Ground-state depletion mechanisms in pulsed barium vapor lasers

Depletion of the metal vapor density from the central region of metal vapor laser tubes has recently been identified as an important factor limiting laser output. We have studied the mechanisms that deplete the ground-state metal density profile by measuring radially resolved population histories of neutral and ionic species during a "burst" (i.e., a pulse train of 20-30 excitation pulses) in a barium vapor laser (BVL). The observed spatiotemporal density behavior during the afterglow of the first shot in the burst agrees well with a simple model for diffusion and recombination, which we have used to show that the primary depletion mechanism during the establishment of steady-state conditions arises from the ambipolar diffusion of ions to the tube wall, and that gas-heating effects are secondary. Analysis of the steady-state (i.e., late-burst) afterglow behavior further reveals that the depletion is significantly greater than that expected from the measured barium ionization, particularly when operating at low barium densities at the wall; this we attribute to a large radial ambipolar field induced by the presence of ionized buffer-gas atoms on axis. The results show that it is important to reduce the time-averaged ionization in order to minimize ground-state depletion. The implications for power scaling of the BVL and other metal vapor lasers are discussed.