  Magneto-optical imaging was used to observe dendritic flux avalanches
connecting the outer and inner edges of a ring-shaped superconducting MgB2
film. Such avalanches create heated channels across the entire width of the
ring, and inject large amounts of flux into the central hole. By measuring the
injected flux and the corresponding reduction of current, which is typically
15%, we estimate the maximum temperature in the channel to be 100 K, and the
duration of the process to be on the order of a microsecond. Flux creep
simulations reproduce all the observed features in the current density before
and after injection events.
