Experimental observations of an n = 1 helical core accompanied by a saturated m/n = 2/1 tearing mode with low mode frequencies in JT-60U

Abstract

We report the experimental finding of n=1 helical cores (HCs) accompanied by saturated m/n=2/1 tearing modes (TMs) with low mode frequencies in JT-60U. The HCs accompanied by TMs were observed after an increase in the mode amplitude and a decrease in the mode frequency of m/n=2/1 precursors with tearing parity. The decreased mode frequency is typically lower than 20 Hz. With various diagnostics, the coupling of n=1 HCs and m/n=2/ 1 TMs has been clearly observed. Because the coherent oscillations in the ion temperature are observed in both the core region and the edge region, the flux surfaces including the m/n=2/1 magnetic island appear to have m=1 helical deformation. It has also been suggested that the m/n=2/1 TM and the HC rotate in the electron diamagnetic direction keeping f m/n=1/1(HC) =2f m/n=2/1(TM) in several plasmas. Here, f m/n=1/1(HC) is the mode frequency of HCs and f m/n=2/1(TM) is the mode frequency of TMs. In addition, the core seems to be shifted to the high-field side when the O-points of the m/n=2/1 magnetic island line up in the midplane, which is confirmed by reconstructions of magnetohydrodynamic equilibria with motional Stark effect measurement and the MEUDAS code. Our observation of m/n=2/1 TMs having HCs contributes to the understanding of the excitation mechanism of HCs in tokamak plasmas.

Non-Resonant n = 1 Helical Core Induced by m/n = 2/1 Neoclassical Tearing Mode in JT-60U

“…In Fig. 6 (a), the n = 1 mode, whose mode frequency is about 200 Hz, is observed in the magnetic fluctuation before 7.55 s. The mode frequency of the m/n = 2/1 NTM is relatively slow (∼ Hz) after the decrease of the mode frequency at 7.55 s, which is the same object as the LFM described in the previous study [20]. And in Figs.…”

“…Because the observed mode frequency with the coupling can be several kHz in this study as well as ∼ 20 Hz in the previous study [20], the resistive wall does not induce the coupling. In addition, the coupling is observed when the mode frequency of an m/n = 2/1 NTM is 6 kHz.…”

“…2) of the coupling because the observed phase profiles can be reproduced in synthetic images assuming the coupling. Note that the same phase from ρ = 0.1 to ρ = 0.5 in ECE measurement is considered due to the m = 1 "helical deformation" mentioned in the previous study [20].…”

“…In this study, the "coupling" means cou- pling satisfying the two characteristics (1) ∼ (2) mentioned in this paragraph. In our previous study [20], though possible excitation mechanisms of HCs were proposed such as the effect of the resistive wall where the resistive wall time τ w ∼ 10 ms [21] in JT-60U or helical equilibria with q min. ∼ 1, these candidates were not investigated.…”

“…∼ 1, these candidates were not investigated. This is mainly because the coupling was only studied with TMs having low mode frequencies (<= 20 Hz), which is mentioned as Low Frequency Mode (LFM) in the previous study [20]. In addition, it is also required to investigate whether the magnetic fluctuation induced by the plasma control system, where the time interval of the control of the plasma position is 250 µs (4 kHz), can induce HCs or not.…”