Modelling of the propagation of an electron beam using PIC algorithm

The TMP-DS (Theory and Modeling of Plasmas - Discharge and Surface) team at the LPGP of the University of Paris-Saclay is developing advanced models for simulating plasma, as well as charged particle beams from their extraction to their final configuration. This master thesis aims to implement and exploit the SESAM [1] model developed at the LPGP to simulate the propagation, focusing, and deflection of high-current electron beams, considering the effects of acceleration due to electrodes and 3D magnetic fields generated by multiple coils. The SESAM model is implemented in a C++ code using parallel programming and it employs a self-consistent Particle-in-Cell (PIC) approach, which considers the effect of the electric field generated by all other charged particles on each particle. The simulation domain used in this study includes a flat finite emissive cathode, a circular anode with a central hole, and a Wehnelt cylinder. It can be extended further including the propagation region beyond the anode and a (virtual) collection wall, grounded at the anode potential. In general, the system benefits from an intrinsic axial symmetry when subjected to axisymmetric magnetic fields, which allows the simulations to be performed on a reduced 2D mesh, or on a 3D mesh when exposed to uniform deflecting magnetic fields. Simulations of the electron beam within the region between the electrodes demonstrated the significant influence of the Wehnelt cylinder voltage in suppressing the beam, the reaching of a saturation regime for the system's extracted current due to the space charge effect, and the influence of the electrode distance and shape of the anode in varying the maximum extracted current. The implementation of axially symmetric focusing magnetic fields has enabled the beam to be focused in a region extending beyond the anode, allowing for the study of focal distance and beam radial size at the focal point for different magnetic fields and field intensities, as well as Wehnelt potential. While some of the magnetic field typologies were unable to achieve considerable focal distances, a simple power study model indicated that it is feasible to achieve complete melting without full vaporisation of localised regions of a virtual aluminium layer perpendicularly exposed to the beam. Furthermore, the system has been investigated by applying a uniform magnetic field perpendicular to the beam, with the objective of making it turn on a lateral wall. This has provided insights into potential future developments of this numerical model for the simulation of high-current electron beams within the context of electron gun technology. [1] A. Revel and T. M. Minea - SESAM « Saclay Electron Solution for BeAm Modelling » software - Déposants CNRS/Univ. Paris-Saclay, date de dépôt 9/01/2023, N° DL 16346-01