Generation of quasistatic magnetic fields in the interaction of counterpropagating laser pulses in a low-density plasma

The effect of generation of quasistatic magnetic fields in the interaction of counterpropagating moderate-intensity laser pulses in a low-density plasma is considered. It is shown that the onset of strong magnetic fields is attributed to small-scale large-amplitude plasma waves excited in the region of interaction of laser pulses under the action of the averaged ponderomotive forces. The spatial distribution of quasistatic magnetic fields is investigated, as well as the structure of the magnetic-field and current lines.     

Nonlinear scattering of two counterpropagating laser pulses in an underdense plasma

A study is made of the interaction (“collision”) of two laser pulses with the same frequency but different durations, propagating toward one another in a low-density plasma. It is found that, in the interaction region, the excitation of small-scale plasma fields localized within a distance on the order of the length of the longer pulse is accompanied by the backscattering of each of the pulses. The frequency shift of the backscattered radiation and its duration depend strongly on the lengths of the interacting pulses. It is shown that the spectrum of the long backscattered radiation “tail” that arises behind the shorter pulse as a result of its interaction with the longer pulse contains satellites shifted from the laser frequency by the plasma frequency.     

Generation of terahertz radiation from a low-density plasma slab irradiated by a laser pulse

The generation of terahertz electromagnetic radiation when a laser pulse propagates through a low-density plasma slab is considered. It is shown that terahertz waves are excited because of the growth of a weakly damped, antisymmetric leaking mode of the plasma slab. The spectral, angular, and energy parameters of the terahertz radiation are investigated, as well as the spatiotemporal structure of the emitted waves. It is demonstrated that terahertz electromagnetic wave fields are generated most efficiently when the pulse length is comparable to the slab thickness.     

Excitation of magnetic fields by a circularly polarized laser pulse in a plasma channel

The excitation of quasistatic magnetic fields by a circularly polarized laser pulse in a plasma channel is considered. It is shown that, to second order in the amplitude of the electric field of the laser pulse, circular rotation of the plane of polarization of the laser radiation in a radially nonuniform plasma gives rise to a nonlinear azimuthal current and leads to the excitation of the radial and axial components of the magnetic field. The dependence of the magnetic field distribution over the plasma channel on the spatial dimensions of the pulse and on the channel width is investigated for a moderate-power laser pulse. The structure of the magnetic fields excited by a relativistic laser pulse in a wide plasma channel is analyzed.     

Generation and interaction of intense counterpropagating plasma flows

Results are presented from experimental studies of the parameters of two counterpropagating (colliding) plasma flows generated by discharges in crossed electric and magnetic fields. It is shown that the conversion efficiency of the energy deposited in the discharges into the energy of directed plasma flows is 0.3–0.6. For discharge current pulses with a duration of ∼10 μs, the energy flux density in the plasma flow reaches ∼10 J/cm² and the total energy of the flow is on the order of 300 J. The density of deuterons in the flows is ∼10¹⁵ cm⁻³, and the flow velocity is ≤2×10⁷ cm/s. The total number of particles carried by the flows is about 10¹⁹. The possibility of using counterpropagating plasma flows to study reactions involving light nuclei (dd, pd, dt, and dHe reactions) in the range of ultralow collision energies is discussed. __________ Translated from Fizika Plazmy, Vol. 29, No. 8, 2003, pp. 714–721. Original Russian Text Copyright ? 2003 by Dudkin, Nechaev, Padalko, Bystritsky, Stolupin, Bystritskii, Voznyak.     

Spectra from stimulated Raman scattering of short relativistically strong laser pulses in an underdense plasma

A set of equations describing large-angle stimulated Raman scattering (SRS) of a short, relativistically strong laser pulse propagating in an underdense plasma is derived and investigated numerically. It is shown that the SRS spectrum depends strongly on the pulse shape. If a pulse with a sharp leading edge excites a strongly nonlinear wake wave, the scattering occurs in relativistic electron flows and is accompanied by the Doppler frequency shift. When the electron flow is directed oppositely to the pulse propagation direction, the frequency upshift is maximum for the direct-backward SRS and decreases with decreasing scattering angle.     

Mechanism of action of moderate-intensity static magnetic fields on biological systems

There is substantial evidence indicating that moderate-intensity static magnetic fields (SMF) are capable of influencing a number of biological systems, particularly those whose function is closely linked to the properties of membrane channels. Most of the reported moderate SMF effects may be explained on the basis of alterations in membrane calcium ion flux. The mechanism suggested to explain these effects is based on the diamagnetic anisitropic properties of membrane phospholipids. It is proposed that reorientation of these molecules during moderate SMF exposure will result in the deformation of imbedded ion channels, thereby altering their activation kinetics. Channel inactivation would not be expected to be influenced by these fields because this mechanism is not located within the intramembraneous portion of the channel. Patch-clamp studies of calcium channels have provided support for this hypothesis, as well as demonstrating a temperature dependency that is understandable on the basis of the membrane thermotropic phase transition. Additional studies have demonstrated that sodium channels are similarly affected by SMFs, although to a lesser degree. These findings support the view that moderate SMF effects on biological membranes represent a general phenomenon, with some channels being more susceptible than others to membrane deformation.     

Electron-Ion collisions in relativistically strong laser fields

Electron-ion collisions in relativistically strong electromagnetic fields are considered. Analytical and numerical analyses both show that all qualitative effects characteristic of collisions in nonrelativistic strong fields [1–3] occur at relativistic intensities of an electromagnetic wave as well. Expressions for Joule plasma heating and for the energy distributions of fast particles are derived from simple analytic considerations and are confirmed by numerical simulations. It is found, in particular, that, due to the relativistic increase in the mass of a scattered electron, Joule heating in ultrarelativistic fields becomes more intense as the field amplitude grows.     

Breaking of a wake wave excited by a narrow laser pulse in a low-density plasma

The spatial structure of a wake wave excited in a low-density plasma by a laser pulse with a small focal spot radius is studied both analytically and numerically. Numerical study shows that, in a small-amplitude laser field, a wake wave breaks after the formation of an off-axis density maximum, which grows in height away from the pulse to become infinitely high after several periods. Analytical and numerical calculations show that the singularity in the density arises from the intersection of the trajectories of neighboring particles. Numerical simulations demonstrate that, as the laser field amplitude increases, the breaking point of the wake wave rapidly approaches the pulse trailing edge. For weakly nonlinear conditions, an analytic dependence of the coordinate of the breaking point on the amplitude and transverse size of the laser pulse is obtained.     

Three-body collisions in a plasma in a strong laser field

Electron-ion collisions in a high-density plasma in strong electromagnetic fields are considered. The applicability condition for the approximate model of pair collisions in strong fields are determined. It is shown that this condition is identical to the condition for the plasma to be transparent. Investigations were carried out by the test particle method generalized to the case of several scattering centers. An accurate calculation of short-range collisions is provided by a “jump” method that is based on the exact solution to the problem of the motion of a particle in a Coulomb potential. This method can also be applied in other approaches to simulating a collisional plasma (such as particle-in-cell and molecular dynamics methods).     

Dynamics of a nonquasineutral plasma in a strong magnetic field

The motion of a nonquasineutral plasma in a strong magnetic field such that is analyzed. It is shown in simple examples that, when the plasma pressure and dissipation are neglected, the only dynamic process in a magnetized plasma is the evolution of the charge-separation electric field and the related magnetic field flux. The equations derived to describe this evolution are essentially the wave Grad-Shafranov equations. The solution to these equations implies that, in a turbulent Z-pinch, a steady state can exist in which the current at a supercritical level is concentrated near the pinch axis.     

Orientation of glutaraldehyde-fixed erythrocytes in strong static magnetic fields

In a uniform static magnetic field up to 8 Telsa, glutaraldehyde-fixed erythrocytes showed an orientation in which their disk plane was perpendicular to the magnetic field. The paramagnetism of membrane-bound hemoglobin was thought to contribute significantly to this orientation. The observation of magnetic orientation is directed toward understanding the fundamental microstructural aspects of the erythrocyte. © 1996 Wiley-Liss, Inc.     

Orientation of skeletal muscle actin in strong magnetic fields

Measurement of birefringence is used to follow actin filament and paracrystal formation in a strong magnetic field. Both F-actin and paracrystals orientate parallel to the field. This confirms that globular proteins arranged in filamentous assemblies can orientate in magnetic fields. This is consistent with the alpha-helical component of the actin subunits being approximately aligned along the actin filament.     

Steady-state vortex structure in a plasma with a strong magnetic field

A study is made of nonquasineutral vortex structures in a plasma with a magnetic field B _z in which the charges separate on a spatial scale equal to the magnetic Debye radius r _B=B _z/4πen _e. The electric field arising due to charge separation leads to radial expansion of the ions, thereby destroying the initial electron vortex. It is shown that the ion pressure gradient stops ion expansion in a nonquasineutral electron vortex and gives rise to a steady structure with a characteristic scale on the order of r _B. With the electron inertia taken into account in the hydrodynamic approximation, the magnetic vortex structure in a hot plas mamanifests itself in the appearance of a “hole” in the plasma density.     

Efficiency of ion acceleration by a relativistically strong laser pulse in an underdense plasma

A particle-in-cell simulation is used to investigate ion acceleration by a femtosecond laser pulse propagating in an underdense plasma slab. In plasma slabs with different thicknesses, the ions are found to be accelerated by different mechanisms. It is shown that, for laser pulse intensities in the range (5–10)×10¹⁹ W/cm², the ions are accelerated near the plasma-vacuum interface. __________ Translated from Fizika Plazmy, Vol. 27, No. 3, 2001, pp. 225–234. Original Russian Text Copyright ￠ 2001 by Kuznetsov, Esirkepov, Kamenets, Bulanov.     

Large-angle stimulated Raman scattering of short laser pulses in plasma

The features of the large-angle stimulated Raman scattering of short laser pulses in a homogeneous underdense plasma are studied analytically. It is found that, for scattering angles that are not too close to zero, a steady-state regime of the convective amplification of unstable waves is established in the frame of reference comoving with the laser pulse. The problem of convective amplification in a two-dimensional region is solved in both weak-and strong-coupling regimes. It is shown that the steady-state envelopes of the scattered radiation and scattering plasma waves are two-dimensional in nature. It is found that, for a given scattering angle, the maximum possible spatial amplification at the trailing edge of the pulse is achieved if the ratio of the transverse to longitudinal size of the pulse is larger than the cotangent of one-half of the scattering angle.     

Generation of high-voltage pulses in a self-oscillating discharge

The possibility of generation of high-voltage pulses in the self-oscillation mode of a beam-plasma discharge is shown. Negative voltage pulses with amplitudes of up to 40 kV are recorded at the plasma facing electrode at a constant negative bias voltage of 200–300 V.     

Analysis of quasistatic MHD perturbations and stray magnetic fields in a tokamak plasma

Mechanisms for the development of quasistatic MHD perturbations in a viscous rotating tokamak plasma are considered. The influence of stray magnetic fields on the stability of MHD modes in the plasma of the TFTR tokamak is analyzed.     

Simulation of the excitation of quasi-plane wake waves in a plasma by a resonant sequence of laser pulses with a variable envelope

Results are presented from full-scale numerical simulations of the excitation of wake waves by a sequence of weakly relativistic laser pulses in a subcritical plasma. Computations were carried out with a 2D3V version of the SUR-CA code that is based on the local-recursive nonlocal-asynchronous algorithm of the particle-in-cell method. The parameters of a train of laser pulses were chosen to correspond to the resonant excitation of the wake field. The curvature of the envelope of the pulses was chosen to depend on the number of the pulse in the train. Numerical simulations showed that there are plane waves during the first period of the plasma wave behind the pulse train.