  Scanning tunneling spectroscopy was performed on single crystals of
superconducting 2H-NbSe2, at 300 mK and in a magnetic field, up to 5 T, applied
parallel to the ab-plane. This novel field geometry allows the quasiparticle
density-of-states spectrum to be measured under finite superfluid momentum,
while avoiding contributions from the vortex-core bound states. At zero field,
we observed a fully-gapped conductance spectrum with both gap-edge peaks and
sub-gap kinks. These spectral features show a systematic evolution with the
applied field: the kinks close in while the peaks move apart in low fields, and
the zero-bias conductance has a two-sloped behavior over the entire field
range, though dipping anomalously at 0.7 T. Our data was analyzed with recent
theoretical models for quasiparticle tunneling into a current-carrying
superconductor, and yielded distinct evidence for multiple superconducting gaps
coming from various Fermi-surface sheets of different topologies, as well as
possible implications on the origin of the coexisting charge-density-wave
order.
