Modeling ion extraction from a free-plasma surface with a flexible conformal mesh

This paper describes a new method for numerical modeling of extraction of high-current ion beams from a plasma source. The challenge in the application is to satisfy simultaneously requirements for space-charge-limited flow and uniform ion flux. The plasma surface must assume a special shape that is not known in advance. The method involves the use of finite-element techniques coupled with a dynamic conformal mesh. Starting from an initial estimate, the flexible mesh is shifted to achieve uniform flux over the emission surface. The approach achieves high accuracy and has the versatility to handle complex emission surfaces in gridded guns. In contrast to trial-and-error approaches, the method proceeds directly to the optimum solution. The technique can also be applied to determine cathode shapes for uniform-flux electron guns. Benchmark calculations using the Trak two-dimensional ray-tracing code are described. The program automatically carries out the surface search.