Study of Coaxial HOM damper for an RF Damped Cavity

东京大学的ISSP和日本电气公司的光子工厂合作研制成功了一种新型的HOM（高阶模）阻尼腔。该腔体通过两端的SiC束流导管吸收HOM。然而，几个模式，其频率低于截止的光束导管，被困在腔体中，并生存与高阻抗。对这些陷阱模进行频率失谐是避免其引起的不稳定性的有效方法之一。然而，对于具有大周长的环，由于其低旋转频率，该方法变得不太有效。在这种情况下，更可取的是降低Q值的捕获模式。为此，我们一直在开发一种简单的和紧凑的尺寸HOM阻尼器。该阻尼器具有一个同轴波导的棒状耦合天线，在波导端部安装一个与波束导管中的吸收体相同材料的SiC小块，以吸收提取的HOM功率。我们制作了两个HOM阻尼器的原型模型，并将它们连接到腔体的水平和垂直端口。在低功率水平下测量RF特性。在测量中，它被证实，HOM阻尼器强烈耦合到六个捕获模式，并大大降低了它们的Q值，而不影响加速模式。并对HOM阻尼器进行了大功率试验。成功地将高达60 kW的输入功率应用于腔体，并且在HOM阻尼器上没有观察到异常加热。我们将继续进行高功率测试，并尝试将输入功率增加到100 kW。此外，我们将在不久的将来在光子工厂储存环上进行HOM阻尼器的实际束流测试。

A new type of HOM (Higher order mode)-damped cavity has been successfully developed by collaboration between ISSP of the University of Tokyo and the Photon Factory of KEK. The cavity can absorb HOM's by SiC beam ducts attached at both ends of the cavity. However, several modes, which have frequency lower than the cutoff of the beam duct, are trapped in the cavity and survive with high impedance. Frequency detuning of these trapped modes is one of proper methods to avoid the instability due to them. For a ring with large circumference, however, the method becomes less effective because of its low revolution frequency. In such a case, it is more preferable to reduce Q-values of the trapped modes.For this purpose, we have been developing a simple and compact-size HOM damper. The damper has a rod-shaped coupling antenna with a coaxial waveguide, A small SiC piece made of the same material as the absorber in the beam duct is mounted on the end of the waveguide in order to absorb the extracted HOM power.We fabricated two prototype models of the HOM damper and attached them to horizontal and vertical ports of the cavity. The RF characteristics were measured in low-power levels. In the measurement, it was confirmed that the HOM damper strongly coupled to six trapped modes and reduced their Q-values drastically without affecting the accelerating modes. We also carried out high-power test of the HOM dampers. Input power up to 60 kW was applied to the cavity successfully, and unusual heating was not observed on the HOM dampers.We will continue the high power test and try to increase the input power up to 100 kW. Furthermore, we will carry out actual beam test of the HOM damper at the Photon Factory storage ring in near future.