A system includes a process chamber, a housing that defines a waveguide cavity, and a first conductive plate within the housing. The first conductive plate faces the process chamber. The system also includes one or more adjustment devices that can adjust at least a position of the first conductive plate, and a second conductive plate, coupled with the housing, between the waveguide cavity and the process chamber. Electromagnetic radiation can propagate from the waveguide cavity into the process chamber through apertures in the second conductive plate. The system also includes a dielectric plate that seals off the process chamber from the waveguide cavity, and one or more electronics sets that transmit the electromagnetic radiation into the waveguide cavity. A plasma forms when at least one process gas is within the chamber, and the electromagnetic radiation propagates into the process chamber from the waveguide cavity.