Photocurrent in superconducting YBa2Cu3O7−x films induced by an infrared free electron laser

Since the discovery of high-temperature cuprate superconductors, there has been much intensive study about the mechanism of them. However, identifying the dynamical mechanism behind them remains one of the great challenges in condensed matter physics. We investigated the high-temperature YBa₂Cu₃O_7−x superconducting films by using a free electron laser (FEL). The method is a type of photoelectron spectroscopy called a free electron laser internal photoemission. The spectrum of the photocurrent induced by FEL was measured in the case of 15 K and 100 K. We estimated the superconductive gap energy of YBa₂Cu₃O_7−x by comparing the photocurrent spectrum of the superconductive state with that of non-superconductive state.     

Effect of beam energy spread on cascade optical klystron undulator radiation

In this paper we analyze the effect of beam energy spread on cascade optical klystron undulator radiation. The analysis suggests that stronger dispersion strength needs a better beam quality in both single stage and cascade optical klystron undulator radiation.     

远红外自由电子激光器的性能和可能的应用(英文)

本文提出了远红外自由电子激光器(FIRFEL)的现实优点:对电子束质量要求不太苛刻;比较经济;对远红外区的非线性和相干性研究是唯一的合适的光源。分析了自由电子激光器增益与电子束和摆动器参量的关系,指出对给定的发射度存在一特定的波长,当波长比特定波长短时,增益将急骤下降。概述了远红外区的重要物理现象以及远红外自由电子激光器的可能的应用,预期这些研究对THz电子学发展是很重要的。     

自由电子激光的计算机模拟

本文使用1(2/2)维电磁相对论粒子程序对自由电子激光进行计算机模拟。在线性段,计算机模拟与用解析理论计算的增长率符合很好。与没有轴向引导磁场的自由电子激光相比,加一弱轴向引导磁场时,Wiggler场的强度可降得较低。加一强轴向引导磁场时,波能量达到饱和水平所需时间较短。     

分光技术在自由电子激光结合里德堡态氢原子飞行时间谱装置中的应用

本文描述了一种应用于自由电子激光结合高里德堡态氢原子飞行时间谱装置中的分光方法,以及该方法应用于小分子(如₂S)光解动力学研究中的必要性. 拉曼-α辐射(121.6 nm),用作H原子产物探测的激光,是在Kr/Ar气介质中利用四波混频产生的. 利用透镜对不同波长的光有不同的折射率,四波混频后的混合光在经过一片离轴的氟化锂透镜后,121.6 nm的激光将会与212.6和845 nm在空间上分开. 在激光到达反应中心前利用挡板挡住212.6和845 nm的激光,只让121.6 nm的光经过反应中心,从而消除212.6 nm激光产生的背景信号对实验的干扰. 结合自由电子激光,成功地研究了H₂S在122.95 nm波长下的光解动力学,采集到了产物时间飞行谱. 本文展示了转换得到的产物总平动能谱,解离机理与121.6 nm波长下的结果相似. 实验结果显示,该方法成功地解决了分子在VUV波段进行光解动力学研究的难题,消除了这些分子在紫外光波段因为强烈吸收而产生的背景信号.     

Corrigendum to “Wave-particle interactions in a resonant system of photons and ion-solvated water” [Phys. Lett. A 381 (2017) 762–766]

Two corrections to Ref. [1] are presented. In particular, a correct version of the derivation of Eq. (14) is given. These corrections do not have any effect on the main results of the original article.     

Development of IR-FEL facility for energy science in Kyoto University

A mid-infrared free electron laser (FEL) has been constructed for energy science in the Institute of Advanced Energy, Kyoto University. The FEL system consists of a compact S-band Linac and an undulator to generate 4–13 μm coherent mid-infrared radiations. The Linac consists of a 4.5 cell rf gun with a thermionic cathode and a 3-m traveling-wave-type accelerator tube fed by 10 MW and 20 MW rf power, respectively. We have succeeded to produce 40 MeV, 40 mA and 3 μs electron beams. Last December, the 9.2 μm spontaneous emission from the undulator generated by 29.5 MeV electron beams was observed for the first time. Further optimization parameters of both the electron beam and the optical cavity are being pursued for an FEL lasing in the near future.     

Hybrid Planar Wiggler with a Period of 2 cm for Higher Harmonic FEL Lasing

A hybrid planar wiggler with a period of 20 mm has been studied as the simplest one which gives the strong field including some higher harmonic components by selecting proper sizes of the ferromagnetic and the permanent magnet. Small gap length of the wiggler and small width of permendur satisfy these conditions to a certain degree.Gain analysis of FEL suggests that for high wiggler field of K>1 1.6, higher harmonic gains are improved primarily due to its strong field, and for low wiggler field of K< 1 1.6, they are mainly due to the modification of the wiggler field distribution.     

毫米波自由电子激光的数值模拟和实验的比较

从波导管毫米波自由电子激光器的设计要求出发,根据Livermore实验室FRED程序的物理思想,编制了空间三维的数值模拟程序(WAGFEL)。为了检验程序的可靠程度,结合ELF装置的实际参数,进行了数值模拟并和实验结果进行了比较。结果表明,把Wiggler磁场B_w增大300 Gs后,WAGFEL程序的模拟结果和Livermore实验室的实验结果基本符合。模拟使用的全部参数,除B_w增大300Gs外,都是ELF的实际参数。模拟时峰值磁场B_w=4050Gs,实验测量峰值磁场B_w=3720Gs,相差在8％左右。WAGFEL程序可以用来从事毫米波自由电子激光器的设计以及基本物理问题的研究。     

Flat electron beams

Electrons from a source in a solenoidal magnetic field emerge from that field with a characteristic angular momentum. This behavior can be used to produce a flat ribbon-like electron beam with zero thickness (to first order). We show that this can be done using a quadrupole triplet and provide analytical expressions for the focal length of each lens. Solutions can be found over a wide range of practical values of the parameters.     

射频超导腔的研究新进展

 射频超导谐振腔已经大规模地应用到粒子加速器领域,其优越之处在于它可以在CW模式或长宏脉冲模式下,提供高的加速梯度。射频超导已经成为自由电子激光和能量回收直线加速器的关键技术。经过30多年的研究发展,解决了超导腔的热崩溃、场致发射等诸多关键问题,目前加速梯度已经超过40 MV/m。高加速梯度的获得是射频超导领域的前沿热点,电抛光＋低温热处理技术使射频超导腔的加速梯度提高3～4 MV/m。最新发展起来的超导腔的干式处理可以改善超导腔的表面状况,提高超导腔的Q值,抑制次级电子发射效应,有可能成为提高超导腔性能的又一有效手段。     

一种测量电子束微脉冲时间抖动的新方法

 提出了一种高灵敏度电子束微脉冲时间抖动的测量方法。该方法采用传统的测量束团位置及电荷量的四条带束流位置探头,用混合环对感应信号进行处理,对共振频率为2.856GHz的谐波分量用MIXER与2.856GHz的参考信号进行相位比较,并由此得到电子束团微脉冲时间抖动值。     

Interaction of a relativistic dense electron beam with a laser wiggler in a vacuum: self‐field effects on the electron orbits and free‐electron laser gain

Employing laser wigglers and accelerators provides the potential to dramatically cut the size and cost of X‐ray light sources. Owing to recent technological developments in the production of high‐brilliance electron beams and high‐power laser pulses, it is now conceivable to make steps toward the practical realisation of laser‐pumped X‐ray free‐electron lasers (FELs). In this regard, here the head‐on collision of a relativistic dense electron beam with a linearly polarized laser pulse as a wiggler is studied, in which the laser wiggler can be realised using a conventional quantum laser. In addition, an external guide magnetic field is employed to confine the electron beam against self‐fields, therefore improving the FEL operation. Conditions allowing such an operating regime are presented and its relevant validity checked using a set of general scaling formulae. Rigorous analytical solutions of the dynamic equations are provided. These solutions are verified by performing calculations using the derived solutions and well known Runge–Kutta procedure to simulate the electron trajectories. The effects of self‐fields on the FEL gain in this configuration are estimated. Numerical calculations indicate that in the presence of self‐fields the sensitivity of the gain increases in the vicinity of resonance regions. Besides, diamagnetic and paramagnetic effects of the wiggler‐induced self‐magnetic field cause gain decrement and enhancement for different electron orbits, while these diamagnetic and paramagnetic effects increase with increasing beam density. The results are compared with findings of planar magnetostatic wiggler FELs.     

Relativistic electron motion in FEL-like fields taking retarded interactions into account

Summary The interaction of two charged particles both with each other (via Lienard-Wiechert retarded potentials) and with the wiggler field of an FEL structure, along which they are launched, is computed by means of the numerical integration of the relativistic motion equations, taking also an incoming laser wave into account. The bunching effect characterizing the collective behaviour of an electron beam in FEL-like fields is simulated by assuming one of the particles to be a suitable macrocharge.     

Energy transmissivity of high-power nanosecond laser pulse focused on glass

The experimental results show that if the pulse energy is low, the damage does not occur and the corresponding transmissivity is 100%. But with the increase of the pulse energy, the damage occurs and the transmissivity decreases to between 100% and 50%, equal to 50% and lower than 50%.The concept of the cut-off time (T_cut) is proposed to give the physics explanation on it, and by taking the free electron density (FED) and the laser-induced damage threshold (LIDT), mathematical simulation of the change law of the T_cut and the corresponding energy transmissivity with the laser energy has been done, respectively. The analysis results indicate that the former definition method can predict all the situations successfully, but the second one cannot predict the situation of the transmissivity being between 100% and 50%, the reasons of this difference also be analyzed.     

北京大学DC-SC光阴极注入器的升级设计

 基于直流电子枪-超导加速腔（DC-SC）光阴极注入器样机的初步实验结果,北京大学提出了新的注入器的改进设计。新注入器核心结构包括皮尔斯枪和3+1/2超导腔。文章给出了它们的详细结构参数,然后采用程序,对注入器的束流动力学进行了模拟。结果发现：新注入器可以提供具有高束流品质、高平均流强的电子束,束团的电荷量100 pC,横向发射度低于2 mm·mrad,脉宽5 ps,rms束斑可达0.5 mm,重复频率81.25 MHz;也可以提供电荷量为300 pC低重复频率的高峰值流强的电子束,其横向发射度小于3 mm·mrad,脉宽约为9 ps,以满足北京大学自由电子激光（PKU-FEL）实验平台的要求。