X-ray emission from a 650-fs laser-produced barium plasma

We have used x rays in the 9-15- band emitted from a solid target of BaF2 irradiated by ∼120 mJ of 248-nm radiation in a 650-fs full width at half maximum Gaussian laser pulse to characterize spectroscopically the resulting ultrashort-pulse laser-produced plasma. The emission was spectrally resolved but space and time integrated. By comparing the spectrum with unresolved- transition-array calculations and measurements of plasma emission from longer-pulse experiments, it is clear that ions as highly stripped as titaniumlike barium are present. We have successfully modeled the observed spectrum by assuming an optically thin source in local thermodynamic equilibrium (LTE) and using the super-transition-array theory [A. Bar-Shalom et al., Phys. Rev. A 40, 3183 (1989)] for emission from a hot, dense plasma. The model indicates that the emitting region is at comparatively low temperature (200-300 eV) and high electron density (1023-1024 cm-3). The degree of agreement between the model and the measured spectrum also suggests that the emitting plasma is near LTE. We conclude that the emission in this band arises from a solid-density plasma formed early in time, and is thus localized in both space and time. This interpretation is reinforced by lasnex [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 11, 51 (1975)] simulations that indicate that emission in this band closely tracks the laser pulse.