Three-dimensional Helical-rotating Plasma Structures in Beam-generated Partially Magnetized Plasmas

Electron beam generated plasmas in an external magnetic field that consist of an emitting cathode and a grounded chamber, or repeller electrode, are often used as ion beam sources in ion implantation devices and linear plasma machines. We modelled beam-generated partially magnetized plasmas using the PPPL LTP-PIC-3D particle-in-cell code on a hybrid CPU-GPU architecture. Due to the highly efficient code, we were able to simulate three-dimensional (3D) beam-generated plasma structures and observed that helical structures are formed at low beam currents. Two distinct plasma regimes are identified. When the gas pressure is higher than a certain threshold, the plasma density produced by the beam is sufficiently high and the quasi-neutrality condition is attained. In this regime, the plasma evolves into spoke-like structures that rotate as a whole and resemble 2D spiral-arms formed by the development of a lower-hybrid instability. Plasma density and potential structures result in enhanced cross-field transport. At pressures lower than this threshold, quasi-neutrality is not achieved and a 3D helical-rotating plasma structure forms due to development of the diocotron instability. Analytical formulas are proposed for the critical threshold pressure between these regimes and for the rotation frequency of the helical structures.