Speaker
Description
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.
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).
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.
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.
Acknowledgements
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).
References
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
| Affiliation | UNIST |
|---|---|
| Country or International Organization | Korea, Republic of |
