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

Orbit-following simulations of fast-ion transport and losses
due to the Alfvén eigenmode burst in the Large Helical Device

R. Seki 1, Y. Todo 1, Y. Suzuki 2, D.A. Spong3, K. Ogawa 1,4, M. Isobe 1,4, and M. Osakabe 1,41
NIFS, NINS, 2Hiroshima Univ. 3ORNL, 4SOKENDAI
e-mail (speaker):seki.ryohsuke@nifs.ac.jp
As an AAPPS-DPP standard, we do not need to add your postal address.
The evaluation of fast-ion confinement is indispensablefor the prediction of heating efficiency in a fusion reactor.
The fast-ion confinement depends not only on thecollisional transport in an equilibrium magnetic field but
also on fast-ion driven instabilities such as Alfvén
Eigenmodes (AEs) which induce the fast-ion transportand losses. It is an important issue to clarify the fast-ion
transport due to the instabilities.
Ahybridsimulation
codefornonlinearmagnetohydrodynamics (MHD) and energetic-particle
dynamics, MEGA, can simulate recurrent bursts offast-ion driven AE instabilities including theenergetic-particle source, collisions, and losses in a
non-axisymmetricthree-dimensional
magnetic
configuration such as the LHD[1]. The multi-phasesimulation, where MHD hybrid simulation and classicalsimulation are performed alternately, was applied to theLHD experiment using the MEGA code. It was foundthat two groups of AEs with frequencies close to thoseobserved in the experiment are destabilized alternately.
The alternate appearance of multiple AEs is similar tothe experimental observation[1].
On the other hand, the role of each AE in fast-ion
transport in the LHD has not been clarified yet.
Therefore, orbit-following simulations of fast-ion
transportandlosses
withtime-dependent
electromagnetic perturbations are performed to clarify
the roles of AEs and a low-frequency MHD mode
observed in the kinetic-MHD hybrid simulation of AE
bursts in the LHD.
Figure 1 shows fast-ion pressure profiles and time
evolution of fast-ion loss rate in orbit-following
simulations with the time-dependent AE amplitude
following the kinetic-MHD hybrid simulation result. The
fast-ion pressure profile flattening in the kinetic-MHD
hybrid simulation can be reproduced by anorbit-following simulation with only the primary single
AE (Fig. 1 (a)). The effects of the other modes arenegligible on the fast-ion pressure profile flattening. The
fast-ion losses in the kinetic-MHD hybrid simulation canbe reproduced by an orbit-following simulation when thelow-frequency MHD mode is considered in addition to
the multiple AEs (Fig. 1 (b)). This indicates theimportant role of the low-frequency MHD mode in thefast-ion losses observed in the kinetic-MHD hybrid
simulation.
In this conference, the analyses of the orbit-following

simulations with a single AE of the constant amplitude as
well as time-dependent amplitude will be shown. We will
discuss the effect of each AE on fast-ion transport.

Figure 1 (a) Fast-ion pressure profiles in time-dependent
AE amplitude simulations. In panel (a), the green(MEGA INIT) and purple (MEGA) lines represent thefast-ion pressure profiles before and after the AE burst in
the MEGA simulation, respectively. (b) time evolution of
fast-ion loss rate in the time-dependent AE amplitude
simulation including the low-frequency MHD mode. Inpanel (b), the green line represents the fast-ion loss ratein the classical simulation without MHD perturbation.
References[1] Y. Todo, et al., Physics of Plasmas 24, 081203(2017).