Helicity dependence in interactions between pairs of arched, laboratory plasma loops

Arched, plasma-filled flux tubes -- or "loops" --- can be generated "on demand" in laboratory vacuum chambers with pulsed-power, magnetized plasma guns. These plasmas exhibit rich dynamics during their brief lifetimes (which are measured in microseconds), ranging from behavior that is well described by ideal MHD --- and thereby more scalable to phenomena observed in the solar atmosphere -- to phenomena that access non-ideal-MHD regimes, e.g., involving kinetic instabilities, Hall MHD, or middling Lundquist numbers [1]. Over the years, a number of different gun designs have been employed to create a wide variety of single- and multiple-loop plasma structures. Among these is a "quad" gun capable of producing two adjacent loops, each of which can be set up to have left-handed (L) or right-handed (R) helicity. There are thus two different co-helicity configurations (LL and RR) and two different counter-helicity configurations (LR and RL); whether the loops have the same or opposite helicity was seen to affect how they interact with each another as they evolve [2]. Here, we describe experiments in which this gun was operated with dual-gas techniques (so that the two loops could initially contain different ion species and thereby be distinguished in fast camera images with optical filters), and B-dot probe measurements were taken of the loop pairs as they evolved and "erupted". Particularly striking are the differences between the two "counterhelicity" configurations (LR and RL); one configuration yields bright, fast, dynamic merged structures, whereas the other stagnates. These results extend previous studies on two-loop interactions and have potential to shed new light on the non-ideal-MHD effects at play.

[1] Bellan, P. M. (2020). Caltech lab experiments and the insights they provide into solar corona phenomena. Journal of Geophysical Research: Space Physics, 125, e2020JA028139.

[2] Hansen, J. F. and Tripathi, S. K. P. and Bellan, P. M. (2004) Co- and counter-helicity interaction between two adjacent laboratory prominences. Physics of Plasmas, 11 (6). pp. 3177-3185.