基于负电子亲和势GaAs光阴极直流高压注入器,设计并搭建了国内首套光阴极量子效率分布测量系统.该系统利用单透镜实现逐点扫描,并采用LabVIEW进行控制和数据读写.实验表明,该系统单点采样时间小于2.3s,分辨率优于0.32 mm.初步测量了GaAs阴极的量子效率分布,观察到量子效率分布及其衰减的不均匀性,量子效率较高区域的衰减速率更低.

离子反轰会造成直流高压电子枪内光阴极的量子效率衰减,降低阴极的工作寿命和稳定性,是限制光阴极直流高压电子枪性能的重要因素之一.基于中国工程物理研究院太赫兹自由电子激光(THz-FEL)的直流高压电子枪,通过束流动力学模拟和阴极量子效率测量,研究了束流偏轴发射对离子反轰的抑制作用及其应用限制.模拟结果表明:束流偏轴发射时,离子主要轰击阴极的电中心附近,光电发射区域的离子通量较轴上发射时降低约95％;但束流发射度增大约4π mm·mrad,横向运动幅度增加约1倍.通过电子枪运行前后阴极量子效率测量,发现束流偏轴发射时离子反轰引起电中心附近量子效率大面积的衰减,而光电发射区域量子效率损失较小,验证了束流偏轴发射抑制离子反轰的可行性.

主要介绍自由电子激光相干强太赫兹源(FEL-THz)装置上的砷化镓光阴极直流高压注入器的研究进展,并讨论其驱动未来高重复频率短波长自由电子激光器的差距.通过综合砷化镓阴极寿命的三大影响因素,提出了其工作寿命的定性物理模型;通过该模型对阴极和注入器进行优化,在直流高压电子枪上得到了5 mA,32 min的连续稳定输出;测量了电子束在4.8 mA下归一化发射度约为4.0π mm·mrad,阴极热发射度约为0.67πmm·mrad,电子束本征横向能量约为92 meV,250 keV电子束在距离阴极90.6 cm处纵向均方根长度约为11.5 ps.这一束流状态已经基本满足FEL-THz需求.

The temperature rise, caused by high power laser, would destroy the activation layer of the NEA GaAs photocathode. In this paper, the temperature rise is studied theoretically and experimentally. With some theoretical analysis, the thermal transfer between the NEA wafer and the puck is proved as the most important factors that affect the temperature. To the thermal transfer, the GaAs vacuum indium soldering chamber is built and the vacuum soldering technology is explored. Some comparative experiments with and without indium soldering are also presented in this paper. The results, both in the atmosphere and in vacuum, indicate that indium provides effective thermal contact between the GaAs cathode and the puck, enhancing the thermal transfer and slowing down the temperature rise. These studies increase the operational lifetime of NEA GaAs illuminated by several-watt laser more than 20 times eventually.

The negative electron affinity (NEA) GaAs photocathode has been demonstrated as an important electron source for high average current accelerators, such as the free electron lasers (FELs) based on energy recovery linacs (ERLs). To increase the life-time of NEA-GaAs, some factors are studied in this paper, such as the vacuum pressure around the cathode, the temperature of the cathode surface and the ion back-bombardment inside DC gun. With these studies, some strategies are applied on the photocathode injector of FEL-THz facility. The cathode operation life-time has been improved at least two orders of magnitude.

As one of the most important high average power electron source materials , the high brightness negative electron affinity (NEA ) gallium arsenide (GaAs) photo‐cathode becomes a researcher focus nowadays .The thermal emittance is the lower limit of emittance ,and its measurement is of great importance to determine the brightness that an injector can provide .In this paper ,based on the terahertz free electron laser (FEL‐T Hz) facility in China Academy of Engineering Physics ,some efforts were made to calculate and measure the thermal emittance of NEA‐GaAs photocathode . Under ultra‐low charge of 28 fC NEA‐GaAs photocathode ,the thermal emittance is (0.603 ± 0.002)μm/mm by the solenoid scan method .

The photocathode lifetime of GaAs in a high voltage DC gun is limited primarily by ion back bombardment. Ions produced in collisions of the electron beam with residual gas in cathode-anode gap and downstream of the anode are accelerated toward the cathode and strike the cathode surface. Systematical studies suggest that ion back bombardment is determined by gas pressure, gun voltage, laser spot and electric field profile. This paper presents a study of ion back bombardment in a high average current DC photocathode gun.This study is based on numerical analysis and particle tracking simulation. The results implies that ion generation can be suppressed by improving vacuum condition as well as gun voltage, and the back bombardment can be reduced by the optimization of laser spot position and electric field profile.

The laser wire (LW) method has been demonstrated as an effective non-interceptive technique for measuring transverse electron beam size of CW FELs and ERLs. To measure the beam size of a CW DC gun, which is built as an electron source of FEL-THz facility in China Academy of Engineering Physics (CAEP), a high repetition LW system is proposed. The first prototype system is going to be installed at the exit of the DC gun, where the energy of electron beam is extremely low. In this paper, the LW system adapted to the FEL beam parameters is discussed, and the main parameters are given. With numerical calculation and Particle-in-Cell simulation, the high repetition LW system is proved to be lower-cost, higher-precision, faster-scanning and lower-influence than normal LW. The experiment proposal of the first high repetition LW is also introduced.

The laser wire（LW） method has been demonstrated to be an efective non-interceptive technique for measuring transverse profile and emittance of electron beams in colliders,storage rings and dumping rings.In this paper,we present an improved design of high repetition LW system for high average power free electron lasers（HAP FELs） and energy recovery linacs（ERLs）.This improved LW utilizes the excess power of the photocathode drive laser,thus making itself much cheaper and simpler.The system main parameters are optimized with numerical calculations and Monte Carlo simulations,indicating that resolutions would be better than 100 μm and scanning time less than 1 minute.Status of the experiment preparation is also presented.