Rapidis Nicholas M, Lewis Samantha M, van Bibber Karl A
Physics Department, University of California, Berkeley, California 94720, USA.
Nuclear Engineering Department, University of California, Berkeley, California 94720, USA.
Rev Sci Instrum. 2019 Feb;90(2):024706. doi: 10.1063/1.5055246.
Many searches for axion cold dark matter rely on the use of tunable electromagnetic resonators. Current detectors operate at or near microwave frequencies and use cylindrical cavities with cylindrical tuning rods. The cavity performance strongly impacts the signal power of the detector, which is expected to be very small even under optimal conditions. There is strong motivation to characterize these microwave cavities and improve their performance in order to maximize the achievable signal power. We present the results of a study characterizing the HAYSTAC (Haloscope At Yale Sensitive to Axion Cold dark matter) cavity using bead perturbation measurements and detailed 3D electromagnetic simulations. This is the first use of bead perturbation methods to characterize an axion haloscope cavity. In this study, we measured impacts of misalignments on the order of 0.001 in. and demonstrated that the same impacts can be predicted using electromagnetic simulations. We also performed a detailed study of mode crossings and hybridization between the TM mode used in operation and other cavity modes. This mixing limits the tuning range of the cavity that can be used during an axion search. By characterizing each mode crossing in detail, we show that some mode crossings are benign and are potentially still useful for data collection. The level of observed agreement between measurements and simulations demonstrates that finite element modeling can capture non-ideal cavity behavior and the impacts of very small imperfections. 3D electromagnetic simulations and bead perturbation measurements are standard tools in the microwave engineering community, but they have been underutilized in an axion cavity design. This work demonstrates their potential to improve understanding of existing cavities and to optimize future designs.
许多对轴子冷暗物质的搜索都依赖于可调谐电磁谐振器的使用。目前的探测器工作在微波频率或接近微波频率，并使用带有圆柱形调谐棒的圆柱形腔体。腔体性能对探测器的信号功率有很大影响，即使在最佳条件下，探测器的信号功率预计也非常小。为了最大化可实现的信号功率，有很强的动机来表征这些微波腔体并提高它们的性能。我们展示了一项使用微珠微扰测量和详细的三维电磁模拟来表征HAYSTAC（耶鲁大学对轴子冷暗物质敏感的卤oscope）腔体的研究结果。这是首次使用微珠微扰方法来表征轴子卤oscope腔体。在这项研究中，我们测量了约0.001英寸量级的失准影响，并证明可以使用电磁模拟来预测相同的影响。我们还对运行中使用的TM模式与其他腔体模式之间的模式交叉和杂化进行了详细研究。这种混合限制了在轴子搜索期间可以使用的腔体调谐范围。通过详细表征每个模式交叉，我们表明一些模式交叉是良性的，并且可能仍然可用于数据收集。测量结果与模拟结果之间观察到的一致性水平表明，有限元建模可以捕捉非理想腔体行为以及非常小的缺陷的影响。三维电磁模拟和微珠微扰测量是微波工程领域的标准工具，但它们在轴子腔体设计中一直未得到充分利用。这项工作展示了它们在增进对现有腔体的理解以及优化未来设计方面的潜力。
