The quasi-stable sheet cavitation produced in a small Venturi channel is investigated using a fast synchrotron X-ray imaging technique aided with conventional high speed photography. The use of X-rays instead of visible light solves cavitation opacity related issues, and X-ray phase contrast-based edge enhancement enables high-definition visualization of the internal two-phase morphology. The simultaneous acquisition of time-resolved velocity and void fraction fields through post-processing the recorded X-ray images reveals, for the first time, the complex diphasic flow structures inside the sheet cavity, which is essentially divided into 6 characteristic parts. Distinct from the current mainstream view, the globallysteady sheet cavitation is found to be characterized by a weak but constantly-existing re-entrant flow that can penetrate the entire cavity. The turbulent velocity fluctuations inside the sheet cavity are also investigated. The turbulence level in the reverse flow region is observed to be as low as in the outer main flow demonstrating the relatively steady status of the reentrant flow. Unlike the streamwise and cross-stream fluctuations, the shear stress appears to be weakly correlated with the velocity gradient. The collapse of vapor phase and the vaporization at the upstream cavity interface are found to be the primary causes of shear stress intensification.Partial cavitation has two main forms of appearance: sheet and cloud cavitation (Pelz et al. 2017). The former generally appears as a quasi-stable sheet cavity with only small vapor shedding at its closure region, i.e. with an open closure. While in the latter case, the cavity is highly unstable as a consequence of periodic shedding of large vapor structures.The transition from sheet to cloud cavitation is, in a classic view, related to the motion of a re-entrant jet that breaks off the sheet cavity from the leading edge and consequently causes a large cavitation cloud to shed (Knapp, 1955). The existence of the re-entrant jet in periodic cloud cavitation was confirmed through the method of dye injection (Le et al. 1993). Kawanami et al. (1997) placed a small obstacle on the suction side of a hydrofoil to prevent the re-entrant jet from moving upstream, and the large cloud shedding disappeared, demonstrating the dominant role of the re-entrant jet. The conditions necessary for the development of the re-entrant jet has been explored by Callenaere et al. (2001). They showed the critical role of adverse pressure gradient at the cavity closure in the onset of the re-entrant jet instability.Regarding the relatively stable sheet cavitation, Gopalan & Katz (2000), Callenaere et al. (2001) and Laberteaux & Ceccio (2001) described that no clear re-entrant jet was observed or only the weak reverse flow existed at the trailing edge

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