Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2024
Location : [in Japanese]
Date : October 20, 2024 - October 24, 2024
In QST Rokkasho Fusion Energy Institute, the accelerator driven fusion neutron source A-FNS project is ongoing for the fusion reactor material study. This accelerator system will irradiate high current deuteron beam to the free surface liquid lithium (Li) target (Fig. 1) and produce intense neutron flux by the nuclear reaction between deuteron and Li. Moreover, it will have to realize Ultra High Vacuum (<10-5 Pa) to keep the availability of superconducting accelerator and High Vacuum (>10-3 Pa) at the target system to prevent the Li boiling. However, contamination and vacuum degradation by liquid Li are concerned because the A-FNS accelerator consists of wide cross-sectional beam ducts for transporting a high current beam with a strong space charge effect. Therefore, the experimental validation of outgas rate, residual gas type and Li vapor behavior are required for individual condition of operational temperature and flow speed of the liquid Li.
For the vacuum characteristics study, we designed the dedicated experimental setup to demonstrate the interface between the beam transport system and the free surface liquid Li target system. Although this setup was 1/10 scale length of the A-FNS accelerator design, the cross-sectional size and the configuration of test ducts were designed same ratio as the actual system. In addition, the vacuum pumps layout and the pressure distribution were confirmed by the 3D Montecarlo vacuum simulation code Molflow+. In order to reduce the Li vapor contamination, Ar gas shield method was introduced by referring the gas jet curtain method in RIKEN [1]. Also, vacuum gauges, thickness monitors (QCM: Quartz Crystal Microbalances) and a residual gas analyzer (QMS: Quadrupole Mass Spectrometers) were installed to observe the vacuum characteristics. Finally, the assembled setup was attached to a liquid Li loop system (Fig. 1).
As a result of this experiment, although the reaching pressure is higher than the design value due to huge outgassing from Li target, a stable Li flow was demonstrated under large differential pumping from 10-2 Pa to 10-4 Pa. In addition, the base pressure and the partial pressure of the accelerator vacuum was kept regardless of the operational temperature (220-250 ℃) and the flow speed (13-18 m/s) of the liquid Li. However, the inflow of Li vapor was detected by the QMS and it was enhanced by rising the Li temperature of 220 ℃ to 250 ℃. On the other hand, the Li vapor flux was decreased by increasing the Li flow speed of 13 m/s to18 m/s. The relation between the vapor flux and the flow speed was verified by the analysis with a 2D thermal conductive equation for the Li flow surface. Regarding the Ar gas shield, the significant Li vapor suppression was not effective. After the experiment, the detector of the thickness monitor in the accelerator vacuum was observed by a Scanning Electron Microscope (SEM) to visualize the vapor condition inflowing from the liquid Li target system. The Li particle seemed droplets which composed of micron clusters. The large droplets did not affect the vacuum pressure but behave as a contaminate particle.
In summary, the designed differential pumping can be realized for the interface between the beam transport system and the free surface liquid Li target system. However, the contamination by the Li vapor will be depended on the operational condition of the Li target. In order to minimize the inflow of the Li vapor, we can conclude that the Li temperature and flow speed should be kept under 250 ℃ and over 18 m/s, respectively. The detail of this experiment and result will be presented.
References
[1] H. Imao et al., “Charge stripping of 238 U ion beam by helium gas stripper” Phys. Rev. ST Accel. Beams, 15, 123501 (2012).
