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

Abstract

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.

Figures (3)

• Figure 1: Experimental setup for interferometry and proton radiography. A probe laser beam with a diameter of 5 mm is used for interferometric measurements. The probe beam is much larger than the plasma region; therefore, two portions of the same beam are utilized, with one passing through the plasma and the other propagating outside the plasma as a reference. After transmission through the target, the beam is split and the plasma-affected portion is overlapped with the reference portion to produce interference fringes. Proton radiography is performed using a short-pulse laser irradiating a 10-$\rm{\mu}$m-thick and 1 mm $\times$ 1 mm aluminum foil target to generate protons via the TNSA mechanism. The proton beam propagates through the central region between the two coil peaks and is recorded on RCF stacks. The proton beam axis is oriented at 45° with respect to the vertical direction.
• Figure 2: Proton radiography of the capacitor--coil target. (a) Proton radiograph obtained with $6.2 \pm 0.5$ MeV protons at $t = t_{0} + 0.59$ ns. The color scale is not absolutely calibrated and qualitatively represents the proton flux, with darker regions indicating higher proton deposition. The coordinate axes are referenced to the target plane. (b) Synthetic proton radiograph calculated assuming a 29 kA current in each coil. (c) Overlay of the synthetic radiograph on the experimental image. Dashed lines indicate the deflection contour derived from the synthetic proton radiograph shown in (b).
• Figure 3: Interferometry measurements for different target geometries at different times. (a-c) Measurements at $t = 1.0$ ns for a no-coil target consisting only of the back and front plates, with the coils removed. (d-f) Measurements at $t = 1.0$ ns and (g-i) at $t = 3.1$ ns after laser irradiation. Panels (a,d,g) show the reference interferograms acquired before the laser shot. Panels (b,e,h) show the interferograms during the shot. Panels (c,f,i) show the inferred line-integrated electron density maps. Density reconstruction is performed only in regions with clear interferometric fringes; areas without reliable fringe information are masked and excluded, resulting in a smaller reconstructed area than the corresponding interferograms. White regions indicate masked areas. Red dashed lines denote the original target position.