Parameter dependences of small edge localized modes (ELMs)

Harrer, G.; Wolfrum, E.; Dunne, M.; Mänz, P.; Cavedon, M.; Lang, P. T.; Kurzan, B.; Eich, T.; Labit, B.; Stober, J.; Meyer, H.; Bernert, M.; Laggner, F. M.; Aumayr, F.

doi:10.1088/1741-4326/aad757

Cited by 49 publications

(72 citation statements)

References 44 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In general, the ELM-free regimes described above require a widening of the operational space towards a higher divertor neutral pressure to achieve simultaneously divertor detachment at higher power. This is the operational domain of small ELMs situated at high shaping, high gas puff levels and close to double null [55] [11]. Here, the additional transport required to avoid large ELMs is thought to be provided by ballooning modes close to the separatrix, which are also thought to be responsible for the small "ELMs" [11].…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2020

Self Cite

View full text Add to dashboard Cite

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.

show abstract

Section: Discussionmentioning

confidence: 99%

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

et al. 2020

Self Cite

View full text Add to dashboard Cite

show abstract

“…In recent experiments, a high density ratio between the foot and top of a pedestal is identified to be a key ingredient for the achievement of grassy ELM operation in EAST [24]. It is observed that high SOL density can induce a transition from type-I ELMs to small type-II-like ELMs in AUG, while maintaining other parameters basically unchanged [25]. Recent study on DIII-D found that high SOL density is essential for access to small grassy ELM regime by flattening the pressure profile near the separatrix to stabilize type-I ELMs [26].…”

Section: Introductionmentioning

confidence: 99%

Dependence of upstream SOL density shoulder on divertor neutral pressure observed in L-mode and H-mode plasmas in the EAST superconducting tokamak

Niu

Chen

et al. 2021

Nucl. Fusion

View full text Add to dashboard Cite

Upstream density profiles in the scrape-off layer (SOL) have been examined in low-confinement mode (L-mode) and high-confinement mode (H-mode) plasmas in the EAST superconducting tokamak. A weak density shoulder forms in the near SOL region in upper single-null configurations when the neutral pressure measured at the lower divertor exceeds a threshold value of 2 × 10−2 Pa in L-mode plasmas. When the neutral pressure is below this threshold, the weak density shoulder is absent and the sidebands of the lower hybrid waves associated with SOL parametric instabilities are reduced. Active detachment control with neon–deuterium seeding demonstrate that the weak density shoulder can form before the onset of the outer divertor detachment as long as the neutral pressure is above the threshold. Furthermore, no remarkable expansion of a shoulder is observed during divertor detachment, suggesting that divertor detachment is not a necessary condition for the formation or growth of a density shoulder. Through the increase in neutral pressure in the lower divertor by an order of magnitude, the weak shoulder was observed to expand into the far SOL and reach the leading edge of the limiter. The results in L-mode discharges identified the neutral pressure in the lower divertor as a primary factor for the formation of an SOL density shoulder in the upper single-null discharges. For the type-I ELMy H-mode plasmas, a similar density shoulder was detected during the inter-ELM phase when the neutral pressure in the lower divertor exceeded a threshold value of 4 × 10−2 Pa. On the other hand, the shoulder was absent when the divertor neutral pressure went below this threshold even though the plasma discharge was conducted with a higher core line-averaged density and divertor collisionality. This is consistent with the observations in L-mode plasmas. The neutral particle ionization of the working gas is thus believed to play a key role during the formation of the SOL density shoulder in the EAST tokamak.

show abstract

“…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. The important ingredients for maintaining the QCE regime are high separatrix density (n e,sep /n GW 0.3) and closeness to double null (together with high triangularity) [6,7,30]. Because there is little variation of separatrix temperature in a given device (T e,sep ≈ 100 eV for H-modes in AUG [31]), high n e,sep translates to high separatrix collisionality (ν * e ∝ n e /T 2 e ).…”

Section: Small Elms and Eda Hmode At Asdex Upgradementioning

confidence: 99%

“…On the other hand, ITER can reach higher pedestal top temperatures and is expected to operate with high ν * e,sep and low ν * e,ped . Such conditions cannot be achieved simultaneously in existing tokamaks; therefore, fundamental uncertainties exist on whether ITER could operate in the QCE regime [6].…”

Section: Small Elms and Eda Hmode At Asdex Upgradementioning

confidence: 99%

“…Certain small ELM regimes are completely free of type-I ELMs, maintain other desirable features, and could, if accessible, signify an attractive option for ITER. One such regime is the quasi-continuous exhaust (QCE) regime, which is routinely operated in ASDEX Upgrade (AUG) and TCV and completely avoids type-I ELMs while maintaining good confinement properties [6,7]. In order to access the QCE regime, it is necessary to operate at high separatrix density (n e,sep 0.3 × n GW ), with high triangularity and close to double null [7].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

MHD simulations of small ELMs at low triangularity in ASDEX Upgrade

Cathey,

Hoelzl,

Harrer

et al. 2021

Preprint

Self Cite

View full text Add to dashboard Cite

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.

show abstract

Parameter dependences of small edge localized modes (ELMs)

Cited by 49 publications

References 44 publications

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

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

Dependence of upstream SOL density shoulder on divertor neutral pressure observed in L-mode and H-mode plasmas in the EAST superconducting tokamak

MHD simulations of small ELMs at low triangularity in ASDEX Upgrade

Contact Info

Product

Resources

About