Persistent Spiral-Arm Structure of a Rotating Plasma in a Stationary Gas

A lagging-spiral-arm structure which is quite regular in shape and persists for a long time is observed when an argon plasma, rotating at a supersonic speed, is injected into an argon gas along a magnetic field line. As the gas pressure increases, two spirals develop due to the velocity shear at the plasma-gas interface, and the ions inside the plasma core are cross-field transported outward along the spirals, forming a low-density-halo region there. The velocity-shear-formation mechanism and the relation to other systems manifesting spiral structures are discussed.