The recent breakthroughs in laser-driving the ${}^{229}\mathrm{Th}$ nuclear transition have created an urgent demand for scalable vacuum-ultraviolet (VUV) sources capable of supporting high power spectral density at the critical 148.38 nm isomer energy. However, generating sufficient photon flux to overcome the low nuclear excitation probability remains a challenge for compact setups. While resonant dispersive-wave emission in gas-filled hollow-core fibers offers a promising route, standard capillaries face a fundamental trade-off: maximizing input coupling requires large core diameters, whereas efficient nonlinear VUV conversion demands high intensities and small cores. Here we resolve this conflict using a gas-filled tapered capillary fiber. This architecture uses a longitudinally decreasing core diameter to combine a large input aperture with adiabatic field concentration, thereby continuously enhancing the nonlinear interaction. Experimentally, we demonstrate a widely tunable source (135–240 nm) that achieves a twofold efficiency increase specifically at a wavelength of 148.38 nm compared with uniform geometries. By providing a scalable route to high-flux VUV generation, this work establishes a critical tabletop tool for advancing solid-state nuclear clocks and time-resolved spectroscopy.

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• Atomic, optical & lattice clocks
• Frequency combs & self-phase locking
• Frequency conversion
• Nonlinear waveguides
• Time & frequency standards
• Ultrafast optics
• Optical fibers
• Optical sources & detectors