This work describes a laboratory plasma experiment and initial results which should give insight into the magnetic dynamics of accretion discs and jets. A high‐speed multiple‐frame CCD camera reveals images of the formation and helical instability of a collimated plasma, similar to MHD models of disc jets, and also plasma detachment associated with spheromak formation, which may have relevance to disc winds and flares. The plasmas are produced by a planar magnetized coaxial gun. The resulting magnetic topology is dependent on the details of magnetic helicity injection, namely the force‐free state eigenvalue αgun imposed by the coaxial gun.

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We report spatially resolved measurements of the oblique merging of two supersonic laboratory plasma jets. The jets are formed and launched by pulsed-power-driven railguns using injected argon, and have electron density ∼ 10 14 cm −3 , electron temperature ≈ 1.4 eV, ionization fraction near unity, and velocity ≈ 40 km/s just prior to merging. The jet merging produces a few-cm-thick stagnation layer, as observed in both fastframing camera images and multi-chord interferometer data, consistent with collisional shock formation [E. C. Merritt et al., Phys. Rev. Lett. 111, 085003 (2013)].

*awetj@lanl.govOne-dimensional radiation-hydrodynamic simulations are performed to develop insight into the scaling of stagnation pressure with initial conditions of an imploding spherical plasma shell or "liner." Simulations reveal the evolution of high-Mach-number (M), annular, spherical plasma flows during convergence, stagnation, shock formation, and disassembly, and indicate that cmand µs-scale plasmas with peak pressures near 1 Mbar can be generated by liners with initial kinetic energy of several hundred kilo-joules. It is shown that radiation transport and thermal conduction must be included to avoid non-physical plasma temperatures at the origin which artificially limit liner convergence and thus the peak stagnation pressure. Scalings of the stagnated plasma lifetime (τ stag ) and average stagnation pressure (P stag , the pressure at the origin, is also found for a wide range of liner-plasma initial conditions.

Measurements of the various plasma configurations produced by a planar magnetized coaxial gun provide insight into the magnetic topology evolution resulting from magnetic helicity injection. Important features of the experiments are a very simple coaxial gun design so that all observed geometrical complexity is due to the intrinsic physical dynamics rather than the source shape and use of a fast multiple-frame digital camera which provides direct imaging of topologically complex shapes and dynamics. Three key experimental findings were obtained: ͑1͒ formation of an axial collimated jet ͓Hsu and Bellan, Mon. Not. R. Astron. Soc. 334, 257 ͑2002͔͒ that is consistent with a magnetohydrodynamic description of astrophysical jets, ͑2͒ identification of the kink instability when this jet satisfies the Kruskal-Shafranov limit, and ͑3͒ the nonlinear properties of the kink instability providing a conversion of toroidal to poloidal flux as required for spheromak formation by a coaxial magnetized source ͓Hsu and Bellan, Phys. Rev. Lett. 90, 215002 ͑2003͔͒. An interpretation is proposed for how the n = 1 central column instability provides flux amplification during spheromak formation and sustainment, and it is shown that jet collimation can occur within one rotation of the background poloidal field.