单口径相干合成系统激光光束的M2因子研究

单口径相干合成系统激光光束的光束质量是一个亟待解决的重要问题.基于二阶矩定义, 文中给出了单口径TEM₀₀, TEM₀₁及TEM₁₀两两相干光束M²因子的解析表达式, 并比较分析了束腰宽度、传输距离、振幅之比,以及源场位置矢量对相干光束M²因子的影响, 得到了诸如源场位置参量d₁<100λ时,各相干光束M²因子恒定,反之, 其随位置参量d₁的增大而增大等一些结论.最后,文章对两TEM₀₀模相干光束M²因子的 部分理论进行了实验验证.     

Generation of High-Current Low-Energy Electron Beams in Systems with Plasma Emitters

Generation and transport of high-current electron beams are investigated in gas-filled diodes with plasma emitters based on arc and glow discharges. A space-charge neutralized beam with a current up to 1 kA was produced in a diode with a plasma emitter based on an arc discharge for an accelerating voltage of 15 kV. The beam is constricted from 8 cm down to 1 cm in diameter by a self-magnetic field and is transported through a distance of over 20 cm with an efficiency of 70%. A beam with a current of 80 A and a current density up to 100 A/cm² was produced in a glow-discharge diode. The beam was transported through a distance of 30 cm in a weak axial magnetic field with induction B = 0.015 T.     

Effects of the dielectric discontinuity on the photodetachment of H− ion

Using the closed orbit theory, we studied the effects of dielectric discontinuity on the photodetachment of H^? ions. The photodetachment cross-section of H^? in a medium with dielectric discontinuity was derived and calculated. The results have shown that the relative dielectric constants of the medium have a significant influence on the photodetachment of H^?. If H^? is kept in a medium with a relative small dielectric constant, the photodetachment cross-section of H^? becomes oscillating only in a small region above the ionization threshold. However, if H^? is kept in a medium with a relative large dielectric constant, the oscillation in the photodetachment cross-section becomes much stronger. Besides, the distance between H^? and the dielectric dividing interface also influences the photodetachment of H^?. For a given dielectric medium, the oscillation in the cross-section decreased with increasing distance between H^? and the dielectric dividing interface. Therefore, we can control the photodetachment of a negative ion by changing the dielectric constant and the ion–interface distance. This study provides a new understanding of the photodetachment process of negative ions in the presence of a dielectric medium.     

Production of secondary radioactive beams from 44 MeV/u Ar projectiles

Secondary beams have been produced through interaction of a 1760 MeV Ar beam with a 99 mg/cm² Be target. An achromatic spectrometer is used to select the magnetic rigidity corresponding to a given beam, and to transport this beam over a distance of about 18m. The beam purity is studied using a solid state ΔE-E telescope. Beams of³⁸S and³⁹Cl are produced with a purity of about 80%, and production rates of 1.5·10^?6 I₀ and 5·10^?5 I_o respectively. Here I₀ denotes the primary beam intensity. Beams of³⁸Ar,³⁹Ar and⁴¹K are produced with about the same abundances as³⁹Cl but with lower purities. It is shown that, by setting properly the experimental parameters, the beam production can be improved by a factor 2 to 5. This could lead to intensities of about 2·10⁶ pps for³⁸S, and of 10⁷ to 10⁸ pps for the four other beams. The possibility of purifying these beams by placing a degrader between the two dipoles of the spectrometer is shown experimentally.     

入射电子能量对低密度聚乙烯深层充电特性的影响

高能带电粒子与航天器介质材料相互作用引起的深层带电现象, 一直是威胁航天器安全运行的重要因素之一. 考虑入射电子在介质中的电荷沉积、能量沉积分布以及介质中的非线性暗电导和辐射诱导电导, 建立了介质深层充电的单极性电荷输运物理模型. 通过求解电荷连续性方程和泊松方程, 可以得出不同能量 (0.1–0.5 MeV) 电子辐射下, 低密度聚乙烯 (厚度为1 mm) 介质中的电荷输运特性. 计算结果表明, 不同能量的电子辐射下, 介质充电达到平衡时, 最大电场随入射能量的增加而减小; 同一能量辐射下, 最大电场随束流密度的增大而增加. 入射电子能量较低时 (≤ 0.3 MeV) , 最大电场随束流密度的变化趋势基本相同. 具体表现为: 当束流密度大于3× 10^-9 A/m²时, 最大场强超过击穿阈值2×10⁷ V/m, 发生静电放电 (ESD) 的可能性较大. 随着入射电子能量的增加, 发生静电放电 (ESD) 的临界束流密度增大, 在能量为0.4 MeV时, 临界束流密度为6×10^-8 A/m². 当能量大于等于0.5 MeV时, 在束流密度为10^-9–10^-6 A/m²的范围内, 均不会发生静电放电 (ESD) . 该物理模型对于深入研究深层充放电效应、评估航天器在空间环境下 深层带电程度及防护设计具有重要的意义. 关键词： 高能电子辐射 低密度聚乙烯(LDPE) 介质深层充电 电导特性     

The TEM101 mode laser beam—A powerful tool for optical levitation of various types of spheres

We point out in this paper that a TEM¹₀₁ mode laser beam is convenient to stably levitate either solid or hollow dielectric spheres or metallic ones. Except a sufficient incident flux, there is no stability condition required to levitate solid transparent spheres having a refractive index larger than the one of the surrounding medium. According to the beam and sphere diameters, the equilibrium position will be centered on the beam axis or off-axis. In the case of hollow dielectric or metallic spheres, the lateral stability requires specific conditions on sphere and beam diameters. These conditions are discussed and then experimentally shown by performing optical levitation of these various kinds of spheres either in vertical or in horizontal beams.     

A coupled-mode theory of heisenberg paramagnets

A new topographic method is presented using sound beams with frequencies at 24 GHz and 35 GHz for imaging crystal defects in nearly perfect dielectric crystals. Large crystals with dimensions up to 20×20×50 mm³ can be investigated. In this method the echo amplitude of longitudinal or transverse phonons is measured at low temperatures during a two-dimensional scan of a movable sound beam. The position dependent echo amplitude is presented as brightness-modulated image. The lateral resolution is limited by the effective diameter of the movable sound beam which is between 0.2 mm and 0.5 mm at present. The contrast pattern observed in natural quartz crystals can mainly be attributed to defects produced during growing time preferable at edges orr-faces andz-faces.     

Loading of a cold atomic beam into a magnetic guide

We demonstrate experimentally the continuous and pulsed loading of a slow and cold atomic beam into a magnetic guide. The slow beam is produced using a vapor loaded laser trap, which ensures two-dimensional magneto-optical trapping, as well as cooling by a moving molasses along the third direction. It provides a continuous flux larger than 10⁹ atoms/s with an adjustable mean velocity ranging from 0.3 to 3 m/s, and with longitudinal and transverse temperatures smaller than 100 μK. Up to 3×10⁸ atoms/s are injected into the magnetic guide and subsequently guided over a distance of 40 cm. Received 19 February 2002 Published online 28 June 2002     

Analysis of propagation of a high-current electron beam from a sectioned plasma-filled diode

We report on the results of analysis of propagation of an electron beam from a plasma-filled diode in the absence of the metal anode between the regions of beam generation and transportation. The diode parameters are 160 kA, 400 kV, and 50 GW. At a distance exceeding 10 cm behind the generation region, a beam current of 100 kA to the target and an energy density of 20 J/cm² are attained for the beam cross-sectional area of about 200 cm². The possibility of varying the beam current and energy density by changing the distance to the target is demonstrated.     

Photon beam softening coefficients evaluation for a 6 MeV photon beam for an aluminum slab: Monte Carlo study using BEAMnrc Code,DOSXYZnrc Code,and BEAMDP code

Determination and understanding the photon beam attenuation by the photon beam modifier and the radiation beam softening for clinical use is more important part of material study for the beam modifier enhancements and the linac improvements. A Monte Carlo model was used to simulate 6 MeV photon beams from a Varian Clinac 2100 accelerator with the flattening filter and the later was replaced by the aluminum slab with variable thickness. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, the gamma criteria was 3% for dose difference and 3 mm for distance to agreement. The purpose was to investigate aluminum material attenuation and beam softening coefficients as a function of the inserted aluminum slab thickness and of off-axis distance. The attenuation and beam softening coefficients were not identical for the same off-axis distance and they varied as a function of aluminum slab thickness. The results of our study were shown that the beam softening coefficients were varied with thickness beam modifier material used for beam softening and the off-axis distance inside the irradiation field. Thereafter, the softening coefficient a ₁ have a maximum of 2.5 × 10^–1 cm^–1 for the aluminum slab thickness of 1 mm, 1.4 × 10^–1 cm^–1 for the aluminum slab thickness of 1.5 mm and 4.47 × 10^–2 cm^–1 for the aluminum slab thickness of 2 mm. The maximum of the second softening coefficient a ₂ was 1.02 × 10^–2 cm^–2 for the aluminum slab thickness of 1 mm, was 1.92 × 10^–2 cm^–2 for the aluminum slab thickness of 1.5 mm and was 1.93 × 10^–2 cm^–2 for the aluminum slab thickness of 2 mm. Our study can be a basic investigation of photon beam softening material that will be used in the future linac configuration and also in the photon beam modifiers.     

电子束产生大尺度等离子体过程的数值模拟研究

建立了一个四组分一维混合模型，对电子束注入大气产生大尺度等离子体的过程进行了数值模拟.结果表明了能量为140keV、流强为50mA/cm²的注入电子束，可以产生线度为0.5m，密度为10¹²cm^-3量级的大气环境下等离子体.电子束所伴随的空间电荷效应由于等离子体的产生会很快消失，不影响后续的等离子体产生过程.电子束注入流强主要影响产生等离子体的密度，而电子束能量则同时影响其空间线度和密度. 关键词： 电子束 碰撞 电离     

1-to-N beam splitter based on photonic crystal branched waveguide structure

A novel beam splitter is proposed based on a two-dimensional (2D) photonic crystal (PC) branched waveguide structure. The beam splitter structure comprises branched waveguide channels and extra dielectric columns. These branched waveguide channels are used to obtain secondary sources and the introduced extra dielectric columns are used to control the phase difference of the secondary point sources. The field distributions of the beam are investigated using the finite-difference time-domain (FDTD). It is found that the number of beams is sensitive to the distance of the waveguide channels ports and extra dielectric columns. By adjusting the positions of the waveguide channels and the parameters of the extra dielectric columns, 1-to-N beam splitters can be realized. These simple, easily fabricated and controllable structures have important potential applications in integrated optical circuits.     

Critical (burst) electron emission from dielectrics, induced by injection of a dense electron beam

A dense pulsed electron beam and nanosecond pulse length has been used to inject negative electric charge into various dielectric materials (single crystals, glasses, composites, plastics) for initiation of electron field emission from the dielectric into a vacuum. It has been shown that upon reaching a critical electric field in the bulk and at the dielectric surface there is intense critical electron emission. The local current density from the emission centers reaches a record value (for dielectrics) of the order of 10⁶ A/cm². The emission occurs in the form of a single gigantic pulse. The measured amplitude of the emission current averaged over the emitting surface is the same order of magnitude as the injected electron current: 10–1000 A. the emission current pulse lages behind the current pulse of the primary electron beam injected into the sample. The delay time is in the range 1–20 nsec and decreases with increasing current density of the injected beam. Direct experimental evidence is found for intense generation of carriers (band or quasifree electrons) in the near-surface layer of the dielectric in a strong electric field due to the Frenkel-Poole effect and collisional ionization of traps, usually various donor levels. This process greatly strengthens the field emission from the dielectric. It has been shown experimentally that the emission is nonuniform and is accompanied by “point bursts” at the surface of the dielectric and ionized plasma spikes in the vacuum interval. These spikes are the main reason that the transition of the field emission into “bursts” is critical, similar to the current which has been previously observed in metals and semiconductors. However there are a number of substantial differences. For example the critical field emission current density needed for the transition into “bursts” is three orders of magnitude less than for metals. If we provide sufficient electron current at the surface or from the bulk of the dielectric to the emission centers, then the critical emission is always accompanied by a vacuum discharge between the surface of the dielectric and a metallic collector. A detailed computer model of the processes in the dielectric during injection of a high-density electron beam has been developed which allows one to understand the complex physical pattern of the phenomenon. Tomsk Polytechnic University. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–67, November, 1997.     

激光冷却获得高亮度的亚稳态惰性气体原子束和原子阱

高强度的亚稳态惰性原子束流在原子分子物理实验研究中具有广泛的应用.使用射频电离方法和激光横向冷却技术制备了高强度的亚稳态氪原子束流,并使用数值模拟方法对横向冷却激光场中的原子径迹进行了分析.通过激光诱导荧光光谱方法测量原子束的束流特性,结果显示,横向冷却后在束流源下游230 cm处的原子束流强度达1.6atoms/(s*sr),束流强度提高了两个量级.利用这种高强度原子束流,我们成功囚禁了1.3×10¹⁰个亚稳态⁸⁴Kr原子,同时冷原子装载速率达到了3.0×10¹¹atoms/s;并利用该装置成功地实现了高亮度的亚稳态氩原子束和原子阱. 关键词： 横向冷却 原子束 原子阱 惰性气体     

Beam delivery characteristics of optical fibres for carbon monoxide lasers

An experimental investigation has been conducted of the propagation characteristics of several types of optical fibres which are candidates for use in optical beam delivery systems for carbon monoxide lasers. Both solid core (chalcogenide) fibres and hollow core (with dielectric or dielectric coated metal) waveguides have been investigated. Such experiments have included an assessment of both the power transmission characteristics, and the effects of waveguide transmission on the optical quality of the beam at the exit of the fibre. The experiments indicated lowest loss for the chalcogenide solid core fibre with a value of 0.4 dB m⁻¹.     

快速H+2在固体中穿行时产生的动力学相互作用力及库仑爆炸图形的演化

模拟了快速H⁺₂在固体中穿行时产生的库仑爆炸过程．作用在单个氢离子H⁺上的力分别由阻止力、屏蔽的库仑力及尾力所组成．借助于等离子-极点近似介电函数,屏蔽的库仑力和尾力可由线性介电响应理论来确定．假定在初始入射时分子轴相对入射速度方向的取向是随机的．模拟结果表明：由于动力学相互作用力的影响,分子离子在穿行过程中其分子轴的取向明显地偏向于入射速度的方向. 关键词：     

Laser-plasma electron acceleration in dielectric capillary tubes

Electron beams and betatron X-ray radiation generated by laser wakefield acceleration in long plasma targets are studied. The targets consist of hydrogen filled dielectric capillary tubes of diameter 150 to 200 microns and length 6 to 20?mm. Electron beams are observed for peak laser intensities as low as 5×10¹⁷?W/cm². It is found that the capillary collects energy outside the main peak of the focal spot and contributes to keep the beam self-focused over a distance longer than in a gas jet of similar density. This enables the pulse to evolve enough to reach the threshold for wavebreaking, and thus trap and accelerate electrons. No electrons were observed for capillaries of large diameter (250???m), confirming that the capillary influences the interaction and does not have the same behaviour as a gas cell. Finally, X-rays are used as a diagnostic of the interaction and, in particular, to estimate the position of the electrons trapping point inside the capillary.     

Dynamics of an electron beam from a high-current picosecond accelerator

The dynamics of a high-current (10²–10⁴ A) electron beam with energies of 10⁵–10⁶ eV and picosecond duration (10⁻¹⁰ s) at the output of the accelerator tube is investigated. The slowing of electrons by the residual positive charge on the surface of the tube is found to have a significant influence in the case of short pulse durations. The distance of the electron beam from the surface of the tube in vacuum is estimated on the basis of a one-dimensional model. It is shown that the electron radiation can travel to a distance of several centimeters from the surface at current densities below 20 A/mm², whereas at high current densities the beam is trapped near the surface. Zh. Tekh. Fiz. 69, 111–115 (May 1999)     

Electron beam irradiation induced changes in liquid-crystal compound 5CB

Electron beam irradiation studies on liquid crystal material 5CB have been carried out at a temperature where the compound exists in the isotropic liquid phase. In situ time-resolved spectroscopic characterization was carried out during the irradiation. Three different transients were observed during the 2-μs electron pulse. After about 50 μs, only one transient species was found to be present, which has an absorption peak at 360 nm. Radiolysed sample exhibits a broad absorption at ∼400 nm. The dielectric measurements show that even a low level of irradiation results in a dramatic increase in the component of dielectric permittivity normal to the long axes of the molecules ε_⊥^′, and a corresponding decrease in the dielectric anisotropy (Δε′=ε_∥^′−ε_⊥^′ ). These studies show that 5CB is prone to substantial radiation damage on exposure to the beam of high-energy electrons.     

Spreading of an SR beam spot (diameter 0.5 μm, 95 eV) photoelectron image on the surface of WO<Subscript>3 − <Emphasis Type="Italic">x</Emphasis></Subscript> films

The 2D-photoemission image of the beam spot was obtained for the first time for the W⁵⁺ oxidation state on the preliminary irradiated WO_{3 − x} thin film surface, created by scanning of the SR beam over the film surface. The W⁵⁺ beam profile intensity was found to spread up to a distance of 3.2 μm for an amorphous film and 5.5 μm for a polycrystalline film, it exceeds considerably the beam spot size. The image saturation dose was reached faster for a polycrystalline film. Among the possible mechanisms explaining this phenomenon, for the case of an almost unchangeable O2s state under irradiation, a choice was made in favor of a photon-generated charge diffusion due to low-energy secondary electrons from photoemission, which produce the “coloration” effect, e⁻ + W⁶⁺ (W⁵⁺) W⁵⁺ → W⁵⁺(W⁴⁺). The O512-eV Auger peak was found to degrade at the distance of 1.5–2 mm outside the beam spot under long-time electron beam irradiation, which is attributed to electron-stimulated oxygen desorption and outdiffusion.