Study of Coaxial HOM damper for an RF Damped Cavity

东京大学的ISSP与韩国KEK的光子工厂合作，成功地研制出一种新型的HOM(高次模)阻尼腔。该腔可以通过在腔的两端连接碳化硅束管来吸收HOM。然而，有几个模式的频率低于束流导管的截止频率，它们被捕获在腔内，并在高阻抗下存活。对这些囚禁模进行频率失谐是避免由它们引起的不稳定性的适当方法之一。然而，对于大圆周的环，由于其旋转频率较低，该方法变得不那么有效。在这种情况下，更可取的是降低捕获模式的Q值。为此，我们一直在开发一种简单紧凑的HOM阻尼器。该阻尼器采用带同轴波导的杆状耦合天线，在波导端安装一小片碳化硅作为吸波管中的吸波材料，以吸收提取的霍姆功率。制作了两个霍姆阻尼器的原型模型，并将其连接到腔体的水平和垂直端口。在低功率电平下测量了射频特性。在测量中，证实了HOM阻尼器与六个俘获模式的强耦合，并在不影响加速模式的情况下显著降低了它们的Q值。我们还对HOM阻尼器进行了高功率试验。成功地将高达60kW的输入功率施加到腔体上，并且没有观察到Hom阻尼器上的异常加热。我们将继续进行高功率试验，并尝试将输入功率增加到100kW。此外，我们还将在不久的将来在光子工厂存储环上进行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.