  Thermo-magnetic instability (TMI) in superconductors is known to destroy the
critical state via magnetic flux avalanches, and hence it deteriorates the
ability of the superconductors to shield external magnetic field. In this work,
we quantify to what extent the shielding current is affected by TMI. We recover
the critical state in one half of a thermo-magnetically unstable NbN film by
coating it with a thin layer of Cu. Suppression of the instability in the metal
coated part is confirmed by the results of a direct real time magneto-optical
imaging. A pattern of discontinuity lines in the observed flux distribution
indicates that only one quarter of the shielding current flows through the
whole sample, including the unstable uncoated part, while three quarters flow
in loops within the stable Cu-coated part.
