Purpose: To evaluate the impact of electromagnetic dispersion in annular parallel plate waveguides for use in the acceleration and focusing of pulsed proton beams in a dielectric wall accelerator (DWA) for proton radiotherapy.
Methods: A 2D axisymmetric model of the waveguide was developed and implemented in COMSOL Multiphysics. The inner radius was chosen to ensure suitable field uniformity within the beam pipe, while the outer radius was chosen based on the transit time of the desired electrical excitation. A broadband reference pulse was used to uniformly excite the outer radius of the waveguide. Measurement of response at the inner radius provided the information required to calculate the transfer function of the system. The desired output pulse was identified as a 1~ns pulse with a linear time gradient imposed to produce longitudinal focusing. The transfer function was used to determine the input excitation required to produce the desired output waveform. The modeled input pulse was validated using the COMSOL model, and the system's sensitivity to changes in the input pulse was studied.
Results: The annular geometry of the waveguide was shown to amplify the electromagnetic input due to dispersion within the structure. Describing the system using a transfer function was shown to adequately predict how the dispersion modifies arbitrary input pulses. Using the transfer function, a suitable input pulse was identified to produce a linearly varying field at the beam pipe.
Conclusion: Adoption of an annular waveguide geometry for electric field delivery in a DWA-based proton radiotherapy device supplies a passive amplification of the field strength thereby reducing electronic strain on upstream systems. The derived transfer function provides a rapid and reliable way of evaluating the impact of changes to the input (e.g. upstream circuit modifications) or output (e.g. feedback from beam dynamics studies) pulses, thereby aiding overall accelerator design.
Funding Support, Disclosures, and Conflict of Interest: This work is financially supported by the Fonds de recherche du Quebec - Nature et technologies, the Walter C. Sumner Foundation, and by NSERC (RGPIN-2019-06746)
Protons, Radiation Therapy, Modeling
TH- External Beam- Particle/high LET therapy: Proton therapy – Development (new technology and techniques)