Inductive Store Pulse Compression System for Driving High Speed Plasma Implosions

The Air Force Weapon Laboratory has investigated and developed inductive pulse compression techniques with fuse opening switches for driving high speed plasma implosions. Experiments have demonstrated the delivery of 7.5 MA to a 5-nH load in < 200 ns from an initial 1.9-MJ 2-?s capacitor bank via inductive pulse compression. Circuit considerations dictate the overall energy efficiency while MHD considerations dictate overall implosion stability and thermalization time. Theoretical considerations along with initial experiment results are presented in this paper.     

The Jon Beam Opening Switch

Plasma opening switches (POS's) have shown excellent characteristics in pulsed power applications. Proposed POS scaling predicts that the fastest opening time for a given conducted current should occur using a high-velocity low-density plasma as the switch medium. The ion beam opening switch (IBOS) uses a charge-neutral ion beam of 100-300 kV, ? 120 A/cm2 as the switch "plasma." Its velocity of up to 600 cm/?s and density of ~1012/cm3 make this a very fast low-density plasma compared with typical 10 cm/?s and 1013/cm3 POS plasmas. The IBOS has conducted ? 70 kA flowing in a parallel-plate transmission line driven by a 4-? pulser. IBOS opening time is load dependent, being ? 4 ns into a 15-nH load and about twice as long into a 4-? electron diode load. However, switch impedance is not zero during the entire conduction time, rising to ? 3 ? by the time of peak current. Peak current conducted before opening does not vary linearly with either injected ion current or switch axial length. Instead, the conduction current scales with plasma density in the switch, and is nearly independent of switch area until the area is restricted to a narrow (~1 cm) strip.     

Efficient radiation production in long implosions of structured gas-puff Z pinch loads from large initial radius

We have proposed and demonstrated successfully a new approach for generating high-yield K-shell radiation with large-diameter gas-puff Z pinches. The novel load design consists of an outer region plasma that carries the current and couples energy from the driver, an inner region plasma that stabilizes the implosion, and a high-density center jet plasma that radiates. It increased the Ar K-shell yield at 3.46 MA in 200 ns implosions from 12 cm initial diameter by a factor of 2, to 21 kJ, matching the yields obtained earlier on the same accelerator with 100 ns implosions. A new "pusher-stabilizer-radiator" physical model is advanced to explain this result.     

准球形电磁内爆动力学研究及能量定标关系浅析

通过改变Z箍缩负载的初始形状和/或质量密度分布,可以实现等离子体的准球形聚心内爆.同柱形箍缩相比,准球形电磁内爆可以将内爆动能集中加载至负载中心较小的空间区域内,获得更高的能量密度,从而在驱动Z箍缩动态黑腔实现聚变点火方面具有潜在优势.准球形电磁内爆的负载和电极结构比柱形Z箍缩更复杂,球面收缩的几何特点使其内爆动力学过程和能量定标关系显著区别于柱形内爆.本文利用解析的薄壳模型推导并分析了理想条件下准球形电磁内爆的动力学行为和能量定标关系,并同二维磁流体力学模拟结果进行了比较.与柱形Z箍缩内爆相比,准球形电磁内爆的动能不仅与驱动电流有关,而且敏感地依赖于负载的初始尺寸.在不显著降低驱动电流和内爆品质的前提下,适当增加负载的初始半径和最大纬度,有利于获得更多的内爆动能和能量加载密度.     

1—4 MA电流驱动的丝阵X光辐射优化的实验研究

进行了1—4MA电流驱动的钨丝阵列负载的Z箍缩实验研究,通过丝阵参数、负载电极结构的优化设计及负载初始装配状态的控制优化X光辐射功率,在单、双层丝阵的内爆实验中分别获得5.3±1.0 TW和5.6±1.1 TW的峰值辐射功率,创同类装置上X光辐射功率的最高纪录. 关键词： Z箍缩')" href="#">Z箍缩 丝阵X光辐射 优化     

Review of Plasma Flow Switch Development

The plasma flow switch utilizes the nonlinear and nonuniform dynamics of a plasma discharge in vacuum to accumulate magnetic energy in times of several microseconds and then release this energy to a load in times of a few hundred nanoseconds. Experiments have been performed with capacitor banks up to 6 MJ, providing currents in excess of 107 A and peak voltages over 0.5 MV. Theoretical models include simple slug dynamics coupled to lumped-circuit analyses, magnetoacoustic considerations of one- and two-dimensional aspects of the plasma flow, and two-dimensional magnetohydrodynamic code calculations. The present article reviews both experimental and theoretical efforts, discusses the use of the plasma flow switch to drive plasma liner implosions and high-energy ion flows, and indicates directions for plasma flow switch applications to very high current, high-energy inductive pulsed power systems.     

Results of radius scaling experiments and analysis of neon K-shellradiation data from an inductively driven Z-pinch

The K-shell radiated energy (yield) from neon Z-pinch implosions with annular, gas-puff nozzle radii of 1, 1.75, and 2.5 cm was measured for implosion times from 50 to 300 ns while systematically keeping the implosion kinetic energy nearly constant. The implosions were driven by the Hawk inductive-storage generator at the 0.65-MA level. Initial neutral-neon density distributions from the nozzles were determined with laser interferometry. Measured yields are compared with predictions from zero-dimensional (0-D) scaling models of ideal. One-dimensional (1-D) pinch behavior to both benchmark the scaling models, and to determine their utility for predicting K-shell yields for argon implosions of 200 to >300 ns driven by corresponding currents of 4 to 9 MA, such as envisioned for the DECADE QUAD. For all three nozzles, the 0-D models correctly predict the Z-pinch mass for maximum yield. For the 1and 1.75-cm radius nozzles, the scaling models accurately match the measured yields if the ratio of initial to final radius (compression ratio) is assumed to be 8:1. For the 2.5-cm radius nozzle, the measured yields are only one-third of the predictions. Analysis of K-shell spectral measurements suggest that as much as 70% (50%) of the imploded mass is radiating in the K-shell for the 1-cm (1.75-cm) radius nozzle. That fraction is only 10% for the 2.5-cm radius nozzle. The 0-D scaling models are useful for predicting 1-D-like K-shell radiation yields (better than a factor-of-two accuracy) when a nominal (≈10:1) compression ratio is assumed. However, the compression ratio assumed in the models is only an “effective” quantity, so that further interpretations based on the 0-D analysis require additional justification. The lower-than-predicted yield for the 2.5-cm radius nozzle is associated with larger radius and not with longer implosion time, and is probably a result of two-dimensional effects     

Enhanced load current delivery from the SHIVA star vacuum inductivestore/plasma flow switch

The SHIVA Star device is a 1313-μF, 120-kV capacitative storage device capable of storing 9.4-MJ electrical energy. The experimental operating voltage is 90 kV. An increased current delivery from the SHIVA Star capacitor band to a plasma-flow-switch-driven cylindrical foil load is reported. Modification of a plasma-flow-switch electrode produces a current delivery that is a factor of two better than the best previous comparable efforts. Peak driving current is 12.3 MA; the peak load current is over 9.4 MA. At peak load current, driving current is 11.2 MA for a current delivery of 85% or more. Current delivery in the experimental system is reported. The modification specified entails partially closing the outflow electrode of a plasma-flow switch     

A long conduction time compact torus plasma opening switch

Experiments to form and accelerate compact toroid (CT) plasmas have been performed on the 0.4-MJ Shiva Star fast capacitor bank at Phillips Laboratory. Theoretical investigations of employing a CT as a very fast opening switch are reported. A particular axisymmetric, geometrically complex switch design is studied with the help of 2-1/2-dimensional magnetohydrodynamic computer simulations. This design, called a magnetically-confined-plasma opening switch (McPOS), accumulates magnetic energy in an inductive store. Because of its intrinsic stability, the switch can conduct current for ten or more microseconds and can open in less than 100 ns-substantially less than the risetime of the capacitively produced electric current. A long conduction time compact torus plasma opening switch     

Railgun experiments with Lexan insulators

A series of railgun experiments has been performed using Lexan insulators in both round and square bores, and with closed-breech and open-breech/injected configurations. Measured armature lengths have been roughly constant at 5-10 cm in a 1-cm bore for all Lexan insulator shots, indicating that the ablated Lexan is not swept up. Projectiles have been observed to reach peak velocity of 5.65 km/s with clean armature structures: i.e. no separated secondary arc or restrike. However, in most cases a secondary arc does occur with Lexan and limits the achievable velocity. Occasionally, stationary secondary arcs have also been observed for a particularly leaky gun assembly. The effect of insulator ablation on performance is discussed, indicating that Lexan may be useful at up to 8-10 km/s for well-sealed railguns     

Plasma density evolution in plasma opening switch obtained by a time-resolved sensitive He-Ne interferometer

To understand the formation process of vacuum gap in coaxial microsecond conduction time plasma opening switch (POS), we have made measurements of the line-integrated plasma density during switch operation using a time-resolved sensitive He-Ne interferometer. The conduction current and conduction time in experiments are about 120 kA and 1 μs, respectively. As a result, more than 85% of conduction current has been transferred to an inductive load with rise time of 130 ns. The radial dependence of the density is measured by changing the radial location of the line-of-sight for shots with the same nominal POS parameters. During the conduction phase, the line-integrated plasma density in POS increases at all radial locations over the gun-only case by further ionization of material injected from the guns. The current conduction is observed to cause a radial redistribution of the switch plasma. A vacuum gap forms rapidly in the plasma at 5.5 mm from the center conductor, which is consistent with the location where magnetic pressure is the largest, allowing current to be transferred from the POS to the load.     

High-rhoR implosions for fast-ignition fuel assembly

Thick, 40 microm plastic shells filled with 25-35 atm of D2 or D3He were imploded on a low-adiabat (alpha approximately 1.3) and with a low-implosion velocity ( approximately 2 x 10(70 cm/s) on the OMEGA laser to generate massive cores of compressed plasma with high areal densities optimal for fast ignition. The targets are driven by 20-kJ relaxation adiabat-shaping laser pulses to keep the inner portion of the shell nearly Fermi degenerate. The measured kinetic energy downshift of proton spectra is in good agreement with the theoretical predictions yielding burn-averaged areal densities of 0.130+/-0.017 g/cm2 and peak rhoR during the burn of about 0.24+/-0.018 g/cm2, the largest rhoR measured on OMEGA to date. The same implosions with empty plastic shells are expected to reach 1.3 g/cm2 across the core (i.e., 2rhoR) enough to stop fast electrons with energies up to 4.5 MeV typical of fast ignition scenarios.     

The Application of a Plasma Erosion Opening Switch to a Nanosecond Generator at the Power Level of 1010 W

This paper is devoted to experimental studies of a short-pulse (80 ns) inductive system with a coaxial plasma erosion opening switch (PEOS), operating at the 2-5 × 1010 W level. Scalings of the PEOS and ion diode characteristics with different parameters (PEOS plasma density and velocity, PEOS electrode geometry, load impedance, type and strength of an external magnetic field) were carried out. It was seen that for the most efficient energy and power switching to the load by the PEOS, the following conditions are preferable: high velocity and low density of the plasma flow, negative polarity of the inner PEOS electrode, coincidence of the switch current and injected plasma flow directions, the absence of an external magnetic field, and the presence of an additional self-field in the PEOS region. Power enhancement of a factor of 3 and pulse shortening by a factor of 2 were obtained under optimal conditions.     

Influence of the gas density on the motion of a free plasma piston in the railgun channel

The influence of the gas density on the acceleration of a plasma armature inside the railgun channel filled with various gases (xenon, air, or helium) under atmospheric pressure is investigated experimentally and theoretically. It is shown that, after the discharge current has reached a steady value, the velocity of the glowing plasma front ceases to grow and remains constant as long as so does the current. The length over which the velocity saturates is equal to a few centimeters, i.e., is much shorter than the railgun channel length. The maximum velocity of the plasma piston meets a predicted limit, which is determined by the drag of the medium and a decrease in the acceleration of the plasma armature when a fraction of the material evaporated from the rails is involved into motion. The plasma composition depends on the electrode material. The velocities measured when the channel is filled with helium (V = 17.5 km/s) or air (V = 9.8 km/s) noticeably exceed the sound speed inside the plasma piston (5–7 km/s).     

Z箍缩动态黑腔形成过程MULTI程序一维数值模拟

Z箍缩动态黑腔能够高效地将Z箍缩丝阵等离子体动能转换为黑腔辐射能,为驱动惯性约束靶丸聚变提供高品质的X射线辐射场.利用一维双温多群辐射磁流体力学程序MULTI-IFE,研究了"聚龙一号"装置驱动电流条件下的Z箍缩动态黑腔形成基本物理过程.数值模拟结果表明,在动态黑腔形成过程中,辐射热波的传播速度比冲击波的传播速度更快,比冲击波更早到达泡沫中心,使中心区域的泡沫在冲击波到达前就已具有较高的辐射温度.对于"聚龙一号"装置动态黑腔实验0180发次采用的负载参数,辐射热波和冲击波在泡沫中的传播速度分别约为36.1 cm/μs和17.6 cm/μs,黑腔辐射温度在黑腔形成初期约80 eV,在冲击波到达泡沫中心前可达100 eV以上,丝阵等离子体外表面发射的X射线能量集中在1000 eV以下.本文给出了程序采用的计算模型、美国"土星"装置丝阵内爆计算结果和"聚龙一号"装置动态黑腔实验0180发次模拟结果.     

“阳”加速器磁驱动平面飞片实验和速度计算校验

脉冲功率驱动源作为磁驱动加载的重要手段,通过调整其电路参数可调节负载电流波形,实现对样品无冲击准等熵加载。在"阳"加速器上,开展了一系列轴对称结构和带状结构构型的磁驱动平面飞片发射实验,电极材料采用不锈钢和LY-12铝。实验中测量了进入负载的电流历史和电极后自由面速度历史,并通过时序控制将两者时间关联起来。本文以测量到的电流历史数据为基础,引入负载电流分布系数,并结合已知的LY-12铝的状态方程数据,计算电极后自由面速度历史和飞片速度历史。通过实验测量自由面速度历史校验负载各个位置的电流分布系数。另外,基于装置参数和实验数据确定了考虑负载电感变化的装置等效电路模型,形成了计算样品压力加载历史和电极后自由面速度历史估算程序。此外,初步分析不同厚度电极的自由面速度历史,获取了电极材料的准等熵加载波剖面信息,观察到一系列准等熵加载下材料动力学性能引起的物理现象。     

Hot ablative implosions driven by 0.53 μm laser radiation

A six beam 0.53 μm laser symmetrically irradiated thin walled gas filled spherical glass shells at 4 × 10¹⁴ W cm^-2, the upper limit for ablatively driven implosions. The high ablation pressure induced sufficient X-ray and thermonuclear product emission for diagnosis of the 200 eV temperature and 3 g cm^-3 density in the implosions. X-ray preheat, Rayleigh Taylor instability and non uniform irradiation are identified as serious limits to performance of the implosions.     

Electromagnetic Wave Propagation in a Magneto-Plasma Filled Coaxial Structure: II, Experimental

Measurements have been made of electromagnetic propagation in a coaxial electrode structure filled with longitudinally magnetized plasma. The annular plasma region had a 9.55 cm outer diameter, a 3.82 cm inner diameter and was approximately 60 cm long. A magnetic field of 300 gauss was employed. Electromagnetic wave frequencies were in the range .5 to 2.4 GHz. The plasma was generated by a continuous glow discharge. The resulting ? vs. ? curves closely follow the predicted curves for the "quasi-TEM" mode. On the basis of this model, plasma frequencies were 40 - 80% of the cyclotron frequency, depending on magnitude of the glow discharge current.     

Mechanism of radiation pulse shortening in plasma relativistic microwave generator

The causes of emission quenching in the plasma relativistic microwave generator are studied by numerical calculations using the particle-in-cell method. The process mechanism in which the plasma boundary moves from the coaxial collector edge with a velocity above 10⁷ cm/s is found. An electron flux with an energy of ～10⁵ eV and a current of ～10³ A is generated from the collector in the formed gap, which heats plasma and increases its potential. Microwave generation stops due to a multiple decrease in the wave reflectance from the collector. The use of the hollow collector is presumably a method for preventing microwave generation quenching.     

单层丝阵负载Z箍缩内爆辐射特性研究

在电流3—4MA的Angara-5-1脉冲装置上进行了单层钨丝阵Z箍缩实验,利用具有坪响应的X射线功率谱仪获得X射线功率,利用X射线纳秒分幅相机获得等离子体内爆辐射区图像.在丝阵直径相同时,实验得到较细的丝直径使得内爆较早,收缩比较大;较大的丝间隙使得内爆早期丝间等离子体不能有效的融合,而是较孤立的等离子体簇向内箍缩;较大的丝直径和丝间隙导致不稳定性波长较大.在丝阵直径不同,丝直径相当时,实验得到较大的丝阵直径内爆启动较早,具有较大的内爆速度,但等离子体在内爆过程中较分散.另外,较大的丝直径和丝阵直径使X射线辐射脉冲时间较宽.