X-ray Spectroscopic Studies of a Solid-Density Germanium Plasma Created by a Free Electron Laser

Gabriel Pérez-Callejo,Hyun-Kyung Chung,Bruce A Hammel,Ming-Fu Lin,Justin S Wark,Tomas Burian,Shenyuan Ren,Sam M Vinko,Tim Brandt Van Driel,Thomas R Preston,Ryan Royle,Oliver Humphries,Vojtěch Vozda

doi:10.3390/app10228153

Abstract

The generation of solid-density plasmas in a controlled manner using an X-ray free electron laser (XFEL) has opened up the possibility of diagnosing the atomic properties of hot, strongly coupled systems in novel ways. Previous work has concentrated on K-shell emission spectroscopy of low Z (<= 14) elements. Here, we extend these studies to the mid-Z(=32) element Germanium, where the XFEL creates copious L-shell holes, and the plasma conditions are interrogated by recording of the associated L-shell X-ray emission spectra. Given the desirability of generating as uniform a plasma as possible, we present here a study of the effects of the FEL photon energy on the temperatures and electron densities created, and their uniformity in the FEL beam propagation direction. We show that good uniformity can be achieved by tuning the photon energy of the XFEL such that it does not overlap significantly with L-shell to M-shell bound-bound transitions, and lies below the L-edges of the ions formed during the heating process. Reasonable agreement between experiment and simulations is found for the emitted X-ray spectra, demonstrating that for these higher Z elements, the selection of appropriate XFEL parameters is important for achieving uniformity in the plasma conditions.

Highlights

Laser (XFEL) pulse of the Linear Coherent Light Source (LCLS) at irradiances of order 1017 Wcm−2 onto foil targets, it was demonstrated that hot plasmas could be created at exactly solid density [1,2]
As the final temperatures produced are insufficient to thermally excite the K-shell of such ions, the observed emission spectrum is gated by the pulse length of the LCLS beam itself, and on such a timescale, the micron scale solid density plasma is effectively confined by its own inertia
L-M bound-bound transitions at energies that could overlap those of the heating X-rays. Such bound transitions were largely absent in the lower Z work, as the M shell electrons of the cold solid are already free for cold Mg and Al, and the measured ionization potential depression (IPD) indicate that rebinding of the M-shell would not have taken place for those elements, except perhaps at the highest ion stages. Given that both the relevant edge structures and bound-bound transitions of the various ion stages of Ge will alter the energy deposition of the FEL as the Ge plasma passes through the various ion stages as it is heated, we might expect a degree of richness and complexity in the way in which the intense LCLS beam interacts with this system: this is the case, and in this energy range, we find that the greatest uniformity of energy deposition occurs for an incident photon energy around 1400 eV, with an associated lower overall electron temperature than obtained with LCLS photon energies immediately above and below this value, and we discuss below the underlying atomic physics underpinning this finding

Summary

Introduction

Laser (XFEL) pulse of the Linear Coherent Light Source (LCLS) at irradiances of order 1017 Wcm−2 onto foil targets, it was demonstrated that hot (up to 200 eV) plasmas could be created at exactly solid density [1,2]. Sci. 2020, 10, 8153 filling the single or double core holes in the ions created by the XFEL, leading, in many cases, to emission all the way up to the hydrogenic species (i.e., Ly-α). As the final temperatures produced are insufficient to thermally excite the K-shell of such ions, the observed emission spectrum is gated by the pulse length of the LCLS beam itself (typically sub-100 fs), and on such a timescale, the micron scale solid density plasma is effectively confined by its own inertia

Methods

Results

Conclusion