The ablation dynamics of wire-array Z-pinch loads critically determine the implosion quality and the resulting X-ray radiation performance. Introducing a tailored prepulse current prior to the main pulse is a well-recognized approach to suppress magnetic Rayleigh-Taylor instabilities and improve implosion uniformity, yet the underlying physics, particularly the evolution of the magnetic field topology and the ablation flow behavior under prepulse regulation, remains insufficiently understood. In this work, a coordinated suite of advanced optical diagnostics is deployed to quantitatively investigate the ablation dynamics of a 4-wire aluminum array (wire diameter 20 μm, array diameter 12 mm) on the “Qin-1” pulsed-power generator. Laser Thomson scattering (LTS) provides space- and time-resolved measurements of plasma velocity, electron temperature, ion temperature, and electron density, while Faraday rotation polarimetry, simultaneously operated with a Mach-Zehnder interferometer, captures the evolution of the magnetic field topology with high sensitivity. Extreme ultraviolet (XUV) framing images, laser shadowgraphy, and a streak camera further trace the macroscopic plasma redistribution.
Under the chosen prepulse-main pulse interval of 400 ns (main-pulse peak current ~280 kA), the prepulse is found to fundamentally alter the initial wire state: the originally cold, dense wire cores are pre-gasified and expand, so that the subsequent main pulse first induces a prolonged phase of core compression dominated by local magnetic fields, thereby delaying the onset of the classical ablation stage. Faraday rotation imaging reveals that the polarity of the magnetic field inside the array reverses, signaling the establishment of a global azimuthal magnetic field, on a timescale far shorter than that required by ablation-plasma convection. This rapid transition indicates that the global field is not formed by resistive convection of coronal plasma but more likely originates from the pre-filled precursor plasma that is promptly ionized by the current and radiation, creating fast-establishing current paths.
LTS measurements reveal a dramatic reduction in ablation flow parameters: under prepulse regulation, the ablation-stream velocity drops to approximately one-third of that in the main-pulse-only case, and the plasma momentum flux falls to only about one-fifth of the value predicted by the classic rocket model. Moreover, the gasified wire-core region exhibits elevated electron temperatures (with the temperature peak located in the core and decreasing radially outward) and a significant bulk radial motion towards the array axis. These observations provide direct evidence that the current distribution is reshaped by the prepulse: instead of being confined entirely in the coronal plasma, a large fraction of the main-pulse current undergoes resistive diffusion and penetrates deeply into the expanded core. By introducing an effective ablation current coefficient α into the rocket model, it is estimated that only ~45 % of the total drive current contributes to accelerating the ablation streams, while the remaining ~55 % flows inside the gasified core, depositing energy through Joule heating and driving the core's macroscopic motion. This spatial redistribution of drive current fundamentally slows down the ablation dynamics and effectively suppresses the growth of axial non-uniformities during the ablation phase, thereby establishing favorable initial conditions for a high-quality implosion. These findings clarify the mechanisms through which prepulse regulation reshapes the magnetic field topology and the current distribution, and highlight the crucial roles of the pre-expanded core and the rapid formation of the global magnetic field in tailoring wire-array Z-pinch performance.