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Effect of ECH/ECCD on Energetic-Particle-Driven MHD Modes in Helical Plasmas

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Effect of ECH/ECCD on Energetic-Particle-Driven MHD Modes in Helical Plasmas PAPER • OPEN ACCESS Effect of ECH/ECCD on energetic-particle-driven MHD modes in helical plasmas To cite this article: S. Yamamoto et al 2020 Nucl. Fusion 60 066018 View the article online for updates and enhancements. This content was downloaded from IP address 130.54.110.24 on 02/06/2020 at 13:08 International Atomic Energy Agency Nuclear Fusion Nucl. Fusion 60 (2020) 066018 (12pp) https://doi.org/10.1088/1741-4326/ab7f13 Effect of ECH/ECCD on energetic-particle-driven MHD modes in helical plasmas S. Yamamoto1, K. Nagasaki2, K. Nagaoka3,4, J. Varela3, A.´ Cappa5, E. Ascasíbar5, F. Castejon´ 5, J.M. Fontdecaba5, J.M. García-Regaña5, A.´ Gonz´alez-Jerez5, K. Ida3,6, A. Ishizawa 7, M. Isobe3,6, S. Kado2, S. Kobayashi2, M. Liniers5, D. Lopez-Bruna´ 5, N. Marushchenko 8, F. Medina5, A. Melnikov9,10, T. Minami2, T. Mizuuchi2, Y. Nakamura7, M. Ochando 5, K. Ogawa3,6, S. Ohshima2, H. Okada2, M. Osakabe3,6, M. Sanders11, J.L. Velasco 5, G. M. Weir8 and M. Yoshinuma3,6 1 National Institutes for Quantum and Radiological Science and Technology, Naka, Japan 2 Institute of Advanced Energy, Kyoto University, Uji, Japan 3 National Institute for Fusion Science, Toki, Japan 4 Graduate School of Science, Nagoya University, Nagoya, Japan 5 National Laboratory for Magnetic Fusion, CIEMAT, Madrid, Spain 6 The Graduate University for Advanced Studies, SOKENDAI, Toki, Japan 7 Graduate School of Energy Science, Kyoto University, Uji, Japan 8 Max-Planck-Institute for Plasma Physics, Greifswald, Germany 9 National Research Centre ‘Kurchatov Institute’, Moscow, Russian Federation 10 National Research Nuclear University ‘MEPhI’, Moscow, Russian Federation 11 Eindhoven University of Technology, Eindhoven, 5600 MB, Netherlands E-mail: [email protected] Received 2 December 2019, revised 19 February 2020 Accepted for publication 11 March 2020 Published 1 June 2020 Abstract The effect of electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) on energetic-particle (EP)-driven magnetohydrodynamic (MHD) modes is studied in the helical devices LHD, TJ-II and Heliotron J. We demonstrate that EP-driven MHD modes, including Alfv´en eigenmodes (AEs) and energetic particle modes (EPMs), can be controlled by ECH/ECCD. In the LHD device, which has a moderate rotational transform and a high magnetic shear, co-ECCD enhances toroidal AEs (TAEs) and global AEs (GAEs), while counter-ECCD stabilizes them, which improves the neutron rate compared with the co-ECCD case. Counter-ECCD decreases the core rotational transform and increases the magnetic shear, strengthening the continuum damping on the shear Alfv´en continua (SAC). In the TJ-II device, which has a high rotational transform, moderate magnetic shear and low toroidal field period, helical AEs (HAEs) appear when the HAE frequency gap of the SAC is changed by counter-ECCD combined with a bootstrap current and neutral-beam-driven current. On the other hand, both co- and counter-ECCD are effective in stabilizing GAEs and EPMs in the Heliotron J device, which has a low rotational transform and low magnetic shear. The experimental results indicate that the magnetic shear has a stabilizing effect regardless of its sign. Modeling analysis using the FAR3d code shows that the growth rates are reduced by both co- and counter-ECCD in Heliotron J, reproducing the experimental results. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1741-4326/20/066018+12$33.00 1 © 2020 IAEA, Vienna Printed in the UK Nucl. Fusion 60 (2020) 066018 S. Yamamoto et al ECH only affects EP-driven MHD modes, and the experimental results show that the effect depends on the magnetic configuration. In Heliotron J, some modes are stabilized withan increase in ECH power in the low-bumpiness magnetic configuration, while some modes are destabilized in the high- and medium-bumpiness magnetic configurations. Keywords: energetic-particle-driven MHD mode, ECH/ECCD, helical plasma (Some figures may appear in colour only in the online journal) applied to a neutral beam injection (NBI) plasma [16]. Further 1. Introduction off-axis electron cyclotron resonance heating (ECRH) power resulted in a strong reduction of the chirping mode amplitude In magnetically confined fusion plasmas, energetic particles [17, 18]. In the Heliotron J helical device, it was demonstrated (EPs) generated by D–T fusion reactions and plasma heat- that EPMs and global AEs (GAEs) were stabilized by both co- ing interact resonantly with shear Alfv´en waves through and counter-ECCD [19, 20]. slowing-down processes when their velocity is comparable In this paper, we report recent progress in the control of EP- to the Alfv´en velocity, resulting in the excitation of EP- driven MHD modes with ECH/ECCD in the helical devices driven magnetohydrodynamic (MHD) modes. The EP-driven LHD, TJ-II and Heliotron J. EP-driven MHD modes, including MHD modes include Alfv´en eigenmodes (AEs) and ener- toroidal AEs (TAEs), GAEs, helical AEs (HAEs) and EPMs, getic particle modes (EPMs), and have been experimentally are observed in tangential NBI-heated plasmas of the hel- observed in tokamaks and helical devices (see the reviews ical devices. Since the stability of the EP-driven MHD modes [1–3]). Since these modes enhance the anomalous transport of depends on the magnetic configuration, comparative studies EP and induce EP loss, which reduces heating efficiency and among the three devices based on similarities and differences damages the first wall, it is important to stabilize and/or con- in the configuration are useful for a comprehensive under- trol the EP-driven MHD modes. Studies have been made on standing of the physics of EP-driven MHD modes in toroidal the physical properties of specific modes, and several external plasmas. We employ ECH and ECCD in order to mitigate actuators have been proposed and demonstrated in tokamaks and eventually suppress the observed EP-driven MHD modes. and helical devices for controlling and/or stabilizing EP-driven Although the EC-driven current in helical devices is not high, modes [4]. Promising control techniques are based on (i) vary- because there are a few 10 kA in LHD and a few kA in TJ-II ing the energetic-ion sources to modify the gradients in the and Heliotron J, due to a strong Ohkawa effect [23], ECCD is energetic ion-distribution [5–10], (ii) localized electron cyclo- effective for tailoring the rotational transform (inverse safety tron heating (ECH) to modify the energetic-ion slowing-down factor: 1/q) profile, particularly at the core region, since the distribution [11–18], (iii) a localized electron cyclotron current poloidal magnetic field generated by ECCD is comparable to drive (ECCD) for modifying the equilibrium [19, 20] and (iv) the vacuum poloidal magnetic field. Localized ECH changes externally applied 3D perturbative magnetic fields for manip- the local electron temperature and its gradient significantly, ulating the energetic-ion distribution and thus the wave drive affecting the driving and damping terms of EP-driven MHD [21, 22]. modes. ECH/ECCD is an ideal technique to control the EP-driven The paper is organized as follows. The EP-driven MHD MHD modes, since it can provide highly localized EC waves modes in the helical plasmas are described in section 2. Exper- with a known location and good controllability. According to imental and theoretical results on the ECCD effect on the EP- linear MHD theory, the changes in electron density and tem- driven modes are given in section 3 and experimental results perature by ECH and in the plasma current by ECCD can on the ECH effect are given in section 4. A summary is presen- affect both the growth and damping rates of the EP-driven ted in section 5. MHD modes through changes in (i) the pressure gradient of the energetic ions, (ii) the structure of the shear Alfv´en con- 2. EP-driven MHD modes in helical plasmas tinua (SAC) through a change in the magnetic configuration and (iii) electron Landau damping. In the DIII-D tokamak, The EP-driven MHD modes are commonly observed in toka- for example, reversed shear AEs (RSAEs) were stabilized by maks and helical devices. The modes excited by the poloidal applying a localized ECH near the minimum of the magnetic mode coupling such as TAE in helical devices has no sig- safety factor in a neutral beam (NB) heated discharge with nificant differences to those in tokamaks [3]. The difference reversed magnetic shear [11, 12]. The effect of ECH on RSAEs between tokamaks and helical devices is related to the mag- is due to an increase in the frequency of a geodesic acoustic netic field structure, that is, tokamaks are two dimensional mode (GAM) through the increase in electron temperature. In (2D) and axisymmetric in an ideal case while helical devices helical devices, ECH/ECCD is also expected to be an effect- are three dimensional (3D). In helical devices, a spectral gap is ive technique to control EP-driven MHD modes. For instance, generated by the toroidal mode coupling as well as the poloidal in the TJ-II helical device, the behavior of AEs was changed mode coupling, which can result in EP-driven MHD modes from a steady mode to a chirping mode when ECH power was such as HAE being excited. However, it should be noted that 2 Nucl. Fusion 60 (2020) 066018 S. Yamamoto et al Table 1. Device and plasma parameters of the LHD, TJ-II and Heliotron J devices in the plasma experiments for EP-driven MHD modes reported in this paper.
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