  Langmuir turbulence is an archetype of wave turbulence in plasma physics. By
means of 1D-1V Vlasov-Poisson simulations, we show that coherent structures,
called Langmuir cavitons, are generated by the long time evolution of Langmuir
weak turbulence, thus illustrating the breakdown of a weak turbulence regime.
These structures correspond to an equilibrium between the pressure forces and
the ponderomotive force resulting from high frequency Langmuir oscillations.
Langmuir cavitons are typical features of strong Langmuir turbulence expected
to be generated at high energy and to saturate when Langmuir energy is of the
order of the plasma thermal energy. Despite this wide-spread belief, here we
observe that cavitons, emerging from weak Langmuir turbulence evolution,
saturate at much lower energies. We show that these Langmuir coherent
structures are characterized by a much larger length scale with respect to the
Debye length. This gives evidence that "large" and "shallow" stable cavitons
should be seen in space plasma observations. The transition toward strong
turbulence is shown to be a consequence of an initial weak turbulent inverse
cascade. Finally, the effective equation of state for ion acoustic oscillations
is tested numerically from the kinetic model.
