提高超导加速腔的性能，从而提升束流品质并降低造价，一直都是超导腔工作者追求的目标。本论文以此为出发点，全面系统地介绍了如何提升稀有同位素束流装置（Facility for Rare Isotope Beams （FRIB））的低β超导加速腔的性能，这个性能提升的过程涵盖了超导加速腔制造的三个流程：从加速腔的设计（加速腔的结构优化），到加速腔的控制（加速电压的控制），最后到加速腔的测试（次级电子倍增现象的抑制）。这些性能优化的方案已经或者将会被应用到FRIB的加速腔上，一些用于验证模型的初步测试结果将会在论文中进行介绍。首先，我们从加速腔设计角度出发，在FRIB加速腔的基础上提出“Race-car”加速腔的设计方案。在维持造价的前提下，通过结构优化我们将加速腔的加速梯度和分流阻抗显著地提高了，使“Race-car”成为了同类型腔中性价比最高的“跑车”。同时，我们还分析了其相应的二级场束流偏转效应和次级电子倍增现象，以确保“Race-car”不仅跑得快，而且跑得稳。其次，我们从加速腔的控制角度出发，首次将自抗扰控制方法引进到低电平控制和调谐器控制之中，并且同传统的比例积分控制方法在仿真和实验中进行了对比。我们还研究了如何设计并使用压电陶瓷快速调谐器来控制加速腔的加速电压。此外，我们首次建立了完整的加速腔加速电压控制的模型。然后，我们从加速腔的测试角度出发，发现了测试中制约FRIB加速腔性能的主要问题——功率耦合器和加速腔中的次级电子倍增现象。我们对该问题进行研究并提出相应的解决方案：（1）推导同轴结构上次级电子倍增现象的物理模型，依据模型对FRIB功率耦合器进行结构优化来抑制次级电子倍增现象；（2）针对FRIB加速腔短路板上的次级电子倍增现象，建立回旋加速模型，发现可以通过优化短路板的曲率来减轻其相应的次级电子倍增现象。最后，我们总结了如何在超导加速腔制造的三个流程中一步步地提高其性能的方法和过程，并指出了该论文研究工作对于提高FRIB低β超导加速腔性能起到的重要作用。
Superconducting RF (SRF) cavities are probably the most expensive components in an accelerator. Scientists have put a lot of efforts into its performance enhancement and cost reduction. Using the low-beta SRF cavities at Facility for Rare Isotope Beams (FRIB) as examples, we have carried out systematic studies to find effective methods to improve their performance. This systematic research includes topics from cavity structural optimization to advanced control techniques, and also effective physics models to predict multipacting suppression. These methods and designs are or will be adopted in FRIB cavity development. Preliminary test results are presented in this thesis as validation of the models.Following a brief introduction on SRF cavity’s fundamentals, we generally describe FRIB project and its procured cavities that were installed at Re-accelerator 3 (ReA3) in Chapter 1. Our research was motivated by the challenging during the commissioning of those SRF cavities, which is mainly discussed in Chapter 2-4.Chapter 2 introduces our progresses on the structural optimization, using FRIB β=0.085 QWR as a sample. The new design significantly enhances the accelerating gradient by reducing magnetic field and increasing shunt impedance. Chapter 3 compares two control algorithms in the low level RF / tuner control system: conventional Proportional Integral (PI) algorithm and an advanced algorithm--Active Disturbance Rejection Control (ADRC). Chapter 4 discusses our important work on the multipacting suppression. Two models were developed: one for coaxial structure that leads to a new optimized coupler to be installed on FRIB cavities; another for the cavity’s short plate to reduce the multipacting on it.In the summarized Chapter 5, we compare the effectiveness of the three optimizations. In the future, we will continue working on optimizing the design of SRF cavity and improving its performance.
