Finite-length effects on beam--plasma instability in collisionless inhomogeneous systems

Streaming instability driven by ion flow inside a finite-length collisionless inhomogeneous system is studied with numerical Particle-In-Cell simulations alongside kinetic equations. The inhomogeneous plasma is produced by introducing a cold beam of ions and thermal electrons in the system. In such systems, we observe that the ion sound waves get modified in a stable ion flow due to acoustic wave reflections from the boundary and that it triggers instability. Such phenomena are known to be a hydrodynamic effect. However, there are also signatures of the two-stream type ion sound instability where kinetic resonances are essential. The study is aimed to quantify the effects of the finite-length system on beam--plasma instability and identify wave modes supported by such systems. The outcome of this study is believed to improve the design aspect of ion beam sources and provide more control over experimental devices.