Michal Červeňák

In this contribution, we present the results of a long-term study on radiofrequency and microwave electromagnetic pulse (EMP) emission, a phenomenon that is still not satisfactorily explained, performed at the PALS iodine laser system (1315 nm, 0.3 ns, up to 700 J on target). Using a comprehensive set of broadband diagnostics, including double ridged horn antennas, target current probe, along with special low-loss cables and oscilloscopes with sampling rates up to 100 GS/s, we observed an unprecedentedly broad EMP spectrum with frequencies significantly exceeding 10 GHz. Moreover, scaling experiments revealed a dependence of the spectral shape and its central frequency on the laser intensity, and this was further correlated with electron emission. This dependence indicates that in addition to the well-known influence of the eigenfrequencies of the experimental hardware (e.g., target, stalk, and chamber), processes associated with the expanding plasma can also play a dominant role, as demonstrated in this study. We believe that these new findings will not only contribute to EMP mitigation and deepen our understanding of fundamental physics, but also support high-tech applications such as EMP hardening, biomedical research, and defense technologies.

Recent experiments at the National Ignition Facility and theoretical modeling suggest that side stimulated Raman scattering (SSRS) instability could reduce laser–plasma coupling and generate considerable fluxes of suprathermal hot electrons under interaction conditions envisaged for direct-drive schemes for inertial confinement fusion. Nonetheless, SSRS remains to date one of the least understood parametric instabilities. Here, we report the first angularly and spectrally resolved measurements of scattered light at laser intensities relevant for the shock ignition scheme (I ∼ 1016 W/cm2), showing significant SSRS growth in the direction perpendicular to the laser polarization. Modification of the focal spot shape and orientation, obtained by using two different random phase plates, and of the density gradient of the plasma, by utilizing exploding foil targets of different thicknesses, clearly reveals a different dependence of backward SRS (BSRS) and SSRS on experimental parameters. While convective BSRS scales with plasma density scale length, as expected by linear theory, the growth of SSRS depends on the spot extension in the direction perpendicular to laser polarization. Our analysis therefore demonstrates that under current experimental conditions, with density scale lengths Ln ≈ 60–120 μm and spot sizes FWHM ≈ 40–100 μm, SSRS is limited by laser beam size rather than by the density scale length of the plasma.