Thursday, November 19, 2009, 16:00
WHGA Auditorium
G. Fussmann, Humboldt-University Berlin
Abstract:
Scientists and laymen have been struggling for centuries to document and
explain the phenomenon of ball lightning. Despite of thousands of eye witness
reports there is up to present no consensus on their real existence nor how
they may be generated in the course of a natural event. However, two recent
experiments that were performed move their possible existence closer to
reality. In the first experiment balls of a few centimetres in diameters
and life times of up to 8 seconds are produced by applying a short electrical
stroke to pure silicon wafers. In the second case larger luminous clouds can
be generated from water discharges. Using this second mechanism we were able
to produce ball-like plasmoids by discharging a capacitor bank via a water
surface. In the autonomous stage after current zero the plasmoids have
diameters up to 0.2 m and lifetimes of some hundred milliseconds. They were
studied by applying high speed cameras, probes, calorimetric measurements,
laser beam deviation and spectroscopy. The plasmoids are found to consist of a
true plasma confined by a cold envelope. Their topology changes from initially
spherical to toroidal during the final phase. The emitted light is mainly
attributed to line radiation from Ca, K and Na impurities occurring in the
tap water. The energy source for the luminescence is provided by chemical
reactions. Ball lightning could thus be produced by a lightning stroke
releasing its energy in a small body of water of about 0.1 l volume.