  We describe a new technique, using thin lines of triplet-state He2 molecular
tracers created by femtosecond-laser field-ionization of helium atoms, for
visualizing the flow of the normal fluid in superfluid 4He, together with its
application to thermal counterflow in a channel. We show that, at relatively
small velocities, where the superfluid is already turbulent, the flow of the
normal fluid remains laminar, but with a distorted velocity profile, while at a
higher velocity there is a transition to turbulence. The form of the structure
function in this turbulent state differs significantly from that found in types
of conventional turbulence. This visualization technique also promises to be
applicable to other fluid dynamical problems involving cryogenic helium.
