Research On The Beam Instability In The Storage Ring With A Higher-harmonic Cavity

The utilization of a higher-harmonic cavity in the storage ring may effectively lengthen the bunch and increase the beam lifetime. Meanwhile, instabilities such as the coupled-bunch instability can be suppressed and the system can operate stably with the adjustment of the tuning angle. Combining actual conditions of HLSII, this dissertation studies mainly around the beam longitudinal instabilities of the double RF-cavity system, which consists of a newly-introduced higher-harmonic cavity and an original fundamental RF-cavity.Considering immediate areas of domestic and foreign research focus, we study with these two methods of Analytic Modeling and Macroparticle Tracking Simulation and excellent effect has been obtained.At the beginning of this paper, the fundamental theory of beam-cavity interaction in the double RF-cavity system is presented in order to clarify the physical environment. We also introduce the definition and the physical interpretation of the optimal bunch lengthening. The parameters’transformation relations are obtained, including the tuning angles, the synchronous phases and the voltages between the higher-harmonic cavity and the fundamental RF-cavity. This will provide guidance for the newly-operated double RF-cavity system.In the method of Analytic Modeling, Robinson instability, parasitic coupled-bunch instability and microwave instability affected by higher order modes are researched systematically. We focus on the analysis of the dipole, quadrupole Robinson instability and the mode coupling between them, which play main roles in beam collective instability. In the study of microwave instability, we obtain the broadband impedance range in which this instability is evitable.In the results of Analytic Modeling, these kinds of instability can be avoided when the beam current and the tuning angle of the higher-harmonic cavity are modulated close to the curve of the optimal bunch lengthening.As an assistant means of Analytic Modeling, the Macroparticle Tracking Simulation can describe an integrated state of motion of the charged particles in the storage ring. This method has two key points:one is that how to determine the number of microparticles so as to keep a balance between efficiency and the accuracy of the simulation outcome; the other is knowing clearly how the electric field experienced by macroparticles evolves. We have built a whole set of macroparticle tracking model and the value of variation of the relative energy spread after multi-turns is calculated.By comparison of these two methods’ results, we discover that they are consistent with each other. Moreover, the bunch length and the ratio of bunch lengthening calculated in different ways indicate that the higher-harmonic cavity has a good performance in bunch lengthening and these results have a good agreement with those done by predecessors.The mechanism of uneven fills ameliorate the beam instability is researched in the last chapter of this paper. Combining actual conditions of HLSII, we design four specific uneven filling patterns and one optimal is obtained by comparing the simulation results of them. We also note that the transient beam loading effect has a weakening on bunch lengthening under the condition of uneven fills.