Maher Morgan J, Lund Christopher M, Bancheri Julien, Cooke David G, Seuntjens Jan
Medical Physics Unit, McGill University, Montréal, Québec, Canada.
Department of Physics, McGill University, Montréal, Québec, Canada.
Med Phys. 2025 Jul;52(7):e17868. doi: 10.1002/mp.17868. Epub 2025 May 12.
Proton therapy (PT) is a beneficial modality for treating certain cancers but remains under utilized due in part to the high cost of existing PT devices. Dielectric wall accelerators (DWAs) are a proposed class of coreless induction accelerators that may present a suitable option for compact and affordable PT. To realize a compact device, acceleration modules must be designed to achieve field strengths approaching 100 MV/m delivered as pulses on the order of nanoseconds.
Here, we examine pulse injection into radial parallel plate waveguides as a means of producing high-intensity, pulsed accelerating fields. We present an approach for understanding the impact of waveguide properties on electromagnetic dispersion as well as a means of accounting for this dispersion to produce suitable accelerating fields.
Geometric and material properties for a set of waveguides were identified based on existing literature and commonly available materials. An analytic model is presented to describe how waveguide geometry and material affect electromagnetic dispersion in a waveguide. Simulations performed in COMSOL Multiphysics are used to calculate a transfer function for the set of waveguides, which provide a means of determining the waveguides output for arbitrary inputs and vice versa. The simulation results are compared to the analytic solution and used to explore alternate matching conditions at the beampipe of the accelerator.
Overall, radial waveguides provide a passive enhancement of the injected pulse, with enhancement of high-frequency components found to be proportional to the square root of the ratio of outer radius to inner radius of the waveguide. Dispersion in the waveguide caused by the radial propagation of the pulse depends on multiple waveguide properties (outer radius, inner radius, material) and leads to reduced enhancement at lower frequencies. The field enhancement in the waveguides reduces the peak voltage required to achieve the desired accelerating field strength. However, dispersion alters the temporal profile of the applied pulse, resulting in a distorted field at the inner radius. Using the transfer function, it is possible to determine the shape of the pulse required to achieve a suitable accelerating field for a given waveguide design.
Passive field enhancement occurred in all waveguides and across all frequencies studied in this work. As such, radial parallel plate waveguides could help to reduce the high voltages required from upstream switching networks. The analytic model can be used to select waveguide parameters that provide a suitable enhancement of the upstream voltage pulse to achieve the high field strengths required for a compact accelerator. However, pulse dispersion must be accounted for. If upstream pulse shaping can be achieved to account for electromagnetic dispersion in the waveguide, pulse injection into radial parallel plate waveguides could be a suitable mechanism for field generation in a DWA.
质子治疗（PT）是治疗某些癌症的一种有益方式，但部分由于现有PT设备成本高昂，其应用仍未得到充分利用。介质壁加速器（DWA）是一类提议的无芯感应加速器，可能为紧凑且经济实惠的PT提供合适选择。为实现紧凑设备，加速模块必须设计成能实现接近100 MV/m的场强，并以纳秒量级的脉冲形式输出。
在此，我们研究向径向平行板波导注入脉冲作为产生高强度脉冲加速场的一种手段。我们提出一种方法来理解波导特性对电磁色散 的影响，以及一种考虑这种色散以产生合适加速场的手段。
基于现有文献和常用材料确定了一组波导的几何和材料特性。提出一个解析模型来描述波导几何形状和材料如何影响波导中的电磁色散。在COMSOL Multiphysics中进行的模拟用于计算该组波导的传递函数，这提供了一种确定任意输入下波导输出的方法，反之亦然。将模拟结果与解析解进行比较，并用于探索加速器束流管道处的替代匹配条件。
总体而言，径向波导对注入脉冲有被动增强作用，发现高频分量的增强与波导外半径与内半径之比的平方根成正比。脉冲径向传播在波导中引起的色散取决于多个波导特性（外半径、内半径、材料），并导致低频处增强降低。波导中的场增强降低了实现所需加速场强所需的峰值电压。然而，色散改变了施加脉冲的时间轮廓，导致内半径处的场发生畸变。利用传递函数，可以确定为给定波导设计实现合适加速场所需的脉冲形状。
在本工作研究的所有波导和所有频率下都发生了被动场增强。因此，径向平行板波导有助于降低上游开关网络所需的高电压。解析模型可用于选择能对上游电压脉冲提供合适增强以实现紧凑加速器所需高场强的波导参数。然而，必须考虑脉冲色散。如果能够实现上游脉冲整形以考虑波导中的电磁色散，那么向径向平行板波导注入脉冲可能是DWA中场产生的合适机制。
