平面调制靶瑞利-泰勒不稳定性初步研究

 瑞利-泰勒不稳定性增长的准确估计是激光聚变的重要研究课题。在神光Ⅱ装置上，利用面向背光照相技术对正弦调制平面靶的瑞利-泰勒不稳定性增长进行了实验研究，得到了清晰的时空分辨图像；采用傅里叶变换取基模法和求波峰波谷差值法分析了实验结果；两种方法得到的靶扰动增长因子相同。实验中平面靶扰动增长较小可能是密度梯度致稳和烧蚀致稳抑制了扰动增长，也可能是扰动进入了非线性阶段而使增长不大。     

烧蚀RT不稳定性非线性过程模拟

重点研究了不同预热对烧蚀RT弱非线性模耦合的影响。首先不同预热情况烧蚀RT不稳定性的线性增长率曲线明显不同。不同预热情况的模拟结果都表明：烧蚀RT的模耦合系数是k／kc的函数，二次和三次谐波的产生系数C（k）和D（k）遵从定标关系C（k）：c1（1-c2k/kc），D（k）=d1-d2k/kc＋d3（k/kc）^2。弱预热和中等预热情况，二次谐波的产生系数约只有经典RT的50％，三次谐波的产生系数约只有经典RT的20％。两体模耦合起主要作用，n次高次谐波可近似看作是由（”一1）次两体模耦合产生的。由于烧蚀RT不稳定性两体模耦合系数小于1，所以高次谐波的产生系数很快衰减。由于非线性作用变弱，单模扰动的非线性饱和阈值明显增大，意味着基模有较长时间的线性增长，另外，向长波长方向的模耦合系数明显大于经典尺丁不稳定性数值，因此，烧蚀RT非线性模耦合容易形成长的尖顶，从而对点火构成严重威胁。     

激光烧蚀瑞利泰勒不稳定性数值研究

 介绍了激光烧蚀流体不稳定性计算程序EUL3D,其计算结果与Takabe 公式、FAST2D程序和LASNEX程序的结果以及日本大阪大学激光烧蚀瑞利—泰勒(RT)不稳定性实验,都较好符合,发现了横向电子热传导烧蚀在长波长扰动的非线性瑞利—泰勒不稳定性演变中起重要作用。在合理近似下,得到了烧蚀RT不稳定性线性增长率的预热致稳公式,此公式除包含了烧蚀对流致稳和密度梯度致稳因素外,还包含了Atwood数变小致稳因素,因此与各种情况的二维计算值都很好符合。     

磁瑞利-泰勒不稳定性非线性演化数值模拟

基于理想磁流体力学模型,采用5阶WENO有限差分格式结合磁场伪散度清除投影算法,发展二维数值模拟程序,利用线性理论对程序进行检验,在此基础上研究磁瑞利-泰勒(MRT)不稳定性非线性演化物理过程和发展规律.结果表明:垂直于加速度方向磁场在线性和非线性阶段对MRT扰动发展都有较强的制稳作用,而平行于加速度方向磁场在线性阶段对MRT扰动发展影响较小,但在MRT扰动非线性发展阶段会对由于流场变化产生的K-H不稳定性有显著的制稳作用,从而抑制总的扰动发展.在MRT非线性发展过程中,由于波模耦合相互作用,单模演化会产生高次谐波;双模演化会产生除基模和谐波以外的其他波模,其扰动幅度甚至超过基模;多模演化过程中发生逆级联过程,即短波长扰动逐渐向长波长扰动发展.     

瑞利-泰勒不稳定性线性增长的密度梯度致稳

 研究了密度梯度对瑞利-泰勒不稳定性的致稳作用,采用有限元算法求解钱得拉塞卡方程本征值问题,得到不同密度分布下理想不可压流体力学量的扰动线性增长率及扰动速度分布。扰动增长率结果与修正的Lindl公式的计算结果比较发现：扰动分布的峰值位于密度梯度标长的取值位置处,波长与密度标长可比拟时,扰动增长率显著偏离Lindl公式,而长波和短波极限情况下,数值解和Lindl公式符合较好。     

激光烧蚀瑞利－泰勒不稳定性模拟

 给出了激光烧蚀流体不稳定性计算程序EUL2D的物理方程,介绍了计算中使用的活动网格和一些技术问题处理。EUL2D程序的计算结果与Takabe公式、FAST2D程序和LASNEX程序的结果较好符合。数值计算日本大阪大学激光烧蚀瑞利－泰勒不稳定性实验,再现了实验结果。发现了横向电子热传导烧蚀在长波长扰动的非线性瑞利－泰勒不稳定性演变中起重要作用。     

高温烧蚀初始印记及其瑞利-泰勒不稳定性发展的研究

直接驱动惯性约束聚变中,高温烧蚀初始印记在压缩过程的不稳定性发展中起着重要作用.利用X射线针孔辅助点投影成像法,研究了不同波形激光脉冲驱动下CH平面靶及其瑞利-泰勒不稳定性发展的情况.结果表明,在相同驱动激光能量作用下,初始阶段激光强度越低,上升越缓慢,高温烧蚀初始印记引起的密度调制幅度越大.提高预脉冲强度能显著抑制高温烧蚀初始印记效应. 关键词： 高温烧蚀初始印记 激光脉冲形状 瑞利-泰勒不稳定性 针孔辅助点投影     

轴向剪切流对Z箍缩等离子体瑞利-泰勒不稳定性的抑制

利用理想磁流体力学模型对有轴向剪切流的Z箍缩等离子体不稳定性进行了分析。给出了可压缩模型的色散关系,分别对可压缩及不可压缩模型中轴向剪切流对Z箍缩等离子体瑞利-泰勒不稳定性的抑制作用进行了比较,讨论了可压缩性对含有轴向剪切流系统不稳定性的影响。结果表明,可压缩性能够减缓瑞利-泰勒P凯尔文-亥姆霍兹(RTPKH)模扰动的增长,因而使得轴向剪切流对系统不稳定性的抑制作用表现得更为突出。计算结果还说明,在RT不稳定性线性增长阶段,等离子体温度较低,使用可压缩模型能够更真实地描述系统的状态。     

CH薄膜泡沫复合平面调制靶制备初步研究

瑞利-泰勒（R—T）流体力学不稳定性是制约惯性约束聚变（ICF）实验点火的重要因素之一。研究证明，通过在有机薄膜或低密度泡沫上人为引入面密度扰动可以模拟靶丸表面的不完整性，通过测量激光对靶烧蚀过程中靶密度扰动的时空分布非线性增长，可估算R—T不稳定性的大小。关于调制薄膜制备的方法国内目前仅有数篇报道，但有关薄膜泡沫复合调制靶的制备还未见相关文献报道。     

二维不可压流体瑞利-泰勒不稳定性的非线性阈值公式

分析了二维情况下平面几何、柱几何和球几何中瑞利-泰勒不稳定性发生非线性偏离的阈值问题，给出了三种几何中密度扰动振幅的定义，以及与界面扰动振幅的关系.由此得到了三种几何中密度扰动的非线性阈值公式，用高精度流体程序对三种几何中的不稳定性进行了数值模拟，验证了得到的非线性阈值公式. 关键词： 瑞利-泰勒不稳定性 非线性阈值 密度振幅     

Weak Nonlinearity of Ablative Rayleigh--Taylor Instability

The weakly nonlinear regime of single mode ablative Rayleigh-Taylor instability is studied, with consideration of preheat effect and the width of the ablation front. The Rayleigh-Taylor linear growth rate agrees well with the direct numerical simulation. For the density perturbation, the amplitude distribution of the fundamental mode has one peak value whereas those of the second and third harmonics have two and three peak values, respectively. Harmonics generation versus wave number is also given and it is close to the result of direct numerical simulation.     

Nonlinear theory of the ablative Rayleigh-Taylor instability

A fully nonlinear sharp-boundary model of the ablative Rayleigh-Taylor instability is derived and closed in a similar way to the self-consistent closure of the linear theory. It contains the stabilizing effect of ablation and accurately reproduces the results of 2D DRACO simulations. The single-mode saturation amplitude, bubble and spike evolutions in the nonlinear regimes, and the seeding of long-wavelength modes via mode coupling are determined and compared with the classical theory without ablation. Nonlinear stability above the linear cutoff is also predicted.     

Suppression of Rayleigh-Taylor instability by gyroviscosity and sheared axial flow in imploding plasma pinches

The synergistic stabilizing effect of gyroviscosity and sheared axial flow on the Rayleigh-Taylor instability in Z-pinch implosions is studied by means of the incompressible viscid magneto-hydrodynamic equations.The gyroviscosity(or finite Larmor radius) effects are introduced in the momentum equation through an anisotropic ion stress tensor.Dispersion relation with the effect of a density discontinuity is derived.The results indicate that the short-wavelength modes of the Rayleigh-Taylor instability are easily stabilized by the gyroviscosity effects.The long wavelength modes are stabilized by the sufficient sheared axial flow.However,the synergistic effects of the finite Larmor radius and sheared axial flow can heavily mitigate the Rayleigh-Taylor instability.This synergistic effect can compress the Rayleigh-Taylor instability to a narrow wave number region.Even with a sufficient gyroviscosity and large enough flow velocity,the synergistic effect can completely suppressed the Rayleigh-Taylor instability in whole wave number region.     

Fourier-space nonlinear Rayleigh-Taylor growth measurements of 3D laser-imprinted modulations in planar targets

Nonlinear growth of 3D broadband nonuniformities was measured near saturation levels using x-ray radiography in planar foils accelerated by laser light. The initial target modulations were seeded by laser nonuniformities and later amplified during acceleration by Rayleigh-Taylor instability. The nonlinear saturation velocities are measured for the first time and are found to be in excellent agreement with Haan predictions. The measured growth of long-wavelength modes is consistent with enhanced, nonlinear, long-wavelength generation in ablatively driven targets.     

表面张力对Rayleigh-Taylor不稳定性谐波的影响

Using the method of the parameter expansion up to the third order, explicitly investigates surface tension effect on harmonics at weakly nonlinear stage in Rayleigh-Taylor instability (RTI) for arbitrary Atwood numbers and compares the results with those of classical RTI within the framework of the third-order weakly nonlinear theory. It is found that surface tension strongly reduces the linear growth rate of time, resulting in mild growth of the amplitude of the fundamental mode, and changes amplitudes of the second and third harmonics, as is expressed as a tension factor coupling in amplitudes of the harmonics. On the one hand, surface tension can either decrease or increase the space amplitude; on the other hand, surface tension can also change their phases for some conditions which are explicitly determined.     

Weakly nonlinear theory for the ablative Rayleigh-Taylor instability

A weakly nonlinear model is proposed for the Rayleigh-Taylor instability in the presence of ablation and thermal transport. The second harmonic generation efficiency of a single-mode disturbance is computed, as well as the nonlinear correction to the exponential growth of the fundamental modulation. Mode coupling in the spectrum of a multimode disturbance is thoroughly analyzed. The ablative stabilization can be clearly discussed because the derived formulas for the evanescent ablation rate are in agreement with previously known results for incompressible, inviscid, irrotational, and immiscible fluids [S. W. Haan, Phys. Fluids B 3, 2349 (1991)]; M. Berning and A. M. Rubenchik, Phys. Fluids 10, 1564 (1998)]].     

Directly driven Rayleigh—Taylor instability of modulated CH targets

Directly driven ablative Rayleigh-Taylor (R-T) instability of modulated CH targets was studied using the face- on X-ray radiography on the Shen-Guang II device. We obtained temporal evolution images of the R-T instability perturbation. The R-T instability growth factor has been obtained by using the methods of fast Fourier transform and seeking the difference of light intensity between the peak and the valley of the targets. Through comparison with the the theoretical simulation, we found that the experimental data had a good agreement with the theoretical simulation results before 1.8 ns, and was lower than the theoretical simulation results after that.     

光反馈诱发长波长垂直腔面发射激光器低功耗偏振开关

基于扩展的自旋反转模型,对光反馈诱发下长波长垂直腔面发射激光器中的低功耗偏振开关进行了理论研究.研究表明:长波长垂直腔面发射激光器在自由运行下未能获得的偏振开关现象,可以通过引入中等强度的偏振旋转光反馈来实现.对比强弱两种不同的线性色散效应,发现了一些有趣的现象:弱线性色散条件下更易于在低注入电流下获得偏振开关,并且产生偏振开关所需的反馈强度具有更大的调控范围;强色散效应中未能始终获得偏振开关,会出现两模共存区,并且偏振开关出现的注入电流值较高.同时,观察到的偏振模跳变和多偏振开关现象类似于短波长垂直腔面发射激光器,因而证实这两类激光器在偏振开关的本质规律上是相似的.此外,还对长波长垂直腔面发射激光器不易在低注入电流下获得偏振开关的原因进行了分析,并给出了合理的解释.     

高阶FD-WENO格式在数值求解Rayleigh-Taylor不稳定性问题中的应用

用高阶加权本质上无振荡有限差分格式(FD-WENO),求解重力作用下高密度比二维流体界面Rayleigh-Taylor不稳定性问题及激光烧蚀Rayleigh-Taylor不稳定性问题,均获得较为理想的数值结果.