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10-15 May 2021
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The Conference will be held virtually from 10-15 May 2021

Toward holistic understanding of the ITER-like RMP ELM control on KSTAR

13 May 2021, 15:08
Virtual Event

Virtual Event

Rapporteur Magnetic Fusion Experiments EX/4 MHD and ELM


Yongkyoon In (Ulsan National Institute of Science and Technology)


KSTAR has clarified a set of unresolved 3-D physics issues that could be addressed in the ITER-like in-vessel 3-row, resonant magnetic perturbation (RMP) configurations. In particular, considering that one of the most critical metrics of RMP ELM-crash control would require the compatibility with the divertor heat fluxes under the given material constraints, a series of intentionally misaligned RMP configurations (IMC)$^{1,2}$ have been explored to reveal the relationship between RMP ELM control and divertor heat fluxes. Specifically, taking advantage of the time-resolved IR camera, each rotating IMC in either 3-row or a combination of 2-row IMCs helped us diagnose the ‘wet’ area of divertor in the vicinity of ELM-crash-suppression; ELM-crash-mitigation, ELM-crash-suppression, and mode-locking.
First of all, we have articulated the contrasting effect of kink (i.e. “away” phasing) vs anti-kink (i.e. “toward” phasing) responses on the ELM-crash suppression, as shown in Figure 1.Contrasting effects of kink vs anti-kink phasing, consistent with ideal MHD modeling. Shown are the time evolutions of (a) kink-phasing (90 deg to 110 deg) and (b) anti-kink phasing (90 deg to 70 deg) at IRMP = 2.2 -2.3 kA, as mapped in the polar plot with the colored arrows in (c) predicted by IPEC$^{3}$.

Starting from a sub-marginal level of RMP current in a typical n=1, 90 deg phasing, the 3-row IMC in kink phasing (in red) becomes more kink-influenced, demonstrating the synergistic benefit of ‘kink’ phasing in ELM-crash-suppression. In contrast, the 3-row IMC in the anti-kink phasing (in green) becomes more insensitive to ELM-crashes at the sub-marginal level of RMP. In a way, this helps us recast the “away” and “toward” phasing as kink and anti-kink phasing respectively, as schematically shown in the lower left inset of Figure 1. Such experimental observation is in excellent agreement with what ideal MHD theory predicts $^{3}$. Previously, we had shown the divertor heat flux broadening with 3-row IMC-driven ELM-crash-suppression in both “away(kink)” and “toward (anti-kink)” phasings, while no such broadening was observed in the 2-row IMCs with top/bottom coils $^{1}$. Now, we have newly observed that the ‘wet’ area of ELM-crash-mitigation got more broadened than that of ELM-crash-suppression (not shown here), based on these 3-row IMC discharges.
Also, we have further investigated whether or not 2-row IMCs would be fundamentally deficient in divertor heat flux broadening during ELM-crash-suppression. In the earlier study, no mid-row was involved in the 2-row IMCs, although the mid-row is much more influential than the other off-mid rows. To clarify this issue, a set of 2-row IMCs, including mid-row, has been explored. Figure 2 shows the time evolutions of various plasma parameters and ‘wet’ area, where each phasing of 2-row IMC varies by the denoted angle in shades incrementally from a typical n=1, 90 degree phasing angle in the anti-kink direction. Throughout the whole IMC application period, the 2-row IMC-driven, ELM-crash-suppression has been accomplished, as shown at the bottom of Figure 2 (a). At the same time, no evidence of the divertor heat flux broadening can be found on the ‘wet’ area in this combination of middle/bottom row IMCs, as shown in Figure 2(b).

Time evolutions of 2-row IMCs with middle/bottom coils in (a), and the measured  wet area of divertor heat fluxes (as defined on the figure in (b)). Shown in (a) are the time traces of (i) TCI-line integrated average electron density $n_e$, (ii) normalized-$\beta_N$, (iii) edge safety factor, $q_{95}$, (iv) RMP coil currents in Top/Middle/Bottom coils, and (v) photodiode signal of $D_{\alpha}$. Here, each shaded region represents a period of rotating RMP, which allows us to diagnose the divertor in a full toroidal angle using an IR camera.

Even for kink phasing in the 2-row IMCs with middle/bottom coils (as shown in Figure 3 (b)), a similar outcome has been obtained. Thus, it is a fair conclusion that the divertor heat flux broadening would require a third row, suggesting that the dispersal of the divertor heat flux in 3-row IMCs cannot be driven by helically structured 2-row IMCs alone. Nonetheless, no physics mechanism of the 3-row IMC-driven, divertor heat flux broadening during ELM-crash-suppression has been understood yet, while several hypotheses are being assessed$^{1}$. Interestingly, we have found that middle/bottom rows are much more effective in suppressing the ELM-crashes than top/mid rows, revealing strong up/down asymmetry in lower-single-null (LSN) plasmas, as shown in Figure 3.

Comparison of Top/Middle vs Bottom/Middle IMCs with the essentially same operational conditions. Shown in both (a) and (b) are the time evolutions of (i) RMP current amplitude per turn, (ii) toroidal phase, (iii) phasing (i.e. phase difference) between rows, and (iv) photodiode signal of $D_{\alpha}$.

Considering that the 3-row IMC-driven, ELM-crash-suppression in kink phasing have been securely obtained at 2.3 kA, a set of 2-row IMCs have been designed to compensate a missing off-mid row current (mid: 2.3 kA, off-mid: 4.6 kA). Surprisingly, such conditions led to a vastly contrasting outcome, proving a much more effective coupling of middle/bottom rows in ELM-crash-suppression than that of top/middle rows. In fact, there was a much lower threshold of ELM-crash-suppression in the combination of middle/bottom rows, even suggesting no need of top row (not shown here). This is reminiscent of the critical influence of X-point on RMP ELM control studied in MAST$^{4}$, though it was related to ELM-crash-mitigation, rather than ELM-crash-suppression.
Overall, the KSTAR has established a new holistic understanding of ITER-like RMP ELM control, elaborating various subtle points in the vicinity of ELM-crash-suppression and ‘wet’ area on divertor. These new findings in 3-D physics is expected to help us further reduce the uncertainty associated with 3-row ITER RMP.

This work was supported by the Korean Ministry of Science and ICT for National Research Fund (NRF-2020M1A7A1A03007919), the UNIST research fund (1.180056.01), and the KSTAR project (NFRI-EN1901-11).

1. Y. In et al., Nucl. Fusion 59 (2019) 126045
2. Y. In et al., Nucl. Fusion 59 (2019) 056009
3. J.K. Park et al, Nature Physics 14 (2018) 1223
4. A. Kirk et al, Nucl. Fusion 55 (2015) 043011

Country or International Organization Korea, Republic of
Affiliation UNIST

Primary authors

Yongkyoon In (Ulsan National Institute of Science and Technology) Hyungho LEE (National Fusion Research Institute) Gunyoung Park (National Fusion Research Institute) YoungMu Jeon (National Fusion Research Institute) Dr Minwoo Kim (NFRI) Kimin Kim (National Fusion Research Institute) Jong-Kyu Park (Princeton Plasma Physics Laboratory) SeongMoo Yang (Princeton Plasma Physics Laboratory) Alberto Loarte (ITER Organization) Yueqiang Liu (General Atomics, PO Box 85608, San Diego, CA 92186-5608, USA) Hyeon K. Park (UNIST) 3D Physics Task Force in KSTAR

Presentation Materials