Femtosecond X-ray Lasers at λ=32.8 and 44.4 nm in a Plasma Formed by Optical Field Ionization in a Krypton Cluster Jet

Elena P Ivanova

doi:10.11648/j.ajpa.20210905.11

Abstract

Due to high efficiency, X-ray lasers based on transitions of Ni-like krypton (Kr8+) are being actively studied. The main focus is on an X-ray laser based on the conventional 3d5/24d5/2 [J=0] – 3d3/24p1/2 [J=1] transition at λ=32.8 nm. Gaseous krypton targets or krypton cluster jets are used in various experiments. X-ray lasers at 32.8 nm in a plasma formed by optical field ionization in a krypton cluster jet are widely used for research of nanoobjects. In this article, the possibility of creating an efficient X-ray laser in Ni-like krypton based on a transition with optical self-pumping 3d3/24f5/2 [J=1] – 3d3/24d5/2 [J=1] at λ=44.4 nm is predicted for the first time. The plasma filament is excited upon interaction of a jet of krypton clusters with an intense pump laser pulse. Optimal conditions to achieve the duration tlas ≤300 fs of the X-ray laser radiation are determined. The optimal electron density is in a rather narrow interval in the range ne ~ 1021 - 2×1021 cm-3. The optimal electron temperature is several keV. It is likely that this explains the fact that no X-ray laser has been observed on this transition in Kr8+ so far. The conversion factor per pulse is found to be ~5×10-5. For an X-ray laser operating on the conventional transition 3d5/24d5/2 [J=0] – 3d3/24p1/2 [J=1] at λ=32.8 nm, tlas ≤ 300 fs can also be achieved; however, the conversion factor for this transition is times ~5 smaller than that for the former transition.

Highlights

X-ray free electron lasers (XFELs) with λ=1-10 nm and a pulse duration of the order of a hundred of femtoseconds allow recording holographic images of intermediate states of nano-objects at times comparable to the times of atomic motions [1,2,3,4]
In 2017, a new generation femtosecond XFEL came into commission in Hamburg [5]
The duration tlas was defined as the full width at half maximum (FWHM) of the X-ray lasers (XRLs) radiation pulse intensity

Summary

Introduction

X-ray free electron lasers (XFELs) with λ=1-10 nm and a pulse duration (tlas) of the order of a hundred of femtoseconds allow recording holographic images of intermediate states of nano-objects at times comparable to the times of atomic motions [1,2,3,4] These works demonstrate the possibility of observing the dynamics of phase transitions in solids – the formation of cracks, the nucleation and separation of phases, and rapid fluctuations in liquids and biological cells. A similar scheme was implemented in the experiment of Sebban et al [14], where a Ni-like krypton plasma was created by the method of OFI of krypton atoms In this experiment, a circularly polarized pump laser beam was focused either into a cell with gaseous krypton or into a jet of gaseous krypton; the XRL on the conventional 3d5/24d5/2 [J=0] – 3d3/24p1/2 [J=1] transition with λ=32.8 nm in (Kr8+) was studied. The duration tlas was defined as the full width at half maximum (FWHM) of the XRL radiation pulse intensity

Formulation of the Problem

Conclusion