X-RAY-EMISSION FROM A 650-FS LASER-PRODUCED BARIUM PLASMA

We have used x rays in the 9-15-angstrom band emitted from a solid tar get of BaF2 irradiated by approximately 120 mJ of 248-nm radiation in a 650-fs full width at half maximum Gaussian laser pulse to characteri ze spectroscopically the resulting ultrashort-pulse laser-produced pla sma. The emission was spectrally resolved but space and time integrate d. By comparing the spectrum with unresolved-transition-array calculat ions 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 assumin g an optically thin source in local thermodynamic equilibrium (LTE) an d 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 mo del indicates that the emitting region is at comparatively low tempera ture (200-300 eV) and high electron density (10(23)-10(24) cm-3). The degree of agreement between the model and the measured spectrum also s uggests that the emitting plasma is near LTE. We conclude that the emi ssion in this band arises from a solid-density plasma formed early in time, and is thus localized in both space and time. This interpretatio n is reinforced by LASNFX [G. B. Zimmerman and W. L. Kruer, Comments P lasma Phys. Controlled Fusion 11, 51 (1975)] simulations that indicate that emission in this band closely tracks the laser pulse.