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Dec 1987

Volume 30, Issue 12, pp. 3641-3847

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Feasibility of numerical tracking of material lines and surfaces in chaotic flows

J. G. Franjione and J. M. Ottino

Phys. Fluids 30, 3641 (1987); http://dx.doi.org/10.1063/1.866449 (3 pages) | Cited 29 times

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The evolution of material lines and surfaces is important in mixing involving fast reactions between fluids. It is shown that direct numerical tracking of the interface presents formidable computational problems in even the simplest chaotic flows because of storage requirements. The results indicate the need for a building block approach for the analysis of mixing problems involving chaotic flows.
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47.52.+j Chaos in fluid dynamics
02.60.-x Numerical approximation and analysis

Bifurcations and bursting in two‐dimensional Poiseuille flow

Javier Jiménez

Phys. Fluids 30, 3644 (1987); http://dx.doi.org/10.1063/1.866450 (3 pages) | Cited 15 times

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The bifurcations to two‐dimensional unsteady behavior of a large amplitude equilibrium shear wave train in two‐dimensional Poiseuille flow are studied by direct simulation of the time evolution of the full Navier–Stokes equations. It is found that the wave train becomes unstable at Re∼5600, and sheds a limit cycle which, at higher Re, seems to undergo further transitions to more complex behaviors. It is shown that the site of the original bifurcation is in the neighborhood of the walls and that it shows some characteristics suggestive of the burst generation mechanism in the boundary layer.
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47.27.Cn Transition to turbulence
47.20.Ky Nonlinearity, bifurcation, and symmetry breaking
47.52.+j Chaos in fluid dynamics
47.27.N- Wall-bounded shear flow turbulence

Local flow properties at a viscous free surface

Hans J. Lugt

Phys. Fluids 30, 3647 (1987); http://dx.doi.org/10.1063/1.866451 (6 pages) | Cited 22 times

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A local analysis has been made about a point at the free surface of an incompressible viscous fluid flow at steady state by means of series expansions of the Navier–Stokes equations. Dividing streamlines, curvature effects, and the role of vorticity have been studied. A single dividing streamline is always perpendicular to the free surface. This means, in particular, that all steady vortices with one end of their axes attached to a free surface are perpendicular to it. Double dividing streamlines of two‐dimensional viscous fluid flows at a free surface always have an angle of 90° between them, whereas double dividing streamlines of inviscid irrotational motion on a free or solid surface are separated by an angle of 60°. For an interface between two immiscible viscous fluids a novel ‘‘refraction law’’ for dividing streamlines has been derived.
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47.15.G- Low-Reynolds-number (creeping) flows
47.35.-i Hydrodynamic waves
68.03.Kn Dynamics (capillary waves)
68.05.-n Liquid-liquid interfaces

The return of strongly anisotropic turbulence to isotropy

Norihiko Nakauchi and Hiroshi Oshima

Phys. Fluids 30, 3653 (1987); http://dx.doi.org/10.1063/1.866452 (8 pages) | Cited 6 times

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Energy spectrum equations for homogeneous axisymmetric turbulence are formulated using the eddy‐damped quasinormal Markovian (EDQNM) approximation. The equations, which consist of two spectrum functions of the two variables k (the magnitude of the wavevector k) and μ (the cosine of the angle between k and the direction of axisymmetry), are computed directly, without any reduction, under initial conditions for different degrees of anisotropy. In this paper, the results for cases of medium anisotropic and nearly two‐dimensional initial conditions are reported. A remarkable result was that in the nearly two‐dimensional case, some characteristic properties of two‐dimensional isotropic turbulence, such as inverse energy cascades, are observed. Also, particular attention is paid to the effects of pressure‐strain correlation and inertial energy transfer on the return of anisotropic turbulence to isotropy.
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47.27.Gs Isotropic turbulence; homogeneous turbulence

A comparison of the implementation and performance of the Nanbu and Bird direct simulation Monte Carlo methods

I. D. Boyd and J. P. W. Stark

Phys. Fluids 30, 3661 (1987); http://dx.doi.org/10.1063/1.866402 (8 pages) | Cited 1 time

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Since Bird [Phys. Fluids 6, 1518 (1963)] first proposed the idea of direct simulation Monte Carlo (DSMC) techniques in order to model rarefied gas flows, a concern has been that his method does not provide a solution to the Boltzmann equation. Several simulation schemes have now been developed and, in a review of these methods, Nanbu [J. Phys. Soc. Jpn. 52, 3382 (1983)] showed that only his method [J. Phys. Soc. Jpn. 49, 2042 (1980)] gave a true solution to the Boltzmann equation. However, Nanbu’s simulation method is also the most computationally expensive scheme and in his review paper he recommended the use of the Bird technique for complicated engineering problems. Nanbu’s method also suffers from the apparent disadvantage that momentum and energy are not conserved at each collision. With the advent of the supercomputer the relative expense of the methods will inevitably be of lesser importance so that the testing of the Nanbu method in an engineering context is desirable. A detailed study of the implementation and performance of the Bird and Nanbu simulation schemes has therefore been carried out with reference to the expansion of a gas into vacuum. The results reported herein show, however, that in addition to increased computational overheads, the Nanbu method suffers from further defects that make its implementation more difficult. It is concluded that the Nanbu method is generally less flexible than that of Bird and that greater care must be exercised if meaningful results are to be obtained using the Nanbu approach.
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02.50.Ng Distribution theory and Monte Carlo studies
47.45.-n Rarefied gas dynamics

Strong shocks in an exponential atmosphere

M. Yousaf

Phys. Fluids 30, 3669 (1987); http://dx.doi.org/10.1063/1.866403 (4 pages) | Cited 4 times

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An approximate method for estimating the strength of the overtaking wave developed by Chisnell and the present author [J. Fluid Mech. 120, 523 (1982)] is applied to the motion of a strong shock wave in a medium with exponentially varying density. The strength of the overtaking wave is calculated for various values of γ, the ratio of specific heats. There is a significant improvement in the accuracy of the propagation parameter for most values of γ.
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47.40.Nm Shock wave interactions and shock effects
51.10.+y Kinetic and transport theory of gases

Linear waves and stability in ideal magnetohydrodynamics

Knut S. Eckhoff

Phys. Fluids 30, 3673 (1987); http://dx.doi.org/10.1063/1.866404 (13 pages) | Cited 7 times

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Linear waves superimposed on an arbitrary basic state in ideal magnetohydrodynamics (MHD) are studied by an asymptotic expansion valid for short wavelengths. It has not been necessary to introduce any assumption beyond the usual regularity assumptions on the arbitrarily given solution which represents the basic state in this paper; it may even be time dependent. The theory also allows for a gravitational potential; it may therefore be applied both in astrophysics and in problems related to thermonuclear fusion. The linearized equations for the perturbations of the basic state are found in the form of a symmetric hyperbolic system. This symmetric hyperbolic system is shown to possess characteristics of nonuniform multiplicity, which implies that waves of different types may interact. In particular, it is shown that mass, Alfvén, and slow magnetoacoustic waves will persistently interact in the exceptional case where the local wavenumber vector is perpendicular to the magnetic field. The equations describing this interaction are found in the form of a weakly coupled hyperbolic system. This weakly coupled hyperbolic system is studied in a number of special cases and detailed analytic results are obtained for some such cases. The results show that the interaction of the waves may be one of the major causes of instability of the basic state. It seems beyond doubt that the interacting waves contain the physically relevant parts of the waves, which often are referred to as ballooning modes, including Suydam and Mercier modes.
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52.30.-q Plasma dynamics and flow
97.10.Kc Stellar rotation

Theory of nonmonotonic double layers

K. Y. Kim

Phys. Fluids 30, 3686 (1987); http://dx.doi.org/10.1063/1.866405 (9 pages) | Cited 15 times

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A simple graphic method of solving the Vlasov–Poisson system associated with nonlinear eigenvalue conditions for arbitrary potential structures is presented. A general analytic formulation for nonmonotonic double layers is presented and illustrated with some particular closed form solutions. This class of double layers satisfies the time stationary Vlasov–Poisson system while requiring a Sagdeev potential, which is a double‐valued function of the physical potential. It follows that any distribution function having a density representation as any integer or noninteger power series of potential can never satisfy the nonmonotonic double‐layer boundary conditions. A Korteweg–de Vries‐like equation is found showing a relationship among the speed of the nonmonotonic double layer, its scale length, and its degree of asymmetry.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
02.30.-f Function theory, analysis

Relativistic particle acceleration by obliquely propagating electromagnetic fields

Elena Villalón and William J. Burke

Phys. Fluids 30, 3695 (1987); http://dx.doi.org/10.1063/1.866406 (8 pages) | Cited 8 times

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The relativistic equations of motion are analyzed for charged particles in a magnetized plasma and externally imposed electromagnetic fields (ω, k), which have wave vectors k that are at arbitrary angles. The particle energy is obtained from a set of nonlinear differential equations, as a function of time, initial conditions, and cyclotron harmonic numbers. For a given cyclotron resonance, the energy oscillates in time within the limits of a potential well; stochastic acceleration occurs if the widths of different Hamiltonian potentials overlap. The net energy gain for a given harmonic increase with the angle of propagation, and decreases as the magnitude of the wave magnetic field increases. Potential applications of these results to the acceleration of ionsopheric electrons are presented.
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52.38.Bv Rayleigh scattering; stimulated Brillouin and Raman scattering
52.27.Ny Relativistic plasmas
41.60.-m Radiation by moving charges
94.20.wj Wave/particle interactions

The nonlinear eikonal relation of a weakly inhomogeneous magnetized plasma upon the action of arbitrarily polarized finite wavelength electromagnetic waves

V. Stefan, N. A. Krall, and J. B. McBride

Phys. Fluids 30, 3703 (1987); http://dx.doi.org/10.1063/1.866407 (10 pages) | Cited 8 times

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Based on a hydrodynamic Maxwell formalism of a weakly inhomogeneous magnetized plasma, a mode–mode coupling eikonal relation is derived. Finite wavelength and arbitrary polarization of a monochromatic driver pump have been taken into account as well as longitudinal and transverse components of the self‐consistent plasma electric field. A weak turbulence theory method is used to solve the resulting inhomogeneous Volterra‐type integral equation in tensorial form, i.e., an expansion of hydrodynamic quantities over the resultant electric field (driver pump and self‐consistent field) up to the third order. The eikonal coupling relation thus obtained is discussed for the case of longitudinal and transverse interactions.
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52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Thermodynamic stability analysis of current‐carrying plasmas

R. P. Brinkmann

Phys. Fluids 30, 3713 (1987); http://dx.doi.org/10.1063/1.866408 (11 pages) | Cited 3 times

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A thermodynamic model is developed for plasma systems that are forced to carry an electrical current which prevents relaxation into global thermodynamic equilibrium. Complementary to earlier approaches in the framework of resistive magnetohydrodynamics, the role of nonresistive dissipation is analyzed by excluding momentum transfer between different particle species (electrons and ions). The general class of steady states compatible with the assumptions is found and their structures and symmetries are discussed. The second law of thermodynamics guarantees the existence of a generalized thermodynamic potential (including contributions of the electromagnetic field) which has a negative time derivate for all dynamical states subject to the equilibrium symmetry and boundary conditions. Applying Lyapunov’s theory, this functional provides a necessary and sufficient stability criterion. The linearized version of this criterion and the corresponding eigenvalue problem are also derived.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Kn Thermodynamics of plasmas
52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.
94.30.cq MHD waves, plasma waves, and instabilities

Thermally driven convective cells and tokamak edge turbulence

D. R. Thayer and P. H. Diamond

Phys. Fluids 30, 3724 (1987); http://dx.doi.org/10.1063/1.866409 (11 pages) | Cited 48 times

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A unified theory for the dynamics of thermally driven convective cell turbulence is presented. The cells are excited by the combined effects of radiative cooling and resistivity gradient drive. The model also includes impurity dynamics. Parallel thermal and impurity flows enhanced by turbulent radial diffusion regulate and saturate overlapping cells, even in regimes dominated by thermal instability. Transport coefficients and fluctuation levels characteristic of the saturated turbulence are calculated. It is found that the impurity radiation increases transport coefficients for high density plasmas, while the parallel conduction damping, elevated by radial diffusion, in turn quenches the thermal instability. The enhancement due to radiative cooling provides a resolution to the dilemma of explaining the experimental observation that potential fluctuations exceed density fluctuations in the edge plasma (eΦ/Te >n/n0).
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52.35.Ra Plasma turbulence
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.55.Fa Tokamaks, spherical tokamaks

Theory of trapped‐particle‐induced resistive fluid turbulence

H. Biglari and P. H. Diamond

Phys. Fluids 30, 3735 (1987); http://dx.doi.org/10.1063/1.866410 (10 pages) | Cited 3 times

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A theory of anomalous electron heat transport, evolving from trapped‐particle‐induced resistive interchange modes, is proposed. The latter are a new branch of the resistive interchange‐ballooning family of instabilities, destabilized when the pressure carried by the unfavorably drifting trapped particles is sufficiently large to overcome stabilizing contributions coming from favorable average curvature. Expressions for the turbulent heat diffusivity and anomalous electron thermal conductivity at saturation are derived for two regimes of trapped‐particle energy: (i) a moderately energetic regime, which is ‘‘fluidlike’’ in the sense that the unstable mode grows faster than the time that it takes for particles in this energy range to precess once around the torus, and (ii) a highly energetic regime, where the trapped species has sufficiently high energy as to be able to interact resonantly with the mode. Unlike previous theories of anomalous transport, the estimates of diffusion and transport obtained here are self‐consistent since the trapped particles do not ‘‘see’’ the magnetic flutter due to their rapid bounce motion. The theory is valid for moderate electron‐temperature, high ion‐temperature (auxiliary heated) plasmas and as such, is relevant for present‐ and future‐generation experimental fusion devices.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties
52.35.Ra Plasma turbulence

Damping of electron cyclotron waves in dense plasmas of a compact ignition tokamak

E. Mazzucato, I. Fidone, and G. Granata

Phys. Fluids 30, 3745 (1987); http://dx.doi.org/10.1063/1.866411 (7 pages) | Cited 17 times

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The absorption of electron cyclotron waves in hot and dense plasmas is investigated by using the relativistic dispersion relation with finite Larmor radius (FLR) corrections. It is shown that the strong reduction caused by FLR effects on the damping of extraordinary waves propagating perpendicularly to the magnetic field becomes insignificant at large values of the parallel component of the refractive index. An application of these results to the auxiliary heating of dense plasmas of a compact ignition tokamak [Plasma Physics and Controlled Nuclear Fusion 1986 (IAEA, Vienna, 1987), Vol. 3] is given. It is shown that the extraordinary mode with oblique propagation and frequency of 190 GHz can be strongly absorbed by dense plasmas with a toroidal magnetic field of 105 kG and a central electron density of 1×1015 cm−3.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.25.Mq Dielectric properties
52.27.Ny Relativistic plasmas
52.20.Dq Particle orbits

Rippling mode in tokamaks

A. Hirose and T. L. Kroeker

Phys. Fluids 30, 3752 (1987); http://dx.doi.org/10.1063/1.866412 (5 pages) | Cited 3 times

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It is shown that toroidicity has stabilizing effects on the rippling mode in tokamaks. For typical tokamak edge parameters, the instability as predicted in slab geometry is unlikely to exist.
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52.35.Kt Drift waves
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.55.Fa Tokamaks, spherical tokamaks

Gyrotron simulations without particles

Thomas M. Antonsen and Baruch Levush

Phys. Fluids 30, 3757 (1987); http://dx.doi.org/10.1063/1.866413 (4 pages) | Cited 1 time

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A new method for simulating gyromicrowave devices is suggested. The electron trajectories are represented in a Fourier series in initial gyrophases. Linear theory is reproduced by using three Fourier amplitudes. The nonlinear regime can be described by a relatively small number of amplitudes.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)

Analytical solutions for the growth of oblique waves in a plasma with a field‐aligned beam

Danny Summers and Richard M. Thorne

Phys. Fluids 30, 3761 (1987); http://dx.doi.org/10.1063/1.866414 (6 pages) | Cited 6 times

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The theory of kinetic instabilities in a plasma when the instabilities are driven by wave‐particle resonance is addressed. Recent progress on the linear theory of resonant oblique wave growth in plasmas is extended to plasmas modeled by three standard forms of distribution, a bi‐Maxwellian, a bi‐Lorenzian, and a loss cone, each incorporating a field‐aligned beam. The wave growth rates are shown to be functions of dimensionless integrals that can be expressed in terms of Bessel functions of argument equal to a normalized wave‐normal variable. A marginal stability criterion is obtained, and accordingly we identify a threshold beam velocity for unstable growth of the waves. The simple analytical results derived can be readily applied to a wide variety of problems on oblique wave growth in space plasmas that hitherto were amenable only to extensive computer calculations.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams

Three‐fluid magnetohydrodynamical simulation of plasma focus discharges

K. Behler and H. Bruhns

Phys. Fluids 30, 3767 (1987); http://dx.doi.org/10.1063/1.866524 (10 pages) | Cited 6 times

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A two‐dimensional, three‐fluid code based on the two‐fluid Potter code [Methods in Computational Physics (Academic, New York, 1970), Vol. 9, p. 340] was developed for simulating the plasma focus discharge. With this code it is possible to treat the neutral gas in addition to the plasma components and to model the ionization and recombination phenomena. Thus the sheet dynamics in a plasma focus can be studied and effects investigated such as the occurrence of residual gas (or plasma) density behind the current sheet in the run‐down phase. This is a prerequisite to the occurrence of leak currents, which are one of the causes limiting the performance of large plasma focus devices. It is shown that fast operating foci with small dimensions behave favorably compared with the ‘‘classical’’ Mather focus [Methods of Experimental Physics (Academic, New York, 1971), Vol. 9B, p. 187] with long coaxial electrodes.
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52.65.-y Plasma simulation
52.50.Dg Plasma sources
FREE

A fluid theory of ion collection by probes in strong magnetic fields with plasma flow

I. H. Hutchinson

Phys. Fluids 30, 3777 (1987); http://dx.doi.org/10.1063/1.866415 (5 pages) | Cited 113 times

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A one‐dimensional fluid theory of Langmuir probe operation in strong magnetic fields is presented. Cross‐field diffusion of ions both into and out of the the collection region is consistently accounted for, in effect taking momentum and particle diffusivity to be equal. The results differ by significant factors from previous analyses, which did not account for outward diffusion but in effect set momentum diffusivity to zero. The differences are especially large when parallel flow of the external plasma is present. It is thus clear that the value assumed for the momentum diffusivity strongly affects the interpretation of recent probe measurements. It is argued that the present results offer a more reliable basis for this interpretation.
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52.70.Ds Electric and magnetic measurements
52.30.-q Plasma dynamics and flow

Refueling pellets as a source of charge exchange neutrals for alpha particle measurements

G. Gerdin

Phys. Fluids 30, 3782 (1987); http://dx.doi.org/10.1063/1.866416 (19 pages) | Cited 3 times

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Alpha interactions with neutrals in the ablation cloud of a refueling pellet are considered as a fast alpha diagnostic through charge‐capture radiation (ACCR) by the alphas or their complete neutralization and escape. Calculations of pellet penetration into a reactor plasma indicate that regions of r/a>0.61 could be reached by refueling pellets with initial radii of 4 mm, hence this would be an outer core diagnostic. A spherically symmetric neutral‐gas shielding model (SSMNGSM) in which ionization is treated self‐consistently and governed by local thermodynamic equilibrium is used in these calculations. Collisional processes are found to govern the alpha–cloud interactions and local equilibrium calculations were performed in which the charge exchange and ionization cross sections are used to obtain the state fractions of the alphas in the cloud. At 1640 Å the ratio of ACCR to cloud bremsstrahlung is found to be 10−5 whereas at 304 Å this ratio ranges from 3×10−3 to 0.5 for the cases considered; no neutralized alphas escape the cloud. Magnetic field effects are considered and they appear to reduce the bremsstrahlung from the cloud by a factor of 300 at 304 Å in regions perpendicular to the magnetic field from the pellet’s surface; escape of neutralized alphas may be possible in these directions. Alpha refueling pellet interactions are potentially useful as outer core fast alpha diagnostics and this could be tested in near term experiments.
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52.55.Pi Fusion products effects (e.g., alpha-particles, etc.), fast particle effects
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Asymptotic solutions of steady magneto‐fluid‐dynamic motion between two rotating disks with a small gap

J. J. Xu and J. T. Woo

Phys. Fluids 30, 3801 (1987); http://dx.doi.org/10.1063/1.866417 (8 pages)

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The steady‐state flow of a conducting fluid between two coaxial rotating disks in the presence of an axial magnetic field is considered for the following conditions: (1) the gap d between two disks is very small compared with the radial extension of the disks R; (2) the angular velocity of the disks is not too high, so that the thickness of the Eckman layer δ is still larger than the gap d, (d/δ)1/4≪1; and (3) the magnetic field B is moderate so that the corresponding Hartman number M≪R2/d2. Under these conditions asymptotic solutions to the problem are obtained in terms of the small parameter Ε=d/R. The results show that to the lowest‐order approximation, the electric properties of the disks are not important to the flow field, while the magnitude of the magnetic field plays an important role in the equilibrium flow profile.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.32.Ef Rotating and swirling flows

Diagnosis of mildly relativistic electron velocity distributions by electron cyclotron emission in the Alcator C tokamak

K. Kato and I. H. Hutchinson

Phys. Fluids 30, 3809 (1987); http://dx.doi.org/10.1063/1.866418 (12 pages) | Cited 15 times

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Electron cyclotron emission from nonthermal electron distributions has been measured with a vertical view and control of multiple reflections. The observed intensities at the first few harmonics provide information that can be used to deduce the electron distribution function. Although harmonic overlap restricts the detail that can be obtained when the distribution is very energetic, a fitting procedure enables useful results to be obtained. The distribution functions during Ohmic runaway and lower‐hybrid current drive and heating are presented. The current‐drive distributions are not inconsistent with theoretical Fokker–Planck calculations, giving perpendicular and parallel ‘‘temperatures’’ around 60 keV and 200 keV, respectively. The runaway cases have quite similar distributions.
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52.27.Ny Relativistic plasmas
52.25.Fi Transport properties
52.70.-m Plasma diagnostic techniques and instrumentation

Experimental studies of the dynamics of a linear high‐beta stellarator

E. R. Hedin, M. E. Koepke, and F. L. Ribe

Phys. Fluids 30, 3821 (1987); http://dx.doi.org/10.1063/1.866419 (4 pages) | Cited 1 time

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In a linear l=1 stellarator [Phys. Fluids 30, 2885 (1987)] internal probes are used to measure the structure and temporal behavior of the plasma and magnetic field. At lower densities (and higher temperatures) diffuse‐profile equilibria are produced that are perturbed by a k≊0 oscillation. At higher densities (and lower temperatures) sharp‐boundary plasmas are produced that are unstable with an exponentially growing m=1, k≊0 mode. Finite Larmor radius (FLR) effects are adduced to explain the absence of the instability in the oscillatory regime.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.55.Jd Magnetic mirrors, gas dynamic traps
52.70.-m Plasma diagnostic techniques and instrumentation

Superthermal electron production from hot underdense plasmas

Shridhar Aithal, Pierre Lavigne, Henri Pépin, Tudor Wyatt Johnston, and Kent Estabrook

Phys. Fluids 30, 3825 (1987); http://dx.doi.org/10.1063/1.866420 (7 pages) | Cited 12 times

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Very‐high‐energy electrons of up to an energy of ∼2.3 MeV have been observed to be emitted from the hot underdense exploding thin foil plasmas created by 10.6 μm CO2 laser radiation at intensity levels up to ∼4×1014 W/cm2. As a supplement to the electron measurements the forward and backward scattered light components were also measured. Correlation of these measurements shows that either Raman scattering or the high‐temperature version of two‐plasmon decay or both, manifesting themselves near the quarter‐critical density region, are responsible for the production of a hot (Th∼135 keV) tail of electrons at least up to energies of 1 MeV. There are no indications that the Raman forward scattering (as distinct from Raman backward scattering) at lower densities plays any significant role. These experimental results are consistent with the results from a l 1/2 ‐dimensional particle‐in‐cell code simulation with a parabolic density profile resembling the experimental conditions. An apparent anomaly is discussed, which is that hot electrons are produced (both in experiments and simulations) at energies higher than the trapping value appropriate to electron plasma waves whose phase velocity is equal to the matching value (C/(3)1/2) at the turning point for the light of half the laser frequency.
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52.38.-r Laser-plasma interactions
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Measurement of plasma wave frequency from absolute stimulated Raman scattering near the quarter‐critical surface in a laser plasma

D. M. Villeneuve, J. E. Bernard, and H. A. Baldis

Phys. Fluids 30, 3832 (1987); http://dx.doi.org/10.1063/1.866421 (4 pages) | Cited 5 times

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Thomson scattering techniques were used to measure the frequency of plasma waves near the quarter‐critical surface in a well‐diagnosed plasma irradiated by a nanosecond CO2 laser with intensity ∼1014 W/cm2. The frequency ωp was shown to be less than ω0/2, in disagreement with the commonly used estimate ωp=ω0/2+ (9)/(8) (ve/c)2ω0. The theory of Afeyan and Williams [Phys. Fluids 28, 3397 (1985)] gives better agreement, and shows that the density scale length is more important than the temperature in determining the frequency shift.
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52.38.-r Laser-plasma interactions
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
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