A pipe source of UV flux has an inner pipe made of UV transmissive material and coated on its outer surface with a UV emitting phosphor. An outer pipe has a cathode array disposed on or near its inner surface, such as an array of thermionic filament cathodes mounted longitudinally or transverse to the length of the pipe, cold cathode arrays formed on the inner surface of the pipe or cold cathode arrays formed on separate substrates which are then attached to the inner surface of the outer pipe. The ends of this two-pipe assembly are hermetically sealed with flanges or end plates at either end of the pipe and evacuated to a pressure below 1×10−3 Torr. Internal spacing rings may be used to provide additional separation between the inner and out pipes. Current from the cathode arrays is accelerated by an anode voltage to strike the UV phosphors when then emit UV light flux which illuminates the inside of the pipe and the fluid material flowing through the inner pipe.
