bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectrum of compressional Alfvén waves
Authors: Mahajan S. M. , Ross David W.
Published: March 2003

The spectrum of compressional Alfvén eigenmodes localized in the potential well created by a combination of the variation in plasma density and the wavenumber k⊥=m/r is obtained, and its importance for radio frequency current drive is discussed. It is found that modes with small parallel wavenumbers and frequencies below the ion cyclotron frequency are attractive for current drive.
A comparison of the full and reduced sets of magnetohydrodynamic equations for resistive tearing modes in cylindrical geometry
Authors: Holmes J. A. , Carreras B. A. , Hender T. C. , Hicks H. R. , Lynch V. E. , Masden B. F.
Published: March 2003

The results of numerical calculations using a full set of resistive magnetohydrodynamic equations are compared with the results of similar calculations using a reduced set of equations derived for tokamaks in the limits of low beta and large aspect ratio ( β∼ε2≪1). The calculations, which are carried out in three-dimensional cylindrical geometry using initial value techniques, allow the comparison of linear stability properties as well as detailed nonlinear studies. The results of these calculations (including linear growth rates and eigenfunctions, the nonlinear evolution of single-helicity magnetic islands, and the nonlinear evolution of multihelicity cases) confirm the validity of the reduced set of equations at low beta in cylindrical geometry.
On tearing modes in a resistive medium
Authors: Rosenau Philip
Published: March 2003

The theory of tearing modes in cylindrical symmetry is reexamined. The resistive medium is characterized by the simplest Ohm’s law. By considering β, the ratio of thermal to magnetic pressure, as a parameter with respect to which the sought-after eigenvalues were scaled, a very simple description was obtained. Many of the classical results were obtained in a more simple way. Among the new results are: (1) the possibility of the existence of a tearing mode on a resistive time scale, which is hardly affected by drift effects; and (2) for modes of an almost marginal growth rate inertia does not play an important role, slab and cylindrical modes are fundamentally different, and finite-β effects play a crucial role.
Unified kinetic theory in toroidal systems
Authors: Hitchcock D. A. , Hazeltine R. D. , Mahajan S. M.
Published: March 2003

A kinetic theory for toroidal systems which includes the effects of collisions as well as instabilities is constructed. This yields a pair of evolution equations; one for the spectrum and one for the distribution function. In addition, this theory yields a toroidal generalization of the usual collision operator which is shown to have many similar properties, conservation laws, and H theorem, to the usual collision operator.
Axisymmetric toroidal equilibria with plasma flow
Authors: Copenhaver C.
Published: March 2003

A general equilibrium equation for an axisymmetric toroidal plasma having both toroidal and poloidal components of the magnetic field and plasma flow is derived. The possibility of a numerical treatment is discussed and polynomial solutions are obtained for several limiting cases. These solutions show that with flow rotation, the pressure and magnetic surfaces are no longer concentric, and that there is a significant displacement between these surfaces. A new result of hydrodynamic interest is the equilibrium equation for a compressible fluid.
Finite Larmor radius stability theory of ELMO Bumpy Torus plasmas
Authors: Berk H. L. , Cheng C. Z. , Rosenbluth M. N. , Van Dam J. W.
Published: March 2003

An eikonal ballooning mode formalism is developed to describe curvature-driven modes of hot electron plasmas in bumpy tori. The formalism treats frequencies comparable to the ion cyclotron frequency, as well as arbitrary finite Larmor radius and field polarization, although the detailed analysis is restricted to E∥ =0. Moderate hot electron finite Larmor radius effects are found to lower the background beta core limit, whereas strong finite Larmor radius effects produce stabilization. The critical finite Larmor radius parameter with weak curvature is FR=k2⊥ ρ2hR/ Δb (1+P′∥/ P⊥) where k⊥ is the perpendicular wavenumber, ρh the hot electron Larmor radius, R the magnetic field radius of curvature at the hot-electron layer, Δb the magnetic field scale length in the diamagnetic well, and P′∥,⊥ are the parallel and perpendicular pressure gradients. The interchange instability arises if 1>FR>1−βcR/ [2Δ(1+P∥/P′⊥)], whereas all modes are stable if FR>1, where βc is the core plasma beta and Δ is the core plasma pressure gradient length.
Rotational stability of tandem mirrors
Authors: Freidberg J. P. , D’Ippolito D. A.
Published: March 2003

The general stability equation for low-m rotational-ballooning instabilities in axisymmetric mirror geometry is derived for an isotropic pressure plasma, including a boundary term which represents the stabilizing effect of an unspecified anchor. The corresponding variational principle, together with a particular quadrupole anchor model, is used to compute the low-β rotational-interchange marginal stability condition for a conventional tandem mirror. The stability criterion is applied to show that low-m rotational modes are stable in the TMX experiment. Also, a mechanism for enhanced stabilization of curvature-driven modes when Er<0 is discussed.
Study of gain, bandwidth, and tunability of a millimeter-wave free-electron laser operating in the collective regime
Authors: Gold S. H. , Black W. M. , Freund H. P. , Granatstein V. L. , Jackson R. H. , Efthimion P. C. , Kinkead A. K.
Published: March 2003

Frequency-resolved measurements of the emission of a collective free-electron laser operating at millimeter wavelengths have shown emission spectra that agree well with theoretical predictions for the collective free-electron laser instability. Broad tunability, moderate emission linewidths, and high single-frequency gain have been observed. In addition, adjusting the axial field in the end region of the interaction has been found in some cases to cause a large increase in measured power and efficiency.
Resistive wall flute stability of magnetically guided relativistic electron beams
Authors: Kleva Robert G. , Ott Edward , Sprangle Phillip
Published: March 2003

The resistive wall stability of an electron beam is studied, and application of the theory to the modified betatron accelerator is considered. Only flute-like perturbations are analyzed (n=0, l≥1, where n and l are the toroidal and poloidal mode numbers, respectively). Included in the analysis are the effects of relativistic velocities, equilibrium and perturbed self-electromagnetic fields, and resonant particle effects due to density and velocity profiles. The principal results are (1) l≥2 modes are much more difficult to stabilize than is the l=1 mode; (2) the velocity gradient along the beam can provide an important stabilizing mechanism; (3) the stabilizing effect of the density gradient is reduced by relativistic effects; and (4) magnetic perturbations can be important even for nonrelativistic beams.
Ion heating due to explosive instability in an ion beam–plasma system
Authors: Sugaya R.
Published: March 2003

Ion heating by the explosive instability in an ion beam–plasma system is studied theoretically. Two electrostatic waves with negative and positive energies become simultaneously unstable by nonlinear wave–particle interaction (nonlinear Landau damping) with plasma ions accompanying ion heating. The relaxation of the ion beam makes the explosion time of the waves much larger and causes saturation of the ion heating. It is shown quantitatively that the decrement of beam energy is transferred into thermal energies of plasma and beam ions by this explosive instability.
Electromagnetic two-stream and filamentation instabilities for a relativistic beam–plasma system
Authors: Lee Huan , Thode Lester E.
Published: March 2003

Previous investigations on the two-stream and filamentation instabilities are based on either the electrostatic or the ordinary-mode approximation. A general relativistic dispersion formulation is presented to study these two instabilities for a scattered electron beam propagating a collisional, bi-Maxwellian plasma. New analytical results that apply to a general beam distribution are obtained for the stability boundary of the filamentation instability. The general dispersion relation uncovers the inadequacy in applying the ordinary-mode approximation in a frame other than the rest frame of the plasma. Analytical expressions for the the growth rates of the filamentation modes in various parameter regimes are obtained. Finally, numerical comparisons are made between the general dispersion results and the earlier results based on the electrostatic and ordinary-mode approximations.
Comment on ‘‘Steady-state planar ablative flow’’ [Phys. Fluids 2
5, 1644 (1982)]
Authors: Sanmartín J. R. , Montañés J. L. , Barrero A.
Published: March 2003

A comment is made on steady-state flow of laser-produced plasmas. (AIP)
Dynamic concentration of positive ions near the axis of an electron plasma column
Authors: Sieger G. E. , Connolly R. C. , Lefevre H. W.
Published: March 2003

A steady-state non-neutral plasma, generated by Penning discharge, is used as a focusing element for MeV ion beams. An almost pure electron plasma column is produced which has nearly constant density (109–1010 cm−3) inside a radius of several millimeters. The background gas is at a pressure of 10−7 torr or less. A model calculation is presented which shows that although orbital damping is negligible, positive ions created by electron-neutral collisions are dynamically concentrated near the axis of the plasma. Light emission by excited ions in the plasma is observed to be brightest in a thin line along the axis. The measured radial profile of relative light intensity is in good agreement with the calculated ion distribution. A radial charge density profile of the plasma, which is inferred from beam deflection measurements, is depressed by several percent near the axis due to positive ions concentrated there.
A numerical-experimental study of confined flow around rectangular cylinders
Authors: Davis R. W. , Moore E. F. , Purtell L. P.
Published: March 2003

A previous numerical study by Davis and Moore of vortex shedding from rectangles in infinite domains is extended to include the effects of confining walls. The major changes to the numerical modeling are the addition of a direct solver for the pressure equation and the use of an infinite-to-finite mapping downstream of the rectangle. The parameters in the problem are now Reynolds number, rectangle aspect ratio, blockage ratio, and upstream velocity profile. As each of these is varied, the effects upon the forces acting on the rectangle and the structure of the wake are discussed. Streakline plots composed of multishaped passive marker particles provide a clear visualization of the vortices. These plots are compared with smoke-wire photographs taken from a wind tunnel test. Strouhal numbers obtained both computationally and experimentally are compared for two values of the blockage ratio. Moving recirculation zones which appear between the wake and the walls are discussed.
A possible plasma-dynamo mechanism driven by particle transport
Authors: Jacobson Abram R.
Published: March 2003

A new plasma-dynamo mechanism is proposed, with application to the outer region of the reversed-field pinch. The mechanism uses space-charge electric fields associated with particle transport to drive steady-state electrical currents which otherwise would resistively decay for lack of an externally applied electric field. The mechanism is suggested by recent observations during sustained discharges in the ZT-40M experiment.
The Ohmic heating of a spheromak to 100 eV
Authors: Jarboe T. R. , Barnes Cris W. , Henins I. , Hoida H. W. , Knox S. O. , Linford R. K. , Sherwood A. R.
Published: March 2003

The first spheromaks with Thomson-scattering-measured electron temperatures of over 100 eV are described. The spheromak is generated by a magnetized coaxial plasma source in a background gas of 30 mTorr of H2, and it is stably confined in an oblate 80 cm diam copper mesh flux conserver. The open mesh design allows rapid impurity transport out of the spheromak. The peak temperature, measured using multipoint Thomson scattering, is observed to rise from approximately 25 eV to over 100 eV in about 0.2 msec due to Ohmic heating from the decaying magnetic fields. Density (∼5×1013 cm−3) and magnetic fields (approximately 2 kG) are measured using interferometry and magnetic probes.
Reconnection rates of magnetic fields including the effects of viscosity
Authors: Park W. , Monticello D. A. , White R. B.
Published: March 2003

The Sweet–Parker and Petschek scalings of the magnetic reconnection rate are modified to include the effect of the viscosity. The modified scalings show that the viscous effect can be important in high-β plasmas. The theoretical reconnection scalings are compared with numerical simulation results in a tokamak geometry for three different cases: a forced reconnection driven by external coils, the nonlinear m=1 resistive internal kink, and the nonlinear m=2 tearing mode. In the first two cases, the numerical reconnection rate agrees well with the modified Sweet–Parker scaling when the viscosity is sufficiently large. When the viscosity is negligible, a steady state which was assumed in the derivation of the reconnection scalings is not reached and the current sheet in the reconnection layer either remains stable through sloshing motions of the plasma or breaks up to higher m modes. When the current sheet remains stable, a rough comparison with the Sweet–Parker scaling is obtained. In the nonlinear m=2 tearing mode case where the instability is purely resistive, the reconnection occurs on the slower dissipation time scale (ψ̇s∼η). In addition, experimental data of the nonlinear m=1 resistive internal kink in tokamak discharges are analyzed and are found to give reasonable agreement with the modified Sweet–Parker scaling.
An alpha-particle-driven Alfvén wave instability in a tandem mirror reactor
Authors: Hanson James D. , Ott Edward
Published: March 2003

Because their energies greatly exceed the plug potential, fusion reaction alpha particles in a tandem mirror reactor will be mirror confined rather than electrostatically confined by the end plugs. Hence, the alpha particle distribution function will have a loss cone. This anisotropy in the distribution function is shown to destabilize Alfvén waves in the central section of a tandem mirror reactor. The wave equation with a modified cold plasma response is used to determine the radial mode structure and real frequency of the Alfvén waves in cylindrical geometry. The growth rate is calculated from the resonant particle part of the plasma response. Destabilization of the wave occurs due to a cyclotron resonance of the alpha particles. The resonant interaction of the wave with the alpha particles will pitch-angle scatter the alphas into the loss cone, effectively widening the loss cone. The instability will cause a substantial loss of alpha particle energy unless the central cell is characterized by small diameter, sharply peaked radial profile, low magnetic field, and large mirror ratio. A recent ‘‘axicell’’ reactor design is found to be stable.
High-power coherent microwave generation from oscillating virtual cathodes
Authors: Kwan Thomas J. T.
Published: March 2003

The formation of an oscillating virtual cathode by a relativistic electron beam and the subsequent generation of coherent microwaves are investigated. If the electron beam is of high quality, the microwaves excited in a cylindrical waveguide are found to occupy a very narrow band (Δω/ω∼5%) primarily in a single transverse magnetic mode. Furthermore, the efficiency of microwave production is demonstrated in our computer simulations to be as high as 20%. It is also shown quantitatively that the efficiency decreases monotonically as the mean scattering angle of the electron beam increases.
Kinetic theory of two-plasmon decay
Authors: Powers L. V. , Berger R. L.
Published: March 2003

The Vlasov–Maxwell theory of two-plasmon decay is developed for homogeneous and weakly inhomogeneous plasmas. Temporal growth rates in homogeneous plasmas and spatial amplification in weakly inhomogeneous plasmas are calculated and compared to fluid theory. For hot plasmas (Te≳10 keV) the kinetic analysis predicts lower growth rates and a reduced range of unstable wavenumbers compared to fluid theory.
Direct generation of the auroral kilometric radiation by the maser synchrotron instability: An analytical approach
Authors: Le Quéau D. , Pellat R. , Roux A.
Published: March 2003

It has recently been shown that radio emission from the Earth, the so-called auroral kilometric radiation, is emitted inside cold plasma depleted regions. These observations have motivated a new interest in direct generation processes. Since it allows the generation of an X mode just above the cutoff frequency of this mode (in agreement with observations) the maser synchrotron instability is a promising candidate. A detailed analytical approach to the understanding of this instability is proposed. The complete dispersion relation is derived in a situation where the cold plasma density is small (εc=ω2pe/ω2c≪1). The physical significance of this instability is discussed; in particular, the difference between the maser synchrotron instability and the classical Weibel instability is stressed. Depending upon the ratio between ‘‘cold’’ and energetic electron components, two regimes, the ‘‘kinetic’’ and the monoenergetic ‘‘ring-like’’ distribution, are considered and the results are compared with the numerical solution of the dispersion relation. Finally the saturation process of this instability is discussed in a homogeneous infinite medium; it is shown that trapping effects can insure an efficient saturation of the maser synchrotron instability.
Bernstein mode excitation in an inhomogeneous plasma by the coupling of two electromagnetic waves near the upper-hybrid frequency
Authors: Braun F. , Leclert G.
Published: March 2003

The equation describing the excitation of a Bernstein wave near the upper-hybrid frequency by the coupling of two electromagnetic waves in an inhomogeneous plasma is derived. The basic assumptions are (i) the three waves propagate perpendicularly to the external magnetic field, and (ii) the Bernstein wave is electrostatic. The excited electric field may be expressed in terms of Airy functions. The power flowing in the upper-hybrid mode is calculated as a function of the coupling length, the inhomogeneity length, and the angle between the incident waves. Numerical results are given for the case of ionosphere.
Time-dependent drift Hamiltonian
Authors: Boozer Allen H.
Published: March 2003

The lowest-order drift equations are given in a canonical magnetic coordinate form for time-dependent magnetic and electric fields. The advantages of the canonical Hamiltonian form are also discussed.
Current drive by the combined effects of electron-cyclotron and Landau wave damping in tokamak plasmas
Authors: Fidone I. , Giruzzi G. , Granata G. , Meyer R. L.
Published: March 2003

The theory of current drive by the combined effects of lower-hybrid and electron-cyclotron wave absorption is investigated. The case of a spatially homogeneous lower-hybrid diffusion coefficient and the electron ordinary mode propagating nearly normal to the magnetic field is considered. The effect of selective electron-cyclotron wave absorption for optimizing lower-hybrid current drive is discussed for two situations, namely, the wave frequency close to or much less than the electron-cyclotron gyrofrequency, which corresponds to particle heating for moderate velocities and mildly relativistic energies, respectively. In the former case, by using the numerical solution of the Fokker–Planck equation it is shown that electron heating is suited for controlling the density of the current-carrying tail. The latter case is investigated for moderate wave powers and is shown to be most appropriate for optimizing the ratio J/P.
Ion tails and lower-hybrid-drift turbulence in the ELMO Bumpy Torus
Authors: Rosenberg M. , Krall N. A.
Published: March 2003

It is demonstrated that a low level of lower-hybrid-drift waves could explain the hot-ion tails observed in EBT; the tail temperature is related to the lower-hybrid-drift wave energy. The effect of these waves on the bulk ion distribution is also analyzed.
Stability of electrostatic drift waves in bumpy tori
Authors: Sanuki Heiji
Published: March 2003

Nonlocal properties of electrostatic drift waves in a straight magnetic field in a collisionless and low-β plasma with a hot-electron component are discussed analytically in detail. Interesting drift wave features, such as eigenfrequency, growth rate, radial wavenumber, and position of localization, are clarified. Effects of the hot-electron component and the ambipolar potential on the stability of drift waves are also studied and related to low-frequency fluctuations measured in ELMO Bumpy Torus and Nagoya Bumpy Torus experiments. It is found that the hot-electron component has a destabilizing effect, but a strong ambipolar field has a stabilizing influence on drift waves inside the hot-electron ring.
Free-boundary stability of straight stellarators
Authors: Barnes D. C. , Cary John R.
Published: March 2003

The sharp-boundary model is used to investigate the stability of straight stellarators to free-boundary, long-wavelength modes. To correctly analyze the heliac configuration, previous theory is generalized to the case of arbitrary helical aspect ratio (ratio of plasma radius to periodicity length). A simple low-β criterion involving the vacuum field and the normalized axial current is derived and used to investigate a large variety of configurations. The predictions of this low-β theory are verified by numerical minimization of δW at arbitrary β. The heliac configuration is found to be remarkably stable, with a critical β of over 15% determined by the lack of equilibrium rather than the onset of instability. In addition, other previously studied systems are found to be stabilized by net axial plasma current.
Radiation spectra from an imploding argon gas puff
Authors: Maxon S. , Wainwright T.
Published: March 2003

The one-dimensional magnetohydrodynamic equations coupled with the time-dependent populations of the average ion model are numerically solved for an argon gas puff in a Z-pinch configuration. Time-dependent emission spectra are obtained and compared with experiment. There is agreement for the radiation emitted by the hot gas adjacent to the shock front and apparent disagreement for the cool, dense snowplowed gas adjacent to the outer boundary.
Measurements of magnetic field fluctuations in the OHTE toroidal pinch
Authors: La Haye R. J. , Carlstrom T. N. , Goforth R. R. , Jackson G. L. , Schaffer Michael J. , Tamano T. , Taylor P. L.
Published: March 2003

Both external and internal magnetic probes have been used in low-current discharges and external probes in high-current discharges to study the magnetic field configuration and its fluctuations in the OHTE toroidal pinch with toroidal field reversal. The equilibrium magnetic field configuration is close to that of the Taylor state in the central half of the plasma (μ≡μ0bJ/B is constant) but differs in the outer half (μ gradually goes to zero at the plasma edge). Measurements of the magnetic fluctuations indicate that the dominant fluctuation mode observed is the resistive internal kink with m=1, n≂18. The measured relative level of magnetic field fluctuations scales as B̃/B∼S−1/2, where S is the magnetic Reynolds number.
Coherent density gradients in water compressed by a modulated shock wave
Authors: Benjamin Robert F. , Trease Harold E. , Shaner John W.
Published: March 2003

A shock wave propagating from liquid metal into water via a corrugated interface produces quasiperiodic perturbations in the compressed water, as determined by shadowgraphy. Theoretical analysis indicates that the shadows are caused by density gradients which occur from the coherent interaction among reverberating pressure waves.
Electron-cyclotron maser radiation from a relativistic loss-cone distribution
Authors: Pritchett P. L.
Published: March 2003

The electron-cyclotron maser instability is studied in a plasma with a loss-cone distribution for parameters relevant to tandem mirror devices. Both linear theory and particle simulations are employed. The effects of finite k⊥ are found to be very significant, and in general, the largest growth rate occurs for radiation emission perpendicular to the magnetic field. The instability persists at much lower densities (ωpe/Ωe<0.1) than indicated by previous analyses, which were limited to the case of parallel emission.
Schlieren photography of second-sound shock waves in superfluid helium
Authors: Torczynski J. R. , Gerthsen D. , Roesgen T.
Published: March 2003

A conventional schlieren system has been used to study second-sound shock waves produced in a shock tube with optical windows. Initially planar shocks are observed to remain so, even after several reflections from the end walls. No interaction of the shocks with the side wall boundary layers is seen, attesting to the thinness of these layers. Second-sound shocks are reflected from the liquid–vapor interface, producing transmitted gasdynamic shocks and reflected second-sound shocks. Several strong shocks are fired with short separation times, generating observable fluctuations in the fluid.
Frequency shift of coherent ion-acoustic waves in the presence of ion-acoustic turbulence
Authors: Ishihara O. , Hirose A.
Published: March 2003

The nonlinear frequency shift of coherent ion-acoustic waves in the presence of background ion-acoustic turbulence is reexamined in the framework of weak turbulence theory. In contrast to previous findings, the frequency shift always remains positive irrespective of the propagation direction of the coherent waves.
Energy balance and temperature in a CO2-laser-produced plasma
Authors: Popil R. , Meyer J. , Ahlborn B.
Published: March 2003

An underdense CO2-laser-produced plasma is experimentally investigated to determine the electron temperature explained in terms of an energy balance. Soft x-ray measurements show a 300 eV thermal and a 2000 eV suprathermal temperature. Ulbricht integrating sphere measurements along with interferometric data for the electron density indicate sufficient laser energy absorption to attain Te of the order of 600 eV. A novel method of analysis of streak records of the radial expansion is implemented where the conditions of a Chapman–Jouguet detonation wave are applied to allow an independent determination of the thermal plasma temperature and absorbed energy. The observed thermal Te is found to be consistent with linear inverse bremsstrahlung absorption using the measured degree of absorption provided by the Ulbricht sphere. The suprathermal temperature can be ascribed to be the result of the parametric processes in the plasma which are known to occur.
Two-dimensional rarefaction of laser-irradiated thin foils
Authors: Ng A. , Celliers P. , Pasini D. , Kwan J. , Parfeniuk D. , DaSilva L.
Published: March 2003

Thin aluminum foils have been irradiated with 0.35 μm, 2 nsec (FWHM) laser pulses focused to a spot diameter of 80 μm. The dynamics of the targets was dominated by axial and lateral rarefactions. These led to premature target burnthrough and disrupted the ablative acceleration process.
On the interaction of second sound shock waves and vorticity in superfluid helium
Authors: Torczynski J. R.
Published: March 2003

Two second sound shock waves were propagated successively into initially quiescent superfluid helium. With long separation times the waveforms are identical, but with shorter times the flow behind the second shock decays anomalously, revealing the presence of fluid disturbances produced near the heater by the first pulse. In order to study the vortical nature of these disturbances, second sound shocks were produced in rotating superfluid helium. Propagation of shocks both along and normal to the axis of rotation was studied, the former case proving to be similar to the experiment with successive shocks.
Coupled two- and three-dimensional instabilities of surface gravity waves
Authors: Su M. Y. , Green A. W.
Published: March 2003

The results of the experimental investigation of the coupling of two- and three-dimensional instabilities of surface gravity waves with initial wave steepness of 0.09≤a0k0≤0.20 are reported. The essentially two-dimensional instabilities (type I) are observed to cause sufficient wave train modulations to trigger the predominantly three-dimensional instabilities (type II). Type II, in turn, limits the growth of the type I and leads to wave breaking and the directional spreading of the wave energy. Hence, type I and II instabilities interact strongly during the evolution of wave trains with moderate initial steepness.
The role of shear layer stability in vortex shedding from cylinders
Authors: Unal M. F. , Rockwell D.
Published: March 2003

Vortex formation in the subcritical regime of shedding, at least to Re=5040, arises from exponential amplification of the disturbance kinetic energy in accord with linear stability theory applied to the separating shear layer.
A generalized BKW solution of the nonlinear Boltzmann equation with removal
Authors: Spiga Giampiero
Published: March 2003

The nonlinear Boltzmann equation for Maxwell molecules is considered in the case when the collision between two particles may lead not only to elastic scattering, but also to a removal event, with the disappearance of both incident molecules. An exact particular solution is found, which generalizes the well-known BKW mode, valid in the conservative case with no removal.
Natural convection near 4 °C in a horizontal water layer heated from below
Authors: Blake K. R. , Poulikakos D. , Bejan A.
Published: March 2003

A numerical study of two-dimensional natural convection in a horizontal water layer heated from below is reported. The density maximum associated with water at 3.98 °C and atmospheric pressure occurs inside the layer, as the top surface is maintained at 0 °C while the bottom surface temperature varies in the range 4 °C–10 °C. Three separate series of numerical simulations document the effect of Rayleigh number, bottom wall temperature and layer horizontal length on the flow pattern and on the net heat transfer rate vertically through the layer. These effects are documented numerically and graphically in the domain104<Ra<3×105, 1/6<H/L<1, where Ra is the Rayleigh number for a fluid with density maximum, and H/L is the (height/length) ratio of the water system selected for analysis. It is found that steady-state convection is present at Rayleigh numbers as low as 2.3×104 when TH =8 °C and H/L=0.5, and as low as 105 when TH =6.3 °C and H/L=0.5. These Rayleigh numbers agree very well with the corresponding critical values recommended by linear stability analysis.
On the spectrum of the transverse derivative of the streamwise velocity in a turbulent flow
Authors: Antonia R. A. , Browne L. W. B. , Chambers A. J.
Published: March 2003

The spectrum of the transverse derivative of the streamwise velocity fluctuation is calculated, assuming local isotropy, and compared with measurements in a turbulent plane jet. Mean square values of the measured derivative do not support isotropy, but the spectrum, corrected for the effect of separation between hot wires, indicates a tendency towards isotropy at sufficiently large wavenumbers.
Analysis of cylindrical Couette flows by use of the direction simulation method
Authors: Nanbu K.
Published: March 2003

Cylindrical Couette flows of a rarefied gas are analyzed by use of Bird’s direct-simulation method. An extremely large sample size is employed to reduce the statistical fluctuations of the obtained solutions to a negligible magnitude. The solutions should, though numerical, be regarded as the exact solutions of the full nonlinear Boltzmann equation. The results are compared with the existing analyses and experiment. The solution of the Navier–Stokes equations for slip boundary conditions is accurate up to Kn∼0.1. The solution for Kn∼1 of the ellipsoidal model equation shows a qualitative agreement with the present solution; it certainly gives the density minimum found here. The experimental data for Kn∼0.04 agree with the present solution. The data for Kn≳0.08 show only a qualtitative agreement. Much more accurate measurements are necessary to ascertain the present solutions.
Nonlinear evolution of drift instabilities
Authors: Lee W. W. , Krommes John A. , Oberman Carl R. , Smith Ralph A.
Published: March 2003

The nonlinear evolution of collisionless drift instabilities in a shear-free magnetic field has been studied by means of gyrokinetic particle simulation as well as numerical integration of model mode-coupling equations. The purpose of the investigation is to identify relevant nonlinear mechanisms responsible for the steady-state drift wave fluctuations. It is found that the saturation of the instability is caused mainly by the nonlinear E×B convection of the resonant electrons and their associated velocity space nonlinearity. The latter also induces energy exchange between the competing modes, which, in turn, gives rise to enhanced diffusion. The nonlinear E×B convection of the ions, which contributes to the nonlinear frequency shift, is also an important ingredient for the saturation.
Steady-state rf-current drive theory based upon the relativistic Fokker–Planck equation with quasilinear diffusion
Authors: Hizanidis K. , Bers A.
Published: March 2003

A simple analytic treatment based on the moments of the relativistic two-dimensional Fokker–Planck equation combined with resonant quasilinear diffusion is used to describe steady-state rf-current drive by waves. For rf diffusion parallel to the magnetic field (as in lower-hybrid current drive) it is shown that current carried by high parallel momenta must have an associated high average perpendicular energy. The model is also applied to unidirectional perpendicular rf diffusion which is relevant to electron–cyclotron current drive.
Modification of the macroscopic stability of a tandem mirror by partial line-tying
Authors: Molvik A. W. , Breun R. A. , Golovato S. N. , Hershkowitz Noah , McVey B. , Post R. S. , Smatlak D. , Yujiri L.
Published: March 2003

Increased stability is observed during experiments with gun injection in the Phaedrus tandem mirror. This is manifested by three effects. First, a magnetic mirror cell with bad average curvature, adjacent to a plasma gun, confines plasma with betas up to 20%. Second, the central cell, also with bad curvature, is stable at betas up to 20%, exceeding the minimum-B interchange stability limit by a factor of 20. Third, the quadrupole magnetic field required for interchange stability is reduced. These effects are shown to agree with a theory of line-tying stabilization, supplemented by minimum-B stabilization.
Stability studies of a hollow plasma in the double cusp experiment
Authors: Baker D. R. , Garner H. R. , Parks P. B. , Sleeper A. M. , Okamura S. , Adati K. , Aoki T. , Fujita H. , Hidekuma S. , Hattori K. , Kawamoto T. , Kumazawa R. , Okubo Y. , Sato T.
Published: March 2003

Axisymmetric cusp end cells have application for the stabilization and plugging of a tandem-mirror-type reactor. Experiments have been performed on the RFC-XX double cusp in Nagoya, Japan to measure the stability of the hollow plasma in the adiabatically confined region of the device. Experiments were performed with both a uniform field central section and with a mirror central section connecting the two cusps. The plasma is produced by an rf discharge of gas puffed into the vessel. Plasma fluctuations were measured by Langmuir probes and magnetic probes. Stable regions of operation with a hollow plasma in the cusp are found for both magnetic configurations. Various types of instabilities are observed: electrostatic drift type waves with m≥1, lower-frequency waves with a magnetic perturbation associated with the density perturbation, an m=0 ion-acoustic mode, and a large amplitude relaxation oscillation.
Raman and two-plasmon decay instabilities in a magnetized plasma
Authors: Barr H. C. , Boyd T. J. M. , Gardner L. R. T. , Rankin R.
Published: March 2003

The effects of a magnetic field on the Raman and two-plasmon decay instabilities are studied in the region of the quarter-critical density of laser produced plasmas where both are coincident. Two-plasmon decay of the incident extraordinary wave into two upper-hybrid waves may now occur in the direction of propagation of the incident radiation driven by the electrostatic component of the extraordinary mode. Maximum growth rates of both the Raman and two-plasmon decay instabilities are increased by the magnetic field. For the two-plasmon decay, the frequency shift from ω0/2, where ω0 is the frequency of the incident radiation, is increased by the magnetic field by amounts which can exceed the thermal shift. This magnetic shift derives from the electromagnetic correction to the dispersion relation of upper-hybrid waves and, consequently, is not found in an electrostatic approximation. For the Raman instability at the reflection point of the scattered extraordinary wave, the red shift of the back-scattered radiation due to the plasma temperature is reduced by any magnetic field present and can be changed to a blue shift if the field is large enough.
Ion beam diagnostics by means of plane Langmuir probes
Authors: Sko/elv Å. , Armstrong Richard J. , Trulsen J.
Published: March 2003

An ion beam normally incident on a plane Langmuir probe in a plasma gives rise to an extra knee in the electron saturation part of the current-voltage characteristics of the probe. Findings from a numerical investigation, together with some experimental results, are presented, confirming that the effect arises from a modulation of the electron collection current because of space-charge effects from beam ions reflected in front of the probe. It is verified that beam energy, temperature, and, in principle, beam density can be derived from the position, relative shape, and size of the extra ion-beam-induced knee of the Langmuir characteristics.
Application of anomalous diffusion in production of negative ions
Authors: Jimbo Kouichi
Published: March 2003

The production of negative hydrogen ions is investigated in the reflex-type negative ion sources. When anomalous diffusion in the positive column was found by Hoh and Lehnert [Phys. Fluids 3, 600 (1960)], it was pointed out that the large particle loss produced by anomalous diffusion is compensated for by the larger particle production inside the plasma. In the present experiments anomalous diffusion was artificially encouraged by changing the radial electric field inside the reflex discharge. Apparent encouragement of negative ion current by the increase of the density fluctuation amplitude is observed. Twice as much negative ion current was obtained with the artificial encouragement as without. On the other hand, the larger extracted negative ion current was observed with a lower electron temperature, which is calculated from the anomalous diffusion coefficient derived from a simple nonlinear theory. This result is consistent with Wadehra’s calculated results [Appl. Phys. Lett. 35, 917 (1979)].
Observations of high-energy electron distributions in laser plasmas
Authors: Keck R. L. , Goldman L. M. , Richardson M. C. , Seka W. , Tanaka K.
Published: March 2003

A very energetic (20–60 keV) electron distribution is observed in laser plasmas produced with both 1054 and 351 nm drivers. This component, believed to be generated by the 2ωp instability, is present at intensities above approximately 2×1014 W/cm2 and contains less than 0.1% of the incident energy. For 1054 nm irradiation, this component appears in addition to the 2 to 7 keV distribution attributed to resonance absorption.
Nonequilibrium statistical mechanics of one-dimensional turbulence
Authors: Qian J.
Published: March 2003

The method of statistical mechanics is applied to the study of the one-dimensional model of turbulence proposed in an earlier paper. The closure problem is solved by the variational approach which has been developed for the three-dimensional case, yielding two integral equations for two unknown functions. By solving the two integral equations, the Kolmogorov k−5/3 law is derived and the (one-dimensional) Kolmogorov constant Ko is evaluated, obtaining Ko=0.55, which is in good agreement with the result of numerical experiments on one-dimensional turbulence.
Thermal stability of radiating fluids: The scattering problem
Authors: Bakan Stephan
Published: March 2003

The problem of convective instability of a radiating fluid layer with scattering is treated with an extension of the Eddington aproximation that allows the inclusion of anisotropic scattering into the solution of the radiative transfer equation. Introduction of scattering by keeping the optical depth of absorption constant reduces the critical Rayleigh number as well as the wavenumber, and thus, reduces the stabilizing influence of thermal radiation. It is shown that in cases of a narrow radiative boundary layer with a large temperature gradient, higher-order expansion terms are sometimes necessary to approximate the solution properly. In certain cases a two layer convection mode with a large critical wavenumber up to 50 sets in the first layer has two cells developing in and near the two radiative boundary layers.
Propagation and absorption of electromagnetic waves in fully relativistic plasmas
Authors: Batchelor D. B. , Goldfinger R. C. , Weitzner Harold
Published: March 2003

The propagation and absorption of electromagnetic waves in a relativistic Maxwellian plasma are investigated by solving the uniform plasma dispersion relation. Both the Hermitian and the anti-Hermitian parts of the plasma conductivity tensor σ are calculated relativistically. The Bessel functions occurring in σ are not expanded, and many cyclotron harmonic terms are included at high temperatures. The dispersion relation is solved numerically for perpendicular propagation, k∥ =0, where the relativistic effects are maximum and are not masked by Doppler broadening, which has been more thoroughly investigated. It is found that relativistic broadening has a substantial effect on wave dispersion, shifting the extraordinary mode right-hand cutoff and the upper-hybrid resonance to higher magnetic field with increasing temperature. Above a critical temperature, the cutoff disappears entirely. There is a broad range of temperatures, 20 keV≤Te ≤500 keV, for which the wavenumber k⊥ differs significantly from both the cold-plasma value and the vacuum value. This has important implications for ray tracing in relativistic plasmas. Wave damping rates are calculated and compared to results from a previous formulation using the Poynting theorem, in which only the Hermitian part of σ is calculated relativistically.
Transport theory: Microscopic reversibility and symmetry
Authors: Molvig K. , Hizanidis K.
Published: March 2003

The general theory of Fokker–Planck equation symmetries and their relation to derived transport theory symmetries is developed. The property of microscopic reversibility implies a symmetry in the Fokker–Planck equation for processes obeying detailed balance. It is shown that this symmetry is not sufficient to guarantee Onsager reciprocity for the full matrix of transport coefficients. The general transport matrix has broken symmetry. A partial symmetry can be identified. The theory is compared to the different formulation given by Onsager.
Analytical expressions for H+, H+2, and H+3 ion densities in a hydrogen glow discharge
Authors: Kunc Joseph A. , Gundersen Martin A.
Published: March 2003

Simple analytical formulas for positive ion densities in high-current, medium-pressure (of order 1 mm Hg) steady-state hydrogen discharges have been obtained. These plasmas have a medium to high degree of ionization (>10−4), electron temperature on the order of 1 eV, and are typical of those that occur in hydrogen thyratrons and glow discharge switches. It is found that the role of charge exchange (H++H2→H+2 +H) may be significant, and limits for the effect of this process in ion production are estimated.
The moving contact line with a 180° advancing contact angle
Authors: Ngan C. G. , Dussan V E. B.
Published: March 2003

A liquid spreading over a dry solid surface with a contact angle of 180° has always been regarded as a special case. It is commonly thought that this represents a unique situation in which a singularity does not arise in an analysis of the dynamics of the liquid when the usual hydrodynamic assumptions are made, i.e., when the liquid is Newtonian and incompressible and obeys the no-slip boundary condition at the solid. In fact, there are strong indications that it is not a special case.
Stabilization of the spheromak tilt instability
Authors: Litwin C. , Sudan R. N. , Turnbull A. D.
Published: March 2003

A stability criterion for the tilt mode of a spheromak-ion ring hybrid configuration has been developed for the case where the ring current is small compared to the spheromak azimuthal current. It is shown that the stability is related to the distortion of the spheromak separatrix.
On the excitation of nonlinear water waves by a moving pressure distribution oscillating at resonant frequency
Authors: Akylas T. R.
Published: March 2003

A theoretical study is made of the free-surface flow induced by a traveling oscillatory pressure distribution acting at the surface of water of finite depth. Depending on the relative values of the water depth and the frequency and speed of the applied pressure, the solution of the linearized water-wave problem may be singular, owing to a resonance phenomenon. Through an asymptotic analysis, it is shown that, close to resonant conditions, the finite-amplitude response is bounded, and it is governed by a forced nonlinear Schrödinger equation. It is deduced that, under certain circumstances, the generated wave disturbance may not reach a steady state; in particular, for deep water, a steady state is never attained.
An analysis of the stability of the compressible Ekman boundary layer
Authors: Spall John R. , Wood Houston G.
Published: March 2003

The linear stability problem for the compressible Ekman boundary layer common to rotating fluids is formulated and the stability properties determined numerically. Three classes of unstable waves are identified (called class A, B, and C), their properties are described. The class C waves have only recently been reported in the literature and are present only in compressible Ekman boundary layers. Most of the calculations presented here are for uranium hexafluoride gas; however, critical Reynolds numbers are also computed for air and ammonia gas. Compressibility is generally found to decrease the critical Reynolds number for each class of wave. A comparison of results for the three different gases shows the stability to be largely unaffected by changes in the gas properties. Maximum growth rate calculations for each wave show the class A and B waves to be the dominant instabilities. OFF
Lognormality of gradients of diffusive scalars in homogeneous, two-dimensional mixing systems
Authors: Kerstein Alan R. , Ashurst William T.
Published: March 2003

Kolmogorov’s third hypothesis, as extended by Gurvich and Yaglom, is found to be obeyed by a diffusive scalar for a class of homogeneous, two-dimensional mixing models. The mixing models all involve the advection of fluid by discrete vortices distributed in a square region with periodic boundary conditions. By computer simulation, it is found that the squared gradient of a diffusive scalar so advected is lognormally distributed, obeys the predicted scaling when a spatial smoothing is applied, and exhibits a power-law range in the spatial autocorrelation. In addition, it is found that the scaling property cuts off at the Batchelor length, as predicted by Gibson. Since the mixing models employed do not incorporate the dynamical features of high-Reynolds-number turbulence, these results suggest that scalar lognormality and associated scaling behavior may be more robust or persistent than the scaling laws of the flow field.