bluuebunny/crossref_metadata_2025 huggingface dataset:
Coherent nonlinear destabilization of tearing modes
Authors: Kleva Robert G. , Drake J. F. , Bondeson Anders
Published: March 2003

The linear stability of a helical quasi-equilibrium containing large-amplitude q=2 magnetic islands in a cylinder is investigated. Three-dimensional tearing modes are driven unstable by the strong current gradient associated with this helical equilibrium.
A class of rotating compact tori equilibria
Authors: Clemente R. A. , Farengo R.
Published: March 2003

A class of semi-analytical equilibria for rotating compact tori with toroidal current density vanishing at the separatrix is presented.
Observation of hot electron ring instabilities in ELMO Bumpy Torus
Authors: Hiroe S. , Wilgen J. B. , Baity F. W. , Berry L. A. , Colchin R. J. , Davis W. A. , El Nadi A. M. , Haste G. R. , Hillis D. L. , Spong D. A. , Uckan T. , Owens T. L.
Published: March 2003

A high-frequency hot electron instability is observed in ELMO Bumpy Torus (EBT) plasmas when the hot electron-to-ion density ratio exceeds 0.4. Both the real frequency and the imaginary frequency are larger than the ion cyclotron frequency. The azimuthal mode number (m) is 7, and the instability rotates in the hot electron curvature drift direction. This instability is identified as a curvature-driven mode. When it is strongly excited, the equilibrium of the hot electron annuli and confined plasmas are destroyed (disruption). Ion heating and neutron bursts are associated with this instability.
The ion feature in a laboratory plasma: Theory and experiment using CO2 laser light scattering
Authors: Grésillon D. , Olivain J. , Truc A. , Lehner T. , Surko C. M.
Published: March 2003

The electron density fluctuation spectrum in the range of frequencies below the ion plasma frequency (the ‘‘ion feature’’) is investigated in plasmas where the ion acoustic waves are weakly damped. The situation typical in laboratory plasmas where the electron temperature is larger than the ion temperature is considered in detail. Analytical expressions are given in the limit of weak damping and the variations of the spectrum with the relevant parameters are readily obtained, including the dependence on frequency, wavenumber, temperature ratio, drift velocity, and the concentrations of light impurity ions. Velocity distributions with non-Maxwellian tails are also considered. An experiment is described in which the scattering of CO2 laser radiation from a discharge plasma is studied. The observed fluctuation spectrum corresponds to the ion feature, in that the dependence of the observed frequencies on wavenumber corresponds to the dispersion relation of ion acoustic waves. They are only observed, however, in the edge plasma where drifting ions enhance the fluctuation level, and not in the plasma center, where light impurities in the argon plasma damp out the ion resonance.
RFP energy evolution and containment in the presence of giant sawteeth
Authors: Watt R. G. , Little E. M.
Published: March 2003

Experimental measurements of internal energy in the ZT-40M Reversed Field Pinch are presented that reinforce the interpretation of the soft x-ray sawtooth oscillations seen in that device as indicating an internal energy loss. Soft x-ray spectra are presented that support the contention that hot electron confinement, as well as energy confinement, is severely degraded by the presence of large scale sawteeth, presumably during the fall time of the soft x-ray flux. The behavior of the equilibrium fields may be explained as the result of a combination of current redistribution and a decrease and recovery in internal βp during a sawtooth.
Bénard convection with time-periodic heating
Authors: Roppo M. N. , Davis S. H. , Rosenblat S.
Published: March 2003

A thin liquid layer, which is heated from below, has its lower boundary modulated sinusoidally in time with amplitude δ. Weakly-nonlinear stability theory shows that the modulation produces a range of stable hexagons near the critical Rayleigh number. For small δ the range is O(δ4) in size and decreases with modulation frequency. These hexagons bifurcate subcritically and correspond to downflow at cell centers.
The evolution of long-wave solutions to the nonlinear Schrödinger equation
Authors: Cramer M. S. , Watson L. T.
Published: March 2003

In water of moderate depth, the behavior of small perturbations superimposed on Stokes wave trains is described by the nonlinear (cubic) Schrödinger equation. In the present study wave-like solutions to this equation are examined, and it is shown that when these perturbations are neutrally stable and sufficiently long, solutions to the Schrödinger equation may be approximated by the well-known Korteweg–deVries equation. As a result, sufficiently long perturbations to Stokes wave trains may be regarded as mathematically analogous to those imposed on a free surface separating two fluids of different densities. This result is established independently by singular perturbation techniques, numerical computation, and comparison of exact stationary wave solutions.
Dispersion of discrete particles by continuous turbulent motions. Extensive discussion of the Tchen’s theory, using a two-parameter family of Lagrangian correlation functions
Authors: Gouesbet G. , Berlemont A. , Picart A.
Published: March 2003

The theory of dispersion of discrete particles by continuous turbulent motions is first expressed in a 3-D formalism, as a synthesis between the 1-D Tchen’s theory of dispersion and the 3-D Batchelor’s theory of diffusion. Then it is exemplified with the aid of a two-parameter Frenkiel family of Lagrangian correlation functions, taking into account the Basset’s term or neglecting it. It is then shown and explained that even dense discrete particles may disperse faster than fluid particles. That work is included in a more general framework aiming at modeling and predicting the behavior of discrete particles in turbulent flows.
On the stability of vortex motions in the presence of magnetic fields
Authors: Fung Y. T.
Published: March 2003

Stability analyses are performed on a general class of vortex flows with density inhomogeneity and magnetic fields. Sufficient conditions for stability and for instability are derived. By examining those conditions and three general types of perturbations to the flow, it is shown that the azimuthal velocity can convey either destabilizing or stabilizing effects depending on the gradient it possesses and the centrifugal force field it creates. For flow subject to azimuthal perturbations, an upper bound on the complex phase velocity, which is reminiscent of the semiellipse theorem in the nonhydromagnetic case, is found for some flow profiles satisyfing a condition. As demonstrated by the sufficient condition for instability and supported by exact solutions to the stability equations, the presence of the magnetic field, regardless of the detailed profile, always stabilizes azimuthal perturbations. For uniform rotation and constant angular Alfvén velocities, azimuthal instabilities can only occur when negative radial density gradients exist within the flow domina. Furthermore, the phase velocity for such instabilities must lie on a semicircle in the complex phase velocity plane regardless of the detailed density profile. Both the semiellipse and semicircle bounds clearly demonstrate that the westward drift phenomenon earlier derived for flows subject to arbitrary disturbances cannot in general be applied to azimuthal modes.
Formation of vorticity fronts in shear flow
Authors: Stern Melvin E. , Voropayev Sergey I.
Published: March 2003

When a cylindrical laminar jet flowing into a resting fluid of the same density has its transport increased to another steady value, an axisymmetric convergence forms between the fast and slow streams. The free boundary surrounding the jet deforms with time, and a steeply sloping vorticity front forms at the nose of the convergent region. This propagates with a speed equal to the average signal velocity (for infinitesimal amplitude disturbances) associated with the laminar flow on either side of the convergence, according to a locally consistent ‘‘shock’’ joining condition which supplements a previous long-wave theory. The prediction is verified by observations of a dyed jet issuing from a round nozzle at Reynolds numbers of approximately 102. It is suggested that the vorticity frontogenesis effect is also important in the turbulent flow over a flat plate.
Marginal stability curve and linear growth rate for rotating Couette–Taylor flow and Rayleigh–Bénard convection
Authors: Dominguez-Lerma M. A. , Ahlers Guenter , Cannell David S.
Published: March 2003

Results of high numerical accuracy are reported for the growth rate σ of axisymmetric disturbances to circular Couette flow between concentric cylinders with the inner one rotating and of two-dimensional disturbances of the pure conduction state of a Boussinesq fluid between horizontal parallel plates heated from below. When σ is positive, the former case leads to Couette–Taylor (CT) flow and the latter to Rayleigh–Bénard (RB) convection. The curve of vanishing growth rate (marginal stability curve) Tc(q) or Rc(q), where q is the wave vector of the disturbance and T and R are the Taylor and Rayleigh number, respectively, is obtained for both cases. The correlation length amplitude ξ0 is calculated by obtaining the curvature of the marginal stability curve. The amplitude σ0 of σ was determined from data with T>Tc(q) or R>Rc(q). In that range, the wave vector qm of maximum σ is also found. For the CT system, the calculations were performed systematically as a function of radius ratio η for 0.975≥η≥0.1. For the RB system, those parameters which depend on the Prandtl number P were obtained for 10−3≤P≤103. Some of the RB results could be compared with analytic calculations and served to test this numerical procedure. It is expected that these results will be useful in calculations using an amplitude equation for the weakly nonlinear state and in studies of wavenumber-selection processes.
Suction effects on turbulence statistics in a heated pipe flow
Authors: Elena Max
Published: March 2003

An experimental study concerning characteristic length scales of the longitudinal velocity fluctuation u′ and the temperature fluctuation θ′ is made in a turbulent pipe flow with a slightly heated porous wall. Frequencies derived, using Taylor’s hypothesis, from time microscales and integral scales, are measured for u′ and θ′ deep in the viscous sublayer. Very near the wall it appears that suction reduces the inflow number which is directly linked to the process of generation of turbulence. Moreover, the skewness and flatness factors of u′ and θ′, and of their time derivatives, are measured. The main results are the relatively high values of flatness with internal intermittency and of skewness with high production of kinetic or thermal turbulence. These values are also reduced by suction.
The structure of normal shock waves in polytropic gases
Authors: Badr Omar
Published: March 2003

The formation and structure of normal shock waves in compressible, polytropic gases is investigated using the continuum gas dynamical equations. A phenomenological model for temperature-dependent viscosity and thermal conductivity is preassumed. The fundamental resulting nonlinear differential equation is solved exactly, and the complete structure of the shock wave is determined. It is found that the flow quantities and characteristics depend upon one parameter, the upstream Prandtl number Pr1, which satisfies the inequality 3γ/(γ+1)≤2Pr1<3, where γ is the ratio of specific heats. For smaller values of Pr1, the strength and compression ratio of the shock increase. Special cases of interest cannot be obtained from this solution and are to be treated independently. Numerical results are given and discussed.
Converging second sound shock waves in superfluid helium
Authors: Torczynski J. R.
Published: March 2003

A spherical heater and conical channel are used to produce converging second sound shock waves. Arrival time and temperature jump data verify the validity of the weak shock relations. The calculated relative velocities (around 10 m/sec) are the largest ever produced in bulk superfluid helium. Comparison of converging and straight channel experiments shows unambiguously that the phenomena associated with the breakdown of superfluidity in second sound shock experiments occur near the heater, not in the bulk fluid.
Variational structure of the Vlasov equation in multidimensional systems
Authors: Dominguez R. R. , Berk H. L.
Published: March 2003

A self-adjoint structure for the Maxwell integral equations, derived for distributions satisfying the Vlasov equation, is demonstrated for a class of plasma equilibria with one ignorable coordinate. It is shown that a general type of self-adjointness is manifest for a class of systems for which there is no magnetic field component in the ignorable direction. The integral kernel also is shown to have a natural differential operator representation.
Parametric instabilities near the ion-cyclotron frequency in an inhomogeneous plasma
Authors: Sharma O. P. , Tripathi V. K. , Liu C. S.
Published: March 2003

A kinetic theory for the parametric instabilities of ion-cyclotron, drift-cyclotron, and drift-cyclotron-loss-cone modes of finite wavenumber in an inhomogeneous plasma is developed. The Vlasov equation is solved in guiding center coordinates for two types of equilibria, viz., Maxwellian and loss-cone velocity distributions. For flute modes (k∥=0) or modes with long parallel wavelengths (k∥vi∥<ω) the coupling coefficients are greatly simplified. Ion-cyclotron waves are seen to decay efficiently into low-frequency drift waves in tokamaks. In a mirror machine a high-amplitude drift-cyclotron-loss-cone mode of negative energy decays into low-frequency positive-energy modes, giving rise to explosive instability.
Approach to equilibrium of a turbulent test-wave system
Authors: Menyuk C. R. , Chen H. H. , Lee Y. C.
Published: March 2003

The validity of weak turbulence theory and microcanonical ensemble theory for describing the evolution of a test-wave system is discussed. In this system, waves interact in triads, and each wave belongs to only one triad except for the test wave which belongs to them all. This study was motivated by a suggestion due to Meiss that this system might be useful for understanding turbulence and by the observation that wave evolution in weak turbulence theory is only weakly dependent on background interactions. It is found that weak turbulence theory fails to describe accurately the behavior of this system while microcanonical ensemble theory succeeds, albeit on a very long time scale.
Reduced equations for nonlinear three-dimensional plasma evolution
Authors: Chu M. S. , Jensen T. H.
Published: March 2003

It is useful to cast the plasma evolution equations for a general three-dimensional configuration in terms of a set of new gauge functions based on the magnetic field. A set of reduced equations may be arrived at in this gauge by making the simple assumption that the compressional motion is decoupled from the shear motion. The equations first proposed by Strauss can be shown as the special case when the perpendicular perturbation wavelength is much shorter than the equilibrium length scale.
Tearing instability in an anisotropic neutral sheet
Authors: Chen James , Palmadesso P.
Published: March 2003

A study is made of the collisionless tearing-mode stability properties of a neutral sheet whose temperature distribution is anisotropic. A kinetic description is used for both ions and electrons. The wave vector k of the perturbation is taken to be parallel to the equilibrium magnetic field B0 and perpendicular to the equilibrium current J0. The analysis is carried out for the low-frequency perturbation (‖ω‖≪ωci). It is found that the conventional technique of matching the inner and outer asymptotic solutions at the electron inner region (two-region approximation), thus neglecting the axis-crossing ion orbits outside the electron inner region, is inadequate for the anisotropic case. An intermediate region in which the axis-crossing ion orbits make the dominant contribution is identified. The eigenvalue equation is solved using both analytic approximations and numerical methods to obtain the eigenmode structure and the linear dispersion relation. Specializing primarily to anisotropic ions with isotropic or weakly anisotropic electrons, it is shown that inclusion of the ion intermediate region can enhance the growth rate by one order of magnitude over the conventional isotropic and anisotropic results based on the neglect of the ion intermediate region.
Numerical solution of the resistive magnetohydrodynamic boundary layer equations
Authors: Glasser A. H. , Jardin S. C. , Tesauro G.
Published: March 2003

Three different techniques are presented for numerical solution of the equations governing the boundary layer of resistive magnetohydrodynamic tearing and interchange instabilities in toroidal geometry. Good agreement among these methods and with analytical results provides confidence in the correctness of the results. Solutions obtained in regimes where analytical methods fail indicate a new scaling for the tearing mode as well as the existence of a new regime of stability.
Parametric excitation of ion Bernstein waves by a fast wave antenna in the ion cyclotron frequency range
Authors: Skiff F. , Ono M. , Wong K. L.
Published: March 2003

Parametric excitation of ion Bernstein waves is observed with an ICRF fast wave induction loop antenna in the ion cyclotron frequency range (ω0∼2Ωi−4Ωi). Important features of the decay process are investigated and discussed.
Thermal effects on the efficiency of Doppler-shift dominated cyclotron masers
Authors: Lin A. T. , Chang W. W. , Lin Chih-Chien
Published: March 2003

It is demonstrated by particle simulations that a relativistic electron beam with 1.25 MeV energy, 1.5 kA current, and 0.5% momentum spread can convert 25% of its energy into electromagnetic radiation based on the Doppler-shift dominated cyclotron maser mechanism. However, the increase in beam thermal spread reduces the available free energy Δγ=γ0−γR by increasing the beam resonant energy γR where γ0 is the initial beam energy and substantially decreases the conversion efficiency. Further increase in thermal spread eventually changes the saturation mechanism from phase trapping to energy depletion.
Interpretation of end-on interferometry in field-reversed configurations
Authors: Tuszewski M. , McKenna K. F.
Published: March 2003

For field-reversed configurations formed from a 20 mTorr static gas filling in the FRX-C device, the density in the end regions is determined by side-on interferometry. This allows us to understand the radial profiles obtained from end-on holograms, and provides a more accurate estimate of the FRC particle confinement time than previously obtained.
Analysis of plasma density profiles and thermal transport in laser-irradiated spherical targets
Authors: Fechner W. B. , Shepard C. L. , Busch Gar. E. , Schroeder R. J. , Tarvin J. A.
Published: March 2003

Measurements are presented for plasma density profiles, critical surface trajectories, and mass-ablation rates, as obtained from experiments in which layered spherical targets were irradiated with 1.05-μm laser light at intensities between 1×1014 and 1×1015 W/cm2. The targets were designed to allow study of thermal transport in overdense plasmas. They were from 70 to 110  μm in diameter, with a variable thickness parylene ablator and an aluminum substrate. Simulated results are consistent with all three sets of data if transport of both thermal and suprathermal electrons is severely inhibited, with a flux limiter of 0.03≤f≤0.06 being appropriate.
Electric phase plane representation of one-dimensional plasma oscillations
Authors: Wang Dong-Jian , Harris Joseph D.
Published: March 2003

In the BGK model of a plasma, the distribution functions may be integrated to give the charge density as a function of the electric potential. The concept of the electric phase plane provides a convenient method of examining the solutions of the Poisson equation in this or any similar one-dimensional model. It is shown in detail how this method is used, the properties of the solutions in the electric phase plane are analyzed and several examples of the potential-field curves corresponding to interesting potential-distance distributions are exhibited.
Influence of intense equilibrium self-fields on the spontaneous emission from a test electron in a relativistic nonneutral electron beam
Authors: Davidson Ronald C. , McMullin Wayne A.
Published: March 2003

A uniform density (n̂b) relativistic nonneutral electron beam with radius Rb propagates parallel to a uniform magnetic guide field B0êz. The current density is assumed to be uniform with J0bz(r)=−n̂beβbc=const within the beam (0<r<Rb). The spontaneous emission from a test electron is calculated for emission in the forward direction (along êz ) including the important influence of equilibrium radial self-electric and azimuthal self-magnetic fields. For an electron beam with νb/γb≪1 (here, νb is Budker’s parameter, and γbmc2=const is the characteristic electron energy), it is found that the spontaneous emission spectrum is peaked for oscillation frequency (ω) and wavenumber (k) satisfying the resonance conditions ω−kvz= ω−≡(ωcb/2) ×[1−(1−s)1/2] and ω−kvz≡ω+≡(ωcb/2)[1+(1−s)1/2], where s=(2ω2pb/ω2cb)(1−βzβb) is the self-field parameter. Here, vz=βzc is the axial electron velocity, ωcb=eB0/γbmc is the electron cyclotron frequency, and ω2pb=4πn̂be2/γbm is the electron plasma frequency-squared. Finally, an applied helical wiggler field δB=δB(cos k0zêx+sin k0zêy) is added to the field configuration, and the influence of equilibrium self-fields on the wiggler-induced spontaneous emission in the forward direction is calculated for emission near the free-electron-laser resonance condition ω−kvz=k0vz.
Characteristics of lateral and axial transport in laser irradiations of layered-disk targets at 1.06 and 0.35 μm wavelengths
Authors: Mead W. C. , Campbell E. Michael , Kruer W. L. , Turner Robert E. , Hatcher C. W. , Bailey D. S. , Lee P. H. Y. , Foster J. , Tirsell K. G. , Pruett B. , Holmes N. C. , Trainor J. T. , Stradling G. L. , Lasinski Barbara F. , Max C. E. , Ze F.
Published: March 2003

Results and analysis are presented for Be-on-Al disk target irradiations at 1.06 and 0.35 μm laser wavelengths with 600–700 psec pulses, 240 μm spot diameter, and 1×1014 W/cm2 absorbed intensity. Absorptions of 32%–39% (1.06 μm) and 90% (0.35 μm) are largely due to inverse bremsstrahlung. The hard x-ray spectra indicate low hot-electron fractions of 10−2 (1.06 μm) and 10−4 (0.35 μm). Backreflected light shows strong hot spots for 0.35 μm irradiations. Multiple absolute and relative x-ray measurements are compared with one- and two-dimensional computer hydrodynamics calculations. Only weak indications of lateral transport are found and limits are set from x-ray imaging and spectral data from targets with and without a surrounding Ti shield. Axial transport appears strongly inhibited at 1.06 μm and mildly inhibited at 0.35 μm wavelength. Measured shock-wave transit times and velocities imply ablation pressures of 7 Mbar (1.06 μm) and 11 Mbar (0.35 μm).
Observation of magnetic fields in laser-produced plasma using the Zeeman effect
Authors: McLean E. A. , Stamper J. A. , Manka C. K. , Griem H. R. , Droemer D. W. , Ripin B. H.
Published: March 2003

The self-generated magnetic field produced when a pulsed Nd:glass laser is focused onto a carbon foil target is measured for the first time using the Zeeman effect. Emission from the CV 1s2s3S1–1s2p3P2 (2270.9 Å) was used for the measurement. At a laser irradiance of ∼5×1012 W/cm2, the magnetic field increased from 100 to 200 kG when the focal spot was changed from a flat-topped distribution to a ring pattern, which caused higher density gradients. A Faraday rotation measurement at approximately 200 kG agreed with the Zeeman effect measurement. Opacity effects in the plasma enhanced the sensitivity, allowing the Zeeman measurement to be made in this magnetic field range.
Pressure fluctuation terms in the turbulence transport equations
Authors: van den Berg B.
Published: March 2003

The transport equations for the six turbulent stress tensor components are discussed, with particular emphasis on the terms containing turbulent pressure fluctuations. The conventional separation of the pressure fluctuation terms is argued. A more logical separation is proposed.
Enhanced dispersion in oscillatory flows
Authors: Kurzweg U. H. , Howell G. , Jaeger M. J.
Published: March 2003

Measurements of the dispersion coefficient in oscillating pipe flows for a wide range of Womersley numbers, tidal displacements, and tube radii are presented. The results are shown to be in good agreement with those of a recent laminar dispersion theory for oscillatory flows.
The stability of two-dimensional linear flows
Authors: Lagnado R. R. , Phan-Thien N. , Leal L. G.
Published: March 2003

A theoretical investigation is made of the linear stability of a viscous incompressible fluid undergoing a steady, unbounded two-dimensional flow in which the velocity field is a linear function of position. Such flows are approximately generated by a four-roll mill device which has many experimental applications, and can be characterized completely by a single parameter λ which ranges from λ=0 for simple shear flow to λ=1 for pure extensional flow. The linearized velocity disturbance equations are analyzed for an arbitrary spatially periodic initial disturbance to give the asymptotic behavior of the disturbance at large time for 0≤λ≤1. In addition, a complete analytical solution of the vorticity disturbance equation is obtained for the case λ=1. It is found that unbounded flows with 0<λ≤1 are unconditionally unstable. An instability criterion relating the initial disturbance wave vector α to the steady flow strain rate E, kinematic viscosity ν, and the parameter λ is obtained. This criterion shows that for all admissible values of E and ν, a wave vector α may be found which corresponds to disturbances that grow exponentially in time. The growth of these disturbances is accompanied by a growth of vorticity oriented along the principal axis of extensional strain in the case λ=1. The results of this investigation also confirm the established fact that simple shear flow (λ=0) is stable to all infinitesimal spatially periodic disturbances.
Analysis of flux-relaxed electron heat waves
Authors: Fechner W. B. , Mayer F. J.
Published: March 2003

It has long been known that a classical description of electron heat conduction results in energy transport speeds which are unphysically large and that the results of model calculations using the classical description are inconsistent with the results of many laser fusion experiments. Some insight into the proper corrective measures is obtained if the plasma kinetic equations are solved via the ‘‘13 moment’’ approximation of Grad, rather than the more usual but incomplete Chapman–Enskog method. The Grad method naturally incorporates time variation of the electron distribution function and leads to a ‘‘flux-relaxation’’ equation for the heat flux, resulting in a hyperbolic temperature equation in which heat flow is restricted to speeds less than a characteristic electron thermal velocity. This paper contains some new insight into the subject of flux relaxation in the form of similarity solutions for the heat flow, as well as numerical solutions for boundary conditions appropriate to laser fusion modeling. Included in the treatment is the effect of hydrodynamics on the flux-relaxed heat flow. The results are that although flux relaxation naturally incorporates classical flux-limiting it does not play a role in the observed ‘‘anomalous’’ flux inhibition.
Field-error induced transport in a pure electron plasma column
Authors: Keinigs Rhon
Published: March 2003

The long confinement times achieved in experiments on pure electron plasmas are explainable in terms of the conservation of canonical angular momentum in azimuthally symmetic systems. A low-density, pure electron plasma is considered, and the effect of disrupting the system’s symmetry by introducing a small, magnetic field error is investigated. It is found that this external perturbation can resonantly drive low-frequency waves that couple back to the field error and produce a change in the plasma’s angular momentum. This results in changing the radius of the column.
Resistive stability of the cylindrical spheromak
Authors: DeLucia J. , Jardin S. C. , Glasser A. H.
Published: March 2003

The growth rates for resistive instabilities in a straight circular cylinder with spheromak profiles are computed by using two complementary methods. The first method employs boundary layer analysis and asymptotic matching, most valid for values of the magnetic Reynolds number S≳105. The second method solves the full linearized resistive magnetohydrodynamic (MHD) equations as an initial value problem, utilizing zone packing around the mode rational surface. Resolution requirements limit this to S≲107. The results from these two methods agree to better than 1 in 103 in the overlap region 107≳S≳105. A scan of parameter space reveals that for parabolic q profiles, the least unstable configurations have q0R/a∼0.67. The Hall term in Ohm’s Law is easily incorporated into both methods. Recalculating the resistive MHD growth rates in the presence of this term shows that the resistive interchange mode is completely stabilized for a large enough value of the ion cyclotron time.
Ballooning instabilities in the ELMO Bumpy Torus (EBT)
Authors: Nguyen K. , Kammash T.
Published: March 2003

The macroscopic stability of a model ELMO Bumpy Torus (EBT) plasma taking into account variations of the field curvature along the magnetic field is numerically investigated in the long-thin limit. When the compression term has a pole in the ring region the background ballooning mode becomes stable for any value of the hot electron beta because of strong line bending which results from the interaction of the hot electron component with the plasma. Axially longer rings give better stability, and the upper bound on the core beta can be improved by increasing the mirror ratio although at the expense of reducing stability of the background ballooning mode.
Alfvén ion-cyclotron instability in tandem-mirror plasmas. I
Authors: Smith Gary R.
Published: March 2003

The linear theory of Alfvén ion-cyclotron instability is studied for ion distributions that model those in tandem-mirror plasmas. The uniform-plasma approximation leads to the following qualitative stability results. Distributions that fill a mirror cell of mirror ratio R have stability limits for beta (plasma pressure/magnetic-field pressure) that increase rapidly with R. Warm ions can damp instability caused by anisotropic hot ions. Injection of neutral beams at angles far from perpendicular to the magnetic field is a method of plasma formation with good stability properties. A plasma composed of ions with different charge-to-mass ratios can achieve beta approximately twice as large as for a single-ion-species plasma. The model distributions used here are convenient also for the eikonal treatment of axially nonuniform plasmas, which are studied in a subsequent paper.
Propagation of pressure waves in bubbly mixtures
Authors: Tan M. J. , Bankoff S. G.
Published: March 2003

The propagation of steady pressure disturbances through a liquid containing small gas bubbles is investigated numerically. The equations of continuity and motion, together with the complete bubble dynamics equation, are employed. The analysis takes into consideration the effects of relative translational motions between the bubbles and the liquid. The results show the possibility of oscillatory shock waves that start with a steep rise in pressure followed by a relaxation region wherein the pressure exhibits damped oscillations about a final equilibrium value. For strong shocks, the predicted frequencies of pressure oscillations are so high that they may be unsusceptible to measurement. For weak shocks (p2/p1<2), the predicted wave forms are in agreement with the results of previous theories.
Feedback effects on stimulated backscattering from laser-produced plasmas
Authors: Mitchel G. R. , Johnson T. W. , Pépin H.
Published: March 2003

Brillouin and Raman backscattering can be enhanced by reflective feedback from underdense plasma structures, such as quarter-critical density profile steepening due to two-plasmon decay. Rapid time modulation of broadband scatter can be explained by modulation of the feedback from a variation of the round trip phase shift due either to motion of the feedback surface, in Brillouin backscattering, or by periodic creation and destruction of the feedback surface itself, as in Raman backscattering.
Small-angle multiple scattering of charged particle beams
Authors: Hughes Thomas P. , Godfrey Brendan B.
Published: March 2003

The scattering of self-pinched particle beams propagating in gas is investigated using Moliere’s treatment of multiple scattering. Nordsieck lengths significantly longer than those predicted on the basis of less accurate scattering formalisms are obtained numerically. An improved analytic expression for the Nordsieck length is derived and agrees well with the numerical results. Experimental data on beam expansion from the ASTRON experiment show good agreement with numerical simulations and analytic results. The evolution of a thin annular beam due to scattering is studied, and the beam is found to acquire a Bennett profile in a distance on the order of one Nordsieck length.
Nonlinear evolution of the resistive interchange mode in the cylindrical spheromak
Authors: DeLucia J. , Jardin S. C.
Published: March 2003

Results are presented of a study of various aspects of the single helicity nonlinear development of the resistive interchange mode in the cylindrical spheromak. A formulation of the helically symmetric resistive magnetohydrodynamic (MHD) equations that partially separates the ideal MHD characteristics is developed. Mode saturation can occur because of the quasilinear flattening of the pressure profile in the vicinity of the mode rational surface. However, this saturation process is defeated when the plasma overheats and in regions of the plasma where the shear is low. Finite fluid compression has significant, and optimistic, consequences on the long-term nonlinear behavior of this mode. For a tearing mode stable cylindrical spheromak equilibrium configuration with an axial beta value of 6%, complete overlap of the m=1 islands occurs in about 3% of the resistive skin time for a magnetic Reynolds number of S=105. For typical parameters of the S-1 device at Princeton, this time corresponds to nearly one millisecond.
Self-organization of the reversed-field-pinch quiescence
Authors: Yoshida Zensho , Uchida Taijiro , Inoue Nobuyuki
Published: March 2003

The self-organization of the reversed-field-pinch quiescence is studied by introducing a deterministic approach to the incompressible dissipative magnetohydrodynamics. The reversed-field configuration is shown to be a quasisteady state that is stable as an attractor. The tokamak-type branch of the attractor and the turbulence in the usual Navier–Stokes system are also discussed.
Stimulated Brillouin scattering in inhomogeneous flowing plasma
Authors: Berger R. L.
Published: March 2003

The threshold, growth rate, and convective amplification of stimulated Brillouin scattering are found in an inhomogeneous flowing plasma for obliquely incident laser light. Absolutely unstable modes exist for special combinations of wave vectors that correspond to forward scattering in a supersonically flowing plasma. Some of the analysis is shown to apply to stimulated Raman scattering at a density extrema. For parameters of interest to laser fusion, these thresholds are lower than for stimulated backscattering in a density gradient.
Macroscopic guiding center stability theorem for nonrelativistic non-neutral electron flow in a cylindrical diode with applied magnetic field
Authors: Davidson Ronald C.
Published: March 2003

A sufficient condition is derived for electrostatic stability of nonrelativistic non-neutral electron flow in a cylindrical diode with applied magnetic field B0êz. The analysis is based on a cold-fluid guiding-center model that treats the electrons as a massless, strongly magnetized fluid with ω2pb(r)/ ω2c =4πn0b (r)mc2/ B20 ≪1, and flow velocity Vb (x, t)=−c∇φ(x, t)×êz/B0. Making use of global conservation constraints satisfied by the fluid-Poisson equations, it is shown that ∂n0b(r)/∂r ≤0 over the interval a≤r≤b is a sufficient condition for stability to small-amplitude electrostatic perturbations. Here n0b(r) is the electron density profile, and the cathode is located at r=a and the anode at r=b. Space-charge-limited flow with E0r (r=a)=0 is assumed. The analysis illustrates the major generality and flexibility of using global conservation constraints to determine a sufficient condition for stability. Nowhere is it necessary to make direct use of a detailed normal-mode analysis or eigenvalue equation.
Electromagnetic ion beam instabilities
Authors: Gary S. Peter , Smith Charles W. , Lee Martin A. , Goldstein Melvyn L. , Forslund David W.
Published: March 2003

The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right-hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T⊥/T∥ gives rise to the left-hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left-hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both right and left elliptically polarized modes.
Evolution of nonlinear ionization wave packets excited in glow discharges
Authors: Ohe Kazuyuki , Hashimoto Masashi
Published: March 2003

The evolution of envelope solitons of ionization wave packets with finite amplitude is observed in positive columns of He glow discharges. The wave packet excited by a high-voltage pulse divides into two or three packets with propagation along the discharge tube. The divided wave packets can be regarded as envelope solitons by evaluating the coefficients of the nonlinear Schrödinger equation for the ionization waves. The growth rate of the modulational instability, measured from the degree of modulation agrees with the numerically calculated one. The number of envelope solitons formed increases with the width of wave packet in the initial stage.
Preionization and start-up in the ISX-B tokamak using electron cyclotron heating at 28 GHz
Authors: Kulchar A. G. , Eldridge O. C. , England A. C. , Bush C. E. , Edmonds P. H. , Kelley G. G. , Loring C. M. , Peng Y-K. M. , Wilgen J. B. , Borowski S. K.
Published: March 2003

A 28-GHz gyrotron is used to produce a plasma at the electron cyclotron resonance in the Impurity Study Experiment (ISX-B) tokamak. The influence of the toroidal magnetic field magnitude, error fields, gas pressure, microwave power, microwave pulse length, and microwave timing is studied for experiments with magnetic field and gas only. Also, experiments with preionization followed by capacitor discharges are carried out in which these quantities are varied, as are the capacitor bank voltages. Optimum conditions of preionization for some of the parameters are determined. A theoretical model that adequately reproduces the data is given. Calculations based on this model show the temporal evolution of the electron temperature and density, the neutral density, and the plasma current. The model adequately accounts for present and previous experimental results and can be used to make predictions for future experiments. The decay of the discharge with no input power is not correctly predicted by the model.
Stimulated Raman scattering, two-plasmon decay, and hot electron generation from underdense plasmas at 0.35 μm
Authors: Figueroa H. , Joshi C. , Azechi H. , Ebrahim N. A. , Estabrook Kent
Published: March 2003

Experimental studies of stimulated Raman back and side scattering, two-plasmon decay, and the generation of high-energy electrons in 0.35 μm laser plasma interaction are presented. To isolate the various phenomena occurring at different densities, we have attempted to control the maximum plasma density by varying the thickness of the foil targets. The scattered light frequency is used as a diagnostic to measure the peak plasma density. Time integrated and time resolved scattered spectra for variable plasma densities are discussed. Effects of self-generated magnetic fields and plasma temperature on SRS and 2ωp decay, respectively, are examined as possible mechanisms responsible for splitting of the backscattered spectrum at ω0/2. A discussion of the effects of a parabolic density profile on the SRS threshold is also included. Finally, the measured energy and angular distribution of the high-energy electrons are discussed. Two-plasmon decay is suggested as the probable mechanism generating the hot electrons.
Propagation of an intense relativistic electron beam through a plasma region into vacuum
Authors: Destler W. W. , O’Shea P. G. , Reiser M.
Published: March 2003

An intense relativistic electron beam (IREB) is injected through a narrow plasma region into a vacuum drift tube at current levels far above the space-charge limit of the drift tube. Experiments show that accelerated co-moving positive ions from the plasma enable the beam to propagate to a collector at the end of the 15 cm diameter drift tube. In one set of experiments, the IREB (1 MeV, 27 kA, 30 nsec) passes through a hydrogen gas cloud with an effective width of approximately 2 cm. The net current measured by a 7 cm diameter collector at the end of a 55 cm long drift tube is 70% of the peak injected electron current. In another configuration, the IREB is injected through a laser-produced carbon plasma. A model is proposed linking electron beam propagation with collective ion acceleration which results in the generation of charge- and current-neutral plasmoids capable of free space propagation. Possible implications of this work to astrophysical processes (cosmic ray acceleration) and laser fusion (observation of high-energy positive ions escaping from target plasma) are discussed.
Ion heating and axially evanescent modes in rf experiments on bumpy tori and other toroidal devices
Authors: Sperling J. L. , Hamasaki S.
Published: March 2003

A fluid model including quasilinear heating, collisions, charge exchange, and spatial transport is used to determine the perpendicular wavenumbers and electric fields which can permit appreciable coupling of rf waves to the ions in ELMO Bumpy Torus-Scale (EBT-S) plasmas. The observed significant ion heating at frequencies above the first harmonic ion-cyclotron frequency in EBT-S is consistent with a quasilinear heating model if rf wave amplitudes vary on a spatial scale which is much smaller than the inverse wavenumber for propagating, cold-plasma waves in the WKB approximation. It is shown that axially evanescent modes could account for the necessary, short perpendicular wavelengths. In general, axially evanescent modes could be a powerful means for heating ions at harmonics of the ion-cyclotron frequency. Charge exchange and transport are the dominant loss mechanisms acting on ions and preclude strong collisional coupling of ion tails or minority ion species to the bulk distribution of the majority ion species in EBT-S. This is true even if the applied rf frequency is comparable to the ion-cyclotron frequency of the minority ion species.
Modeling the pressure redistribution terms
Authors: Morris Philip J.
Published: March 2003

A model is developed for the pressure redistribution terms in the Reynolds stress equation. It is shown that, following some basic assumptions, the same general form of the model is obtained whether or not the turbulence is regarded as being homogeneous. The unknown coefficients in this model and those for models of previous investigators are evaluated on the basis of data from basic experiments.
Oscillatory and steady convection in dielectric liquid layers subjected to unipolar injection and temperature gradient
Authors: Castellanos A. , Atten P. , Velarde M. G.
Published: March 2003

The effect of a temperature variation of both dielectric constant and ionic mobility on the stability of a horizontal dielectric liquid layer subjected to an electric field, weak unipolar injection, and heating from below is analyzed. Regions of steady and oscillatory convective instability are delineated by means of a Galerkin method as well as by direct computer integration of the thermoelectrohydrodynamic equations. A heuristic description of all relevant physical phenomena involved in the problem is also given.
Some analytical results for thermal transpiration and the mechanocaloric effect in a cylindrical tube
Authors: Lang H. , Loyalka S. K.
Published: March 2003

Analytical expressions for the phenomenological coefficients for thermal transpiration and the mechanocaloric effect in a cylindrical tube, relating to flow of a rarefied gas, are obtained by considering the BGK model. Neumann iteration and variational techniques are used in the near free molecular regime and the method of elementary solutions is used in the near continuum regime. The previously reported results of Ferziger and Loyalka on Poiseuille and thermal creep flows are extended and augmented.
Electron-cyclotron maser and whistler instabilities in a relativistic electron plasma with loss cone distribution
Authors: Tsang Kang
Published: March 2003

The electron-cyclotron maser and whistler instabilities are investigated in a unified treatment. The dispersion relation is solved numerically and extensive parametric studies are performed. It is found that growth rates for both instabilities increase with perpendicular hot electron temperature and the size of the loss cone.
Equation of state for relativistic plasma waves
Authors: Pegoraro F. , Porcelli F.
Published: March 2003

It is shown that in a fully relativistic plasma an isothermal equation of state must be assumed in order to obtain from fluid equations the correct dispersion equation of longitudinal plasma waves. This isothermal equation of state contrasts with the adiabatic equation that applies to a non-relativistic plasma. For arbitrary values of the ratio α between the particle rest mass and particle temperature, a polytropic equation of state must be introduced. The dependence of the polytropic index γ on α can be derived from the relativistic Vlasov equation and can be shown to join the adiabatic (α≫1) and the isothermal (α≪1) regimes smoothly.
Plasma resistivity in the presence of a reversed-field pinch dynamo
Authors: Schoenberg K. F. , Moses R. W. , Hagenson Randy L.
Published: March 2003

A global calculation of reversed-field pinch (RFP) plasma resistivities, ηw∥ and ηk∥, based on power balance and magnetic helicity balance, respectively, is discussed. For reasonable RFP beta values (β<0.1), ηk∥ provides an estimate of plasma resistivity parallel to the magnetic field, and agrees with the Spitzer resistivity within a factor of two. The expression (1−ηk∥/ηw∥) indictes the fraction of ohmic heating power absorbed by fluctuations in the discharge. An examination of 180 kA discharges on the Los Alamos ZT-40M experiment suggests that approximately one-third of the ohmic heating power drives fluctuations, including the dynamo effect, during discharge sustainment.