The primary impetus for the work reported in this paper is to develop efficient overmoded waveguides (OMWGs) that employ broadband downtaper transitions that would be compatible with the severe space constraints in high-field NB magnets. Further, it is essential these would be readily manufacturable, as high precision corrugated metallic downtapers for the sub-mmw regime are very difficult to produce. We have simulated numerous alternatives to corrugated circular OMWGs, including most of the previously reported alternatives (except for many of the low-power fiberoptics options) and several novel designs. We conclude that corrugated circular metallic OMWGs are the best of the reported options to date (except from a cost perspective) for diameters down to ~1.5λ, but the corrugation parameters for small OMWGs need to be significantly different from the previously published guidelines that have worked well for large OMWGs. With numerically optimized small OMWGs, easily manufacturable smooth downtapers appear to work as well as corrugated downtapers in many cases relevant to MAS-DNP probes. Our example simulations will be for the 170–230 GHz range, but the lessons and results should be readily applicable to other ranges by simple scaling.

本文报道的工作的主要推动力是开发高效的过调制波导 (OMWG)，该波导采用宽带向下锥度过渡，与高场 NB 磁体中严格的空间限制兼容。此外，重要的是这些易于制造，因为用于亚毫米波范围的高精度波纹金属锥度非常难以生产。我们模拟了波纹圆形 OMWG 的多种替代方案，包括大多数先前报道的替代方案（许多低功率光纤选项除外）和几种新颖的设计。我们的结论是，对于直径低至 ~1.5λ 的波纹圆形金属 OMWG 是迄今为止报道的最佳选择（从成本角度除外），但小型 OMWG 的波纹参数需要与之前发布的指南显着不同，这些指南对于大型 OMWG 效果很好。借助数值优化的小型 OMWG，在许多与 MAS-DNP 探头相关的情况下，易于制造的光滑下锥度似乎可以与波纹下锥度一样工作。我们的示例模拟将针对 170–230 GHz 范围，但经验教训和结果应该可以通过简单的缩放轻松应用于其他范围。