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Dec 1983

Volume 26, Issue 12, pp. 3445-3659

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Circular waves on a stationary disk in rotating flow

Ö. Savaş

Phys. Fluids 26, 3445 (1983); http://dx.doi.org/10.1063/1.864124 (4 pages) | Cited 15 times

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Circular waves of a new type are observed in the disk boundary layer occurring on the end of a cylindrical cavity during impulsive spindown to rest. These waves occur around z(Ωi/ν)1/2≊3.6 and are reminiscent of Tollmien–Schlichting waves of an ordinary boundary layer. It is concluded that these waves are manifestations of the instabilities excited in the Bödewadt‐type boundary layer developed over the disk.
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47.35.-i Hydrodynamic waves
47.32.Ef Rotating and swirling flows
47.15.Fe Stability of laminar flows
47.20.-k Flow instabilities

A Lagrangian two‐time probability density function equation for inhomogeneous turbulent flows

S. B. Pope

Phys. Fluids 26, 3448 (1983); http://dx.doi.org/10.1063/1.864125 (3 pages) | Cited 22 times

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An exact equation for the Lagrangian two‐time velocity joint probability density function (pdf) is derived from the Navier–Stokes equation. The pdf equation contains as an unknown the conditional expectation of the fluid acceleration. A linear Markov model is proposed which leads to a modeled equation that is consistent both with Kolmogorov’s theory in the inertial subrange and with Reynolds‐stress models. The dissipation rate is obtained from the joint pdf in a way that is consistent with the modeled dissipation equation. A Monte Carlo method can be used to solve the modeled two‐time pdf equation for inhomogeneous turbulent flows.
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47.27.-i Turbulent flows
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion

Breaking of liquid films and threads

Joseph B. Keller

Phys. Fluids 26, 3451 (1983); http://dx.doi.org/10.1063/1.864126 (3 pages) | Cited 21 times

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Breaking of nonuniform liquid films and threads is analyzed by means of a simple equation of motion. The results are applied to the bursting film between two merging gas bubbles in a liquid, to growing holes in liquid films on solid surfaces, and to the pinching off of a liquid jet. Two of the results are compared with exact hydrodynamic calculations.
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68.15.+e Liquid thin films
68.03.Cd Surface tension and related phenomena
47.10.-g General theory in fluid dynamics

Vortex shedding from a spinning cylinder

F. Díaz, J. Gavaldà, J. G. Kawall, J. F. Keffer, and F. Giralt

Phys. Fluids 26, 3454 (1983); http://dx.doi.org/10.1063/1.864127 (7 pages) | Cited 26 times

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An experimental investigation of a two‐dimensional turbulent wake behind a spinning cylinder at Re=9000 is carried out to determine the influence of the rotation on the initial development of the flow. Spectral analysis of the velocity data measured in the near wake shows that for peripheral velocities up to the value of the free‐stream velocity, a distinct Kármán vortex activity exists within the wake, whereas for greater peripheral velocities, the Kármán activity deteriorates and disappears for values in excess of twice the free‐stream velocity.
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47.27.wb Turbulent wakes
47.32.Ef Rotating and swirling flows

Equilibrium statistics of two‐dimensional viscous flows with arbitrary random forcing

Philip Duncan Thompson

Phys. Fluids 26, 3461 (1983); http://dx.doi.org/10.1063/1.864128 (10 pages) | Cited 3 times

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A method to solve the Liouville equation for an ensemble of two‐dimensional viscous flows that are driven by random forcing with arbitrary statistics is outlined. By appropriate transformations of both the dependent variable (probability distribution) and independent variables, and by expansion of the solution in eigenfunctions of the separable part of the Liouville operator, it is found possible to reduce the problem to that of solving a simultaneous system of nonhomogeneous linear algebraic equations. The equilibrium kinetic energy and energy‐transfer spectra can be calculated directly from the equilibrium probability distribution.
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51.10.+y Kinetic and transport theory of gases
05.20.Dd Kinetic theory
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion

Shock‐wave series for real fluids

Philip A. Thompson

Phys. Fluids 26, 3471 (1983); http://dx.doi.org/10.1063/1.864129 (4 pages) | Cited 3 times

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Power series based on the Rankine–Hugoniot equation relate the states upstream and downstream of a shock. The usual forms for such series are limited to very weak shocks, e.g., to a shock Mach number approximately 1.1. Improvement is obtained by restricting the shock series to a form which is exact, for the special case of a perfect gas, with the lowest‐order term only. Series in this form show reasonable accuracy for real fluids up to shock Mach numbers of order 1.5.
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47.40.-x Compressible flows; shock waves
02.30.Lt Sequences, series, and summability
05.70.Ce Thermodynamic functions and equations of state

Effects of the relativistic correction to the resonance condition on electron cyclotron current drive

R. A. Cairns, J. Owen, and C. N. Lashmore‐Davies

Phys. Fluids 26, 3475 (1983); http://dx.doi.org/10.1063/1.864130 (7 pages) | Cited 27 times

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The effect on electron cyclotron current drive of the relativistic correction arising from the velocity dependence of the electron cyclotron frequency has been investigated. This correction is shown to be important, particularly for waves propagating close to perpendicular to the magnetic field, for plasmas in the keV range and above. The steady‐state current due to the asymmetric heating mechanism described by Fisch and Boozer has been calculated. The most striking effect of the relativistic correction is that the current driven in the plasma in response to a given wave amplitude is, in an inhomogeneous field, no longer antisymmetric about the cyclotron resonance layer. This means that it is possible to drive a nonzero total current without strong absorption, and also that the total current depends strongly on whether the wave is incident from the low‐field or the high‐field side.
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52.50.Gj Plasma heating by particle beams
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.55.Fa Tokamaks, spherical tokamaks
52.55.Hc Stellarators, torsatrons, heliacs, bumpy tori, and other toroidal confinement devices

Quasilinear theory of current‐driven ion‐acoustic turbulence in a magnetized collisional plasma

A. S. Sakharov and S. Kuhn

Phys. Fluids 26, 3482 (1983); http://dx.doi.org/10.1063/1.864131 (6 pages)

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The stationary spectra of current‐driven ion‐acoustic turbulence in a magnetized plasma (with the electron drift velocity vd along B0) are studied within the framework of quasilinear theory. In particular, a plasma with strongly magnetized electrons (ωce>kdvTe) and unmagnetized ions (ω>ωci) well above the threshold of instability (vd/vTe≫vs/vTe, νin/ωpi) is considered. It is found that the waves are mainly excited in a narrow angular region almost perpendicular to vd (i.e., cos θ≂vs/vd≪1), which is in agreement with experimental results by Gekelman and Stenzel [Phys. Fluids 21, 2014 (1978)] and Batanov et al.
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52.35.Ra Plasma turbulence
52.35.Dm Sound waves
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Fi Transport properties

Ponderomotive effects in nonneutral plasmas

B. M. Lamb and G. J. Morales

Phys. Fluids 26, 3488 (1983); http://dx.doi.org/10.1063/1.864132 (9 pages) | Cited 5 times

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The ponderomotive effects which arise in a single species plasma, i.e., a nonneutral plasma, are considered. The important difference from a neutral plasma is that quasineutral density cavities given by δn/n0≂−‖E‖2/16πn0T cannot arise in a single species plasma. Instead, it is found that the ponderomotive force is balanced by self‐consistent space charge fields, and results in δn≂∇2‖E∥‖2/16πmω2, where ω is the frequency and E∥ refers to the wave electric field parallel to the confining magnetic field. In addition to the density rearrangement, the zero order E×B rotation is modified. The self‐consistent nonlinear state has been found for a large‐amplitude l=0 axially standing Gould–Trivelpiece wave. The linear dispersion relation is modified by the presence of the large‐amplitude wave and results in a nonlinear frequency shift. This shift produces an interesting hysteresis effect in the nonlinear resonant response of the plasma when the frequency of the external driver is swept slowly.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Mq Dielectric properties

Synchrotron emission from runaway electron distributions

D. Winske, Th. Peter, and D. A. Boyd

Phys. Fluids 26, 3497 (1983); http://dx.doi.org/10.1063/1.864110 (11 pages) | Cited 11 times

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Synchrotron emission from a relativistic anti‐loss‐cone (runaway) distribution is investigated numerically and compared with various analytical approximations. The results are applied to recent measurements of enhanced emission during current‐drive experiments on the Princeton Large Torus (PLT) as well as to impulsive solar microwave bursts.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.25.Fi Transport properties

Equilibrium statistical mechanics treatment of a ‘‘modified’’ two‐dimensional guiding center plasma

R. Calinon and D. Merlini

Phys. Fluids 26, 3508 (1983); http://dx.doi.org/10.1063/1.864111 (7 pages)

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Equilibrium properties of a two‐dimensional guiding center plasma are reinvestigated in detail by means of the BBGKY hierarchy. A new equation of state is presented and compared with the results of the Monte Carlo simulation and the correlation energy is computed. A heuristic generalization of the the dispersion relation is investigated by means of different truncation schemes of the hierarchy. Analytical and numerical results support the appearance of periodic states for a neutral system in an unbounded domain at a moderately high value of the coupling parameter γc∼8.
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52.40.Kh Plasma sheaths
52.25.Kn Thermodynamics of plasmas
05.20.-y Classical statistical mechanics

An electromagnetic integral equation: Application to microtearing modes

R. Farengo, Y. C. Lee, and P. N. Guzdar

Phys. Fluids 26, 3515 (1983); http://dx.doi.org/10.1063/1.864112 (9 pages) | Cited 21 times

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The integral equation technique previously developed for electrostatic drift waves to study low‐frequency electromagnetic perturbation is extended. When σe≫σi (as is the case for tearing modes) the problem can be reduced to the simultaneous solution of an integral and a differential equation. Using a Fourier representation for ϕ(x), a differential equation is derived from Ampere’s law for a modified Green’s function that contains the magnetic effects. This equation is solved simultaneously with an integral equation (corresponding to the quasineutrality condition in k space) to obtain the eigenvalues and corresponding eigenfunctions. When applied to the study of microtearing modes this method gave, for the same values of the parameters, larger growth rates than those of the usual differential approximation.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
02.30.Rz Integral equations

Nonlinear gyrokinetic equations

Daniel H. E. Dubin, John A. Krommes, C. Oberman, and W. W. Lee

Phys. Fluids 26, 3524 (1983); http://dx.doi.org/10.1063/1.864113 (12 pages) | Cited 176 times

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Nonlinear gyrokinetic equations are derived from a systematic Hamiltonian theory. The derivation employs Lie transforms and a noncanonical perturbation theory first used by Littlejohn for the simpler problem of asymptotically small gyroradius. For definiteness, only electrostatic fluctuations in slab geometry are considered; however, there is a straightforward generalization to arbitrary field geometry and electromagnetic perturbations. An energy invariant for the nonlinear system is derived, and several limiting forms are considered. The weak turbulence theory of the equations is examined. In particular, the wave kinetic equation of Galeev and Sagdeev can ony be derived by an asystematic truncation of the equations, implying that this equation fails to consider all gyrokinetic effects. The equations are simplified for the case of small but finite gyroradius and put in a form suitable for efficient computer simulation. Although it is possible to derive the Terry–Horton and Hasegawa–Mima equations as limiting cases of our theory, several new nonlinear terms absent from conventional theories appear and are discussed. The resulting theory is very similar in content to the recent work of Lee. However, the systematic nature of our derivation provides considerable insight into the structure and interpretation of the equations.
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52.25.Dg Plasma kinetic equations
52.25.Gj Fluctuation and chaos phenomena
52.20.Dq Particle orbits
52.35.Ra Plasma turbulence

Energy conservation and related constraints in drift wave turbulence

David R. Thayer and K. Molvig

Phys. Fluids 26, 3536 (1983); http://dx.doi.org/10.1063/1.864114 (4 pages) | Cited 1 time

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The problem of energy conservation for the renormalization of the drift wave instability in a sheared magnetic field is considered. It has been suggested previously that there is a connection between a certain constraint on the nonlinear term in the drift kinetic equation and energy conservation. Arguments are presented to dissolve this connection; and in turn, energy conservation is formulated in the physically meaningful statistically averaged sense. Finally, energy conservation is proven for the system of nonlinear equations, renormalized by the normal stochastic approximation, describing the drift wave instability.
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52.35.Kt Drift waves
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Ra Plasma turbulence
52.25.Dg Plasma kinetic equations

Turbulent relaxation of compressible plasmas with flow

John M. Finn and T. M. Antonsen

Phys. Fluids 26, 3540 (1983); http://dx.doi.org/10.1063/1.864115 (13 pages) | Cited 28 times

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Relaxation of compressible plasmas to an equilibrium with flows is studied. The magnetohydrodynamic (MHD) equations with large parallel thermal conductivity and ergodic field lines show that ∫v⋅B is an invariant even with compression. Also, S=∫ρ ln( p/ργ) is the only entropy‐like invariant of the MHD equations with infinite parallel thermal conductivity; S increases in time with finite thermal conductivity. The other invariants are energy, helicity ∫A⋅B, mass ∫ρ, and possibly angular momentum. Equilibria are found by extremizing energy while conserving these invariants or by maximizing entropy with the energy and other invariants as constraints. These invariants are complete in the sense of generating all equilibria that form after relaxation with ergodic field lines. For parallel flows, there are three classes of solutions characterized by the sign of dρ/dB2 and the mirror mode parameter. A sufficient condition for stability is derived. This condition is never satisfied by the class with dρ/dB2>0, indicating the possibility of unstable resistive interchanges. The class with dρ/dB2<0 is stable if generalizations of the local firehose and mirror criteria are satisfied, and if generalized Taylor helicity eigenvalues exceed those of the equilibrium. A comparison of these conditions with those of Frieman and Rotenberg is discussed.
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52.30.-q Plasma dynamics and flow
52.35.Ra Plasma turbulence
52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.

Steepest‐descent moment method for three‐dimensional magnetohydrodynamic equilibria

S. P. Hirshman and J. C. Whitson

Phys. Fluids 26, 3553 (1983); http://dx.doi.org/10.1063/1.864116 (16 pages) | Cited 174 times

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An energy principle is used to obtain the solution of the magnetohydrodynamic (MHD) equilibrium equation J×B−∇p=0 for nested magnetic flux surfaces that are expressed in the inverse coordinate representation x=x(ρ, θ, ζ). Here, θ are ζ are poloidal and toroidal flux coordinate angles, respectively, and p=p(ρ) labels a magnetic surface. Ordinary differential equations in ρ are obtained for the Fourier amplitudes (moments) in the doubly periodic spectral decomposition of x. A steepest‐descent iteration is developed for efficiently solving these nonlinear, coupled moment equations. The existence of a positive‐definite energy functional guarantees the monotonic convergence of this iteration toward an equilibrium solution (in the absence of magnetic island formation). A renormalization parameter λ is introduced to ensure the rapid convergence of the Fourier series for x, while simultaneously satisfying the MHD requirement that magnetic field lines are straight in flux coordinates. A descent iteration is also developed for determining the self‐consistent value for λ.
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52.55.Fa Tokamaks, spherical tokamaks
52.55.Hc Stellarators, torsatrons, heliacs, bumpy tori, and other toroidal confinement devices
52.30.-q Plasma dynamics and flow

Equilibrium and stability properties of high‐beta torsatrons

B. A. Carreras, H. R. Hicks, J. A. Holmes, V. E. Lynch, L. Garcia, J. H. Harris, T. C. Hender, and B. F. Masden

Phys. Fluids 26, 3569 (1983); http://dx.doi.org/10.1063/1.864117 (11 pages) | Cited 40 times

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Equilibrium and stability properties of high‐beta torsatrons are investigated using numerical and semianalytical techniques based on the method of toroidal averaging. The averaged equilibria are compared with those obtained using full three‐dimensional codes. Good agreement is obtained, thus validating the averaged method approach. The stability of plasmas for configurations with different aspect ratios and numbers of field periods is studied. The role of the vertical field is also studied in detail. The main conclusion is that for moderate‐aspect‐ratio torsatrons (Ap≲8), the self‐stabilizing effect of the magnetic axis shift is large enough to open a direct path to the second stability region.
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52.55.Fa Tokamaks, spherical tokamaks
52.55.Hc Stellarators, torsatrons, heliacs, bumpy tori, and other toroidal confinement devices

Ballooning instabilities in hot electron plasmas

T. M. Antonsen, Y. C. Lee, H. L. Berk, M. N. Rosenbluth, and J. W. Van Dam

Phys. Fluids 26, 3580 (1983); http://dx.doi.org/10.1063/1.864118 (15 pages) | Cited 9 times

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The stability of short‐wavelength, low‐frequency modes in a hot electron plasma is investigated. Particular attention is devoted to the effect of the dependence of perturbed quantities along the equilibrium magnetic field lines. Two types of modes in a bumpy torus are considered: modes which are nearly flutes in each mirror cell but vary in phase from cell to cell, and modes which have significant variation in each cell. Generally speaking, allowing perturbations to vary along the field lines is destabilizing. However, sufficiently large hot electron gyroradii can stabilize all short perpendicular wavelength modes.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Jd Magnetic mirrors, gas dynamic traps

Destabilization of the hot‐electron precessional mode in tandem mirrors and bumpy tori

D. E. Baldwin and H. L. Berk

Phys. Fluids 26, 3595 (1983); http://dx.doi.org/10.1063/1.864119 (7 pages) | Cited 14 times

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The high‐frequency precessional mode of a hot‐electron‐stabilized magnetic configuration has previously been shown to be stable in a window of core‐plasma mass. Under conditions of frequency matching, the resulting stable negative‐energy precessional wave can be destabilized by coupling to positive‐energy shear‐Alfvén waves. Coupling is avoided when the hot‐electron precession frequency exceeds the core‐plasma ion gyrofrequency.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.20.Dq Particle orbits
52.55.-s Magnetic confinement and equilibrium

Numerical simulation of axisymmetric spheromak merging

Tetsuya Sato, Y. Oda, S. Otsuka, K. Katayama, and M. Katsurai

Phys. Fluids 26, 3602 (1983); http://dx.doi.org/10.1063/1.864120 (10 pages) | Cited 7 times

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Axisymmetric merging of spheromaks is studied extensively by means of a magnetohydrodynamic code. Merging simulations of identical and different spheromaks created by the Princeton slow induction method have revealed that (1) the total magnetic energy decreases rather quickly through reconnection while the magnetic helicity is reasonably conserved; (2) a hollow structure appears in the radial q profile when two different spheromaks merge, while the q profile is doubled when the identical spheromaks merge; and (3) the postmerging toroidal flux becomes the sum of the premerging fluxes, while the postmerging poloidal flux remains the same as the larger of premerging fluxes. It is also observed that kinetic energy once converted from the magnetic energy through reconnection is returned back to the agnetic energy near the end of the merging process, this indicating a relaxation toward a lower‐energy force‐free equilibrium.
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52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.
52.65.-y Plasma simulation
52.30.-q Plasma dynamics and flow

Computer simulations of upper‐hybrid and electron cyclotron resonance heating

A. T. Lin and Chih Chien Lin

Phys. Fluids 26, 3612 (1983); http://dx.doi.org/10.1063/1.864121 (7 pages) | Cited 7 times

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A 2 1/2 ‐dimensional relativistic electromagnetic particle code is used to investigate the dynamic behavior of electron heating around the electron cyclotron and upper‐hybrid layers when an extraordinary wave is obliquely launched from the high‐field side into a magnetized plasma. With a large angle of incidence most of the radiation wave energy converts into electrostatic electron Bernstein waves at the upper‐hybrid layer. These mode‐converted waves propagate back to the cyclotron layer and deposit their energy in the electrons through resonant interactions dominated first by the Doppler broadening and later by the relativistic mass correction. The line shape for both mechanisms has been observed in the simulations. At a later stage, the relativistic resonance effects shift the peak of the temperature profile to the high‐field side. The heating ultimately causes the extraordinary wave to be substantially absorbed by the high‐energy electrons. The steep temperature gradient created by the electron cyclotron heating eventually reflects a substantial part of the incident wave energy. The diamagnetic effects due to the gradient of the mode‐converted Bernstein wave pressure enhance the spreading of the electron heating from the original electron cyclotron layer.
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52.65.-y Plasma simulation
52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.50.Gj Plasma heating by particle beams
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma

Beam–ion and alpha‐particle effects on microinstabilities in tokamaks

G. Rewoldt and W. M. Tang

Phys. Fluids 26, 3619 (1983); http://dx.doi.org/10.1063/1.864122 (5 pages) | Cited 4 times

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The analysis of electromagnetic kinetic toroidal eigenmodes for general magnetohydrodynamic (MHD) equilibria has been extended to include the effects of non‐Maxwellian equilibrium distribution functions. This is necessary to properly represent the response of the hot‐beam–ion species produced during neutral‐beam injection heating and the response of the alpha‐particle species in a thermonuclear plasma. The influence of these components on low‐frequency microinstabilities is investigated for realistic cases typical of the PLT and PDX tokamak experiments.
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52.35.Kt Drift waves
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Pi Fusion products effects (e.g., alpha-particles, etc.), fast particle effects

Theory of axially symmetric probes in a collisionless magnetoplasma: Aligned spheroids, finite cylinders, and disks

J. Rubinstein and J. G. Laframboise

Phys. Fluids 26, 3624 (1983); http://dx.doi.org/10.1063/1.864123 (4 pages) | Cited 7 times

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A theory is presented for current collection by electrostatic probes in a collisionless, Maxwellian plasma containing a uniform magnetic field B, where the probes are spheroids or finite cylinders whose axis of symmetry is aligned with B, or disks perpendicular to B. The theory yields upper‐bound and adiabatic‐limit currents for the attracted particle species. For the repelled species, it yields upper and lower bounds. This work is an extension of existing theory for spherical probes by Rubinstein and Laframboise.
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52.70.Ds Electric and magnetic measurements

A general theory of magnetically insulated electron flow

C. W. Mendel, D. B. Seidel, and S. A. Slutz

Phys. Fluids 26, 3628 (1983); http://dx.doi.org/10.1063/1.864133 (8 pages) | Cited 42 times

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A theory of magnetically insulated flow is developed where two‐ or three‐dimensional spatial variation and temporal variation are allowed as long as the spatial variations in the flow direction are over many electron gyrolengths and temporal variations are over many electron gyroperiods. These criteria are met in most flows of interest. The theory is based upon describing the electron dynamics in terms of parameters which are constants of motion in one‐dimensional, time‐independent flows.
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41.60.-m Radiation by moving charges
41.75.Fr Electron and positron beams

Detailed spectra of high‐power broadband microwave radiation from interactions of relativistic electron beams with weakly magnetized plasmas

Keith G. Kato, Gregory Benford, and David Tzach

Phys. Fluids 26, 3636 (1983); http://dx.doi.org/10.1063/1.864134 (14 pages) | Cited 32 times

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Prodigious quantities of microwave energy distributed uniformly across a wide frequency band are observed when a relativistic electron beam (REB) penetrates a plasma. Typical measured values are 20 MW total for Δν≂40 GHz with preliminary observations of bandwidths as large as 100 GHz. An intense annular pulsed REB (I≂128 kA; r≂3 cm; Δr≂1 cm; 50 nsec FWHM; γ≂3) is sent through an unmagnetized or weakly magnetized plasma column (nplasma∼1013 cm−3). Beam‐to‐plasma densities of 0.01≤nbeam/nplasma≤2 are used, the higher values of this range being an unconsidered region for most previous theoretical and experimental efforts. For these higher nb/np values, the observed emission with ω≫ωp and weak harmonic structure is wholly unanticipated from Langmuir scattering or soliton collapse models. A model of Compton‐like boosting of ambient plasma waves by the beam electrons, with collateral emission of high‐frequency photons, qualitatively explains these spectra. Power emerges largely in an angle ∼1/γ, as required by Compton mechanisms. As nb/np falls, ωp−2ωp structure and harmonic power ratios consistent with soliton collapse theories appear. With further reduction of nb/np only the ωp line persists. Thus a transition occurs in spectral behavior from the weak to strong turbulence theories advocated for type‐III solar burst radiation, and further into a regime we characterize as superstrong REB–plasma interactions, is observed. For frequencies slightly below the broadband region, an ωp line is observed with high power (approximately 1 MW); the line disappears in an external Bz∼400 G. Changing γb over a range of 2.2–3.7 has little effect on the spectra.
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52.40.Mj Particle beam interactions in plasmas
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.70.-m Plasma diagnostic techniques and instrumentation
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