Observation of Nonlinear Waves Induced by a Charged Object in Both Flowing and Stationary Low-Density Plasmas

We report the laboratory observation of non-linear waves excited in both flowing and stationary plasma. The experiments are carried out in the Space Plasma Simulation Chamber (SPSC) at the Naval Research Laboratory (NRL). A cylindrical object is mounted on a translation stage, allowing it to be positioned consistently within the large plasma volume. The object is biased with a square wave voltage pulse. A 40-tip Langmuir probe array is mounted on a three-axis motion stage inside the vacuum chamber to track the waves in all directions. A series of biased rings placed at the end of the SPSC are used to create a radial electric field along with an applied axial magnetic field resulting in an azimuthal plasma flow. The inferred flow speed (assuming the driven flow is the E x B speed) changes in a wide range by different combinations of radial electric fields and axial magnetic fields. In flowing plasma, we observe the regular generation of precursor solitons propagating upstream, against the direction of the plasma flow. The ability to detect such solitons in space environments could offer a novel method for identifying and tracking small-scale space debris in the Earth's ionosphere and lower magnetosphere. In stationary plasma, a half-cylindrical shock front is generated in front of the object and expands radially outward. As time progresses, the radius of the shock front increases, while its velocity gradually decreases. This study contributes to the advancement of our understanding of higher-dimensional shock structures in both space and laboratory low-density plasmas.