Toward development of X-ray lases in water window

Abstract

Extensive experimental efforts have been spent on the development toward lasing to the ground state in CVI ions at 3.37 nm, which lies well within the `water window'. The propagation of 100 fs pulses, with an intensity of 4×1016 W/cm2, in optically ionized gases was studied both experimentally and numerically. Ionization-induced refraction was identified to be responsible for the much shorter propagation distance in N2 than in H2. Three dimensional particle-in-cell simulations also showed the roles played by the forward Raman and ionization scattering instabilities in further affecting the propagation. A modified igniter-heater technique, which involves the combination of a 200 ps Bessel beam and a 10 ns spherical beam, was successfully developed in creating plasma waveguides with 450 um diameter and over 1019 cm-3 axial density, ideal for Raman amplification. Much narrower plasma waveguides, with less than 10 um diameter, were produced using the combination of two Bessel beams of 100 fs and 200 ps duration, aiming at guiding the ultrashort and ultra-intense pump pulses in soft x-ray lasers. Spectroscopic study was performed for the plasmas created by a 100 fs, >1019 W/cm2 pulse in an ethane gas jet. A series of population inversions in CVI ions, including 3-2, 4-2, 5-2 and 5-4, were identified, especially when the level of pre-pulse was effectively suppressed by two RG850 saturable absorbers. Preliminary experiments were conducted to demonstrate the guiding of a 25 ps pulse in plasma waveguides over many Rayleigh lengths, opening the possibility of extending the plasmas having favorable conditions and thus increasing the gain to measurable level.