  We construct groundstates of the string with non-zero mass gap and
non-trivial chromo-magnetic fields as well as curvature. The exact spectrum as
function of the chromo-magnetic fields and curvature is derived. We examine the
behavior of the spectrum, and find that there is a maximal value for the
magnetic field $H_{\rm max}\sim M_{\rm Plank}^2$. At this value all states that
couple to the magnetic field become infinitely massive and decouple. We also
find tachyonic instabilities for strong background fields of the order ${\cal
O}(\mu M_{\rm Plank})$ where $\mu$ is the mass gap of the theory. Unlike the
field theory case, we find that such ground states become stable again for
magnetic fields of the order ${\cal O}(M^2_{\rm Plank})$. The implications of
these results are discussed.
