Verification of accuracy of contact-probe distance meter for lithium target of fusion neutron source

•Experiments for developing the Li target system in the Fusion Neutron Source (FNS).•Measurements of free-surface fluctuation using a non-contact laser-probe method and electro-contact probe distance meter.•The 2D distribution was obtained from the non-contact measurement.•From the comparison between two methods, the over or under evaluation of the contact-type distance meter was confirmed.•These differences are expected to be caused by a high surface tension of Li and a collision phenomenon. An accelerator based intense neutron sources such as the IFMIF, A-FNS and DONES produce neutron flux by a stripping reaction between lithium (Li) and deuteron for testing various materials for fusion reactor. In these fusion neutron sources, a liquid Li free-surface jet is applied as the liquid Li target. For the development of the Li target system, the variation in the thickness of the Li jet was measured using the Li loop of Osaka University. At the Osaka Univ., a lot of knowledge about the surface fluctuation of the Li jet had been obtained by using the electro-contact type distance meter (referred to as the probe apparatus herein). Besides, for the stable and safety operation of the facility, the probe apparatus is planned to be used as a safety interlock system for beam irradiation in actual intense neutron source because of its simple structure. Therefore, it is desirable to verify its accuracy. Especially, it is expected that false detection and undetected waves, which is particular to the contact-type apparatus, could cause errors. Then in this study, the measurement using non-contact type distance meter using optical comb (referred to as the laser probe herein) was conducted, and then the error of the results of the probe apparatus is evaluated by comparing results.