7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Parity transition of MHD fluctuations in helical plasmas
Y. Takemura1, S. Ito2, K. Watanabe1, S. Sakakibara1, S. Ohdachi11National Institute for Fusin Science,2
Graduate school of Engineering, Nagoya University
e-mail (speaker):takemura.yuki@nifs.ac.jp

Parity transitions in the radial structure of fluctuations
caused by MHD instabilities are observed as common
phenomena in toroidal magnetic confinement plasmas.
Conventionally, only the even-to-odd parity transition isobserved in tokamak[1] and helical devices[2]. However,for the first time in the Large Helical Device (LHD), theodd-to-even parity transition has been discovered. Bothtransitions of the LHD are considered to be observed
when the instability with a magnetic island appears or
disappears under the appearance of the resistive
interchange mode.
The parity of the mode structure is related to thetopology of the magnetic equilibrium. The even-to-odd
and odd-to-even parity transitions in the radialdisplacement profile are associated with the occurrence
and disappearance of magnetic islands. Understanding
the mechanism of parity transitions that contribute to theappearance and disappearance of magnetic islands,
which cause severe confinement degradation and, in theworst case, can lead to disruptions, is an important task
for fusion science.
Figures 1 (a) and (b) show the magnetic island width
evaluated from the peak spacing of the density
fluctuation profile and magnetic fluctuation frequency
around the even-to-odd parity transition. Before t = 4.06
s, the even-parity mode is observed, and t ~ 4.065 s, themagnetic island width starts to increase, and theeven-to-odd parity transition is observed. Figures 1 (c)and (d) show the change in the magnetic island widthand fluctuation frequency around the odd-to-even parity
transition. From t ~ 4.61 s, the magnetic island width
begins to decrease, and from t ~ 4.615 s, the even-parity
mode dominates. During the even-to-odd parity
transition, the magnetic island width increased slightly
after the increase in magnetic fluctuation amplitude, andthe fluctuation frequency decreased gradually. On theother hand, during the odd-to-even parity transition, themagnetic island width started to decrease together withthe decrease in magnetic fluctuation amplitude, and thefluctuation frequency increased discontinuously. Figure 2
shows the relationship between amplitude and frequency
of a magnetic fluctuation during both parity transitions,which shows the different behavior of both transitions.
The odd-to-even parity transitions is observed only
after even-to-odd transitions. Additionally, it is foundthat higher amplitude of resonant magnetic perturbation
imposed into plasmas leads to a higher likelihood of theodd-to-even parity transitions.
References[1] V. Igochine et al., Nucl. Fusion 59, (2019).
[2] Y. Takemura et al., Phys. Plasmas 29, 092505 (2022).

(a)

(b)

(c)

(d)

Figure 1. A typical discharge of (a)(b) theeven-to-odd and (c)(d) the odd-to-even parity
transition. (a)(c) magnetic island width evaluatedfrom peak spacing of the density fluctuations
profile and magnetic fluctuation amplitude. (b)(d)magnetic fluctuation frequency.

Figure 2. Relationship between amplitude andfrequency of a magnetic fluctuation during
even-to-odd (open red circles) and odd-to-even(open blue circles) parity transitions. Closed red(closed blue) circles mean parameters before
even-to-odd (after odd-to-even) parity transition.