Fig. 1. (a) Side view of short-pulse laser–wire target interactions. The meanings of the colors, symbols, and arrows are explained in the text. (b) and (c) Density projections of 10 µm diameter copper wire driven by the surface return current at different stages (different t) in regions II and I, respectively.

Fig. 2. PICLS 2D simulations of return current formation in a solid hydrogen wire at 160 fs. (a) 2D distribution of the current density in the y direction, in units of en_cc (∼0.084 MA/μm²). (b) 2D distribution of the magnetic field in the z direction, in units of 100 T. (c) Zoom-in of the current density in the region around y = 40 µm in (a). (d) Lineout of return current density at y = 40 µm in (a). (e) Lineout of B_z at y = 40 µm in (b).

Fig. 3. (a) Total current and magnetic field distributions in the radial direction at y = 5 µm from the PIConGPU simulation at 45 fs. (b) Time variation of surface return current component at an offset of 5 µm to the laser spot from the 3D PIConGPU simulations.

Fig. 4. (a) and (b) Lineouts at y = 60 µm of hydrogen electron density distribution and electron temperature distribution, respectively, at different times from the 2D PIC simulation. The geometry of the simulation box is shown in Fig. 2. x = 0 µm indicates the center of the hydrogen slab as shown in Figs. 2(a) and 2(b). (c) Time variations of magnetic pressure and plasma thermal pressure on the plasma surface.

Fig. 5. Electron temperature distributions in the radial direction at times of 20, 50, and 100 fs.

Fig. 6. (a) Hydrogen density distributions along the radius at different times. (b)–(d) Time variations of the density compression factor, electron temperature, and pressure of hydrogen, respectively, at the target center.

Fig. 7. (a) Plot of transient shock velocity as a function of radius and atomic number Z. (b) Plot of plasma β as a function of target radius and atomic number Z. The two dashed contours correspond to β = 1 and β = 10.

Fig. 8. (a) Density variation with time and radius in a 10 µm radius copper wire according to the hydrodynamic simulation. (b) EOS of copper at the peak density attained by convergent shock compression according to the hydrodynamic simulation. The dashed line indicates ρ/ρ₀ = 4. (c) Electron pressure calculated from P_e = n_ek_BT_e at different times in a 10 µm width slab, as simulated by the PIC method. Here, x = 0 µm represents the center position of the slab target. The offset is 10 µm from the laser incidence position. Time 0 marks the point at which the maximum laser intensity reaches the target front surface. (d) Corresponding evolution of surface return current density distribution at different times.

Fig. 9. (a) Experimental shadow radius of the target. The different colors indicate the distances to the laser spot. The uncertainty, as represented by the error bars, arises from (i) the statistical variations across effective multiple experimental runs, where run numbers for each delay (from 0, 5, 10, 20, 35, 50 to 100 ps) are 1, 1, 2, 3, 4, 2, and 2, respectively, (ii) the resolution limit (2 µm) of the probing, and (iii) the different laser target overlaps, with not all shots having the same initial conditions. (b) Plot of the hydrogen density distribution at different times simulated by the hydrodynamic method in Sec. III C. The dashed line shows the shadow radius obtained by the ray tracing method. (c) Color map showing the time evolution of the density in a uniform isochoric heating process with an initial temperature of 300 eV. The dashed lines show the ray tracing results for different initial temperatures.

Fig. 10. PIConGPU 3D simulations of return current formation in a solid hydrogen wire at 100 fs. The base coordinate can be seen in Fig. 1(a). (a) 2D x–y distribution of the magnetic field in the z direction (z = 0 µm). (b) 2D x–y distribution of the current density in the y direction (z = 0 µm). (c) 2D x–z distribution of the magnetic field in the z direction (y = 5 µm). (d) 2D x–z distribution of the current density in the y direction (y = 5 µm). (e) Lineout of B_z at z = 0 µm in (c). (f) Lineout of j_y at z = 0 µm in (d).

Fig. 11. Average bulk electron temperature distribution along the y axis. The star symbols are the results from 2D PIC simulations at 480 fs after the laser peak arrival time. The orange curve is the exponential fitting to these data points.

Fig. 12. Experimental shadow radius of the target. The different colors indicate the distances to the laser spot.