  We demonstrate the high-efficiency generation of water-window soft x-ray
emissions from polyethylene nanowire array targets irradiated by femtosecond
laser pulses at the intensity of 4*10^19 W/cm^2. The experimental results
indicate more than one order of magnitude enhancement of the water-window x-ray
emissions from the nanowire array targets compared to the planar targets. The
highest energy conversion efficiency from laser to water-window x-rays is
measured as 0.5%/sr, which comes from the targets with the longest nanowires.
Supported by particle-in-cell simulations and atomic kinetic codes, the physics
that leads to the high conversion efficiency is discussed.
