
 Ion temperature (Ti) profiles are commonly observed to increase in peaking, leading to higher central Ti, after impurity seeding in EAST H-mode plasma. Argon can be more efficient to raise Ti than neon. Toroidal rotation can also be enhanced in scenarios with NBI heating. More significant increased toroidal rotation is brought by seeding argon than seeding neon. Turbulence is experimentally observed to be suppressed. Extensive modelling by quasilinear gyrokinetic code QuaLiKiz is performed to explain above observations. It is found that the enhanced Ti can be always explained by the turbulence stabilization. But the mechanism of turbulence stabilization is related to heating methods and the seeding impurity species. In the pure RF (ECRH+LHW) heating scenarios, where only TEM exists, argon can stabilize TEM more significantly than neon due to its higher charge and heavier mass. In the scenarios with increasing NBI power, ion heat flux can be dominated by ITG, thus the enhanced Ti is mainly attributed to ITG stabilization. In these cases, except argon’s ability to more efficiently stabilize TEM, more evident increased toroidal rotation brought by argon seeding can be also beneficial to stabilize turbulences.

Mechanism of enhanced ion temperature by impurity seeding in EAST H-mode plasma


 A significant change in the background ELM behavior prior to neon (Ne) seeding has been observed in a series of repetitive radiative divertor experiments in EAST. With similar operational parameters, the ELM behavior before Ne seeding changes from large to mixed ELMs, and finally evolves to pure small ELMs. Meanwhile, a significant increase in both Ne and high-Z impurity (W and Mo) emissions has been observed in the bulk plasma, suggesting the retention and recycling of Ne impurity from the wall surface. Experimental results show that the variation in background ELM behavior is highly correlated with the occurrence of high-Z impurity accumulation. The increased accumulation of high-Z impurities leads to a lower electron temperature both in the plasma core and edge, accompanied by a higher and more peaked electron density in the plasma core. Pedestal linear stability analysis reveals that the decreased pedestal electron temperature and thus the lower pressure gradient and lower edge current density are the primary reason for the change of background ELM behavior. The concentration of recycling Ne in the bulk plasma is estimated to be ~1% in the discharges with pure small ELMs.

Impacts of recycling impurity on the background ELM behavior during repetitive radiative divertor experiments in EAST

Abstract 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. 

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


 Achieving the divertor detachment helps to alleviate excessive heat load and sputtering problems on the target plates, thereby prolonging the lifetime of divertor components for fusion devices. In order to provide a rapid but relatively reliable prediction of plasma parameters along the flux tube for future devices design, a one-dimensional (1D) modeling code for the impurity seeded detached divertor operational point has been developed based on the Python language, which is a fluid model based on the previous work.[1] The experimental observation of the onset of divertor detachment by neon (Ne) and argon (Ar) seeding in EAST is well reproduced by using the 1D modeling code. The comparison of the 1D modeling and 2D simulation by SOLPS-ITER code for CFETR detachment operation with Ne and Ar seeding also shows good consistency. The predictions of the radiative power loss and requirement of the corresponding impurity concentration for achieving divertor detachment via different impurity seeding for the high heating power scenarios on EAST and CFETR Phase II by the 1D model are also presented. Based on the predictions, the optimized parameter space for divertor detachment operation on EAST and CFETR has also been determined. Such a simple but reliable 1D model can provide a reasonable parameter inputs for a detailed and accurate analysis by 2D or 3D modeling tools through rapid parameter scanning.

Calculation and prediction of divertor detachment via impurity seeding by using 1D model

Abstract One critical issue in steady-state and high-performance operation in future tokamak fusion reactors is to develop H-mode operational regimes that mitigate or eliminate the large-amplitude ELMs to decrease the transient heat flux on the divertor and first wall. This paper reviews recent advances in extending and understanding the access and sustainment of natural and impurity-induced small/no-ELM H-mode regimes in EAST. Highly reproducible stationary grassy ELM regime with good energy confinement and excellent tungsten impurity exhaust capability has been obtained in EAST with water-cooled metal wall, compatible with low rotation, high normalized density, high bootstrap current fraction, radiative divertor and fully non-inductive operations, which are the conditions required for future fusion reactor-level plasmas. Grassy-like small-ELM H-mode regime has been achieved in helium (He) plasma experiments, which will be conducted during the pre-fusion power operation phase in ITER. The exploration for type-II ELM regime in EAST is also briefly introduced. Three spontaneous no-ELM H-mode regimes frequently observed in EAST are introduced, including an enhanced-recycling no-ELM regime, a non-inductive low-recycling no-ELM regime and a transient spontaneous no-ELM regime at relatively low pedestal collisionality ( $${v}_{e}^{*}&lt;1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>v</mml:mi> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mrow> <mml:mrow /> <mml:mo>∗</mml:mo> </mml:mrow> </mml:msubsup> <mml:mo>&lt;</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> ). These no-ELM regimes show different characteristic signatures of pedestal fluctuation and profiles, and the physical mechanisms behind have been studied. In addition, small/no-ELM regimes with impurity seeding have also been achieved in EAST. The ELM behavior changing from mixed small and large ELMs to pure grassy ELMs has been observed with neon (Ne) seeding. A highly reproducible no-ELM H-mode regime with radiative divertor has been demonstrated through CD 4 seeding in EAST. These results are believed to be helpful for the application of small/no-ELM H-mode regimes to future tokamak fusion reactors. 

/pdf/recent-advances-in-developing-natural-and-impurity-induced-3k1by566.pdf

Recent advances in developing natural and impurity-induced small/no-ELM H-mode regimes in EAST

Improved confinement caused by Ne injection is investigated in EAST. The safety factor q profile and electron-scale turbulence are studied with a polarimeter-interferometer system and CO2 laser collective scattering system in a series of Ne injection experiments in EAST. Ne with a suitable injection rate can lead to negative magnetic shear in the plasma core, which promotes electron-scale turbulence suppression. The continuous injection of Ne under the reversed shear condition further reduces the electron-scale turbulence intensity. Core electron temperature increase and confinement improvement are observed. QuaLiKiz is applied to model the influence of an impurity and reversed q on turbulence. The combination of stabilization effects of negative magnetic shear and Ne on turbulence is much stronger than the impurity’s own effect, which could explain the experimentally observed electron temperature (T e) increase and turbulence suppression. Besides, the relationship between pedestal structure induced by Ne injection and lower hybrid wave (LHW)-driven current deposition is discussed. These results provide a new perspective to improve the confinement in the case of Ne injection, i.e. to adjust the Ne injection to modify the ideal plasma equilibrium.

Observation of electron-scale turbulence suppression under weak magnetic shear with neon seeding in EAST plasma

This paper describes scalings of the H-mode confinement database of neutral beam injection (NBI)-lower hybrid wave (LHW) and electron cyclotron resonant heating-LHW plasmas in the EAST tokamak. Fundamental information of the EAST H-mode database and the details of calculation of the thermal energy confinement time in EAST are presented. The result of the scaling model τE,EASTS,H,NBI - LHW of the thermal energy confinement time in NBI-LHW plasmas with root-mean-square-error RMSE=5.5% , compared with the multi-machine scaling model τth,98y2 derived from the multi-machine database (mainly NBI heating) with RMSE=12.9% , is obtained. The scaling τE,EAST,H,NBI - LHW presents a similar dependency on plasma current and power loss and a lower dependency on plasma density and elongation, but stronger dependency on toroidal field than in τth,98y2 . The scaling τE,EAST,H,ECRH - LHW demonstrates a similar dependency on plasma current and power loss and lower dependency on plasma density, but a negative dependency with large error bars on elongation compared with τE,EAST,H,NBI - LHW in NBI-LHW plasmas. The comparison of EAST models with the more recent multi-tokamak scaling ITPA20-IL shows a similar engineering dependency on plasma current, power loss and plasma density, but opposite dependency on toroidal field.

https://iopscience.iop.org/article/10.1088/1741-4326/acb15f/pdf

The scalings of the thermal energy confinement time in EAST H-mode plasmas

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

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Observation of electron-scale turbulence suppression under weak magnetic shear with neon seeding in EAST plasma

The scalings of the thermal energy confinement time in EAST H-mode plasmas

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