基于卷绕型带状线的感应电压叠加器的初步实验

 提出了一种基于卷绕型带状线和感应电压叠加器的重复频率脉冲电子束加速器的技术方案。介绍了一台感应电压叠加器感应单元的结构设计,并建立相应的电路模型,对其响应特性进行了模拟研究。介绍了卷绕型带状线的设计原理,制作了一台输出阻抗约3 Ω、脉冲宽度约230 ns的固态化卷绕型带状脉冲形成线。利用该脉冲发生器作为馈源,对感应电压叠加器感应单元的响应特性进行了实验研究,表明感应单元响应良好。对4级感应电压叠加器分别进行了单次脉冲和5 Hz重复频率的实验研究,结果表明叠加器的输出电压约为输入电压幅值的4倍,电流效率约80%,重复频率条件下,脉冲序列重复性较好。     

4MV/80kA IVA型脉冲X射线照相装置研制进展

介绍了西北核技术研究院研制的4 MV脉冲X射线闪光照相装置("剑光二号")系统组成和实验结果。装置基于感应电压叠加器(IVA)驱动阳极杆箍缩二极管(RPD)技术,主要由前级脉冲功率源、感应电压叠加器和RPD等组成。前级脉冲功率源由两台3.2 MV低电感Marx发生器和四路同轴水介质线组成。每台Marx同时给两路脉冲形成线(特征阻抗6Ω、电气长度30 ns)充电,充电峰值时间约370 ns。每路水介质线采用两级脉冲压缩,为感应腔馈入约1 MV/160 kA/60 ns电脉冲。电触发SF6气体开关、自击穿水开关分别用作主同步开关和脉冲陡化开关。感应电压叠加器采用四级1.5 MV感应腔串联,每级感应腔采用单点馈入结构。次级采用真空绝缘传输线实现电压叠加和功率传输,特征阻抗由30Ω线性增大至120Ω。采用4 MV电压下综合性能较优的RPD来产生强脉冲X射线。装置目前达到技术指标:输出电压4.3 MV、脉冲前沿(10%~90%)21 ns、半高宽约70 ns、二极管电流85 kA,X射线半高宽约55 ns,整机延时(从Marx触发器输出到X射线产生)约749 ns,标准偏差约7 ns。当RPD阳极采用直径2 mm钨针时,正前方1 m处剂量约15.5 rad(LiF),正向焦斑约1.4 mm。     

感应电压叠加器驱动阳极杆箍缩二极管型脉冲X射线源

 介绍了自行研制的用于闪光照相且基于感应电压叠加器和阳极杆箍缩二极管的X射线源的组成、结构和主要参数。输出电压3 MV的Marx发生器给阻抗7.8 Ω水介质脉冲形成线充电,产生脉宽约70 ns,电压约1 MV的高功率脉冲,经过峰化开关和预脉冲开关后分成3路馈入三级感应电压叠加器感应腔进行电压叠加,感应电压叠加器次级采用真空绝缘传输线,阻抗从40 Ω变成60 Ω,驱动阳极杆箍缩二极管,二极管阴极为石墨,阳极为直径1.2 mm的钨杆,石墨阴极产生的电子束在电流自磁场作用下发生箍缩,轰击阳极,产生小焦斑脉冲Ｘ射线。该装置在Marx充电电压为±35 kV时,二极管电压约2.0 MV,二极管电流约为50 kA,半高宽约80 ns;X射线半高宽约为40 ns,剂量约为28 mGy,焦斑约为0.95 mm。利用该X射线源拍摄到了炸药爆炸产生的层裂碎片不同飞行时间的图像。     

感应电压叠加器驱动阳极杆箍缩二极管型脉冲X射线源

介绍了自行研制的用于闪光照相且基于感应电压叠加器和阳极杆箍缩二极管的X射线源的组成、结构和主要参数。输出电压3 MV的Marx发生器给阻抗7.8 Ω水介质脉冲形成线充电,产生脉宽约70 ns,电压约1 MV的高功率脉冲,经过峰化开关和预脉冲开关后分成3路馈入三级感应电压叠加器感应腔进行电压叠加,感应电压叠加器次级采用真空绝缘传输线,阻抗从40 Ω变成60 Ω,驱动阳极杆箍缩二极管,二极管阴极为石墨,阳极为直径1.2 mm的钨杆,石墨阴极产生的电子束在电流自磁场作用下发生箍缩,轰击阳极,产生小焦斑脉冲Ｘ射线。该装置在Marx充电电压为±35 kV时,二极管电压约2.0 MV,二极管电流约为50 kA,半高宽约80 ns;X射线半高宽约为40 ns,剂量约为28 mGy,焦斑约为0.95 mm。利用该X射线源拍摄到了炸药爆炸产生的层裂碎片不同飞行时间的图像。     

多脉冲励磁下铁氧体及非晶磁环的磁特性

感应脉冲加速器的磁芯通常为铁氧体或非晶材料,而感应腔磁芯在工作脉冲下的磁性能是决定感应加速脉冲波形好坏的重要因素。搭建了低压多脉冲实验平台对铁氧体和非晶小磁环分别进行MHz重复频率的多脉冲励磁,对励磁线圈上的电压电流波形进行监测,绘制了多脉冲励磁下磁环的磁化曲线,并结合含磁芯线圈动态电感量的递推公式计算出磁环在多脉冲励磁过程中磁导率的变化曲线;在高压三脉冲实验平台上对铁氧体磁芯和非晶磁芯实验感应腔进行了高压三脉冲实验,得到的磁芯多脉冲磁化规律与低压实验的结果一致。最后对两种磁环在多脉冲励磁下的磁性能差异进行了对比分析。     

感应电压叠加器模块对方波脉冲的响应

为了解感应电压叠加器(IVA)对馈入脉冲的响应特性,尤其对上升前沿、平顶的影响,从理论和实验两个方面对IVA模块进行了研究。介绍了IVA的工作原理,利用集总参数方法建立了相应的电路模型,通过拉普拉斯变换分析了感应电压叠加器对方波脉冲上升前沿和平顶的响应,并在一个特定的IVA模块上进行了实验研究。选择输出阻抗约1.2Ω、脉宽约1μs的脉冲形成网络作为馈源,在匹配负载上得到的波形与输入波形在幅值、上升前沿方面达到了很好的吻合,平顶出现略微的顶降,与理论预期相一致。     

固态化高功率长脉冲驱动源重频特性

基于固态化磁开关、低阻抗脉冲形成网络和感应电压叠加等关键技术, 提出并研制了一台固态化高功率长脉冲驱动源。在前期通过2 GW单次实验验证技术方案的基础上, 研制了中等电压等级的重复频率初级电源;改进了两级磁脉冲压缩系统的复位和绝缘特性;优化了系统整体电路结构, 利用感应电压叠加器完成充电磁开关和脉冲升压的双重功能;设计了合理的复位路径, 实现了各部分磁芯的在线直流复位;并开展了重频运行研究。在电阻负载上获得了输出功率2.1 GW、脉宽约170 ns、重复频率20 Hz及运行时间1 s的实验结果, 脉冲波形的重叠一致性好。     

感应电压叠加器中水介质开关脉冲自击穿特性

 设计了应用于感应电压叠加器中的水介质主开关与峰化开关,主开关的电极结构为“平板 环型”,峰化开关的电极结构为“环型”。研究了开关在300 ns脉冲电压下的自击穿特性,给出了不同工作电压下开关的间隙距离。实验结果表明,主开关与峰化开关的结构合理、击穿特性稳定、相互配合良好,峰化开关可以有效地峰化主脉冲,降低预脉冲的幅值。主开关的实验结果同时验证了Martin经验公式的适用性。     

双脉冲电子束源实验

2MeV直线感应加速器注入器系统由电子束产生器和脉冲功率系统组成。电子束产生器包括由感应腔组成的阴极电压叠加器、阳极电压叠加器和真空二极管及束输运系统。脉冲功率系统则包含初级功率源Marx、次级功率源Blumlein线和触发系统，其作用是为感应腔提供一个具有数十纳秒平顶宽度的高压脉冲，激发感应腔在感应腔间隙上获得一个加速电场。在2MeV注入器功率系统中，4个Blumlein线的充气开关是由发散装置的输出触发信号进行导通控制的。通过控制发散输出触发信号到达Blumlein线开关的时间，即可以实现Blumlein线开关在不同时间内触发导通，使Blumlein线依据所设定的时间顺序输出激励脉冲，从而在真空二极管上获得高压脉冲串。由于功率系统采用的是182C结构，即一根Blumlein线驱动两个感应腔，因此最多可以实现四脉冲串列。     

强脉冲γ射线能谱测量仪研制

研制出了一种用于强脉冲γ射线能谱测量的能谱仪,在感应电压叠加器实验平台上,测量了阳极杆箍缩二极管产生的强脉冲γ射线能谱。选择适合强脉冲γ射线测量的Si-PIN探测器阵列和铅过滤片作为测量系统,通过在能谱仪的前部放置不同厚度的铅衰减片,测量了感应电压叠加器的强脉冲γ射线的强度。理论计算了不同能量的光子经过不同厚度的铅衰减片以后在探测器阵列上的能量沉积,得到了探测器的灵敏度曲线。利用探测器上的理论计算的电荷量和实验波形的对应关系,求解了该加速器的能谱。光子的最高能量约为1.44 MeV,平均能量为0.68 MeV,其中能量在0.4~0.8 MeV范围内的光子数最多,占74.3%。     

Pauli paramagnetic effects on vortices in superconducting TmNi2B2C

The magnetic field distribution around the vortices in TmNi2B2C in the paramagnetic phase was studied experimentally as well as theoretically. The vortex form factor, measured by small-angle neutron scattering, is found to be field independent up to 0.6Hc2 followed by a sharp decrease at higher fields. The data are fitted well by solutions to the Eilenberger equations when paramagnetic effects due to the exchange interaction with the localized 4f Tm moments are included. The induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor.     

3000% high-field magnetoresistance in super-lattices of CoFe nanoparticles

We report on magnetotransport measurements on millimeter-large super-lattices of CoFe nanoparticles surrounded by an organic layer. Electrical properties are typical of Coulomb blockade in three-dimensional arrays of nanoparticles. A large high-field magnetoresistance, reaching up to 3000%, is measured between 1.8 and 10 K. This exceeds by two orders of magnitude magnetoresistance values generally measured in arrays of 3d transition metal ferromagnetic nanoparticles. The magnetoresistance amplitude scales with the magnetic field/temperature ratio and displays an unusual exponential dependency with the applied voltage. The magnetoresistance abruptly disappears below 1.8 K. We propose that the magnetoresistance is due to some individual paramagnetic moments localized between the metallic cores of the nanoparticles, the origin of which is discussed.     

Multi-core orthogonal fluxgate sensor

A multi-core orthogonal fluxgate sensor is presented, in which the sensing element is formed by multiple insulated thin-wires of a soft ferromagnetic material. Experiments were conducted to investigate the characteristics of the sensor. The results showed that the sensor sensitivity increased exponentially with increasing the number of cores in the sensing element. Such an increase was neither due to increase in the excitation current passing through the sensing element because the number of cores was increased, nor due to the increase of the ferromagnetic material in the sensing element. The results further showed that the sensitivity increases with increasing the number of cores exponentially only if the multiple cores of the sensing element are closely packed, and that there is a resonance in the sensing output corresponding to an external field, which becomes significant as the number of cores reaches a certain large value and varies in its corresponding external magnetic field with the frequency of the excitation current. All the results indicated that in multi-core orthogonal fluxgate sensor, driven by the high frequency of the excitation current passing through the multiple cores, there is a dynamic magnetic interaction between the cores in the sensing element, which increases the sensitivity.     

Delocalized quasiparticles in the vortex state of an overdoped high-T(c) superconductor probed by 63Cu NMR

We report the spin Knight shift (K(s)) and the nuclear spin-lattice relaxation rate (1/T1) in the vortex state as a function of magnetic field (H) up to 28 T in the high-Tc superconductor TlSr2CaCu2O6.8 (Tc = 68 K). At low temperatures well below Tc, both K(s) and 1/T1 measured around the middle point between the two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of states (DOS) around the saddle point is found to be kappaN(0)square root[H/H(c2)], with kappa = 0.5-0.7 and N0 being the normal-state DOS.     

Effects of anisotropy and layering in the high-temperature superconductors

The key building blocks of the high-temperature copper-oxide superconductors are the CuO₂ layers, on which superconductivity tends to be localized and along which the normal-state electrical conductivity is highest. Separating these layers (or bilayers, trilayers, etc.) are locally nonsuperconducting layers, which can be modeled as proximity junctions or tunnel junctions. In this paper I summarize some consequences of a theoretical model for strongly anisotropic high-temperature superconductors in which the interlayer regions are treated as Josephson junctions. In such a model, the vortex lines threading through the structure are best visualized as stacks of two-dimensional pancake vortices connected by Josephson strings. The two-dimensional pancake vortices are centered on the layers and have Abrikosov cores, while the Josephson strings are confined to the junctions and have Josephson cores. Outside the cores, the field and current distributions can be calculated from the anisotropic Ginzburg-Landau (or London) theory. Various features of the flux-pinning anisotropy can be explained using these approaches.     

Effect of charged deep states in hydrogenated amorphous silicon on the behavior of iron oxides nanoparticles deposited on its surface

Langmuir-Blodgett technique has been used for the deposition of ordered two-dimensional arrays of iron oxides (Fe₃O₄/Fe₂O₃) nanoparticles onto the photovoltaic hydrogenated amorphous silicon (a-Si:H) thin film. Electric field at the a-Si:H/iron oxides nanoparticles interface was directly in the electrochemical cell modified by light soaking and bias voltage (negative or positive) pretreatment resulting in the change of the dominant type of charged deep states in the a-Si:H layer. Induced reversible changes in the nanoparticle redox behavior have been observed. We suggest two possible explanations of the data obtained, both of them are needed to describe measured electrochemical signals. The first one consists in the electrocatalytical effect caused by the defect states (negatively or positively charged) in the a-Si:H layer. The second one consists in the possibility to manipulate the nanoparticle cores in the prepared structure immersed in aqueous solution via the laser irradiation under specific bias voltage. In this case, the nanoparticle cores are assumed to be covered with surface clusters of heterovalent complexes created onto the surface regions with prevailing ferrous or ferric valency. Immersed in the high viscosity surrounding composed of the wet organic nanoparticle envelope these cores are able to perform a field-assisted pivotal motion. The local electric field induced by the deep states in the a-Si:H layer stabilizes their “orientation ordering” in an energetically favourable position.     

Vortex structure in superconductors with a many-component order parameter

Abstract

The phenomenological theory of superconductors with a many-component order parameter (OP) is developed. On the basis of a generalized Ginzburg-Landau functional, equations for a two-component-OP superconductor are derived. It is shown that such a superconductor is specified by three length dimensionality parameters—penetration depth λ, correlation length ζ, and width d of the boundary between two superconducting-phase domains. With λ ? d ? ζ, the equations for the OP of a superconductor in a magnetic field can be explored analytically. The transition from the superconducting to the mixed phase may occur not only by the formation of ordinary Abrikosov vortices, but also owing to vortices that have two cores, each transferring a half-integral flux quantum. The total flux transferred by a vortex certainly constitutes an integral quantum. The cores of such a dimer are interconnected by two domain walls, which exercise confinement within the dimer. The distance between the cores in the dimer is of the order of d. Within a domain wall that separates two superconducting-phase domains, a dimer may fall apart into two vortices with a half-integral flux quantum.

For many-component-OP superconductors in a magnetic field, vortex structures of a more complicated nature than a dimer may occur. An individual core may transfer a fractional flux quantum, but the structure as a whole transfers an integral flux quantum. Confinement of individual cores occurs owing to a complicated system of domain walls determined by the topological charges of these vortices.

Under certain conditions, on attaining field H _c1, vortices may arise first in the domain walls, carrying a fractional flux quantum, and then within the superconducting domains.     

基于JASMIN的地下水流大规模并行数值模拟

针对具有精细网格剖分、长时间跨度特征的地下水流模拟中计算时间长、存储开销大等瓶颈问题,基于MODFLOW三维非稳定流计算方法,提出基于网格片的核心算法以及基于影像区的通信机制,并在JASMIN框架上研制了大规模地下水流并行数值模拟程序JOGFLOW.通过河南郑州市中牟县雁鸣湖水源地地下水流的模拟,对程序正确性和性能进行了验证;通过建立一个具有精细网格剖分的假想地下水概念模型对可扩展性进行测试.相对于32核的并行程序,在512以及1 024个处理机上的并行效率分别可达77.2%和67.5%.数值模拟结果表明,JOGFLOW具有较好的计算性能与可扩展性,能够有效使用数百上千计算核心,支持千万量级以上网格剖分的地下水流模型的大规模并行计算.     

Tesla变压器耦合系数的静磁场解法

 为研究Tesla变压器耦合系数与各参量的关系,采用静磁场分析方法,从柱坐标系磁场Laplace方程出发,推导出磁场级数表达式的系数矩阵方程组,计算出磁芯磁场的轴向分布和间隙磁场的轴向、径向分布。引入了一种平均耦合系数概念——次级绕组每匝线圈具有独立的耦合系数,用全部单匝耦合系数的平均值作为Tesla变压器的耦合系数。重点研究了平均耦合系数与磁芯纵横比、半径比、初级绕组-磁芯长度比、磁芯材料磁导率的相对变化关系。结果表明:增大纵横比、减小半径比是提高Tesla变压器耦合系数的有效方法;增大磁芯材料的磁导率可提高耦合系数,但效果随磁导率增大而降低;初级绕组长度与磁芯长度之比约为0.7时,耦合系数达到最大值。     

Rigid piston approximation for computing the transfer function and angular response of a fiber-optic hydrophone

The transfer function of a fiber-optic hydrophone (FOH) is computed for various fiber core radii. The hydrophone is modeled as a rigid disk, with plane waves impinging at normal or oblique incidence. The total sound field is written as the sum of the incident field and the field diffracted from the hydrophone. The diffracted field is approximated by the field generated by a vibrating planar piston in an infinite rigid baffle. For normal incidence and a pointlike fiber core, an analytical solution is presented. For finite fiber core radii, and for oblique incidence, the transfer functions are computed numerically. The calculated transfer functions exhibit an oscillatory frequency dependency that is most pronounced for small fiber cores. The solution for a core radius of 2.5 microm can be very well approximated by the analytical solution for a pointlike core at frequencies of up to 30 MHz. The results for normal incidence can be directly employed to deconvolute ultrasonic pressure signals measured with an FOH. From the transfer functions for oblique incidence, the angular response of the hydrophone is calculated. The angular response obtained here differs significantly from the model commonly used for piezoelectric hydrophones. The effective hydrophone radius derived from the angular response shows a strong frequency dependency. For low frequencies, it is found to be larger than the outer fiber radius, whereas it generally lies between the outer radius and the fiber core radius for frequencies above 10 MHz.