| With the continuous development and actual demands of accelerator technology and related application disciplines,the requirements for the brightness and beam quality of the current synchrotron radiation(SR)light source and collider based on the electron storage ring are getting higher and higher.Among,various beam instabilities in the beam collective effect will greatly deteriorate the beam quality and simultaneously decrease performance of an electron storage ring.As a consequence,a large number of accelerator researchers have been committed to exploring the mechanism of beam dynamics and further seeking various solutions to effectively suppress or overcome beam instability.Note that for different electron storage rings,and the beam instability and bunch characteristics are significantly different because of their uniqueness.For Hefei light source-Ⅱ(HLS-Ⅱ),the potential-well distortion and microwave instability in single-bunch effect and the transverse and longitudinal coupling bunch instability in multi-bunch effect are more prominent.Therefore,in this paper,the study of longitudinal beam dynamics and related beam characteristics measurements are carried out on the HLS-Ⅱ storage ring,which will provide a mature technical scheme and research ideas for the future longitudinal beam diagnosis and related beam dynamics research of the diffraction-limited storage ring(DLSR)light source under construction,namely Hefei Advanced Light Facility(HALF).Firstly,according to the actual beam diagnostic requirements of the HLS-Ⅱ storage ring,a longitudinal beam diagnostic system based on a streak camera is improved and built.A series of offline and online experiments were performed to effectively analyze the performance of the whole measurement system,including static resolution,linear scanning range,gain voltage curve,phase noise,etc.A scanning module with a trigger frequency of 136 MHz is developed for the odd bunch filling mode unique to the HLSⅡ ring to overcome the head-tail overlapping problem so as to achieve absolute bunch length measurement.In terms of the streak camera based beam diagnostic system,a temporal beam characteristic parameter extraction(T-BCPE)method is proposed to measure the longitudinal beam coupling impedance of the electron storage ring.The impedance analysis model is well-established for the HLS-Ⅱ ring,for which the contribution of inductance and resistive impedance in the longitudinal coupling impedance can be evaluated by extracting the bunch lengthening factor and asymmetry factor in the longitudinal beam parameters in the potential-well distortion region below the microwave instability(MWI)threshold current.On the basis of the 102 MHz scanning module,the inductive impedance and resistive impedance of the HLS-Ⅱ storage ring are measured to be 107±28 nH and 1430±242 Ω,respectively,and the effective longitudinal broadband coupling impedance is obtained as 3.05±0.80 Q.Subsequently,the longitudinal impedance of HLS-Ⅱ ring was also measured by using the 136 MHz scanning module.The experimental results are in well consistency with the measurement results using the 102 MHz scanning module.In the experiment,we found that there are problems of low measurement accuracy and large error in determining the MWI threshold based on the conventional measurement methods of current dependence for energy spread broaden,bunch lengthening factor and horizontal beam size.Consequenltly,a novel longitudinal bunch asymmetry characterization-based measurement method is proposed to achieve high-precision measurement of MWI threshold current,and the practicability,feasibility,and reliability of this new method are verified under different RF voltages.In addition,the longitudinal beam dynamics and bunch behaviors evolution above the MWI threshold current are explored,and the scaling factor and longitudinal broadband impedance of HLS-Ⅱ under normal operating RF voltage of 123.6 kV are obtained as 0.65±0.07 and 8.74±0.77 Ω,respectively.In order to further analyze the longitudinal beam dynamics and bunch characteristics,the RF phase modulation technology is introduced into the HLS-Ⅱ storage ring to improve the beam lifetime and perform longitudinal beam manipulation.The theoretical analysis model of first-order RF phase modulation(RF-PM)based on HLS-Ⅱis established.The applicability and feasibility of this RF phase modulation scheme are effectively verified by observing the evolution of longitudinal beam behaviors,island phenomena,beam dilution effect,transverse beam size and emittance change,and beam lifetime improvement under different modulation frequencies and amplitudes.As a result,the optimum beam lifetime improvement factor of the HLS-Ⅱ ring can be obtained to be 1.87 in the case of suitable RF-PM amplitude of 0.02 rad and modulation frequency range of 20.1～21.0 kHz.Of course,it has certain reference value for manipulation of longitudinal bunched beam,analysis of RF error noise,investigation of machine debugging,and exploration of the origin and sources of beam loss in storage rings.Besides,an optical pulse picking system is developed for the study of single-bunch characteristics in normal multi-bunch operation mode.The function of selecting a specific single bunch or bunch train in multiple bunches is realized,which provides an important research platform for the analysis of coupling interaction between multiple bunches and related longitudinal beam dynamics.Finally,the current research work is briefly summarized,and the future research directions and ideas are expanded and prospected. |
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