  The Thirty Meter Telescope (TMT) has been designed to include an adaptive
optics system and associated laser guide star (LGS) facility to correct for the
image distortion due to Earth's atmospheric turbulence and achieve
diffraction-limited imaging. We have calculated the response of mesospheric
sodium atoms to a pulsed laser that has been proposed for use in the LGS
facility, including modeling of the atomic physics, the light-atom
interactions, and the effect of the geomagnetic field and atomic collisions.
This particular pulsed laser format is shown to provide comparable photon
return to a continuous-wave (cw) laser of the same average power; both the cw
and pulsed lasers have the potential to satisfy the TMT design requirements for
photon return flux.
