bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal spectral calculation using eddy-damped quasi-normal Markovian theory
Authors: Vignon Jean-Marc , Cambon Claude
Published: March 2003

The eddy-damped quasi-normal Markovian theory is applied to the computation of spectral transfers of turbulent kinetic energy and temperature variance. This model is then used to predict spectral decay in three situations for which experimental measurements are available.
Nonlinear gravity-capillary stern waves
Authors: Vanden-Broeck Jean-Marc
Published: March 2003

Gravity-capillary stern waves generated by the flow from beneath a horizontal flat plate are considered. An exact relation is derived among the integral properties of these waves, the draft based Froude number F, and the Weber number W. It is used to compute the amplitude of the waves far from the plate. A problem of nonuniqueness is discovered and discussed. In addition, a perturbation solution for large F is given.
Fluctuations, turbulence, and transports in the presence of drift waves
Authors: Okuda H. , Sato T. , Hasegawa Akira , Pellat R.
Published: March 2003

Turbulence spectrum and plasma diffusion have been studied using theoretical analysis and numerical simulations that use a drift wave model in which electrons are assumed to follow the Boltzmann distribution, while ions are assumed to move two dimensionally in the plane perpendicular to the magnetic field. For a plasma near local thermal equilibrium, it is found that while the ion density diffusion is negligibly small, the energy diffusion is much larger and is comparable to the test particle diffusion. In the presence of strong turbulence, δn∼n0, where δn and n0 are fluctuating density and average density, fluctuation energy is found to cascade toward long wavelength modes which accompany the density diffusion. The resultant turbulence spectrum is isotropic in two dimensions perpendicular to the magnetic field. However, the particle diffusion coefficient remains small. When the ion density gradient is maintained by freezing the electron background density, it is found that the turbulence spectrum cascades toward smaller wavenumbers only in the direction perpendicular to the density gradient. As a result an anisotropic turbulence spectrum is formed indicating the appearance of zonal flow in this direction.
Destruction of trapping oscillations by sideband instability
Authors: Shoucri Magdi
Published: March 2003

The nonlinear evolution of a beam-plasma instability is studied numerically by direct solution of the Vlasov–Poisson equations. It is shown that the trapped particle oscillations which follow the saturation of a single launched wave can be destroyed by growing sidebands. The results are in agreement with previously reported experimental results.
Interaction between a Langmuir wave and a ballistic perturbation
Authors: Gervais F. , Olivain J. , Quéméneur A. , Trocheris M.
Published: March 2003

The theoretical solutions of the Landau–Vlasov initial value problem giving mode-mode coupling usually neglect the free-streaming contribution. The problem is solved theoretically including the ballistic terms. A mode, resulting from the interaction between the ballistic perturbation of the frequency ω′ associated with a launched wave and the Landau component of frequency ω′′ of a second launched wave, appears only if ω′′≳ω′. Its frequency is ω=ω′′−ω′ and its phase velocity is identical to that of the Landau mode ω/k=ω′′/k′′. The amplitude of this mode is calculated as a function of distance from the launching probe and this is compared with the amplitude measured experimentally in a plasma column.
Vortex formation during radio frequency heating of plasma
Authors: Motley R. W.
Published: March 2003

Experiments on a test plasma show that the linear theory of waveguide coupling to slow plasma waves begins to break down if the rf power flux exceeds approximately 30 W/cm2. Probe measurements reveal that within 30 μsec, an undulation appears in the surface plasma near the mouth of the twin waveguide. This surface readjustment is part of a vortex, or off-center convective cell, driven by asymmetric rf heating of the plasma column.
Time resolved density profiles of laser-heated plasmas
Authors: Willi O. , Evans R. G. , Raven A.
Published: March 2003

By including the ponderomotive force of the laser light in a simple one-dimensional fluid simulation code, the predicted electron density profiles, for irradiances of 3×1014–2×1016 W cm−2, are in extremely good agreement with measurements. Four distinct phases are observed in the fluid behavior near the critical density and these are compared with analytic models. Furthermore, the laser electric field near the critical density is found to be modulated in time with a period of the order of 10 psec.
Axial laser heating of three meter theta pinch plasma columns
Authors: Hoffman A. L. , Lowenthal D. D.
Published: March 2003

A 3 m long plasma column formed and confined by a fast rising solenoidal field was irradiated from one end by a powerful pulsed CO2 laser. It was found that beam trapping density minima could be maintained for the length of the laser pulse if the plasma diameter exceeded about 1.5 cm. The erosion of the density minimum was governed by classical diffusion processes. Three meter long plasmas in 2.6 cm bore plasma tubes could be fairly uniformly heated by 3.0 kJ of CO2 laser irradiation. Best results were obtained when heating began before or during the theta pinch implosion phase and the plasma fill pressure exceeded 1.0 Torr H2. Plasma line energies of about 1 kJ/m could be obtained in a magnetic field rising to 6 T in 4.7 μsec.
Filamentation of a heavy-ion beam in a reactor vessel
Authors: Lee Edward P. , Yu Simon , Buchanan H. L. , Chambers F. W. , Rosenbluth M. N.
Published: March 2003

A heavy-ion beam driver for inertial confinement fusion is subject to filamentation instability over a broad range of beam and plasma background conditions. The case of a beam injected into a gas-filled reactor vessel, where finite pulse length and propagation distance play an important role in limiting mode growth, is analyzed. The effects of transverse thermal spread, spherical convergence to the pellet, and finite magnetic decay rate of eddy currents are included in this treatment. It is concluded that a cold beam will be severly disrupted unless the product of magnetic plasma frequency and propagation time is not large compared with unity. If this condition is not met, mode growth may still be limited to about six e folds by adding transverse velocity spread such that the pulse tail is in a state of pinch equilibrium. However, this approach causes much of the pulse to be lost by thermal expansion.
Monte Carlo hybrid modeling of electron transport in laser produced plasmas
Authors: Mason Rodney J.
Published: March 2003

A new Monte Carlo hybrid scheme for the transport of electrons in laser produced plamas is described in detail. Comparison is made with alternate schemes. The hot electrons are followed as collisional particles-in-cell. The cold electrons are a donor-cell fluid. The self-consistent E field is computed approximately by either a moment method which seeks to establish quasi-neutrality, or by plasma-period dilation that stretches the local plasma period up to the time step. Application is made to the transport of laser driven electrons in foil-like geometries. Results from a large number of computer simulations are used to confirm the quasi-neutrality of the scheme, to demonstrate important aspects of the hot electron collisionality, to explore the influence of return current E fields from classical and ion-acoustic resistivity, and finally, to examine the efficiency of transport inhibition from thermal convection in the presence of density dips. Attention is drawn to the need for an even more fundamental particle description for the thermals.
A random flight model of inhomogeneous turbulent dispersion
Authors: Durbin P. A.
Published: March 2003

The effect of finite memory time cannot be ignored in many dispersion problems: It is proposed that a random flight model will be more satisfactory than the diffusion approximation in such situations. A calculation of dispersion in open channel flow is presented.
Memory effects in the motion of a suspended particle in a turbulent fluid
Authors: Gitterman M. , Steinberg V.
Published: March 2003

The noninstantaneous behavior of the velocity correlation function for a turbulent fluid leads to memory effects in the equation of motion of a particle suspended in such a fluid. This effect is a direct consequence of the fluctuation-dissipation theorem, which connects the correlation properties of the random force with a memory function. As a result, the equation of motion of a suspended particle in a turbulent flow is an integro-differential rather than a differential equation, and the diffusion coefficient of a suspended particle and that of a fluid do not coincide even in the simplest model. The velocity correlation function of a particle and its diffusion coefficient are found for a simple model.
A variational principle for turbulent flow
Authors: Stern Melvin E.
Published: March 2003

A general control theory is proposed to select the realized stationary state whenever the equations of motion are densely nonunique (degenerate). For the turbulent flow in a pipe with given pressure gradient, the theory implies that the discharge is an extremum. Quantitative results, such as von Kárḿan’s constant, emerge when this variational principle is combined with inequalities pertaining to the mean field. The control theory is also applied to fully turbulent thermal convection, and a variational principle for this problem is obtained which is also consistent with measurements.
Linear centrifugal instability of a thick Stokes’ layer
Authors: Duck P. W.
Published: March 2003

The linear centrifugal instability of a Stokes’ layer, produced by a circular cylinder torsionally oscillating in an infinite viscous fluid is investigated. The problem is studied over a range of ratios of Stokes’ layer thickness to cylinder radius, and the results are seen to agree reasonably well with previously reported experimental measurements.
Inertial and equilibrium shear retardation of electrohydrodynamic instability growth
Authors: Hoburg James F.
Published: March 2003

The effects of fluid inertia and equilibrium shear rate on a normal field, surface-coupled electrohydrodynamic instability in polar geometry are described. Instability growth is retarded and takes on a complex (propagating) part to degrees dependent upon the ratio of viscous diffusion time to electroviscous time and the product of rotation rate and viscous diffusion time. Implications relate directly to an electrically augmented mechanical shear mixing device and tradeoffs between mixing quality and material processing rate.
Transport equations for fast electrons in a moving Newtonian fluid
Authors: Buchler J. Robert
Published: March 2003

The equations for the transport of relativistic electrons are derived inclusive of all terms of order (v/c). The equations are valid in all regimes, from diffusion to streaming. The moments of the electron distribution function, as well as the electron velocities, are measured in the fluid frame. No special geometry or symmetry is assumed.
Nonlinear space-charge waves on cylindrical electron beams and plasmas
Authors: Hughes Thomas P. , Ott Edward
Published: March 2003

Nonlinear space-charge waves in a strongly magnetized cylindrical plasma are investigated. Soliton and periodic wave solutions are obtained. For the case of an intense unneutralized electron beam, a significant decrease in the phase velocity of the slow space-charge wave is found at large amplitudes. The importance of this result for some collective ion acceleration schemes is discussed.
Stability of the noncircular screw pinch
Authors: Miller G.
Published: March 2003

The magnetohydrodynamic stability of a slightly noncircular screw pinch with arbitrary cross-sectional shape is investigated analytically and numerically using two models: the surface-current model and the force-free model. For the surface-current model the noncircular correction to the instability growth rate is given in analytical form: (1) for the case nq≃m, where q is the safety factor and m and n are the azimuthal and longitudinal mode numbers of the instability, and (2) for the special case of purely transverse perturbations (n=0). For the force-free model, the noncircular correction to the marginal stability condition is obtained. This is done analytically for nq≃m, and for fixed boundary internal modes.
Enhanced transport in tokamaks due to toroidal ripple
Authors: Boozer Allen H.
Published: March 2003

A method for evaluating transport in nonsymmetric systems is developed and applied to a previously little studied ripple collisionality regime of tokamaks. This collisionality regime, the ripple plateau, is the regime of primary importance both for present day and reactor scale tokamaks. The results can be directly applied to related systems like the toroidal Z pinch.
Resonant parametric excitations driven by lower-hybrid fields
Authors: Villalón E.
Published: March 2003

Three-wave parametric excitation in inhomogeneous plasmas is examined in a two-dimensional geometry relevant to supplementary rf heating of tokamaks. The stabilization of resonant parametric excitation due to a linear mismatch in wavenumbers and to the Landau-damping rates of the decay waves is analyzed, assuming that the magnitude of the pump field is constant in time and in the spatial region where the resonant interaction takes place. Both types of temporally growing modes and spatially amplified instabilities are studied, using a WKB analysis. It is shown that either by increasing the strength of the mismatch K′ or the width of the pump L, the growth rate of the fastest growing normal mode will decrease. When the excited waves are slightly damped, it is shown that there exists a finite value of the product K′L, such that, above it, no temporal normal modes are excited. The amount of spatial amplification is also reduced by the mismatch in wavenumbers and by the damping rates of the excited waves. Because of the finite spatial extent of the pump electric field, the amplification length is found to be smaller than or equal to L, depending on the strength of the mismatch and damping rates.
Large structure with a characteristic upstream interface in turbulent boundary layers
Authors: Bandyopadhyay Promode
Published: March 2003

In a reasonably high-Reynolds-number boundary layer, a class of large structure is occasionally formed that has a characteristic upstream interface slope. A consideration, based on the dynamics of a hairpin vortex in such a layer, is shown to be able to account for this slope.
A similarity transformation for a turbulent spot in a laminar boundary layer
Authors: Sokolov M. , Antonia R. A. , Chambers A. J.
Published: March 2003

A similarity transformation, which allows for the effect of Reynolds number on the growth of a turbulent spot in a boundary layer, is in satisfactory agreement with the measurements.
Profile modification by light pressure in plasmas expanding with uniform, time-dependent temperature
Authors: Sanmartín J. R. , Montañes J. L.
Published: March 2003

Profile modification of laser plasmas, in the transition layer at critical density and in the flow on the overdense side, is studied. Assuming isothermal flow and low absorption within the layer, compression transitions are proved impossible and cavities possible only in subsonic flow. The overdense flow adjusts itself for a rarefaction transition in a manner (formation of plateaus, bumps, or cavities) critically dependent on how the (spatially uniform) temperature varies with time. Spherical effects and evidence for the results are considered.
Equilibrium shapes of a pair of equal uniform vortices
Authors: Saffman P. G. , Szeto R.
Published: March 2003

The shapes and properties of two equal corotating uniform vortices, rotating steadily about each other, are calculated. An integrodifferential equation for the bounding contour is solved numerically, using Newton’s method. The results compare well with those obtained from a simple model. It is shown that steady solutions do not exist if the vortices are too close. The stability to two-dimensional disturbances is discussed qualitatively and the critical separation at which the system becomes unstable is calculated. Some comments are made on the stability of a vortex pair of equal counter rotating uniform vortices.
Mixing of passive scalar stripes by a three-dimensional multi-mode vector field
Authors: Fournier J. L. , Gence J. N. , Comte-Bellot G.
Published: March 2003

An attempt is made to generalize the work of Corrsin and Karweit by considering the pure convection of a passive scalar by a three-dimensional multi-mode and time-independent random isotropic vector field made up of different Fourier modes. It can be roughly concluded that an initially nonisotropic scalar field has a tendency to return to isotropy under the influence of the mixing. However, it appears that the ratio of the Taylor scales corresponding to two perpendicular directions tends to a value less than one when t is growing. This result is independent of the number of modes.
Modulation solitons in inhomogeneous media
Authors: Larsen L. H.
Published: March 2003

The propagation of a modulation soliton in a medium with inhomogenities is considered. It is shown that the center of the wave packet obeys a Newtonian force equation with the inhomogenity acting as the force. The model equation is compared with numerical solutions of the inhomogeneous nonlinear Schrödinger equation.
Electron beam dynamics in combined guide and pump magnetic fields for free electron laser applications
Authors: Friedland L.
Published: March 2003

The propagation of a cold relativistic electron beam in a free electron laser with an axial guide magnetic field is considered. The possibility of several steady-state helical trajectories for the electrons is shown, and the stability against perturbations and accessibility of such steady states is considered. Necessary and sufficient conditions for the stability are derived and indicate the importance of the transition region at the entrance of the laser. Possible modes of operation of the laser in different steady-state regimes are suggested and illustrated by numerical examples.
Alfvén-ion-cyclotron instability in mirror machines
Authors: Watson Duncan C.
Published: March 2003

The Alfvén-ion-cyclotron instability is studied for finite mirror-confined plasmas with high beta without field reversal. Variation perpendicular to field lines is modeled by an effective k⊥. Variation along a representative field line is treated using the Wentzel-Kramer-Brillouin approximation in two ways. First, the local dispersion relation is expanded about a wavenumber and frequency corresponding to absolute instability at the machine midplane. This yields a parabolic k∥(s) and a frequency correction. Second, the local dispersion relation is evaluated exactly as a function of position, and the appropriate phase-integral condition is used to fix the frequency. This condition is chosen using a generalized WKB formulation which is outlined. The two ways of obtaining the mode frequency agree closely. Stability boundaries are drawn in β⊥−β∥ space for two representative finite plasmas. The long thin approximation is used to model finite-beta well deepening. For ease of computation, the bi-Maxwellian ion velocity distribution is used. At high β, the stability boundaries are affected by the appearance of an additional root, with a larger parallel wavenumber and a lower frequency.
Simulation studies of the collisionless tearing instabilities
Authors: Katanuma I. , Kamimura T.
Published: March 2003

Collisionless tearing instabilities and the consequent enhanced transport are investigated theoretically and also numerically, using a two-and-one-half-dimensional particle simulation code in a slab geometry. The effects of electrostatic fields on the instability are also considered. The initial current is found to diffuse along the perturbed magnetic field lines, the observed diffusion along the perturbed magnetic field lines, the observed diffusion coefficient being in good agreement with the theoretical prediction. The growth of the instability is observed to divide into three phases; a linearly unstable phase, a quasi-stable phase, and a nonlinear phase similar to the Rutherford phase. Electrostatic effects have a tendency to enhance the tearing mode growth rate. In multi-mode tearing, a coalescence of magnetic island is observed.
Influence of wall impedance on the electron cyclotron maser instability
Authors: Uhm Han S. , Davidson Ronald C.
Published: March 2003

The influence of finite-wall impedance on the cyclotron maser instability is investigated for a hollow electron beam. The stability analysis is carried out within the framework of the linearized Vlasov–Maxwell equations, assuming that the beam thickness is much less than the mean radius of the beam. The formal dispersion relation for azimuthally symmetric electromagnetic perturbations including the important influence of arbitrary wall impedance is obtained. One of the most important features of the analysis is that, for a purely resistive wall, the instability growth rate is substantially reduced by a very small wall resistivity. Moreover, the range of axial wavenumbers corresponding to instability increases rapidly as the wall resistivity is increased. Cyclotron maser stability properties for a dielectric loaded waveguide are also investigated. For an appropriate choice of the dielectric constant ε and the thickness of the dielectric material, it is shown that the instability bandwidth can be more than double that for a perfectly conducting waveguide.
Observations of neoclassical and anomalous resistivity in toroidal discharges
Authors: Etzweiler J. F. , Brouchous D. A.
Published: March 2003

Plasma resistivity measurements parallel to the magnetic field have been made on the Wisconsin supported octupole and the Wisconsin levitated octupole. Plasmas ranged in density from 108 to 1012 cm−3 and varied from highly collisionless to highly collisional. The measured resistivity was found to agree with neoclassical resistivity theory in the collisional and transition regimes. Highly collisionless plasmas were found to have anomalously high resistivity which obeyed the Te1/2/ne scaling of the transition regime.
Superadiabatic and stochastic ion motion in the presence of a wave in a mirror-machine plasma
Authors: Smith Gary R. , Byers J. A. , LoDestro L. L.
Published: March 2003

The method of overlapping resonances is used to determine whether a single electrostatic wave in a mirror-machine plasma causes stochastic ion motion or its opposite, superadiabatic motion. Inclusion of the grad-B drift velocity does not alter the result found by Rosenbluth and by Timofeev: high-energy ions move superadiabatically and low-energy ions move stochastically, the boundary occurring at a perpendicular kinetic energy of approximately 8 keV. Numerical calculations of ion orbits reveal large fractional changes in the magnetic moment during a bounce period; this result sheds doubt on the quantitative validity of a quasi-linear diffusion equation, although qualitative validity may hold. Results from the 2XIIB experiment contradict the theoretical prediction of superadiabaticity. Replacement of the slab approximation used in this paper by a model including the azimuthally asymmetric quadrupole field may remove this discrepancy.
Neutral stability calculations for boundary-layer flows
Authors: Nayfeh Ali H. , Padhye Atul
Published: March 2003

An analysis is presented of the parallel neutral stability of three-dimensional incompressible, isothermal boundary-layer flows. A Taylor-series expansion of the dispersion relation is used to derive the general eigenvalues. These equations are functions of the complex group velocity. These relations are verified by numerical results obtained for two- and three-dimensional disturbances in two- and three-dimensional flows.
Reynolds number dependence of skewness and flatness factors of turbulent velocity derivatives
Authors: Van Atta C. W. , Antonia R. A.
Published: March 2003

The influence of fluctuations in the rate of local turbulent energy dissipation on higher-order structure functions for small separation distances r and on moments of turbulent velocity derivatives is examined using the hypotheses of Kolmogoroff and Obukhov for the probability density and variance of the dissipation fluctuations. The predicted variation of the skewness and flatness factors with Rλ represents the available experimental data fairly well over a relatively wide range of Rλ when the constant μ introduced in the variance hypothesis is suitably chosen. The predicted variation of S with K fits the data very well. The present analysis breaks down for moments of sufficiently high order probably due to basic shortcomings of the hypotheses.
Bispectrum and nonlinear wave coupling
Authors: Kim Y. C. , Beall J. M. , Powers E. J. , Miksad R. W.
Published: March 2003

The relationship between nonlinear wave coupling and the properties of the bispectrum are investigated for the case of three-wave coupling. In particular, the dependence of the phase of the bispectrum, the direction of power flow between modes, and the sign of the skewness parameter, on the nonlinear amplitude variation and coupling coefficient, is analytically investigated and found to be in good agreement with experimental observations.
Hamiltonian of a slightly relativistic plasma
Authors: Jones Roger D. , Pytte A.
Published: March 2003

In the past, treatments of plasma to order v2/c2, where v is the thermal speed of the particles, have relied on a Lagrangian formalism or on coarse graining techniques. It is shown that a Hamiltonian formulation where only particle degrees of freedom are considered is possible and a Hamiltonian to order v2/c2 is obtained. This Hamiltonian is valid within the limits of the random phase approximation.
Scattering of suprathermal ions by partially ionized impurities
Authors: Andrews P. L. , Goldston R. J.
Published: March 2003

The pitch-angle scattering rate of suprathermal ions has been used as a diagnostic to measure Zeff in tokamak plasmas. The interpretation of these measurements is made somewhat obscure, however, by the presence of partially ionized impurities. Beam ions can penetrate the electron clouds of impurity ions, and experience a scattering potential intermediate between the unshielded nuclear potential and the reduced field observed well outside the electron cloud. This problem is treated by using the Thomas–Fermi statistical model for the charge density of the electron cloud of the impurities and the impulse approximation for the angular deviation resulting from a collision. An analytic approximation to the exact Thomas–Fermi potential for an ion of arbitrary charge state is found, and is used to calculate the addition required to the classical Coulomb logarithm for 10%–90% ionized impurities and for incident energies in the range (10−2–103) keV×Zn4/3, where Zn is the nuclear charge of the impurity. The correction can be comparable in magnitude to the classical result, e.g., for 40 keV protons incident on 30 times ionized tungsten ions, the addition to 1nΛ is approximately 30%.
Influence of the distribution function on eigenoscillations and stability of a beam
Authors: Hofmann Ingo
Published: March 2003

Eigenfrequencies are calculated for the transverse oscillations of a round beam radially confined in a solenoidal magnetic field. Under the assumption of a linear restoring force in the equilibrium beam, with partial or total neutralization by an immobile background charge, the influence of different distribution functions on stability is investigated. It is found that the well-known extended instabilities of a microcanonical (Kapchinskij–Vladimirsky) distribution are replaced by apparently insignificant patches of instability if the distribution function is broadened, hence the loss-cone is partially filled up. The water-bag distribution indicates transition to only stable eigenfrequencies and it is found that this transition is accompanied by suppression of negative energy oscillations, which are responsible for the instabilities of loss-cone or nonmonotonically decreasing distributions. The method employed consists of infinite series expansion for the eigenfunctions and approximating the infinite determinant dispersion relation by rapidly converging finite order subdeterminants.
Electron beam heating of a mirror confined plasma
Authors: Robertson Scott , Fisher Amnon , Roberson Charles W.
Published: March 2003

In an experiment to study the heating of a mirror confined hydrogen plasma (Bz=2–4.3 kG, ne=2×1013 and 2×1014 cm−3) by an intense pulsed relativistic electron beam (1 MV, 100 kA, 50 nsec), the effect of the diode magnetic field gradient on the strength of the beam plasma interaction has been examined. It is found that at constant beam current, the energy transferred to the plasma as measured by diamagnetism is increased when the beam is either launched from a smaller cathode or is launched into a converging magnetic field, both of which increase the beam density. At high plasma density the diamagnetism agreed with the prediction of Thode for the two-stream instability at the lowest values of nb/np investigated, but failed to scale upward as rapidly as predicted. At low plasma density, anomalous resistivity and ion heating are observed which is consistent with the appearance of the ion acoustic instability.
Magnetic shear stabilization of the diocotron modes of a relativistic electron beam
Authors: Ott Edward , Wersinger J.-M.
Published: March 2003

The stability of the diocotron modes of an annular, relativistic, un-neutralized electron beam is examined when the guide magnetic field is sheared. Stabilization results from the reduction in the electron velocity shear. The ’’low density limit’’ is found to be a surprisingly good approximation in the case of a slab, nonrelativistic model. The case of an annular relativistic beam propagating in a cylinder is analyzed in this limit. Magnetic shear stabilization is found to be a promising means of obtaining stable beam propagation.
Resonant particle effects on finite Larmor radius stabilization
Authors: Seyler C. E. , Freidberg J. P.
Published: March 2003

It is shown that the threshold for instability of a one-dimensional Vlasov-fluid screw pinch is identical to that in ideal magnetohydrodynamics, for arbitrary ion gyroradius. That is, there is no absolute finite Larmor radius stabilization, although the growth rates for the Vlasov-fluid model can be much lower. It is the ions in resonance with the real part of the mode frequency which are responsible for maintaining the instability beyond the value of ρi/a for which ’’finite Larmor radius stabilization’’ would have occurred. An analytical expression for the growth rate near the marginal point is derived using the finite Larmor radius expansion. The expression exhibits the expected scaling with β and Larmor radius.
Tearing-mode instability in a field-reversed ion layer
Authors: Uhm Han S. , Davidson Ronald C.
Published: March 2003

Tearing-mode stability properties are investigated for azimuthally symmetric perturbations about an intense proton layer (P layer) immersed in a background plasma. The analysis is carried out within the framework of a hybrid-kinetic model. It is assumed that the layer is thin, with the radial thickness (2a) much smaller than the mean radius (R0). Tearing-mode stability properties are investigated for the choice of ion layer distribution function in which all ions have the same perpendicular energy in a frame rotating with angular velocity ωϑ and a step-function distribution in axial velocity vz. Moreover, the stability analysis is carried out for low frequency perturbations with ‖ω‖≲ω̂ci where ω is the complex eigenfrequency and ω̂ci is the ion cyclotron frequency. The dispersion relation is derived including the important dielectric influence of the background plasma and the background layer electrons. It is shown that an axial velocity spread stabilizes perturbations with sufficiently short axial wavelength. Moreover, the tearing-mode growth rate is significantly reduced whenever np/nb≳≳1, where np is the background plasma density and nb is the density of layer ions.
Weakly localized two-dimensional drift modes
Authors: Choi Duk-In , Horton Wendell
Published: March 2003

The structure of drift modes in tokamaks is investigated taking into account modes that are weakly ballooning on the magnetic surfaces. The toroidal mode theory of Conner, Hastie, and Taylor is used to find the surface eigenfunctions and shear damping as a function of the system parameters.
Radiation from a strongly turbulent plasma: Application to electron beam-excited solar emissions
Authors: Goldman Martin V. , Reiter George F. , Nicholson Dwight R.
Published: March 2003

The emission of radiation at the plasma frequency and at twice the plasma frequency from beam-excited strong Langmuir turbulence, for the case of low-density high-velocity warm beams, is considered. Under these conditions, Langmuir wave packets undergo (direct) collapse in a time short compared with one e folding of a beam mode. The wave packet energy density threshold for collapse depends only on the beam temperature and velocity, not on the beam density. Upper and lower limits on the volume emissivity for harmonic emission from these collapsing wave packets are found. Within most of this range, the emissivity is large enough to account for observations of second harmonic radiation during type III solar radio wave bursts. The radiation at the fundamental is many orders of magnitude larger than predicted by weak turbulence theory.
Finite amplitude solitary Alfvén waves
Authors: Ray Dipankar
Published: March 2003

Solitary Alfvén waves with small but finite β have been studied by Yu and Shukla [Phys. Fluids 21, 1457 (1978)]. A more rigorous study is made and conditions for uniqueness are established.
Spontaneous emission of plasma-frequency radiation in tokamaks
Authors: Freund H. P. , Lee L. C. , Wu C. S.
Published: March 2003

Extraordinary mode radiation near the electron plasma frequency can occur in tokamaks via Cerenkov resonance with runaway electrons. A numerical study is presented which shows the emission to be of sufficiently broad bandwidth and high intensity to account for the observations in TFR and Alcator.
Nonlinearly stable equilibrium statistical states for spheromaks
Authors: Vahala George
Published: March 2003

Even though the eigenfunctions of the curl operator do not form a complete set, one still obtains the force-free spheromak field as the only nonlinearly stable state for spherical plasmas.
Nonlinear drift tearing modes
Authors: Monticello D. A. , White R. B.
Published: March 2003

It is shown analytically and with a numerical code that the nonlinear growth and saturation of magnetic islands in tokamaks are not affected by diamagnetic corrections. Diamagnetic effects change the linear growth and introduce mode rotation, which persists in the nonlinear regime. The analysis is used to construct a model for the effect of feedback stabilization schemes in the nonlinear regime.
Strong turbulence effects on the kinetic beam-plasma instability
Authors: Rowland H. L.
Published: March 2003

The results of a series of one-dimensional fully kinetic Vlasov simulations on the effect of strong turbulence on the kinetic beam-plasma instability are presented. Previous results on the initial stabilization of the instability from a different simulation code are confirmed. Past this initial stabilization, it is shown that if the energy in the beam resonant waves at stabilization exceeds 0.1 to 0.2 of the initial thermal energy, strong coupling of the high frequency fields to the electrons takes place. The localized field structures and ion cavities that are formed are short-lived and the beam-plasma instability can periodically reappear. If this level is not exceeded, the coupling to the electrons is reduced and the structure formed by the strong turbulence is more stable. The variations in time of the beam resonant waves are also reduced.
Strongly turbulent stabilization of electron beam-plasma interactions
Authors: Freund H. P. , Haber I. , Palmadesso P. , Papadopoulos K.
Published: March 2003

The stabilization of electron beam interactions due to strongly turbulent nonlinearities is studied analytically and numerically for a wide range of plasma parameters. A fluid mode coupling code is described in which the effects of electron and ion Landau damping and linear growth due to the energetic electron beam are included in a phenomenological manner. Stabilization of the instability is found to occur when the amplitudes of the unstable modes exceed the threshold of the oscillating two-stream instability. The coordinate space structure of the turbulent spectrum which results clearly shows that soliton-like structures are formed by this process. Phenomenological models of both the initial stabilization and the asymptotic states are developed. Scaling laws between the beam-plasma growth rate and the fluctuations in the fields and plasma density are found in both cases, and shown to be in good agreement with the results of the simulation.
Convective amplification of universal drift modes
Authors: Hinton F. L. , Rosenbluth M. N.
Published: March 2003

An analytical solution of the wave equation for universal drift modes is derived. More exact analytical expressions for the dispersion relation and the damping rate are obtained than have been obtained previously. The spatial amplification of an incident wave is derived from the solution of the scattering problem. The dependence of the convective amplification factor on the shear is then determined.
Two-dimensional electrostatic E×B trapping
Authors: Hirshman S. P.
Published: March 2003

The nonlinear motion of a charged particle in a uniformly magnetized, two-dimensional plasma is analyzed in the presence of finite amplitude electrostatic waves traveling in the plane perpendicular to the magnetic field. In a strongly magnetized plasma, no particle trapping occurs in a single electrostatic traveling wave of arbitrary amplitude, since the induced E×B motion is orthogonal to the direction of wave propagation. However, particle orbits can be trapped when there are at least two mutually orthogonal components of the electrostatic field with finite wave amplitude cE⊥/B⩾ω/k⊥ such that a particle drifts one wavelength in a waveperiod. Exact integrability of the two-wave system implies that there is no stochastic particle behavior. Perturbation of this nonlinear equilibrium by additional waves with incommensurate phase velocities will destroy the coherent island structure and lead to stochastic diffusion. The threshold amplitude for strong nonlinear behavior in the coherent wave case helps to explain the many-wave turbulence criterion for the onset of particle diffusion in a two-dimensional plasma.
Streaklines in a shear layer perturbed by two waves
Authors: Williams David R. , Hama Francis R.
Published: March 2003

Streaklines are computed for a shear flow, which is perturbed by two waves in the form of a fundamental and a subharmonic, with various combinations of the two. It is shown that the streakline gives an erroneous impression of wave amplification and presence of discrete vortices. Under certain circumstances, moreover, the streakline development could be misinterpreted as evidence of vortex merging that never exists in the assumed flow field.
Study of density fluctuations in plasmas by small-angle CO2 laser scattering
Authors: Slusher R. E. , Surko C. M.
Published: March 2003

Small-angle CO2 laser scattering techniques are described which provide detailed information about density fluctuations in plasmas. These techniques are nonperturbative and measure the frequencies, wave vectors, and amplitudes of the density fluctuations. The theories of the scattering and the heterodyne and homodyne detection processes are described in detail for the case of small-angle scattering. Several models are developed to describe recent experiments involving both coherent and stochastic fluctuations. The laser, detector, and physical arrangement used in the experiments are described in detail. Experimental studies of driven Bernstein waves, ion acoustic turbulence in a positive column discharge, and fluctuations in tokamaks are described to illustrate these techniques. It is demonstrated that CO2 laser scattering is useful in studying fluctuations with wavelengths between 0.01 and 1 cm and frequencies from 1 kHz to tens of giga-Hertz in plasmas with mean densities ranging from 1010 to 1017 cm−3.
Effects of ion dynamics on tearing modes
Authors: Lee X. S. , Mahajan Swadesh M. , Hazeltine R. D.
Published: March 2003

The eigenmode equations of several tearing modes are modified to include the ion dynamic terms, which are derived from a simple self-consistent ion kinetic theory. The stabilizing tendency of the ion acoustic term, noted previously by Bussac and co-workers, is confirmed, and the effect of an additional ion-electron friction term on the dispersion relation is discussed. Quantitative expressions for the change in mode frequency are given.
Coupling of drift modes in a torus
Authors: Waltz R. E. , Pfeiffer W. , Dominguez R. R.
Published: March 2003

The coupling of electrostatic drift modes due to the ion, magnetic curvature drift in a torus is examined analytically and numerically. It is found that this toroidal effect is not sufficient to destabilize the electron drift modes for values of shear, temperature gradients, and current drive that are typical of tokamaks and within the limits of the theoretical approximations.
Rotation of a classical toroidal plasma
Authors: Wen Shih-liang , Pao Young-ping
Published: March 2003

The toroidal and poloidal velocity components of an axisymmetric toroidal plasma are calculated using the classical formulae of stress tensors in the magnetohydrodynamic equations. Explicit formulae are obtained and a criterion regarding whether the toroidal velocity component is exponentially increasing or decaying is established. The special case of a large aspect-ratio plasma is also examined.
Tensorial volume of turbulence
Authors: Lin A. , Wolfshtein M.
Published: March 2003

A new simple equation is proposed, for a tensorial volume of turbulence, replacing the scale of turbulence equation. The equation was applied to two simple cases: a logarithmic boundary layer and the decay of isotropic homogeneous turbulence.
Spherical source expansion of a diatomic gas
Authors: Chatwani Ashok U. , Fiebig Martin
Published: March 2003

The effect of the intermolecular potential model on the final rotational and translational temperatures of a diatomic gas expanding from a spherical-sonic source has been examined by simulating the flow with the help of the direct simulation Monte-Carlo method.
Comments on ’’Effect of transonic flow in the ablation cloud on the lifetime of a solid hydrogen pellet in a plasma’’
Authors: Lengyel L. L.
Published: March 2003

A recently proposed model for scaling pellet ablation rates in hot plasmas, utilizing transonic flow equations may be of limited applicability because of basic assumptions used in the model. (AIP)
Comments on ’’Electromagnetic wave propagation in inhomogeneous plasmas’’
Authors: Bornatici M. , Engelmann F.
Published: March 2003

The clain of Antonsen and Manheimer that certain mode conversion and reflection effects are lost when a WKB approach is used is disputed because of their disregard of relativistic line broadening. (AIP)
Measurements of the complex plasma loading impedance of the magnetosonic cavity modes
Authors: Hwang D. Q. , Gould R. W.
Published: March 2003

The complex plasma loading impedance of the fast magnetosonic wave in the Caltech tokamak has been investigated at low power (between 10 and 300 W). The magnetosonic cavity modes are observed both as voltage maxima at the output of a six-turn receiving probe, and as maxima in the input resistance of the transmitting antenna. When the real and imaginary parts of the measured plasma loading impedance are plotted in the complex impedance plane, the resulting curves are approximately circles, indicating a resonance phenomenon. The observed plasma loading resistance at the various cavity modes is as high as 3 to 4 times the basic antenna resistance (∼0.4 Ω). This means that efficient energy coupling into the tokamak and low losses in the antenna system are possible. The estimated cavity Q’s are between 400 and 700.
Comments on ’’Pressure gradient-velocity correlations for flows with two- and three-dimensional turbulence’’
Authors: Bradshaw P.
Published: March 2003

The claim of RAJ that the ’’wall-effect’’ contribution to pressure gradient-velocity correlations is significant only in the viscous sublayer is disputed; also criticized is his summary of the historical development. (AIP)
Comments on ’’Resistive drift-Alfvén waves in sheared magnetic fields’’
Authors: Itoh Kimitaka , Inoue Sanae
Published: March 2003

In contradition of the result obtained by Catto et. al. the collisional drift-Alfvén mode in a sheared magnetic field is shown to be stable. The validity of an expansion used by Catto et al. is questioned. (AIP)
A completeness observation on the stability equations for stratified viscous shear flows
Authors: Herron Isom H.
Published: March 2003

The DiPrima–Habetler completeness theorem applies to the system of equations governing the linearized stability of stratified viscous shear flows.
Cooling of a pure electron plasma by cyclotron radiation
Authors: O’Neil T. M.
Published: March 2003

It has been suggested that a magnetically confined pure electron plasma might be cooled to the liquid and crystal states. Here, cyclotron radiation is considered as a possible cooling mechanism. The plasma and some cooled resisting medium are assumed to reside inside a conducting cavity. When the cyclotron motion of the electrons resonantly drives a cavity mode which is damped by the cooled medium, energy is transferred from the electrons to the medium. Attention is focused on the case where all of the electrons experience a sharp (i.e., high-Q) resonance with a single cavity mode. An interesting result is that the rate of energy loss per electron can exceed, by a factor of Q, the radiation rate for an electron executing cyclotron motion in unbounded space. The maximum value of Q is limited by cyclotron damping of the mode on the plasma itself and can be large for a non-neutral plasma at a density well below the Brillouin limit (i.e., for ωp<<Ω, where ωp is the plasma frequency and Ω is the cyclotron frequency).
Spontaneous emission of radiation from localized Langmuir perturbation
Authors: Freund H. P. , Papadopoulos K.
Published: March 2003

The radiation at the first and second harmonics of the electron plasma frequency (ωe) from localized Langmuir oscillations is computed for a field-free plasma. The localized perturbations are assumed to have cylindrical symmetry about the direction of propagation of the localized perturbation. It is shown that when the wavelengths of the excited modes are much greater than the scale lengths of the Langmuir perturbation, a quadrupole radiation pattern is recovered for emission at 2ωe. An application to type III solar radio bursts is discussed briefly.