Effect of wave envelope of pulse-train laser on production of x-ray laser gain in recombining lithium-like aluminum plasmas

We have investigated the effect of the envelope of the pulse-train laser on the gain coefficients of the 3d-4f (15.5 nm) and 3d-5f (10.6 nm) lines of lithium-like aluminum ions. An aluminum slab target was irradiated by neodymium glass laser light, which consisted of 16 pulses; each pulse was 80 ps in width and separated by 270 ps nominally. Three types of pulse-train envelope have been investigated: with Type (A); sustained 16 pulses, Type (B); attenuated latter 8 pulses and Type (C); eliminated latter 8 pulses. The pulse-train laser was focused on a line, whose length was controlled using a variable aperture. Time-integrated x-ray spectra were measured for 1.25-cm-long and 2.5-cm-long plasmas using an x-ray CCD camera. The rates of the increase of the CCD counts due to lengthening the plasma from 1.25 cm to 2.5 cm were reduced to gain coefficients using Linford's equation. As the irradiation energy is decreased from similar to 2.3 J/cm to similar to 1.2 J/cm, the gain coefficients increase. The degree of the increase of the gain coefficient depends on the type of pulse-train envelope. In both cases of the 3d-4f and 3d-5f lines, the gain coefficients for Type (A) increase steeply below similar to 1.6 J/cm. On the other hand, the gain coefficients for Type (B) gradually increase below similar to 2.3 J/cm. The gain coefficient for Type (C) increases slightly in case of the 3d-4f line and is almost constant in case of the 3d-5f line.