Vacuum-ultraviolet photoabsorption imaging system for laser plasma plume diagnostics

We describe a recently designed and constructed system based on a 1 m normal incidence vacuum monochromator with corrected (toroidal) optics that produces a wavelength tuneable and collimated vacuum-ultraviolet (VUV) (lambda = 30 - 100 nm) beam. The VUV continuum source is a laser-generated gold plasma. The primary function of the system is the measurement of time resolved "images'' or spatial distributions of photoabsorption/photoionization in expanding laser plasma plumes. This is achieved by passing the beam through the sample of interest (in our case a second synchronised plasma) and recording the "footprint'' of the attenuated beam on a charge coupled device. Using this VUV photoabsorption imaging or "shadowgraphy'' technique we track and extract column density distributions in expanding plasma plumes. We can also measure the plume front velocity. We have characterized the system, particularly in relation to spectral and spatial resolution and the experimental results meet very well the expectations from ray tracing done at the design phase. We present first photoabsorption images and column density distributions of laser produced Ca plumes from the system.