Influence of Conical Wire Array Geometry on Flow and Temperature Profiles Measured via Thomson Scattering and Optical Techniques

Conical wire arrays with different opening angles are used as load of a 400kA, 1kA/ns generator. The differences in opening angle allow the study of the influence of the array geometry on the jet properties. The characterization of the jets is performed using a combination of advanced diagnostic techniques, including moiré schlieren deflectometry, visible self-emission spectroscopy, and optical Thomson scattering. The results reveal that, under the experimental conditions, the plasma jets exhibit electron temperatures ranging from $8$ to $17$ eV, increasing along the axial direction. In contrast, the ion temperature decreases from approximately $35$ eV near the base of the jet to about $20$ eV at higher axial positions. The electron density profile peaks at $\sim 4 \times 10^{18}$ cm$^{-3}$ in the central lower region of the jet and decreases with height exponentially with a characteristic lenght $L_n = $2.86 mm. This behavior is reproducible and independent of the conical array geometry. However, the cone opening angle significantly affect the jet propagation velocity, with larger opening angles producing higher axial velocities ($V_{ϕ=40^\circ} \approx 125\pm3$ km/s, $V_{ϕ=20^\circ} \approx 98\pm5$ km/s), demonstrating that the cone geometry provides effective control over the jet propagation velocity.

💡 Research Summary

This paper presents a comprehensive experimental study of plasma jets generated by aluminum conical wire arrays driven by a 400 kA, 1 kA ns⁻¹ pulsed power generator. Three different opening angles (ϕ = 20°, 30°, 40°) were investigated to determine how the geometry of the array influences jet properties such as flow velocity, temperature, density, and dimensionless plasma parameters. A key innovation in the experimental design was the inclusion of a 3 mm thick metallic lid with a 5 mm central aperture placed over the upper electrode. This aperture blocks lateral plasma flares produced during the wire ablation phase, ensuring that only the axial jet passes through and is diagnosed, thereby improving reproducibility and purity of the measurements.

Three complementary diagnostics were employed. (1) Moiré‑Schlieren deflectometry using a 532 nm Nd:YAG laser and two Ronchi gratings measured line‑integrated electron‑density gradients. The fringe shifts were processed with Fourier filtering and an onion‑peeling inversion assuming cylindrical symmetry, yielding radial electron‑density profiles with a detection limit of ≈6.7 × 10¹⁷ cm⁻³. Symmetry checks showed less than 5 % variation between opposite sides of the jet. (2) Visible self‑emission spectroscopy captured Al III lines (particularly the 452.9 nm transition). After correcting for instrumental broadening, Stark‑broadened line widths were converted to local electron densities, providing an independent validation of the Moiré results. (3) Optical Thomson scattering (TS) was performed with a 532 nm, 1 J, 4 ns probe beam aligned along the jet axis; scattered light was collected at 90° by a linear fiber bundle of 25 fibers spaced 200 µm apart, defining 25 sampling volumes each 420 µm long. Spectra were recorded with a 2400 l mm⁻¹ grating spectrometer and a gated ICCD (6 ns exposure). A Bayesian fitting routine, constrained by the electron density obtained from the other diagnostics, extracted ion temperature (Ti) and placed an upper bound on electron temperature (Te).

The combined data reveal several robust trends. Electron temperature increases from ≈8 eV near the base to ≈17 eV toward the jet head, while ion temperature shows the opposite behavior, decreasing from ≈35 eV to ≈20 eV over the same axial range. The electron density peaks at the jet axis at the base with ne ≈ 4 × 10¹⁸ cm⁻³ and decays exponentially with a characteristic length Ln ≈ 2.86 mm, reaching ≈6 × 10¹⁶ cm⁻³ at z ≈ 13 mm. Importantly, this density profile is essentially independent of the opening angle, indicating that geometry does not significantly affect the mass loading of the jet.

Flow velocity, obtained from the Doppler shift of the TS spectra, is strongly dependent on the cone angle. For ϕ = 20°, the axial velocity at z = 6 mm is 98 ± 5 km s⁻¹; for ϕ = 40°, it rises to 125 ± 3 km s⁻¹. The velocity increases roughly linearly with axial position for all angles, with a slope that grows by ≈3 km s⁻¹ mm⁻¹ for each 10° increase in ϕ. Consequently, the Mach number (Ms = V/Cs) also rises with opening angle, remaining supersonic (Ms > 15) throughout the measured region.

Dimensionless parameters calculated using Ln as the characteristic length show a high Reynolds number (Re ≈ 10⁴–10⁵), confirming that viscous effects are negligible at the system scale. The magnetic Reynolds number (Rm ≈ 10³) indicates that magnetic fields are largely frozen‑in, with diffusion only becoming relevant at much smaller scales. The Knudsen number is small (Kn ≈ 0.01), supporting a fluid description.

The authors discuss the implications for laboratory astrophysics. By varying the cone angle, one can tune the jet’s initial kinetic energy and Mach number, providing a controllable platform to emulate acceleration and collimation processes observed in astrophysical jets from young stellar objects or active galactic nuclei. The observed Ti > Te near the base, gradually converging downstream, suggests ongoing energy exchange between ions and electrons as the jet expands and radiative cooling becomes more effective.

Future work could explore larger opening angles, alternative wire materials, or the application of external magnetic fields to further manipulate jet collimation and stability. The present study establishes that conical wire‑array geometry is an effective lever for controlling jet propagation speed while leaving density and temperature gradients largely unchanged, thereby enhancing the reproducibility and relevance of Z‑pinch jet experiments for astrophysical modeling.