bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of synchrotron radiation loss in inhomogeneous tokamak plasmas
Authors: Fidone I. , Meyer R. L. , Giruzzi G. , Granata G.
Published: July 2002

Synchrotron radiation in a tokamak with inhomogeneous plasma parameters is investigated to determine the role of the temperature profile on the global loss. Using reactor relevant parameters, it is shown that synchrotron radiation is profile dependent, namely, at constant thermal energy, the emitted radiation increases with the peak temperature. An approximated analytical formula for the global loss is found.
The wiggler-free free-electron laser: A single-particle model
Authors: Fruchtman Amnon
Published: July 2002

A general single-particle formalism is developed that is applicable for describing thermal and nonlinear effects both in the wiggler-free free-electron laser (WFFEL) and in the cyclotron autoresonance maser (CARM). The general formalism is used in the present paper for a linear analysis. The WFFEL interaction is shown to result from the coupling of the electromagnetic wave to two slow waves: a left-hand polarized wave and a longitudinal wave. This coupling results from the spatial periodicity of the electron flow in the WFFEL which follows the gyrophase coherence of the beam. The comparison between the WFFEL and the CARM is extended to the case where the wave phase velocity is different from the velocity of light in vacuum, and conditions are found for the gain in the WFFEL to be larger. An exact dispersion relation is derived for the WFFEL which employs an electron beam with thermal spread in the transverse velocity.
Effects of cold electron emission on the plasma sheath
Authors: Schwager L. A. , Hsu W. L. , Tung D. M.
Published: July 2002

The use of surfaces that emit cold electrons into a plasma has been shown to reduce the sheath potential drop by at least a factor of 2.6. This effect can be used to mitigate sputtering by causing a reduction of the mean energy of ions striking the emissive surface. The present study has been motivated by the severe sputtering problems that may occur at the divertor plate of a full power fusion reactor. In addition, secondary electron emission is a common occurrence at the bounding surface of a plasma, and so should be accounted for in the plasma analysis. Kinetic theory shows that, for a hydrogen plasma (singly charged, mean ion mass of 2.5 amu) with an electron temperature of Te and ion temperature of 0.1Te, emission reduces the sheath potential drop from 3.9Te to 1.5Te. This reduction occurs when the surface emits a cold electron current that is increased to nine times the incident ion current. For comparison with theory, thermionic electron emission has been studied in a hydrogen plasma (composed mostly of H3+) powered by inductive coupling from an rf coil. The plasma was in contact with an electrically isolated surface of tungsten impregnated with barium scandate. Plasma potential was measured with a Langmuir probe and with a mass and energy analyzer that sampled the energy distribution of ions passing through an aperture in the emissive surface. As the surface temperature was increased, the surface potential floated up to the plasma space potential so that the ion energy at the surface dropped from 5Te for temperatures below 570 °C to less than 1Te for temperatures above 680 °C. The Richardson equation, which determined emission current as a function of surface temperature, and the above kinetic theory, was used to predict that the reduction in potential drop should occur from 420 °C to 510 °C. The abrupt reduction in ion energy over a narrow range of surface temperature was observed experimentally as predicted; however, the sudden reduction began at a surface temperature of 600 °C rather than 420 °C. The delayed onset of emission was probably caused by the sputtering removal rate of the impregnate at the surface exceeding the replenishment rate from the bulk for disk temperatures below the normal operating range.
Feedback suppression of the radiative condensation instability
Authors: Sen A. K.
Published: July 2002

A new scheme of feedback suppression of the radiative condensation instability in the tokamak edge plasma is presented. The physics basis of this scheme is the modulation of the parallel electron thermal flux at the instability frequency via a feedback circuit. The latter consists of a temperature sensor, amplifiers, phase shifters, and a suitable form of insulated segmented poloidal limiter sections used as suppressors. The necessary feedback power is the order of a kilowatt for small tokamaks and a few megawatts for large reactor-type machines.
Mode coupling effects on alpha-particle-driven long wavelength Alfvén wave instability
Authors: Gang F. Y. , Leboeuf J.-N.
Published: July 2002

It is demonstrated both analytically and numerically that mode couplings play an important role in the nonlinear evolution of alpha-particle-driven long wavelength Alfvén wave instabilities. The mode coupling process is characterized by a beat between two linearly unstable Alfvén waves having opposite frequencies, which generates a linearly stable, static (zero frequency) mode. The backreaction of the static mode tends to stabilize the Alfvén instabilities by eliminating the phase shift between the alpha pressure and the Alfvén fluctuations.
Magnetothermal instability with generalized Ohm’s law
Authors: Bora M. P. , Talwar S. P.
Published: July 2002

The problem of thermal instability, having bearing on the formation of astrophysical condensations, is investigated for a hydromagnetic fluid obeying generalized Ohm’s law, both for self-gravitating and nongravitating configurations. Effects of finite Larmor frequency, resistivity, and finite plasma frequency on the stability of the system are studied and the condition of instabilities are derived for a temperature-dependent and density-dependent heat-loss function. It is found that the condition of instability for propagation parallel to the ambient magnetic field is independent of finite resistivity, Hall current, and electron inertia effects and also of the magnetic field strength. For transverse propagation, however, the instability criterion involves the field strength, resistivity, and electron inertia terms. The Hall current is found to modify the growth rates for generally inclined propagation only. Both monotonically unstable and overstable modes of instability arise in the system depending on the dependence of the heat-loss function on the local density and temperature.
Finite-Larmor-radius-corrected collision operator: A new approach to solving the kinetic equation
Authors: Auerbach S. P.
Published: July 2002

The kinetic equation for a magnetized plasma is usually treated by transforming the kinetic operator ∂/∂t+v ⋅ ∇x+(e/m)(E+v×B/c) ⋅ ∇v to guiding-center variables, while evaluating the collision operator at the particle position. This introduces apparent nonlocality into a local equation. A method is presented for transforming the kinetic equation into a local equation, i.e., an equation in which the kinetic operator and the collision operator are evaluated at the same point (the guiding-center position). The collision operator is transformed thereby into a new operator, called the FLR-corrected collision operator, which is a power series in the displacement between a particle and its guiding center. This transformation to a local equation, plus the use of the guiding-center distribution function, make the method significantly more efficient than standard techniques. The method is illustrated by a calculation of transport caused by ion–neutral collisions. The technique is sufficiently general that it could be applied in a variety of contexts, for example, neoclassical transport, or the effect of collisions on a wave.
An analytical model for creation and decay of strong shock waves caused by a trapezoidal laser pulse
Authors: Loeb A. , Eliezer S.
Published: July 2002

An analytic hydrodynamic model for the formation and decay of strong shock waves (0.1–10 TPa) is derived in this paper by using self-consistent and well-defined assumptions. A trapezoidal pressure profile as a function of time applied to the target surface is used as the initial condition. This model permits the calculation of the pressure, velocity, and acceleration of the shock front, as well as the calculation of the pressure gradient immediately behind the shock front, as functions of time and position. This model is applied to laser-generated shock waves with maximum intensities of 5×1013 and 5×1014 W/cm2. Good agreement with previous computer simulations of solid–laser interaction is obtained.
A convergent spectral representation for three-dimensional inverse magnetohydrodynamic equilibria
Authors: Hirshman S. P. , Weitzner H.
Published: July 2002

By rearranging terms in a polar representation for the cylindrical spatial coordinates (R,φ,Z), a renormalized Fourier series moment expansion is obtained that possesses superior convergence properties in mode number space. This convergent spectral representation also determines a unique poloidal angle and thus resolves the underdetermined structure of previous moment expansions. A conformal mapping technique is used to demonstrate the existence and uniqueness of the new representation.
Radiation heating in laser-irradiated planar targets
Authors: Ng A. , Parfeniuk D. , DaSilva L. , Pasini D.
Published: July 2002

Planar targets of different Z were irradiated with 0.53 μm laser light at intensities ≤5×1013 W/cm2. The target rear surface was observed to be first heated by the laser-generated shock wave. This was followed by a strong second heating which was attributed to radiation transport through the decompressed target. This additional heating might also be responsible for enhancing early laser transmission through thick targets.
Beam stability in a stellatron
Authors: Chernin D.
Published: July 2002

Collective effects in the l=2 stellatron [Phys. Rev. Lett. 50, 507 (1983)], a high-current electron accelerator, are studied. A thin-beam model is employed, and only long-wavelength, low-frequency modes are considered. The eigenvalue problem is formulated in general and solved analytically in the smooth approximation; comparison with a numerical solution of the eigenvalue problem is excellent. It is found that the dispersion relation in the smooth approximation is identical in form to that for the modified betatron, under a simple substitution. An analytical expression for the transition energy, obtained earlier by a simple dynamical argument, is confirmed. The stellarator field is found to reduce the growth rate of the negative mass instability.
Relaxation model for helicity injection in Ohmic and oscillating current-driven reversed field pinch discharges
Authors: Finn John M.
Published: July 2002

A model for studying current drive by helicity injection in reversed field pinches (RFP’s) is proposed. In this model, the helicity supplied by the external circuits is injected into an exterior vacuum region that models the resistive edge region where skin currents can decay. Full Taylor relaxations allow this injected helicity to penetrate into the bulk of the plasma. Results on Ohmically driven discharges are shown, benchmarking the model with current experiments. Results on oscillating current drive are also presented, including studies of the dependence upon the driving voltages, the edge resistivity, and the number of relaxations per oscillation cycle.
Formation of a large-scale potential structure along a mirror field resulting from parallel plasma flows
Authors: Serizawa Yoichi , Sato Tetsuya
Published: July 2002

A one-dimensional electrostatic particle simulation model was constructed to investigate an evolution of the parallel electrostatic potential in the presence of a plasma flow along the mirror field. It is shown that a steady-state, large-scale dc potential structure is created as a result of a continuous plasma flow injection, and that the resultant potential difference increases in proportion to the ion bulk flow energy. In the magnetospheric environment, the potential difference is found to be able to exceed 2.5 kV.
Toroidal viscosity and ambipolarity in the neoclassical transport theory of axisymmetric tori
Authors: Catto Peter J.
Published: July 2002

To self-consistently evaluate both the toroidal rotation and the radial electric field in a tokamak the two distinct constraints of toroidal angular momentum conservation and ambipolarity must be invoked. A gyrokinetic derivation of the neoclassical transport equations that permits a large toroidal rotation speed is employed. Explicit forms for the toroidal viscosity are obtained so that both constraints can be imposed. To keep the E×B drift speed smaller than the ion thermal speed and still consider large toroidal rotation, the ratio of the poloidal to toroidal magnetic field components is assumed to be small. For larger poloidal fields the derivation is restricted to small rotation. In general, it is also found necessary to retain the poloidal variations in the electrostatic potential and density. A tokamak plateau regime calculation is performed using the general formulation and illustrates the importance of retaining finite poloidal gyroradii effects.
Comment on ‘‘Self-organization and energy relaxation in a three-dimensional magnetohydrodynamic plasma’’ [Phys. Fluids 2
9, 1161 (1986)]
Authors: Montgomery David , Matthaeus W. H.
Published: July 2002

Certain objections regarding the article of Horiuchi and Sato [Phys. Fluids 29, 1161 (1986)] are presented. One class of objections is technical and the second concerns matters of attribution.
Effects of pressure anisotropy on plasma transport
Authors: Zawaideh Emad , Najmabadi Farrokh , Conn Robert W.
Published: July 2002

In a recent paper [E. Zawaideh, F. Najmabadi, and R. W. Conn, Phys. Fluids 29, 463 (1986)] a new set of generalized two-fluid equations is derived that describes plasma transport along the field lines of a space- and time-dependent magnetic field. These equations are valid for collisional to weakly collisional plasmas; they reduce to the conventional fluid equations of Braginskii [S. I. Braginskii, Reviews of Plasma Physics, edited by M. A. Leontovich (Consultants Bureau, New York, 1965), Vol. 1, p. 205] for highly collisional plasmas. An important feature of these equations is that the anisotropy in the ion pressure is explicitly included. In this paper, these generalized transport equations are applied to a model problem of plasma flow through a magnetic mirror field. The profiles of the plasma parameters (density, flow speed, and pressures) are numerically calculated for plasmas in different collisionality regimes. These profiles are explained by examining the competing terms in the transport equation. The pressure anisotropy is found to profoundly impact the plasma flow behavior. As a result, the new generalized equations predict flow behavior more accurately than the conventional transport equations. A large density and pressure drop is predicted as the flow passes through a magnetic mirror. Further, the new equations uniquely predict oscillations in the density profile, an effect missing in results from the conventional equations.
Dressed Langmuir solitons
Authors: Deeskow P. , Schamel H. , Rao N. N. , Yu M. Y. , Varma R. K. , Shukla P. K.
Published: July 2002

The existence of a two-parameter set of dressed Langmuir solitons is shown numerically, resolving a discrepancy among earlier investigations.
Heating and current drive with high-phase-velocity compressional Alfvén waves
Authors: Li Y. M. , Mahajan S. M. , Ross D. W.
Published: July 2002

It is shown that high-phase-velocity compressional Alfvén waves have the desirable features needed for efficient current drive in fusion-reactor-like conditions; the energy deposition is low on the α particles, and high on the hot electrons in the plasma interior.
Ballooning modes or Fourier modes in a toroidal plasma?
Authors: Connor J. W. , Taylor J. B.
Published: July 2002

The relationship between two different descriptions of eigenmodes in a torus is investigated. In one the eigenmodes are similar to Fourier modes in a cylinder and are highly localized near a particular rational surface. In the other they are the so-called ballooning modes that extend over many rational surfaces. Using a model that represents both drift waves and resistive interchanges the transition from one of these structures to the other is investigated. In this simplified model the transition depends on a single parameter which embodies the competition between toroidal coupling of Fourier modes (which enhances ballooning) and variation in frequency of Fourier modes from one rational surface to another (which diminishes ballooning). As the coupling is increased each Fourier mode acquires a sideband on an adjacent rational surface and these sidebands then expand across the radius to form the extended mode described by the conventional ballooning mode approximation. This analysis shows that the ballooning approximation is appropriate for drift waves in a tokamak but not for resistive interchanges in a pinch. In the latter the conventional ballooning effect is negligible but they may nevertheless show a ballooning feature. This is localized near the same rational surface as the primary Fourier mode and so does not lead to a radially extended structure.
Diagnosing superstrong turbulence in plasma by forbidden line measurements
Authors: Levron David , Benford Gregory , Baranga Andrei Ben-Amar , Means James
Published: July 2002

Observations of satellites of forbidden helium lines yield a new diagnostic of superstrong turbulence in a relativistic beam–plasma system. Comparing the satellite-to-allowed line intensities allows estimates of the volume fraction of atoms that experience high Langmuir fields, i.e., 〈E2〉≥4πnTe. An independent estimate follows from counting the total number of events, and using the calculated optical efficiency of the system, this ratio is estimated independently—agreement is good. This ratio may be the packing fraction of high field cavitons. Its value (∼0.1) implies that collision processes may be important to caviton dynamics.
A Lagrangian for water waves
Authors: Balk A. M.
Published: July 2002

A Lagrangian for strongly nonlinear unsteady water waves (including overturning waves) is obtained. It is shown that the system of quadratic equations for the Stokes coefficients, which determine the shape of a steady wave (discovered by Longuet-Higgins 100 years after Stokes derived his system of cubic equations) directly follows from the canonical system of Lagrange equations. Applications to the investigation of the stability of water waves and to the construction of numerical schemes are pointed out.
Instabilities of the sidewall boundary layer in a differentially driven rotating cylinder
Authors: Hart John E. , Kittelman Scott
Published: July 2002

We report experiments on the instability of the sidewall Stewartson layer that arises in the fluid next to the vertical boundary of a cylinder that is rotating about its axis at rate Ω, and is driven by slow differential rotation of a contact lid. There are three primary instabilities that come in sequentially as the driving is increased: vertically traveling axisymmetric rolls, quasistationary diagonal rolls inclined backward (retrograde, or opposite to the direction of the basic rotation Ω) with height, and prograde-traveling waves that tilt forward with height. These instabilities are markedly different from those observed in laboratory studies or computational simulations of pressure-driven flow down a rotating channel. The instability mechanisms appear to be related to our particular method of driving these flows.
Quasisteady streaming with rarefaction effect induced by asymmetric sawtooth-like plane waves
Authors: Yano Takeru , Inoue Yoshinori
Published: July 2002

The nonlinear plane acoustic wave emitted from a harmonically oscillating plate into an ideal gas of semi-infinite extent develops into a sawtooth-like wave, as long as the energy dissipation is negligibly small everywhere except for discontinuous shock fronts. The present authors have recently studied the strongly nonlinear propagation process and, in particular, numerically shown that, contrary to the result of the conventional weakly nonlinear theory, streaming (mean mass flow) due to shocks occurs in the direction of wave propagation, and thereby the gas near the plate is rarefied as time proceeds [J. Acoust. Soc. Am. 94, 1632 (1993)]. In this paper, the analysis of strongly nonlinear problem is advanced by extending the numerical computation up to about 190 periods of oscillation of plate, which is about three times longer than the previous one. It is demonstrated that, in the course of time, a quasisteady state is established, where a low-density and high-entropy region formed near the plate continues to grow at almost constant rate and the quasisteady streaming endures outside the region. Furthermore, the weakly nonlinear problem is analytically examined by a perturbation method up to O(M3) [M(≪1) is the acoustic Mach number]. The result shows that a sawtooth-like profile loses its symmetry in the second-order, and this causes weak streaming of O(M2). The decrease in density of the gas can be related to the accumulation of the third-order effects of production of entropy and generation of reflected wave at each shock front.
Lagrangian and Eulerian view of the bursting period
Authors: Podvin Bérengère , Gibson John , Berkooz Gal , Lumley John
Published: July 2002

Low-dimensional models for the turbulent wall layer display an intermittent phenomenon with an ejection phase and a sweep phase that strongly resembles the bursting phenomenon observed in experimental flows. The probability distribution of inter-burst times has the observed shape [E. Stone and P. J. Holmes, Physica D 37, 20 (1989); SIAM J. Appl. Math. 50, 726 (1990); Phys. Lett. A 5, 29 (1991); P. J. Holmes and E. Stone, in Studies in Turbulence, edited by T. B. Gatski, S. Sarkar, and C. G. Speziale (Springer, Heidelberg, 1992)]. However, the time scales both for bursts and interburst durations are unrealistically long, a fact that was not appreciated until recently. We believe that the long time scales are due to the model’s inclusion of only a single coherent structure, when in fact a succession of quasi-independent structures are being swept past the sensor in an experiment. A simple statistical model of this situation restores the magnitude of the observed bursting period, although there is a great deal of flexibility in the various parameters involved.
Stability characteristics of a periodically unsteady mixing layer
Authors: Hajj Muhammad R.
Published: July 2002

In nature, in many technological applications and in some laboratory experiments, the basic state of shear flows can be time-varying. The effects of such variations on the stability characteristics of these flows are not well understood. In previous work, Miksad et al. [J. Fluid Mech. 123, 1 (1982)] and Hajj et al. [J. Fluid Mech. 256, 385 (1992)], it has been shown that low-frequency components, generated by nonlinear difference interactions, play an important role in the redistribution of energy among spectral components. In particular, phase modulation was found to be the most effective mechanism in energy transfer to the sidebands of unstable modes. In this work, the effects of small-amplitude low-frequency mean flow unsteadiness on the stability of a plane mixing layer are determined. By extending earlier analytical arguments, it is shown that periodicity in the mean flow causes modulations of the most unstable modes. The analysis is then verified experimentally by comparing levels of amplitude and phase modulations in mixing layers with steady and unsteady basic flows. The results show that small-amplitude low-frequency unsteadiness results in enhanced modulations of the fundamental mode. These modulations cause variations in the growth rates of the unstable modes and energy redistribution among them.
Two-phase flow structure of sheet cavitation
Authors: Stutz B. , Reboud J.-L.
Published: July 2002

An experimental study of flow within sheet cavities is performed in a cavitation tunnel equipped with a Venturi-type test section. The flow is investigated by means of a double optical probe allowing void fraction, velocity, and chord length of the vapor structures to be measured. Laser velocimetery, wall pressure measurements, and visualization techniques are also used to characterize the liquid flow around the cavity. The consistency of the experimental results was checked though mass and momentum balances. The effects of Reynolds and cavitation numbers are analyzed. Special attention is given to the dynamic behavior of the flow, and to the vapor flow rate within the cavities. The measurements show a complex two-phase flow characterized by the presence of an extended reversed flow occurring along the solid surface and a regular decrease in void fraction along the cavity. The phase transitions seem to be mainly restricted by the dynamic of the bubbles and thermodynamic effects.
Decaying quasi-two-dimensional viscous flow on a square domain
Authors: van de Konijnenberg J. A. , Flór J. B. , van Heijst G. J. F.
Published: July 2002

A comparison is made between experimental, numerical and analytical results for the two-dimensional flow on a square domain. The experiments concern the flow at the interface of a two-layer stratified fluid, evoked by either stirring the fluid with a rake, or by injecting additional fluid at the interface. Two numerical simulations were performed with initial conditions and boundary conditions that correspond approximately with those met in the experiments. The analytical results concern the calculation of the lowest modes of a decaying Stokes flow on a square domain. At late times, the experimental and numerical flows resemble the fundamental mode of the Stokes flow. The damping of small-scale structures by viscous diffusion plays an important role even if the initial Reynolds number is high. In particular, viscous diffusion is likely to be responsible for the appearance of an almost linear relationship between vorticity and stream function in the experiments and the simulations.
Numerical simulation of the flow behind a rotary oscillating circular cylinder
Authors: Baek Seung-Jin , Sung Hyung Jin
Published: July 2002

A numerical study was made of flow behind a circular cylinder in a uniform flow, where the cylinder was rotationally oscillated in time. The temporal behavior of vortex formation was scrutinized over broad ranges of the two externally specified parameters, i.e., the dimensionless rotary oscillating frequency (0.110⩽Sf⩽0.220) and the maximum angular amplitude of rotation (θmax=15°, 30°, and 60°). The Reynolds number (Re=U∞D/ν) was fixed at Re=110. A fractional-step method was utilized to solve the Navier–Stokes equations with a generalized coordinate system. The main emphasis was placed on the initial vortex formations by varying Sf and θmax. Instantaneous streamlines and pressure distributions were displayed to show the vortex formation patterns. The oscillatory forcing was in the vicinity of the lock-on range, which can be applied to flow feedback control afterwards. The vortex formation modes and relevant phase changes were characterized by measuring the lift coefficient (CL) and the time of negative maximum CL (t−CLmax) with variable forcing conditions.
The interaction of near-critical swirling flows in a pipe with inlet azimuthal vorticity perturbations
Authors: Rusak Z.
Published: July 2002

The interaction of a near-critical axisymmetric incompressible swirling flow in a straight pipe with small inlet azimuthal vorticity perturbations is studied. Certain flow conditions that may reflect the physical situation are prescribed along the pipe inlet and outlet. It is first demonstrated that under these conditions a regular-expansion solution in terms of the small azimuthal vorticity perturbations has a singular behavior around the critical swirl. This singularity infers that large-amplitude disturbances may be induced by the small perturbations when the incoming flow to the pipe has a swirl level around the critical swirl. In order to understand the nature of flows in this swirl range, a small-disturbance analysis is developed. It shows that under the prescribed inlet/outlet conditions, a small but finite inlet azimuthal vorticity perturbation breaks the transcritical bifurcation of solutions of the Euler equations at the critical swirl into two branches of perturbed solutions. When the azimuthal vorticity perturbations are positive these branches show a regular behavior. However, when they are negative, the perturbed branches fold at limit points near the critical swirl, with a finite gap between the two branches, and no near-columnar equilibrium state can exist for an incoming flow with swirl close to the critical level. The flow must develop large disturbances in this swirl range. Beyond this range, two equilibrium states may exist under the same inlet/outlet conditions. When the negative inlet vorticity perturbations become larger in their size, this special behavior uniformly changes into a branch of single equilibrium state for each incoming swirl. The relevance of the results to the appearance of the axisymmetric vortex breakdown in a pipe and the control of this phenomenon using inlet vorticity perturbations is also discussed. The results suggest that, in general, positive inlet azimuthal vorticity perturbations may be used to delay vortex breakdown to higher swirl levels whereas negative perturbations induce the appearance of vortex breakdown at levels below the critical swirl.
Spectral scaling of static pressure fluctuations in the atmospheric surface layer: The interaction between large and small scales
Authors: Albertson John D. , Katul Gabriel G. , Parlange Marc B. , Eichinger William E.
Published: July 2002

Static pressure fluctuations measured in the atmospheric surface layer over a grass covered forest clearing are studied in the context of Townsend’s 1961 hypothesis regarding the effect of the outer region on the inner region. It is shown that large-scale pressure features are actively straining the inertial-scale pressure fluctuations, thus invalidating the direct extension of Kolmogorov’s 1941 hypothesis to the spectral scaling of pressure within the inertial subrange. A parameter describing the large scale pressure fluctuations is added to the set of variables responsible for inertial-range pressure differences and dimensional analysis is employed to derive an improved scaling law for pressure spectra which more closely matches these and previous experimental results. An examination of the Poisson equation for pressure is conducted and found to support the dimensional and experimental results.
On peaking and breaking of standing waves excited in synchronous Faraday resonance
Authors: Sekerzh-Zen’kovich S. Ya. , Bordakov G. A. , Flores-Espinoza R. , Lizárraga-Celaya C.
Published: July 2002

Asymptotic solutions in the Lagrangian variables that describe a peaking surface standing weakly nonlinear dispersive wave are reported. This wave can be considered as a standing wave of greatest height. The waves are excited parametrically in a vertically oscillating vessel and have the frequency equal to that of the excitation.
Complex dynamics of combustion flows by direct numerical simulations
Authors: Gummalla M. , Vlachos D. G.
Published: July 2002

The dynamics of a well characterized hydrogen–air combustion flow is studied at high pressures, with detailed chemistry and multicomponent transport, using numerical bifurcation theory, local stability analysis, and time-dependent simulations. We report, for the first time, complex dynamics including oscillatory multiplicity, isolas of periodic orbits, and chaotic behavior. The relatively wide range of conditions identified, over which such complex dynamics occurs, could allow for experimental verification of numerical predictions.
Compressible normal modes of a resistive nonuniform plasma
Authors: Califano Francesco , Chiuderi Claudio , Einaudi Giorgio
Published: July 2002

The theory of resistive dissipation of linear compressible modes in a nonuniform plasma is presented in a unified way, following a normal mode approach. The familiar modes of a homogeneous ideal plasma are shown to be substantially modified by the presence of the resistivity and nonuniformity. It is then argued that the most suitable criterion for mode identification is based on the asymptotic properties of the solutions. The modes are then classified as ideal or resistive according to the asymptotic absence or presence of the resistivity. After a brief summary of the results from previous articles, made for the sake of completeness, the article concentrates on the new results concerning the compressible ideal solutions, the only ones that exhibit a resonant behavior. The properties of the solutions are examined by treating an explicit example of a nonhomogeneous situation. It is shown that the nature of the modes varies in the direction of nonuniformity, being essentially a fast mode in the asymptotic region, a slow mode in the vicinity of the so-called cusp resonance, and an Alfvénic mode near the Alfvénic resonance point.
Nonlinear evolution of alpha-particle-driven Alfvén turbulence from a reduced model
Authors: Wising F. , Gang F. Y.
Published: July 2002

The coupled nonlinear evolution of kinetic Alfvén turbulence (KAW) and anomalous alpha-particle transport is analyzed using a reduced turbulence model [Phys. Fluids B 4, 3152 (1992)]. Due to the nonlocal nature of the nonlinear interaction (bulk ion Compton scattering), the saturated spectrum consists of two well separated peaks, one in the linearly unstable and one in the stable regime. The two peaks represent the energy input from the alphas and energy dissipation to the electrons, respectively, and are in balance at saturation. Without a particle source, the turbulence-induced anomalous alpha transport leads to a flattening of the initial alpha density profile, on a time scale slow compared to the fluctuation time scale. In the presence of a particle source, a stationary alpha density profile is built up self-consistently as a result of the competition between the alpha-particle source and anomalous transport. The anomalous alpha diffusivity from the reduced model for KAW, using reactor size parameters, is found to be Danα ∼ 1 m2/s.
Stability analysis of the ion-temperature-gradient-driven mode in noncircular tokamak geometry
Authors: Guo S. C. , Romanelli F.
Published: July 2002

The tokamak shaping effects on the toroidal branch of the ion-temperature-gradient-driven mode in the long wavelength limit (k2χρ2i≪ε1/2T) are studied. A model equilibrium with noncircular cross section is employed, where elongation, Shafranov shift, and triangularity are all taken into account. It is shown that each Fourier term in the expansion of the equilibrium magnetic field is associated with a new branch of the dispersion relation.
Response to ‘‘Comment on ‘Validity of the field line resonance expansion’ ’’ [Phys. Fluids B 4, 2713 (1992)]
Authors: Hansen P. J.
Published: July 2002

The comments of Thomson and Wright on the original paper by Hanson and Goetz (Ref. 1), are answered. It is contended that if low orders are considered in the expansion, the results of Ref. 1 will be valid. (AIP)
Mode conversion of fast magnetosonic waves and minority ion heating in a longitudinally inhomogeneous magnetic field
Authors: Sakai O. , Yasaka Y.
Published: July 2002

Mode conversion process in ICRF (Ion Cyclotron Range of Frequencies) in a longitudinally inhomogeneous two-ion-species plasma is clarified in the HIEI tandem mirror experiments. Measurements of wave number parallel to the magnetic field reveal that the fast magnetosonic wave of low-field-side incidence converts into the slow ion-cyclotron wave near the ion–ion hybrid resonance. The propagation of the slow wave is detected between the ion–ion hybrid resonance and minority ion cyclotron resonance layers, which shows good agreement with the calculated dispersion relation based on one- and two-dimensional plasma models. Simultaneous minority ion heating is observed, and it is due to the presence of the slow wave that resonates with minority ions at the minority ion cyclotron resonance.
Relaxation oscillations induced by amplitude-dependent frequency in dissipative trapped electron mode turbulence
Authors: Terry P. W. , Ware A. S. , Newman D. E.
Published: July 2002

A nonlinear frequency shift in dissipative trapped electron mode turbulence is shown to give rise to a relaxation oscillation in the saturated power density spectrum. A simple non-Markovian closure for the coupled evolution of ion momentum and electron density response is developed to describe the oscillations. From solutions of a nonlinear oscillator model based on the closure, it is found that the oscillation is driven by the growth rate, as modified by the amplitude-dependent frequency shift, with inertia provided by the memory of the growth rate of prior amplitudes. This memory arises from time-history integrals common to statistical closures. The memory associated with a finite time of energy transfer between coupled spectrum components does not sustain the oscillation in the simple model. Solutions of the model agree qualitatively with the time-dependent numerical solutions of the original dissipative trapped electron model, yielding oscillations with the proper phase relationship between the fluctuation energy and the frequency shift, the proper evolution of the wave number spectrum shape and particle flux, and a realistic period.
Analysis of loop voltage evolution in current drive experiments in the Phaedrus-T tokamak
Authors: Litwin C. , Hershkowitz N. , Wukitch S. , Intrator T. , Vukovic M. , Brouchous D. , Breun R. , Harper M.
Published: July 2002

The loop voltage response in the low-frequency current drive experiments is analyzed in order to extract information about the current drive profile and efficiency.
Current drive due to fluid helicity
Authors: Avinash K. , Fukuyama A.
Published: July 2002

The discrepancy in the expression for quasi-linear ponderomotive force as obtained by Klima and Petrazilka [Czech. J. Phys. B 30, 1002 (1980)] and by Fukuyama et al. [Phys. Fluids B 5, 539 (1993)] are resolved by showing that the extra terms in the expression of Klima and Petrazilka are due to fluid helicity of the fluctuations.
Magnetic field diffusion in semi-ideal magnetohydrodynamics
Authors: Gruzinov A. V. , Isichenko M. B. , Diamond P. H.
Published: July 2002

Diffusion of magnetic fields in semi-ideal magnetohydrodynamics (σ∥=∞, σ⊥ is finite) is studied. Though the magnetic field is frozen into an auxiliary medium, current layers are shown to develop for generic initial conditions. The final stage of the magnetic field evolution is described by a two-dimensional isotopological diffusion equation for the magnetic streamfunction ψ(x,y): ∂tψ={ψ,χ}, where χ is the streamfunction of the auxiliary medium given by ∇2χ={ψ,∇2ψ}, and {,} is a two-dimensional Jacobian.
Self-similar hydrodynamic flow in the laser light to x-ray conversion layer of a laser-heated solid
Authors: Oparin A. M. , Sigel R.
Published: July 2002

Intense short-wavelength laser light may be converted into thermal soft x rays in the dense plasma formed by irradiation of a solid high-Z material. Under certain conditions, the hydrodynamic flow in the conversion layer is self-similar, and profiles of the hydrodynamic variables may be readily calculated by solving the appropriate hydrodynamic equations. It is found that the structure of the conversion layer depends on the type of equilibrium that determines the atomic physics processes of radiation emission. Varying the conditions between the limits of local thermodynamic equilibrium (LTE) and coronal equilibrium (CE) shows, that in the latter case, the radiation comes mainly from a thin layer in the dense part of the conversion layer.
Wave propagation in a drifting plasma
Authors: McKinstrie C. J. , Startsev E. A.
Published: July 2002

Electromagnetic wave propagation in a drifting plasma is considered as a paradigm of vector wave propagation. In the drift frame, the dispersion tensor has a diagonal representation: one eigenmode is electrostatic and two eigenmodes are electromagnetic. The dispersion equations for all three eigenmodes are simple and well known. However, in the laboratory frame the dispersion tensor is rather complicated and two of the eigenmodes are neither electrostatic nor electromagnetic. We show explicitly that this complexity is due to the way in which the electric and magnetic fields transform between the drift frame and the laboratory frame, and discuss how this transformation can be accomplished most efficiently.
Transition to high mode induced by reduction of magnetic stress
Authors: Shaing K. C. , Zhang Y. Z.
Published: July 2002

The transition to the high mode (H mode) of plasma operation in tokamaks and stellarators can be triggered by reduction of the magnetic stress. The magnetic stress can be reduced directly, by stabilization of the magnetic perturbation, or by increasing the plasma density and ion temperature if all of the other parameters are fixed. This implies the existence of thresholds in density and ion temperature. The theory is applied to a class of H-mode phenomena observed experimentally.
Mode conversion of fast Alfvén waves at the ion–ion hybrid resonance
Authors: Ram A. K. , Bers A. , Schultz S. D. , Fuchs V.
Published: July 2002

Substantial radio-frequency power in the ion-cyclotron range of frequencies can be effectively coupled to a tokamak plasma from poloidal current strap antennas at the plasma edge. If there exists an ion–ion hybrid resonance inside the plasma, then some of the power from the antenna, delivered into the plasma by fast Alfvén waves, can be mode converted to ion-Bernstein waves. In tokamak confinement fields the mode-converted ion-Bernstein waves can damp effectively and locally on electrons [A. K. Ram and A. Bers, Phys. Fluids B 3, 1059 (1991)]. The usual mode-conversion analysis that studies the propagation of fast Alfvén waves in the immediate vicinity of the ion–ion hybrid resonance is extended to include the propagation and reflection of the fast Alfvén waves on the high magnetic-field side of the ion–ion hybrid resonance. It is shown that there exist plasma conditions for which the entire fast Alfvén wave power incident on the ion–ion hybrid resonance can be converted to ion-Bernstein waves. In this extended analysis of the mode conversion process, the fast Alfvén waves can be envisioned as being coupled to an internal plasma resonator. This resonator extends from the low magnetic-field cutoff near the ion–ion hybrid resonance to the high magnetic-field cutoff. The condition for 100% mode conversion corresponds to a critical coupling of the fast Alfvén waves to this internal resonator. As an example, the appropriate plasma conditions for 100% mode conversion are determined for the Tokamak Fusion Test Reactor (TFTR) [R. Majeski et al., Proceedings of the 11th Topical Conference on RF Power in Plasmas, Palm Springs (American Institute of Physics, New York, 1995), Vol. 355, p. 63] experimental parameters.
Pulsed currents carried by whistlers. VIII. Current disruptions and instabilities caused by plasma erosion
Authors: Stenzel R. L. , Urrutia J. M.
Published: July 2002

In a large magnetized laboratory plasma (n≃1012 cm-3, kTe⩾1 eV, B0⩾10 G, 1 m × 2.5 m), the transient processes of switch-on currents to electrodes are investigated experimentally. The current rise time lies between the ion and electron cyclotron periods (electron magnetohydrodynamics). The initial current scales linearly with applied voltage and is not limited by the electron saturation current of the positive electrode, but by the ion saturation current of the return electrode. The collection of electrons in the flux tube of the positive electrode gives rise to a space charge electric field, which expels the unmagnetized ions, erodes the density, and disrupts the current. Repeated current oscillations arise from a feedback between current, density, and potential oscillations. The dependence of the transient and unstable electrode currents on externally variable parameters is investigated in the present paper. A companion paper [Urrutia and Stenzel, Phys. Plasmas 4, 36 (1997)] presents in situ measurements of plasma currents, plasma parameters, and microinstabilities. These results are relevant to the physics of pulsed Langmuir probes, current collection from tethered electrodes in space, and plasma erosion switches.
Asymptotic growth in the linear Richtmyer–Meshkov instability
Authors: Wouchuk Juan Gustavo , Nishihara Katsunobu
Published: July 2002

An analytic model for the asymptotic growth in the linear Richtmyer–Meshkov instability is presented. Analytic formulae for the interface velocity are obtained both in the weak and the strong shock limits, whether a shock or a rarefaction are reflected. For weak shocks, the irrotational approximation is used. For strong shocks, the vorticity in the bulk must be also taken into account. It is seen that this bulk vorticity actually lowers the velocity predicted by the irrotational approximation. An explicit approximate formula is given in this case. It agrees very well with a previously reported numerical solution. Perturbation freeze-out is also considered in the weak shock limit. It is concluded that this instability is driven by the vorticity left by the shocks at the interface and in the fluids.
Stimulated Raman scattering in non-Maxwellian plasmas
Authors: Bychenkov V. Yu. , Rozmus W. , Tikhonchuk V. T.
Published: July 2002

Inverse bremsstrahlung heating of laser-produced plasmas can result in a non-Maxwellian electron distribution function. This leads to a modification of Langmuir wave collisional and Landau damping coefficients and may result in changes in the stimulated Raman scattering threshold and scattered light frequency spectra. The importance of these effects for current laser–plasma interaction experiments is discussed.
The finite length diocotron mode
Authors: Fine K. S. , Driscoll C. F.
Published: July 2002

A simple model is presented of a finite length electron plasma column supporting a small amplitude diocotron wave with mode number m=1. The electrons are contained inside conducting cylinders in an axial magnetic field, with negative voltages on end cylinders providing axial containment. The m=1 diocotron mode is the E×B drift orbit of an offset electron column around the cylinder axis, due to radial electric fields from image charges on the wall. The model predicts that the mode frequency will be higher than that of an infinitely long column due to θ-drifts from the radial containment fields at the plasma ends. The predicted dependencies on plasma length, radius, and temperature agree well with experiments, where frequency increases up to 2.5× are observed. For very short plasmas, these containment fields predominate over the image charge fields, and the plasma orbit is called the “magnetron” mode. The shift in the magnetron frequency due to image charges is also calculated.
Nonlinear saturation of the neutral driven interchange mode
Authors: McCarthy D. R. , Ramagos M. , Catto Peter J. , Krasheninnikov S. I.
Published: July 2002

The nonlinear behavior of a flutelike interchange mode driven by a background neutral pressure gradient in the collisionless regime has been studied both numerically and analytically. It is found that the mode saturates when the current produced by the friction force of the perturbed E×B drift against the neutrals balances the equilibrium ion current. Based upon this physical picture of the saturation mechanism, an analytical expression for the magnitude of the fluctuation levels as a function of the collisionality and magetization of the plasma is derived. An important result from these simulations is that the mode always grows to a state where kyLN∼1.