Experimental characterization of short-wavelength Ni-like soft-x-ray lasing toward the water window

We report experimental results of studies of Ni-like x-ray lasing at wavelengths as short as 4.5 nm. The performance of x-ray lasing pumped by various types of pulse train that were composed of 100-ps pulses was investigated with a double-slab target placed in series to double the gain length. Two opposing laser beams separated by an appropriate time difference irradiated the double targets for quasi-traveling-wave pumping. Well-collimated double-target amplification was successfully demonstrated for Yb and Hf lasing at 5.0 and 4.7 nm whose gain-length products were 11 and 6, respectively. The Ni-like lasing lines of Ta have also been observed at 4.5 nm.