7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Development of laser-produced Au plasma for water window x-ray radiation
sourcesJ. Wang1, M. Kishimoto1, T. Johzaki1,2, T. Higashiguchi3, A. Sunahara2,4, K. Mizushima1,K. Yamasaki1 and S. Namba1
1
Department of advanced science and engineering, Hiroshima University
2
Institute of Laser Engineering, Osaka University
3
Department of Electrical and Electronic Engineering, Utsunomiya University
4
Center for Material under Extreme Environment, Purdue University
e-mail (speaker): d213009@hiroshima-u.c.jp

Soft X-ray sources developed by laser produced plasma(LPP) are highly anticipated for their potential use indiagnostic and manufacturing nano-devices [1]. In
particular, soft X-rays in the so-called water-window(WW) region (2.3-4.4 nm) produces characteristicfeatures for biological diagnostics, as the K absorption
edges for C (4.4 nm), N (3 nm) and O (2.3 nm) lie in thiswavelength range, exhibiting high transmission contrasts
between these elements, which are the main constituentsof living organisms. Consequently, it is utilized as WWX-ray microscopy for biological diagnostic in many
circumstances. Because it not only allows imaging ofrelatively thick living organ samples without sample
pretreatments (such as dehydration and sectioning),
observing organelles in the cytoplasm with high contrast
also becomes achievable.
The imaging resolution and quality for WW X-ray
microscopy are highly limited by the WW photon
fluence reaching to the sample surface. However, only
large facilities like free electron laser or synchrotron
radiation can produce output over 1014 photons/ (sr ∙ s ∙µm2 ∙ 0.1% Band width) for a high-quality imaging [2].
Since the widespread deployment of WW X-ray
microscopy is hindered because of the associated costs
and availability of the required devices, it is crucial toenhance the photon fluence produced by the table-topdevices.
One of the methods is to use laser-produced Au
plasma. Due to the n=4-4 and 4-5 transitions in thehighly ionized Au ions attributed to Au 20+ - Au30+, X-ray
emissions emitted from countless energy levels forms theunresolved transition array (UTA) with a peakwavelength lying in the WW region [2]. On the otherhand, WW X-ray emission enhances when generating
laser-produced Au ions in a low pressure N2 gas
atmosphere, benefiting from the KLL-Auger electrons
emitted from N atom interact with the highly charged Au
ions [3]. These make Au a suitable material for LPP
scheme to develop a WW X-ray source.
To farther enhance the WW emission from the Au
plasma, optimization of laser-target interactions and laser
pulse features is then craved. An electron temperature of~400 eV in the plasma is required to generate highly
charged Au ions for WW emissions. However, since thelaser ablation intensity is not linear associated with the
X-ray emission area. In this work, we investigate thedependence of WW spectrum on laser pulse energy andlaser ablation spot size. The dependence of WW
emission on Au target thicknesses is also investigated totest different ablation processes and their plasma
expansion behaviors. Simulation for laser ablation
process is conducted by using STAR2D and FAC code.
The experiment setup is shown in Fig. 1. A seeder
injected table-top Nd:YAG laser (1064 nm, 6.2 ns, 10 Hz,>1013 W/cm2) was used in this work. The laser spot
focused though a f = 10-mm lens onto the Au target ismeasured to have a minimum FWHM= 15 μm. Au foils
and deposition targets with different thicknesses ranging
from 300 nm to 0.1 nm were used for plasma generation.
A grating incident spectrometer, a pinhole camera with
Ti filters, a Si photodiode with integration circuit were
installed to characterize the spatiotemporal plasma
behavior.

Fig. 1 Schematic of the experiment setup.
The results show a minimum Au target thickness of ~1μm is obtained for a laser intensity of ~1013 W/cm2 byobserving the intensity drop in the WW spectra. Au
targets produced by thermal evaporation are found tohave a higher conversion efficiency than commercial foil
targets for WW X-ray radiation. In addition, optimallaser spots for fixed laser energies (240 mJ and 650 mJ)
are found for an Au target ~1 mm in front of the focal
point, where suitable conditions for plasma temperatureand plume volume coupling are achieved.
References[1] J. Kirz, C. Jacobsen, and M. Howells, Q Rev Biophys, vol. 28, pp.
33–130, 1995.
[2] H. Ohashi et al. Appl. Phys. Lett., vol. 104, no. 23, 2014.
[3] C. John et al., Opt. Lett., vol. 44, no. 6, p. 1439, 2019