Laser plasma imaging and spectroscopy to enhance radiance for water window source development

Laser produced plasmas (LPPs) offer a compact and affordable route to soft X-ray (SXR) illumination of biological samples for microscopy in the water window (WW), the spectral region between 2.34 and 4.38 nm. The radiance of an LPP source scales with laser repetition rate and individual plasma radiance. The latter is optimised by maximizing conversion efficiency from laser light to SXR radiation while also reducing the emission area of the plasma. This is the first detailed study reporting plasma emission area and its contribution to radiance improvement for a range of solid metal target materials. Based on a 6 ns 1064 nm laser, radiance across the water window region is optimised by illuminating a molybdenum target with 38 mJ to 77 mJ of laser energy, creating a plasma emission area of 65 mu m(2) to 115 mu m(2). An estimate of the absolute radiance is provided for comparison with bending magnets at synchrotrons.