  We present an analytic study of the physics of the glasma which is a strong
classical gluon field created at early stage of high-energy heavy-ion
collisions. Our analysis is based on the picture that the glasma just after the
collision is made of color electric and magnetic flux tubes extending in the
longitudinal direction with their diameters of the order of 1/Q_s (Q_s is the
saturation scale of the colliding nuclei). We find that both the electric and
magnetic flux tubes expand outwards and the field strength inside the flux tube
decays rapidly in time. Next we investigate whether there exist instabilities
against small rapidity-dependent perturbations for a fixed color configuration.
We find that the magnetic background field exhibits an instability induced by
the fluctuations in the lowest Landau level, and it grows in the time scale of
1/Q_s. For the electric background field we find no apparent instability while
the possible relation to the Schwinger mechanism for particle pair creations is
suggested.
