Experimental Search for a Topological Gaseous Plasmon Polariton at the Large Plasma Device*

A change in the topology of waves on opposite sides of an interface guarantees the existence of topologically protected waves localized at that interface. We conducted an experiment at the Large Plasma Device (LAPD) to search for such topological waves. Specifically, we searched for a gaseous plasmon polariton (GPP), an electromagnetic wave that owes its existence to topology. The GPP has been theoretically predicted to exhibit chiral motion and is localized along the boundary between a magnetized plasma and a vacuum. In our experiment, we observed signatures of helical propagation of GHz microwaves [JM1] along the sharp density gradient near the plasma edge. We analyze the phase coherence of the electric field fluctuations and demonstrate that the experimental observations are consistent with the theoretical predictions of a GPP.