A modulational instability analysis is carried out for a new kind of integr
o-differential nonlinear Schrodinger equation which describes nonlinear wav
e-envelope propagation for several different situations, such as the transp
ort of very intense charged-particle beams in accelerating machines, large
amplitude electromagnetic wavepacket propagation in nonlinear media (optica
l fibres, plasmas, etc.), Langmuir-wave-envelope propagation in warm plasma
s, and collective dynamics in mesoscopic physics. The modulational instabil
ity analysis is extended from configuration space into phase space, by usin
g the Wigner transform. It is shown that the propagation of a wavepacket in
a nonlinear medium, governed by the above nonlinear Schrodinger equation,
can be described, in phase space, in terms of a kinetic-like theory similar
to the one based on the Vlasov equation which is used for describing both
collective plasma dynamics and collective longitudinal dynamics of charged-
particle beams in accelerating machines. Remarkably, the phenomenon of Land
au damping is recovered for the longitudinal charged-particle beam dynamics
(extending in this way a previous analysis carried out within the Thermal
Wave Model [R. Fedele and G. Miele, Il Nuovo Cim. D13, 1527 (1991)]) but is
also predicted for other physical situations concerning electromagnetic no
nlinear wave envelope propagation and mesoscopic physics. Furthermore, the
concept of a coupling impedance associated with the wavepacket propagation
is also introduced in analogy to the one of charged-particle bunches. This
approach provides stability charts fully similar to the ones describing cha
rged-particle beams in accelerating machines. These new results generalize
the conventional theory of the modulational instability associated with the
nonlinear Schrodinger equation and show clearly the stabilizing role of La
ndau damping during the development of the modulational instability.