Abstract—Methodology for designing the 2.45 GHz microwave coupling system to optimize the hydrogen plasma density in an ECR plasma reactor is presented. Two different plasma generator systems have been studied by experiments and 3D simulations to find the criteria to reach an optimized design. The experimental work includes detailed measurements and calculations of the Electron Energy Distribution Functions (EEDF) and ultrafast photography diagnostics to estimate spatial distributions of plasma unbalanced charge density, potential and electric field for both cases. It demonstrates that to simulate in 3D the distribution of the resonant stationary electric field along the entire microwave driver system can be use to improve the design in order to reach higher plasma densities and temperatures.