Laboratory Demonstration of Collisionless Blob Formation via Laser Produced Plasma Self-Focusing*

We observe the formation and propagation of an ion gryoradius scale blob-cavity structure at the interface between a super-Alfvénic laser produced plasma (LPP) and an ambient magnetized plasma as a result of Larmor coupling. The magnetic pressure gradients along the edge of the diamagnetic cavity focus the quasi-perpendicularly expanding LPP into a collimated flow that maintains late-time enhanced density compared to ballistic expansion. The Larmor electric fields generated in this expansion act upon the ambient plasma, driving an E × B motion of the ion and electron populations. This E × B drift results in a pile-up of the ambient ions resulting in a "blob". A secondary, miniature diamagnetic cavity is maintained between the collimated LPP and the blob and separates a relatively large distance from the bulk diamagnetic cavity. Three-dimensional particle-in-cell simulations replicating the experimental conditions reproduce the density increase of the ambient ions and reveal the individual mechanisms that lead to both the self-focusing of the LPP flow and creation of the blob-mini cavity structure. In these studies a novel, collisionless mechanism for blob production has been demonstrated that can help uncover dynamics involved in blobs observed during cometary transits and ionospheric disturbance.