The present invention provides a surface wave plasma source including an electromagnetic (EM) wave launcher comprising a slot antenna having a plurality of antenna slots configured to couple the EM energy from a first region above the slot antenna to a second region below the slot antenna, and a power coupling system is coupled to the EM wave launcher. A dielectric window is positioned in the second region and has a lower surface including the plasma surface. A slotted gate plate is arranged parallel with the slot antenna and is configured to be movable relative to the slot antenna between variable opacity positions including a first opaque position to prevent the EM energy from passing through the first arrangements of antenna slots, and a first transparent position to allow a full intensity of the EM energy to pass through the first arrangement of antenna slots.