Comparison of divertor behavior and plasma confinement between argon and neon seeding in EAST

Abstract

The exhaust of excessively high heat and particle fluxes on the divertor target is crucial for EAST long-pulse operation. In the recent EAST experiments, stable partial energy detachment around the upper outer strike point with H 98,y2 ∼ 1 was achieved with either Ne or Ar seeding from the upper outer divetor target in the upper single null configuration with ITER-like tungsten divertor. With either Ar or Ne seeding, the electron temperature around the upper outer strike point (T et,UOSP) was maintained at around 5 eV, the peak temperature of divertor target surface around the upper outer strike point (T div,UO) decreased significantly, and material sputtering was well suppressed. It was observed that there was less Ar seeding needed for partial energy detachment onset than Ne seeding, which shows that Ar is more efficient in the cooling of T et on the upper outer divertor than Ne. However, there was no detachment on the upper inner divertor with T et around strike point (T et,UISP) remaining >10 eV with either Ar or Ne seeding from the upper outer divertor. Accompanied with the disappearance of double peak phenomenon of ion flux density on the upper inner divertor target (j s,UI), the peak T div,UI around the strike point increased to around 300 °C. Although the heat flux on the upper inner divertor target (q t,UI) is still in the acceptable level, either Ar or Ne seeding only from the upper outer divertor target is not enough to protect the upper inner divertor target from sputtering under current EAST conditions. On the other hand, Ar seeding always causes confinement degradation in the partial energy detachment state. It was observed that there is a slight confinement improvement (∼10%) with Ne seeding, which may be due to density peaking, dilution effects and stabilization of the ion temperature gradient mode.

Achievements of actively controlled divertor detachment compatible with sustained high confinement core in DIII-D and EAST

“…Also, a slight decrease of the plasma stored energy (∼8%) was observed after 10 s, which was not seen with neon seeding in an 18 s long-pulse H-mode discharge with P heat ∼ 5.5 MW (shot #101825). According to the previous detachment experiments with an upper divertor in the EAST, it has shown that the compatibility of detachment and core confinement with Ne seeding is better than that with Ar seeding [35] under present plasma conditions on the EAST.…”

“…A lower pedestal and a steeper-gradient increase inside the core plasma were observed during Ar seeding, similar to the DIII-D detached high β P scenario with both ITB and ETB, to be shown later in section 4. The strong ITB formation mechanism in DIII-D to be shown in section 4.3 and the EAST results in reference [35], suggesting that neon seeding may offer a more promising tool for favorable core-edge integration in the high β P plasma conditions in EAST, with respect to Ar seeding. In the EAST 2019 winter campaign, the compatible core-edge integration in the high β P scenario has been extended and demonstrated with H 98 > 1.2 at β P ∼ 2.5, β N ∼ 2.0, f bs ∼ 50%, f gw ∼ 0.7 at moderate q 95 ∼ 6.7 (EAST #94437) [33,34], with the peak heat flux on the divertor reduced by ∼30%, using a mixture of 50% neon and 50% D 2 seeding.…”

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

“…The enhanced ion temperature induced by impurity seeding through the stabilization of turbulence has been observed in several magnetic fusion devices, such as DIII-D [3,4], JET [5][6][7], WEST [8], ASDEX-U [9,10], JT-60U [11], Alcator C-Mod [12], LHD [13,14] and HL-2A [15]. EAST has demonstrated a compatible core and edge integration in high β P scenarios sustained by active impurity seeding [16] and a large number of divertor feedback control experiments have also been performed on EAST [17][18][19]. In these experiments, the central T i is commonly observed to increase.…”

Evolution of the high-field-side radiation belts during the neon seeding plasma discharge in EAST tokamak

“…During 2019 EAST campaign, a series of detachment experiments were carried out by Ne or Ar mixed with D 2 seeding from the UO-GP. [28] The effect of different magnetic configurations on plasma detachment with impurity seeding is studied by changing the B t direction. In this paper, two representative shots with different magnetic configurations are presented to show the characteristics of radiation belts during neon seeding.…”