Author: Julien Bancheri, Chau Giang Bui, David G Cooke, Christopher M Lund, Morgan J Maher, Jan P. Seuntjens, Jason Z Yuan 👨🔬
Affiliation: Princess Margaret Cancer Centre & University of Toronto, Medical Biophysics, University of Toronto, University of Toronto, Medical Physics Unit, McGill University, Department of Physics, McGill University 🌍
Purpose: The dielectric wall accelerator (DWA) offers a low-cost solution for proton therapy, using non-resonant waveguides to accelerate particles. Radial waveguides (RWGs), consisting of a dielectric annulus between conductive plates, transport electronically-switched nanosecond pulses from the outer radius to produce an accelerating electric field at the beam pipe (inner radius) as particles pass. Previous work has shown reduced electric field strength due to inefficient power transfer with discrete excitation points. This study introduces patterned inhomogeneities in the RWG’s dielectric to enhance power transfer.
Methods: COMSOL Multiphysics simulations were performed on RWGs with five excitation ports along the outer periphery. Three RWGs are considered: a homogeneous reference; a design incorporating planar Luneburg lenses; a rotationally symmetric design with variable-sized air holes in the RWG’s dielectric. The frequency-dependent power transfer is determined with a 2.35 GHz FWHM bandwidth Gaussian envelope modulated by a cosine wave at carrier frequencies from 1.5 to 5 GHz — repeated for different port widths.
Results: Inhomogeneities in the RWG improved power transfer over the reference for most parameters.The lens design enhanced power transfer by factors of 1.18 - 2.71, with larger enhancement for smaller ports and higher frequencies. The rotationally symmetric hole design showed a port-dependent average enhancement of 1.21 - 1.33 for frequencies below ~3 GHz, however performance declined beyond this, becoming worse than the reference at 5 GHz. The best-performing design (lens, smallest port width, 4.5GHz) transmitted 71.3% of the applied power to the beam pipe.
Conclusion: Including air hole defects can achieve improved power transmission to the DWA’s beam pipe. Smaller air holes enhance power transfer for small port angles at a larger frequency range, making this a suitable starting design for field strengths of 10 - 30 MV/m. Continued studies are needed to determine geometric requirements for achieving field strengths near 100 MV/m.