Research And Design Of Front-end Module Of Low-level Control System For The THz-FEL

High-quality electron beam is one of the key factors for terahertz free-electron laser device to obtain high-quality terahertz radiation signals.The stability of the microwave power source has an important impact on the quality parameters of the electron beam clusters.Many accelerator devices introduced low-level radio frequency control system to ensure the stability of the acceleration electric field amplitude and phase.The front-end module is used as the signal preprocessing and signal generation module of the low-level radio frequency control system.Its ability to preprocess the signal and the quality of the generated signal directly affects the operation of the control system.Huazhong University of Science and Technology is studying a high-frequency and low-level control system to improve the stability of the microwave power system of the existing terahertz free electron-laser device.This paper focused on the front-end module of low-level radio frequency control system.Low-level radio frequency control system has high requirements for synchronization,and the front-end module needs to generate high-precision and coherent intermediate frequency signal,clock signal,operation signal and local oscillator signal.In this paper,the coherent direct analog signal synthesis method was adopted.The four signals were respectively generated by their respective synthesis units.Only one reference signal source was used,and the two-stage series structure was adopted to realize the coherent signal output.Based on the initial development framework and device selection,a front-end module signal synthesis simulation model was built,the interference caused by harmonics and phase noise to the signal was analyzed,and a simplified phase noise model was established to quantify the phase noise interference.Combining the initial plan and noise analysis,the filter optimizeed the signal output quality,so that the signal can reach the system index,and the simulation results verified the rationality and feasibility of the design plan.For the 2.8GHz bandpass filter applied to the working unit in the optimized scheme,the physical structure design of the cavity bandpass filter was completed.The six-resonator filter was selected,combining with theoretical calculations,the modeling and simulation of single cavity resonator,metal probe,dual cavity coupling structure,and full-wave six resonator were completed,the overall physical structure was optimized,and the size parameters were determined.The processed physical filter was tested,and its S-curve and spectrum test showed that this filter has narrow band pass and high rejection performance,and can play a role in suppressing harmonics in the working unit.Based on the optimized front-end module signal synthesis overall plan,the frequency conversion devices used in the plan were compared and selected.The space layout of the device was designed,and the device layer and power layer were integrated and installed inside the chassis,completing the construction of the front-end device.The performance parameters of the front-end device were tested,and the output signal can meet the requirements of low noise,high stability and coherence.At the same time,the front-end device was tested for long-term stability.After the developed front-end device was embedded in the high-frequency and low-level control system,it can cooperate with the control system to realize the stability control of the acceleration field,which improves the stability of the microwave power system.