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

日本东北大学核科学实验室（LNS）的一个1.2 GeV担架助推器（STB）环主要用于核物理实验。机顶盒不仅是光源环，而且具有类似存储环的功能。限制机顶盒环性能的诸多问题之一被认为是强纵向耦合束不稳定性。实际上非常强的同步振荡信号已被观察到约3.6 GHz的阈值电流约10毫安。为了抑制不稳定性，研究了三次谐波朗道腔的适用性。对于全能量运行，首先进行了解析建模和仿真。结果表明，如果使用三个谐波腔，在最大环电流为100 mA时可以获得接近最佳的串长。对于200 MeV的注入能量，进行了几乎相同的分析，结果表明，在几乎所有电流和调谐角度下，不稳定或丢失的宏观粒子的能量扩散都增加了。虽然仿真研究表明需要更多的谐振腔来抑制不稳定性，但作为研究的第一步，我们制造了一个谐振腔并在机顶盒中进行了测试。谐振腔在一定的调谐角度下对耦合束不稳定性引起的强同步振荡信号进行了抑制。然而，不认为这种抑制是由于通常的朗道阻尼造成的，因为由于调谐角大，谐波腔中从未激发出显著的电压。利用刚束模型进行了跟踪仿真。结果表明，谐波腔的高阶模态可以抑制不稳定性。对于较低能量的操作，谐振腔的效应可能也很有趣，但目前还没有明显的改善。在未来的发展中，对加速过程中的谐振腔的研究也将进一步深入。

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.