Beam Dynamics Studies and RF Cavity Design for Proton Radiotherapy Machines

Southerby, Matthew and Apsimon, Robert and Burt, Graeme (2024) Beam Dynamics Studies and RF Cavity Design for Proton Radiotherapy Machines. PhD thesis, Lancaster University.

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Abstract

In recent years, there has been a rigorous effort to make proton radiotherapy a more viable and accessible source of cancer treatment. Due to the dose deposition rate of protons, there are an array of advantages of using protons over conventional x-rays. In order to treat deep seated tumours in all body sizes, protons of energy 250 MeV are required. As hospitals and treatment centers often have limited space, and budgets, proton machines must be compact and as affordable as possible. This thesis discusses a conceptual design of an S-band rf cavity for proton radiotherapy. The energy range is 150 - 250 MeV. The work is completed in conjunction with AVO and the Cockcroft Institute, in addition to Lancaster University. As the accelerator is the last stage of an all-linac machine, the beam emittance is relatively low, expanding the possible design space of solutions. For a limit on the cavity aperture of 2.5 mm, multiple cavity designs are explored, with respect to constraints such as cavity length, available rf power, and risk of rf breakdown. Firstly, a single cell is designed with the cell shape defined by splines, allowing for lossless increase in rf efficiency. The optimisation is completed using using multi-objective genetic algorithms and various visualisation techniques are explored to best represent the design space. The transverse beam dynamics are explored utilising a novel technique, and two newly developed focusing schemes are explored analytically. The schemes are solved such that the maximum cavity length for a given beam emittance is obtained. In order to quickly assess the performance of a cavity with respect to a given input power, a novel fast tracking algorithm is developed. The algorithm uses the on-axis electric field to approximate the energy gain of a particle over one rf cell assuming constant particle velocity. The tracking code is expanded to incorporate 6D phase space tracking, and is bench-marked relative to well-known tracking codes. With the limit on the cavity length, both Standing and Traveling Wave structures are explored with respect to input rf power. Cavity types are compared with respect to important design parameters, such as required power and fill time. The conceptual design is completed with the addition of matching cells and fine tuning of the structure, to allow for efficient power coupling. Finally, an approximation of the transmission is obtained using the fast tracking algorithm - confirming the high transmission required for linear proton machines.