Time-Resolved Interferometric Measurements of Plasma Density Evolution in Laser-Driven Capacitor-Coil Targets

Laser-driven capacitor-coil targets provide a compact platform for generating strong magnetic fields and are widely used in magnetized high-energy-density plasma experiments. In addition to magnetic-field generation, these targets also produce plasma in the coil region, which can influence the subject physical processes, interact with secondary targets or external plasmas in their applications. However, direct, time-resolved measurements of the plasma density surrounding the coil remain limited. Here, we report interferometric measurements of the plasma density evolution in laser-driven capacitor-coil targets irradiated by the University of Osaka LFEX laser. Two-dimensional electron density maps reveal two distinct plasma sources loading the coil region: plasma generated in the coil itself and plasma produced by laser ablation of the target plates. These results provide quantitative information on plasma loading and evolution in capacitor-coil targets and are directly relevant to the design and modeling of magnetized high-energy-density plasma experiments.

💡 Research Summary

This paper presents a comprehensive, time‑resolved study of plasma density evolution in laser‑driven capacitor‑coil targets, a platform widely used to generate strong magnetic fields for high‑energy‑density (HED) plasma experiments. Using the LFEX laser at Osaka University (≈ 750 J total energy, 1.5 ps pulse, 1053 nm wavelength) the authors irradiated a target consisting of two parallel copper plates (50 µm thick, 1.5 × 1.5 mm²) separated by 600 µm and connected by two U‑shaped copper coils (wire cross‑section 100 × 50 µm², coil radius 300 µm). The laser, focused to a ~100 µm spot at 48° incidence, produced a peak intensity of ~4 × 10¹⁸ W cm⁻², generating super‑thermal electrons that establish a voltage between the plates and drive a current of tens to hundreds of kilo‑amps through the coil, creating quasi‑static magnetic fields of tens to hundreds of tesla.

To diagnose the plasma environment, the authors employed two complementary diagnostics: (1) a self‑referenced interferometric system using a 520 nm probe beam (5 mm diameter) and Fourier‑domain phase‑retrieval (IDEA software) to obtain line‑integrated electron density maps, and (2) proton radiography using a TNSA‑generated proton beam (≈ 6 MeV) produced by a secondary short‑pulse laser on a 10 µm Al foil. The proton beam traversed the region between the coil peaks at a 45° angle and was recorded on radio‑chromic film stacks, allowing reconstruction of the electromagnetic field distribution.

Interferometric data at 1.0 ns and 3.1 ns after the main laser pulse reveal two distinct plasma sources loading the coil region. In a control target without coils, only a low‑density plasma (line‑integrated density ≈ 4 × 10²⁰ m⁻²) expands from the laser‑irradiated front plate with an estimated velocity of ~1000 km s⁻¹, consistent with laser‑ablation‑driven expansion. In the coil‑containing target, dense plasma lobes (2–4 × 10²¹ m⁻²) appear localized near the coil peaks already at 1 ns, indicating plasma generation directly in the coil. By 3.1 ns the plasma fills the inter‑coil region, reaching line‑integrated densities up to ~10²² m⁻², showing continued contribution from both coil‑generated and ablation‑driven sources.

Proton radiography at 0.59 ns shows strong deflection patterns around the coil, consistent with magnetic‑field‑dominated deflection. Synthetic radiographs generated with PlasmaPy, using Biot–Savart calculations for two U‑shaped current paths, reproduce the experimental features when a current of 29 ± 5 kA per coil is assumed. This current is lower than values reported in similar OMEGA‑EP experiments, attributed to differences in laser energy, pulse duration, and focal conditions.

The coexistence of coil‑generated plasma and ablation‑driven plasma has several important implications. First, the presence of a conducting plasma around the coil modifies the effective conductivity, current distribution, and magnetic‑field topology, potentially altering the peak field strength and its temporal evolution. Second, for experiments that rely on the coil to magnetize secondary targets (e.g., magnetized inertial confinement fusion or laboratory astrophysics), the evolving plasma can pre‑fill the interaction region, change background density and collisionality, and affect magnetic‑field penetration. Third, in magnetic‑reconnection studies, early‑time coil plasma can pre‑condition the reconnection layer, while later plasma loading can modify density gradients and influence current‑sheet formation and reconnection rates.

Overall, the work provides the first direct, time‑resolved, two‑dimensional electron‑density measurements of plasma surrounding laser‑driven capacitor‑coil targets, quantifies the associated currents via proton radiography, and highlights the necessity of incorporating plasma loading effects into the design and modeling of magnetized HED experiments. The data and analysis presented constitute a valuable benchmark for future theoretical and simulation efforts aimed at accurately predicting magnetic‑field generation and plasma dynamics in these compact, high‑field sources.