bluuebunny/crossref_metadata_2025 huggingface dataset:
Decay of a lower-hybrid wave to two lower-hybrid waves
Authors: Reiman Allan
Published: March 2003

The threshold and initial evolution for the decay of a lower-hybrid wave to two lower-hybrid waves are determined. Included are the effects of convective losses in a three-dimensional lower-hybrid resonance cone geometry and the effects of inhomogeneity appropriate to lower-hybrid heating in a tokamak. It is found that the decay to two lower-hybrid waves differs from other such parametric instabilities previously studied in that its instability threshold can have a strong dependence on the minor radius of the tokamak. This is due to the quasi-absolute nature of the instability. While its threshold power is high in small devices, the instability poses a serious threat in reactor size devices.
Interaction between large and small scale motions in a two-dimensional turbulent duct flow
Authors: Rajagopalan S. , Antonia R. A.
Published: March 2003

The organized structure in a two-dimensional turbulent duct flow has been studied experimentally using measurements of the wall shear stress fluctuation τ′w and the longitudinal velocity fluctuation u′. The correlation between the high and low frequency parts of τ′w and u′ indicates a definite phase relation. A simple criterion based on the amplitude of the derivative of the low-pass filtered u′ signal is used to educe the ensemble-averaged shapes of τ′w and u′ within the organized structure. One of the main features of the τ′w and u′ signatures of the organized structure is the reduction in these quantities, followed by a sudden increase toward the back of the structure. The normal velocity fluctuation v′ exhibits a sharp decrease associated with the rapid acceleration in u′. A peak in the value of the ensemble-averaged Reynolds shear stress is associated with the sharp changes in u′ and v′.
Further results related to the turbulent boundary layer with slot injection of helium
Authors: LaRue John C. , Libby Paul A.
Published: March 2003

Data from an experiment involving the slot injection of helium into a turbulent boundary layer in air are analyzed in terms of unconditioned and conditioned Favre averages. The conditioning is based on two levels of helium concentration so that the contributions to the unconditioned statistics from air, helium, and a mixture of these two gases can be determined. The distributions of intermittency associated with the two helium levels establish the domains of influence of air, helium, and the mixture.
Theoretical pressure correlation functions in turbulent boundary layers
Authors: Meecham W. C. , Tavis M. T.
Published: March 2003

A Poisson equation is constructed for the pressure in terms of velocity fluctuations and is solved. The pressure correlation has a second-order velocity fluctuation contribution (usually assumed dominant in past work) and a fourth-order velocity correlation contribution. The velocity correlations are assumed to be those, suitably weighted, found from homogeneous turbulence measurements. Results are computed for two boundary layers: an idealized (canonical) turbulent boundary layer and a deliberately thickened boundary layer. The pressure variances are found to be essentially the same as the measured values. The correlations are positive except the streamwise, x correlations. The canonical layer shows a pressure variance which peaks very close to the wall, near the laminar sublayer. The fourth-order velocity fluctuations may generally be neglected for the thickened boundary layer; close to the wall they are an important part of the variance and correlation for the canonical layer.
Observation of shock-generated turbulence in a magnetized plasma by CO2 laser scattering
Authors: Gold S. H. , DeSilva A. W. , Huba J. D.
Published: March 2003

Small angle CO2 laser scattering is used to sense density fluctuations produced by microinstabilities in a collisionless plasma driven by a magnetic implosion in a high voltage theta pinch. Turbulent density fluctuations are measured at a wavelength near the Debye length within and behind the current sheet of the imploding shock. Two main scattering features are found. The earlier feature, occurring at the leading edge of the current sheet, is only about 30 nsec wide and is narrowly radial. It appears to be driven by ions reflected from the shock front and is believed due to an ion beam driven electron cyclotron instability. The later feature, peaking about 70 nsec after the first, is broader in time and has a maximum rotated about 25° from the azimuthal (current sheet) direction, with a half-width of 40°. It appears to be due to the ion acoustic instability, driven by relative drift of electrons and ions in the current sheet.
Bifurcation and ’’strange’’ behavior in instability saturation by nonlinear three-wave mode coupling
Authors: Wersinger J.-M. , Finn J. M. , Ott Edward
Published: March 2003

A simple model of nonlinear saturation of an unstable mode is studied. The model consists of the resonant three-wave coupling equations with the highest frequency wave linearly unstable and the two lower frequency waves damped. As the damping of the stable waves is increased, the solutions go through an interesting succession of qualitative changes, including bifurcations to increasingly complex periodic solutions and the appearance of apparently chaotic (i.e., aperiodic) solutions. These qualitative features are shown to be explainable on the basis of a one-dimensional mapping which is numerically derivable from the original system of differential equations.
Electron temperature gradient driven microtearing mode
Authors: Gladd N. T. , Drake J. F. , Chang C. L. , Liu C. S.
Published: March 2003

The linear stability theory of an electron temperature gradient driven microtearing mode, an instability recently proposed as a possible cause for anomalous electron thermal transport in tokamaks, is considered. The theory is electromagnetic and is carried out within the context of a slab model with a sheared magnetic field. In contrast to the linear theory of drift waves, where any magnetic shear is completely stabilizing, shear may actually increase the growth rate of microtearing modes. The crucial feature required for instability is the energy dependence of electron-ion collisions. The mode is shown to be unstable for electron temperature gradients and degrees of collisionality typical of present day tokamaks. It is found that previous theories of these modes were based on assumptions which are not, in general, justified; a case in point being the fact that the usually neglected electrostatic effects are actually quite important in producing instability.
Wave-particle transport from electrostatic instabilities
Authors: Gary S. Peter
Published: March 2003

The second-order theory for electrostatic microinstabilities driven by currents both across and parallel to a uniform magnetic field in a Vlasov plasma is considered. Both electrons and ions are taken as magnetized, and propagation is in the plane defined by the drift velocities and the magnetic field. A consistent procedure is used to compare wave-particle exchange frequencies of momentum and energy for the lower hybrid density drift, ion cyclotron electron density drift, universal density drift, ion acoustic current, and ion cyclotron current instabilities. In this model, resistivities and heating frequencies of the universal instability are substantially greater than those due to the other drift modes, and wave-particle transport due to the ion cyclotron electron density drift instability is larger than that of the lower hybrid density drift instability at Te≳Ti.
Ponderomotive laser-plasma effects subject to strong magnetic fields and collisions
Authors: Shkarofsky I. P.
Published: March 2003

The self-generated magnetic field in laser-plasma interactions is allowed to be sufficiently strong so that the cyclotron frequency exceeds the electron-ion collision frequency. Kinetic theory is applied to determine the ponderomotive terms in the transport coefficients of current flow, heat flow, and electron pressure tensor in a plasma with Z≳≳1. In particular, perpendicular to the magnetic field and parallel to the gradient, a B−10 dependence can occur in the current and heat flow. The source terms in the equation for the self-generated magnetic fields are derived, and several terms differ from those of a weak magnetic field. The moment relations with ponderomotive contributions are given including the contribution to the collisional momentum transfer. The time-averaged isotropic part of the electron velocity distribution function is allowed to have an arbitrary form.
Nonlinear saturation and thermal effects on the free electron laser using an electromagnetic pump
Authors: Lin A. T. , Dawson J. M.
Published: March 2003

The production of visible laser radiation by a low energy relativistic electron beam (MeV) is most readily accomplished through the use of a high frequency (far infrared) electromagnetic pump (λr=λ0/4γ02); such radiation can be generated by reflecting the radiation the relativistic electron beam generates when it interacts with a rippled static magnetic field. It is important to investigate what limits the efficiency in this situation. It is found that with a weak pump field, the pump energy can be amplified by a factor of 4γ20 and the pump intensity by 16γ40 with the instability being terminated by pump depletion. With a strong pump field, particle trapping in the longitudial potential wave stops the growth of the instability and the initial momentum spread (temperature) limits the saturation amplitude.
Dispersion through a flowing fluid with variable viscosity and diffusion coefficient
Authors: Shukla J. B. , Parihar R. S.
Published: March 2003

In the study of dispersion of a solute through a flowing fluid it is shown that, for a given mean velocity, the Taylor diffusion coefficient decreases as the viscosity or the thickness of the peripheral layer decreases and as the molecular diffusion coefficient of this layer increases.
Stimulated Brillouin rescattering
Authors: Speziale T. , McGrath J. F. , Berger R. L.
Published: March 2003

Rescattering of stimulated Brillouin scattered light is investigated. The net reflection coefficient for light scattered by a plasma slab is calculated and compared with the single scatter result.
Energy deposition of fast α particles in a fully ionized deuterium-tritium plasma
Authors: Khandekar D. C. , Sahni D. C.
Published: March 2003

The transport equation of Antal and Lee has been solved analytically to obtain the energy flux of α particles in a fully ionized deuterium-tritium plasma having uniform temperature, density, and composition in space and time. The energy deposited by α particles in the plasma is then obtained analytically when small angle scattering is the only mechanism for energy transfer. The results agree well with the earlier numerical results obtained by Cooper and Evans and Antal and Lee using numerical integration and the Sn method, respectively.
Higher-order estimates of conditional eddies in isotropic turbulence
Authors: Tung T. C. , Adrian R. J.
Published: March 2003

Conditional eddies in isotropic turbulence are examined by estimating the velocity field around a point where the velocity is specified. The results indicate the low-order estimates qualitatively resolve the large scale structure for all probable levels of the fluctuation intensity.
Transport properties of a two-temperature helium plasma
Authors: Kannappan D. , Bose T. K.
Published: March 2003

The viscosity, thermal conductivity, and electrical conductivity have been computed for a two-temperature helium plasma with electron to heavy particle temperature ratios of 1 to 3, at pressures from 0.001 to 1000 atm, and with electron temperatures ranging from 5000 to 35 000 °K.
Approximate solution in gasdynamics
Authors: Sirovich L. , Chong T. H.
Published: March 2003

One-dimensional unsteady gasdynamics is considered. An approximation based mainly on the interaction of simple and entropy waves is adopted. A discussion supporting this approximation, based in part on shock expansion theory, is given. By the use of certain transformations the approximation leads to solution in terms of quadratures. Excellent agreement with exact numerical results is obtained over a wide range of cases.
Iterative approach to strong turbulence theory
Authors: Knorr Georg , Mond Michael
Published: March 2003

Starting from the equations for turbulence, relations are derived which allow the computation of the correlation function of turbulent modes. This is achieved by using the following ideas: (1) The turbulence equations are interpreted as stochastic equations. (2) The directness of interaction of the different modes is ordered according to the number of modes necessary to establish the feedback of a given mode on itself. (3) A non-Markovian dynamical friction is introduced and is determined by the condition that the average stochastic force should be zero. In lowest order, Kraichnan’s direct interaction approximation is recovered.
Higher corrections to the direct interaction approximation in turbulence theory
Authors: Mond Michael , Knorr Georg
Published: March 2003

The next higher approximation to the direct interaction approximation is calculated making use of an approach to the turbulent equations which has been reported earlier. The equations of turbulence are interpreted as stochastic equations and an ordering scheme is set up which depends on increasingly indirect feedback of a mode on itself. In the lowest approximation, the direct interaction approximation is recovered. A calculation of the next higher approximation beyond the direct interaction approximation is presented. The higher approximations are of increasing order and are a formal expansion in ’’powers’’ of Green’s functions which appear in convolution integrals.
Quantitative representation of nonrepetitive temporal behavior
Authors: Nakagawa Y. , Teuber D. L.
Published: March 2003

Analytical representations suitable to analyze the nonrepetitive pulse-like temporal behavior of physical quantities are derived. The representation utilizes solutions of a linear model equation in which the temporal variation is subject to time-dependent driving and dissipative forces. The property of solutions is described, and it is shown that such representations can provide a basis for quantitative comparisons of behaviors and a basis for physically meaningful interpretations of the results. Observations of solar flares in the soft x-ray flux have been analyzed with this method.
Studies of sidescatter and backscatter from pre-ionized plasmas
Authors: Herbst M. J. , Clayton C. E. , Peebles W. A. , Chen F. F.
Published: March 2003

Stimulated Brillouin scattering at 180° and sidescatter at 90° are observed when CO2 laser light is incident on an arc-pre-ionized, underdense plasma target. Scattering occurs in short spikes, sometimes early in the pulse, sometimes delayed by as much as 700 nsec. This time behavior depends critically on the state of pre-ionization and is explained by the evolution of a laser-driven ionization wave. Using ruby laser interferometry, it is possible to infer the sources of backscatter and sidescatter within this disturbance. In addition, by using a smooth input pulse, it is found that the spikiness of Brillouin scatter is not due solely to mode beating in the input beam.
Amplification of radiation near cyclotron frequency due to electron population inversion
Authors: Lee L. C. , Wu C. S.
Published: March 2003

Amplification of electromagnetic waves via the cyclotron maser mechanism by a population of weakly relativistic electrons is studied. The effect of a tenuous population of low energy background plasma is included. It is found that both the ordinary and extraordinary modes can be excited by the weakly relativistic electrons with a loss-cone distribution. The growth rate for the extraordinary mode is much higher than that for the ordinary mode. Velocity spread in the energetic electron distribution function may reduce the growth rate by a factor of approximately 10 from that in the monoenergetic case. The maximum growth rate for the fast extraordinary mode (X mode) occurs near the upper hybrid cutoff frequency. Numerical results are obtained and discussed.
Linear analysis of the double-tearing mode
Authors: Pritchett P. L. , Lee Y. C. , Drake J. F.
Published: March 2003

The linear behavior of the double-tearing mode is investigated within the framework of magnetohydrodynamics. A two-space-scale analysis in which resistive solutions valid near the rational surfaces are joined to ideal solutions outside these regions is performed and used to derive the dispersion relation for the mode. If the separation of the rational surfaces at x=±xs is sufficiently small [xs/a<(kya)−7/9S−1/9], the growth rate is predicted to scale as S−1/3, and the structure of the mode proves to be essentially identical with that of the m=1 tearing mode in cylindrical geometry. With increasing separation, the mode makes a transition to the S−3/5 scaling and structure of the standard tearing mode. These predictions are confirmed by direct numerical solution of the magnetohydrodynamic equations, and the S−1/3 scaling is shown to be correlated with violations of the constant-ψ approximation. Possible physical implications of the double-tearing mode are discussed.
Magnetic driving energy of the collisional tearing modes
Authors: Adler E. A. , Kulsrud R. M. , White R. B.
Published: March 2003

The change in the magnetic energy density produced by a collisional tearing mode is calculated exactly. The driving energy for the mode is found to come entirely from the region inside the tearing layer, although there is also a displacement of energy in the outer region which integrates to zero. The total change in magnetic energy is exactly equal to the change in a quadratic form related to a variational principle for the full resistive equations.
Model of ablative acceleration at moderate laser intensities
Authors: Felber F. S.
Published: March 2003

An analytic model of pellets accelerated by ablation produced by lasers of moderate intensity (∼108 to ≳1012 W/cm2) is presented. The solutions for partially ionized ablation of hydrogen support earlier estimates indicating that the pellet fueling requirements of a demonstration fusion reactor can probably be met practicably by existing laser technology. The method of solution can be applied to ablative acceleration by partially ionizing irradiation of targets of other materials as well.
An exact solution for the oscillating two-stream instability
Authors: Kaup D. J.
Published: March 2003

An exact solution for the oscillating two-stream instability is presented for the case where the initial pump profile has a constant phase. The main features exhibited by the solution are (i) a very rapid partial pump depletion when the scaled pump energy is approximately π/2, 3π/2, 5π/2, etc., (ii) filamentation of a square pump profile undergoing any such partial depletion, and (iii) stability when the scaled pump energy is just above nπ, and instability when it is just below nπ.
Stability effects of an external conducting open sheet on magnetically confined plasmas
Authors: Weston V. H.
Published: March 2003

The effect of different outer conducting configurations on the stability of a cylindrical plasma is treated with emphasis on the k∼0, n=1 mode corresponding to perturbation of the form exp(ikz±inϑ). The conducting configuration is comprised of an open segment of a cylinder with a gap in it. The problem is formulated in terms of a singular-differential integral equation which is then reduced to a regular Fredholm integral equation which can be solved numerically. Results are given for a circular segment (−α<ϑ<α, r=A) for various values of the half-angle α and radius A, both in the presence of and absence of a complete circular outer conductor. The stability effects of the various geometries are compared through their resulting value for the minimum vacuum energy integral.
Self-trapping of intense optical beams in plasmas
Authors: Felber F. S.
Published: March 2003

Two new results concerning the propagation of intense optical beams in plasmas are presented. The first is an analytic solution of the relativistic nonlinear equations governing the steady-state diffraction of intense, circularly polarized, electromagnetic beams in warm, quasi-neutral plasmas. The beams cannot self-focus, but can self-trap (propagate without diverging) at powers above Pc=2×105 (λD/λ)2 MW, where λD/λ is the Debye length/vacuum optical wavelength ratio. The second result is an exact solution of the relativistic nonlinear equations for a self-trapped beam propagating with constant cross section; this solution yields self-consistent density and intensity profiles and a relativistic beam-plasma dispersion relation.
Climbing water films in experiments on Rayleigh–Taylor instabilities
Authors: Popil R. , Curzon F. L.
Published: March 2003

The climbing liquid films observed in Rayleigh–Taylor instability experiments were investigated by a new capacitance method which yielded an improved similarity law for the height of the film as a function of time. This law permits the adverse effects of climbing films to be minimized.
Study of x-ray emissions from a copper plasma produced by a laser
Authors: Sinha B. K. , Gopi N.
Published: March 2003

Using a 50 MW, 60 nsec ruby laser, variation of x-ray emissions from a copper plasma with laser flux has been studied in a purely collisional regime for laser-plasma interaction. Scaling laws of x-ray intensity and plasma temperature versus laser flux are reported. Relative contributions of recombination, bremsstrahlung, and line emissions were calculated on the basis of the steady-state corona model for the ionization states. The emitted recombination radiation was found to be approximately 104 times greater than the bremsstrahlung radiation. The temperature estimated from bremsstrahlung ratios was seen to be highly erroneous.
Helical relativistic electron beam Vlasov equilibria
Authors: Lai H. M.
Published: March 2003

Three existing helical relativistic electron beam models are discussed and compared. Both Yoshikawa’s and Lawson’s models are shown to be derivable from appropriate Vlasov equilibria. A new helical Vlasov equilibrium with energy spread is presented and studied. Unlike Auer’s axial current model in which the allowance of an energy spread limits the total current in the relativistic beam case, the present model, with the addition of an azimuthal current, permits solutions with arbitrarily large current. On the other hand, like the model studied by Kan and Lai, the present model leads to non-hollowed-out beam solutions in which, the larger the beam current, the more force-free is the magnetic field configuration.
Reflection and absorption of electromagnetic waves obliquely incident on a half-space relativistic plasma
Authors: Imre Kaya , Özizmir Ercüment
Published: March 2003

Reflection and absorption characteristics of electromagnetic waves obliquely incident upon a half-space relativistic Vlasov plasma are investigated. Numerical results comparing the properties of the s- and p-polarized waves are given. It is shown that the response of plasma can be best understood by studying the locations of the roots κt and κl of the transversal and longitudinal dispersion functions in the complex wavenumber plane. In general, s-polarized waves are reflected and p-polarized waves are absorbed more efficiently by the plasma. Total reflection occurs when κt becomes zero for the case of s-polarization and when κl becomes zero for the case of p-polarization. Total penetration of p-waves is obtained for sufficiently large frequencies at the Brewster angle ϑB. These waves exhibit a sharp absorption peak at the characteristic angle ϑc at which the transversal transmission terminates. For a given temperature, p-waves are totally absorbed in the plasma at a specific frequency at which ϑB=ϑc. Absorption peaks are broader at higher temperatures. If both transversal and longitudinal modes propagate undamped at some value of angle of incidence ϑ, there exists a secondary smooth and broad absorption peak at larger values of ϑ for the p-waves. The magnitude of the secondary peak can also be appreciable at the ultrarelativistic temperatures.
Analytic study of azimuthal asymmetries in relativistic hollow e-beams
Authors: Epstein B. G. , Poukey J. W.
Published: March 2003

A study is made of zero-thickness relativistic hollow electron beams propagating in a vacuum guide field. The azimuthal variation in surface density for a beam emitted from an off-centered circular cathode is calculated exactly by solving Laplace’s equation in a region between two eccentric conductors. The cathode problem is used as an initial state for solving the off-centered beam propagation problem in the drift region. The ϑ-bunching effect of the azimuthal asymmetries is estimated and related to specific experiments.
Vortex simulations of the Rayleigh–Taylor instability
Authors: Baker Gregory R. , Meiron Daniel I. , Orszag Steven A.
Published: March 2003

A vortex technique capable of calculating the Rayleigh–Taylor instability to large amplitudes in inviscid, incompressible, layered flows is introduced. The results show the formation of a steady-state bubble at large times, whose velocity is in agreement with the theory of Birkhoff and Carter. It is shown that the spike acceleration can exceed free fall, as suggested recently by Menikoff and Zemach. Results are also presented for instability at various Atwood ratios and for fluids having several layers.
Radiation effects in a nonequilibrium corner-expansion flow of ionizing argon
Authors: Igra O.
Published: March 2003

The two-dimensional, steady, inviscid, nonequilibrium corner-expansion flow of ionized argon is solved numerically using the method of characteristics. The solution takes into account the chemical and thermal nonequilibrium processes as well as the appropriate radiation losses associated with the flow. It is shown that for the flow conditions considered (pre-corner values of temperature and electron number density are of the order of 1 eV and 1017 cm−3, respectively) the radiation losses play a minor role. Based on a comparison with experimental results, it is apparent that the proposed numerical solution can be used for the solution of supersonic, nonequilibrium plasma flows.
Thermal diffusion of ions drifting in an electric field through a neutral gas
Authors: Weinert Ulrich
Published: March 2003

A simple perturbation theory is applied to investigate the influence of an external temperture gradient on the drift of ions moving through a neutral gas under the influence of a homogeneous electric field. Based upon the two-temperature model, an expression is given for the field-dependent thermal diffusion factor.
Alteration of Pfirsch–Schlüter transport in tokamaks by all four external sources
Authors: Burrell Keith H.
Published: March 2003

Tokamak energy and particle transport are calculated for a plasma in the Pfirsch–Schlüter regime containing all possible external sources: particle, momentum, heat, and heat momentum. The effect of the last source has never been considered previously; its presence makes possible simultaneous control of the impurity flux into the plasma and of the heat flux from the plasma. Examples are given showing that such control is possible with existing neutral beam or rf sources.
Parametric excitation of ion quasi-mode by the pump near the ion cyclotron frequency
Authors: Ono M. , Porkolab M. , Chang R. P. H.
Published: March 2003

Parametric excitation of a nonresonant ion quasi-mode in a low density (i.e., ωpi≈Ωi) plasma is observed when the pump frequency is ω0≳2Ωi. From the interferometry measurement, the lower sideband is identified to be the cold lower hybrid wave and the low-frequency mode is shown to be a nonresonant ion quasi-mode. The dependence of the excitation process and the threshold upon density has been measured and was found to agree well with theory. The ion heating associated with this decay process is also observed. Such processes may take place near the plasma surface during ion cyclotron range of frequency heating experiments in tokamaks.
Resonant absorption by a magnetic plasma at a rippled critical surface
Authors: David F. , Pellat R.
Published: March 2003

When laser light is incident on a plasma, unstable filamentary structures may develop, producing a rippling of the critical surface. Such density ripples lead to resonant absorption of normally incident, p-polarized laser light. In order to compute the absorption coefficient, the eikonal approximation for the ray path is adopted and the standard results for oblique incidence (characterized by the value of the plasma dielectric constant at the cutoff frequency of the incident light) are applied. Explicit results are given for various angles of incidence and ripple scale lengths. A one-dimensional plasma in a nonuniform magnetic field is then considered. In agreement with published results, it is also shown that the laser light absorption is mainly dependent on the magnetic field value at cutoff. Finally, the effect of a rippled critical surface and a magnetic field of thermoelectric origin (∇n×∇T) are combined and the resulting absorption coefficient is computed. The results are given for various angles of incidence, ripple scale lengths, and ratios of the electronic cyclotron to plasma frequencies.
Observations of fluctuating ωp emission in Alcator tokamaks
Authors: Hutchinson I. H. , Kissel S. E.
Published: March 2003

Measurements are presented of fluctuating millimeter-wave radiation from the Alcator tokamaks. Its characteristics are (1) rapid modulation, approaching 100% with rise time 3–4 μsec, (2) very narrow line width, <5 GHz at the plasma frequency, and (3) extremely large intensity, up to 200 times thermal. These characteristics distinguish this radiation from the steady ωpe emission previously documented and are interpreted as indicating nonlinear conversion of electrostatic oscillations as the origin of the radiation.
A channel model for long arcs in air
Authors: Latham Don J.
Published: March 2003

The results of an empirically based model for a vertical direct current arc discharge in 1 atm air are presented. The arc is found to be quasi-steady state for currents between 20 and 5000 A. There are two distinct core temperature ranges, one near 7000 °K and the other near 12 000 °K, and the transition between these occurs over a current range between 40 and 150 A. Arc temperature and energy balance curves are presented for several values of total arc current.
Ideal gasdynamics in Hamiltonian form with benefit for numerical schemes
Authors: Buneman Oscar
Published: March 2003

It is shown that the ideal flow equations can be put in Hamiltonian form by means of Clebsch-type variables. This suggests an efficient numerical scheme for storing the information on the state of the gas flow and for updating this information in time.
Modification of plasma solitons by resonant particles
Authors: Karpman V. I. , Lynov J. P. , Michelsen P. , Pécseli H. L. , Rasmussen J. Juul , Turikov V. A.
Published: March 2003

A consistent theory of plasma soliton interaction with resonant particles is developed. A simple derivation of a perturbed Korteweg–de Vries equation with the interaction term is presented. It is shown how the known limit cases (such as Ott–Sudan’s, etc.) can be derived from the general equations and what their regions of applicability are. Some effects caused by the soliton-particle interaction (amplitude change-rate, tail formation, etc.) are analyzed by means of a recently developed perturbation method. The analytical results are compared with a direct numerical integration of the perturbed Korteweg–de Vries equation. Laboratory measurements carried out in a strongly magnetized, plasma-filled waveguide and results from particle simulation are interpreted in terms of the analytical results.
Formation of the shock wave near the wave front of nonlinear waves
Authors: Ishii R.
Published: March 2003

Wave fronts of nonlinear waves in a vibrationally relaxing gas are treated. The flow is induced by a piston advancing with finite acceleration into a relaxing gas. Particular attention is paid to the formation of a shock wave near the characteristic wave front in the course of propagation. It is found that the shock may be either a shock front or an imbedded shock according to the undisturbed gas and piston conditions. The conditions for the formation of the shock front are obtained. The initial development of the shock wave is also investigated in detail for the shock front.
Unstable universal drift eigenmodes in toroidal plasmas
Authors: Cheng C. Z. , Chen Liu
Published: March 2003

The eigenmode equation describing ballooning collisionless drift instabilities is analyzed both analytically and numerically. A new branch of eigenmodes, which corresponds to quasi-bound states due to toroidal coupling effects such as ion ∇B drifts, is shown to be destabilized by electron Landau damping for typical tokamak parameters. This branch cannot be understood by the strong coupling approximation. However, the slab-like (Pearlstein–Berk-type) branch is found to remain stable and experience enhanced shear damping.
Adiabatic interaction of coherent waves with weak turbulence in plasmas
Authors: Vaclavik J. , Appert K.
Published: March 2003

The method of correlation functions, together with a Fourier-transform technique, is used to construct self-consistent equations describing the adiabatic interaction of coherent waves with weakly turbulent plasmas. The formalism is applied to study the propagation of a low-frequency ion-acoustic wave in the presence of high-frequency ion-acoustic turbulence. It is found that the turbulent waves increase the phase velocity of the coherent wave in the case where they all propagate so that each has a velocity component in a given direction. In the opposite case, the phase velocity of the coherent wave increases or decreases depending on whether the turbulence peaks at low or high values of kλD.
Scattering of high frequency waves due to density fluctuations
Authors: Hsu J. Y. , Chiu S. C. , Chan V. S.
Published: March 2003

A theory of the scattering of high frequency waves by density fluctuations is formulated. In the plasma interior, the usual wave kinetic equation is used to derive a Fokker–Planck equation with the background fluctuations assumed to be given. The possible excitation of electrostatic drift waves by the high frequency waves is also considered and a quasi-linear equation is demonstrated. Stochastic changes in the wave frequency and the group velocity result in the corresponding diffusion and friction in phase space.
Nonlinear coupling of tearing modes with self-consistent resistivity evolution in tokamaks
Authors: Carreras B. , Hicks H. R. , Holmes J. A. , Waddell B. V.
Published: March 2003

The nonlinear interaction of tearing modes of different helicity in tokamaks is studied for realistic values of resistivity and parallel heat conduction. The self-consistent evolution of the resistivity is taken into account through the electron heat conduction equation. For equilibrium q profiles inferred from electron temperature profiles measured before a tokamak disruption, the essential result is that the (m=2;n=1) mode nonlinearly destabilizes other modes on a rapid time scale. Because of the development of magnetic islands of different helicity, the toroidal current density is severely deformed. These islands overlap and field lines become stochastic in a sizable plasma volume, flattening the temperature profile in this region through parallel heat transport. The deformation of the toroidal current produces a rapid decrease in the self-inductance of the plasma, and the voltage at the limiter decreases, becoming increasingly negative. An extensive survey of equilibria and initial conditions has been conducted and a simple prescription for their nonlinear stability properties is given.
Rotational instability in a linear theta pinch
Authors: Ekdahl C. , Bartsch R. R. , Commisso R. J. , Gribble R. F. , McKenna K. F. , Miller G. , Siemon R. E.
Published: March 2003

The m=1 ’’wobble’’ instability of the plasma column in a 5-m linear theta pinch has been studied using an axial array of orthogonally viewing position detectors to resolve the wavelength and frequency of the column motion. The experimental results are compared with recent theoretical predictions that include finite Larmor orbit effects. The frequency and wavelength characteristics at saturation agree with the predicted dispersion relation for a plasma rotating faster than the diamagnetic drift speed. Measurements of the magnetic fields at the ends of the pinch establish the existence of currents flowing in such a way that they short out the radial electric fields in the plasma column. The magnitude of rotation, the observed delay in the onset of m=1 motion, and the magnitude of end-shorting currents can all be understood in terms of the torsional Alfvén waves that communicate to the central plasma column the information that the ends have been shorted. The same waves are responsible for the torque which rotates the plasma and leads to the observed m=1 instability. Observations of the plasma in the presence of solid end plugs indicate a stabilization of high-m number modes and a reduction of the m=1 amplitude.
Effects of hot electrons on magnetohydrodynamic modes in ELMO Bumpy Torus
Authors: Nelson D. B.
Published: March 2003

The effect of hot electrons on magnetohydrodynamic instabilities is studied using a fully electromagnetic, finite β, Vlasov–Maxwell model consisting of three species in an inhomogeneous slab geometry with gravity. If no hot electrons are present, the Vlasov model agrees with well-known results from the magneto-hydrodynamic model. If the density of hot electrons is small, but their temperature is sufficiently large, the Vlasov model predicts stabilization of magnetohydrodynamic modes in certain parameter regimes. For ELMO Bumpy Torus parameters this occurs at β=10%–20%, which is in agreement with experimental observations. Comparison is also made with a model that assumes the hot electrons form a rigid non-interacting current. It is found that while the latter model can qualitatively predict the low β stability threshold, it is overly optimistic in describing the finite β behavior.
Trapped particle dynamics and efficiency optimization in free electron lasers
Authors: Kwan Thomas J. T.
Published: March 2003

The dynamics of trapped particles in free electron lasers has been investigated. From the results of computer simulations, it is found that the efficiency of energy extraction from the electron beam maximizes at a particular initial intensity of the electromagnetic radiation. Based on the physical argument of the dynamics of trapped particles, a criterion which accurately predicts the optimum intensity of the electromagnetic radiation has been obtained.
Tokamak plasma variations under rapid compression
Authors: Holmes J. A. , Peng Y-K. M. , Lynch S. J.
Published: March 2003

Changes in plasmas undergoing large, rapid compressions are examined numerically over the following range of aspect ratios A: 3≳A≳1.5 for major radius compressions of circular, elliptical, and D-shaped cross sections; and 3≲A≲6 for minor radius compressions of circular and D-shaped cross sections. The numerical approach combines the computation of fixed boundary magnetohydrodynamic equilibria with single-fluid, flux-surface-averaged energy balance, particle balance, and magnetic flux diffusion equations. It is found that the dependences of plasma current Ip and volume-averaged poloidal beta β̄p on the compression ratio C differ significantly in major radius compressions from those proposed by Furth and Yoshikawa. The present interpretation is that compression to small A dramatically increases the plasma current, which lowers β̄p and makes the plasma more paramagnetic. Despite large values of volume-averaged toroidal beta β̄t (≳30% with safety factor q≈1 at the magnetic axis, q≈3 at the plasma edge), this tends to concentrate more toroidal flux near the magnetic axis, which means that a reduced minor radius is required to preserve the continuity of the toroidal flux function F at the plasma edge. Minor radius compressions to large aspect ratio agree well with the Furth–Yoshikawa scaling laws.
Guiding-center Hamiltonian for figure-8 particles in axisymmetric field-reversed configurations
Authors: Mynick H. E.
Published: March 2003

The guiding-center Hamiltonian K is derived for so-called ’’figure-8’’ particles which are present in field-reversed mirror configurations, using a formalism developed previously. For such particles, the gyro-orbit cannot be approximated by a circle, and thus standard approches to guiding-center theory are totally inapplicable. K manifests this intrinsic difference by a quite different dependence on the gyro-action, and by familiar effects such as mirroring and magnetic-gradient drifts being controlled by the radial derivative of the magnetic field strength B at the point of field reversal, rather than by B itself, as occurs in standard guiding-center theory.
On Kelvin–Helmholtz instability
Authors: Miles John W.
Published: March 2003

It is shown that the wave energy for both modes in an inviscid Kelvin–Helmholtz model is positive in a reference frame moving with the density-weighted mean of the upper and lower flow speeds. This fact is significant in connection with the role of negative energy waves in hydrodynamic stability.
A simplified eigenvalue equation for ideal magnetohydrodynamics
Authors: Kadish Abraham
Published: March 2003

A change of dependent variable renders the eigenfunction equation of ideal magnetohydrodynamics more amenable to analysis of the discrete spectrum. The resulting equation and boundary value problem are discussed.
Electromagnetic instabilities in a focused ion beam propagating through a z-discharge plasma
Authors: Ottinger P. F. , Mosher D. , Goldstein Shyke A.
Published: March 2003

A beam-plasma system consisting of a focused light ion beam propagating through a z-discharge plasma is analyzed for electromagnetic velocity space instabilities. In particular, the Weibel (k⋅B0=0, k⋅Vz≃0) and the whistler (k×B0≃0, k⋅Vz=0) instabilities are studied. It is found that unstable modes do not grow fast enough to affect the beam propagation in a pellet-fusion-reactor chamber. For k⋅Vz≳0, the instabilities convect with a velocity much less than the beam streaming velocity.
Kinetic theory of relativistic plasmas
Authors: Gould Robert J.
Published: March 2003

The thermalization of particle kinetic motion by binary collisions is considered for a plasma with kT∼(10–100) mc2, where m is the electron mass. At this temperature, the principal mechanism for relaxation of electron motion is via radiationless electron-electron collisions (Mo/ller scattering). Ions are nonrelativistic, but are energetic enough so that their Coulomb scattering can be treated in the Born approximation. Relaxation times are computed and Boltzmann-equation Fokker–Planck operators are derived for the various binary-collision processes. The expression for the rate of kinetic energy exchange between electron and ion gases is derived for the case where the gases are at different temperatures.
The behavior of the density oscillator
Authors: Alfredsson Per-Henrik , Lagerstedt Torgny
Published: March 2003

A density oscillator is studied which consists of two fluids of different density separated by a horizontal wall with the dense fluid at the upper side. A small hole connects the fluids, through which fluid is interchanged with distinct pulses. The pulse time is studied experimentally and a dimensional analysis is performed to find the relevant parameters governing the pulse time.
Equilibrium for cylindrical, magnetically insulated ion diodes assuming adiabatic turn-on
Authors: Swegle John , Ott Edward
Published: March 2003

Equilibrium values of diode current are calculated for cylindrical, magnetically insulated vacuum ion diodes as functions of applied voltage, magnetic flux, and aspect ratio. The analysis is based upon the assumption of a slowly (adiabatically) applied voltage so that electrons are confined to a sheath near the cathode where they undergo E×B drifts in the electric field (E) and the magnetic field (B) in the anode–cathode gap. One-dimensional cases in which the applied magnetic field is either axial or azimuthal are treated. A previous analysis, conducted under the assumption of cycloidal electron orbits, found that equilibrium solutions often do not exist. It is found that this difficulty is absent when E×B drift orbits are assumed.
Enhancement of stimulated Raman backscattering by magnetoresonance effects
Authors: Birkett D. S. , Marshall T. C.
Published: March 2003

It is observed experimentally that stimulated Raman backscattering is enhanced whenever the electron gyrofrequency corresponds to the pump frequency or the scattered frequency in the electron frame of motion.
Uniqueness and inversion of the ballooning representation
Authors: Hazeltine R. D. , Hitchcock D. A. , Mahajan Swadesh M.
Published: March 2003

Scale-separation arguments underlying the balloning representation are employed to demonstrate that its coefficients are uniquely determined by the original function, contrary to certain suggestions in the literature. An explicit formula for inverting the series is displayed.
Modulational instability and the Fermi-Pasta-Ulam recurrence
Authors: Janssen Peter A. E. M.
Published: March 2003

The long-time behavior of the modulational instability of the nonlinear Schrödinger equation is investigated. Linear stability analysis shows that a finite amplitude uniform wave train is unstable to infinitesimal modulational perturbations with sufficiently long wavelengths while it is stable for perturbations with short wavelengths. Near the threshold for instability, the long-time behavior of the unstable modulation is obtained by means of the multiple time scale technique. As a result, the Fermi–Pasta–Ulam recurrence is rediscovered, in agreement with recent experiments and with a numerical solution of the problem at hand.
Current generation with low-frequency waves
Authors: Fisch Nathaniel J. , Karney Charles F. F.
Published: March 2003

Various types of traveling waves may be injected into a tokamak to continuously sustain the toroidal current. Interest in this problem arises from the possibility of operating tokamak reactors in the steady state. The low-frequency waves most suitable for this task are identified in terms of the power cost for deployment in a reactor. Means of exciting these waves and tradeoffs with design criteria are discussed. A comparison is made with the alternative attractive regime of high-frequency waves. Conclusions are based, in part, on the numerical solution of the two-dimensional Fokker–Planck equation with an added quasi-linear term due to the waves.
Finite-wavelength oscillations of a diffuse-profile, Vlasov-fluid plasma column
Authors: Sanchez Richard
Published: March 2003

An equation of motion is derived which describes finite-wavelength (i.e., axial wavelength ∼ plasma radius) oscillations of a Vlasov-ion, fluid-electron plasma in a diffuse, near-ϑ pinch geometry. The equations include simultaneous geometric and ion kinetic effects. For a screw-pinch geometry, and with isothermal electrons, these equations are shown to be equivalent to the guiding-center equations. Boundary conditions and a closed-form solution for the sharp-profile case are discussed.
Nonlinear magnetohydrodynamic stability
Authors: Bauer F. , Betancourt O. , Garabedian P.
Published: March 2003

Ways have been found to significantly upgrade the resolution of an equilibrium and stability code that is based on the variational principle of ideal magnetohydrodynamics. Nonlinear saturation of ballooning modes for tokamaks has been demonstrated using a revised version of the code. Second stability regions are calculated from the nonlinear dependence of δW on β. An l=2,3 torsatron is described that appears to have no ideal magnetohydrodynamic instabilities for β as high as 5%.