Identification of rippling interface between laser-ablated plasma and ambient uniform plasma at rest
We present laboratory experiments on the early stage of the formation process of quasi-perpendicular supercritical magnetized collisionless shocks using laser-produced plasmas in an ambient uniform nitrogen (N) gas with 1~Torr under the external magnetic field with controlled polarity. An expanding aluminum (Al) piston plasma arising from laser irradiation interacts with a magnetized ambient N plasma, producing a bright, asymmetric self-emission whose global morphology reverses with the magnetic-field polarity. Two-dimensional ideal-magnetohydrodynamics simulations reproduce the main features of the observed structures and indicate that the ripple formation is unlikely to be driven by the Kelvin--Helmholtz instability, but is instead associated with vortex formation at the rounded leading edge of the piston plasma. We show that the bright, rippling emission delineates the Al-N interface, whose intrinsic width is consistent with an electron kinetic scale, and that its upstream side is predominantly composed of N plasma. This identification allows the formation of a supercritical collisionless shock in the magnetized N plasma to be discussed with minimal Al contamination.