Collapse of the critical state in superconducting niobium*

Giant abrupt changes in the magnetic flux distribution in niobium foils were studied by using magneto-optical visualization, thermal and magnetic measurements. Uniform flux jumps and sometimes almost total catastrophic collapse of the critical state are reported. Results are discussed in terms of thermomagnetic instability mechanism with different heat removal channels. Video figures are available at: http://www.cmpgroup.ameslab.gov/supermaglab/video/Nb.html