Low Energy Linacs for 3D X-ray Scanning Applications

Smith, Samuel and Burt, Graeme and Apsimon, Robert (2024) Low Energy Linacs for 3D X-ray Scanning Applications. PhD thesis, Lancaster University.

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Abstract

Typical cargo scanning Linacs used to scan shipping containers are usually designed at S-band (2-4 GHz) frequencies and have energies of 3-6 MeV in order to obtain sufficient contrast during inspections. To scan smaller and thinner containers and to reduce the footprint of scanning systems, there is an interest in lower energy (1-3 MeV) and higher frequency (5-6 GHz) devices. In this thesis, a design of such a device is presented, with a compact (11.9 cm), five cell, 2 MeV, bi-periodic, C-band linac selected as the final design. Multi-objective optimisation methods and spline modelling techniques are used to optimise the cells with a Pareto analysis used to select a final design, also allowing for rapid design adjustment. A novel coupling method using nose cone slants is developed, giving an improvement of 28 % in the coupling factor between cells (0.7 % - 0.9 %), with minimal effect on the peak fields or shunt impedance (1.3 %). MO methods are also employed to optimise the cell lengths and RF amplitudes to increase the capture efficiency of the linac to over 90 % using re-capture methods. A complete thermal analysis is presented showing that the linac can handle up to 1.2 kW average power with less than 2 % error in the electric field. The study includes CFD simulations and an improved method for estimating the heat transfer coefficient by including bends when performing calculations which agrees with the CFD analysis. The design is then integrated into a full RF system that allows for three linacs to be fired and synchronised, using three frequencies with a 3 MHz separation (5.712 ± 3 MHZ) on three sections of one 10 µs RF pulse. It is then shown that this system is capable of generating quasi-3D images in a CT-like setup using 3D image reconstruction techniques.