The rapid heating of a thin titanium foil by a high intensity, subpicosecond laser is studied by using a 2D narrow-band x-ray imaging and x-ray spectroscopy. A novel monochromatic imaging diagnostic tuned to 4.51 keV Ti $K\alpha$ was used to successfully visualize a significantly ionized area ($⟨Z⟩>17\pm 1$) of the solid density plasma to be within a $\sim 35\mu \mathrm{m}$ diameter spot in the transverse direction and $2\mu \mathrm{m}$ in depth. The measurements and a 2D collisional particle-in-cell simulation reveal that, in the fast isochoric heating of solid foil by an intense laser light, such a high ionization state in solid titanium is achieved by thermal diffusion from the hot preplasma in a few picoseconds after the pulse ends. The shift of $K\alpha$ and formation of a missing $K\alpha$ cannot be explained with the present atomic physics model. The measured $K\alpha$ image is reproduced only when a phenomenological model for the $K\alpha$ shift with a threshold ionization of $⟨Z⟩=17$ is included. This work reveals how the ionization state and electron temperature of the isochorically heated nonequilibrium plasma are independently increased.

• Received 29 January 2018
• Revised 10 March 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.155002