Prototype 1 MeV X -band linac for aviation cargo inspection

Aviation cargo unit load device (ULD) containers are typically much smaller than standard shipping containers, with a volume of around 1 m3. Standard 3-6 MeV x-ray screening linacs have too much energy to obtain sufficient contrast when inspecting ULDs, hence a lower 1 MeV linac is required. In order to obtain a small physical footprint, which can be adapted to mobile platform applications, a compact design is required, hence X-band radio-frequency technology is the ideal solution. A prototype 1.45 MeV linac cavity optimized for this application has been designed by Lancaster University and Science and Technology Facilities Council (STFC), manufactured by Comeb (Italy) and tested at Daresbury Laboratory using an e2v magnetron, modulator, and electron gun. The cavity is a bi-periodic π/2 structure, with beam-pipe aperture coupling to simplify the manufacture at the expense of shunt impedance, while keeping the transverse size as small as possible. The design, manufacture, and testing of this linac structure is presented. In order to optimize the image it is necessary to be able to modify the energy of the linac. It can be changed by altering the rf power from the magnetron but this also varies the magnetron frequency. By varying the beam current from 0-70 mA the beam energy varied from 1.45 to 1.2 MeV. This allows fast energy variation by altering the focus electrode bias voltage on the electron gun while keeping the dose rate constant by varying the repetition frequency. Varying the beam energy by varying the rf power and by varying the beam current are both studied experimentally. The momentum spread on the electron beam was between 1% and 5% depending on the beam current of 0-70 m