雷射激發光陰極電子注射器系統可以產生高品質高能量電子束的電子加速器系統。它在許多以加速器技術為基礎的科學實驗設施都有非常重要的應用，其中包括高增益自由電子雷射、線型粒子對撞機、超快電子繞射等。在台灣，國家同步輻射研究中心已成功開發一套2.998 GHz的光陰極電子注射器系統，主要用於新型光源研究。該中心的研究人員利用注射器中線性加速器的微波相位調整，透過一個稱為速度群聚的機制產生了25 MeV、250 pC的次皮秒級的相對論電子束。利用這種高亮度電子束通過週期長度為10公分的U100平面聚頻磁鐵，產生了100-500μm波長範圍內的兆瓦級的同調太赫茲輻射。然而，研究發現電子槍腔體內的高加速場是系統中不可忽視暗電流的主因。在光陰極附近，部分發射條件接近主電子束的場發射電子，可在下游直線加速器結構中被加速至高能量。然而，這些無預期的高能電子撞擊加速器中真空容器，成為輻射劑量過量的主要來源。因此，我們考慮利用電子槍下游用作調整電子束橫向發射度的螺線型磁鐵對不同能量電子的聚焦特性不同，在直線加速器的入口安裝準直系統，以限制這些多餘電子傳輸至直線加速器結構中。本論文使用3D空間電荷追蹤程式IMPACT-T，為主束和暗電流建立了光陰極射頻電子槍系統模型並尋找最佳操作參數，分析在不同光陰極發射位置及射頻相位下的粒子軌跡，並選擇最佳準直器位置。最後在光陰極處生成十萬顆暗電流粒子對不同孔徑大小的準直器進行分析，透過選擇最佳孔徑尺寸，大幅降低暗電流傳輸至線性加速器，有效地降低輻射劑量。

The laser driven photocathode electron injector system is an electron accelerator system that can produce high-quality relativistic electron beams. It has very important applications in many scientific experimental facilities that are based on accelerator technology, including high-gain free electron lasers (FEL), linear colliders, ultrafast electron diffraction (UED), etc. In Taiwan, the National Synchrotron Radiation Research Center (NSRRC) has successfully developed a 2.998 GHz photocathode electron injector system, which is mainly used for novel light source research. Researchers at the center used microwave phase adjustment of a linear accelerator in an injector to generate a 25 MeV, 250 pC sub-picosecond relativistic electron beam through a mechanism called velocity bunching. They used this high-brightness electron beam to drive a U100 undulator with a period length of 10 cm to generate megawatt-level coherent terahertz radiation in the wavelength range of 100-500 μm. However, it has been found that the high acceleration field in the electron gun cavity is the main cause of the dark current in the system that cannot be ignored. Near the photocathode, some field-emitted electrons with emission conditions close to those of the main electron beam can be accelerated to high energies in the downstream linear accelerator structure. However, these undesirable high-energy electrons hit the vacuum vessel in the accelerator and become a major source of excessive ionization radiation dose. Therefore, we consider using the solenoid downstream of the electron gun, which is used to adjust the transverse emittance of the electron beam, to have different focusing characteristics on electrons of different energies and install a collimation system at the entrance of the linear accelerator to limit the transmission of these excess electrons into the linear accelerator structure. This research used the 3D space charge tracking program IMPACT-T to establish a photocathode rf gun system model for the main beam and dark current and to search for the optimal operating parameters. By analyzing the particle trajectories at different cathode emission positions and rf phases, the optimal collimator position can be determined. Finally, one hundred thousand dark current particles are generated on the cathode to analyze different aperture sizes of the collimator. Selecting the optimal aperture size, significantly reduces the transmission of dark current to the booster linac, effectively lowering the radiation dose.

摘要 1
Abstract 2
List of Figures 8
List of Tables 10
Chapter 1. Introduction 11
1.1. Background 11
1.2. Motivation 14
Chapter 2. Photoinjector Theory 16
2.1. Mode Type and Electromagnetic Field Distribution in Cylindrical Cavity 16
2.2. Physical Quantities Related to the Accelerating Cavity 19
2.2.1. Shunt impedance and cavity loss[3] 19
2.2.2. Quality Factor 20
2.3. TM010 Mode in Cylindrical Cavity 23
2.3.1. Mechanics of photocathode rf gun 23
2.3.2. Electromagnetic field in pillbox 24
2.3.3. Shunt impedance in pillbox 26
2.3.4. Quality factor in pillbox 27
2.4. 1.6-cell Photo-cathode RF Gun Cavity 27
2.4.1. Modes of coupled cavities[6] 28
2.4.2. SUPERFISH calculation 30
2.5. Beam dynamics 32
2.5.1. Emittance 32
2.5.2. RF Acceleration 34
2.5.3. RF Effects in Longitudinal Space Dynamics 36
2.5.4. RF Effects in Transverse Space Dynamics 37
2.5.5. Space Charge Effect 38
2.6. Field Emission 40
2.6.1. Fowler-Nordheim Theory 40
2.6.2. Field Emission Enhancement 42
2.6.3. Dark current 43
Chapter 3. Literature study 45
3.1. PITZ Gun 45
3.1.1. Dark Current in Photocathode RF Guns at PITZ 46
3.1.2. Single Particle Transmission Trajectories of Dark Current in PITZ Gun 48
3.2. SwissFEL Gun 50
3.2.1. Single Particle Transmission Trajectories of Dark Current in SwissFEL Gun 51
3.2.2. Dark Current Reduction by Collimator in SwissFEL Gun 53
Chapter 4. Modeling of dark current generation and transport in photoinjector 55
4.1. Simulation Code – IMPACT-T 55
4.2. IMPACT-T Model of Photoinjector 56
4.3. Modeling Field Emission in Photocathode RF Gun 58
Chapter 5. Simulation results 60
5.1. Photoinjector Operation Condition 60
5.2. Trajectories of Particles at Various Launching Conditions 61
5.3. Trajectories of Different Initial Phases of Dark Current 66
5.4. Beam Distribution of a Bunch of Beam 68
5.5. Reduction of Dark Current Using Collimators 71
Chapter 6. Conclusions 75
Reference 76
Appendix 78
Code 78
A. Impact-T code 78
A-1. Single Particle 78
A-2. 100000 particles 84
B. Python code 90
B-1. Scan gun phase 90
B-2. Code for generate a bunch of beam of dark current 93
B-3. Particle energy spectrum 96

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