Exemplary systems according to embodiments of the present technology include a housing that defines a process chamber and a waveguide cavity. A first conductive plate is disposed within the housing. The system also includes a second conductive plate positioned within the housing and at least partially defining the waveguide cavity. The second conductive plate is vertically translatable within the housing to adjust a distance between the first conductive plate and the second conductive plate to affect modes of electromagnetic radiation propagating within the waveguide cavity. The systems also include one or more electronics sets that are configured to transmit the electromagnetic radiation into the waveguide cavity to produce plasma from at least one process gas delivered within the process chamber.