On the jets, kinks, and spheromaks formed by a planar magnetized coaxial gun

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: s1d formation of an axial collimated jet fHsu and Bellan, Mon. Not. R. Astron. Soc. 334, 257 s2002dg that is consistent with a magnetohydrodynamic description of astrophysical jets, s2d identification of the kink instability when this jet satisfies the Kruskal–Shafranov limit, and s3d 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 fHsu and Bellan, Phys. Rev. Lett. 90, 215002 s2003dg. 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. © 2005 American Institute of Physics. fDOI: 10.1063/1.1850921g