The results of space-resolved investigations on false amplification ob
served in soft-x-ray laser experiments that are due to axial plasma ex
pansion are presented. Space-resolved observations show a difference i
n the spontaneous-emission intensities of the N Balmer-alpha line for
plasmas with different active media lengths, which can be a cause of o
verestimation of gain coefficients. The deviations of the intensities
are found to be small for optically thick lines and for lower pump int
ensities. Modeling calculations that include the effects of axial expa
nsions are also performed that support the experimental observations.
The different N Balmer-alpha intensity profiles observed for short and
long active media lengths are attributed to expansion of the cylindri
cal plasma in the axial direction, which tends to force plasmas with s
maller volumes to expand and cool at an enhanced rate. (C) 1996 Optica
l Society of America.