Instabilities in strong magnetic fields in string theory

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 Planck}) 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.