Optimisation studies for a high gradient proton Linac for application in proton imaging : ProBE: Proton Boosting Linac for imaging and therapy

Pitman, Sam and Burt, Graeme (2019) Optimisation studies for a high gradient proton Linac for application in proton imaging : ProBE: Proton Boosting Linac for imaging and therapy. PhD thesis, Lancaster University.

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Abstract

Proton beam therapy is an alternative to traditional X-ray radiotherapy utilised especially for paediatric malignancies and radio-resistant tumours; it allows a precise tumour irradiation, but is currently limited by knowledge of the patient density and thus the particle range. Typically X-ray computed tomography (CT) is used for treatment planning but CT scans require conversion from Hounsfield units to estimate the proton stopping power (PSP), which has limited accuracy. Proton CT measures PSP directly and can improve imaging and treatment accuracy. The Christie Hospital will use a \SI{250}{MeV} cyclotron for proton therapy, but \SI{330}{MeV} protons are needed to image the largest adult. In this thesis the feasibility of a pulsed linac upgrade to provide \SI{100}{MeV} acceleration in a cyclinac set up is studied. Space constraints require a compact, high gradient (HG) solution that is reliable and affordable. An overview of accelerator physics and beam dynamics are presented alongside the phenomenology of breakdown in high gradient RF structures. Both a small and large aperture solution are investigated. The small aperture option aims to keep the beam size to a minimum using focussing magnets between cavities and accelerate with a very high gradient. The large aperture solution aims to occupy more of the available space with accelerating structures and less with focussing magnets. This way the optics are simpler and the beam size is larger throughout the linac. The small aperture optimisation investigated S-, C- and X-band cavities. Firstly with simple pill-box structures then looking at the effect of nose cones on RF efficiency and breakdown limits. Multi-cell structures are then investigated employing side-coupling for standing wave (SW) cavities and various different magnetic coupling slots for backward travelling wave structures (bTW). Limited by 100~MW/m a 15~cm bTW solution was proposed with a calculated gradient of 65~MV/m. Unfortunately to be used with a cyclotron, which typically have large emittance, infeasibly strong magnets would be required. The large aperture optimisation only considers S- and C-band structures as they exhibit higher shunt impedance with larger apertures than X-band cavities. Side-coupled SW structures and magnetically coupled bTW structures are re-optimised for a larger aperture. An S-band side-coupled SW structure is subsequently identified as being the optimum for this energy range. A full 11-cell structure design with input coupler and end cells is presented calculated to reach 54~MV/m. A beam dynamics study is also presented considering both the small and large aperture schemes. For the large aperture scheme a particle tracking study is also presented. Mechanical engineering considerations are presented with a novel disk design manufacturing each individual cell from two machined copper disks. Thermal analysis of the temperature distribution inside the cavity is presented alongside heat transfer calculations for the cavity cooling system. Finally the conditioning and high power test of a similar S-band medical structure is presented with comparison made to the prototype structure designed in this study.