Visualizing color plasma instabilities

I discuss recent advances in the understanding of non-equilibrium gauge field dynamics in plasmas which have particle distributions which are locally anisotropic in momentum space. In contrast to locally isotropic plasmas such anisotropic plasmas have a spectrum of soft unstable modes which are characterized by exponential growth of transverse (chromo)-magnetic fields at short times. The long-time behavior of such instabilities depends on whether or not the gauge group is Abelian or non-Abelian. I will report on recent numerical simulations which attempt to determine the long-time behavior of an anisotropic non-Abelian plasma within hard-loop effective theory. For novelty I will present an interesting method for visualizing the time dependence of SU(2) gauge field configurations produced during our numerical simulations.     

Nature of mechanical instabilities and their effect on kinetic friction

It has long been recognized that kinetic friction F(k) between two solids must be due to instabilities, sudden "pops" of certain degrees of freedom. Here, such pops are studied with a focus on boundary lubrication. The pops' characteristics and consequently the friction-velocity relationship depend qualitatively on dimensionality, commensurability, and details of the lubricant wall interaction. It is found that F(k) should be small between commensurate surfaces. F(k) is large for incommensurate surfaces, unless the lubricant's motion is confined to 1D. The effects of thermal noise are discussed and computer simulations are employed to show the relevance of the predictions to less idealized models.     

Z箍缩等离子体不稳定性的数值研究

利用二维辐射磁流体程序模拟了钨丝阵Z箍缩等离子体腊肠不稳定性的演化：分析了存在不稳定性与没有不稳定性条件下等离子体内爆的差异；研究了不同初始密度扰动对x射线输出 功率和能量的影响.数值结果表明，在一定初始密度扰动范围内，这种不稳定性对x射线输 出的总能量影响不显著，但对x射线功率随时间变化的曲线有明显的影响.此外，还探讨了Z 箍缩等离子体数值模拟中低密度区的数值处理对数值结果的影响. 关键词： Z箍缩 腊肠不稳定性 数值模拟     

Hard-loop dynamics of non-Abelian plasma instabilities

Non-Abelian plasma instabilities may be responsible for the fast apparent quark-gluon thermalization in relativistic heavy-ion collisions if their exponential growth is not hindered by nonlinearities. We study numerically the real-time evolution of instabilities in an anisotropic non-Abelian plasma with an SU(2) gauge group in the hard-loop approximation. We find exponential growth of non-Abelian plasma instabilities both in the linear and in the strongly nonlinear regimes, with only a brief phase of subexponential behavior in between.     

激光等离子体不稳定性及其抑制方案研究

受激拉曼散射、受激布里渊散射等激光等离子体不稳定性（LPI）是激光等离子体物理领域最重要的研究课题之一。特别是在激光驱动的惯性约束聚变中,LPI会造成相当份额的激光能量损失,破坏辐射对称性,产生的超热电子还会预热靶丸,进而影响压缩效率和聚变能量增益。近期,在美国国家点火装置上开展的实验表明对LPI物理过程的充分理解和有效控制对成功实现ICF点火至关重要。我们对近期LPI方面的一系列研究进展进行了简单介绍与讨论。首先,回顾了描述LPI过程的三波耦合理论,由此得出了LPI在线性阶段的增长率。接着讨论了一些复杂情景下的LPI物理过程,譬如LPI的非线性发展阶段、级联LPI、多光束LPI以及LPI间的非线性耦合。最后,着重介绍了一系列抑制LPI的技术方案,包括束匀滑技术、光束时域整形、宽带激光、偏振旋转激光以及外加磁场等。     

Nonabelian plasma instabilities in Bjorken expansion

Plasma instabilities are parametrically the dominant nonequilibrium dynamics of a weakly coupled quark-gluon plasma. In recent years the time evolution of the corresponding collective colour fields has been studied in stationary anisotropic situations. Here I report on recent numerical results on the time evolution of the most unstable modes in a longitudinally expanding plasma as they grow from small rapidity fluctuations to amplitudes where non-Abelian self-interactions become important.     

Cyclotron instabilities of a two-component electron plasma

A study is made of instability at frequencies close to the electron cyclotron frequency _B and its multiples, subject to the presence of two different groups of electrons. It is shown that a mixture of hot and cold electrons ( _ph ² _pc ² ) in the region of frequencies s _B, s2 can be unstable with respect to waves of the flute type (k _z=0) with maximum increment _max ( _ph/_pc). _B, if there exists an interval of transversal velocities in whichF/ >0. When the curvature of the magnetic field is taken into account, even waves with _B can be unstable in such a plasma. The effect of spatial inhomogeneity of the hot component on flute-type instability and on two-beam cyclotron instability is also examined.The author extends his thanks to A. B. Mikhajlovskij for his valuable comments and discussions.     

Low-frequency current instabilities in nonisothermal magnetoactive plasma

The excitation of current instabilities in the lower ionosphere is studied for magnetic-field-aligned inhomogeneities of arbitrary length with allowance for electron-temperature perturbations and the dependence of electron collision frequency on electron temperature. It is found that the threshold velocity of Farley-Buneman instability has a minimum for a finite degree of inhomogeneity elongation when the temperature dependence of the electron collision frequency is sufficiently strong.Scientific-Research Radio-Physics Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 1, pp. 25–36, January, 1993.     

Apparent thermalization due to plasma instabilities in the quark-gluon plasma

Hydrodynamical modeling of heavy-ion collisions at RHIC suggests that the quark-gluon plasma (QGP) "thermalizes" in a remarkably short time scale, about 0.6 fm/c. We argue that this should be viewed as indicating fast isotropization, but not necessarily complete thermalization, of the nonequilibrium QGP. Non-Abelian plasma instabilities can drive local isotropization of an anisotropic QGP on a time scale which is faster than ordinary perturbative scattering processes. As a result, we argue that theoretical expectations based on weak-coupling analysis are not necessarily in conflict with hydrodynamic modeling of the early part of RHIC collisions, provided one recognizes the key role of non-Abelian plasma instabilities.     

Microwave imaging of magnetohydrodynamic instabilities in fusion plasma

Microwave imaging diagnostics are extremely useful for observing magnetohydrodynamic (MHD) instabilities in magnetic fusion plasmas. Two imaging diagnostics will be available on the WEST tokamak. A method was developed to reconstruct electron density maps from electron density profiles measured by ultrafast reflectometry, a technique based on FM-CW radar principle. It relies on plasma rotation to perform 2D reconstruction. An Electron Cyclotron Emission Imaging (ECEI) diagnostic will image directly the temperature fluctuations. It will be equivalent to 24 stacked vertically radiometers, each probing a spot of few centimetres. These two complementary techniques will contribute to the validation of MHD models.     

Anisotropic N=4 super-Yang-Mills plasma and its instabilities

We present a type-IIB supergravity solution dual to a spatially anisotropic finite-temperature N=4 super-Yang-Mills plasma. The solution is static and completely regular. The full geometry can be viewed as a renormalization group flow from an ultraviolet anti-de Sitter geometry to an infrared Lifshitz-like geometry. The anisotropy can be equivalently understood as resulting from a position-dependent θ?term or from a nonzero number density of dissolved D7-branes. The holographic stress tensor is conserved and anisotropic. The presence of a conformal anomaly plays an important role in the thermodynamics. The phase diagram exhibits homogeneous and inhomogeneous (i.e., mixed) phases. In some regions the homogeneous phase displays instabilities reminiscent of those of weakly coupled plasmas. We comment on similarities with QCD at finite baryon density and with the phenomenon of cavitation.     

Decay and modulation instabilities of beat waves in a plasma

Summary A large-amplitude beat wave excited at the beat frequency of two copropagating laser beams is extremely unstable and suffers strong three- and fourwave parametric instabilities with large growth rates. The relativistic effects are seen to be negligible on both the decay and modulation instabilities. However, the growth rates of these instabilities are sensitive functions of the plasma parameters. The authors of this paper have agreed to not receive the proofs for correction.     

Laser pulse modulation instabilities in partially stripped plasma

The laser pulse modulation instabilities in partially stripped plasma were discussed based on the phase and group velocities of the laser pulse and the two processes that modulation instabilities excited. The excitation condition and growth rate of the modulation instability were obtained. It was found that the positive chirp and competition between normal and abnormal dispersions play important roles in the modulation instability. In the partially stripped plasma, the increased positive chirp enhances the modulation instability, and the dispersion competition reduces it.     

Nonlinear simulations of combustion instabilities with a quasi-1D Navier-Stokes code

As lean premixed combustion systems are more susceptible to combustion instabilities than non-premixed systems, there is an increasing demand for improved numerical design tools that can predict the occurrence of combustion instabilities with high accuracy. The inherent nonlinearities in combustion instabilities can be of crucial importance, and we here propose an approach in which the one-dimensional (1D) Navier-Stokes and scalar transport equations are solved for geometries of variable cross-section. The focus is on attached flames, and for this purpose a new phenomenological model for the unsteady heat release from a flame front is introduced. In the attached flame method (AFM) the heat release occurs over the full length of the flame. The nonlinear code with the use of the AFM approach is validated against analytical results and against an experimental study of thermoacoustic instabilities in oxy-fuel flames by Ditaranto and Hals [Combustion and Flame 146 (2006) 493-512]. The numerical simulations are in accordance with the experimental measurements and the analytical results and both the frequencies and the amplitudes of the resonant acoustic pressure modes are reproduced with good accuracy.