电子温度各向异性对霍尔推力器BN绝缘壁面鞘层特性的影响

建立多价态多组分等离子体一维流体鞘层模型,引入电子温度各向异性系数并考虑出射电子速度分布,研究了电子温度各向异性对霍尔推力器中的BN绝缘壁面鞘层特性和近壁电子流的影响.分析结果表明,相比于纯一价氙等离子体鞘层参数,推力器中的多价态氙等离子体鞘层电势降略有降低,电子壁面损失增加,临界二次电子发射系数减小.推力器中的电子温度各向异性现象可以显著地加大出射电子能量系数,进而降低鞘层电势降,增强电子壁面相互作用.数值结果表明,空间电荷饱和机制下电子温度各向异性对鞘层空间电势分布影响显著. 关键词： 霍尔推力器 电子温度各向异性 空间电荷饱和鞘层     

霍尔推力器放电通道溅射腐蚀计算

 为了预示霍尔推力器的寿命,建立了推力器粒子束放电通道的2维电磁场模型,模拟的推进剂为氙。利用PIC方法跟踪粒子在电磁场中的运动。磁场的求解采用拉普拉斯方程,电场的求解采用泊松方程。电子由阴极喷入通道,并在电磁场中与原子发生电离碰撞生成离子。在跟踪离子的过程中记录下撞击到内外壁面的离子个数、角度和能量。利用记录下的参数进行腐蚀计算,得到当溅射阈值能量分别为10,20,30,40,50 eV时通道壁面的腐蚀速率。推力器放电通道出口附近的最大腐蚀速率约为1.7×10^-9 m/s。     

霍尔推力器中呼吸振荡激发机理及影响因素

呼吸振荡作为霍尔推力器中的一种低频、大振幅放电不稳定性,对推力器的性能及寿命有严重的影响.本文利用包含了离子径向扩散和电子壁面相互作用的双区“捕食者-被捕食者”(Predator-Prey, P-P)模型,对霍尔推力器中呼吸振荡的激发机理和影响因素开展了研究.研究结果表明,电子与壁面之间相互作用导致的能量耗散对呼吸振荡有抑制作用,而近阳极区的离子径向扩散对呼吸振荡有激发作用.依赖于近阳极区的离子径向扩散强度,模式振荡频率以及放电电流的振荡峰值呈现非单调变化的趋势.此外,在推力器放电通道长度一定的情况下,呼吸振荡的激发与电离区长度的变化无关,而振荡的频率(周期)随着电离区长度的增大而增大(减小).本文的研究结果将为霍尔推力器中呼吸振荡激发机理的认识以及呼吸振荡抑制新方法的提出提供理论支撑.     

Hall推力器内饱和鞘层下电子与壁面碰撞频率特性

利用二维粒子模拟方法研究Hall推力器内电子与壁面的碰撞频率.研究发现,饱和鞘层状态下的电子与壁面的碰撞频率较经典鞘层下大大增加,甚至高出经典鞘层状态下电子与壁面碰撞频率两个数量级,这样饱和鞘层状态下电子与壁面的碰撞频率对近壁电流的贡献将不容忽略.进一步分析造成饱和鞘层状态下电子与壁面碰撞频率增加的原因后认为,饱和鞘层状态下电子与壁面碰撞频率的增加是鞘层电势降过低和壁面发射的二次电子回流造成的. 关键词： 饱和 鞘层 碰撞 频率     

壁面二次电子发射对霍尔推力器放电通道绝缘壁面双鞘特性的影响

为进一步揭示霍尔推力器放电通道绝缘壁面鞘层的特性, 利用考虑了壁面二次电子分布函数的一维稳态流体鞘层模型, 研究了壁面二次电子发射对近壁双鞘特性的影响. 分析结果表明, 由于壁面发射的二次电子对近壁鞘层中的电子密度有增加作用, 存在一个临界二次电子发射系数σ_dc使得: 当σ≤σ_dc时, 鞘层为单层的正离子鞘结构; 当σ>σ_dc时, 鞘层表现为双层的正离子鞘和电子鞘相连结构, 连接点对应于垂直于壁面方向上电势分布的拐点. 然而, 当σ进一步增大到0.999时, 鞘层转变为三层的正离子鞘-电子鞘-正离子鞘交替结构. 数值结果表明: 随着σ的增加, 电子鞘与离子鞘的连接点向远离壁面的方向移动, 电子鞘的厚度逐渐增加; 随着壁面出射电子能量系数a的增加, 近壁区鞘层的厚度也逐渐增加. 关键词： 霍尔推力器 双鞘 壁面二次电子发射     

霍尔推进器中振荡鞘层对电子与壁面碰撞频率的影响研究

利用二维粒子模拟方法研究振荡鞘层对近壁电导的影响.研究结果表明,当二次电子发射系数大于1时,鞘层处于振荡状态.在振荡鞘层状态下,电子与壁面的碰撞通量沿平行与壁面方向剧烈的周期性振荡,振荡的波长为电子静电波波长量级,电子与壁面的碰撞频率高出经典鞘层状态下电子与壁面碰撞频率1—2个数量级,此时的碰撞频率对通道中电流的贡献不可忽略.振荡鞘层相对与经典鞘层增大了电子与壁面的碰撞频率,但是振荡鞘层的存在,仍然会使一部分慢电子无法穿越鞘层的势垒而打到壁面. 关键词： 霍尔推进器 振荡鞘层 二次电子     

电子温度对霍尔推进器等离子体鞘层特性的影响

采用二维粒子模拟方法研究了霍尔推进器通道中电子温度对等离子体鞘层特性的影响, 讨论了不同电子温度下电子数密度、鞘层电势、电场及二次电子发射系数的变化规律. 结果表明: 当电子温度较低时, 鞘层中电子数密度沿径向方向呈指数下降, 在近壁处达到最小值, 鞘层电势降和电场径向分量变化均较大, 壁面电势维持一稳定值不变, 鞘层稳定性好; 当电子温度较高时, 鞘层区内与鞘层边界处电子数密度基本相等, 而在近壁面窄区域内迅速增加, 壁面处达到最大值, 鞘层电势变化缓慢, 电势降和电场径向分量变化均较小, 壁面电势近似维持等幅振荡, 鞘层稳定性降低; 电子温度对电场轴向分量影响较小; 随电子温度的增大, 壁面二次电子发射系数先增大后减少. 关键词： 霍尔推进器 等离子体鞘层 电子温度 粒子模拟     

30cm口径离子推力器磁场设计

为了对30cm口径离子推力器的磁场设计提出合理建议,研究了四极磁场结构下,不同尺寸的磁极宽度和磁极间距对磁极表面磁场强度和放电室电子约束长度的影响,并利用Maxwell-3D磁场分析软件得到柱段和锥段永磁体分别呈30°,60°和90°夹角时的放电室磁场强度分布,根据不同磁场强度计算了电子温度、离子密度以及电离率等推力器放电参数。结果表明,当推力器放电电压为30V时,磁极长度设计为0.008m且磁极间距取为0.12m,电子约束路径大约为50m;柱段和锥段永磁体分别呈30°,60°和90°夹角时,放电室磁场等势线基本在0.002~0.005T之间;永磁体夹角为60°时磁场分布和磁空区相比30°和90°夹角更为合理,此时的电子温度约在2~6eV,等离子体密度约在4×1017~8×1017 m-3,电子碰撞频率比率约在0.2~1.8范围内。     

磁场强度对霍尔推力器放电特性影响的实验研究

为探索霍尔推力器通道内优化磁场的标准,本文研究了磁场强度对其放电特性的影响规律.通过在保持磁场形貌不变（“聚焦”形）下改变磁场强度的大小,使用朗缪尔探针、光谱仪等测量手段分析了工作于不同磁安特性曲线段的推力器放电特性.研究表明：当磁场强度小于优化值时,电子横越磁场的传导以近壁传导机制为主;反之,当磁场强度大于优化值时,放电电流反常变化,而现有的电子输运传导机制不能解释这种现象. 关键词： 霍尔推力器 磁场强度 放电特性     

非磁化等离子体中的电子碰撞频率

电子碰撞频率是非磁化等离子体的一个重要参数,它对等离子体与电磁波相互作用的性质具有较大影响。基于产生等离子体的气压条件不同,其电子碰撞频率具有较大变化范围。研究了它对非磁化等离子体吸波能力的影响,得出了当电子碰撞频率与等离子体频率和入射波频率相当时,等离子体对电磁波具有较大吸收的结论,给出了对应于最大吸收的最佳碰撞频率值。分析了通过选择放电气体来调整等离子体的电子碰撞频率。     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.