A modular ion source design relies on relatively short modular anode layer source (ALS) components, which can be coupled together to form a longer ALS. For long ion sources, these shorter modular components allow for easier manufacturing and further result in a final assembly having better precision (e.g., a uniform gap dimensions along the longitudinal axis of the ion source). Modular components may be designed to have common characteristics so as to allow use of these components in ion sources of varying sizes. A modular gas distribution system uniformly distributes a working gas to the ionization region of the module ion source. For each gas distribution module, gas distribution channels and baffles are laid out relative to the module joints to prevent gas leakage. Furthermore, gas manifolds and supply channels are used to bridge module joints while uniformly distributing the working gas to the ALS.