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

Enhancement of ECCD by the current condensation effect for stabilizing large
magnetic islands caused by neoclassical tearing modes in tokamak plasmas
Liu Tong1, Wang Zheng-Xiong1, Wei Lai1, Wang Jialei2 and Reiman Allan3
1

Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams of the Ministry ofEducation, School of Physics, Dalian University of Technology, China
2
National Institute for Fusion Science, National Institutes of Natural Sciences, Japan
3Princeton Plasma Physics Laboratory, Princeton University, Princeton, USAe-mail (speaker): liutong@dlut.edu.cn

In order to achieve self-sustained burning in ITER andfuture large-scale devices, good confinement will beessential. These devices will operate at low collisionality
and at sufficiently high pressure to drive substantial
bootstrap current. The large bootstrap current poses athreat in the form of neoclassical tearing modes (NTMs).
With the aim of the mitigation and/or active control of
NTM for disruption avoidance, electron cyclotron
current drive (ECCD) has been brought forward.
Although ECCD is proven to be effective to stabilize
NTMs both in experiments and in simulations,
off-normal events that can cause extremely large islands
is still challenging to control in future large-scale devices
such as ITER.
Recently, Reiman et al. found a radio frequency current
condensation effect that can nonlinearly enhance thestabilizing efficiency of ECCD so that the extremely
large islands are expected to be effectively controlled.
The radio frequency current condensation effect reported
in [1] is modelled in the nonlinear resistive MHD code
MHD@Dalian Code [2-4]. A series of numericalinvestigations have been performed to investigate theenhancement of ECCD by the current condensation
effect during the control of NTMs in tokamak plasmas.
In the numerical model, both the parallel transport andthe perpendicular transport of electron temperature areconsidered. The EC driven current and driven perturbed
electron temperature can nonlinearly evolve within thegiven magnetic configuration and eventually reachsaturation states. The input power threshold of ECCD
and the fold bifurcation phenomenon (shown in figure 1)
are numerically verified via nonlinear simulations. The
numerical results show good agreements with theanalytical results. Moreover, spatial distributions of EC
current for the two solutions at different condensed levelare displayed. The control effectiveness of ECCD forlarge NTM islands has been evaluated while considering
the current condensation effect. While taking into
account current condensation effect, for a given input
power, a larger island can be more effectively stabilized
than a smaller one, which suggests a reassessment of theprevious idea that the ECCD should always be turned on
as early as possible. The potential physics mechanism
behind the ECCD control have all been discussed indetail. Furthermore, the condensation effect is found tohave favorable effects on the radial misalignment ofECCD. In the consideration of the situation for
extremely localized control needs, a highly peaking
heating profile is adopted to verify that the foldbifurcation phenomenon still exists and the current
condensation effect can still take effect in this extremecondition.
The work described here is the first to use a nonlinear
resistive MHD code in the modeling of the current
condensation effect. The investigations manifested thatthe current condensation effect can be more effective for
the ECCD control of large NTM islands. Considering thelarger the island is, the effectiveness and efficiency ofECCD is more enhanced, the condensation effect is
expected to play important roles in dealing with
off-normal events in future large-scale devices.

Figure 1. The fold bifurcation phenomenon.
References[1] Reiman A. H. and Fisch N. J. 2018 Phys. Rev. Lett.

121 225001[2] Liu T. et al 2018 Nucl. Fusion 58 076026[3] Liu T. et al 2020 Nucl. Fusion 60 106009[4] Liu T. et al 2022 Nucl. Fusion 62 056018