基于逐圈位置频谱的束流不稳定性

Beam instabilities based on spectrum of turn-by-turn position

利用上海光源储存环中受激电子束团位置信号频谱数据,确定了束团在加速器中运行的工作点及品质因数,获得了储存环垂直方向阻抗和横向阻尼率,分析了不稳定性产生的原因。阐述了加速器中受激束团的运动特点,以RLC振荡电路为基础,对束团运动进行简单的建模,利用模型中频率响应函数对频谱中的脉冲波形进行拟合,得到了用于表示运动阻尼及频率分散程度的品质因数。实验结果表明:利用该方法拟合得到的数据与原数据的相关性可达98%以上,所拟合波形与原波形整体相似度高。对不同流强下的数据进行了研究,随着流强的逐渐增加,工作点和品质因数同步减小,不稳定性加剧,流强达到阈值后,频谱中单峰分裂为双峰,束流品质降低。

Spectrum of excited electron beam position was analyzed to determine tune value and quality factor in Shanghai Synchrotron Radiation Facility（SSRF）. Vertical impedance and transverse damping rate of SSRF storage ring were educed. In light of these results, sources of beam instability are discussed and characteristics of the motion of excited single bunch are presented. Frequency response function of RLC circuit was used to fit peaks in the spectrum of beam position signal. Based on the fitting results, quality factor, which represents damping state of the motion, dispersion of frequency, and degree of instability, was ac- quired. Experiment results show that, the correlation of fitted data and original data can be up to 98%, and the profile of fitted figure is quite similar to original one, which means the fitting performance is better than Gaussian fitting. Data of several beam currents were analyzed. Tune value and quality factor will decrease accordingly if beam current increases, thus the beam instability is stronger. While the current increases beyond a certain threshold, a split appears in the peak of spectrum which indicates beam quality decreases.