The Wide-Open Waveguide represents a novel type of Crab Cavity, proposed for the hadronic versions of the FCC [1]. Compared to the bulk-niobium cavities under production for the High Luminosity LHC, the WOWCC is a copper cavity, whose inner surface is to be coated with a niobium thin film [2]. This approach is advantageous for both manufacturing and cryogenic aspects. To match the bulk SRF performance, the niobium film must however be dense, defect free and homogeneous throughout the complex 3D inner geometry of the cavity. This challenge has been addressed by a concerted RnD effort over the past years. Experimentally, the achievement of the requisite film quality has been demonstrated on both flat and complex shaped samples using the HiPIMS coating technique [3]. Numerical studies guided the design of a coating set-up with six cylindrical cathodes and movable magnets [4]. Heat-load management was likewise addressed. Currently, the full-scale coating set-up has entered a testing phase for both the mechanical and electrical components, which is proceeding successfully [5]. A first coating of the full-scale cavity prototype is thus foreseen in the coming months. A dedicated in-situ X-Ray Fluorescence diagnostic to assess the film-thickness distribution in the cavity is likewise coming to fruition.

[1] K. Papke et al, Phys. Rev. Accel. Beams 22, 072001, 2019

[2] S. Calatroni, Phys. C, 441, 95, 2006, https://doi.org/10.1016/j.physc.2006.03.044

[3] F. Manke et al, HiPIMS Today Conference, online, 03/2023, breakout room presentation

[4] F. Manke et al, 18th Conference for Plasma Surface Engineering, Erfurt, 09/2022

[5] F. Manke et al, 6^th CERN SRF workshop, 02/2023