Speaker
Description
The interaction of a high-intensity laser with matter generates plasma. The subsequent laser-plasma interaction is responsible for the emission of radiation spanning from radiofrequencies up to gamma-rays. The X-rays emitted by the plasma are mainly related to the electrons. Under some assumptions, it is possible to consider a Maxwellian velocity distribution for these electrons, and the continuum X-ray emission can be described as a thermal bremsstrahlung and recombination spectrum, which is strictly related to the plasma electron temperature. Laser-generated plasmas can reach electron densities of ~ $10^{23} \ \text{cm}^{-3}$ and temperatures of several hundreds of electronvolts; consequently, a major component of the emitted electromagnetic spectrum lies in the soft X-ray region of 0.15 – 2 keV [1].
X-ray emission plays a fundamental role in indirect-drive inertial confinement fusion, where it is used to uniformly compress the fuel capsule and achieve ignition [2]. Moreover, laser-induced plasmas represent a source of soft X-rays with characteristics unmatched by conventional sources, enabling applications in high-resolution imaging, radiobiology, and micromachining [3]. Its analysis can of course give important information about the laser-produced plasma they are coming from. Therefore, the characterization of this emission is of great importance for both plasma diagnostics and a wide range of applications.
In this work, we present the soft X-ray diagnostic system developed at the ABC laser facility at the ENEA Research Centre in Frascati. The setup consists of an array of eight Ge(Li) PIN diodes equipped with filters of different materials and thickness, allowing measurements of the X-ray emission spectrum produced during laser-matter interaction experiments. By exploiting the different spectral responses of each detector with its filter, plasma electron temperature can be inferred from the ratios of the signals measured by diode pairs [4]. The theoretical framework underlying the temperature reconstruction method is discussed, together with the optimization of the filter configuration and the evaluation of the associated measurement uncertainties. A dedicated analysis procedure has been developed to combine information from multiple detector channels and obtain a robust estimate of the plasma temperature and its error.
Experimental results obtained during campaigns at the ABC facility on aluminum and two-photon polymerized foam targets are presented. The information regarding soft X-ray emission from the filtered-diode array are compared with independent measurements performed using a transmission diffraction grating [5], showing good agreement and validating the diagnostic approach. These results demonstrate the effectiveness of the system as a compact and reliable tool for soft X-ray detection and plasma temperature characterization in laser-plasma experiments.
References
[1] P. Gorenstein and W. H. Tucker, Annu. Rev. Astron. Astrophys. 14, 373 (1976).
[2] J. Lindl, Phys. Plasmas 2, 3933 (1995).
[3] I. C. E. Turcu and J. B. Dance, X-rays from Laser Plasmas: Generation and Applications (John Wiley & Sons, Chichester, 1999).
[4] N. G. Basov, Heating and Compression of Thermonuclear Targets by Laser Beam (Cambridge University Press, Cambridge, 1986).
[5] M. Salvadori et al., J. Instrum. 14, C03007 (2019).
Acknowledgements
Activities partially funded through the ReMade@ARI project by the European Union as part of the Horizon Europe call HORIZON-INFRA-2021-SERV-01 under grant agreement number 101058414 and co-funded by UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee (grant number 10039728) and by the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number 22.00187. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the UK Science and Technology Facilities Council or the Swiss State Secretariat for Education, Research and Innovation (SERI). Neither the European Union nor the granting authorities can be held responsible for them.