Plasma and Fusion Research: Rapid Communications

Volume 19, 1202008 (2024)

Response of Energetic-Particle-Driven
Magnetohydrodynamics (MHD) Instability to Modulated ECH in
Heliotron J
Yao ZHONG1,2) , Kazunobu NAGASAKI2) , Zhibin WANG1) ∗ , Shinji KOBAYASHI2) ,Shigeru INAGAKI2) , Takashi MINAMI2) , Shinichiro KADO2) , Shinsuke OHSHIMA2) ,Fumiyoshi KIN2) , Chenyu WANG2) , Yuji NAKAMURA3) , Shigeru KONOSHIMA2) ,Toru MIZUUCHI2) , Hiroyuki OKADA2) , Nikolai MARUSHCHENKO4) and Jian CHEN1)
1)Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China2)
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan3)
Graduate School of Energy Science, Kyoto University, Uji, Kyoto 611-0011, Japan4)
Max-Planck Institute for Plasma Physics, Greifswald, Germany(Received 30 September 2023 / Accepted 29 November 2023)

The response of energetic particle (EP)-driven magnetohydrodynamic instability to electron cyclotron heating (ECH) was experimentally studied in Heliotron J. The neutral beam injection power remained constantthroughout the experiment, and the ECH power was systematically modulated. When the ECH power was gradually decreased to a certain threshold, the excitation of an EP mode with a frequency of approximately 100 kHz
was observed. The response to ECH showed a close linkage with variations in electron density and temperature.
In addition, a distinct delay response eﬀect concerning plasma pressure suggests a complex delay response ofmode excitation and suppression.
c 2024 The Japan Society of Plasma Science and Nuclear Fusion Research⃝

Keywords: MHD instability, energetic-particle-driven mode, ECHDOI: 10.1585/pfr.19.1202008
Energetic particles (EPs) are generated in magnetically confined plasmas through processes, such as the D–T
fusion reaction or auxiliary heating methods that include
neutral beam injection (NBI). These EPs can excite certain
types of magnetohydrodynamic (MHD) waves, including
Alfven eigenmode and EP mode (EPM) [1], through thegradient of EP distribution function in the resonance region. These MHD waves can lead to the transport and lossof EPs, which in turn reduces the heating eﬃciency andconfinement performance. In addition, these lost EPs canaﬀect the walls of devices, causing serious wall damage.
Therefore, understanding and controlling the interactions
between EP and MHD waves are crucial in magneticallyconfined fusion plasmas. Various methods have been developed to suppress or mitigate EP-driven MHD instabilities [2]. Electron cyclotron heating (ECH) and electron
cyclotron current drive eﬀectively stabilize these MHD instabilities [3–5]. In this study, we analyzed the eﬀect ofECH on the suppression of EP-driven MHD modes in Heliotron J. ECH exerted a complicated eﬀect on EP-driven
modes. Almost all factors related to MHD stability can
be aﬀected by ECH, including the driving and dissipative
terms. The ECH can change ion, electron Landau damping, radiation damping, and continuous spectrum damping(by changing the pressure) by changing the electron temperature (Te). Conversely, the change in Te also aﬀects
the electron drag collision, which aﬀects the slowing down
period of the local energy.
Heliotron J is a medium-sized helical-axis heliotron
device. It has a major radius of R = 1.2 m, a minor radiusof a = 0.17 m, and operates at a magnetic field strength of
B = 1.25 T [6, 7]. In this experiment, the NBI power wasfixed at 90 kW for coinjection, and the ECH was operated
with modulation. The maximum power of modulation was
192 kW, and the minimum power varied from 158 kW to113 kW. The ECH modulation frequency was set at 50 Hz,
with a duration of 10 ms for the maximum and minimum
power levels.
Figure 1 illustrates the temporal evolution of plasma
discharge. An EP-driven mode was observed in the frequency range of 95 - 103 kHz, with some delays after themodulated ECH power reached its minimum amplitude.
The electron cyclotron emission (ECE), which reflects thecore Te, was plotted; the optical thickness at the core region was approximately 2. The ECE signal increased withan increase in the ECH power, whereas the average electron density measured using a microwave interferometerdecreased. Furthermore, after reducing the ECH power,
the mode was excited with a delay of 6.0 ms. Subsequently,∗ Corresponding author’s e-mail: wangzhb8@mail.sysu.edu.cn1202008-1

c 2024 The Japan Society of Plasma⃝
Science and Nuclear Fusion Research

Plasma and Fusion Research: Rapid Communications

Volume 19, 1202008 (2024)

Fig. 2 Dependence of mode amplitude on ECH power.

Fig. 1 Time evolution of (a) the power spectrum density of amagnetic probe, (b) line-averaged density (ne), (c) mode
intensity (I), (d) ECE, (e) ECH signal, and (f) NBI signal.

after a short delay of 1.5 ms, the mode disappeared and was
eﬀectively suppressed after an increase in the ECH power.
According to the hybrid EP-MHD simulation results calculated using MEGA [8] and the shear Alfven continua calculated using STELLGAP [9] and previous experimental
results obtained under the same magnetic configuration [4],the observed mode was likely an EPM with m/n = 1/2,
where m and n refer to the poloidal and toroidal mode numbers, respectively.
The amplitude of the observed mode exhibited a distinct correlation with the ECH injection power. As theECH power was increased, mode suppression became evident. When the minimum ECH amplitude was set at
100 kW, a distinct mode was observed. Once the ECH
power surpassed the threshold, eﬀective mode suppression
occurred (Fig. 2). The threshold was between 110 and138 kW. These results highlight the critical role of ECH
power in the control and mitigation of the observed mode.
The establishment of the threshold range for mode suppression provides valuable insights into the optimized operation of the Heliotron J device and management of theassociated MHD instabilities driven by EPs.
A delay response eﬀect was observed in the responseof the mode amplitude to ECH modulation. This behavior is illustrated in Fig. 3, which also displays the evolutionof mode intensity ⟨I⟩ as a function of the product of lineaveraged electron density ⟨ne⟩ and ECE intensity, whichserves as an indicator of the core plasma pressure. At
high amplitudes of ECH modulation, the mode intensity
remained low. However, as the ECH power decreased, theFig. 3 Evolution of normalized intensity (I) with the product of⟨ne⟩ and ECE intensity.

mode strength rapidly increased, as indicated by the redcircle in Fig. 3.
The mode intensity remained high as the plasma pressure began to rise, and mode suppression eventually occurred at high pressure levels. To understand the dominantfactors influencing this response, we investigated mode intensity as a function of either the electron density ⟨ne⟩ or
ECE intensity. The analysis reveals that compared with theelectron density, Te plays a more important role in determining mode strength. This result suggests that the delay
response eﬀect is closely correlated with variations in Te.
The diﬀerence in the product of ⟨ne⟩ and ECE intensity indicates that mode excitation suppression is related not only
to bulk plasma pressure but also fast ion confinement.
In addition to the EP mode, the intervals of elevated
ECH power were characterized through broadband fluctuations. The increase in broadband fluctuation may be related to mode suppression. EP-driven modes suppress turbulence [10]. In future work, we will conduct relevant experiments to further study the relationship between mode
suppression and broadband fluctuation.

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Plasma and Fusion Research: Rapid Communications

Volume 19, 1202008 (2024)The authors are grateful to the Heliotron J groupfor their support in conducting the experiment. They
are also grateful to Dr. Jacobo Varela and Dr. Panith
Adulsiriswad for their useful comments on the theoretical simulation. This work was conducted with support from the NIFS Collaborative Research Program(NFIS10KUHL030), Grant-in-Aid for Scientific Research,
MEXT (Kiban (A) 22H00117), and JSPS Core-to-Core
Program, A. Advanced Research Networks, PLADyS.
This study was supported by the Chinese ScholarshipCouncil (Nos. 202206380110).

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