A device comprises a controlled source of microwave energy 10, at least one means of modulating the microwave energy 30, a means of measuring forward going and reflected microwave energy 60, 70, 80, 90, a means of detecting magnitude and/or phase 120, a means of providing a suitable gas mixture to the antenna with a suitable flow rate 400-460, a microwave cable assembly 200 such as a coaxial cable, an applicator/ antenna 300-350, a means controlling the system 140, and a user interface unit 150. The device is configured to create a high impedance at the plasma generating region in the applicator 300 when gas and microwave energy are delivered thereto to create a non-thermal plasma. The device may also include an amplifier and variable attenuator. The system may also contain a tuning mechanism 100 such as a stub tuner to enable the plasma to be struck and maintained in an efficient manner. The plasma delivery antenna structure 300 be used for insertion into human orifices, or may be used as one of a plurality of structures for sterilising large surfaces or hospital wards is also disclosed.
