Research On Automatic RF Conditioning System For HUST-FEL LINAC

Huazhong University of Science and Technology carried out the development of freeelectron laser terahertz source based on linear accelerator.In order to obtain high quality beam,linear accelerator uses the accelerating structure that combines S-band standing-wave RF gun with traveling-wave accelerating tube.Both the RF gun and accelerating tube are high-gradient structures to make the accelerator a more compact device.However,high accelerating field strength are overcritical on vacuum level,cleanliness and wall smoothness of the cavity.However,accelerator processing and installation can't meet this operational requirements.So before normal operation the accelerator must be conditioned by high RF power.Linear accelerator is conditioned by progressively improving RF power that feeds into the cavity to remove gas,impurities and inner surface burrs,ensuring that the device can work in high vacuum,high field strength and high power environment.The accelerator conditioning is very time-consuming that needs to duty round the clock.Automatic RF conditioning control can monitor the accelerator operating status that include dark current,vacuum level and reflected RF power in real time,and automatically adjust the incident RF power,pulse width and pulse frequency according to operating status,greatly reducing the manpower and avoiding damaging the device under the abnormal situation by rapid interlock protection.Based on the existing accelerator,a study of the RF power coupling and field analysis,RF signal acquisition and its demodulation algorithm,the arcing detection algorithm and the hardware and software scheme of the control of automatic conditioning system is presented.Based on the introduction of the high power system,the acceleration structure is simulated by the electromagnetic simulation software CST to analyze the power coupling process,the field distribution and the electric field intensity.In order to obtain the characteristics of arcing in the process of the condition,the reflected power is continuously collected and analyzed by using the virtual oscilloscope with high precision and high sampling rate.Aiming at the characteristics of arcing,the combination of the reflection power average comparison and the dark current detection are used as the arcing signal detection method.In order to realize the real-time online acquisition and processing of S-band RF signal,the RF signal is downconverted to reduce the data acquisition bandwidth requirement,the data acquisition and IQ demodulation are combined by the Non-IQ demodulation algorithm.The experimental results show that the Non-IQ demodulation algorithm can suppress the timing jitter and noise interference contrasting to the conventional IQ demodulation algorithm,and get flat amplitude waveform.On this basis,using PXIe-5122 high-speed data acquisition card for data acquisition,and taking advantage of simple programming control of LabVIEW and powerful data processing functions of MATLAB to prepare data acquisition and control software.In addition,in exceptional circumstances the RF power source need to be cut off timely to protect the equipment.Therefore,using LabVIEW to prepare control software to achieve the RF signal generator remote control.Finally,the automatic conditioning control system is developed according to the existing control system of the accelerator.