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

Photoionization experiments in UVSOR-III for study of divertor plasmas in anuclear fusion reactor and of interstellar plasmas in a context of astrobiology
M. Kobayashi1, S. Yoshimura1, J. Takahashi2, H. Iwayama3, N. Kondo3, M. Katoh3,4, K. Kobayashi2,I. Sakon5, K. Sawada6, H. Nakamura1
1National Institute for Fusion Science, 2 Yokohama National University, 3 UVSOR Synchrotron

Facility, Institute for Molecular Science, 4 Hiroshima Synchrotron Radiation Center, HiroshimaUniversity, 5 Tokyo University, 6 Graduate School of Engineering, Shinshu University
e-mail (speaker): kobayashi.masahiro@nifs.ac.jp
In nuclear fusion reactors, density of plasmas andneutral hydrogen in the divertor region will increase by
more than one order of magnitude compared to thepresent-day devices due to the increasing plasmapressure in the scrape-off layer (SOL) as well as to thehigh ratio of divertor size to ionization mean free path ofhydrogen [1]. At the same time, the photon emission
from plasmas in confinement region (Bremsstrahlung)and in the SOL (line emissions from highly charged ions)will increase in wide wavelength range, from EUV tovisible and infrared regions, because of the higher
plasmas density required to sustain nuclear fusion
reaction. Such situation leads to enhanced interaction
between hydrogen atoms/molecules and the photons in
the divertor region, such as photoionization,photoexcitation (electronically, vibrationally, androtationally). While the effects have not received muchattention until now due to the negligible interaction in
the present nuclear fusion experiments, theatomic/molecular processes will be altered under theinfluence of the increased numbers of photons especiallyin very low temperature range below ~10 eV [2, 3].
There exist partially ionized plasmas in aninterstellar space, where ionization is caused by cosmic
rays or by interstellar radiation field [4]. In thiscircumstance, photon-induced processes, such asphotoionization,photoexcitation,photolysis,
photodissociation etc, also play important roles inchemical evolution of prebiotic molecules related to theorigin of life in space [5]. It is also noted that collective
phenomenon of plasmas under the effects of magnetic
fields will provide additional effects on the evolution.
Characteristics and roles of the photoionized plasmas are,however, not yet fully understood.
To address these issues, we attempt to generate
photoionized plasmas by using the synchrotron radiation
source UVSOR-III [6] to simulate the above mentioned
plasmas in the divertor region in nuclear fusion reactors
and in the interstellar space. The beamline BL7B is used
to generate beam from 30 to 500 nm. A gas cell has been
installed inside the irradiation chamber to maintain highpressure of sample gas while keeping a good vacuum
condition for the beamline. A high gas pressure isnecessary to enhance the photoionization events throughan interaction between the photons and neutral
atoms/molecules. Fig.1 shows the gas cell used in thepresent experiments. The beam from BL7B is injected tothe cell through a hole at the front end (φ 2 mm and 60mm length). The beam is damped at the end of the gascell. An off-axis parabolic mirror is installed at the frontend to collect emission from the gas along the beam axis.
An electrode of Langmuir probe is situated at the centerof the cell and biased to +/- 18 V to measure saturation
current of the plasmas. The gas is fed to the cell throughthe input terminal at the top of the cell.
As a sample gas, Argon (ionization potential of
14.5 eV) was introduced. The gas pressure inside the gas
cell could be increased to an order of 1 Pa while theambient pressure outside of the cell was kept at an orderof 10-4 Pa and the pressure at the upstream of thebeamline was at an order of 10-6 Pa. The photon flux ofthe beam was measured with a photodiode, and wasestimated at an order of 1015 ~ 1016 photons/s/mm2. At
the conference the detail plan of the research and theexperiments will be presented.

Fig.1: Photo of the gas cell for photoionization
experiments.
This work has been financially supported by JSPS
KAKENHI grant number 22K18272.
References[1] M. Kotschenreuther et al., Physics of Plasmas, 14(2007) 072502.
[2] K. Sawada et al., Contributions to Plasma Physics, 60(2020) e201900153.
[3] M. Kobayashi et al., Contributions to Plasma Physics,
60 (2020) e201900138.
[4] E.F.van Dishoeck et al., Faraday Discuss 133 (2006)
231.
[5] K. Kobayashi et al., Astrobiology 21 (2021) 1479.
[6] H. Ota et al.,J. Phys.: Conf. Ser. 2380 (2023) 012003.