[en] TRASCO-ADS is a national funded programme in which INFN. ENEA and Italian industries work on the design of an accelerator-driven subcritical system for nuclear waste transmutation. TRASCO is the Italian acronym for transmutation (TRASmutazione) of waste (SCOrie). One of the most critical aspects of an accelerator-driven system (ADS) is the interface between the UHV environment of the accelerator and the pressurised system of the spallation target (a contained plenum of Pb-Bi eutectic at 400 deg C with a gas ceiling at about 1 bar). The window is the physical interface between the two environments. Thermo-mechanical and radioprotection issue pointed out that the window is a critical issue. The windowless interface consists of a system where the spallation target and the linac are separated only by a progressive dynamic pumping system that connects the vacuum of the linac accelerator and the coolant flow of the spallation target. Theoretical consideration and calculation results are presented in this paper. The need for a validation of the model gave the motivation for an experimental set-up whose results are also presented. The extrapolation to the full system needs accurate analysis for a compliant dimensioning of the system in the interface region. (authors)