Self-focusing of a laser beam in a strongly ionized plasma

In the presence of a strong Gaussian laser beam, the non-linearity in the dielectric constant of a strongly ionized plasma has been investigated. The non-linearity arises due to the heating and redistribution of the electrons; the loss of electron energy gained from the field has been assumed to be due to thermal conduction. This self-induced non-linearity causes a self-focusing and oscillatory waveguide propagation of the beam even when the non-linear dielectric constant does not fall in the saturating range. In a typical case of a 10¹⁰ W laser, the enhancement of axial intensity by a factor of 25 has been predicted in a length of 0.6 cm.Works supported by N.S.F. (USA).On leave from Malviya Regional Engeeniring College, Jaipur-4, India.     

Steady state self-focusing, self-modulation of laser beam in an inhomogeneous plasma

Laboratory as well as PIC simulation experiments reveal strong flow of energetic electrons co-moving with laser beam in laser plasma interaction. Equation governing the evolution of complex envelope in slowly varying envelope approximation is nonlinear parabolic equation. Variational approach is used to solve this problem and a Lagrangian for the problem is set up. Assuming a trial Gaussian profile, authors solve the reduced Lagrangian problem for beam width and curvature. Two scale lengths for inhomogenity along the direction of propagation, one for nonlinearity and other for diffraction management are introduced. Self-focusing, self-modulation as well as self-trapping of the laser-electron-beam plasma system is studied under variety of parameters.     

Self-focussing of a laser beam in a multiply ionized,absorbing plasma

Using a numerical code we have studied self-focussing of an intense laser beam which is propagating through a multiply-ionized absorbing plasma. The effecient self-focussing threshold has been calculated as a function of various pulse and plasma parameters.     

Steady state dynamics of a moving model cell

Crawling cell motility results due to treadmilling of a polymerized actin network at the leading edge. Steady state dynamics of a moving cell are governed by actin concentration profiles across the cell. Branching of new filaments implicating Arp2/3 and capping of existing filaments with capZ or Gelsolin are central to the robust functioning of the actin network. Using computer simulations, steady state concentration profiles of globular actin (G actin) and filamentous actin (F actin) are computed. The profiles are in agreement with experimentally observed ones. Simulations unveil that there is an optimal capping and branching rate for which the velocity of the model cell is maximum. Our simulations also indicate that the capping of actin filaments results in an increase in nucleation of new filaments by Arp2/3-induced branching and is in agreement with a recently observed monomer gating model. We observe that Arp2/3 and capping protein exhibit a functional antagonism with respect to the actin network treadmilling.     

Steady state response of an infinite beam on a viscoelastic foundation with moving distributed mass and load

Compared with the moving concentrated load model, it is more realistic and proper to use the moving distributed mass and load model to simulate the dynamics of a train moving along a railway track. In the problem of a moving concentrated load, there is only one critical velocity, which divides the load moving velocity into two categories: subcritical and supercritical. The locus of a concentrated load demarcates the space into two parts: the waves in these two domains are called the front and rear waves,respectively. In comparison, in the problem of moving distributed mass and load, there are two critical velocities, which results in three categories of the distributed mass moving velocity. Due to the presence of the distributed mass and load, the space is divided into three domains, in which three different waves exist. Much richer and different variation patterns of wave shapes arise in the problem of the moving distributed mass and load. The mechanisms responsible for these variation patterns are systematically studied. A semi-analytical solution to the steady-state is also obtained, which recovers that of the classical problem of a moving concentrated load when the length of the distributed mass and load approaches zero.     

Simulation of the dynamics of strongly interacting macroparticles in a weakly ionized plasma

The method of molecular dynamics is used to study the dynamic behavior of a nonideal system of particles interacting through screened Coulomb potential. The behavior of the self-diffusion coefficient of particles as a function of the nonideality parameter is investigated. The conditions of the crystallization of such a system are discussed, as well as the possibility of using the crystallization criterion, based on the dynamic characteristics of the system.     

Steady state of photoexcited plasma in semiconductors

The nonequilibrium macroscopic state of a polar semiconductor under constant laser illumination is studied. The values of the relevant steady-state thermodynamic parameters are determined. They are the solutions of generalized transport equations derived using the nonequilibrium statistical operator method. Numerical results are obtained for the case of GaAs.     

Steady state and transient Overhauser effect in systems of three spins

The rate equations describing spin polarization in a system of three spins are derived and solved for the case of a free radical dissolved in a solvent containing two nuclear spins. Triple irradiation experiments indicate that a nuclear spin A can be effectively coupled to an electron spin C via a second nuclear spin B and measurements of both the steady state and transient Overhauser effects are in accord with the theoretical predictions for a three-spin system. The ‘three spin effect’ is found to operate only in dilute solutions of free radicals in which case the probabilities for transitions between different nuclear or electronic energy levels can be determined. It was found to be effective for fluorine nuclei—in the presence of both protons and a free radical and for carbon [13] nuclei in the presence of either protons or fluorine nuclei and a free radical. Detailed measurements have been performed for CHFCl₂, para-difluorobenzene, and meta-fluorotoluene containing the tritertiary butyl phenoxyl radical.     

强激光部分离化等离子体成丝不稳定性

从部分离化等离子体和通常的全离化等离子体的差异是存在束缚电子出发,分析强激光在部分离化等离子体中的传播和折射指数,其束缚电子加强成丝不稳定性的发展.对钕玻璃三倍频激光金靶等离子体的原子成丝不稳定性进行了计算和分析.结果表明强激光部分离化等离子体的原子成丝不稳定性显著高于相对论成丝不稳定性. 关键词： 激光等离子体相互作用 部分离化等离子体 成丝不稳定性     

Self-focusing of elliptically shaped high-power laser beams in a fully ionized plasma

The effect of self-focusing of a high-power light beam with an elliptical cross-section in a strongly ionised plasma has been evaluated. The mechanism of self-focusing considered is non-uniform heating of plasma by an electromagnetic wave having transverse variation of amplitude along its wave front. The heating causes redistribution of carriers which lead to a nonlinear relationship between the electron density and the electric vector and hence to a field-dependent effective dielectric constant. It is found that the beam gets focused at different focal points in different directions exhibiting the effect of astigmatism. There are several critical powers, below thex=0,y=0 planes; at higher powers one dimension focuses while the other defocusses, an oscillatory waveguide is formed in both dimensions. Above the highest critical power both dimensions self-focus, the dimensionless beam width parametersf ₁ andf ₂ forx andy focusing reach different minima corresponding to different self-focusing distances beyond which both dimensions continue diverging. The effect of energy loss through absorption from the beam has also been considered. It has been observed that absorption brings about a reduction in the extent of self-focusing and favours defocusing of the beam.     

Quantum electron self-interaction in a strong laser field

The quantum state of an electron in a strong laser field is altered if the interaction of the electron with its own electromagnetic field is taken into account. Starting from the Schwinger-Dirac equation, we determine the states of an electron in a plane-wave field with inclusion, at leading order, of its electromagnetic self-interaction. On the one hand, the electron states show a pure quantum contribution to the electron quasimomentum, conceptually different from the conventional classical one arising from the quiver motion of the electron. On the other hand, the electron self-interaction induces a distinct dynamics of the electron spin, whose effects are shown to be measurable in principle with available technology.     

Rydberg matter as a metastable state of strongly nonideal plasma

The phase diagram of cesium is complemented by an isolated region of metastable states of a strongly nonideal plasma.     

Steady state ion acceleration by a circularly polarized laser pulse

The steady state ion acceleration at the front of a cold solid target by a circularly polarized flat-top laser pulse is studied with one-dimensional particle-in-cell (PIC) simulation. A model that ions are reflected by a steady laser-driven piston is used by comparing with the electrostatic shock acceleration. A stable profile with a double-flat-top structure in phase space forms after ions enter the undisturbed region of the target with a constant velocity.     

Transition radiation of modulated electron beam in strongly inhomogeneous plasma

A formula for the transition radiation of a modulated electron bean in a strongly inhomogeneous plasma is derived in a given-current approximation. Nonmonotonic variation of the transition-radiation power is detected when the inhomogeneity parameter for the beam emerging from the plasma is varied. The analytic results are confirmed by numerical calculations.Kiev State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 2, pp. 194–199, February, 1994.     

Growth and stabilization of a laser ripple on laser beam in a collisionless unmagnetized plasma

Summary In this paper we have studied the growth and stabilization of a radially symmetric ripple superimposed on a laser beam in a collisionless unmagnetized plasma. The saturation of the growth rate is due to pump depletion effect. On acoount of the nonuniform intensity distribution of the main beam, a d.c. component of the ponderomotive force becomes finite and it redistributes the carriers in the plasma in a plane transverse to the beam propagation. We have set up and solved the wave equations for main beam and ripple by using the WKB and paraxial ray approximations. As the ripple grows in the plasma, it draws energy from the background laser field. It is seen that the relative intensity of the laser ripple becomes critical due to the counteracting behaviour of the self-focusing and depletion of the main beam. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

激光在等离子体中的坍塌行径

从椭圆偏振激光场波包所满足的非线性控制方程出发,利用场论方法,构造该非线性方程的拉格朗日密度函数,得到激光场波包的能量集中具有准粒子特性,并给出守恒的粒子数、动量和能量。讨论了调制不稳定发展后期的波包坍塌动力学问题,利用归一化的粒子数密度分布作为权重,分析了激光场的波包。结果表明,激光场波包的尺度在有限的时间进程中,将塌缩到一个很小的值,证明了激光场具有塌缩行径。