  Muon-spin rotation spectroscopy has been used to measure the internal
magnetic field distribution in NbSe2 for Hc1 << H < 0.25 Hc2. The deduced
profiles of the supercurrent density indicate that the vortex-core radius in
the bulk decreases sharply with increasing magnetic field. This effect, which
is attributed to increased vortex-vortex interactions, does not agree with the
dirty-limit microscopic theory. A simple phenomenological equation in which the
core radius depends on the intervortex spacing is used to model this behaviour.
In addition, we find for the first time that the in-plane magnetic penetration
depth increases linearly with H in the vortex state of a conventional
superconductor.
