Prepulse effect for recombining plasma produced by ultrashort high-intensity lasers

We compare the space-resolved characteristics of carbon plasmas produced using a single ultrashort high-intensity laser pulse (> 10(19) W/cm(2)) of less than 800 fs duration and those using a combination of a 100 ps duration prepulse and an ultrashort laser pulse. Profiles of electron density and electron temperature as functions of position from the original target surface are derived from the measured one-dimensional space-resolved soft X-ray spectra. The electron density was estimated from the Inglis-Teller limit and from a comparison of the experimental and the calculated profiles of Lyman series lines. The electron density gradually increases up to the critical density of the incident laser light near the target surface, and the density gradient is much steeper when the plasma is produced using the single ultrashort pulse. The electron temperature was estimated from the intensity ratio of the H-like Ly-beta line to the He-like He-beta line and that of the H-like Ly-alpha line to its satellite lines. The plasmas produced using the prepulse additionally are relatively uniform with moderate electron density and temperature, and a larger portion of the K-shell lines from carbon ions is produced in the recombining phase, rather than in the heating phase. With a suitable choice of prepulses, the spatial and temporal profiles of the plasma density and temperature can be controlled, which can facilitate applications of the laser-produced plasmas to various kinds of research, such as producing a gain medium for X-ray lasers.