Time evolution of transient plasma states from nanowire arrays irradiated at relativistic intensities

The interaction of matter with high-power lasers at relativistic intensities is a key challenge in high-field physics, with broad implications for astrophysics, particle acceleration, and exotic phenomena like radiation reaction and vacuum polarization. While particle-in-cell (PIC) simulations are crucial for understanding these ultrafast interactions, experimental time-resolved studies of solid targets are limited. Intense laser-matter interactions also create bright X-ray sources, vital for high-energy-density (HED) physics experiments and inertial confinement fusion (ICF). Nanowire array targets offer increased laser absorption and volumetric heating, leading to significantly enhanced X-ray emission compared to flat foils, reaching energy densities comparable to ICF implosions. However, scaling these targets to high-energy laser facilities is challenging due to stringent laser contrast requirements. This research investigates the time-resolved X-ray spectra from nanowires driven by relativistic laser intensities to understand the dynamics and evolution of these HED systems on picosecond timescales, comparing experimental findings with PIC simulations.

Previous research has highlighted the potential of nanowire arrays as efficient X-ray sources and platforms for ultra-HED studies. Studies like Bargsten et al. (2017) demonstrated energy penetration into nanowire arrays at relativistic intensities, reaching terabar pressures. Other work has shown the enhanced X-ray emission from nanowires compared to flat foils (Kulcsar et al., 2000; Purvis et al., 2013). However, experimental studies detailing the time-resolved plasma dynamics of nanowire targets at kilojoule-scale facilities have been lacking. This gap in knowledge necessitates the current study, aiming to provide experimental data to benchmark existing computational models.

The experiment utilized the high-contrast Orion petawatt laser at AWE, UK, irradiating Ni nanowire arrays (12.5 µm length, 400 or 1000 nm diameter, ~15% fill fraction) with a frequency-doubled Nd:glass short-pulse laser (100 J, 600 fs, 10 µm focal spot) at ~10²⁰ W cm⁻² intensity (a₀ ≈ 6). A contrast of 10¹⁸ at 100 ps was maintained. Time-resolved X-ray spectra were collected using an ultrafast X-ray streaked spectrometer (XRSS) diagnostic with ~1 ps time resolution and E/ΔE ~ 500 (7.4–8.4 keV energy range). The collected emission included Kα, Heα, and Lyα lines from high ionization states. To analyze the spectra, the FLYCHK collisional radiative code was employed to generate spectra over a range of temperature and density conditions. A grid of 10 temperatures (0–4 keV) and 10 densities (0.1–10 g cm⁻³) was used. A subset of weighting points within this grid was chosen as fitting parameters, with interpolation used to reduce the number of free parameters. The spectral weightings were determined by minimizing a weighted least-squares cost function. The centroid of the fitted emitting plasma distributions over time, and the plasma distribution at key points in the evolution, was determined. The time evolution of the ion populations was determined from the fitted weights, input atomic densities and relative contributions of ion constituents from each point in the distribution.

The study found that nanowires exhibit brighter and prolonged emission compared to flat foil targets, primarily from high-lying charge states. A detailed analysis of the time-resolved X-ray spectra allowed for the reconstruction of the plasma temperature and density evolution. The observed evolution comprises six key phases: (I) initial laser heating, (II) laser reflection and energy redistribution, (III) heating of dense wire cores, (IV) nanowire recollision, (V) high-temperature inertially confined stagnation, and (VI) expansion and cooling. The nanowires’ volumetric heating leads to a larger volume of slightly lower-temperature plasma compared to flat foils. This results in enhanced Ni²⁷⁺ populations at early times and prolonged high-temperature stagnation. In contrast, flat foils rapidly reach maximum ionization before cooling and expanding. The 1000 nm diameter nanowire exhibited characteristics of both bulk and nanowire interactions, showing multiple ionization peaks in the time history. The analysis using FLYCHK provided insights into time-resolved ion populations, including the predicted population of fully stripped Ni²⁸⁺ ions. A late-time jump in highly charged ion stages was observed for nanowires, resulting from collisional heating, a feature absent in flat foil targets. The experimental data is in good agreement with previous computational studies. The study demonstrated a method for determining plasma conditions in rapidly evolving samples with significant density and temperature gradients.

The results demonstrate a novel method for characterizing the complex plasma evolution in near-solid-density, high-temperature conditions from single-shot emission. This method is valuable for benchmarking and improving physical models, especially given the computational expense and complexity involved in simulating the wide-ranging spatio-temporal scales in these interactions. The sustained high temperatures and densities in the nanowire plasma bridge the regimes typically associated with PIC and radiation-hydrodynamic codes. The volumetric heating effects in the nanowire targets significantly alter the plasma dynamics compared to flat foils, providing a unique platform for ultra-HED studies. The discrepancies between the model and late-time experimental emission may be attributed to low signal levels, the limitations of the fitting model's resolution, or the absence of certain high-lying satellite states in the FLYCHK model.

This research successfully characterized the complete time evolution of complex plasma conditions in relativistic laser-irradiated nickel nanowire arrays using time-resolved X-ray spectroscopy. The findings highlight the unique dynamics of nanostructured laser targets and their potential as extremely bright short-pulse probes and platforms for ultra-HED plasma experiments. Future research should focus on optimizing nanostructure design and analysis techniques to further enhance the performance and understanding of these systems.

The main limitations stem from the signal-to-noise ratio, the energy resolution of the X-ray spectrometer, and intensity broadening effects during electron time-of-flight. The zero-dimensional nature of the FLYCHK code is another limitation; The model's ability to resolve bimodal distributions in temperature and density for the 1000 nm nanowires was limited. Irregular nanowire spacing could also influence the observed evolution.