Tamed stability and transport using controlled non-axisymmetric fields in KSTAR

Abstract

Meticulously orchestrated non-axisymmetric fields (δB) enabled KSTAR to explore various paths to tame plasma stability and transport in a very rigorous manner. Given an extremely low level of intrinsic non-axisymmetry, KSTAR has now established high-precision 3D field control capability that can not only robustly suppress edge localized modes (ELM) using resonant magnetic perturbation (RMP), but also exclusively alter plasma rotation without invoking particle and energy transport. In highly shaped plasmas (triangularity of δ ~ 0.6), we have secured low-n RMP-driven, ELM-crash suppressions in a wide range of edge safety factor at q95 = 3.4–6.4. One of the best n = 1 RMP-driven, ELM-crash suppressions has been sustained for more than 30 s (comparable to wall saturation time), satisfying a low edge collisionality (ν* ~ 0.2) at Zeff = 1, close to ITER-target. Besides a routinely used three-row RMP configuration, we have newly succeeded in suppressing ELM-crashes using n = 1 off-midplane RMPs only, whose helical structure in vacuum appears nearly orthogonal to a typical configuration. Nonetheless, when the plasma response is factored, the off-midplane RMP configuration remains dominantly resonant. With RMP configuration fixed, a gradual torque control between ‘perpendicular’ and tangential components of neutral beams probed the onset of ELM-crash-suppression, strongly endorsing the existence of ω⊥,e ~ 0 at pedestal top as necessary condition for ELM-crash-suppression, consistent with direct measurement of ECEI. In support of ITER, KSTAR has demonstrated broadened divertor heat fluxes during ELM-crash-suppression, as well as during ELM-crash-mitigation, using intentionally misaligned RMP configurations. However, we have found that such a misaligned configuration, as had effectively broadened the divertor heat fluxes during ELM-crash-mitigation, did not show a similar broadening during ELM-crash-suppression. This suggests that the divertor heat flux during ELM-crash-suppression, governed by a bifurcated state of δB, may not be appropriately projected, based on the results of ELM-crash-mitigation, in which the linear plasma response of δB prevails.

Test of the ITER-like resonant magnetic perturbation configurations for edge-localized mode crash suppression on KSTAR

“…However, once the near-scrapeoff-layer (SOL) peaks are normalized, the divertor heat fluxes under 3-row IMCs have been broadened but nothing similar appears in 2-row RMPs, as shown in Figure 6 (b) and (d). This reminds us of a similarly contrasting observation of divertor heat fluxes between ELM-crash-suppression and ELM-crash-mitigation [5], in which the divertor heat flux dispersal was observed only during ELM-crash-suppression, while nothing similar appears during ELM-crash-mitigation. So far, when the IMC with "away" phasing is reinforced by the global 'kink-resonant' influences, such broadening has been speculated to be readily available in a non-linear plasma simulation [5].…”

“…Although the choice of 3-row ITER RMP is greatly meritorious in this aspect, the physics question about the broadening mechanism needs to be further clarified. While the IMC with "away" phasing greatly benefited from a global 'kink-resonance' that would interact with edge RMP coupling [5], it remains to be answered what causes the IMC with "toward" phasing to be similarly accompanied by divertor heat flux broadening. Based on Fig.…”

Pedestal electron collisionality and toroidal rotation during ELM-crash suppression phase under n = 1 RMP in KSTAR

“…1 The application of resonant magnetic perturbation (RMP) is proposed to relieve the heat flux onto the PFC by controlling the ELM-crash. Since the first demonstration of ELM-crash suppression by the RMP technique in DIII-D experiment, 2 various tokamaks have confirmed that the RMP would be an ideal tool to control the ELM-crashes effectively in either suppression or mitigation: KSTAR, [3][4][5][6][7] ASDEX Upgrade, 8,9 MAST, 10 and JET. 11 The access conditions to ELM-crash suppression have been investigated to establish robust and reliable ELM-crash control.…”

“…18 KSTAR has provided results to understand the physics and effects of RMP-driven ELM-crash suppression based on the high reproducibility of the suppression. [4][5][6][7]19,20 In KSTAR, the RMP-driven ELM-crash suppression phase is highly sustainable, in that the longest suppression phase was stably sustained up to the time scale of wall saturation by deuterium fuel ($30 s). 6 Therefore, KSTAR provides one of the best testbeds to address the remaining uncertainties of ELM-crash control in ITER.…”

“…[4][5][6][7]19,20 In KSTAR, the RMP-driven ELM-crash suppression phase is highly sustainable, in that the longest suppression phase was stably sustained up to the time scale of wall saturation by deuterium fuel ($30 s). 6 Therefore, KSTAR provides one of the best testbeds to address the remaining uncertainties of ELM-crash control in ITER. Note that KSTAR is the only major tokamak that is equipped with an in-vessel mid-plane row of coils, a similar configuration to the ITER RMP coils.…”