The International Fusion Materials Irradiation Facility (IFMIF) has been conceived as a high-flux 14 MeV neutron source for testing candidate fusion reactor materials. In the current design, neutrons are generated by irradiating a target with a deuteron beam and high-speed free-surface flow of liquid metal lithium (Li) is adopted as the target. To reveal the stability of the Li flow, we have examined characteristics of surface waves at a location 175 nm downstream from a nozzle exit, which corresponds to the center of the beam irradiated region. In this study, the characteristics of surface waves just downstream of the nozzle exit were measured experimentally, since the initial growth of surface waves exerts a definite influence on the surface behavior of the Li flow in the downstream region. Experiments were carried out with a focus on surface oscillations of the Li flow using the lithium circulation loop at Osaka University. These oscillations are measured using an electro-contact probe apparatus, which can detect electrically a contact between the probe tip and the Li surface and provide local height data of surface waves. The apparatus was installed at a location 15 mm downstream from the nozzle exit and scanned the Li surface by moving along the liquid-depth direction. The experiments were performed for the velocity range of 3-15 m/s under argon gas atmosphere at a pressure of 0.13 MPa. The contact signal recorded in the experiment was used to analyze the characteristics of surface waves, and then the root-mean-square wave amplitude and the frequency of surface waves were calculated. It was found that the root-mean-square wave amplitudes of surface waves increased with a rise in the flow velocity, and reached approximately 0.18 mm at 14-15 m/s. And also, obtained frequencies were analyzed using a linear stability theory, and the variation of frequencies was examined with the mean flow velocity.

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