Argon–seeded detachment during ELM control by RMPs in KSTAR

Abstract

In this study, we demonstrate argon-seeded discharges that exhibited a detached divertor during the full suppression and mitigation of edge localized modes (ELMs) by an ITER-like, three-row resonant magnetic perturbation (RMP) configuration in KSTAR. During the ELM suppression phase, the peak heat flux on the divertor target was successfully reduced from 1.6 MW/m2 to 0.5 MW/m2 via argon seeding. Further, the ion saturation current densities corresponding to the particle fluxes on both targets were reduced by more than 50%. During the RMP grassy-ELM regime, a further reduction to 0.1 MW/m2 in the divertor heat load was successfully achieved. A highly localized radiation zone near the x-point was also observed during divertor detachment. The calculated degree of detachment (DoD) based on the two-point model increased to levels of approximately 3 and 2.3 for the outer target and inner target cases, respectively. These results provide valuable information regarding the effect of mid-Z impurities on RMP-detachment-compatible discharges.

Radiatively tamed divertor thermal loading in resonant magnetic perturbation (RMP)-driven, ELM-crash-suppressed plasmas

“…with increased radiation remains in the divertor volume itself, as shown in figure 6. This is an important difference compared to similar studies including the prior observations in KSTAR [16,18,19], in which a significant proportion of the additional radiation created by impurities comes from the confined core plasma [18].…”

“…Unfortunately, when applied with RMP, such impurity/gas fueled plasmas often lead to a loss of ELM-crash-suppression with a small reduction of the divertor heat flux with respect to the otherwise similar ELM-crash-suppressed conditions (typically less than a factor of 2) as found in KSTAR [15], and EAST [16]. Larger reductions, while sustaining RMP-driven, ELM-crash suppression, have been obtained in DIII-D and KSTAR [18,19] but not through the increase of radiation in the divertor region but in the confined plasma. ITER is expected to operate with a partially detached high density divertor plasma so as to provide acceptable divertor power load handling [20], while maintaining a low core radiation to sustain the high confinement plasmas required for high fusion gain operation.…”

Integrated RMP-based ELM-crash-control process for plasma performance enhancement during ELM crash suppression in KSTAR

“…One of the potential candidates is the KSTAR divertor plasma detachment control based on the simplified real-time modeling [66], reducing the heat load on PFCs. However, the impurity seeding, the primary actuator of the detachment control, has a side effect on core plasma performance [67] and affects conditions to access ELM crash suppression by changing electron density and recycling conditions [68,69]. Therefore, incorporating the detachment control requires upgrading the integrated process to balance plasma detachment, ELM crash suppression, and plasma performance enhancement.…”

Plasma performance and operational space with an RMP-ELM suppressed edge

“…This is because the heat flux width scales inversely with I [120], spreading the divertor power dissipation over a wider area at low I (assuming fixed separatrix density). These dependencies provide an explanation for the relative difficulty of AUG and DIII-D achieving edge-integrated RMP-ELM suppression, as compared to EAST and KSTAR [40,121,122], though the dynamics of the seeded impurities also play an essential role in divertor integration [123,124] as does the divertor geometry. Overall, additional effort is required to understand the n e,ped threshold, as its scaling to ITER and future devices is the key question confronting the possibility of integrating a dissipative divertor with the RMP scenario in high current scenarios.…”