DESIGN OF A VHEE RADIOTHERAPY MACHINE

Abstract

Very High Energy Electron Radiotherapy (VHEE-RT) is a proposed complementary treatment to traditional x-ray radiotherapy and Hadron beam therapies. The VHEE modality could allow for medical irradiations which are less sensitive to changes in the patient geometry, and the high relative dose rates achievable with electron therapies allow for innovative treatment techniques. Typical x-ray therapies use machines which can be placed in treatment rooms which generally have a 3x3 metre floor area, however proton treatment facilities are generally far larger and require significant construction work. VHEE therapy would require accelerating electrons to energies far higher than is currently available in clinical environments, with the VHEE treatment beams proposed to have energies of 100 MeV or higher. To commercialise VHEE and potentially use existing treatment suites the space constraints require a compact high gradient linear accelerator (linac) solution that is both reliable and affordable. This report will begin with an overview of radio-therapeutics and accelerator radio-frequency (RF) physics. This is presented along with a description of breakdown and other high electromagnetic field phenomena which are important to consider when designing high electric field gradient RF structures. The bulk of the contents of this thesis present a design covers a design approach towards the development of a high gradient accelerating structure for medical electron applications. The process began with the design of a 12 GHz on-axis single cell RF cavity, with simulation and analysis of its RF performance for different geometries, this geometry was then optimised for high shunt impedance and sufficiently low surface fields to allow for a high accelerating gradient. This was performed via an iterative approach. This iterative process consisted largely of manipulation of the geometry of re-entrant cavity nose cones which provide loading of the electric field close to the accelerating axis. The resulting on-axis single cell was then coupled with an off-axis coupling cavity to form a bi-periodic off-axis single cell design. The optimal design of a Coupled Cavity Linac (CCL) single cell was investigated guided by the constraints of maximum surface field ratios which must be balanced with the demand for high shunt impedance. This type of cavity design was chosen as it offers the potential for high shunt impedances as well as reliability of operation in medical devices. The significant surface field properties studied in this work were the surface electric field ratio E_s/E_acc=2.6, surface magnetic field ratio H_s/E_acc=5.7 mA/V, and surface modified Poynting vector ratio S_c/E_acc^2 = 4.2x 10^-4 A/V. This set of design parameters allows for a balanced single cell, with accelerating gradient of E_acc=60 MV/m, whilst maintaining a sufficiently high shunt impedance per unit length of r'_s=135 M\Omega /m, and a monopole passband dispersion bandwidth of 340 MHz. The single cell design is then taken further by producing a multi-cell design following the example of a 5-cell CCL (Coupled Cavity Linac). The final part of the work dealt with producing a set of test RF cavity components for fabrication. The goal of the experimental and fabrication design work was intended in include sensitivity studies, assess the viability of fabricating such cells, and perform analyses of cavity frequency and field distribution on axis. The main accelerating cells which had been sent for manufacture have been produced. In 2022 these parts arrived with the collaborative partner of the project Elekta, several CCL half-cells were delivered, some example images of these parts are presented in this thesis. An experimental programme is expected to continue in the near future.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Roger Jones (Supervisor) & Guoxing Xia (Supervisor)