  Laser spectroscopy of the $^{\mathrm{229m}}$Th nuclear clock transition is
necessary for the future construction of a nuclear-based optical clock.
Precision laser sources with broad spectral coverage in the vacuum ultraviolet
are needed for this task. Here, we present a tunable vacuum-ultraviolet
frequency comb based on cavity-enhanced seventh-harmonic generation. Its
tunable spectrum covers the current uncertainty range of the
$^{\mathrm{229m}}$Th nuclear clock transition.
