bluuebunny/crossref_metadata_2025 huggingface dataset:
The impedance and energy efficiency of a coaxial magnetized plasma source used for spheromak formation and sustainment
Authors: Barnes Cris W. , Jarboe T. R. , Marklin G. J. , Knox S. O. , Henins I.
Published: July 2002

Electrostatic (dc) helicity injection has previously been shown to successfully sustain the magnetic fields of spheromaks and tokamaks. The magnitude of the injected magnetic helicity balances (within experimental error) the flux lost by resistive decay of the toroidal equilibrium. Hence the problem of optimizing this current drive scheme involves maximizing the injected helicity (the voltage-connecting-flux product) while minimizing the current (which multiplied by the voltage represents the energy input and also possible damage to the electrodes). The impedance (voltage-to-current ratio) and energy efficiency of a dc helicity injection experiment are studied on the CTX spheromak [Phys. Fluids 29, 3415 (1986)]. Over several years changes were made in the physical geometry of the coaxial magnetized plasma source as well as changes in the external electrical circuit. The source could be operated over a wide range of external charging voltage (and hence current), applied axial flux, and source gas flow rate.

A database of resulting voltage, helicity injection, efficiency, electron density, and rotation has been created. These experimental results are compared to an ideal magnetohydrodynamic theory of magnetic flux flow. The theory is parametrized by the dimensionless Hall parameter, the ratio of electric to mass current. For a constant Hall parameter the theory explains why the voltage depends quadratically on the current at constant flux. The theory also explains the approximately linear dependence of the impedance-to-current ratio on the current-to-flux ratio of the source. The current-to-flux ratio itself (the energy-per-unit helicity of the source) is bounded below by considerations of force balance. While the rotation of the flow is not understood, the density of the sustained spheromak is shown to be related to the mass flow in the source, supporting the constant Hall parameter assumption. The overall efficiency of sustainment through dc helicity injection is limited by the usual Ohmic resistive decay, by the force-balance limits on the current-to-flux ratio, by the losses of the external electrical circuit, and by the fundamental limitations on the achievable impedance of flux flow in a magnetized plasma. Even so, ratios of spheromak magnetic energy to capacitor bank energy of over 17% have been achieved on CTX. Ignoring external circuit losses the efficiency of electrostatic helicity injection for converting energy received by the coaxial source to the energy of the spheromak magnetic field has exceeded 70%.
Neoclassical transport of energetic minority tail ions generated by ion-cyclotron resonance heating in tokamak geometry
Authors: Chang C. S. , Hammett G. W. , Goldston R. J.
Published: July 2002

Neoclassical transport of energetic minority tail ions, which are generated by high-powered electromagnetic waves of the ion cyclotron range of frequencies (ICRF) at the fundamental harmonic resonance, is studied analytically in tokamak geometry. The effect of Coulomb collisions on the tail-ion transport is investigated in the present work. The total tail-ion transport will be the sum of the present collision-driven transport and the wave-driven transport, which is due to the ICRF-wave scattering of the tail particles as reported in the literature. The transport coefficients have been calculated kinetically, and it is found that the large tail-ion viscosity, driven by the localized ICRF-heating and Coulomb slowing-down collisions, induces purely convective particle transport of the tail species, while the energy transport is both convective and diffusive. The rate of radial particle transport is shown to usually be small, but the rate of radial energy transport is larger and may not be negligible compared to the Coulomb slowing-down rate.
Energetic electron measurements in the edge of a reversed-field pinch
Authors: Ingraham J. C. , Ellis R. F. , Downing J. N. , Munson C. P. , Weber P. G. , Wurden G. A.
Published: July 2002

The edge plasma of the ZT-40M [Fusion Technol. 8, 1571 (1985)] reversed-field pinch has been studied using a combination of three different plasma probes: a double-swept Langmuir probe, an electrostatic energy analyzer, and a calorimeter–Langmuir probe. The edge plasma has been measured both with and without a movable graphite tile limiter present nearby in the plasma. Without a limiter a fast nonthermal tail of electrons (T≂350 eV) is detected in the edge plasma with nearly unidirectional flow along B and having a density between 2% and 10% of the cold edge plasma (T≂20 eV). The toroidal sense of this fast electron flow is against the force of the applied electric field. A large power flux along B is measured flowing in the same direction as the fast electrons and is apparently carried by the fast electrons. With the limiter present the fast electrons are still detected in the plasma, but are strongly attenuated in the shadow of the limiter. The measured scrape-off lengths for both the fast electrons and the cold plasma indicate cross-field transport at the rate of, or less than, Bohm diffusion. Estimates indicate that the fast electrons could carry the reversed-field pinch current density at the edge and, from the measured transverse diffusion rates, could also account for the electron energy loss channel in ZT-40 M. The long mean-free-path kinetic nature of these fast electrons suggests that a kinetic process, rather than a magnetohydrodynamic process that is based upon a local Ohm’s law formulation, is responsible for their generation.
The concept of a kinetic transport theory
Authors: Stubbe Peter
Published: July 2002

In fluid theory, the kinetic equation is converted into a system of moment equations. Any wanted moment is thereby coupled with other moments in an infinite set of equations. In plasma kinetic theory, the linearized Fourier-transformed kinetic equation is solved for the distribution function, and the wanted moments are obtained afterward as functions of the electric field strength. A complete knowledge of the frequency and wavenumber electric field spectrum is needed for these moments to be determined as functions of t and r. This standard kinetic approach is not suitable beyond the weak turbulence limit, in which the interaction of waves is treated in the random phase approximation. In situations where regular structures in (t, r) space arise due to phase correlations, recourse is commonly taken to fluid theory. In the present paper the plasma kinetic theory is treated differently by taking the electric field not as the cause of the process to be studied, but as a mediator between different moments. An arbitrary number of moment equations can be obtained in this way. Only those that, in the fluid limit, pass over to the fluid equations will be used. Outside the fluid limit, these so-called kinetic transport equations represent a continuation of the fluid equations into the kinetic regime. One purpose of kinetic transport theory is to extend the plasma kinetic theory beyond the weak turbulence limit. In contrast to standard plasma kinetic theory, the formulation of kinetic transport theory allows an application to neutral gases by setting the particle charge to zero. Therefore a second purpose of kinetic transport theory is to extend the transport equations for neutral gases beyond the fluid limit and to define the transport coefficients for highly rarefied gases.
Complete fluid equations for low-n singular modes in axisymmetric toroidal plasmas
Authors: Glasser Alan H.
Published: July 2002

A system of equations governing the singular region of magnetohydrodynamic (MHD) instabilities in a collisional, axisymmetric toroidal plasma with magnetic shear has been derived from Braginskii’s fluid equations. This is an extension of earlier work, incorporating anisotropic resistivity, anisotropic ion viscosity, anisotropic ion and electron thermal conductivity, and diamagnetic and E×B drifts. The equations describe the narrow layer in the neighborhood of a singular surface for tearing and interchange instabilities with low toroidal mode number n. Ordering assumptions allow for the maximum range of physical effects while retaining the principal terms in the earlier work and ensuring the validity of the fluid equations. The equations are complete in the sense that all terms, and only those terms, consistent with this carefully constructed ordering are included.
Unified theory of ballooning instabilities and temperature gradient-driven trapped ion modes
Authors: Xu X. Q. , Rosenbluth M. N.
Published: July 2002

A unified theory of temperature gradient-driven trapped ion modes and ballooning instabilities is developed using kinetic theory in banana regimes. All known results such as electrostatic and purely magnetic trapped particle modes and ideal magnetohydrodynamic ballooning modes (or shear Alfvén waves) are readily derived from the present single general dispersion relation. Several new results from ion–ion collision, finite beta stabilization of ion temperature gradient-driven trapped particle modes, and trapped particle modification of ballooning modes are derived and discussed. The interrelationship between these modes is established.
On toroidal equilibrium of non-neutral plasma
Authors: Avinash K.
Published: July 2002

The equilibrium of a non-neutral cloud in a toroidal magnetic field is analyzed. In the zero mass limit it is shown that, because of plasma diamagnetism and electrostatic hoop force, there is a net force on the cloud along the major radius. In a large-aspect-ratio conducting vessel, this force is balanced by image charges that produce a compensating force toward the major axis. As a result, shift of potential minimum (or maximum) from the geometric center of the vessel (in the poloidal plane) is toward the major axis. A partial differential equation governing the two-dimensional axisymmetric equilibrium is obtained and solved in the large-aspect-ratio limit. Further, it is shown that modification of this equilibrium owing to first-order guiding center drifts is small, i.e., drift surfaces are still closed, however, they are further displaced toward the major axis from the potential surfaces by a small distance. Finally, a brief comparison of the properties of the equilibrium of neutral and non-neutral plasma is given.
Bohm criterion and ion-acoustic sound barrier
Authors: Riemann K.-U.
Published: July 2002

In the limit of a small Debye length λD the plasma boundary layer is split up into a thin space charge sheath and a quasineutral presheath. A necessary condition for the formation of a stationary sheath is the validity of Bohm’s criterion at the sheath edge. In its most simple form, it requires that the ions enter the sheath region at least with ion sound velocity vB. Usually, the quasineutral plasma approximation is terminated by a singularity at v=vB and Bohm’s criterion is fulfilled in the equality form. In widening geometries, however, the plasma flow may become supersonic within the presheath, and the sheath edge exhibits no singularity. This hydrodynamic result is in apparent contradiction to general kinetic considerations. The contradiction is exhibited (and finally resolved) with the example of a collisionless cylindrical column, where ionization is restricted to a small region near the axis.
Nonlinear reversed-field pinch dynamics with nonideal boundaries
Authors: Ho Y. L. , Prager S. C.
Published: July 2002

The nonlinear behavior of the reversed-field pinch bounded by a resistive shell or a distant conducting wall is investigated with a three-dimensional magnetohydrodynamic code. Nonlinear interaction between modes enhances fluctuation levels as the conducting wall is removed. The enhanced fluctuation induced v×b electric field, which produces the dynamo effect, suppresses toroidal current, and enhances surface helicity dissipation. Thus, loop voltage must increase to sustain the current and maintain helicity balance.
Fluid model equations for the tokamak plasma edge
Authors: Vold E. L. , Najmabadi F. , Conn R. W.
Published: July 2002

Increasing evidence that the edge plasma plays a crucial role in global tokamak confinement motivates this study of two-dimensional (2-D) (θ,r) computational models of fluid transport in the edge plasma region. Fluid plasma equations, boundary conditions, and a coupled neutral–plasma model are developed assuming toroidal symmetry. A plasma potential equation is presented for consistent drift flow solution in the nonambipolar case. Simplified plasma equations are implemented in a 2-D computational model. Results show large poloidal flux dominated by drift flow near the separatrix and parallel flow near the tokamak divertor target. Simultaneously large poloidal gradients in plasma potential and electric fields are seen. These may play a role in driving observed turbulent fluctuations in the edge plasma.
Initial value problem for plasma oscillations
Authors: Li Jin , Spies Günther Otto
Published: July 2002

The solution of the initial value problem for the linearized one-dimensional electron Vlasov–Poisson equations in a field-free homogeneous equilibrium is examined for small and for large ratios κ of Debye length and wavelength, assuming initial perturbing distribution functions varying on the same velocity scale as the equilibrium. Previously known approximations of the initial evolution (which, unlike the time-asymptotic one, does not depend on analyticity assumptions) are extended to longer times, and to arbitrary stable or unstable equilibria: In the quasifluid regime (small κ), the electric field, within an additive error O(κ2), and independently of the initial data, performs an oscillation near the plasma frequency that corresponds to an eigenmode if it is unstable or marginal, but to an approximate eigenmode arising from the continuous spectrum otherwise. If other unstable or marginal modes are present, these influence only the time-asymptotic behavior because their amplitudes are O(κ2) initially. In the ballistic regime (large κ), there are no instabilities and the perturbing density, now within an error O(κ−2), is the Fourier transform of the initial perturbing distribution function, thus following an arbitrary decay law that is independent of the equilibrium. The errors are shown to be time-independent, implying that either approximation is relevant at least until the perturbing density has essentially damped out. Hence the dominating damping mechanism (in the stable case) is Landau damping if κ≪1, but ballistic particle mixing if κ≫1.
The effect of induced spatial incoherence on the absolute Raman instability
Authors: Guzdar P. N. , Tan W. , Lee Y. C. , Liu C. S. , Lehmberg R. H.
Published: July 2002

A numerical and analytical study of the Raman instability in a homogeneous plasma is presented in which the pump has been modeled to include the effects of broad bandwidth and the induced spatial incoherence (ISI) method of beam smoothing. For a time-averaged homogeneous growth rate γ̄0 and a bandwidth σ, there is a significant reduction in Raman backscattering when σ≳2γ0, for γ̄20 near threshold intensity. However, for γ̄20 very large compared to the threshold, neither ISI nor bandwidth affects Raman scattering.
Lower-hybrid turbulence in a nonuniform magnetoplasma
Authors: Stenzel R. L.
Published: July 2002

An experimental study of a pressure-gradient-driven instability in a large discharge plasma (1 m diam, 2.5 m length, ne≂1012 cm3, B≂14 G) is presented. When the electron diamagnetic drift vd=∇(nkTe)×B/neB2 exceeds the sound speed cs≂(kTe/mi)1/2 ion-acoustic-like waves (Te≫Ti) are driven unstable. The growth rate maximizes near the lower-hybrid frequency ωlh≂(ωceωci)1/2 and the waves propagate essentially across B (k∥≪k⊥≂ωlh/cs). The sound waves grow to large amplitudes (δn/n≳50%) and saturate by wave steepening (λD<λ⊥<rce) and refraction (∇Te≠0) away from the destabilizing drift vd. Magnetic fluctuations result from electron diamagnetic currents and opposing Hall currents associated with the wave density fluctuations. Ions are essentially unmagnetized (νin/ωci>1) and slow compared to the magnetized electrons, vi/vd≂(me/mi)1/2≪1. In spite of the large amplitude waves little acceleration of electrons or ions is observed. The experiment employs a new technique of conditional averaging with digital oscilloscopes.
A note on a Hamiltonian formulation of force-free toroidal magnetic fields
Authors: Abraham-Shrauner B. , Lewis H. Ralph
Published: July 2002

Recently, a formalism was presented for representing magnetic fields with toroidal topology in terms of field-line invariants [Phys. Fluids B 2, 2551 (1990)]. The representation of a field with a nonvanishing toroidal component depends on a transformation from toroidal coordinates to a set of variables consisting of a poloidal angle function, a toroidal flux function, and a toroidal angle function. A restricted class of such transformations was considered for vacuum fields. Here it is shown that this class of transformations for a vacuum or force-free field always requires that the field be axisymmetric.
Cyclotron resonance phenomena in a pure electron plasma
Authors: Gould Roy W. , LaPointe Michael A.
Published: July 2002

An experiment designed to elucidate the features of cyclotron resonance in a rotating cylindrical pure electron plasma column is described. The density is well below the Brillouin limit and varies with radius, as does the rotational angular velocity. Thus the steady state is not the rigid-rotor equilibrium which is frequently studied theoretically. The readily observed modes are found to have kz≊0 and m=1,2,3,4... (eikzz+im θ) with frequencies which are within a few percent of the cyclotron frequency (140 MHz). A single m=1 mode is found that is downshifted from the cyclotron frequency by an amount equal to the m=1 diocotron frequency. For each of the higher m values, bands of closely spaced discrete modes are found which are upshifted from the cyclotron frequency by the Doppler effect of rotation. The bands of discrete modes are explained as radially trapped azimuthally propagating Bernstein modes in a rotating plasma and approximate theories for these modes are outlined. By comparing the results with experiments, plasma parameters such as the ratio of Larmor radius to plasma scale length, the central rotation frequency, and the ratio of peak to average density can be inferred.
Experimental program on the 20 TW laser system
Authors: Watteau J. P. , Bonnaud G. , Coutant J. , Dautray R. , Decoster A. , Louis-Jacquet M. , Ouvry J. , Sauteret J. , Seznec S. , Teychenné D.
Published: July 2002

Pulses of 20 TW have been generated at 1064 nm using the chirped pulse amplification technique coupled to a 90 mm output aperture Nd–silicate glass amplification line. The chirped pulses are compressed to 1.2 psec with an energy of 24 J using large holographic gold-coated 1740 lines/mm diffraction gratings. In a first step, the beam is focused by an aspherical lens, but the maximum measured irradiance is limited to 1018 W/cm2 by self-focusing effects in the air path and in the lens. Higher values up to 1019 W/cm2 will need propagation of the compressed beam in vacuum and focusing by reflective optics. The first experiments deal with laser–plasma coupling, with emphasis on the power contrast ratio. Further plans include multiple Compton scattering and x-ray laser generation. Theoretical works on ionization processes and particle accelerator are also presented.
Unified fluid/kinetic description of plasma microinstabilities. Part II: Applications
Authors: Chang Zuoyang , Callen J. D.
Published: July 2002

The unified fluid/kinetic equations developed in Part I [Phys. Fluids B 4, ▪▪▪ (1992)] of this work are used to study plasma-drift-type microinstabilities. A generalized perturbed Ohm’s law is derived (for a sheared slab magnetic field model) that is uniformly valid for arbitrary collisionality ω/ν and adiabaticity ω/k∥vt. Applications to electron drift waves, ion temperature gradient modes (ηi modes) and electron electromagnetic modes (microtearing modes) are addressed. It is shown that the unified equations exhibit their simple and uniformed features in the plasma microinstability analysis. The well-known results of these instability branches in both fluid and adiabatic limits are easily recovered using the new fluid/kinetic equations. Generalization of the two-field Hasegawa–Wakatani turbulent equations [Phys. Rev. Lett. 50, 682 (1983)] to include electron temperature fluctuations and linear Landau damping effects is also discussed. Finally, a new method is presented to facilitate the study of magnetic trapped particle modes using the present kinetic closure procedure. It is found that by including the trapped particle effects in the closure relations, the usual separation of the fluid equations into trapped and untrapped components becomes unnecessary.
Validity of the field line resonance expansion
Authors: Hansen P. J. , Goertz C. K.
Published: July 2002

The field line resonance expansion is studied with a magnetohydrodynamic (MHD) model applicable to Earth’s magnetotail. Using a Frobenius series solution, this expansion is found to be inconsistent. The inconsistency is seen by evaluating the terms in the model equations that the expansion neglects as small. Their effect is to provide a solution more singular than that determined by the expansion procedure. A tentative alternative expansion procedure, retaining these neglected terms approximately, yields a nonsingular solution, suggesting that resonant mode conversion rather than field line resonance is the appropriate phenomenon.
Current collection by an active spherical electrode in an unmagnetized plasma
Authors: Ahedo E. , Martínez-Sánchez M. , Sanmartín J. R.
Published: July 2002

A theoretical model for the steady-state response of anodic contactors that emit a plasma current Ii and collect electrons from a collisionless, unmagnetized plasma is presented. The use of a (kinetic) monoenergetic population for the attracted species, well known in passive probe theory, gives both accuracy and tractability to the theory. The monoenergetic population is proved to behave like an isentropic fluid with radial plus centripetal motion, allowing direct comparisons with ad hoc fluid models. Also, a modification of the original monoenergetic equations permits analysis of contactors operating in orbit-limited conditions. Besides that, the theory predicts that, only for plasma emissions above certain threshold current a presheath/double layer/core structure for the potential is formed (the core mode), while for emissions below that threshold, a plasma contactor behaves exactly as a positive-ion emitter with a presheath/sheath structure (the no-core mode). Ion emitters are studied as a particular case. Emphasis is placed on obtaining dimensionless charts and approximate asymptotic laws of the current-voltage characteristic.
Mode coupling in nonlinear Rayleigh–Taylor instability
Authors: Ofer Dror , Shvarts Dov , Zinamon Ze’ev , Orszag Steven A.
Published: July 2002

This paper studies the interaction of a small number of modes in the two-fluid Rayleigh–Taylor instability at relatively late stages of development, i.e., the nonlinear regime, using a two-dimensional hydrodynamic code incorporating a front-tracking scheme. It is found that the interaction of modes can greatly affect the amount of mixing and may even reduce the width of the mixing region. This interaction is both relatively long range in wave-number space and also acts in both directions, i.e., short wavelengths affect long wavelengths and vice versa. Three distinct stages of interaction have been identified, including substantial interaction among modes some of which may still be in their classical (single mode) ‘‘linear’’ phase.
Simulations of wave–particle interactions in stimulated Raman forward scattering in a magnetized plasma
Authors: Bertrand P. , Ghizzo A. , Karttunen S. J. , Pättikangas T. J. H. , Salomaa R. R. E. , Shoucri M.
Published: July 2002

Stimulated Raman forward scattering in a high-temperature, magnetized plasma is investigated with relativistic Vlasov–Maxwell simulations and with envelope and test particle calculations. The parameters correspond to Raman current drive by free-electron lasers in reactor grade tokamak plasmas. The phase velocity of the Raman excited plasma wave is large, and therefore the Landau damping is initially weak. The electron plasma wave grows to a large amplitude and accelerates electrons to high energies. Simultaneous pump depletion weakens the driving ponderomotive force, which leads to a collapse of the plasma wave if the number of the interacting electrons is large enough. Spatially the wave–particle interaction takes place in a distance of a few wavelengths of the plasma wave. The electron energies can largely exceed the kinetic energy at the phase velocity of the electron plasma wave. Short-wavelength amplitude modulations of the plasma wave appear at high amplitudes. Efficient generation of the nonresonant anti-Stokes wave and of the second Stokes wave are also observed. Analytical growth rates and envelope calculations explain well the early evolution of the Raman process. Later on, nonlinear wave–particle interactions and relativistic effects start to dominate, and the system is not satisfactorily described by simple envelope equations.
Subcritical reactive drift wave turbulence
Authors: Nordman Hans , Pavlenko Vladimir P. , Weiland Jan
Published: July 2002

The existence of self-sustained toroidal ηi-mode turbulence below the instability threshold has been shown by mode coupling simulations. Significant transport and a tendency for long-wavelength condensation are observed. The subcritical behavior is explained as a result of nonlinear interaction between modes of positive and negative energy by analytical calculation of the coupling factors.
Characterization of laser-produced plasma density profiles using grid image refractometry
Authors: Craxton R. S. , Turner F. S. , Hoefen R. , Darrow C. , Gabl E. F. , Busch Gar. E.
Published: July 2002

Grid image refractometry (GIR) is proposed as a technique for determining the two-dimensional density profiles of long scale-length laser-produced plasmas. Its distinctive feature is that an optical probe beam is broken up into ‘‘rays’’ by being passed through a grid before traversing the plasma. The refraction angles of the rays are measured by imaging the plasma at two or more object planes and are integrated to yield the phase front. For cylindrically symmetric plasmas the density profile is then determined using Abel inversion. The feasibility of GIR is illustrated by an experiment in which a thick CH target was irradiated with ∼100 J of 527 nm radiation and diagnosed with a 20 ps, 263 nm probe. The resulting density profile is substantially larger than any that have previously been reported using interferometry and compares quite closely with hydrodynamic simulations.
Cumulative hose instabilities of a magnetically self-focused slab electron beam
Authors: Whittum David H.
Published: July 2002

Transverse instabilities are investigated for the slab analog of the Bennett equilibrium of a magnetically self-focused relativistic electron beam propagating through a plasma. Two kinds of plasma are considered: a channel matched to the beam and a broad uniform plasma. The high-frequency electrostatic transverse two-stream instability and the lower-frequency resistive-hose instability are treated analytically with a distributed tune (‘‘mass’’) model. Asymptotic growth is computed for each instability in various regimes, and analytic results are benchmarked against numerical solutions of the linearized system and particle-in-cell simulations. It is found that in the limit of large collisionless skin depth the electrostatic hose is the more virulent mode of beam breakup. Numerical examples are discussed for parameters in the range considered for plasma focusing of the asymmetric beams of interest in high-energy physics.
On the stability of combustion and laser-produced ablation fronts
Authors: Bychkov V. V. , Goldberg S. M. , Liberman M. A.
Published: July 2002

A correct approach to the problem of a reduction of the growth rate of the Rayleigh–Taylor (RT) instability in an ablation wave is demonstrated in this paper by considering a slow combustion wave in a gravitational field. It is shown that both the supplementary condition required in the model of discontinuity and the reduction of the instability growth rate can be obtained only by solving the complete system of equations, including the equation of thermal conductivity and energy release which are responsible for the wave propagation and the finite thickness of the wave front. The point is that there is no stabilization of the growth rate of RT instability by a mass flow in the limit of zero thickness of the wave front. The reduction of the growth rate can be obtained rigorously for a finite thickness of the wave front only.
Nonlinear self-reinforced growth of tearing modes with multiple rational surfaces
Authors: Persson M. , Dewar R. L. , Maschke E. K.
Published: July 2002

The nonlinear evolution of tearing modes with multiple rational surfaces is discussed. It is demonstrated that, in the presence of small differential rotation, the nonlinear growth might be faster than exponential. This growth occurs as the rotation frequencies of the plasma at the different rational surfaces equilibrate.
X-ray radiographic measurements of radiation-driven shock and interface motion in solid density material
Authors: Hammel B. A. , Griswold D. , Landen O. L. , Perry T. S. , Remington B. A. , Miller P. L. , Peyser T. A. , Kilkenny J. D.
Published: July 2002

Time resolved x-ray radiographic measurements at high photon energy (∼7 keV) are used to observe radiation-driven shock propagation and interface motion in solid density plastic samples, produced by indirect drive on the Nova laser. Measurements of x-ray transmission through the shock compressed material are used to infer a density of approximately three times solid. In addition, by doping a section of a sample with high-Z material (Br) for radiographic contrast, the shocked particle velocity was measured by observing the motion of the interface between the doped and nondoped materials resulting from acceleration by a shock.
Production and application of dense Penning trap plasmas
Authors: Barnes D. C. , Nebel R. A. , Turner Leaf
Published: July 2002

A new paradigm for producing well-confined, dense-thermonuclear plasmas is described. The convergence of a radial beam distribution of a Penning-trap-confined plasma produces a dense inertially confined non-neutral plasma. The equilibrium, stability, classical transport, and particle-handling properties of such a concept are developed. The application of this approach to controlled fusion using a pure electron plasma to form a central virtual cathode in which ions are electrostatically confined is discussed. On one hand, extreme plasma control is required, placing the major uncertainty on issues of machine precision. On the other hand, development is characterized by the manufacture and testing of extremely small and inexpensive systems. Thus, it would seem that a timely experimental test of this concept would be ineluctable. Success at such experiments might indicate an alternate path to practical fusion applications.
Results of an experimental study of the properties of ion-beam modes found in a double plasma device containing a hemispherical separating screen
Authors: Garner Charles E. , Oleson Norman L.
Published: July 2002

Ion-beam modes have been generated in a double plasma device containing argon in which the driver and target plasmas were separated by a hemispherical screen biased negatively with respect to both plasmas. Making the driver plasma positive with respect to the target plasma resulted in the formation of an ion beam flowing from the driver plasma to the target plasma and causing the formation of a focal region in the target plasma. Both single pulses and pulse trains were excited in the target plasma by application of positive voltage pulses and/or sine waves to the hemispherical grid. Detection of these modes was accomplished using a positively biased spherical probe capable of axial and rotational movement. The wave amplitude and the phase velocity along the axis of the target plasma were measured over a distance of more than two hemispherical grid radii. Since no theoretical treatments have as yet been published for the hemispherical case, the measured values have been compared with theoretical values for a spherical separating screen.
A theory of cavity excitation by modulated electron beam in connection with application to a klystron amplifier
Authors: Uhm Han S. , Park G. S. , Armstrong C. M.
Published: July 2002

Based on an equivalent circuit representation of a cavity impedance, a theoretical model of the cavity excitation by a modulated electron beam is developed. The beam current modulation is initiated by the first cavity and amplified in drift section between the first and second cavities. Properties of the second cavity excitation by the beam are described in terms of the phase shift Ψ and amplitude φ2 of induced voltage in the cavity. The phase shift Ψ and amplitude φ2 are determined in terms of the frequency difference (η), the cavity Q value, the voltage multiplication factor (χ), and magnitude (φ1) of the first cavity voltage. The voltage multiplication factor is proportional to the beam intensity, Q value, and distance between the first and second cavities. On the other hand, it is inversely proportional to frequency and capacitance of the cavity, and beam electron velocity. Because the induced voltage in the second cavity is in phase with the first cavity voltage at upshifted cavity frequency, amplitude of the current modulation at the upshifted frequency is larger than that at the downshifted cavity frequency. Experimental study is carried out by making use of a klystron amplifier operable from 4.4 to 5 GHz. Experimental data from the klystron agree remarkably well with amplifier performance profiles predicted by the theory.
Nonlinear analysis of generalized cross-field current instability
Authors: Yoon Peter H. , Lui Anthony T. Y.
Published: July 2002

Analysis of the generalized cross-field current instability is carried out in which cross-field drift of both the ions and electrons and their temperatures are permitted to vary in time. The unstable mode under consideration is the electromagnetic generalization of the classical modified-two-stream instability. The generalized instability is made of the modified-two-stream and ion-Weibel modes. The relative importance of the features associated with the ion-Weibel mode and those of the modified-two-stream mode is assessed. Specific applications are made to the Earth’s neutral sheet prior to substorm onset and to the Earth’s bow shock. The numerical solution indicates that the ion-Weibel mode dominates in the Earth’s neutral sheet environment. In contrast, the situation for the bow shock is dominated by the modified-two-stream mode. Notable differences are found between the present calculation and previous results on ion-Weibel mode which restrict the analysis to only parallel propagating waves. However, in the case of Earth’s bow shock for which the ion-Weibel mode plays no important role, the inclusion of the electromagnetic ion response is found to differ little from the previous results which treats ions responding only to the electrostatic component of the excited waves.
Bounce-averaged Monte Carlo energy and pitch angle scattering operators
Authors: Tolliver J. S.
Published: July 2002

Beginning with a linearized Coulomb collision operator, the derivations of bounce-averaged energy and pitch angle scattering terms that have been suitably discretized for Monte Carlo use are described. The derivation assumes that the electrostatic potential is constant along a magnetic field line. The collision operators are applied to electron transport in ELMO Bumpy Torus.
Adiabatic beam dynamics in a modified betatron
Authors: Roberts G. A. , Rostoker N.
Published: July 2002

Electron beams in modified betatrons were previously analyzed by using a paraxial treatment. This treatment is valid only for a beam where particles have an axial velocity much larger than their perpendicular velocity. However, in accelerators such as the UCI Modified Betatron, the beam does not satisfy the paraxial assumption. Another treatment based on the guiding center equations is presented. In this treatment the paraxial assumption is not necessary. Corrections to the conventional ‘‘betatron condition’’ are found and compared with recent experiments. Additionally, this treatment can describe nonparaxial quasiconfined particles. The effect of these quasiconfined particles is examined, and it is shown that they provide fields that are necessary to stabilize the beam.
Raman scattering in inhomogeneous laser-produced plasma
Authors: Simon A. , Seka W. , Goldman L. M. , Short R. W.
Published: July 2002

A model of Raman scattering in inhomogeneous laser-produced plasma is described, which invokes enhanced Thomson scattering. The enhancement is caused by pulses of hot electrons arising from laser–plasma interactions at the critical or quarter-critical surfaces. A simple model predicts the locations of two enhanced frequency bands, one between ω0 and ω0/2, and the other between 2ω0 and ω0. The model is shown to fit the results of eight experiments, including a new experiment using the six-beam Omega laser facility [Phys. Rev. Lett. 48, 1179 (1982); Phys. Fluids 27, 2181 (1984)], converted to 351 nm. In this latter experiment, simultaneous measurement of the upscattered and downscattered bands is carried out, and good agreement is found with the enhanced Thomson model.
Variational quadratic form for low-frequency electromagnetic perturbations. II. Application to tandem mirrors
Authors: Berk H. L. , Lane B. G.
Published: July 2002

In this paper the low-frequency quadratic form governing the linear response of electromagnetic trapped particle modes is applied to a tandem mirror geometry and several problems are treated. It is shown that the long-wavelength magnetohydrodynamic (MHD) wall stabilization mechanism persists even when the line bending energy is suppressed by allowing an electrostatic response. In another problem, the amount of charge uncovering needed to suppress electrostatic trapped particle modes is determined as the beta of the plasma rises to its critical MHD beta value βcr and the required charge uncovering is shown to increase substantially as beta approaches βcr. A third problem treats trapped particle instabilities driven by rotation and field line curvature in diffuse and steep boundary models. The steep boundary model offers the possibility of an enhanced ‘‘robust’’ finite Larmor radius (FLR) stabilization due to an amplification of the finite Larmor radius magnetic compressional term when steep pressure gradients are present. Otherwise, one finds relatively small regions of stability in parameter space when electric field drifts are comparable to diamagnetic drifts. Detailed stability plots are presented for parameters applicable to the Tara experiment [Nucl. Fusion 22, 549 (1982)].
Modulational instability of obliquely modulated ion-acoustic waves in a two-ion plasma
Authors: Chhabra R. S. , Sharma S. R.
Published: July 2002

Using the KBM perturbation technique, the stability of oblique modulation of ion-acoustic waves in a two-ion plasma is studied. It is found that the presence of a small amount of lighter ion impurities significantly changes the instability domain in the ω-φ plane. The effect of the concentration and mass of impurity ions on the modulational instability is discussed in detail. The threshold amplitude for instability and nonlinear frequency shift of the wave are also calculated. The predictions of the theory are found to be in good agreement with the experimental observations.
Propagation of a sound wave in a rarefied polyatomic gas
Authors: Banankhah A. , Loyalka S. K.
Published: July 2002

The propagation of a sound wave in a polyatomic gas is studied by using the Hanson and Morse polyatomic gas model [Phys. Fluids 10, 345 (1967)] in the Wang-Chang and Uhlenbeck equation, with Maxwell’s diffuse boundary conditions. The model equation and the associated boundary conditions are transformed into a system of integral equations, which is then solved numerically. The computed results for attenuation and the speed of sound for nitrogen, oxygen, and air at 300°K, and a wide range of rarefaction parameters, are compared with the experimental data of Greenspan [J. Acoust. Soc. Am. 31, 155 (1959)], and of Meyer and Sessler [Z. Phys. 149, 151 (1957)], and previously reported theoretical results.
The wall region of a turbulent boundary layer
Authors: Phillips W. R. C.
Published: July 2002

An analysis is undertaken of a nominally two-dimensional turbulent boundary layer in zero pressure gradient. In accord with Yajnik [J. Fluid Mech. 42, 411 (1970)], the well known law-of-the-wall similarity relationship for the mean velocity, Reynolds stress, and turbulence fields in the wall region is shown to be only the leading term in an expansion, the higher-order terms of which are dependent upon streamwise Reynolds number. The asymptotic behavior of the streamwise velocity, Reynolds stress, and turbulence intensities is deduced to leading order for two parts of the wall region, one immediately next to the wall and the other far enough from it for the effects of viscosity to be unimportant. A continuous function is then deduced for the velocity that precisely satisfies and links the asymptotics in each region; the corresponding distributions of Reynolds stress and turbulence intensities follow. Agreement with experiment at high Reynolds number is excellent. No ad hoc turbulence assumptions are made.
Resistive fluid turbulence in diverted tokamaks and the edge transport barrier in H-mode plasmas
Authors: Hahm T. S. , Diamond P. H.
Published: July 2002

The thermal and particle diffusivities driven by resistive fluid turbulence in diverted tokamak edge plasmas are calculated. Diverted tokamak geometry is characterized by increased global shear near the separatrix and the tendency of field lines to linger near the x point. For resistive fluid turbulence, the dominant effect is increased global shear, which causes a reduction in the effective step size of the turbulent diffusion process and corresponding improvements in heat and particle confinement close to the separatrix. Stability of resistive kink modes resonant near separatrix is also ensured by the increased global shear. The relevance of these considerations to the L→H transition and to the edge transport barrier in H-mode plasmas is discussed.
Electron beam hollowing instability simulations
Authors: Godfrey Brendan B.
Published: July 2002

Intense electron beam hollowing instabilities, although routinely observed in axisymmetric computer simulations of beam propagation in air, are not often seen in experiments because of competing nonaxisymmetric effects. Recent experiments with the IBEX accelerator 4 MeV, 70 kA electron beam [R. B. Miller, M. G. Mazarakis, J. W. Poukey, and R. J. Adler, IEEE Trans. Nucl. Sci. 30, 2722 (1983)] provided detailed measurements of hollowing instability scaling over the pressure range of 10–80 Torr. Simulations performed in support of those experiments are reported here. Both experiment and simulation show large amplitude hollowing at the lowest pressures, decreasing in magnitude as the pressure is increased. The instability is suppressed above about 120 Torr. In addition, the simulations indicate that hollowing can be reduced by matching the beam radius at injection with its equilibrium value in air. The good agreement between experiment and simulation validates existing hollowing instability scaling laws.
Superthermal electron production from hot underdense plasmas
Authors: Aithal Shridhar , Lavigne Pierre , Pépin Henri , Johnston Tudor Wyatt , Estabrook Kent
Published: July 2002

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.
Current sustainment by synchrotron radiation in a tokamak device
Authors: Fidone I. , Granata G. , Johner J.
Published: July 2002

Current drive using anisotropic synchrotron radiation in a tokamak device is considered. A relativistic formulation of the current drive efficiency, which includes both momentum transfer and asymmetric resistivity, is presented. The explicit computation of the current drive efficiency is performed for a Maxwellian plasma using Trubnikov’s theory of radiation [in Reviews of Plasma Physics, edited by M. A. Leontovich (Consultants Bureau, New York, 1979), Vol. 7, p. 345]. The effect of the various synchrotron harmonics on the total generated current is examined, and it is shown that most of the contribution comes from a few harmonics near n=ω/ωc≫1 for which the corresponding resonant velocities are in the same direction. The general properties of the synchrotron radiation and its capability to generate current in a reactor are also briefly discussed.
Poloidal ion motion during formation of toroidal high beta plasmas
Authors: Antoniades J. A. , Goldenbaum G. C. , Weber B. V.
Published: July 2002

Spectroscopic measurements of the ion motion in a fast compression high beta tokamak experiment show paramagnetic ion motion. This motion is initially caused by magnetic field gradient drift, which does not produce any net paramagnetic current opposing the plasma diamagnetic current. After the crowbar of the fast rising toroidal magnetic field a larger paramagnetic polarization drift develops, which contributes to the disappearance of the diamagnetic well.
Linear theory of an electron cyclotron autoresonance maser with a ‘‘phase filter’’
Authors: Chen Liu , Kho T. H. , Chen K. R. , Lin A. T.
Published: July 2002

The linear growth rate of an electron cyclotron autoresonance maser (CARM) with a phase filter, constructed by imposing a transverse magnetostatic wiggler, is derived from relativistic kinetic theory. The theoretical results confirm previous findings from computer simulation showing substantial enhancement of the growth rate near the autoresonance condition. This theory provides an additional, more readily accessible tool for designing a novel phase filter-assisted CARM. The mathematical formalism developed to treat the complex electron motion in this device is introduced.
Association between weakly relativistic ion modulation modes in a collisionless plasma
Authors: Nejoh Yasunori
Published: July 2002

Association between weakly relativistic ion modulation modes described by the Korteweg–de Vries equation in the small wavenumber region and by the nonlinear Schrödinger equation in the finite wavenumber region is found in a collisionless unmagnetized plasma. It is shown that the stability criterion for the modulation of the ion wave envelope depends on the relativistic effect. The present results may explain the observations obtained in space plasmas.