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

Simulation of Cs layer on plasma grid in ITER-scaled negative ion source forlong-term operation
Masafumi Yoshida1,2, Shunsuke Hayashi2,
Department of Electrical Engineering, National Institute of Technology, Ube College,
2
Advanced Course of Production Systems Engineering, National Institute of Technology, Ubee-mail (speaker): yoshida-m@ube-k.ac.jp
1

Negative ion based neutral beam injection (N-NBI) is a
powerful application for heating and current driving offusion plasma. For ITER, a beam operation for up to3600 seconds is required in N-NBI. [1] Up to now, 500
keV, 154 A/m2 for 118 s beam production was achieved
in QST. [2] One of the key issues for an achievement of
the requirements in ITER is stabilization of the current of
the negative ion beam. Negative hydrogen ions (H) aremainly produced on the plasma facing grid (PG) coveredby cesium (Cs), which lowers work function to increasea production yield of negative ions, namely surface
production process. In this work, we focus on the Cs
transport and Cs layer at the PG for 3600 s in ITER scale
negative ion source. Time trends of Cs layer at the PG
with plasma operation is discussed by using Cstransportation code. [3, 4]
In this model, the temperatures of the chamber wall andPG are set to 30 oC and 180 oC, respectively. The plasma
length simulated in this study is 3600 s. The
temperatures of the chamber wall and PG rises to 60 oC
and 220 oC, respectively, due to heat of plasma. At thebeginning of the calculation, Cs is initially input. During
the plasma operation, Cs is kept seeding in the negatice
ion source. Cs input in the source is neutralized before
the plasma operation (vacuum phase). In the plasma
operation (plasma phase). a part of Cs is the ionized.
They are transported in the source, and is finally reached
to 4 regions; chamber wall, plasma grid, loss from thebeam apertures and flowing in the source. The amount ofCs at the PG correlates with the thickness of Cs layer,
which is depended on the production yield of thenegative hydrogen ions.
Figure 1 shows time trends of Cs monolayer at the PG.
During the plasma operation (t > 0 s), Cs monolayer isgradually increased without the saturation of thethickness. In addition, with increasing the amount of theinitial Cs input mass (N0), the Cs monolayer at 3600 s
was increased and the layer became above 0.5 monolayer,
which was an optimal layer for higher production yieldof negative hydrogen ions. Figure 2 summarized the Cslayer at the PG at 3600 s under various conditions suchmax
as 𝑁0 and maximum temperature in the PG (𝑇PG). It
was found that as Cs accumulated in the negative ionmax
source, 𝑇PG
was needed to raise to stabilize thethickness of 0.5 monolayer of Cs layer at the PG. This isto prevent over pilling-up of Cs due to a large amount ofCs incoming to the PG. Therefore, it is suggested that thePG maximum temperature and the initial Cs input massis also one of key parameter for keeping the optimal
thickness of Cs layer on the PG in ITER.

Figure 1. Time trends of cesium monolayer at the PG
max
for 3600 s (𝑇wall= 60 oC).

Figure 2. Contour map of Cs monolayer at PG at 3600 smax
(𝑇wall= 60 oC).

References[1] Progress in the ITER physics basis, Nucl. Fusion, 47 S18(2007).
[2] J. Hiratsuka, et al., Rev. Sci. Instrum. 91, 023506 (2020)[3] M. Yoshida, et al., Rev. Sci. Instrum. 87 02B144 (2016).
[4] M. Yoshida, et al., AIP Conf. Proc. 2052 040008 (2018).