X-Ray Spectral Determination of Electron Density in Dense Laser-Excited Targets*

Abstract

Plasmas produced by subpicosecond laser excitation of solid targets can produce bright short x-ray pulses[1] for x-ray laser pumping[2]. This type of plasma is unusual because it is hot (electron temperature in keV range), dense (near solid density) and highly non-equilibrium. It is particularly advantageous to use subpicosecond excitation to investigate this type of plasma, since expansion of the plasma can be quite small under this condition. Furthermore, it has been found that the prepulse associated with typical high-power ultraviolet laser systems does not Interfere with the subpicosecond excitation for ultraviolet transmitting solids, such as MgF2 and SiO2. This follows, since the intensity for appreciable absorption is roughly five orders of magnitude higher than the intensity associated with the laser prepulse. Also for laser radiation with wavelength of 248 nm, the critical density is 1.8 × 1022 cm−2, only a factor of 44 lower than the electron density of a completely stripped MgF2 solid target. Therefore, the laser radiation naturally interacts with a plasma of very high density. Finally, x-ray spectroscopy is particularly useful for diagnosing the conditions of the plasma for very short time intervals that are considerably outside of the range available for conventional streak-camera measurements. Since typical radiative lifetimes for kilovolt transitions are of the order of 100 fs, the emitted radiation can provide valuable information in the early hlgh-density phase of the plasma. In this experiment, we examine the dielectronic satellite from lithium-like ions in order to determine the electron density of the plasma.