7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Experimental study of divertor detachment during RMP ELM control

Haewon Shin1,2, Junghoo Hwang2, Yoonseong Han2, Giwook Shin3, Hyungho Lee3,
Kil-Byoung Chai1, and Wonho Choe2*
1
Nuclear Physics Application Research Division, KAERI, 2 Department of Nuclear and Quantum
Engineering, KAIST, 3 Korea Institute of Fusion Energy
e-mail (speaker): hwshin@kaeri.re.kr
Realization of nuclear fusion power faces manydifficulties such as improving the high energy
confinement and controlling heat loads below an
acceptable level on divertors. One significant challenge
lies in edge-localized modes (ELM) crash control
compatible with detached plasmas to avoid damages todivertor materials including tungsten. Furthermore,evaluating the heat and particle fluxes on the divertor
through analysis of experimental data remains crucial forthe optimal design of fusion reactors.
To address this concern, various ELM control techniques
have been proposed, with resonant magnetic perturbations(RMPs) considered the most promising [1]. However, thecompatibility of RMP-induced ELM control with adetachment to manage divertor heat fluxes in highperformance tokamaks like ITER remains uncertain.
In this study, we report on an experimental
demonstration of argon-seeded discharges that exhibit adivertor detachment during the ELM control by an ITERlike, three-row RMP configuration in KSTAR [2]. The
results suggest that argon seeding may show a potential
suitable technique for impurity-seeded detachment inITER. Furthermore, in the small ELM phase, which isconsidered a favorable regime for impurity exhaust, an
additional reduction in heat and particle fluxes on thetargets was observed compared with the ELM suppressionphase.
The target discharge was an H-mode plasma with an Ipof 0.5 MA and a BT of 1.8 T. Neutral beam injection (NBI)
was used as a main heating source and a maximum power
level was 5 MW. RMP coil currents IRMP started to ramp
up at 4.0 s, and reached the maximum current of 2.0
kA/turn on 3-rows with a n = 1, +90º phasing
configuration for ELM control. After IRMP reached its
maximum value, we injected argon gas at a constant ratefrom 6.0 s through the gas pipeline connected to themidplane and introduced prefilled and additional fuelgases into the plasma from the midplane and divertor,respectively.
During the application of RMP, complete suppression of
ELM was achieved, followed by mitigation until the endof the discharge. Remarkably, during the RMP-driven
ELM suppression phase, successful argon-seededdetachment was accomplished. In this phase, the heat flux
showed a continuous reduction of 70% compared to thepre-argon seeding case.
The ELM suppression phase abruptly transitioned to theELM mitigation phase 0.6 s after the argon was injected.
During the ELM mitigation phase, the heat flux wasreduced by approximately 90% when compared to the preargon seeding case.
The degree of detachment (DoD) based on the two-point

model was calculated. The DoD increased to levels of
approximately 3 and 2.3 for the outer target and innertarget cases, respectively. Definitive evidence ofdetachment was discernible in the 2-D radiation
distribution as shown in Figure 1. The concentrated
radiation zone shifted gradually from the inner target plate
towards the X-point during the argon seeding.

Figure 1 Tangentially reconstructed 2-d radiated power
distributions from the imaging bolometer [2].
In addition, experiments using the KAERI plasma beamirradiation facility (KPBIF) have been actively carried out
to investigate various plasma-surface interactions bysimulating divertor plasmas [4]. Currently, it is operatingin continuous-wave mode and it will be upgraded tosimulate ELM-like conditions by developing a pulse
power supply using capacitor banks.
Acknowledgments
This work was supported by the National Research
Foundation of Korea (NRF) funded by the Korean
government (Ministry of Science and ICT) (RS-202200155950 and RS-2022-00155960) and R&D Program of'KSTAR Experimental Collaboration and Fusion Plasma
Research (EN2201-13)' through the Korea Institute ofFusion Energy (KFE) funded by the Government fundsand ‘National Research Council of Science & Technology(NST)’-‘Korea Atomic Energy Research Institute(KAERI)’ Postdoctoral Fellowship Program for Young
Scientists at KAERI in South Korea.
References[1] T.E. Evans et al., Nuclear Fusion 58 (2018) 126010.
[2] H. Shin, J. Hwang, Y. Han, G. Shin, H. Lee, K.-B.
Chai and Wonho Choe, Nucl. Fusion 63 (2023)
044003.
[3] H.H. Lee et al., Nucl. Mater. Energy 12 (2017) pp.
541-547.
[4] K.-B. Chai and D. Kwon, Plasma Phys. Control.
Fusion 62 (2020) 035007.