  We study the propagation of 0.05-1 TW power, ultrafast laser pulses in a 10
meter long rubidium vapor cell. The central wavelength of the laser is resonant
with the $D_2$ line of rubidium and the peak intensity in the $10^{12}-10^{14}
~W/cm^2$ range, enough to create a plasma channel with single electron
ionization. We observe the absorption of the laser pulse for low energy, a
regime of transverse confinement of the laser beam by the strong resonant
nonlinearity for higher energies and the transverse broadening of the output
beam when the nonlinearity is saturated due to full medium ionization. We
compare experimental observations of transmitted pulse energy and transverse
fluence profile with the results of computer simulations modeling pulse
propagation. We find a qualitative agreement between theory and experiment that
corroborates the validity of our propagation model. While the quantitative
differences are substantial, the results show that the model can be used to
interpret the observed phenomena in terms of self-focusing and channeling of
the laser pulses by the saturable nonlinearity and the transparency of the
fully ionized medium along the propagation axis.
