Observation of intense filaments in fully developed turbulence

We study the structure of the large events which control the tails of the pdf of the velocity derivatives in an helium experiment. For R(lambda) < 700, we find that these events have a remarkable structure, which is consistent with that of intense vortex filaments (''worms'') observed previously in numerical studies. The characteristics of such objects (form, size and circulation distributions, scaling with the Reynolds number) are in good agreement with reported numerical findings. These objects do not contribute significantly to the overall dissipation but they control an important part of the intermittency in the inertial range. Above R(lambda) approximate to 700, the structure of such events dramatically changes; we interpret this observation as an instability. We finally offer a physical interpretation of a previous experiment showing that around R(lambda) approximate to 700, the flatness of the velocity derivatives shows a transitional behaviour.