Experimental Observation of a Field-Aligned Ion Beam Produced by Magnetic Reconnection of Two Flux Ropes

An ion beam field-aligned to the background guide field (B₀ = 330 G) was observed in a reconnection experiment on the Large Plasma Device (LAPD). To the authors’ knowledge, it is the first experimental observation of the generation of a field aligned ion beam in a reconnection experiment. Two kink-unstable flux ropes (L = 11 m, dia = 7.6 cm) collided periodically as a by product of the kinking. The magnetic field due to the rope currents was at most 30 Gauss. Magnetic reconnection occured midway between the ropes at each collision. Sub-Alfvénic ion beams with energies of up to 15 eV were observed from measurements of the local ion energy distribution function using an ion retarding field energy analyzer (RFEA). The appearance of the beam is correlated with the oscillation of the ropes. The ions are not heated, but appear to be energized by both the inductive fields from magnetic reconnection and the space charge fields of the ropes. The acceleration mechanism appears to be non-local. The ions get a "kick" from the inductive field , then drift into regions of large space charge field and subsequently back into the reconnection region. This happens over a distance of approximately 1 meter. The results and interpretation of the measurements are supported by three-dimensional gyrokinetic particle simulations which can track the motion of individual ions (see adjacent poster by Sydora et al.) Interestgly when the inductive field is switched off in the simulation the beam vanishes. A previous flux rope experiment [W. Gekelman et al., Astrophys. J. 853, 33 (2018)] determied that Ohm's law must be non-local as well.