The dynamics of strongly magnetized plasma jets on COBRA

Radial foil implosions on pulsed-power generators [1] can produce strongly collimated plasma jets. The typical ratio of the jet length to its radius is at least 20. These jets have large Reynolds and magnetic Reynolds numbers. Due to favorable scaling [2], they can be considered as a reasonable embodiment of collimated astrophysical plasma jets generated by protostars, planetary nebula or galactic nuclei. However, many of these astrophysical jets seem to strongly diverge near their base while flow collimation occurs much later downstream (e.g. M2-9). Using a set of twisted pins to add a strong axial magnetic field (≫ 10 T) at the base of a jet produced by radial foils, we show that conical plasma jet can be formed consistently and repeatedly on COBRA. While these jets arc strongly divergent at their base, the jet collimates rapidly as the axial magnetic field weakens. Using the 3D PERSEUS code [3], we show that the conical part of the jet is formed by the imposed axial field and the top part of jet, while extremely hollow, is collimated by the same mechanisms ruling non-magnetized plasma jets [4]. Code results are scaled to lengths similar to planetary nebula size and show how scaling affects the plasma dynamics of strongly magnetized plasma jets.