飞秒激光-金属薄膜靶相互作用中靶前后超热电子能谱的比较

采用电子谱仪测量了飞秒激光-金属薄膜靶相互作用中靶前和靶后产生的超热电子能谱.结果显示：靶前超热电子能谱的峰出现在约430 keV处，靶后超热电子能谱的峰出现在约175 keV处；靶前超热电子的有效温度分别为218 keV和425 keV，靶后超热电子能谱出现“软化”现象，其有效温度分别为96 keV和347 keV.靶前和靶后超热电子能谱明显不同是由于超热电子输运穿越过密等离子体和冷材料的靶，并在靶后建立Debye鞘，鞘电场使靶后超热电子能谱峰向低能端移动，鞘电场和自生磁场导致靶后超热电子能谱产生“软化”，估算出的鞘电场小于激光电场.     

电子束放射照相的特性与参数优化

短脉冲强激光产生的电子束具有源尺寸小、脉宽窄、准单能谱等特点, 在放射照相诊断中具有独特作用. 本文通过分析电子在材料中散射并采用蒙特卡罗方法数值模拟, 研究了100 keV到几百MeV能量电子束对有厚度起伏或存在界面的靶的透视, 并与质子、X射线束透视结果比较, 给出了电子束放射照相的特性与参数优化: 基于电子在材料中非弹性散射或能量损失, 选用能量使其射程与靶厚度接近的电子束来诊断靶厚度不均匀性; 基于电子在材料中的弹性散射, 选用射程超过靶厚度的电子束来诊断靶界面.     

闪烁法测量热电子等离子体电子温度

在稳态磁镜装置中,用电子束和等离子体相互作用的方法产生了热电子氢等离子体。利用闪烁谱仪测量了高能电子轫致辐射能谱,从能谱的斜率得到热电子的温度为46keV。测量了热电子温度与电子束能量、氢气压强、磁场强度的关系,对测量结果进行了分析讨论。     

黑腔靶中超热电子特性研究

近几年来,在“神光”装置上进行了1.053μm激光与平面靶及一系列柱形黑腔靶相互作用实验。用一台多道滤波—荧光X光能谱仪(FFS)测得各种靶发射的超热X射线谱,由谱推导超热电子温度T_h和超热电子总能量E_h当照射靶单束激光能量E_(tar)为400～670J、脉宽τ=650～1150ps时,发现黑腔内明显存在两群服从Maxwell分布高能电子(T_h=35～45keV;T_(hh)=150～350kev),而且E_(he)占E_(tar)的份额为10％～12％。实验还表明:腔内的E_(he)与非线性过程特征量(SRS)有较好的线性关系,因此推断出腔内超热电子产生的主要机制是受激Raman散射。在相同照射条件下,黑腔靶产生的超热电子比平面靶严重。     

高压直流连续波光阴极注入器中电子束流发射度研究

 高平均功率自由电子激光研究中，电子束质量是关键。针对高平均功率自由电子激光目标参数，提出了直流高压连续波光阴极注入器，给出了注入器的束流动力学过程。为了降低输出束流横向发射度，采用特殊结构设计的静电加速腔，加速电压1MV，最大加速梯度10MV/m。用PARMELA程序进行了粒子动力学模拟，电子束束团电荷为0.5nC，束团长度10ps时，注入器输出束流归一化发射度均方根值为5.8mm·mrad。     

超短脉冲强激光与固体靶相互作用产生超热电子的能谱研究

较详尽地描述了能对激光与靶物质相互作用所产生的超热电子进行直接测量的电子磁谱仪的建造及其相应的参量,并利用这台磁谱仪测量了超短脉冲强激光与靶物质相互作用后出射的超热电子的能谱,在5×1015W/cm2的激光强度下,超热电子的最大能量大于500keV,能谱结构呈双温Maxwell分布,能谱的峰值出现在50keV附近,超热电子的数量在相同的能量范围内远大于γ射线的数量     

TRIUMF ARIEL 电子直线加速器设计

 TRIUMF 的三期升级工程(ARIEL) 计划建造一个 50 MeV 平均流强为10 mA的电子直线加速器作为注入器,通过光裂反应生成放射性核素。电子直线加速器包括两个主要部分：注入器和后加速器,注入器完成电子能量从100 keV到10 MeV的转换,随后的后加速器将电子能量从10 MeV加速到50 MeV。电子源拟采用重复频率为650 MHz的热电子枪提供初始能量为100 keV,束长为 171 ps的电子束。束流动力学模拟了几种不同的设计方案以获得最优化的设计,模拟显示通过对腔体以及聚焦元件的仔细设计以及电子枪出射电子的参数选择, 电子束能量在达到50 MeV时束长可以被聚焦到 11.75 ps (对应于1.3 GHz 频率下5.5°) ,并且可以使电子束在超导低温柜中的尺寸保持在1.26 cm以下。     

Energy spectra of protons driven by ultra-short laser interaction with thin gold foils

在能量11 mJ、波长744 nm、脉宽120 fs、功率密度6×1016 W/cm2的超短脉冲装置上,开展了超短脉冲激光与2.1 μm和5.0μm金薄膜靶相互作用产生质子束的实验研究.利用Thomson谱仪测量了产生的质子能谱,发现利用2.1 μm金薄膜靶时,质子能谱由于质子源数量不足而在74 keV附近出现单能峰,5.0 μm的金薄膜靶产生的质子计数和能谱均比2.1 μm的金薄膜靶产生的低,主要原因是超热电子穿过薄膜靶时出现的能量损失和几何倾斜降低了电子回流所致.     

超短激光与金薄膜靶相互作用产生的质子能谱

在能量11 mJ、波长744 nm、脉宽120 fs、功率密度61016 W/cm2的超短脉冲装置上,开展了超短脉冲激光与2.1 m和5.0 m金薄膜靶相互作用产生质子束的实验研究。利用Thomson谱仪测量了产生的质子能谱,发现利用2.1 m金薄膜靶时,质子能谱由于质子源数量不足而在74 keV附近出现单能峰,5.0 m的金薄膜靶产生的质子计数和能谱均比2.1 m的金薄膜靶产生的低,主要原因是超热电子穿过薄膜靶时出现的能量损失和几何倾斜降低了电子回流所致。     

磁压缩对高频稳定性的要求

 曲柄式磁压缩系统是北京大学SASE自由电子激光装置中非常重要的部分，通过其对电子束团的压缩为扭摆器提供高流强、短脉冲的电子束，使电子束在扭摆器内较短的距离实现饱和出光。曲柄式磁压缩需要利用偏离高频峰位的加速相位使得电子束产生能量-位置关联，主要讨论高频相位抖动与能量-位置关联的相互关系，高频相位抖动使得束团的能量-位置关联不同，即束团内电子能量随位置分布不同。进而研究其对磁压缩性能的影响，即能量-位置关联不一样会导致磁压缩得到的束团长度出现涨落。     

L波段强流单束团注入器的束流动力学计算

自由电子激光器要求高亮度、低能散度的电子束注入波荡器(Undulator)。本文叙述提供高亮度电子束的高频电子直线加速器中的注入器部分的设计计算。注入器由L波段(1300MHz)的十二分频和三分频两个谐振腔预聚束器和一个基波频率的变相速聚束器组成。粒子运动方程中考虑了空间电荷效应和束流负载效应。电子枪的注入参数:脉冲宽度T=4ns;电流I=5A;电子的初始动能E_0=100keV;电子束分布为高斯型。参数优化设计结果:单束团宽度小于25ps,峰值电流达400A以上,电子的平均归一化能量>4,束团内的能量差小于200keV。     

Bunch length measurement using a traveling wave RF deflector

RF deflectors can be used for bunch length measurement with high resolution. This paper describes a completed S-band traveling wave RF deflector and the bunch length measurement of the electron beam produced by the photocathode RF gun of the Shanghai DUV-FEL facility. This is the first time that such a transverse RF deflector has been developed and used to measure the bunch length of picosecond order in China. The deflector's VSWR is 1.06, the whole attenuation 0.5 dB, and the bandwidth 4.77 MHz for VSWR less than 1.1. With a laser pulse width of 8.5 ps, beam energy of 4.2 MeV, and bunch charge of 0.64 nC, the bunch lengths for different RF input power into the deflector were measured, and an averaged rms bunch length of 5.25 ps was obtained. A YAG crystal is used as a screen downstream of the deflector, with the calibrated value of 1 pix = 136 μm.     

高能同步辐射光源直线加速器初期调束取得重要进展

高能同步辐射光源（HEPS）是中国第一台第四代高能同步辐射光源,其加速器由直线加速器、增强器、储存环及输运线组成。报道了HEPS直线加速器的初期束流调试重要进展。HEPS直线加速器是一台500 MeV S波段常温直线加速器,由热阴极电子枪、聚束系统、主直线加速器构成。在按时完成设备加工、安装和老练的基础上,于2023年3月9日启动束流调试,当天实现束流全线贯通。3月14日束流能量达到500 MeV,束团电荷量达到2.5 nC。经过测量,直线加速器出口束流能散0.4%,能量稳定度0.06%,水平和垂直几何发射度分别为233 nm和145 nm。目前直线加速器束团电荷量可达到7.0 nC,相关束流调试正在进行。     

Storage ring free electron laser dynamics in presence of an auxiliary harmonic radio frequency cavity

In a Storage Ring Free Electron Laser (SRFEL) there is a strong interdependence between the laser beam and the electron beam from which the laser is generated. The Super ACO storage ring has a second Radio Frequency (RF) cavity at the 5th harmonic of the main RF cavity. It is used to shorten the bunch length, thereby enhancing the laser gain. Employing this RF harmonic cavity instabilities are observed with a strong effect on both the laser radiation properties and the electron beam behaviour. In this paper, we first present beam characteristics of Super-ACO as influenced by the harmonic cavity, and the instabilities of the beam due to this RF cavity. Then we discuss the FEL properties in presence of the harmonic RF cavity. In general the harmonic cavity functions as intended, and it is observed that the laser suppresses the instabilities caused by the harmonic cavity in the absence of the FEL.     

Electro-optical sampling non-synchronous delay scanning measurement of electron beam bunch length at BFEL

The Electro-optical sampling delay scanning technique can be used for electron beam bunch length measurement.A novel non-synchronous delay scanning technique based on the electro-optical sampling measurements is presented.Based on Beijing Free Electron Laser(BFEL),the electron beam bunch length was measured with the electro-optical sampling technique for the first time in China.The result shows that the electron beam bunch length at BFEL is about 5.6±1.2 ps.     

A new method to generate relativistic comb bunches with tunable subpicosecond spacing

We propose and analyze a scheme to produce comb bunches, i.e. a bunch consisting of micro-bunch trains, with tunable subpicosecond spacing. In the scheme, the electron beam is first deflected by a deflecting cavity which introduces a longitudinal-dependent linear transverse kick to the particles. After passing through a drift space, the transverse beam size is linearly coupled to the longitudinal position of the particle inside the beam, and a mask is placed there to tailor the beam, then the mask distribution is imprinted on the beam's longitudinal distribution. A quadrupole magnet and another deflecting cavity are used in the beam line to compensate the transverse angle due to the first deflecting cavity. Analysis shows that the number, length, and spacing of the trains can be controlled through the parameters of the deflecting cavity and the mask. Such electron bunch trains can be applied to an infrared free electron laser, a plasma-wakefield accelerator and a supper-radiance THz source.     

Design studies on a 500 kV DC gun photo-injector for the BXERL test facility

BXERL is a proposal for a test facility(Beijing X-ray Energy Recovery Linac),which requires its injector to provide an electron beam of 5 MeV,77 pC/ bunch at a repetition rate of 130 MHz(average current of 10 mA).In this paper,we present the design of the injector,which consists of a 500 kV photocathodeDC gun equipped with a GaAs cathode preparation device,a 1.3 GHz normal conducting RF buncher,two solenoids,and one cryomodule containing two 1.3 GHz 2-cell superconducting RF cavities as the energy booster.The detailed beam dynamics show that the injector can generate electron bunches with a RMS normalized emittance of 1.49 7rmm-mrad,a bunch length of 0.67 mm,a beam energy of 5 MeV and an energy spread of 0.72%.     

Deducing the electron-beam diameter in a laser-plasma accelerator using x-ray betatron radiation

We investigate the properties of a laser-plasma electron accelerator as a bright source of keV x-ray radiation. During the interaction, the electrons undergo betatron oscillations and from the carefully measured x-ray spectrum the oscillation amplitude of the electrons can be deduced which decreases with increasing electron energies. From the oscillation amplitude and the independently measured x-ray source size of (1.8±0.3) μm we are able to estimate the electron bunch diameter to be (1.6±0.3) μm.     

Emittance measurement & optimization for the photocathode RF gun with laser profile shaping

The Laser Undulator Compact X-ray source(LUCX) is a test bench for a compact high brightness X-ray generator,based on inverse Compton Scattering at KEK,which requires high intensity multi-bunch trains with low transverse emittance.A photocathode RF gun with emittance compensation solenoid is used as an electron source.Much endeavor has been made to increase the beam intensity in the multi-bunch trains.The cavity of the RF gun is tuned into an unbalanced field in order to reduce space charge effects,so that the field gradient on the cathode surface is relatively higher when the forward RF power into gun cavity is not high enough.A laser profile shaper is employed to convert the driving laser profile from Gaussian into uniform.In this research we seek to find the optimized operational conditions for the decrease of the transverse emittance.With the uniform driving laser and the unbalanced RF gun,the RMS transverse emittance of a 1 nC bunch has been improved effectively from 5.46 πmm.mrad to 3.66 πmm.mrad.