Authors: Shreya Dwivedi (UW-Madison), Joe Olson (UW-Madison), Jeremiah Kirch (UW-Madison), Paul Gradney (UW-Madison), Hui Li (LANL), Ellen Zweibel (UW-Madison), Jan Egedal (UW-Madison) and Cary Forest (UW-Madison)

The astrophysical problem of jet-environment interaction is being studied under controlled laboratory conditions on the Big Red Ball (BRB) at WiPPL, with the aim of understanding how the ambient plasma medium governs the structure, propagation, and stability of a magnetically driven jet. The larger propagation volume and insulating inner wall of the 3 m diameter spherical vessel offer key advantages over previous laboratory jet experiment vessels for studying the jet’s complete plasma current structure without wall-mediated short circuits. We present 3D magnetic field reconstruction, radial J×B force density profiles, and return current structure derived from high-frequency (10 MHz) magnetic probe arrays, characterizing the jet’s collimation and propagation through background plasma. Hydrogen plasma jets are formed via a magnetic tower mechanism, in which a high bias voltage (2–3 kV) applied across planar electrodes in the presence of a poloidal magnetic field (∼ 10 mWb) drives gas discharge that evolves from plasma current arches into a collimated jet. Jets (density ∼ e+19 m^−3 at ∼11 eV) are injected into hydrogen plasma backgrounds across six distinct ambient densities (∼ e+17 m^−3 at ∼5 eV). Jets with axial magnetic field of average peak amplitude ∼250 G remain collimated for longer durations when launched into lower density plasma: persisting for ∼34 μs at 1.0×e+17 m^−3 compared to ∼24 μs at 6.2×e+17 m−3 (each at 3 kV bias). The magnetic-front propagation speed decreases from 90 km/s to 46 km/s across a representative range of ambient densities from 2.1×e+17 m^−3 to 5.2×e+17 m^−3 for jets biased at 2 kV, suggesting that ambient plasma inertial loading plays a significant role in governing jet magnetic structure propagation.