Dynamics of the dendritic flux instability in $\chem{YBa_2Cu_3O_{7-\delta}}$ films

We have studied the dynamics of magnetic-flux avalanches in superconducting $\chem{YBa_2Cu_3O_{7-\delta}}$ films by means of a fast magneto-optic pump-probe technique. Two regimes of propagation are found: in regime I, directly after the nucleation of the avalanches by a femtosecond laser pulse, the velocity v of the flux dendrites depends strongly on the magnetic field, and values of v up to 180$\un{km/s}$ are observed. Within some ten nanoseconds after nucleation regime II is reached, where the propagation velocity has dropped by one order of magnitude and is nearly independent of sample temperature and magnetic field. Comparison with previous experiments on flux jumps in superconductors shows good qualitative agreement.