Abstract
An extensive study of intrinsic and controlled non-axisymmetric field (δB) impacts in KSTAR has enhanced the understanding about non-axisymmetric field physics and its implications, in particular, on resonant magnetic perturbation (RMP) physics and power threshold (Pth) for L–H transition. The n = 1 intrinsic non-axisymmetric field in KSTAR was measured to remain as low as δB/B0 ~ 4 × 10−5 even at high-beta plasmas (βN ~ 2), which corresponds to approximately 20% below the targeted ITER tolerance level. As for the RMP edge-localized-modes (ELM) control, robust n = 1 RMP ELM-crash-suppression has been not only sustained for more than ~90 τE, but also confirmed to be compatible with rotating RMP. An optimal window of radial position of lower X-point (i.e. Rx = m) proved to be quite critical to reach full n = 1 RMP-driven ELM-crash-suppression, while a constraint of the safety factor could be relaxed (q95 = 5 0.25). A more encouraging finding was that even when Rx cannot be positioned in the optimal window, another systematic scan in the vicinity of the previously optimal Rx allows for a new optimal window with relatively small variations of plasma parameters. Also, we have addressed the importance of optimal phasing (i.e. toroidal phase difference between adjacent rows) for n = 1 RMP-driven ELM control, consistent with an ideal plasma response modeling which could predict phasing-dependent ELM suppression windows. In support of ITER RMP study, intentionally misaligned RMPs have been found to be quite effective during ELM-mitigation stage in lowering the peaks of divertor heat flux, as well as in broadening the ‘wet’ areas. Besides, a systematic survey of Pth dependence on non-axisymmetric field has revealed the potential limit of the merit of low intrinsic non-axisymmetry. Considering that the ITER RMP coils are composed of 3-rows, just like in KSTAR, further 3D physics study in KSTAR is expected to help us minimize the uncertainties of the ITER RMP coils, as well as establish an optimal 3D configuration for ITER and future reactors.
Highlights
A seemingly negligible level of intrinsic nonaxisymmetric field has been proven to be quite a significant challenge for high-β plasmas [1, 2], while an introduction of significant level of controlled δB has been confirmed to be effective in controlling edge-localizedmode (ELMs) [3]. (Here, B0 is the magnetic field strength at geometric center of tokamak, while β refers to a ratio of plasma kinetic pressure to applied magnetic field pressure.) Despite substantial progress that have been made for the last couple of decades, the understanding of such a ‘double-sided’ impact of non-axisymmetric field on stability and transport needs to be enhanced for ITER and future reactors [4,5,6]
The uncertainties of non-axisymmetric field physics are expected to be better resolved, when the intrinsic components do not compete with controlled counterparts
Throughout this study, we have found the optimal window of radial position of lower X-point, whose role has been found to be as pivotal as that of q95
Summary
A seemingly negligible level (δB/B0 ~ 10−4) of intrinsic nonaxisymmetric field (δB) has been proven to be quite a significant challenge for high-β plasmas [1, 2], while an introduction of significant level (δB/B0 ~ up to 10−3) of controlled δB has been confirmed to be effective in controlling edge-localizedmode (ELMs) [3]. (Here, B0 is the magnetic field strength at geometric center of tokamak, while β refers to a ratio of plasma kinetic pressure to applied magnetic field pressure.) Despite substantial progress that have been made for the last couple of decades, the understanding of such a ‘double-sided’ impact of non-axisymmetric field on stability and transport needs to be enhanced for ITER and future reactors [4,5,6]. Since a low level of intrinsic non-axisymmetry leads to a weak influence of neoclassical toroidal viscosity (NTV), faster plasma rotation (up to Mach number ~0.8 (deuterium)) in KSTAR [15], in comparison with other devices of similar size at nearly the same level of heating input power, may be understood. A systematic scan of power threshold for L–H transition shows that KSTAR appears lower than the projected level by a multi-device empirical scaling, as potentially another evidence of the merit of low intrinsic non-axisymmetry [20].
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