Magnetically-driven plasma jet experiments on the Big Red Ball

Plasma jet experiments using planar coaxial electrodes with bias magnetic field, emulating the central object–accretion disk system, are conducted on the BRB at WiPPL. Hydrogen plasma jets (density ∼ 10⁻¹⁹ m⁻³ at 11 +- 2eV) at high bias voltage (2–3 kV) are injected into different H plasma backgrounds (density ∼ 10⁻¹⁷ m⁻³ at ∼ 5 eV). Axial component B_z propagates downstream from the gun, undergoing collimation while elongating. The induced toroidal field B_ϕ shows pinching near the column axis, indicative of plasma compression or helicity injection. The current density J_z is strongly collimated, extending up to ∼ 90–100 cm from the gun, approximately propagating along collimated B_z lines. Later, both B_z and J_z distributions broaden and weaken, suggesting relaxation or enhanced interaction with the background plasma. This J − B topology is consistent with earlier plasma jet experiments. B_z with average peak amplitude ∼ 240 G remain collimated for longer durations when jets are launched into lower density plasma: lasting ∼ 34 μs at 1. x 10⁻¹⁷ m⁻³ compared to ∼ 24 μs at 6.2 x 10⁻¹⁷ m⁻³ (each at 3 kV bias). The propagation speed of B_z [90, 76, 46] km/s decreases with increasing background density ∼ [2.1, 3.1, 5.2] x 10⁻¹⁷ m⁻³, for jets biased at 2 kV. Magnetic field analysis suggests that the plasma jets likely operate in a regime with a low force-free eigen-value α_gun, placing them below the Kruskal–Shafranov threshold for the current-driven ideal kink instability.