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

Observation of fast ion profile stiffness due to the Alfvén eigenmodeS. Kamio1,2, Y. Fujiwara1,2, K. Nagaoka3, H. Nuga3, H. Yamaguchi3, R. Seki3,K. Ogawa3, Y. Kawamoto3, M. Isobe3, E. M. Granstedt2, and M. Osakabe3
1
University of California, Irvine, Irvine, California, USA
2
TAE Technologies, Foothill Ranch, California, USA3
National Institute for Fusion Science, Toki, Japane-mail (speaker): skamio@tae.com

Fast-ion transport by Alfvén eigenmodes (AEs) is one
key issue in the discussion of fast-ion confinement. In
the DIII-D tokamak experiment, stiff transport offast-ions increased with increasing neutral beam (NB)
injection power when the amplitudes of multiple
interacting AEs exceeded a certain threshold [1]. Theseexperiment results are supported by simulation studies
that predict monotonically degrading fast-ion
confinement and profile stiffness with increasing beam
power [2]. To investigate the universality of the fast-ion
profile stiffness dependence on AE amplitude, anexperiment was performed at the Large Helical Device(LHD) to scan the injection current of the NB and vary
the AE amplitude. Since LHD uses only external fieldcoils to generate the confinement field, the variation ofthe confinement field with the NB injection power issignificantly smaller plasma equilibrium with the NB
injection power is significantly smaller than for tokamak
devices; this control of the plasma parameters whilechanging NB power better allows for an isolating of thefast-ion dependence on AEs. The relationship between
NB injection power, Pt-NB, and AE amplitude shown inFig. 1 (a) agrees with the trend obtained in DIII-D [3].
Under the experimental conditions, the AE amplitude
increased linearly with NB injection power beyond anNB power threshold of about Pt-NB = 1.6 MW. Fig. 1 (b)shows the neutron emission rate, Sn, with variable NB
injection power for both the experiment and from a
Fokker-Plank simulation [4]. The result shows Sn has alinear dependence on Pt-NB; this is expected because thefast-ion density will increase with Pt-NB, resulting in ahigh beam-thermal neutron emission rate. However, Sn
did not increase linearly with Pt-NB in the experiment;
this suggests that large amplitude AEs degrade fast-ionconfinement. Fig. 1 (c) shows the fast-ion profiles
measured by fast-ion D alpha (FIDA) measurement [5].
The blue shifted FIDA intensity between 663-665 nm,
corresponding to the energy range of 98-166 keV in thectr-direction, was used for estimating the radial profile ofthe fast-ion density. There is direct evidence of stiffeningof the fast-ion profile and degradation of the fast-ion
confinement. This is consistent with the experimentallyobserved reduction in the expected neutron emission rate
shown in Fig. 1 (b). However, it is possible that
AE-induced transport is not the primary cause of theobserved degradation. We will discuss an alternative
fast-ion confinement degradation mechanism at
high-power NB injection.
References[1] W. W. Heidbrink et al., PRL 99, 245002 (2007).
[2] Y. Todo et al., Nucl. Fusion 56 (2016) 112008.
[3] C. S. Collins et al., PRL 116, 095001 (2016).
[4] H. Nuga et al., J. Plasma Phys. (2020) 86,815860306.
[5] Y. Fujiwara et al., Nucl. Fusion 60 (2020) 112014.

Figure 1: Injection NB power dependencies of (a) AE amplitude, (b) neutron emission rate Sn, and (c) fast-ion profiles.