Investigating preplasma dynamics with x-ray spectroscopy

We present experimental results from the PHELIX laser facility in Darmstadt, Germany, utilizing both components of the PHELIX laser system: a nanosecond (ns) laser pulse and a 500 fs laser pulse. Our study aimed to understand the interaction dynamics of Cu targets by inducing intentional preplasma formation with the ns laser and heating electrons to a suprathermal distribution with the sub-ps laser.

The experimental method involved recording time-integrated x-ray line emission spectra of highly ionized Cu atoms, focusing on the He-like Rydberg series and Lyman-alpha lines. By varying preplasma conditions, we explored the effects of delays between the laser pulses on preplasma expansion dynamics, the impact of different energies in the ns pulse on preplasma formation intensity, and the influence of varying conditions of the ultrafast optical parametric amplifier (uOPA) on the laser contrast of the short-pulse laser.

Our results show distinct changes in the x-ray line emission spectrum under different preplasma conditions. We interpret these spectral signatures in terms of the plasma dynamics and its transient conditions. Additionally, we propose a comparative analysis of our findings with potential experiments at an X-ray Free-Electron Lasers (XFEL), such as the Linac Coherent Light Source (LCLS).

This study enhances our understanding of laser-plasma interactions and provides insights into optimizing experimental conditions for future high-energy density physics research with both optical lasers and XFELs.