Investigation of a Spoke-Design Waveguide Structure for Dielectric Wall Proton Accelerators
Abstract
Purpose
The dielectric wall accelerator (DWA) offers a low-cost solution for proton therapy with non-resonant waveguides. Radial waveguides (RWGs) transport electronically-switched nanosecond pulses and produce an accelerating electric field in the beam pipe. Previous work has established that the accelerating gradient is greatly diminished when the RWG is excited at discrete points. Spoke-design waveguides use discretely excited striplines converging on a central ring surrounding the dielectric beam pipe. In this study, we examine the parameters and performance of a spoke-design waveguide and seek optimization of output amplitude.
Methods
COMSOL Multiphysics was used to analyze spoke-design waveguides. Three parameters were varied: outer radius (rout), inner ring radius (rinner) and number of spokes (N). Corresponding designs of the RWG with the same outer radius, port width (equivalent to rinner) and number of ports (equivalent to N) served as reference. All designs used a dielectric material with εr = 4.8. The input is a 1 V peak, 2 ns FWHM Gaussian pulse. Peak output voltage amplification was measured in two cases: varying port width with constant rout; and varying rout while maintaining port width.
Results
The spoke-design waveguide had a higher output voltage than the reference. Comparing the best-performing cases of the spoke-design and reference under constant port width, the peak voltage amplification of the spoke-design is 1.044 - 4.189 times higher. For constant rout, peak voltage amplification of the spoke-design is 1.044 - 1.396 times higher than the reference. Altering the number of ports resulted in similar performance of the spoke-design and reference.
Conclusion
The spoke waveguide can achieve higher amplitude output pulses compared to the reference RWG. At larger outer radii, the spoke-design waveguide delivers the input pulse to the beam pipe more efficiently. Larger port widths also performed better on the spoke-design waveguides. Experimental validation is underway to confirm simulations.