Z箍缩丝阵负载微型装配系统

 在利用金属丝阵Z箍缩产生强X射线辐射的方法中,为获得超强X射线,一般采用直径几μm的钨丝构成单、双或多层圆柱面状丝阵负载,其中丝阵负载的结构和装配精度是关键因素。为制备出均匀、平直的丝阵负载,进行了Z箍缩丝阵负载微型装配系统的研制,建立了基于机器视觉、微力传感器和精密运动控制系统的Z箍缩丝阵负载微型装配实验系统。该系统成功实现了双层直径5 μm钨丝丝阵的装配,且张力控制精度达0.002 N。     

Z箍缩内爆的MARED程序数值模拟分析

MARED程序是模拟Z箍缩内爆过程的二维三温辐射磁流体力学程序,它适用于不同装置条件和不同负载参数.利用MARED程序对Z箍缩内爆进行模拟,结合丝阵Z箍缩实验分析表明:相同负载质量条件下,钨丝阵内爆产生的X射线辐射功率远大于铝丝阵产生的X射线功率;相同负载电流条件下,负载质量越大,计算得到的X射线功率越低;X射线功率随着负载电流增加而增加.MARED程序能够较好地反映Z箍缩内爆动力学过程,特别是不稳定性发展的二维图像,能够给出与不稳定性简化模型的理论分析及实验结果定性一致的演化规律.MARED程序模拟丝阵填充泡沫形成辐射场的初步计算得到了与Sandia实验室模拟Z装置上丝阵填充泡沫定性一致的结果.     

强光一号Z箍缩产生keV级特征X射线初步研究

 分析了高功率Z箍缩产生keV级特征X射线辐射的物理机理和用于计算等离子体K层辐射二能级模型，给出了采用二能级模型进行数值模拟的结果，描述了强光一号装置驱动Z箍缩负载的脉冲功率源特性参数，介绍了所设计研制的双层喷Ne气和双层铝丝阵两类Z箍缩负载的结构与参数，并对下一步拟开展的实验研究工作进行了说明。利用特制真空X射线二极管测量了强光一号双层喷氖气Z箍缩实验产生的X射线波形，实验结果表明，当气室内初始气压在0.8 MPa时，喷Ne气Z箍缩可获得较好的keV量级的X射线辐射。     

箍缩背光照相

 利用PPG-Ⅰ脉冲功率装置（500 kV,400 kA,100 ns）驱动X箍缩负载,得到了μm量级的亚纳秒脉冲X射线辐射点源。在阴阳极轴线和回流柱上同时安装X箍缩负载,前者可作为背光照相的X射线点源;后者可作为被拍照的目标X箍缩。获得了X箍缩发展过程不同时刻的时间序列图像,在背光照相的图像中可以清晰观察到X箍缩交叉点处等离子体的外爆、箍缩以及最终的崩溃阶段。将阴阳极轴线上的X箍缩负载用Z箍缩丝阵负载代替,实现了对丝阵负载Z箍缩放电起始阶段的X射线背光照相,观察到了最初的各单丝电爆炸、等离子体膨胀及融合过程,同时观察到了双丝Z箍缩发展过程中的等离子体不稳定性。实验结果有助于深入理解Z箍缩发展的物理过程,同时可为有效的模拟建模分析提供基本的实验数据。     

X箍缩背光照相

利用PPG-Ⅰ脉冲功率装置(500 kV,400 kA,100 ns)驱动X箍缩负载,得到了μm量级的亚纳秒脉冲X射线辐射点源。在阴阳极轴线和回流柱上同时安装X箍缩负载,前者可作为背光照相的X射线点源;后者可作为被拍照的目标X箍缩。获得了X箍缩发展过程不同时刻的时间序列图像,在背光照相的图像中可以清晰观察到X箍缩交叉点处等离子体的外爆、箍缩以及最终的崩溃阶段。将阴阳极轴线上的X箍缩负载用Z箍缩丝阵负载代替,实现了对丝阵负载Z箍缩放电起始阶段的X射线背光照相,观察到了最初的各单丝电爆炸、等离子体膨胀及融合过程,同时观察到了双丝Z箍缩发展过程中的等离子体不稳定性。实验结果有助于深入理解Z箍缩发展的物理过程,同时可为有效的模拟建模分析提供基本的实验数据。     

强光一号Z箍缩实验研究

 在强光一号装置驱动电流峰值1.4~2.1 MA、上升时间80~100 ns条件下,研究了喷Kr气、喷Ne气、W丝阵和Al丝阵负载Z箍缩等离子体的辐射特性和聚爆过程。实验研究中,用分压器和两个罗戈夫斯基线圈分别测量二极管的电压和流过负载的电流波形,用过滤型X射线二极管和100~1 400 eV平能谱响应闪烁探测器测量X射线时间谱,用镍薄膜量热计测量X射线总能量,用纳秒时间分辨软X射线图像诊断系统记录了Z箍缩的聚爆过程,用时空分辨的椭圆弯晶谱仪诊断了Z箍缩等离子体产生的keV级特征X射线的能谱分布。其中,喷Kr气负载的X射线辐射总能量大于60 kJ,峰值功率约1.7 TW,总能量转换效率可达23%;W丝阵负载的辐射总能量大于30 kJ,峰值功率约1.30 TW,总能量转换效率约为12.5%;喷Ne气负载的keV级辐射总能量5.6 kJ,峰值功率约256 GW,能量转换效率可达2%; Al丝阵负载的keV级辐射总能量2.3 kJ,峰值功率约94 GW,能量转换效率可达0.8%。喷气负载在Z箍缩聚爆过程中存在“拉链”现象,丝阵负载在Z箍缩聚爆过程中存在先驱现象。     

丝阵Z箍缩可见光辐射区径向变化过程

 在“强光一号”上进行的Z箍缩实验中,利用1维可见光成像系统获取了丝阵内爆可见光辐射区径向变化过程图像。采用的负载包括单层钨丝阵、单层铝丝阵,驱动电流为1.3~1.5 MA,上升时间89~140 ns。实验结果表明:丝阵内爆产生轴向先驱等离子体柱,尺寸为0.3~0.5 mm;丝阵在内爆和内爆到芯及冷却飞散阶段,外围区域始终存在较弱的可见光辐射;获得的可见光条纹像提供了丝阵等离子体内爆可见光轨迹,内爆径向压缩比为2.84～7.84,丝阵内爆速度为4.60×10⁶～1.73×10⁷ cm/s。     

快Z箍缩钨丝阵内爆物理研究

 通过理论和实验研究了快Z箍缩电磁内爆动力学物理过程及其辐射特性，完成了1维和2维辐射磁流体动力学数值模拟程序。利用阳加速器、强光一号脉冲功率装置、S-300装置和Angara-5-1装置，开展了实验研究，发展了一系列观察Z箍缩等离子体辐射的诊断系统。研制了单层、双层钨丝阵及单层丝阵加聚氘乙烯芯等负载。重点研究了能量耦合和内爆动力学规律。     

强光一号兆安电流钨丝X箍缩实验研究

为了获得更高亮度的X射线点源,在"强光一号"装置上开展钨丝X箍缩实验研究.基于能量平衡方程,估算了1 MA电流热斑等离子体平衡半径为1—10μm.实验中,测试钨丝直径为25—100μm,丝根数为2—48,负载线质量为0.18—6.9 mg/cm;负载电流峰值为1—1.4 MA,10%—90%前沿为60—70 ns."强光一号"装置上匹配的X箍缩负载为30或32根25μm钨丝X箍缩,这种负载一定概率产生单个脉冲X射线辐射,辐射时刻位于电流峰值附近,典型参数为keV X射线脉宽1 ns,辐射功率35 GW,产额40 J,热斑尺寸～30μm.然而,兆安电流X箍缩通常产生多个热斑及多个脉冲X射线辐射.keV能段首个脉冲X射线辐射时刻与负载线质量正相关,并受到负载丝直径的影响.多个X射线脉冲可能由二次箍缩和局部箍缩产生,多个热斑可能由交叉点处微Z箍缩的长波长扰动和短波长扰动引起.与百千安电流X箍缩相比,兆安电流X箍缩热斑亮度更高,但X射线辐射脉冲的单一稳定性还有待于进一步改善.     

PTS装置Z箍缩负载设计分析

介绍了PTS装置丝阵负载设计思路,用简化的薄壳模型(零维)和辐射磁流体力学模型(一维)数值模拟了丝阵Z箍缩内爆过程,给出了PTS丝阵负载设计参数,分析了初步优化结果,确定了优化选择的箍缩常数.经脉冲功率驱动器与负载优化匹配设计,PTS实验平台能够获得100TW量级的X射线功率,最佳插头能量转换效率10%.预计PTS实验平台的X射线输出能力基本达到Saturn装置长脉冲模式放电的水平.结果将为PTS开展Z箍缩研究提供有价值的参考. 关键词： PTS装置 Z箍缩丝阵负载 零维薄壳模型 一维辐射磁流体力学数值模拟     

Experimental observations of the spatial anisotropy of the neutronemission in a medium energy plasma focus

The results of experiments carried out on the DPF-78, a 60 kV, 28 kT plasma focus device are presented. The primary objective of these experiments was to investigate the spatial anisotropy of the neutron emission and to correlate the total neutron yield with hard X-ray emission and the presence of hot spots. To influence the plasma parameters, gas fillings of deuterium with doping of neon, argon, and krypton were used. An admixture of gases with an equivalent mass density of 5 mbar of D₂ was employed throughout the experiment. A novel technique for neutron detection was used, which allowed the recording of neutron signals at close distance to the focus (65 cm), with minimal dispersion of the signal due to time of flight and yet high X-ray rejection. A set of these detectors was placed at different angular positions to investigate the polar distribution in the neutron emission. It was identified for the first time that in a medium energy plasma focus device, the neutron emission is composed of two periods, similar to that reported in high energy devices. It was found that the first period occurs immediately before the maximum compression and lasts less than 50 ns while the second period lasts between 150 and 200 ns. It was found that the polar distribution of the neutron emission shows a strong anisotropy during the second period and depends on the Z of the doping gas. No correlation was found between the total neutron yield (about 10¹⁰) and the hard X-ray emission or appearance of hot spots     

CeF3闪烁探测器对DD中子的相对灵敏度

 用国内近年新研制的CeF₃闪烁体和常用闪烁体ST401分别配特性相同的光电倍增管，构成两种闪烁体探测器，在强度不随时间变化的DD中子源场中测量了这两种闪烁探测器的相对灵敏度，测量结果表明：CeF₃闪烁探测器对DD中子的灵敏度比同尺寸ST401的灵敏度低一个量级以上。     

Nuclear fusion in collapsing bubbles-is it there? An attempt to repeat the observation of nuclear emissions from sonoluminescence

We have repeated the experiment of Taleyarkhan et al. [Science 295, 1868 (2002)]] in an attempt to detect the emission of neutrons from d-d fusion during bubble collapse in deuterated acetone. Using the same cavitation apparatus, a more sophisticated data acquisition system, and a larger scintillator detector, we find no evidence for 2.5-MeV neutron emission correlated with sonoluminescence form collapsing bubbles. Any neutron emission that might occur is at least 4 orders of magnitude too small to explain the tritium production reported in Taleyarkhan et al. as being due to d-d fusion. We show that proper allowance for random coincidence rates in such experiments requires the simultaneous measurement of the count rates in the individual detectors.     

Nuclear Reactions under Irradiation of Deuterated Structures by X Rays

Measurements of emission from nuclear reaction products (neutrons and protons) have been carried out appearing in the deuterated structures of textured CVD diamond, palladium, titanium, and zirconium under irradiation with a beam of X rays using independent methods (neutron detector based on He-3 counters, Si surface-barrier semiconductor detectors and CR-39 track detector). The possibility of enhancement of both DD reaction and multi-particle deuterium fusion by the beam of X rays with energy ranging 20–30 keV in solid deuterated targets has been established. Analysis of X-ray fluorescence spectra of the target bombarded by beams of ions has revealed “additional” peaks, the occurrence of which cannot be related to any of the known elements, and requires separate study.

     

Response of converter semiconductor detectors on neutron radiation

Active detectors based on Si semiconductors are being developed for individual radiation protection purposes in mixed neutron and photon fields. These devices are constructed as combined converter semiconductor detectors. Incident neutrons interact with the converter or detector nuclei and produce charged particles that can deposit energy in the semiconductor and cause a signal. The aim of this work is the determination of the response of such detectors to incident neutrons by experiment and by computation. The computer model consists of the neutron interaction simulation and the calculation of the ion transport. An analysis of measured and computed pulse height distributions for 5 MeV incident neutrons is presented as well as a comparison.     

HT-7上氘等离子体放电时中子注量的测量与分析

利用三氟化硼（BF₃） 和氦三（3He）正比计数管组成的快速时间分辨中子注量探测系统,对HT-7超导托卡马克上氘等离子体放电时光中子和聚变中子的产生机理进行研究.结合γ射线、硬X射线等相关诊断的实验结果,分析了纯欧姆放电和低杂波辅助加热放电时,中子注量信号随时间演化的典型特征.结果表明：HT-7在投入大功率低杂波辅助加热等离子体放电时,能够产生数量可观的氘-氘（D-D）聚变中子. 关键词： 中子注量 聚变中子 光中子 托卡马克     

Calibration of ITER Instant Power Neutron Monitors: Recommended Scenario of Experiments at the Reactor

Instant power is a key parameter of the ITER. Its monitoring with an accuracy of a few percent is an urgent and challenging aspect of neutron diagnostics. In a series of works published in Problems of Atomic Science and Technology, Series: Thermonuclear Fusion under a common title, the step-by-step neutronics analysis was given to substantiate a calibration technique for the DT and DD modes of the ITER. A Gauss quadrature scheme, optimal for processing “expensive” experiments, is used for numerical integration of ²³⁵U and ²³⁸U detector responses to the point sources of 14-MeV neutrons. This approach allows controlling the integration accuracy in relation to the number of coordinate mesh points and thus minimizing the number of irradiations at the given uncertainty of the full monitor response. In the previous works, responses of the divertor and blanket monitors to the isotropic point sources of DT and DD neutrons in the plasma profile and to the models of real sources were calculated within the ITER model using the MCNP code. The neutronics analyses have allowed formulating the basic principles of calibration that are optimal for having the maximum accuracy at the minimum duration of in situ experiments at the reactor. In this work, scenarios of the preliminary and basic experimental ITER runs are suggested on the basis of those principles. It is proposed to calibrate the monitors only with DT neutrons and use correction factors to the DT mode calibration for the DD mode. It is reasonable to perform full calibration only with ²³⁵U chambers and calibrate ²³⁸U chambers by responses of the ²³⁵U chambers during reactor operation (cross-calibration). The divertor monitor can be calibrated using both direct measurement of responses at the Gauss positions of a point source and simplified techniques based on the concepts of equivalent ring sources and inverse response distributions, which will considerably reduce the amount of measurements. It is shown that the monitor based on the average responses of the horizontal and vertical neutron chambers remains spatially stable as the source moves and can be used in addition to the staff monitor at neutron fluxes in the detectors four orders of magnitude lower than on the first wall, where staff detectors are located. Owing to low background, detectors of neutron chambers do not need calibration in the reactor because it is actually determination of the absolute detector efficiency for 14-MeV neutrons, which is a routine out-of-reactor procedure.     

Uncertainties induced by statistical fluctuations in fast monoenergetic neutron yield measurements

A plastic scintillation detector was used to measure the yield of deuterium-deuterium (DD) neutrons or deuterium-tritium (DT) neutrons. Collisions of fast neutrons with hydrogen nucleus in a scintillator generated recoil protons, the energies of which were fully deposited in the scintillator. The statistical fluctuation of the protons' number and that of the protons' total energy were two sources of measurement uncertainty. Based on DT neutrons, this paper represents the algorithms of computing the probability density functions of the two sources. Uncertainties of the measurement induced by statistical fluctuations were finally computed by constructing the probability density functions of the proton number and that of the neutron number.     

Measurement of the neutron spectrum of a Pu-C source with a liquid scintillator

The neutron response function for a BC501A liquid scintillator (LS) has been measured using a series of monoenergetic neutrons produced by the p-T reaction. The proton energies were chosen such as to produce neutrons in the energy range of 1 to 20 MeV. The principles of the technique of unfolding a neutron energy spectrum by using the measured neutron response function and the measured Pulse Height (PH) spectrum is briefly described. The PH spectrum of neutrons from the Pu-C source, which will be used for the calibration of the reactor antineutrino detectors for the Daya Bay neutrino experiment, was measured and analyzed to get the neutron energy spectrum. Simultaneously the neutron energy spectrum of an Am-Be source was measured and compared with other measurements as a check of the result for the Pu-C source. Finally, an error analysis and a discussion of the results are given.     

Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in²³⁵U(n _th,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.