The generation of superradiance pulses by high-current subnanosecond electron bunches moving in a periodic slow-wave system: Theory and experiment

Cherenkov superradiance observed when an electron bunch rectilinearly moves through a slow-wave periodic system is studied theoretically and experimentally. The simulation based on averaged equations and the direct numerical simulation using the PIC-code KARAT show that the peak power of the microwave pulses varies as the total number of the particles in a bunch squared. This finding is confirmed experimentally. Ultrashort (300 ps wide) high-power (up to 140 MW) pulses are generated at a frequency of 39 GHz. As an electron source, the high-current subnanosecond RADAN-303 accelerator is used. It injects 0.5-to 1.5-ns-wide electron bunches of current up to 2 kA and energy 200–300 keV. The simulation suggests that the power of the electromagnetic pulses can be increased further (up to 300 or 400 MW) by optimizing the accelerating voltage pulse shape.     

用ICCD作光开关测量储存环中光学速调管谐波超辐射

 用高通断比ICCD作为快速光开关,在中国科学技术大学的800MeV电子储存环上建造了一套光学速调管谐波超辐射测量系统,从每隔106个自发辐射脉冲1个谐波超辐射脉冲所形成的脉冲链里选出超辐射脉冲,测量谐波辐射的相干增强因子和谱宽。     

Characterization of ultrashort electron pulses by electron-laser pulse cross correlation

An all-optical method to determine the duration of ultrashort electron pulses is presented. This technique makes use of the laser pulse ponderomotive potential to effectively sample the temporal envelope of the electron pulse by sequentially scattering different sections of the pulse out of the main beam. Using laser pulse parameters that are easily accessible with modern tabletop chirped-pulse amplification laser sources, it is possible to measure the instantaneous duration of electron pulses shorter than 100 fs in the energy range that is most useful for electron diffraction studies, 10-300 keV.     

新型金属丝靶电子源实验研究

电子束在基础科学研究、工农业生产和医疗领域发挥了重要作用。提出了一种新型的电子源技术方案:高功率激光脉冲轰击金属丝靶,可以产生大量能量在百keV量级的热电子,一部分热电子在丝靶表面自生电磁场的作用下沿着丝靶运动,丝靶后方可以获得指向性良好的电子束。实验上成功在金、钨和铜丝靶后方获得了电子束团,测量了束团束斑、电荷量和能谱。铜丝靶单发实验收集到的电子束团总电荷量可达3 nC,能量分布在0～240 keV区间内,能谱在100 keV附近呈现峰值。提出了微波压缩方案,设计了2腔微波聚束腔,利用ASTRA对微波腔压缩过程进行了模拟计算。结果显示,可以将电荷量1 nC、长度55 ps的束团压缩至27 ps,满足后续微波加速器对电子源的要求。     

神龙二号加速器及其关键技术北大核心CSCD

神龙二号加速器是一台以M H z 猝发率猝发工作的三脉冲直线感应电子加速器.该加速器输出的三脉冲电子束,相邻两脉冲间最小时间间隔300 n s ,而且可调,每个脉冲电子束的电子能量1 8 -20 M e V 、束流强度大于等于2 k A .当电子束与轫致辐射转换靶相互作用时,可产生三个强X 光脉冲,X 光斑点尺寸小于等于2 m m （ F W H M ）,距靶1 m 处照射量大于等于7 . 7 4 × 1 0^- 2C /k g （ 0 0 R ）.该加速器涉及的主要关键技术包括三脉冲功率源设计、三脉冲强流高品质电子束源的产生、加速场建造、束流传输线设计、轫致辐射转换靶设计、测量与诊断技术等.     

A review of Ni-like X-ray laser experiments undertaken using the VULCAN laser at the Rutherford Appleton Laboratory

Recent progress using the VULCAN laser at the Rutherford Appleton Laboratory to pump X-ray lasing in nickel-like ions is reviewed. Double pulse pumping with ∼100 ps pulses has been shown to produce significantly greater X-ray laser output than single pulses of duration 0.1–1 ns. With double pulse pumping, the main pumping pulse interacts with a pre-formed plasma created by a pre-pulse. The efficiency of lasing increases as there is a reduced effect of refraction of the X-ray laser beam due to smaller density gradients and larger gain volumes, which enable propagation of the X-ray laser beam along the full length of the target. The record shortest wavelength saturated laser at 5.9 nm has been achieved in Ni-like dysprosium using double pulse pumping of 75 ps duration from the VULCAN laser. A variant of the double pulse pumping using a single ∼100 ps laser pulse and a superimposed short ∼1 ps pulse has been found to further increase the efficiency of lasing by reducing the effects of over-ionisation during the gain period. The record shortest wavelength saturated laser pumped by a short ∼1 ps pulse has been achieved in Ni-like samarium using the VULCAN laser operating in chirped pulse amplified (CPA) mode. Ni-like samarium lases at 7.3 nm.     

X波段短脉冲相对论返波管设计与初步实验

 对基于短电子束脉冲超辐射机理的X波段相对论返波管进行了优化设计和粒子模拟，结果表明：在超辐射机理作用下，该器件能实现高峰值功率和高功率转换效率的微波辐射。在小型Tesla脉冲源基础上设计了阻抗变换段、二极管、磁场系统等装置，建立了一套小型窄脉冲电子加速器，以此为实验平台在低磁场条件下进行了器件的初步实验研究。在磁场0.73 T、束压约380 kV、束流约4.5 kA、脉宽3.1 ns条件下，实验获得的微波脉冲峰值功率约360 MW，脉宽1.10 ns，上升沿800 ps，频率9.15 GHz，功率转换效率为21%。     

Nd:YVO₄ amplifier for ultrafast low-power lasers

An Nd:YVO₄ amplifier consisting of two modules end pumped at 808?nm at 30?W total absorbed power has been designed for efficient, diffraction-limited amplification of ultrafast pulses from low-power seeders. We investigated amplification with a 50?mW, 7?ps Nd:YVO₄ oscillator, a 2?mW, 15?ps Yb fiber laser, and a 30?mW, 300?fs Nd:glass laser. Output power as high as 9.5?W with 8?ps pulses was achieved with the 250?MHz vanadate seeder, whereas the 20?MHz fiber laser was amplified to 6?W. The femtosecond seeder allowed extracting Fourier-limited 4?ps pulses at 7?W output power. To our knowledge, these are the shortest pulses from any Nd:YVO₄ laser device with at least 7?W output power. This suggests a novel approach to exploit the gain bandwidth of vanadate amplifiers with high output power levels. Such amplifier technology promises to offer an interesting alternative to high-power thin disk oscillators at few picoseconds duration, as well as to regenerative amplifiers with low-repetition-rate fiber seeders.     

Sum and difference frequency generation of white light continuum with the ps pulses of Nd:YAG laser in a thick BBO crystal

The white light continuum (WLC) generated in water/D₂O mixture by pumping with the fundamental of ps Nd⁺³:YAG laser has been used as a variable frequency source for the sum frequency generation as well as for its amplification. 35 ps long pulses with 8 mJ energy at 1064 nm were mixed collinearly with the WLC generated by the same laser beam in a 20 mm thick BBO crystal. The obtained tunable output has been identified as the sum frequency between the fundamental and a portion of the WLC with the required phase matching. Theoretical simulations are also given along with a few initial experiments to use this combination for the difference frequency generation (optical parametric amplification) under non-collinear geometry.     

High-gradient millimeter-wave accelerator on a planar dielectric substrate

We report the first high-gradient studies of a millimeter-wave accelerator, employing for the first time a planar dielectric accelerator, powered by means of a 0.5-A, 300-MeV, 11.424-GHz drive electron beam, synchronous at the 8th harmonic, 91.392 GHz. Embedded in a ring-resonator circuit within the electron beam line vacuum, this structure was operated at 20 MeV/m, with a circulating power of 200 kW, for 2 x 10(5) pulses, with no sign of breakdown, dielectric charging, or other deleterious high-gradient phenomena. We also present the first measurement of the quadrupolar content of an accelerating mode.     

Generation of high-power ultrashort electromagnetic pulses on the basis of effects of superradiance of electron bunches

One of the promising methods for generation of ultrashort electromagnetic pulses (with duration of about ten periods of high-frequency oscillations) is radiation from spatially localized electron ensembles (bunches), which can be considered a classical analog of Dicke superradiance known in quantum electronics. In classical electronics, superradiance can be related to various mechanisms of stimulated radiation. Until now, cyclotron, undulator, and ?erenkov (in the case of interaction with both copropagating and counterpropagating waves) superradiance of electron bunches as well as superradiance during stimulated scattering of a pump wave have been studied theoretically and experimentally. As a result of these studies based on high-current RADAN and SINUS accelerators and their modifications, a new class of oscillators producing pulsed electromagnetic radiation has been created. They have such unique characteristics as pulses of high peak power (up to 1 GW and 3 GW in the millimeter-and centimeter-wave ranges, respectively) and ultrashort duration (from 300 ps to 1 ns, respectively). In this case, regimes with a peak radiation power exceeding the electron-beam power are experimentally realized. Regimes with high (kilohertz) pulse repetition rate and high average power (up to 2.5 kW) are obtained.     

二次发射微波电子枪的模拟计算及其特性分析

 采用URMEL-T程序进行了二次发射微波电子枪的腔内电磁计算和特性分析。同时利用URMEL-T得出的腔内轴向电场及自编程序,模拟电子在该高频场作用下的运动。计算表明二次发射微波电子枪确实具有相位选择性,进而探讨了腔形尺寸、射频电场强度对相位选择性和产生二次倍增的有关条件的影响,以及输出电子的能量稳定性。模拟结果表明,这类电子枪（MPG）可得到高电流密度（5303A/cm²）及短脉冲（3.15~10ps）的电子束。     

Influence of consecutive picosecond pulses at 532 nm wavelength on laser ablation of human teeth

The interaction of 40 ps pulse duration laser emitting at 532 nm wavelength with human dental tissue (enamel, dentin, and dentin–enamel junction) has been investigated. The crater profile and the surface morphology have been studied by using a confocal auto-fluorescence microscope (working in reflection mode) and a scanning electron microscope. Crater profile and crater morphology were studied after applying consecutive laser pulses and it was found that the ablation depth increases with the number of consecutive pulses, leaving the crater diameter unchanged. We found that the thermal damage is reduced by using short duration laser pulses, which implies an increased retention of restorative material. We observe carbonization of the irradiated samples, which does not imply changes in the chemical composition. Finally, the use of 40 ps pulse duration laser may become a state of art in conservative dentistry.     

The optical limitation effect and features of luminescence kinetics in hybrid nanosystems with CdSe/ZnS quantum dots and organic agents

The optical limitation effect in three-component systems formed in colloidal solutions of semiconductor CdSe/ZnS quantum dots with participation of fullerene C₆₀ and perylene, has been experimentally investigated. The first and second harmonics of a YAG:Nd³⁺ laser operating in the single-shot mode with a pulse duration of 7 ns and laser pulses subjected to stimulated Brillouin scattering (SBS)-stimulated Raman scattering (SRS) compression with a duration of about 20 ps near 560 nm have been used as radiation sources. It is shown that the optical limitation efficiency in the systems studied is determined by electron transfer. This is confirmed by the analysis of luminescence quenching.     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.     

Demonstration of a 17-GHz, high-gradient accelerator with a photonic-band-gap structure

We report the testing of a high gradient electron accelerator with a photonic-band-gap (PBG) structure. The photonic-band-gap structure confines a fundamental TM(01)-like accelerating mode, but does not support higher-order modes (HOM). The absence of HOM is a major advantage of the PBG accelerator, since it suppresses dangerous beam instabilities caused by wakefields. The PBG structure was designed as a triangular lattice of metal rods with a missing central rod forming a defect confining the TM(01)-like mode and allowing the electron beam to propagate along the axis. The design frequency of the six-cell structure was 17.14 GHz. The PBG structure was excited by 2 MW, 100 ns pulses. A 16.5 MeV electron beam was transmitted through the PBG accelerator. The observed electron beam energy gain of 1.4 MeV corresponds to an accelerating gradient of 35 MV/m, in excellent agreement with theory.     

Photo-triggered pulsed cavity compressor for bright electron bunches in ultrafast electron diffraction

Temporally resolved observation of microscopic structural dynamics of solids with ultrafast electron diffraction (UED) requires extremely short pulsed, highly charged, monoenergetic electron beams with sufficient transverse coherence length of several unit cells of the investigated samples. However, Coulomb repulsion defeats these parameters in free propagation of an electron pulse initially bright on the photo cathode. We demonstrate a new electron pulse compressor design based on a simple and compact RF structure incorporating a pair of gallium arsenide photoconductive semiconductor switches that are triggered by femtosecond laser pulses, thereby providing a longitudinal voltage gradient of up to 20?V/ps. Our proof of principle experiment achieved compression of bunches containing 26,000 electrons to a duration of below 750?fs and a beam diameter of 300???m in the temporal and spatial focus of the device while maintaining the good beam collimation required for time resolved electron diffraction experiments. The simplicity of the compressor provides a strong incentive for its further development toward practical implementation in sub-relativistic UED experiments requiring the highest possible source brightness.     

30 MeV电子束轰击旋转钽靶产生轫致辐射分析

提出了一种基于射频直线加速器的多脉冲X光照相系统,有望用于材料动态性能诊断等流体物理动力学研究。基于射频加速器的特点,该套照相系统能够产生时间跨度10 s以上、数个脉冲间隔可调、脉宽为几十至一百ns的脉冲电子束,产生电子束束斑半高宽尺寸小于1 mm。通过蒙特卡罗模拟程序Geant4,分析计算了特定的几何布局以及不同厚度及电子束束斑条件下,电子束打靶后在靶中的能量沉积,靶中的电子束散射对X光焦斑的影响,以及1 m处的照射量,探讨了这套X光照相系统的应用可行性。结果表明,在30 MeV,400 nC电子束轰击厚度为1 mm的靶条件下,1 m处照射量约为9.1 R,靶厚在1~2 mm范围内并未引起X光焦斑的明显增大。较小横向尺寸的电子束会引起靶体局部升温严重,将会制约脉冲数量;采用旋转靶能够提升脉冲数量,通过分析二维旋转靶的应力,分析了靶材升温以及钽/钽合金屈服强度对脉冲间隔的限制作用。     

Tunable,sub-nanosecond KrF-Raman laser in the ultraviolet

A 0.5 cm^–1 bandwidth injection-locked KrF laser pumps a rare-gas Brillouin cell to produce a reflected pulse with a leading edge risetime of 1 ns, tunable from 248.1 to 248.7 nm. Consistent with Lamb theory of laser amplifiers, subsequent excimer amplification of this pulse produces an intense 500 ps spike on the pulse leading edge. Stimulated Raman scattering then separates the spike from the parent pulse, yielding a tunable short pulse at the first Stokes (S ₁) wavelength. Varying the Raman cell length results in a variable Raman threshold and an adjustable short pulse duration: 250 ps pulses at energies of 3–4 mJ at 268 nm with a 50 cm methane cell and 350 ps, 5 mJ pulses from a 100 cm cell are measured with a streak camera. First pass Raman conversion of the spike toS ₁ followed by second pass backward Raman amplification, where the parent 248 nm pulse serves as the pump beam for the reflectedS ₁ pulse, yields simultaneousS ₁ pulses of 20–25 mJ in the 800 ps range andS ₂ pulses of 550 ps at 5–6 mJ near 290 nm. This laser will avoid collision effects during laser excitation and enable quantitative, single pulse imaging of OH radicals in turbulent combustion because of its high pulse energy.     

Generation and detection of tunable ultrashort infrared and far-infrared radiation pulses of high intensity

We report on generation and detection of intense pulsed radiation with frequency tunability in the infrared and far-infrared spectral regions. Infrared radiation is generated with a transversally electrically excited high pressure CO₂ laser. A laser pulse of a total duration of about 300 ns consisted, due to self mode locking, of a series of single pulses, some with pulse durations of less than 450 ps and peak powers larger than 20 MW. Using these pulses for optical with durations less than 400 ps were obtained. For detection a new ultrafast superconducting detector was used.