Continuum lowering in 100-fs laser produced plasmas

Abstract

We present what we believe to be the first measurements of continuum lowering in high-density plasmas produced by 100-fs laser pulses. Continuum lowering arises in dense plasmas when the excited states of an ion are perturbed by the close proximity of the neighboring ions, and can be a useful density diagnostic. It is a fundamental atomic physics concept in high-density plasmas important to work in X-ray lasers, ICF plasma diagnostics, astrophysics and plasma simulations. In our experiments, a 10-Hz, 100 mJ 100-fs laser system is used to create the plasmas studied, essentially providing a delta-function heat pump which terminates before significant hydrodynamic motion occurs. Continuum lowering was observed both in the high-density, high-temperature expanding plasma plume and in the solid, low-temperature shocked material of the target. In the expanding plasma, we used XUV emission spectroscopy to observe the suppression of high-lying excited levels of He-like and H-like boron. In the compressed plasma behind the shock wave, we used XUV absorption spectroscopy to measure the shifts in inner-shell absorption edges in boron (K-edge) and in aluminium (L-edge).