軸向耦合結構熱陰極射頻電子槍之設計與分析

Design and Analysis of a Thermionic Cathode Radio Frequency Electron Gun with On-axis Coupled Structure

魏子喻(Zih-Yui Wei)

指導教授 ： 柳克強 ； 劉偉強

國立清華大學/原子科學院/先進光源科技學位學程/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00259

摘要

Abstract

Thermionic-cathode radio-frequency electron gun (RF gun) has been a compact electron source of accelerators for more than two decades. In an RF gun, electrons pulled out from the cathode by the high gradient field setup in its microwave cavity are rapidly accelerated to relativistic energy such that space charge effects are less significant. Motivated by the need of a high quality electron source for the NSRRC ultrashort beam facility, we studied a 2998 MHz, 1/2+1/2+1 cell, thermionic cathode RF gun with on-axis coupled structure (OCS RF gun). OCS RF gun features simplicity of frequency tuning during fabrication due to its axial symmetry. We started with circuit analysis that provides a reliable model through the investigation. Microwave properties and electron beam characteristics of designed OCS RF gun has been evaluated by 2D simulations of SUPERFISH and PARMELA as well as 3D simulations of HFSS and CST-PS. Operating parameters are optimized to obtain a 2.5 MeV electron beam with quasi-linear energy chirp that allows aggressive bunch compression in the RF linac located downstream after proper beam selection. We have also investigated power distribution of back-streamed electrons that eventually lost onto the surface of thermionic cathode. A prototype of the designed OCS RF gun structure has been fabricated for low power microwave test. Measured frequencies and longitudinal electric field profile of the cavity structure agree well with the results predicted in the simulation studies.

關鍵字

電子槍 ； 軸向耦合 ； 射頻 ； 熱陰極 ； 光注射器 ； 加速器

並列摘要

Abstract

Thermionic-cathode radio-frequency electron gun (RF gun) has been a compact electron source of accelerators for more than two decades. In an RF gun, electrons pulled out from the cathode by the high gradient field setup in its microwave cavity are rapidly accelerated to relativistic energy such that space charge effects are less significant. Motivated by the need of a high quality electron source for the NSRRC ultrashort beam facility, we studied a 2998 MHz, 1/2+1/2+1 cell, thermionic cathode RF gun with on-axis coupled structure (OCS RF gun). OCS RF gun features simplicity of frequency tuning during fabrication due to its axial symmetry. We started with circuit analysis that provides a reliable model through the investigation. Microwave properties and electron beam characteristics of designed OCS RF gun has been evaluated by 2D simulations of SUPERFISH and PARMELA as well as 3D simulations of HFSS and CST-PS. Operating parameters are optimized to obtain a 2.5 MeV electron beam with quasi-linear energy chirp that allows aggressive bunch compression in the RF linac located downstream after proper beam selection. We have also investigated power distribution of back-streamed electrons that eventually lost onto the surface of thermionic cathode. A prototype of the designed OCS RF gun structure has been fabricated for low power microwave test. Measured frequencies and longitudinal electric field profile of the cavity structure agree well with the results predicted in the simulation studies.

並列關鍵字

RF Gun ； On-axis Coupled ； Radio Frequency ； Thermionic Cathode ； Photo Injector ； Accelerator

參考文獻

[2] S. V. Benson, J. M. Madey, J. Schultz, M. Marc, W. Wadensweiler, G. A. Westenskow, et al., "The Stanford Mark III infrared free electron laser," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 250, pp. 39-43, 1986.

[4] C. McKee and J. M. Madey, "Computer simulation of cathode heating by back-bombardment in the microwave electron gun," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 296, pp. 716-719, 1990.

[5] T. Kii, T. Yamaguchi, R. Ikeda, Z.-W. Dong, K. Masuda, H. Toku, et al., "Experiment and analysis on back-bombardment effect in thermionic RF gun," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 475, pp. 588-592, 2001.

[7] W. Lau, J. Chen, J. Hwang, A. Lee, C. Liang, T. Wu, et al., "CONSTRUCTION OF A THERMIONIC RF GUN LINAC SYSTEM FOR ULTRASHORT ELECTRON PULSE GENERATION."

[8] N. Huang, W. Lau, C. Liang, A. Lee, W. Cheng, and S. Yang, "Driver linac design for a femtosecond head-on inverse Compton scatteringX-ray source," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 637, pp. S76-S79, 2011.