Stabilization of high intensity beam by higher harmonic cavity in electron synchrotron

东北大学核科学研究室的1.2GeV拉伸-助推器（STB）环主要用于核物理实验。机顶盒不仅是光源环，还具有类似存储环的功能。限制STB环性能的许多问题之一应该是强纵向耦合束团不稳定性。实际上，在3.6GHz附近已经观察到非常强的同步辐射振荡信号，阈值电流约为10 mA。为了抑制这种不稳定性，研究了三次谐波朗道腔的适用性。结果表明，如果采用三个谐波腔，在最大环电流为100 mA时，可以获得接近最佳的束团延长。对于200 MeV的注入能量，进行了几乎相同的分析，结果表明，对于几乎所有的电流和调谐角度，都发生了来自不稳定性或丢失宏观粒子的能量扩散增加。虽然模拟研究表明，需要更多的腔来抑制不稳定性，一个谐波腔制造和测试的STB作为研究的第一步。由于耦合束团不稳定性引起的强同步辐射振荡信号在特定的调谐角下被谐振腔极大地衰减。然而，不认为这种抑制是由于通常的朗道阻尼，因为由于大的调谐角，在谐波腔中从未激励显著的电压。为了理解这种情况，使用刚性束团模型进行了跟踪仿真。结果表明，谐振腔的高次模可以抑制这种不稳定性。对于低能运转，谐振腔的影响也可能是有趣的，但还没有看到明显的改善。对于未来的发展，谐振腔在加速过程中的研究也将取得进展。

A 1.2 GeV Stretcher-Booster (STB) ring at Laboratory of Nuclear Science(LNS), Tohoku University, has been mainly operated for experiments of nuclear physics. The STB is not the light source ring but also has storage ring-like function. One of the many issues limiting performance of the STB ring is supposed to be strong longitudinal coupled-bunch instability. Actually very strong synchrotron oscillation signal have been observed around 3.6 GHz with a threshold current of about 10 mA. In order to suppress the instability, applicability of a third-harmonic Landau cavity has been studied.For the full energy operation, analytic modeling and simulations were first performed. The results show that if three harmonic cavities are used, near-optimal bunch-lengthening may be obtained at the maximum ring current of 100 mA. For injection energy of 200 MeV, almost same analysis were performed and showed that increased energy spread from the instability or lost macro-particles occurred for nearly all currents and tuning angles. Although the simulation study indicates that more cavities are required to suppress the instability, one harmonic cavity was manufactured and tested in the STB as the first step of the study. The very strong synchrotron oscillation signal due to the coupled-bunch instability was drastically damped by the harmonic cavity at a specific tuning angle. However, it is not considered that this suppression is due to the usual Landau damping, since a significant voltage is never excited in the harmonic cavity because of the large tuning angle. A tracking simulation using rigid-bunch model was performed to understand this situation. The result showed that a higher order mode of harmonic cavity can suppress the instability. For the lower energy operation, the effect of harmonic cavity might be also interesting, but no obvious improvement has been seen yet. Toward the future development, a study of the harmonic cavity in acceleration process will be also progressed.