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

Emission of high rovibrational molecules from tungsten divertor
S. Saito1, H. Nakamura2, S. Sawada2, K. Hoshino2,M. Kobayashi2, M. Hasuo2, Y. Homma2, and S. Yamoto2
1
Yamagata University, Yonezawa, Japan
2
National Institute for Fusion Science, Toki, Japan
3Nagoya University, Nagoya, Japan
4
Shinshu University, Nagano, Japan
5
Keio University, Yokohama, Japan
6
Kyoto University, Kyoto, Japan
7
National Institutes for Quantum and Radiological Science and Technology, Rokkasho, Japan
8National Institutes for Quantum and Radiological Science and Technology, Naka, Japane-mail (speaker): saitos@yz.yamagata-u.ac.jp
Introduction
In the case of detached plasma, Molecular Assisted
Recombination (MAR), such as charge exchange anddissociative recombination (DAR), plays a significant rolein the particle transport within the edge plasma. It is
widely recognized that the rate coefficient of MAR canvary by several orders of magnitude depending on therovibrational states of hydrogen molecules.
Hydrogen molecules are produced on the wall throughthe hydrogen recycling process. While there is apossibility that the hydrogen molecules generated on thewall could impact MAR in the edge plasma, this process
has not been thoroughly investigated yet due to thedifficulties in experimentally determining therovibrational states of these hydrogen molecules. In thispaper, therefore, we employ a molecular dynamics
simulation [1-4] to estimate the rovibrational states ofrecycled hydrogen molecules on the tungsten divertor in
the DEMO reactor under detached plasma conditions.
Subsequently, we utilize a neutral transport code [5],developed by Prof. Sawada’s group, incorporating theinformation regarding the generated molecules on the wallobtained from the MD simulation.
Simulation Results
The simulation results [6] reveal that molecules in highrovibrational states are emitted even in cases of low
incident energy, which are expected to be the dominantconditions during detached plasma. Figure 1 illustrates thedistributions of rotational states for emitted hydrogenmolecules under various incident energy conditions. It isevident that certain molecules exhibit high rotational
states, even in scenarios involving low incident energy
injection.
The trajectory analysis of the simulation reveals theemission mechanism as follows: 1. In cases of low
incident energy, the incident hydrogen atom is unable topenetrate the target material. 2. The incident atom
becomes trapped at the surface due to the influence ofsurface binding energy. 3. The incident atom traverses thesurface until it comes into contact with other hydrogenatoms situated on the surface. 4. Upon contact with theseother hydrogen atoms, a hydrogen molecule is formed,
subsequently detaching from the surface.
To clarify the effects of such emitted hydrogenmolecules from divertor of DEMO reactor, we alsodevelop a neutral transport simulation in edge plasma withthe emission distributions by MD simulation for thedivertor. As shown in Figure 2, the distribution ofrotational states for hydrogen molecules in the edge
plasma, situated 2 cm from the divertor, demonstrates thepresence of molecules exhibiting high rotational states.
0.0600Count / Number of Trials

1.

Ein=0.1eV
Ein=1eV
Ein=10eV

0.0100

Ein=100eV
0.00100.000505101520

Rotational state J

2530

Figure 1 Distributions of rotational states of emittedhydrogen molecules from tungsten divertor.

2.

Figure 2 Distribution of rotational states of edge plasmanear the divertor obtained by neutral transport
simulation.
References[1] S. Saito, et. al., Contrib. Plasma Phys. e201900152 (2020).
[2] S. Saito, et. al., Plasma Fusion Res. 15, 2403073 (2020).
[3] S. Saito, et. al., Jpn. J. Appl. Phys. 60, SAAB08 (2021).
[4] H. Nakamura, S. Saito, et. al., Jpn. J. Appl. Phys. 61,SA1005 (2022).
[5] K. Sawada, S. Saito, et. al., Contrib. Plasma Phys.
e201900153 (2020).
[6] S. S. Saito, et. al., Synopsis of IAEA-FEC 2023, Contribution ID: 1733, https://conferences.iaea.org/event/316/contributions/28538/