Department of Oncology, Medical Physics Division, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2, Canada.
Med Phys. 2013 Apr;40(4):041713. doi: 10.1118/1.4794928.
To use a finite-element method (FEM) model to study the feasibility of producing a short s-band (2.9985 GHz) waveguide capable of producing x-rays energies up to 10 MV, for applications in a linac-MR, as well as conventional radiotherapy.
An existing waveguide FEM model developed by the authors' group is used to simulate replacing the magnetron power source with a klystron. Peak fields within the waveguide are compared with a published experimental threshold for electric breakdown. The RF fields in the first accelerating cavity are scaled, approximating the effect of modifications to the first coupling cavity. Electron trajectories are calculated within the RF fields, and the energy spectrum, beam current, and focal spot of the electron beam are analyzed. One electron spectrum is selected for Monte Carlo simulations and the resulting PDD compared to measurement.
When the first cavity fields are scaled by a factor of 0.475, the peak magnitude of the electric fields within the waveguide are calculated to be 223.1 MV∕m, 29% lower than the published threshold for breakdown at this operating frequency. Maximum electron energy increased from 6.2 to 10.4 MeV, and beam current increased from 134 to 170 mA. The focal spot FWHM is decreased slightly from 0.07 to 0.05 mm, and the width of the energy spectrum increased slightly from 0.44 to 0.70 MeV. Monte Carlo results show dmax is at 2.15 cm for a 10 × 10 cm(2) field, compared with 2.3 cm for a Varian 10 MV linac, while the penumbral widths are 4.8 and 5.6 mm, respectively.
The authors' simulation results show that a short, high-energy, s-band accelerator is feasible and electric breakdown is not expected to interfere with operation at these field strengths. With minor modifications to the first coupling cavity, all electron beam parameters are improved.
使用有限元方法（FEM）模型研究产生短 s 波段（2.9985GHz）波导的可行性，该波导能够产生高达 10MV 的 X 射线能量，适用于直线加速器-MR 以及传统放射治疗。
利用作者所在团队开发的现有波导 FEM 模型，模拟用速调管代替磁控管作为功率源。将波导内的峰值场与已发表的电击穿实验阈值进行比较。通过对第一加速腔的射频场进行缩放，近似模拟对第一耦合腔的修改效果。在射频场中计算电子轨迹，并分析电子束的能谱、束流和焦点。选择一个电子能谱进行蒙特卡罗模拟，并将得到的 PDD 与测量值进行比较。
当第一腔场缩小时，波导内电场的峰值幅度计算为 223.1MV/m，比该工作频率下公布的击穿阈值低 29%。最大电子能量从 6.2MeV 增加到 10.4MeV，束流从 134mA 增加到 170mA。焦点的 FWHM 略有减小，从 0.07mm 减小到 0.05mm，能谱的宽度略有增加，从 0.44MeV 增加到 0.70MeV。蒙特卡罗结果表明，在 10×10cm²的场中，dmax 为 2.15cm，而瓦里安 10MV 直线加速器为 2.3cm，而半影宽度分别为 4.8mm 和 5.6mm。
作者的模拟结果表明，短、高能、s 波段加速器是可行的，并且在这些场强下预计不会发生电击穿干扰。通过对第一耦合腔进行微小修改，所有电子束参数都得到了改善。
