双层钨丝阵的Z箍缩动力学过程研究

基于Z箍缩零维模型、丝阵碰撞假设和箍缩动能最大化的优化原则，对美国Sandia实验室Z装置的典型双层钨丝阵负载进行了优化分析，结果表明它基本上是优化的.在丝阵碰撞假设下，利用一维三温辐射磁流体力学程序对其箍缩过程进行了数值模拟，模拟结果再现了Z箍缩过程的一维时空演化过程，得到了内外层丝阵的运动轨迹和驱动电流在它们之间的转移规律，并将计算得到的宏观结果与实验结果进行了比较.表明丝阵相互作用的碰撞假设有合理性. 关键词： Z箍缩 双层钨丝阵 零维模型 一维辐射磁流体     

基于MULTI2D-Z程序的Z箍缩动态黑腔形成过程模拟

对辐射流体力学程序MULTI-2D进行改造,增加磁场演化方程程序模块,自洽地在运动方程模块中增加洛伦兹力,在能量方程模块中增加欧姆加热,将它改造成辐射磁流体力学程序MULTI2D-Z.验证了新增磁场程序模块的可靠性,并发现温度和密度的增大会抑制磁场的扩散,负径向速度梯度的流体对流也会抑制磁场的扩散.利用改造好的MULTI2D-Z程序模拟了峰值为8 MA的脉冲电流驱动的钨丝阵Z箍缩动态黑腔形成过程.得到了X光功率(约30 TW)和能量(约300 k J)、泡沫辐射温度(约120 eV)、箍缩轨迹等模拟结果.在动态黑腔形成过程中,磁场主要分布在钨主体等离子体中;辐射向内传播,烧蚀泡沫柱而使它膨胀;辐射热波在被撞击的泡沫柱中传播,其传播速度比物质温度传播得快,当辐射热波传播到中心轴时泡沫柱中的辐射场变得比较均匀,并且除了冲击波处外辐射温度与物质温度基本上没有分离.这些模拟结果可增强人们对磁场扩散和对流规律以及动态黑腔形成机制的理解,同时表明了MULTI2D-Z程序可成为Z箍缩及其应用的新的程序模拟工具.     

回流罩结构Z-箍缩实验的数值模拟

采用二维磁驱动数值模拟程序(MDSC2)对大电流脉冲功率装置FP-2上的回流罩结构Z-箍缩实验exp90和exp60进行了数值模拟。数值模拟表明,回流罩结构Z-箍缩实验测量电流/回路电流不是负载套筒电流,回流罩结构Z-箍缩实验中回路电流不完全从负载套筒通过,回路电流和负载套筒电流之间存在一个结构系数,提出了边界磁场强度与回路电流关系的新公式。采用具有结构系数的边界磁场强度公式和磁流体力学程序能正确模拟exp90和exp60两个回流罩结构Z-箍缩实验,模拟的套筒内径运动速度和实验测量速度相一致。回流罩结构Z-箍缩实验的结构系数为一常数,仅由回流罩的初始结构确定。90 mm和60 mm内直径套筒的结构系数分别为0.87和0.90。在套筒初始厚度、绝缘材料等其它条件相同的情况下,套筒内径越大,回流罩结构Z-箍缩实验的结构系数越小。     

Z箍缩丝阵负载区域的似稳态磁场分布

 将负载区域的电流（丝阵电流、阴极板电流、阳极板电流和回流柱电流）离散成电流线或电流面等电流微元,根据毕奥-萨伐尔定律,计算所有电流微元在指定场点的磁场,再通过叠加给出该点的总磁场。研究结果发现:在丝阵外围区域,仅由丝阵电流所产生的磁场偏离无限长直导线磁场公式的值,但全部电流所产生的总磁场与公式给出的值很接近。同时,研究了不同负载结构参数下的磁场分布,结果表明：增加丝根数有助于减小单根丝表面的局部磁场,改善丝阵外围磁场分布的均匀性。     

基于等离子体粒子模拟的喷气Z箍缩过程物理研究

给出了喷气Z箍缩动力学过程在二维柱坐标系下的等离子体粒子模拟物理模型,编写了相应的程序.对低电流驱动下的稀薄喷气Z箍缩动力学过程进行了验证性的等离子体粒子模拟,得到了许多微观的Z箍缩物理信息,如负载中的电流(密度)、电磁场、粒子位置和密度的时空演化,以及总的z箍缩拖尾质量和拖尾电流等信息.发现在Z箍缩过程中,模拟得到的等离子体电流随时间的变化反映出了等离子体箍缩到心和反弹的过程特征,磁场随径向的变化与长直导线电流给出的磁场很接近;电子所受到的电场力和磁场力(洛伦兹力)是相当的,而离子所受到的力主要是电场力;电子首先在z方向加速,然后在自身运动产生电流的磁场的作用下向轴心箍缩,而离子是在电子和离子电荷分离所产生的电场力的作用下向轴心运动;在压缩到轴心附近时,电子首先因静电排斥而飞散,而离子则在惯性的作用下继续向轴心箍缩,而后滞止飞散.Z箍缩等离子体的拖尾质量在20%左右,拖尾电流最大时在7%左右.     

Effects of drive current rise-time and initial load density distribution on Z-pinch characteristics

A two-dimensional, three-temperature radiation magneto-hydrodynamics model is applied to the investigation of evolutional trends in x-ray radiation power, energy, peak plasma temperature and density as functions of drive current rise-time and initial load density distribution by using the typical experimental parameters of tungsten wire-array Z-pinch on the Qiangguang-I generator. The numerical results show that as the drive current rise-time is shortened, x-ray radiation peak power, energy, peak plasma density and peak ion temperature increase approximately linearly, but among them the x-ray radiation peak power increases more quickly. As the initial plasma density distribution in the radial direction becomes gradually flattened, the peak radiation power and the peak ion-temperature almost exponentially increase, while the radiation energy and the peak plasma density change only a little. The main effect of shortening drive current rise-time is to enhance compression of plasma, and the effect of flattening initial load density distribution in the radial direction is to raise the plasma temperature. Both of the approaches elevate the x-ray peak radiation power.     

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.     

微型磁探针在Z箍缩负载电流诊断中的应用

介绍了磁探针测量等离子体电流的设计原理,针对Z箍缩实验负载的实际结构特点和现场布局,制作出应用于诊断脉冲功率装置Z箍缩实验负载通过电流的微型磁探针,并通过建立相同结构尺寸的模拟负载装置的方法,实现了对其灵敏度的标定。实验结果显示:在脉冲功率装置峰值电流1.2MA、电流上升时间60ns时,由微型磁探针测得的负载电流与加速器监测电流存在12%的幅度差异,电流峰值时刻存在5ns的差异,说明微型磁探针技术测量Z箍缩负载电流的结果是可靠的。     

Z箍缩等离子体内爆X光辐射功率角分布

 在Z箍缩实验中,利用ST1432红光闪烁体、多模石英光纤探头和大电流光电倍增管探测器等研究了丝阵等离子体内爆过程中沿轴向与径向不同方位角的X光辐射功率分布。采用的负载为单层钨丝阵和单层铝丝阵,驱动电流1.5～1.8 MA,上升时间60～90 ns。实验结果表明:Z箍缩等离子体X光辐射强度有轴向和径向分布不均匀性;单层钨丝阵轴向X光辐射强度大于径向辐射强度;单层铝丝阵径向X光辐射强度大于轴向辐射强度。     

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     

Z箍缩动态黑腔形成过程和关键影响因素数值模拟研究

在Z箍缩动态黑腔研究中, 认识黑腔形成物理过程及主要特征, 明确优化黑腔辐射的关键参数, 是实验物理设计的重要基础. 本文针对W丝阵填充CH泡沫转换体的负载构型, 利用一维辐射磁流体程序, 在8 MA驱动电流条件下开展了动态黑腔形成过程和关键影响因素的数值模拟研究. 结果表明, 丝阵等离子体与泡沫转换体相互作用产生局部高压力区是驱动冲击波传播和形成动态黑腔的关键物理过程. 由于辐射超声速传播及其与冲击波波阵面的空间分离, 产生了辐射温度较高而物质未受明显压缩的动态黑腔区域. 丝阵等离子体碰撞泡沫转换体前的状态分布决定了动态黑腔辐射场的主要特征, 可以通过改变负载参数调整并优化黑腔辐射波形. 综合考虑黑腔峰值辐射温度和有效维持时间两个参数, 选择匹配质量的丝阵和泡沫, 使丝阵质量略小于泡沫, 可以获得相对优化的动态黑腔辐射波形. 同时, 合适的丝阵/泡沫初始半径比也是优化动态黑腔辐射的重要影响因素.     

X-ray imaging measurements of capsule implosions driven by a Z-pinch dynamic hohlraum

The radiation and shock generated by impact of an annular tungsten Z-pinch plasma on a 10-mm diam 5-mg/cc CH(2) foam are diagnosed with x-ray imaging and power measurements. The radiative shock was virtually unaffected by Z-pinch plasma instabilities. The 5-ns-duration approximately 135-eV radiation field imploded a 2.1-mm-diam CH capsule. The measured radiation temperature, shock radius, and capsule radius agreed well with computer simulations, indicating understanding of the main features of a Z-pinch dynamic-hohlraum-driven capsule implosion.     

Simulations on the multi-shell target ignition driven by radiation pulse in Z-pinch dynamic hohlraum

We present the first simulation results of a multi-shell target ignition driven by Z-pinch dynamic hohlraum radiation pulse. The radiation pulse is produced with a special Z-pinch dynamic hohlraum configuration, where the hohlraum is composed of a single metal liner, a low-Z plastic foam, and a high-Z metallic foam. The implosion dynamics of a hohlraum and a multi-shell target are investigated separately by the one-dimensional code MULTI-IFE. When the peak drive current is 50 MA, simulations suggest that an x-ray pulse with nearly constant radiation temperature (～ 310 eV) and a duration about 9 ns can be obtained. A small multi-shell target with a radius of 1.35 mm driven by this radiation pulse is able to achieve volumetric ignition with an energy gain (G) about 6.19, where G is the ratio of the yield to the absorbed radiation. Through this research, we better understand the effects of non-uniformities and hydrodynamics instabilities in Z-pinch dynamic hohlraum.     

Characteristics of high-power radiating imploding discharge with cold start

A qualitative model of the dynamics of a multiterawatt radiating Z-pinch with cold start and high rate of current rise is proposed. The model is used to analyze discharges with currents I ～ 2–5 MA (with dI/dt > 10¹³ A/s) through uniform or structured plasma-producing loads, including wire arrays. The most important consequence of cold start is that spatially nonuniform plasma production is prolonged to almost the entire current rise time. Under these conditions, the Ampére force begins to play a dominant role in the plasma dynamics before the plasma-producing load is completely transformed into an accelerated plasma. The results of computations of wire-array vaporization are presented. A formula is proposed for estimating the highest attainable velocity of plasma flow into a heterogeneous liner driven by the Ampére force. It is shown that local imbalance between radial motion of the produced plasma and supply of the plasma-producing substance to be ionized leads to axially nonuniform breakthrough of magnetic flux into the liner, which precedes plasma collapse. The magnetic-flux breakthrough gives rise to a chaotic azimuthal-axial plasma structure consisting of radial plasma jets of relatively small diameter, which is called a radial plasma rainstorm. The breaking-through azimuthal magnetic flux obstructs further current flow in the breakthrough region. Analyses of Z-pinch implosion based on the theory of Rayleigh-Taylor instability or the snowplow model are incorrect under the plasma-rainstorm conditions. The processes taking place in a stagnant Z-pinch include conversion of the energy carried by the current-generated magnetic field into turbulent MHD flow of the ion component of the plasma, its convective mixing with magnetic field, heating, energy transfer from ions to electrons, and emission from the plasma. Under typical experimental conditions, emission plays a key role in the energy balance in an imploding pinch. Z-pinch is modeled by an electric-circuit component that has a time-dependent nonlinear impedance and consumes the magnetic energy supplied by a generator through a magnetically insulated transmission line (MITL). The peak power reached in the circuit is comparable to the peak soft X-ray power output emitted by the pinch in terms of magnitude and timing. Optimum matching conditions are formulated for the generator-MITL-pinch circuit.     

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。     

用X-pinch对双丝Z箍缩进行轴向X射线背光照相

利用PPG-1脉冲电流源(400 kA, 100 ns), 同时驱动X-pinch和双丝Z箍缩负载. 用X-pinch作为X射线源, 对双丝Z箍缩进行了轴向背光照相的初步实验. 得到了双丝Z箍缩在r-θ 平面上随时间演化的图像, 观察到了丝阵Z箍缩所有早期过程, 包括丝芯膨胀、冕层等离子体产生、等离子体向丝阵中心的运动、先驱等离子体形成. 利用阶跃光楔滤片, 对上述背光图像进行了等离子体质量面密度的标定, 首次得到了在r-θ平面上双丝Z箍缩等离子体质量面密度的时空分布图像.     

X-ray backlighting of two-wire Z-pinch plasma using X-pinch

Two 50-μm Mo wires in parallel used as a Z-pinch load are electrically exploded with a pulsed current rising to 275 kA in 125 ns and their explosion processes are backlighted using an X-pinch as an x-ray source.The backlighting images show clearly the processes similar to those occurring in the initial stages of a cylindrical wire-array Z-pinch,including the electric explosion of single wires characterised by the dense wire cores surrounded by a low-density coronal plasma,the expansion of the exploding wire,the sausage instability (m=0) in the coronal plasma around each wire,the motion of the coronal plasma as well as the wire core toward the current centroid,the formation of the precursor plasma column with a twist structure something like that of higher mode instability,especially the kink instability (m=1).     

Ion viscous heating in a magnetohydrodynamically unstable Z pinch at over 2 x 10(9) Kelvin

Pulsed power driven metallic wire-array Z pinches are the most powerful and efficient laboratory x-ray sources. Furthermore, under certain conditions the soft x-ray energy radiated in a 5 ns pulse at stagnation can exceed the estimated kinetic energy of the radial implosion phase by a factor of 3 to 4. A theoretical model is developed here to explain this, allowing the rapid conversion of magnetic energy to a very high ion temperature plasma through the generation of fine scale, fast-growing m = 0 interchange MHD instabilities at stagnation. These saturate nonlinearly and provide associated ion viscous heating. Next the ion energy is transferred by equipartition to the electrons and thus to soft x-ray radiation. Recent time-resolved iron spectra at Sandia confirm an ion temperature Ti of over 200 keV (2 x 10(9) degrees), as predicted by theory. These are believed to be record temperatures for a magnetically confined plasma.     

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。