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

Characterization of solid hydrogen-pellet penetrationin fusion plasmas of the large helical device
DAI Xiaoyue1, YAMADA Hiroshi,1 ISOBE Yugo1, SAKAMOTO Ryuichi2
1
Graduate School of Frontier Sciences, The University of Tokyo,
2
National Institute for Fusion Science.
e-mail (speaker): dai.xiaoyue23@ae.k.u-toyko.ac.jp
Characteristics of penetration depth and velocity ofsolid hydrogen-pellet are discussed with the results fromthe large helical device (LHD) experiment.
The solid hydrogen pellet injection is a promising
method for fueling to maintain the fusion plasmas confined by the magnetic field. This method is expected tobe highly efficient direct fueling for future large-scalefusion reactors since pellets penetrate into a core region
beyond a hot scrape-off layer. Fuel particles are deposited in the core region through the ablation of pellets
and the following drift motion of high-density plasmoid.
Therefore, the integrated understanding of physical properties such as pellet velocity and mass, target plasma
parameters, and the operational magnetic field is a prerequisite for establishing control of the fueling position.
The aim of this study is the comprehensive characterization of the penetration of the pellet and resultant fueldeposition.
The experiment with pellet injection was conductedby varying the target plasma conditions in the LHD.
Through these, a data set consisting of parameter regions
and pellet velocities over a wide range of plasma temperature and density, magnetic field strength, andECH/NBI heating power was created. The neutral gas
shielding (NGS) model [1] has been used as a basic
theoretical prediction model for pellet ablation. According to this theory, a representative NGS model scaling
law for LHD was derived based on the NGS model using
the modeled electron temperature and density profiles asshown in Eq. (1).
λ/a = 0.027Te0-0.62 ne0-0.13 Vp0.34 (1)whereλ/a, Te0, ne0, and Vp are the penetration lengthnormalized by plasma minor radius, electron temperature, electron density, and pellet velocity. The
negative dependence of this scaling law on electron
temperature and density and the positive dependence onpellet velocity agree with the expression obtained by
Baylor et al. [1]. Figure1 demonstrates a comparison
between the experimental data and the predictions of thescaling law for the LHD plasmas. A systematic difference between the NBI-heated plasmas and the ECHheated plasmas is observed, which is potentially attributed to the presence of fast ions from the NBI.
Furthermore, a regression analysis was performed on
the dependence of the pellet penetration length including
the magnetic field strength B and the fast ion density
nf.ion, which have not been treated in the NGS model, forthe NBI-heated plasma, resulting in Eq. (2) (see Fig.2).
λ/a ∝ Te0-0.30 𝑛#e0-0.19 Vp0.24 B0.38 nf.ion-0.25 (2)Fig. 1 Comparison of pellet penetration length between theexperiment and prediction from the scaling of LHD(Eq.(1)).

Fig. 2 Comparison of pellet penetration length between
experimental and scaling law values in NBIheated plasmas(Eq.(2)).

The dependence on temperature, density, and velocity isin qualitative agreement with the results of Baylor et al.
In addition, significantly, positive magnetic field dependence and negative fast ion density dependence werealso obtained. This suggests that the fast-ion elution by
NBI is related to the shielding properties of the plasmoids, which are constrained by the magnetic field. The
scaling is further discussed with the results from thenumerical computation with HPI2 code [2].
References[1] L. R. Baylor et al. Nucl. Fusion 37,445(1997).
[2] B. Pégourié et al. Plasma Phys, Control Fusion, 4717(2004).