to achieve simultaneous exhaust of excessive transient and steady-state heat fluxes on the divertor target for the divertor protection in the future fusion reactors. The sustained large ELM control and stable partial detachment have been achieved concurrently with argon (Ar) or neon (Ne) seeding in EAST. With Ne seeding, the large ELMs with frequency fELM ~ 100 Hz disappear and a stable ELM-free state with H98,y2 > 1 is maintained. Meanwhile, the electron temperature Tet around the lower outer strike point decreases from more than 70 eV during the large ELM burst to less than 5 eV in the stable ELM-free phase. In addition, a slight improvement of plasma confinement is observed in the partially detached state, mainly attributed to the increased electron density ne and ion temperature Ti in the core region. In the pedestal region, the density gradient and the electron temperature show subtle variation. The effective charge number Zeff increases significantly after Ne seeding, leading to a decrease in the edge bootstrap current and the pedestal pressure gradient, and thus the stabilization of ELMs. With Ar seeding, the large ELMs are also suppressed at first, but soon transit to type-III ELMs with a high fELM ~ 1 kHz, highly correlated with the energy confinement degradation. The steady-state and transient heat fluxes on the divertor can be both well reduced with Ar/Ne seeding in EAST.

Section: Pedestal Stability Analysis For Ar Seedingsupporting

confidence: 89%

Section: Pedestal Stability Analysis For Ar Seedingmentioning

confidence: 68%

Section: Introductionmentioning

confidence: 99%

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Compatibility of large ELM control and stable partial detachment with neon/argon seeding in EAST

Lin

Yang

et al. 2023

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“…It is interesting to observe that the N 4+ radiation keeps at a constant level during the duration of small ELMs, which suggests that the impurity screening effect is magnified with a pronounced detachment phase in the closed divertor as discussed in section 3.2. The ELM frequency initially declines upon reaching the shadow region, similar to the results from 100% neon seeding experiments in the mid-plane of the HL-2A tokamak [52,53] and EAST [54]. As the radiation continues to increase, the high-frequency small ELMs (defined as f > 2 kHz) emerge, as delineated by the ELM frequency and amplitude in figures 10(c) and (d).…”

Section: The Effect Of Radiative Divertor On Elmssupporting

confidence: 71%

Facilitated core-edge integration through divertor nitrogen seeding in the HL-2A tokamak

Wu,

Cheng,

et al. 2024

Divertor detachment with the sustainable high-performance plasmas has been achieved through divertor nitrogen seeding in the HL-2A tokamak. The closed divertor structure facilitates achieving high divertor neutral pressure due to its efficient particle containment effect as demonstrated by both experimental and SOLPS simulation results, which aids in achieving the divertor detachment. The radiative divertor is conducive to high-frequency small ELMs operation, characterized by the reduced pedestal ion temperature gradient ∇T_i and the enhanced electron density gradient ∇n_e. The presence of minority nitrogen at the edge plasma, which results from the divertor nitrogen seeding, contributes to the rise of ion temperature T_i in the confinement region by suppressing the turbulence through the impurity dilution effect and E×B shear. The increased ion temperature T_i and electron density n_e compensate the energy loss resulted from the increased edge radiation during detachment, which contributes to the confinement sustainment combined with reduced pedestal energy loss caused by small ELMs. This work advances our understanding on the fundamental physics governing the closed divertor detachment with sustainable high-performance plasmas through divertor nitrogen seeding.

“…In DIII-D SAS divertor experiments, low frequency and larger ELMs appeared with neon seeding [53]. In EAST, neon seeding even led to an anomalous transition from type-III ELMs to low-frequency large-amplitude ELMs [54]. More importantly, in all these cases the pedestal density height and gradient increase significantly, which may play an important role in the trigger of large ELMs.…”

Section: Statistical Analysis Of Neon Content and Elm Behaviormentioning

confidence: 97%

Compatibility of divertor detachment and ELM suppression in DIII-D high- β

Wu,

Wang,

Wang

et al. 2024

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Integration of transient and steady-state divertor heat fluxes control with a high-performance core is necessary for future fusion reactors. In recent DIII-D high- β p experiments, divertor detachment and simultaneous edge localized mode (ELM) suppression are demonstrated while the plasma confinement quality is maintained high in ITER-similar shape. By optimizing the neon injection in high- β p scenario with ITER-similar shape, deep detachment and ELM suppression are achieved with a high-performance core ( β N ∼ 2.8, β p ∼ 2.3) at q 95 ∼ 7.5. Partial divertor detachment and suppression of large ELMs are achieved at q 95 ∼ 6. The stability analyses suggest that with low neon injection, the density pedestal becomes higher and steeper and the T i profile also increases, therefore the increased edge pressure and higher current density destabilize the Peeling-Ballooning mode (PBM), which would lead to a large ELM collapse. With strong neon gas puffing, the significantly reduced pedestal pressure and current density, due to the degraded T e pedestal, lead to the stabilization of PBM and ELMs are suppressed. For both cases, the coupling between the large radius internal transport barrier (ITB) and edge pedestal is the key reason for maintaining high global performance. The formation of large radius ITB compensates for pedestal degradation. Such results could provide an attractive scenario to well control the transient and steady-state heat flux onto the divertor plates while maintaining good plasma performance, which is an important step toward the steady-state operation of future fusion reactors.