  The propagation of intense picosecond laser pulses in air in the presence of
strong nonlinear self-action effects and air ionization is investigated
experimentally and numerically. The model used for numerical analysis is based
on the nonlinear propagator for the optical field coupled with the rate
equations for the production of various ionic species and plasma temperature.
Our results show that the phenomenon of plasma-driven intensity clamping, which
is paramount in femtosecond laser filamentation, holds for picosecond pulses.
Furthermore, the temporal pulse distortions are limited and the pulse fluence
is also clamped. The resulting unique feature of the picosecond filamentation
regime is the production of a broad, fully ionized air channel, continuous both
longitudinally and transversely, which may be instrumental for numerous
applications.
