bluuebunny/crossref_metadata_2025 huggingface dataset:
Mode conversion and absorption of lower hybrid waves in inhomogeneous magnetic fields
Authors: Cho Suwon , Swanson D. G.
Published: July 2002

When a cold lower hybrid wave propagates in an inhomogeneous plasma, it undergoes a total mode conversion to the warm wave which is then coupled with the ion Bernstein wave around ion cyclotron harmonic frequencies. Both types of linear mode conversion problems can be described by fourth-order differential equations if the two processes are separated from each other. The combination of the two types of mode conversion when the lower hybrid turning point is in the vicinity of a cyclotron harmonic requires a sixth-order equation with a quadratic coefficient, which is not solvable analytically nor numerically. Another type of sixth-order equation, which has only linear coefficients and represents the three-wave problem marginally, is solved analytically without including the absorption term. Finally, the damping and the absorption of the wave are compared for the unmagnetized dispersion relation, the full magnetized dispersion relation, and the mode conversion analysis as the wave propagates in a magnetically confined plasma with slab geometry.
Analysis of a wide-band rotating-beam free-electron laser
Authors: Schuetz Luise S. , Ott E. , Antonsen Jr. T. M.
Published: July 2002

A stability analysis is presented in the Compton regime for a rotating-beam free-electron laser with azimuthally periodic wiggler field. The analysis includes both the sum and difference resonances, and the effect of particle energy and guiding center spread. It is found that the device has potential as a broadband amplifier.
Ponderomotive and fluid response in a cold magnetized plasma
Authors: Skoric M. M. , Kono M.
Published: July 2002

A fluid ponderomotive theory of a nondissipative cold magnetized plasma is presented. It is shown that nonlinear low-frequency dynamics is self-consistently determined by ponderomotive potential force and magnetization terms, both uniquely related to the ponderomotive potential. A compact and general fluid relation between the ponderomotive potential and the linear high-frequency response is derived. Finally, turning to the eikonal representation, a relation to earlier results is established.
Ensemble phase-averaged equations for bubbly flows
Authors: Zhang D. Z. , Prosperetti A.
Published: July 2002

A system of averaged equations describing the motion of a mixture of spherical compressible bubbles in an inviscid liquid is derived by the ensemble-averaging method introduced in an earlier paper [Zhang and Prosperetti, J. Fluid Mech. 267, 185 (1994)]. The averaging procedure introduces new terms in the equations, among which a contribution to the liquid stress tensor is of special interest. An extension of the well-known Rayleigh–Plesset equation to the case of bubbles interacting with the flow is also found. The general system of equations is closed in a rigorous manner in the dilute limit generalizing and correcting earlier averaged equations models. The results are illustrated by considering the problem of linear pressure wave propagation in a nonuniform bubbly liquid. Gradients of the bubble concentration are shown to dampen or amplify the wave strength.
Metamorphosis of a hairpin vortex into a young turbulent spot
Authors: Singer Bart A. , Joslin Ronald D.
Published: July 2002

Direct numerical simulation was used to study the formation and growth of a hairpin vortex in a flat-plate boundary layer and its later development into a young turbulent spot. Fluid injection through a slit in the wall triggered the initial vortex. The legs of the vortex were stretched into a hairpin shape as it traveled downstream. Multiple hairpin vortex heads developed between the stretched legs. New vortices formed beneath the streamwise-elongated vortex legs. The continued development of additional vortices resulted in the formation of a traveling region of highly disturbed flow with an arrowhead shape similar to that of a turbulent spot.
An experimental and theoretical investigation of instabilities in hypersonic flat plate boundary layer flow
Authors: Wendt Volker , Simen Martin , Hanifi Ardeshir
Published: July 2002

As part of an investigation on hypersonic boundary layer transition, the first successful instantaneous hot-wire and fluctuating pressure measurements have been conducted in the Ludwieg-tube facility of DLR at a free-stream Mach number of M∞=5. The disturbance amplitudes [root-mean-square (RMS) values] of the fluctuations of Pitot pressure and mass flow are smaller than the values measured in conventional, i.e., continuously driven or blowdown, hypersonic wind tunnels. Hot-wire measurements in the laminar boundary layer on a flat plate in the same wind tunnel at Mach 5 did not show the expected dominance of high-frequency second mode instabilities, predicted by linear stability theory and found in other experiments. On the other hand, the growth of natural disturbances within a broad frequency band up to 50 kHz could be observed on the flat plate. The measured amplification rates are in good agreement with theoretical results for three-dimensional first mode disturbances. The theoretical investigations were based on a local and nonlocal instability analyses of similarity profiles for flat plate boundary layer flow at M∞=5. Whereas the local analysis is limited to parallel model flows, nonlocal analysis additionally takes into account the nonparallel basic flow and the nonuniform disturbance flow evolution. Surprisingly, the first rather than second mode instability wave growth is found to be dominant in the Ludwieg-tube experiment. This result deviates from measurements obtained in other wind tunnels. It indicates that not only transition locations but also instability wave growth for hypersonic boundary layer flows are strongly dependent on the disturbance environment of the individual wind tunnel. Due to the very low initial, high-frequency disturbance amplitude level in the free stream of the Ludwieg-tube, second mode instabilities may not grow to significant values or not be present at all. As such a free-stream disturbance spectrum can also be expected under free-flight conditions, the Ludwieg tube may be a suitable wind tunnel to adequately simulate boundary layer flow transition along smooth surface, hypersonic flight vehicles.
Motion of a two-dimensional monopolar vortex in a bounded rectangular domain
Authors: van Geffen J. H. G. M. , Meleshko V. V. , van Heijst G. J. F.
Published: July 2002

In this paper we describe results of a study of the two-dimensional motion of a distributed monopolar vortex in a viscous incompressible fluid in a bounded rectangular domain with free-slip and no-slip boundary conditions. In the case of free-slip walls the motion of the vortex center can be satisfactorily modelled by a single point vortex in an inviscid fluid. Comparison of the results of both models reveals a good quantitative agreement for the trajectories of the vortex centers and of the period of one revolution around the center of the domain, for moderate viscous effects (Re=1000 and more). In a domain with no-slip walls the distributed monopolar vortex moves to the center of the domain along a curved but not smooth trajectory due to the interaction of the monopole and the wall-induced vorticity.
Numerical determination of turbulent fractal dimensions
Authors: Nicolleau F.
Published: July 2002

This paper focuses on the fractal dimension of a line embedded in a homogeneous turbulent field. Based on an intuitive picture motivated by fractal theory, the introduction provides a physical approach for analysing the fractal dimension. The analysis is first applied to the case of an isotropic turbulent field which is suddenly subjected to steady rotation. It is shown that for a non-rotating field the fractal dimension D′ of the line increases with time. When expressed as a function of t/td, where td is the time of decay, the fractal dimension depends on both the Reynolds number and the rotation rate. However, using a suitable combination of linear (rotation rate Ω) and non-linear (td) mechanisms to define a characteristic time t*=td(1+αtdΩ), the dependence of the fractal dimension on rotation rate can be absorbed for Rossby numbers Ro≳1.54. The fractal dimension of a line immersed in periodic channel flow is also considered and the role of a no-slip wall is analysed. It shows that the fractal dimension increases rapidly from unity on the solid surface, and that this sharp and substantial increase occurs with 15 wall units. In the cases considered, this region corresponds to zones where the rms value of the velocity fluctuation is maximum.
Reynolds number effects in a turbulent pipe flow for low to moderate Re
Authors: den Toonder J. M. J. , Nieuwstadt F. T. M.
Published: July 2002

We present in this paper high resolution, two-dimensional LDV measurements in a turbulent pipe flow of water over the Reynolds number range 5000–25000. Results for the turbulence statistics up to the fourth moment are presented, as well as power spectra in the near-wall region. These results clearly show that the turbulence statistics scaled on inner variables are Reynolds-number dependent in the aforementioned range of Reynolds numbers. For example, the constants in the dimensionless logarithmic mean-velocity profile are shown to vary with Reynolds number. Our conclusion that turbulence statistics depend on the Reynolds number is consistent with results found in other flow configurations, e.g., a channel flow. Our results for the pipe flow, however, lead nevertheless to quite different tendencies.
The importance of higher-order effects in the Barenblatt–Chorin theory of wall-bounded fully developed turbulent shear flows
Authors: Srinivasan Radhakrishnan
Published: July 2002

Barenblatt [J. Fluid Mech. 248, 513 (1993)] proposed a Reynolds number (R)-dependent power law for the mean velocity near the wall in fully developed turbulent shear flows. In an appropriate region, as R→∞, the envelope of the velocity profiles reduces to a logarithmic law with an additive constant that is significantly larger than that of the universal logarithmic law. This discrepancy is removed upon including a higher-order term in the power-law exponent. Using the friction (instead of average) velocity in the definition of R destroys the invariance of the nonvanishing higher-order terms of the power law as R→∞.
Scale-similar models for large-eddy simulations
Authors: Sarghini F. , Piomelli U. , Balaras E.
Published: July 2002

Scale-similar models employ multiple filtering operations to identify the smallest resolved scales, which have been shown to be the most active in the interaction with the unresolved subgrid scales. They do not assume that the principal axes of the strain-rate tensor are aligned with those of the subgrid-scale stress (SGS) tensor, and allow the explicit calculation of the SGS energy. They can provide backscatter in a numerically stable and physically realistic manner, and predict SGS stresses in regions that are well correlated with the locations where large Reynolds stress occurs. In this paper, eddy viscosity and mixed models, which include an eddy-viscosity part as well as a scale-similar contribution, are applied to the simulation of two flows, a high Reynolds number plane channel flow, and a three-dimensional, nonequilibrium flow. The results show that simulations without models or with the Smagorinsky model are unable to predict nonequilibrium effects. Dynamic models provide an improvement of the results: the adjustment of the coefficient results in more accurate prediction of the perturbation from equilibrium. The Lagrangian-ensemble approach [Meneveau et al., J. Fluid Mech. 319, 353 (1996)] is found to be very beneficial. Models that included a scale-similar term and a dissipative one, as well as the Lagrangian ensemble averaging, gave results in the best agreement with the direct simulation and experimental data.
Axisymmetric capillary waves on thin annular liquid sheets. I. Temporal stability
Authors: Mehring C. , Sirignano W. A.
Published: July 2002

A reduced-dimension approach is employed to analyze the nonlinear distortion and disintegration of axisymmetric thin inviscid annular liquid sheets in a surrounding void with nonzero gas-core pressure at zero gravity. Linear and nonlinear solutions for the free motion of periodically disturbed infinite linearly stable and unstable sheets are obtained and compared in this first paper. (The forced motion of semi-infinite annular sheets exiting from a nozzle or atomizer is considered in the second paper.) Both sinuous and dilational modes are studied. Both modes are dispersive unlike the planar case where only the dilational mode is dispersive. These modes are coupled even in the linear representation although for sufficiently large annular radius, a pure dilational linear oscillation is found. The sinuous oscillation always excites the dilational mode. Nonlinear effects can modify the wave shapes substantially, causing an increase in breakup time for the dilational mode and a decrease in breakup time for the sinuous mode. The capillary sheet instability due to the nonlinear interaction of harmonic and subharmonic dilational disturbances, originally observed on planar sheets, is also observed and analyzed for the annular geometry. Parametric studies on the influence of annular radius, disturbance wavelengths, and their ratios are reported.
Edge plasma fluctuations and transport in a reversed-field pinch
Authors: Brunsell P. R. , Maejima Y. , Yagi Y. , Hirano Y. , Shimada T.
Published: July 2002

Edge plasma fluctuations are studied with inserted triple Langmuir probes and magnetic coils in the TPE-1RM20 reversed-field pinch [Y. Yagi et al., in Plasma Physics and Controlled Nuclear Fusion Research 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. 2, p. 611]. Two-point measurements show that density and potential fluctuations have relatively low mode numbers (m<3, n<40). High coherence (γ=0.5) with magnetic field fluctuations and similar mode spectra suggest that density and potential fluctuations are mainly caused by electromagnetic turbulence. Broadband magnetic fluctuations are dominated by m=0, low-n modes and internally resonant m=1 and m=2 modes. A coherent (f=20–30 kHz) m=0, low-n mode is also observed. Particle flux driven by electrostatic electric field fluctuations is 50%–100% of total flux obtained from Dα line intensity measurement. Low-frequency fluctuations (f<100 kHz) give the main contribution to the total flux. Electrostatic fluctuation driven electron energy flux is only of the order of 10% of total nonradiative power loss.
Dynamic compression of hydrogen for probing the molecular-atomic phase transition
Authors: Aoki T. , Meyer-ter-Vehn J.
Published: July 2002

Low-entropy, cylindrical implosion of cryogenic hydrogen is simulated to explore the equation of state (EOS) close to the transition from molecular to atomic structure. The simulations are based on the Los Alamos EOS library. Concerning isentropic compression, optimum time-shaped pulses are derived analytically and are used in numerical simulations. Small cylinders of cryogenic hydrogen, suitable for laser experiments, are considered with 100 μm radius and about a millimeter long; the energy to be invested into such cylinders to reach the transition is about 1 J. Multiple-shock compression by an imploding liner is also considered; the liner velocity has to be close to 1 km/s. Liner compression produces more uniform compressed configurations with longer lifetime (about 10 ns), though at somewhat higher entropy. Rarefaction shocks occur in the expansion stage and might be useful as a diagnostic.
Magnetic and velocity fluctuation measurements in the REPUTE-1 reversed-field pinch plasma
Authors: Ejiri Akira , Ohdachi Satoshi , Oikawa Toshihiro , Shinohara Shunjiro , Yamagishi Kenichi , Toyama Hiroshi , Miyamoto Kenro
Published: July 2002

Magnetic and velocity fluctuations are studied in the REPUTE-1 [Plasma Phys. Controlled Fusion 28, 805 (1986)] reversed-field pinch (RFP). The first measurement of velocity fluctuation in an RFP plasma has been done using a Doppler shift of the O v(O4+, 278.1 nm) line. The fluctuation level increases as the radius of the viewing chord increases. Magnetic fluctuation measurements by an insertable probe reveal that the radial cross correlation of toroidal field fluctuation changes its sign at the radius slightly inside the reversal surface. The level of magnetohydrodynamic dynamo term is estimated from magnetic fluctuations at the surface correlation changes and oxygen velocity fluctuations measured with the chord distance of 115 mm. The dynamo term and that due to resistivity are the same level. This fact is consistent with Ohm’s law on which magnetohydrodynamic dynamo models are based.
Confinement improvement mechanism in very high mode (VH mode)
Authors: Shaing K. C.
Published: July 2002

The mechanism for confinement improvement in the toroidal-rotation-induced very high mode (VH mode) may be the turbulence suppression due to the shear of the toroidal angular frequency associated with parallel flow. This explanation is in contrast to the plausible hypothesis that high-mode (H-mode) confinement improvement may be caused by the turbulence suppression due to the shear of the poloidal angular frequency associated with E×B and parallel flows. Here, E is the electric field and B is the magnetic field. A theory for VH mode, based on the rapid change of the radial gradient of the toroidal flow, is presented.
Analytic calculation of certain scattering parameters from a mode conversion analysis of X-mode–O-mode coupling
Authors: Ng C. S. , Swanson D. G.
Published: July 2002

Certain fast wave scattering parameters from a sixth order mode conversion equation, which represents the coupling of five propagating wave branches in an inhomogeneously magnetized plasma, are shown to be independent of absorption. However, the mode conversion coefficient C23 between the X-mode and the O-mode where both propagate in the same direction is not one of these. A recently developed analytical method is applied to calculate C23 and one of the nonzero reflection coefficients. Empirical formulas are found for these two coefficients. The result shows that C23 is not exactly independent of absorption, but for many cases has an unusually weak dependence. This explains a previous numerical result showing that C23 is independent of absorption to numerical accuracy. The coefficient C32 is also calculated by the same method and is shown to be equal to C23 as required by a proven reciprocity relation. The weak dependence of C23 on absorption has to be taken into consideration by any theory that attempts to treat a five branch problem as two separated three branch problems.
Three-dimensional theory of a dense thick-beam free electron laser
Authors: Gong Jianming , Zhen Jianhong
Published: July 2002

Based on both the coupled wave theory and the fluid dynamics, a three-dimensional theory of a dense, thick-beam free electron laser is presented which includes combined effects of the electron beam thickness, the radial gradients of the wiggler field, the self-fields of the beam, and the electrostatic field self-consistently. The noncoupling dispersion relations of the scattered wave and the electron plasma wave are, respectively, derived to decide their intersections in the ω-k plane, which tell the resonant conditions of the free electron laser. The coupling dispersion equation is then derived, which can be used to investigate the growth rate and the combined effects on the growth rate of the laser to detailed applications. From the analytic dispersion equation, it is found that the growth rate is proportional to the square root of the product of the beam density and the square of the perpendicular velocity amplitude.
Theory of current drive by parallel acceleration of electrons in a weakly relativistic plasma
Authors: Chiu S. C. , Karney C. F. F. , Mau T. K. , Lin-Liu Y. R.
Published: July 2002

A weakly relativistic theory of current drive by parallel acceleration of electrons in toroidal geometry is presented. Analytic expressions of efficiency are obtained for superthermal and subthermal electrons. By using a few adjustable parameters for intermediate energies, an accurate formula for the efficiency is obtained by fitting numerical results. This includes trapping effects and is valid for all frequencies up to the lower hybrid range of frequencies. It is found that for low-frequency fast-wave current drive, the relativistic and nonrelativistic efficiencies agree to within 15% up to 500 keV, although the relativistic corrections to power absorption and current density can be much larger separately. Relativistic effects on the efficiency become prominent at lower energies for Landau damping.
Magnetic flux generation near an O-line in collisionless reconnection— A new dynamo process
Authors: Cai H. J. , Lee L. C.
Published: July 2002

Magnetic reconnection in a current sheet with electron gyroradius scale is studied by a full particle simulation, in which the ion-electron mass ratio is set to be mi/me=10−1836. A new dynamo process has been found in which magnetic flux emerges outward from the magnetic O-line in the magnetic island formed by reconnection. The generation of new flux is associated with plasma compression in the magnetic island. The generated new flux is estimated to be ∼B0ρe, independent of the ion-electron mass ratio, where B0 is the magnetic field far away from the current sheet, and ρe is the electron gyroradius based on B0 and the electron thermal velocity. The plasma inertia and momentum transport due to the off-diagonal elements of plasma pressure tensor can lead to E⋅J<0 near the magnetic O-line, which makes the dynamo process possible. It is also found that the characteristic time scales of the reconnection process have a power-law dependence on the ion-electron mass ratio.
The Tokamak Physics Experiment
Authors: Davidson Ronald C. , Goldston Robert J. , Neilson George H. , Thomassen Keith I.
Published: July 2002

The mission of the Tokamak Physics Experiment (TPX) [Nevins et al., Plasma Physics and Controlled Nuclear Fusion, Würzburg (International Atomic Energy Agency, Vienna, 1992), Vol. 3, p. 279] is to develop the scientific basis for an economically competitive and continuously operating tokamak fusion power source. This complements the primary mission of the International Thermonuclear Experimental Reactor (ITER) [ITER Document Ser. No. 18 (International Atomic Energy Agency, Vienna, 1991)], the demonstration of ignition and long-pulse burn, and the integration of nuclear technologies. The TPX program is focused on making the demonstration power plant that follows ITER as compact and attractive as possible, and on permitting ITER to achieve its ultimate goal of steady-state operation. This mission of TPX requires the development of steady-state regimes with high beta, good confinement, and a high fraction of a self-driven bootstrap current. These regimes must be compatible with plasma stability, strong heat-flux dispersion in the divertor region, and effective particle control.
Turbulent transport across invariant canonical flux surfaces
Authors: Hollenberg J. B. , Callen J. D.
Published: July 2002

Net transport due to a combination of Coulomb collisions and turbulence effects in a plasma is investigated using a fluid moment description that allows for kinetic and nonlinear effects via closure relations. The model considered allows for ‘‘ideal’’ turbulent fluctuations that distort but preserve the topology of species-dependent canonical flux surfaces ψ♯,s≡∫dF⋅B♯,s, where B♯,s≡∇×[A+(ms/qs)us], in which us is the flow velocity of the fluid species. Equations for the net transport relative to these surfaces due to ‘‘nonideal,’’ dissipative processes are found for the total number of particles and total entropy enclosed by a moving canonical flux surface. The corresponding particle transport flux is calculated using a toroidal axisymmetry approximation for the ideal surfaces. The resulting net transport flux includes classical, neoclassical-like, and anomalous contributions and shows for the first time how these various contributions should be summed to obtain the total particle transport flux.
Nonlinear instability and chaos in plasma wave–wave interactions. I. Introduction
Authors: Kueny C. S. , Morrison P. J.
Published: July 2002

Conventional linear stability analyses may fail for fluid systems with an indefinite free-energy functional. When such a system is linearly stable, it is said to possess negative energy modes. Instability may then occur either via dissipation of the negative energy modes, or nonlinearly via resonant wave–wave coupling, leading to explosive growth. In the dissipationless case, it is conjectured that intrinsic chaotic behavior may allow initially nonresonant systems to reach resonance by diffusion in phase space. In this and a companion paper (submitted to Phys. Plasmas), this phenomenon is demonstrated for a simple equilibrium involving cold counterstreaming ions. The system is described in the fluid approximation by a Hamiltonian functional and associated noncanonical Poisson bracket. By Fourier decomposition and appropriate coordinate transformations, the Hamiltonian for the perturbed energy is expressed in action-angle form. The normal modes correspond to Doppler-shifted ion-acoustic waves of positive and negative energy. Nonlinear coupling leads to decay instability via two-wave interactions, and to either decay or explosive instability via three-wave interactions. These instabilities are described for various integrable systems of waves interacting via single nonlinear terms. This discussion provides the foundation for the treatment of nonintegrable systems in the companion paper.
Laminar crossed-field flow stability in slow wave cavities: A singularity-free approach
Authors: Riyopoulos Spilios
Published: July 2002

A linear stability approach free from the singularities of the cold fluid theory is applied to laminar E×B flows in slow wave cavities. Employing time scale separation in the appropriate frame of reference, one avoids singularities in the guiding center motion near the drift and drift-cyclotron resonances ω−ku0(x)=nΩ, n=0,±1. Instead of an a priori Fourier mode expansion, a boundary value problem is solved with a traveling wave boundary condition at the wall. The space-charge potential is obtained in closed form using the Green’s function approach. Due to the incompressibility of the flow, charge perturbations and energy exchange take place at the free flow boundaries (surface perturbations). A local expansion of the Green’s function in terms of the perturbation amplitude and its derivative at the boundary yields the self-consistent closure. The loaded cavity mode profiles are everywhere free from singularities, exhibiting local maxima at the flow boundaries, not at the resonant layer(s). The small signal growth is biexponential ∝eΓt2, where Γ scales as the diocotron frequency squared Ω2D=ω4p/Ω2, and is independent of the frequency detuning from resonance. The method is compared to the normal mode analysis and the vacuum mode expansion technique.
Influence of molecular processes on the hydrogen atomic system in an expanding argon–hydrogen plasma
Authors: Meulenbroeks R. F. G. , Engeln R. A. H. , Box C. , de Bari I. , van de Sanden M. C. M. , van der Mullen J. A. M. , Schram D. C.
Published: July 2002

An expanding thermal arc plasma in argon–hydrogen is investigated by means of emission spectroscopy. The hydrogen can be added to the argon flow before it enters the thermal arc plasma source, or it can be flushed directly into the vacuum expansion vessel (1–20 vol % H2). The atomic state distribution function for hydrogen, measured at a downstream distance of 20 mm, turns out to be very different in the two cases. For injection in the arc, three-particle recombination is a primary source of hydrogen excitation, whereas measurements with hydrogen injected into the vessel clearly point to a molecular channel (dissociative recombination of formed ArH+) populating atomic hydrogen levels.
Theory of ideal magnetohydrodynamic ballooning stability of a poloidally rotating plasma in a sheared electric field
Authors: Medvedev M. V. , Diamond P. H.
Published: July 2002

The ideal magnetohydrodynamic ballooning stability of a poloidally rotating plasma in a sheared radial electric field is investigated for an axisymmetric toroidal configuration. An analytical criterion for the ballooning instability of such a system is derived. Electric field shearing is the dominant effect. The sign of the electric field gradient enters the stability criterion via the angular momentum gradient à la Taylor–Couette flow. Thus, Er′<0 is stabilizing while Er′≳0 is destabilizing. Centrifugal force effects contribute a weak destabilization. The modifications in the ballooning mode stability boundary are calculated.
Variational calculation of alpha-driven bootstrap current in a deutrium–tritium tokamak reactor
Authors: Jhang Hogun , Chang C. S.
Published: July 2002

A general analytic expression for bootstrap current driven by fusion-born alpha particles is obtained in a deutrium–tritium tokamak plasma using a kinetic variational theory. A self-adjoint form of linearized Coulomb scattering operator has been used, which includes both energy and pitch angle collisions. The formula is applied to predict alpha-driven bootstrap current in a reactor condition similar to that of the International Thermonuclear Experimental Reactor Conceptual Design (ITER CDA) parameters [ITER Documentation Series, Number 18 (International Atomic Energy Agency, Vienna, 1991)]. The result agrees well with results obtained using another technique [Hsu et al., Phys. Fluids B 4, 4023 (1992)].
Soft x-ray laser in a steep density gradient MgXI plasma
Authors: Ma Guo-Bin , Tan Wei-Han
Published: July 2002

The hydrodynamic behaviors of a steep density gradient Mg plasma produced by a short driving laser pulse is simulated, and it is shown that the temperature of a shorter scale-length (steeper density gradient) plasma decreases faster. The rate equations of the energy level population are solved using the collisional radiative model. The ground states of MgX, MgXII, and MgXIII, and the 21 energy levels of MgXI, of which the principal quantum number n≤6, are considered in our calculation. The gain coefficients of several possible laser transitions at wavelengths less than 200 Å in the MgXI plasma are presented. Finally the possibility of achieving a soft x-ray laser near the water-window is discussed.
Scaling laws for two-dimensional divertor modeling
Authors: Catto Peter J. , Krasheninnikov S. I. , Connor J. W.
Published: July 2002

To gain insight into divertor operation, similarity techniques are employed to investigate whether model systems of equations plus boundary conditions admit scaling transformations that lead to useful divertor scaling laws. These can be used to perform similarity experiments or more fully exploit large computer simulations. Fluid plasma models of the divertor region are adopted that ignore anomalous processes. We consider neutral descriptions in both the short and long mean-free path limits. As usual, the more approximations that are made, the more scaling transformations are allowed, leading to fewer independent dimensionless parameters that need to be considered, thereby imposing fewer divertor similarity constraints. The simplest model considered balances electron heat conduction with impurity radiation and places the fewest constraints on similarity. To be able to model the onset of detached divertor operation in short mean-free path regimes, a fluid neutral description is employed that balances plasma pressure by neutral pressure. In this model the constraints on divertor similarity are most severe. A less constrained long mean-free path or Knudsen neutral model is also considered. It models the onset of detached divertor operation by balancing plasma pressure by momentum transfer to the neutrals that are randomized by collisions with the deep slot sidewalls. The simpler models have relaxed divertor similarity constraints, but all models remain severely restricted by the collisionality constraints.
Effects of static magnetic fields on rf-driven plasma sheaths
Authors: Riyopoulos Spilios
Published: July 2002

The physics of sheath formation during rf capacitor discharges in magnetized plasmas is examined, for arbitrary angle between the dc magnetic field and the oscillating rf electric field. Observations from particle simulations show that the induced dc sheath potential under given rf drive increases with increasing angle between the electric and magnetic fields, reaching a maximum nearly twice the unmagnetized dc potential for nearly perpendicular E and B. Analytic study of the ion dynamics in the time-averaged sheath field reveals that the ion motion is unstable, yielding unbound ion transport across the magnetic lines. The effective ion mass m*, employed for the acceleration along the electric field, is obtained as a function of the magnetic angle and the relative magnetic strength. The magnetized presheath ions are partially demagnetized inside the sheath. The ratio of the effective masses mB/m*, where mB stands for fully magnetized ions, parametrizes the sheath potential and sheath thickness. The sheaths behave as unmagnetized in the limit of parallel E and B.
On the efficiency of energy transfer by nonlinear magnetohydrodynamic waves propagating along magnetic slabs
Authors: Huang Ping
Published: July 2002

A finite-difference numerical code is utilized to study the behavior of magnetohydrodynamic (MHD) body and surface waves on magnetic slabs. The waves are excited by finite amplitude velocity perturbations, imposed either inside the slab or in field-free or magnetized surroundings. The paper investigates the efficiency of energy leakage from the slab to the surroundings and vice versa. In the former, waves are converted to acoustic waves or MHD fast waves depending upon the surroundings and, in the latter, to body and surface waves propagating along the slab. The dependence of the former case on the perturbations and physical parameters, such as plasma β and the slab thickness, has been studied in detail. It is found that energy leakage to the surroundings is of the order of 50%. The dependence of the latter case on β and external magnetic field is also studied.
Radial profile of plasma potential with various biased electrode ring configurations in a toroidal plasma
Authors: Chaube Nilima R. , Jain K. K.
Published: July 2002

An experimental study on behavior of radial profile of the floating potential with different biased electrode ring configurations has been carried out in a currentless magnetized toroidal plasma. Radial profile of the floating potential has been measured by biasing single ring of various sizes and two rings. It is observed that floating potential profile of a well shaped with controllable depth, hill-cum-well shaped, and almost flat positive potential can be obtained. Results on parameter dependence studies of floating potential on the bias voltage, magnetic field, and gas pressure are presented.
Bäcklund transformations and Painlevé analysis: Exact solutions for the nonlinear isothermal magnetostatic atmospheres
Authors: Khater A. H. , Callebaut D. K. , Ibrahim R. S.
Published: July 2002

The equations of magnetohydrostatic equilibria for a plasma in a gravitational field are investigated analytically. For equilibria with one ignorable spatial coordinate, the equations reduce to a single nonlinear elliptic equation for the magnetic potential μ̃, known as the Grad–Shafranov equation. Specifying the arbitrary functions in the latter equation, one gets the nonlinear elliptic equation. Analytical solutions of the elliptic equation are obtained for the case of a nonlinear isothermal atmosphere in a uniform gravitational field. The solutions are obtained by using the Bäcklund transformations technique and Painlevé analysis, which are adequate for describing parallel filaments of diffuse, magnetized plasma suspended horizontally in equilibrium in a uniform gravitational field.
A variational approach to parametric instabilities in inhomogeneous plasmas II: Stimulated Raman scattering
Authors: Afeyan Bedros B. , Williams Edward A.
Published: July 2002

The theory of temporally unstable modes or bound states associated with stimulated Raman scattering in inhomogeneous plasmas is recast in the form of a variational principle for the pump strength or intensity of the incident laser. This Minimum Pump Strength–Variational Principle (MPS–VP) formalism allows the unification of disparate results on growth rates, frequency shifts, and threshold conditions, which in the past relied on specific and restrictive assumptions on the density profile, scattering geometry, temperature, and damping rates. The variational approach leads to generalizations of these results in a uniform manner. Various levels of sophistication in the choice of trial and dual functions are explored. Simplifications and short cuts that will be used throughout this series of papers are tested here, and their regions of validity explored. The principle new result of some practical interest is the growth rate of Raman sidescattering occurring anywhere at the peak or on the flanks of a parabolic density profile such as can be found in an exploding foil experiment. This new calculation includes any density at or below quarter-critical, at any temperature, and at low or high damping rate limits.
Effects of fast beam ions on ion-acoustic solitons and shocks excited in a double plasma device
Authors: Honzawa Tadao , Nagasawa Takeshi
Published: July 2002

The limitation in amplitude of ion-acoustic solitons or laminar shocks excitable in a double plasma device is clarified to result from the effect of pulsed fast beam ions, generated by the pulse externally applied for wave excitation. Beam ions faster than the soliton, which suppress the growth of the proto-soliton amplitude at an early stage of wave excitation, are always generated in the case of the double plasma (DP)-excitation, when a pulse high enough is applied so as to excite a large amplitude soliton. Controlling the generation of such fast beam ions by adjusting the rise time of the applied pulse, ion-acoustic solitons or laminar shocks of high Mach numbers up to M≃1.4 are observed to be formed even in a plasma with a finite ion temperature such as Ti≃Te/20.
Density irregularities in the weakly ionized nonuniform plasma
Authors: Karashtin A. N. , Gurevich A. V. , Zybin K. P.
Published: July 2002

A rigorous theory of small disturbances in a weakly ionized nonuniform plasma in crossed electric and magnetic fields is developed. Regular inhomogeneities both of plasma density and collision frequency are taken into account. The analysis of eigenvalues and eigenmodes is performed in the electrostatic approximation. It is shown that the inhomogeneity of collision frequency is of fundamental significance, is responsible for the creation of boundary conditions, and determines the range of existence and spatial structure of eigenmodes. It is also shown that there is no localized mode associated with steep plasma density gradients. The dispersion relation and altitude behavior of eigenmodes are investigated in the WKB approximation in the case of the exponential profile of collision frequencies, which is natural for the ionospheric E-region.
Study of a low-pressure nitrogen plasma boundary layer over a metallic plate
Authors: Bourdon A. , Vervisch P.
Published: July 2002

The modeling of a low-pressure nitrogen plasma boundary layer over a flat plate is presented and compared with numerous measurements carried out on this test case. Shortcomings of state-of-the art physical and chemical models are discussed. In particular, wall conditions for a nitrogen plasma over a metallic plate are studied in detail. It is shown that in given flow conditions, the accommodation coefficient of the vibrational mode on the metallic plate is very small. A complex recombination process of N+ ions at the wall is proposed to explain the low values of N number densities measured.
Tearing instabilities in multiple current sheets for current-dependent resistivity models
Authors: Birk G. T. , Konz C. , Otto A.
Published: July 2002

A generalized analytical treatment of the resistive tearing mode in multiple current sheets for current-dependent electrical resistivities is given. This generalization allows for taking into account anomalous dissipation caused by current-driven microturbulence. The growth rates of the mode are found to be larger than the growth rates for constant resistivity. Numerical simulations show the linear and nonlinear development of the instability, where, in particular, the assumption of periodic configurations necessary in the analytical treatment can be relaxed. The results indicate that reconnection in neighboring magnetic flux systems mainly results in Ohmic heating and small tongue outflow rather than plasmoid-like eruptions.
Energy release and plasma heating by forced magnetic reconnection
Authors: Vekstein G. E. , Jain Rekha
Published: July 2002

A simple model of forced magnetic reconnection in a force-free magnetic field is considered, which allows the calculation of the magnetic energy release during the current sheet reconnection. The dependence of this energy on characteristics of the magnetic configuration has been studied, and it was found that the released energy becomes very large when the field is near the marginal tearing stability. A persistent plasma heating provided by ongoing external driving and internal reconnection is most efficient when the time-scales of these processes are comparable. Possible implications of the obtained results for the problem of solar coronal heating are briefly discussed.
Collisional transport in non-neutral plasmas
Authors: Dubin Daniel H. E.
Published: July 2002

Several recent experiments have measured collisional transport in non-neutral plasmas (heat conduction, test particle diffusion, and viscosity) that is from 10 to 104 times larger than predicted by classical theory. New guiding center theories of collisional transport have been developed that agree with the measurements. The experiments operate in the guiding center regime rc≪λD, where rc is the cyclotron radius and λD is the Debye length. In this regime, classical transport theory is irrelevant because it implicitly assumes the opposite ordering, λD≪rc, although this ordering is not always satisfied in neutral plasmas.
Flow reversal, convection, and modeling in the DIII-D divertor
Authors: Boedo J. A. , Porter G. D. , Schaffer M. J. , Lehmer R. , Moyer R. A. , Watkins J. G. , Evans T. E. , Lasnier C. J. , Leonard A. W. , Allen S. L.
Published: July 2002

Measurements of the parallel Mach number of background plasma in the DIII-D tokamak divertor [M. A. Mahdavi et al. in Proceedings, 16th International Conference, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997) Vol. I, p. 397] were performed using a fast scanning Mach probe. The parallel particle flow shows evidence of complex behavior such as reverse flow, i.e., flow away from the target plate, stagnant flow, and large scale convection. For detached discharges, measurements confirm predictions of convective flow towards the divertor target plate at near sound speed over large regions in the divertor. The resulting convected heat flux is a dominant heat transport mechanism in the divertor. For attached discharges with high recycling, particle flow reversal in a thin region at or near the outer separatrix, thereby confirming the existence of a mechanism by which impurities can be transported away from the divertor target plates. Modeling results from the two-dimensional fluid code UEDGE [G. D. Porter and the DIII-D Team, “Divertor characterization experiments and modelling in DIII-D,” in Proceedings of the 23rd European Conference on Controlled Fusion and Plasma Physics, 24–28 June 1996, Kiev, Ukraine (European Physical Society, Petit-Lancy, Switzerland, 1996), Vol. 20C, Part II, p. 699] can reproduce the main features of the experimental observations.
Properties of plasmas generated by electrical breakdown in flames
Authors: Uhm Han S.
Published: July 2002

Properties of electrical discharge in flames and influence of plasma electrons on gas neutrals are investigated by making use of the ionization cross section of air. An analytical expression of air ionization rate is obtained from tabulated data of the ionization cross section of oxygen and nitrogen, and is compared with air ionization rate measured with the applied electric field. The influence of gas temperature on electrical discharge properties is investigated by making use of electron energy gain in the electric field. Electrical breakdown occurs whenever ionization of neutrals dominates the electron attachment of oxygen molecules. It is found that the breakdown electric field in flames is inversely proportional to the flame temperature Tg, thereby easily generating plasmas in flames. A swarm of low-energy electrons in flames would allow a significant population of electronically excited states of flame molecules to be formed. The analysis shows that the electronic excitation of flame molecules may also considerably reduce the breakdown field. Plasma electrons generate atomic oxygen by the electron attachment of oxygen molecules in high-pressure flames. An example calculation shows that more than 63% of oxygen molecules are converted into atoms within 760 ms dwelling time for the plasma with density of np=1013 cm−3 and temperature of Te≈2.5 eV. Oxygen atoms are the most reactive radicals in flames for material oxidation.
Improved boundary layer analysis of forced magnetic reconnection due to a boundary perturbation
Authors: Ishizawa A. , Tokuda S.
Published: July 2002

Linear boundary layer analysis of forced magnetic reconnection due to an externally imposed boundary perturbation has been reworked. This improved analysis introduces correct asymptotic matching to take into account the effect of inertia in the inner layer precisely, and adopts a time dependent boundary perturbation which is suitable for this analysis. The improved analysis demonstrates a new reconnection process and clarifies the role of stability against the tearing modes in the process. The initial evolution of this new reconnection process is characterized by some significant features which affect the subsequent evolution toward a fully reconnected state. One is that the reconnected flux increases on the same time scale as the boundary perturbation, which excludes the Sweet–Parker time scale obtained by use of the constant-ψ asymptotic matching. Another is that an induced surface current on a resonant surface is in such a direction as to oppose the progress of the reconnection, because the equilibrium is stable against tearing modes in the absence of the boundary perturbation.
The roles of shear and cross-correlations on the fluctuation levels in simple stochastic models
Authors: Krommes John A.
Published: July 2002

Highly simplified models of random flows interacting with background microturbulence are analyzed. In the limit of very rapid velocity fluctuations, it is shown rigorously that the fluctuation level of a passively advected scalar is not controlled by the rms shear. In a model with random velocities dependent only on time, the level of cross-correlations between the flows and the background turbulence regulates the saturation level. This effect is illustrated by considering a simple stochastic-oscillator model, both exactly and with analysis and numerical solutions of the direct-interaction approximation. Implications for the understanding of self-consistent turbulence are discussed briefly.