PASER在混合气体激活介质中的理论计算(英文)

在受激辐射粒子加速(PASER)中,受激介质中处于激发态的电子跃迁到基态时辐射的光子能量直接以量子形式转移给从它旁边经过的电子、首先对单个电子柬团穿过受激介质时产生的电场进行了理论推导,并分别对受激的二氧化碳混合气体和受激氟化氩混合气体进行了计算、计算结果表明加速梯度可以达到1 GV/m,比现有的普通加速器(如盘和波导加速结构)加速梯度高两个数量级。此外,通过二维模型进一步分析了电子微柬团串穿过受激的混合气体介质获得的能量增益。结果表明电子束团串可显著吸收受激介质中的量子态能量,且吸收的能量与相互作用长度成正比、在相互作用长度等于0.5 m时对柬团参数和其它的量对能量交换的影响进行了分析、通过理论计算给出了电子获得最大能量增益时的优化参数、     

自由电子激光器

本文从理论和实验方面评述了自由电子激光器。首先讨论高速电子束穿过螺线摆动器时发射的光谱。当光脉冲与电子一起穿过螺线摆动器时，电子受迫发出同样频率的光。计算了由这种感应辐射而获得的增益。然后介绍了斯坦福关于感应辐射自由电子激光器实验的一些具体细节。最后讨论有关自由电子激光器的一些问题。     

激光尾波场电子加速及新型辐射源（特邀）

受益于超短超强激光技术的持续迅猛发展,飞秒强激光为人类提供了全新的实验手段与极端的物理条件,使激光物质相互作用进入到一个极端非线性的强场超快新范畴,催生了大量新原理、新现象,推动了技术变革。飞秒强激光驱动的等离子体尾波场加速原理是一种具有超高加速梯度的粒子加速新原理,该技术的加速梯度可达100 GV/m,相比于传统射频加速器提高了3个数量级以上,可在厘米量级的加速长度内获得GeV量级的高品质高能电子束,极大地降低了加速器的成本,为发展新一代粒子加速技术和新型超快辐射源提供了新机遇和新途径。从飞秒强激光驱动等离子体尾波场中的电子注入、能量啁啾控制和高品质电子束产生以及基于高品质电子束的betatron X射线辐射、高能伽马射线和小型化自由电子激光这几个方面介绍了激光等离子体尾波场电子加速的若干主要研究进展,并对未来进行了展望。     

远红外自由电子激光器中超短光脉冲的产生

探讨了射频型远红外波导自由电子激光器（ＦＩＲ—ＦＥＬ）中产生超短光脉冲的可能性。由于短电子束辐射出与其长度相当的光脉冲．因此提出了选择适当的电子束起始相速度来确保正增益和光脉冲折射率沿相对滑移方向单调增加，这样当光脉冲渡越折射率渐变的“增益介质”—电子束团时．光脉冲前后沿将向中心相向移动而使其径向宽度被压缩。则可能获得远红外波段的高功率超短光脉冲。     

基于激光脉宽精确控制获得准直高能电子束的研究

研究了紧聚焦的线偏振飞秒强激光脉冲剧烈加速初始静止的低能相对论电子的效应,发现通过调控激光脉冲宽度可以使电子在激光脉冲纵向有质动力下获GeV量级的能量增益,并进一步研究了被加速电子脱离激光束后的速度偏向角和能量增益受激光脉宽变化的影响,发现当激光脉宽在8λ_0到10λ_0之间时可以获得准直性好的高能电子束,当激光脉宽超过10λ_0时,电子能量增益变化不大且准直性不好。     

预群聚电子束团辐射特性

研究了预群聚电子束团沿周期性理想金属光栅表面平行移动时产生的史密斯-帕塞尔辐射(SPR).详细分析了电子束团能量、束团群聚长度、具有不同电流密度分布的电子束团以及光栅结构对辐射特性的影响,同时研究了多个群聚束团的辐射.分析发现,电子束团能量增加、束团群聚尺寸变小以及光栅周期变短,辐射能量角分布峰值所对应的频率明显向高频方向移动;光栅周期与电子束团参量相同的条件下,三角形光栅辐射能量角分布峰值所对应的频率明显高于正弦光栅;群聚束团尺寸越小辐射能量越大,并且束团的纵向群聚长度对辐射能量与辐射频率起主要作用.研究结果表明,通过合理选择光栅尺寸与束团参量,可获得工作频率锁定的太赫兹(THz)波段辐射.     

磁场与z轴无关摆动器自由电子激光的自发辐射与受激辐射

本文讨论了磁场与z轴无关摆动器自由电子激光中的能量损失和能量离散以及它们相应的意义.分析、计算了自发辐射和受激辐射.结果说明:自发辐射正比于电子能量离散;光功率增益正比于平均电子能量损失.     

自由电子激光器的进展

一、进展概况高功率短波长和波长连续可调高功率激光器一直是激光研究的两个主要方向,自由电子受激辐射正是为实现这种目的的一种新途径.原理上,利用自由电子受激辐射可以实现从微波至X射线、γ射线宽波长连续可调的高功率、高效率微波与光激射器.因此,它已引起人们的普通重视.早在1933年,有人根据量子理论提出利用"摆动"或"波动"电子束与电磁波相互作用产生"受激散射"的概念.两个散射区域分别为:(1)康普顿散射:两波互作用(或单个电子互作用);(2)拉曼散射:三波互作用(或集团电子互作用).1951年莫茨(Motz)提出自由电子受激辐射理论.他认为,当电子行经电场或磁场时产生辐射,辐射的频率取决于电子的速率,电子能量从1兆电子伏到1千兆电子伏范围内可产生从微波至硬X射线频谱.他认为,将电子束聚焦,可以使一群电子相干地辐射,相干辐射功率比非相干辐射约高     

基于相对论超短电子束的THz源

THz波的产生有多种途径,清华大学加速器实验室正致力于发展基于相对论电子束的高峰值功率的THz辐射源。本文分别从高峰值流强的单束团与基于激光整形的多脉冲束团序列出发,介绍实验室在宽谱高电场梯度和准单频可调谐THz辐射等方面的研究进展。高亮度电子束团由新一代光阴极电子枪产生,经3 m行波加速管加速,能量可达~50 MeV,采用速度压缩机制可使束团长度达到百飞秒量级。该超短相对论束团可通过多种机制产生高峰值功率的THz辐射,目前,实验室已通过相干渡越辐射(CTR)获得单脉冲能量18μJ的THz辐射输出。同时,提出了一种结合欠压缩工作模式和漂移压缩方法的新束团压缩方案,可进一步将束团压缩至几十飞秒,有望获得频率范围为0.1 THz~30 THz的超宽谱THz辐射。本文同时介绍了多脉冲超短电子束序列的束团测量实验结果,通过模拟研究验证了基于该束团序列CTR产生高功率、可调谐准单频THz辐射的可行性,并进行了相应的实验方案设计。     

自由电子激光辐射和迴旋辐射的比较研究

在自由电子激光器中一般存在两种受激辐射形式:自由电子激光辐射和迴旋辐射。本文从理论上阐明,如果电子束的初始横向速度较大,自由电子激光器的参数选用不当,则迴旋辐射将占优势,特别当电子束能量较低时,产生迴旋辐射的倾向更大。但是迴旋辐射并不具有双重多普勒频率上漂移的特性,由于受轴向磁场强度的限制,在电子束能量较高时,其辐射频率要比自由电子激光辐射频率低得多。因此在自由电子激光器的实验研究中,如何区别这两种辐射,并有效地抑制迴旋辐射,是一个十分重要的课题。     

Enhancement of Coherent THz Smith-Purcell Radiation by Resonance Overlapping

Frequency-locked coherent Smith-Purcell Radiation (SPR) is emitted when a train of pre-bunched electron beam passes close to the surface of a metallic grating, which develops an energy density spectrum restricted to harmonics of the bunching frequency. For the lamellar grating with narrow grooves, the radiation spectrum from a single electron can also have a feature of narrow band, which is related to the grating structure and the beam energy. The combination of them is proposed in this paper. By properly choosing the parameters, the peak frequency of single electron radiation can be overlapped with the harmonics of the bunching frequency of the electron bunch train, leading to the generation of extremely intense narrow-band THz radiation.     

An analysis of oblique angle stimulated Cherenkov radiation with some experimental results

This paper considers the possible amplification in a dielectric medium of a plane wave intersected by an electron beam at the Cherenkov angle. An oscillator is designed in which a Fabry-Perot resonator's axis is placed at this angle. The gain is found to be inversely proportional to the wavelength, suggesting the possibility of a vacuum ultraviolet or soft X-ray laser. The randomization of the phase coherence between the wave and the electrons due to multiple scattering of the electrons by the atoms of the dielectric is minimized by limiting the length of interaction, and increasing the electron beam energy. Early unpublished experiments are given in which quartz resonators were placed in the Mark III Linear Accelerator's electron beam at Stanford University. From this, other subsequent experimental work, and the theory derived here. two devices are considered which utilize gas as a dielectric instead of a solid.     

一种SASE-FEL用摇摆器机制探讨

利用带电粒子在均匀等离子体中传播时,在其后激发的尾波场作为自发辐射自放大自由电子激光(SASE-FEL)的摇摆器,称为尾波摇摆器。分析了此摇摆器在工作机制及其相应的周期、场强度,根据SASE-FEL理论计算了其增益长度Lg,并和APS的SASE实验参数作了比较。     

Scaling characteristics of the XeF (C→A)excimer laser

The scaling characteristics and medium properties of an injection-controlled XeF(C→A) laser pumped by a 10-ns-high current density electron beam have been investigated. A five-component laser gas mixture, consisting of F₂, NF₃ , Xe, Kr, and Ar was optimized for the scaled laser conditions, resulting in 0.8-J output pulses at 486.8 nm, corresponding to an energy density of small-signal-gain measurements combined with kinetic modeling permitted the characteristics of the dependence of net gain on the electron-beam energy deposition and gas mixture composition, resulting in an improved understanding of XeF(C→A) laser operation     

Cs-N2系统中精细结构能量交换的作用

为了讨论缓冲气体对碱原子精细结构能级间的能量交换加速作用, 利用激光感应荧光光谱对Cs(Rb)-N₂系统中的精细结构碰撞能量转移过程进行了实验研究, 获得了不同条件下碱原子D₁线和D₂线的荧光变化数据。结果表明, 在Cs-N₂系统中, N₂分子更多参与精细结构能量交换的加速过程; 在340K时, 系统具有高的荧光转换效率; 在Rb-N₂系统中, N₂分子主要参与猝灭过程, 对精细结构碰撞的增益作用不明显。这一结果可为半导体抽运碱金属激光器的高效运行提供参考数据。     

Visible and ultraviolet radiation generation using a gas-loaded free-electron laser

Gas loading of a free-electron laser modifies the phase-matching condition while the small-signal gain expression remains the same when written in appropriate form. This permits a wider parameter space than the vacuum FEL, which is particularly advantageous at shorter wavelength operation. Scattering of the electrons by the gas limits the interaction length, but available gains are still high enough to allow oscillation build-up. For example, a 0.5-μm wavelength helical wiggler FEL utilizing a 41 MeV electron beam is restricted to a length of 14 cm, has a small signal gain of 21 percent, and builds up to saturation in less than 1μs. Tunability of this device over several microns is easily obtained by changing the gas pressure.     

Free-electron interactions with light using the inverse Cerenkov effect

An analysis and experimental verification of momentum modulation of relativistic electrons by laser light using the inverse Cerenkov effect is presented. As an alternative to the free-electron laser for achieving energy exchange between particles and photons, the inverse Cerenkov effect uses the index of refraction of a gaseous medium to retard the phase velocity of an electromagnetic wave, enabling the electrons to remain in a field of constant phase. The momentum modulation converts to charge-density modulation by allowing the electrons to drift, thus forming electron bunches separated by optical wavelengths. An analysis is presented for the maximum amount of energy exchange, the energy exchange distribution, and the optimum bunching distance. A computer simulation of the interaction process is also given. These results are compared with the observed momentum modulation of a 102 MeV electron beam by a 30 MW Nd:YAG 1.06 μm laser in both hydrogen and methane gases. Initial observation of coherent optical radiation from a 57 MeV electron beam using the same laser system is also presented. Laser-driven particle accelerators and optical klystrons are possible applications of this interaction.     

Novel approaches to FEL operation: The gas-loaded FEL and a high efficiency FEL design

Two novel methods for improving free-electron laser (FEL) oscillator performance are discussed: (a) The gas-loaded FEL (GFEL) allows operation at snorter wavelengths for a given accelerator energy and wiggler. Experimental results of laser operation with a gas retention foil in the electron beam line and with the introduction of gas to the wiggler cavity are presented, (b) An FEL design utilizing a time-ramped microwave field to accelerate electrons as they lose energy to radiation allows for high conversion efficiencies. Parameter constraints for such an FEL are discussed, leading to a structure that integrates a wiggler with a linac. It is shown that conversion efficiencies of 50% at λ = 10 μm with a 2m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain     

光抽运XeF(C-A)蓝绿激光器

利用分段表面放电作为光抽运源 ,采用光解离XeF2 技术 ,研制了一台 XeF(C A) 蓝绿激光器。抽运源有效激活长度为 6 0cm ,单位长度的沉积功率为 4 5MW /cm。在 2 5 0PaXeF2 / 6 0kPaAr/ 40kPaN2 条件下 ,采用平凹腔 ,输出耦合为 4% ,获得了XeF(C A)激光输出。对XeF(C A)激光特性参数进行了测量 ,输出能量为百毫焦耳量级 ,最大能量达 16 7mJ,激光脉宽～ 6 0 0ns ,辐射光谱范围 470～ 5 0 0nm ,发散角水平方向为 1 7mrad ,垂直方向为3 7mrad。     

横流CO_2激光器小信号增益顺流分布的解析解

本文利用一个简化模型,在推导出电子分布函数的基础上,给出了横流CO_2激光器小信号增益沿气流方向分布的近似表达式,导出了小信号增益与顺流位置、气流速度、气压、顺流电极长度、放电电流之间的关系。理论与实验基本符合。