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 pop
ulation histories of neutral and ionic species during a ''burst'' (i.e
., a pulse train of 20-30 excitation pulses) ina barium vapor laser (B
VL). The observed spatiotemporal density behavior during the afterglow
of the first shot in the burst agrees well with a simple model for di
ffusion and recombination, which we have used to show that the primary
depletion mechanism during the establishment of steady-state conditio
ns arises from the ambipolar diffusion of ions to the tube wall, and t
hat 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 tha
t it is important to reduce the time-averaged ionization in order to m
inimize ground-state depletion. The implications for power scaling of
the BVL and other metal vapor lasers are discussed. (C) 1997 American
Institute of Physics.