bluuebunny/crossref_metadata_2025 huggingface dataset:
Changes in compression dynamics for seeded plasma focus pinches
Authors: Venneri F. , Boulais K. , Gerdin G.
Published: July 2002

The effects of radiation emission on the dynamic development of a neon seeded dense plasma focus are observed. The radiative cooling of the discharge is found to be responsible for the >10 times the increase in density of the pinch.
Oscillating magnetic islands in a rotating plasma
Authors: Persson M. , Bondeson A.
Published: July 2002

The nonlinear evolution of tearing modes in the presence of sheared mass flow is studied as an initial value problem. Under certain conditions, when the mode is driven unstable primarily by the mass flow, the nonlinear evolution leads to a dynamic state in which the size and shape of the magnetic islands are oscillatory.
TEXT tokamak edge turbulence modeling
Authors: Leboeuf J.-N. , Lee D. K. , Carreras B. A. , Dominguez N. , Harris J. H. , Hedrick C. L. , Hidalgo C. , Holmes J. A. , Ruiter Jason , Diamond P. H. , Ware A. S. , Ritz Ch. P. , Wootton A. J. , Rowan W. L. , Bravenec R. V.
Published: July 2002

Experimental results on the Texas Experimental Tokamak (TEXT) [Nucl. Technol./Fusion 1, 479 (1982)] and the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] indicate that electrostatic fluctuations can explain the edge transport of particles and perhaps energy. Certain mechanisms for drive (radiation and ionization, as well as density and temperature gradients) and stabilization (velocity shear) are suggested by these results. The experimental fluctuation levels and turbulence characteristics can be reproduced by considering the nonlinear evolution of fluid-type equations, incorporating thermal and ionization drives.
Comparison of steady-state and perturbative transport coefficients in TFTR
Authors: Efthimion P. C. , Barnes C. W. , Bell M. G. , Biglari H. , Bretz N. , Diamond P. H. , Hammett G. , Heidbrink W. , Hulse R. , Johnson D. , Kusama Y. , Mansfield D. , Medley S. S. , Nazikian R. , Park H. , Ramsey A. , Rewoldt G. , Scott S. D. , Stratton B. C. , Synakowski E. , Tang W. M. , Taylor G. , Zarnstorff M. C. , Zweben S. J.
Published: July 2002

Steady-state and perturbative transport analysis are complementary techniques for the study of transport in tokamaks. These techniques are applied to the investigation of auxiliary-heated L-mode and supershot plasmas in the tokamak fusion test reactor (TFTR) [R. J. Hawryluk et al., Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 11th International Conference, Kyoto, 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51.]. In the L mode, both steady-state and perturbative transport measurements reveal a strong temperature dependence that is consistent with electrostatic microinstability theory and the degradation of confinement with neutral beam power. Steady-state analysis of the ion heat and momentum balance in supershots indicates a reduction and a significant weakening of the power-law dependence on the transport in the center of the discharge.
Review of soft x-ray lasers and their applications
Authors: Skinner Charles H.
Published: July 2002

The emerging technology of soft x-ray lasers is in a transition phase between the first laboratory demonstrations of gain and the acceptance of soft x-ray lasers as practical tools for novel applications. Current research is focused on several fronts. The operational wavelength range has been extended to the ‘‘water window,’’ important for applications in the life sciences. Gain has also been generated with substantially simpler technology (such as a 6 J laser) and this augurs well for the commercial availability in the near future of soft x-ray lasers for a variety of applications. Advanced soft x-ray laser concepts are being developed from investigations into ultrahigh intensity laser/matter interactions. The first applications of soft x-ray lasers to x-ray microscopy and holography have begun. In this paper a brief historical perspective of x-ray laser development will be followed by a review of recent advances in recombination, collisional, and photopumped systems and applications. A summary of current gain-length performance achieved in laboratories worldwide is presented. Near-term prospects for applications to novel fields are discussed.
Spectroscopic measurements of the parameters of the ablation clouds of deuterium pellets injected into tokamaks
Authors: McNeill D. H. , Greene G. J. , Newburger J. D. , Owens D. K.
Published: July 2002

Spectroscopic measurements have been made of the parameters of the luminous cloud surrounding deuterium pellets injected into the Princeton Large Torus (PLT) [Phys. Rev. Lett. 55, 1398 (1985)] and the Tokamak Fusion Test Reactor (TFTR) [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1987), Vol. 1, p. 171], with the measurements on the latter described formally here for the first time. The electron densities determined from the Stark broadening of the Balmer alpha and beta lines ranged from 5×1015–1×1018 cm−3, while the temperatures obtained from the line to continuum intensity ratios and the ratio of the intensities of the Balmer alpha and beta lines ranged from 0.9 to 3 eV. Balmer alpha emission powers as high as 100 kW were measured. The electron temperature (1.5±0.2 eV) and density (3±1×1017 cm−3 ) at the time of maximum emission from intact pellets were essentially the same for the two tokamaks, although the volumes of both the discharges and the pellets were roughly ten times larger in TFTR than in PLT. The data were taken in Ohmically heated plasmas, except for a limited number of discharges in PLT with low-power (approximately half the Ohmic heating power) ion cyclotron heating for which the results are the same as with Ohmic heating alone. Various factors that may influence the results of measurements of this type are discussed, including self-absorption, low-density plasma surrounding the most intense region of an ablation cloud, fluctuations in the emission intensity, and non-Maxwellian electron distributions. The results of the present experiments are similar to measurements of the pellet cloud parameters in other machines.
Nonlinear electron-cyclotron power absorption
Authors: Farina Daniela , Pozzoli Roberto
Published: July 2002

The process of nonlinear power absorption of a localized electron-cyclotron wave is investigated on the basis of the analysis of the electron motion in the wave field, by means of a relativistic Hamiltonian formalism under adiabatic conditions, for a wave frequency close to a cyclotron harmonic, and in perpendicular propagation. The computation of the absorbed power density relies on the investigation of the electron energy transitions due to a single crossing of the radiation beam. The absorbed power profiles are explicitly given for the ordinary mode at the first cyclotron harmonic and the extraordinary mode at the second cyclotron harmonic, and are shown to be in good agreement with those obtained by numerical simulation codes. The behavior of the absorbed power versus the frequency and the amplitude of the wave is interpreted by means of the main features of the nonlinear interaction process in momentum space.
Magnetic turbulent electron transport in the reversed-field pinch
Authors: Schoenberg Kurt F. , Moses Ronald W.
Published: July 2002

A model of magnetic turbulent electron transport is presented. The model, based on test particle diffusion in a stochastic magnetic field, entails a Boltzmann description of electron dynamics in the long mean-free-path limit and quantitatively describes the salient features of superthermal electron measurements in the reversed-field pinch (RFP) edge plasma. Included are predictions of the mean superthermal electron energy, current density, and distribution function asymmetry. The distribution function asymmetry leads directly to the observed power flow asymmetry in present generation RFP’s. A discussion of the transport model, the assumptions implicit in the model, a comparison of model predictions with experiment, and the relevance of this work to the general issue of magnetic turbulent transport in toroidal systems are presented.
Fast magnetic-field penetration into plasmas due to the Hall field
Authors: Fruchtman A. , Maron Y.
Published: July 2002

The enhancement of magnetic-field penetration into short-duration plasmas by the dissipationless Hall field is examined. Magnetic-field penetration along a background magnetic field is focused on, where the inductive Hall electric field enables the magnetic field to penetrate as a whistler wave. It is shown that the magnetic-field evolution, when governed simultaneously by both whistler wave propagation and collisional diffusion, is described by a diffusion equation with a complex diffusion coefficient. The imaginary part of this coefficient is proportional to the Hall resistivity associated with the background magnetic field. In the collisionless limit the governing equation is equivalent to the Schrödinger equation for a free particle, and the magnetic field propagates the way a free-particle wave packet expands by dispersion rather than by diffusion. This study was motivated by the enhanced magnetic-field penetration recently observed in the anode plasma of a magnetically insulated ion diode.
Thermally driven edge magnetic turbulence
Authors: Craddock G. G. , Terry P. W.
Published: July 2002

As a model for the hot [βe(Ls/iLT)2 ≳1] turbulent tokamak edge, the nonlinear saturated state of the high-βe limit of microtearing turbulence is examined. This mode couples magnetic and electron temperature fluctuations. Using mixing length analysis and a variational calculation of the renormalized eigenmode equations, a magnetic fluctuation level and turbulent radial correlation length are derived. The resulting anomalous thermal diffusivity for auxiliary heating scales as T7/4e n−3/20 I−5/3p P 5/6H R7/3, where Te is the electron temperature, n0 is the density, Ip is the toroidal plasma current, PH is the heating power, and R is the major radius.
Control of rotation velocity profile of tokamaks by application of slow electromagnetic waves
Authors: Jensen T. H. , Leonard A. W.
Published: July 2002

Transport of a tokamak plasma may depend sensitively on its rotation velocity profile. Therefore a means for controlling the rotation profile may be useful. The paper describes a study of utilizing rotating islands, established deliberately by currents in external coils, as ‘‘stirrers’’ of the plasma and thereby influence the velocity profile. One of the main issues of the study is how large a stirring force rotating islands can provide. It is suggested that even relatively narrow islands can provide substantial stirring forces.
Stabilization of collisional drift-tearing modes at the breakdown of the constant-Ψ approximation
Authors: Migliuolo S. , Pegoraro F. , Porcelli F.
Published: July 2002

The linear growth rate of the collisional drift-tearing mode is found to be a nonmonotonic function of the stability parameter Δ′. It reaches a maximum for Δ′≊0.5Δ′1=mωA/ √ω̂*e(ω̂*e−ωdi) ≳ 0, where ωA, ω̂*e, ωdi are the Alfvén, electron drift wave, and ion diamagnetic frequencies, and becomes negative for Δ′ ≥ Δ′1, corresponding to the regime where the constant-Ψ approximation breaks down. A second mode, identified as the diamagnetic modification of the ε1/3η mode near the condition for ideal magnetohydrodynamic (MHD) marginal stability, becomes unstable for Δ′≥Δ′2 = (ω̂*e/ωAεη)1/2 ≳ Δ′1, leaving a stable window in values of Δ′. Applications to the stability of modes with poloidal and toroidal mode numbers m = n = 1 are presented.
Collisional losses of charged particles from end-plugged mirrors
Authors: Sadamoto Yoshiro
Published: July 2002

Particle- and energy-loss rates of charged particles from end-plugged mirrors with a multispecies plasma are obtained for arbitrary values of plug potentials. The loss rates are derived on the basis of an approximate solution of the Fokker–Planck equation. The solution satisfies the true loss boundary condition. The mean loss energy of the charged particles is also discussed. A comparison is made with the earlier analytical and numerical results.
Electron acceleration in a plasma wave above a laser-irradiated grating
Authors: Laberge M. , Meyer J.
Published: July 2002

A plasma with a modulated density of ñ/n=8% was produced by illuminating a grating with a ruby laser at an intensity of 1010 W/cm2. A CO2 laser of intensity 3×1011 W/cm2 then irradiated this modulated plasma and generated plasma waves. The phase speeds of the waves are vph = ±ω0/kg, where kg is the wave number of the grating and ω0 is the frequency of the CO2 laser. Electrons were injected at an energy of 92 keV in one of the plasma waves. In order for the phase speed of the wave to synchronize with the accelerating electrons, a grating with constantly increasing line spacing (chirped) was used. Some electrons accelerated by 70 keV in 70 μm (gradient of 1 GeV/m) were detected, but the number of accelerated electrons was three orders of magnitude less than the expected value. This discrepency is the result of a large electric field perpendicular to the surface of the grating, which deflects the electrons onto the grating. This detrimental field is produced when fast electrons are emitted by the plasma, and leave it positively charged. Some techniques to remedy this difficulty are proposed.
A sufficient criterion for Kelvin–Helmholtz instability in the magnetopause boundary-layer region
Authors: Parhi S.
Published: July 2002

The Kelvin–Helmholtz instability has been investigated for the magnetopause boundary-layer region by the linearized method. The plasma in magnetosheath and magnetopause is assumed to be semi-infinitely extended homogeneous, nondissipative, and incompressible. It is observed that, if one relation of two plasma speeds on the two sides of the magnetopause, wave number, and boundary-layer thickness exceeds a certain threshold, the instability sets in. This new analytically sufficient criterion for excitation of instability in the three-layer plasma flow generalizes the corresponding Chandrasekhar’s instability criterion for two-layer plasma flow. The known results have been recovered and modified, the new results have been discovered. It is proved that the velocity threshold for the onset of instability is low when the magnitude of the magnetosheath and boundary-layer region magnetic field and the angle between them are small. Also the threshold depends on the direction of plasma flow. The following results are observed numerically. The growth of the instability is sensitive to the magnetic field direction in the magnetosheath. A slight variation in the magnetic field direction in the second region can substantially change the relative velocity threshold for instability. When the ratio of the density of the second and third layer (magnetosphere) increases or that of the first and third layer decreases, the threshold decreases. Apart from this a necessary criterion for instability is obtained for a particular case.
Modification of stimulated Brillouin, saturated Raman scattering and strong Langmuir turbulence by nonlocal heat transport
Authors: Rose Harvey A. , DuBois D. F.
Published: July 2002

The linear instability analysis of stimulated Brillouin backscatter (SBS) is revisited to study the effect of nonlocal electron heat transport that enhances the density modulation over that caused by the ponderomotive pressure. It is found that the intensity threshold is lowered by a factor of roughly the inverse of 1+1000 (ne/nc)ln Λ(Z3/2/TeV2)/λμ, where TeV is the electron temperature in eV and λμ is the laser wavelength in microns, provided that λμTeV2(nc/ne)≫50 ln Λ√Z. A similar reduction in the threshold for Langmuir decay is found, which, in the case of stimulated Raman scattering (SRS), may lead to a reduced saturation level for SRS.
Neutral beam injectors for the International Thermonuclear Experimental Reactor
Authors: Cooper William S.
Published: July 2002

The International Thermonuclear Experimental Reactor (ITER) has been proposed as the next major step in the development of fusion power [International Thermonuclear Experimental Reactor (ITER), Establishment of ITER: Relevant Documents, International Atomic Energy Agency, Vienna (1988)]. During the Conceptual Design Activity, deuterium neutral beams were chosen for heating, driving current, and controlling the current profile in the central region of the ITER plasma. In order to penetrate to the center of the ITER plasma, an energy of 1.3 MeV is required, an order of magnitude higher than in existing neutral beam systems. The neutral beam system must deliver 75 MW of D0 to the plasma, with a pulse length that ultimately will reach two weeks. The neutral injection system will consist of nine modules, and will be based on D− ion sources and novel high-current dc accelerators optimized for flexible operation and control of breakdowns. The negative ions can be converted to neutrals in gas or plasma targets, with a resulting system efficiency of 40% to 50%. The successful development and operation of neutral beam injectors for ITER will be a major step toward the application of neutral beams on power reactors.
A hybrid Zakharov particle simulation of ionospheric heating
Authors: Clark K. L. , Payne G. L. , Nicholson D. R.
Published: July 2002

A one-dimensional hybrid simulation model incorporating one of the Zakharov equations for the high-frequency waves and a particle-in-cell (PIC) simulation of the ions is described and applied to ionospheric heating using realistic parameters. Results from the hybrid simulation are compared with a Zakharov simulation that incorporates a phenomenological model of ion damping. Both the hybrid and Zakharov simulations predict the formation of solitonlike waves. The early time behavior of the two simulations is different due to the high noise level in the hybrid simulation, but the late time behavior is quite similar.
Electron beam tracking in a preformed density channel
Authors: Murphy D. P. , Pechacek R. E. , Taggart D. P. , Fernsler R. F. , Hubbard R. F. , Slinker S. P. , Meger R. A.
Published: July 2002

High-current charged particle beams can be guided by reduced density channels. Such guiding occurs when the distribution of plasma currents in the density channel causes a net attractive force to be exerted on the beam. In particular, a relativistic electron beam (REB) injected parallel to a spatially offset, reduced density channel is pulled toward the channel. The force exerted on the beam is predicted to increase as the beam current increases and as the offset between the beam and the channel increases out to offsets equal to the channel radius. An experiment with a 1 MV, ≊10 kA beam was performed that demonstrates this effect.
Development of a second cyclotron harmonic gyrotron operating at submillimeter wavelengths
Authors: Idehara T. , Tatsukawa T. , Ogawa I. , Tanabe H. , Mori T. , Wada S. , Brand G. F. , Brennan M. H.
Published: July 2002

The development of a high-frequency, step-tunable gyrotron operating at submillimeter wavelengths is described. The gyrotron design was optimized for operation at the second harmonic of the electron cyclotron frequency in the TE261 cavity mode, whose resonant frequency is 384 GHz. Experimental results show that second harmonic operation can occur without mode competition as long as the beam current is low (Ib ≲0.8 A), but as the current is increased, the fundamental TE231 cavity mode increases and eventually (Ib ≳1 A) suppresses the second harmonic. The competition between the two modes is studied in detail. The starting current for second harmonic operation is also studied experimentally and compared with calculated results. Other resonances have also been examined. With the present superconducting magnet, the maximum frequency achieved is 402 GHz (second harmonic operation in the TE551 cavity mode) at several kilowatts.
Pulsed currents carried by whistlers. Part I: Excitation by magnetic antennas
Authors: Stenzel R. L. , Urrutia J. M. , Rousculp C. L.
Published: July 2002

Time-varying plasma currents associated with low-frequency whistlers have been investigated experimentally. Pulsed currents are induced in the uniform, boundary-free interior of a large laboratory plasma by means of insulated magnetic antennas. The time-varying magnetic field is measured in three dimensions and the current density is calculated from R∇×B(r,t)=μ0J, where J includes the displacement current density. Typical fields B(r,t) and J(r,t) induced by a magnetic loop antenna show three-dimensional helices due to linked toroidal and solenoidal field topologies. Constant amplitude and phase surfaces assume conical shapes since the propagation speed along B0 is higher than oblique to B0. The wave vector is highly oblique to B0 while the energy flow is mainly along B0. The electric field in the wave packet contains both inductive and space-charge contributions, the latter arising from the different dynamics of electrons and ions as explained by physical arguments. The dominant electric field in a whistler packet is a radial space-charge field. Neither the field topology nor the propagation characteristics are sensitive to the induced magnetic field amplitude up to Bwave≲B0. The results are relevant to both the basic properties of whistlers and to applications such as large loop antennas and electrodynamic tethers in space plasmas.
Arbitrary mode number boundary-layer theory for nonideal toroidal Alfvén modes
Authors: Berk H. L. , Mett R. R. , Lindberg D. M.
Published: July 2002

The theory of toroidicity-induced Alfvén eigenmodes (TAE) and kinetic TAE (KTAE) is generalized to arbitrary mode numbers for a large aspect ratio low-beta circular tokamak. The interaction between nearest neighbors is described by a three-term recursion relation that combines elements from an outer region, described by the ideal magnetohydrodynamic equations of a cylinder, and an inner region, which includes the toroidicity and the nonideal effects of finite ion Larmor radius, electron inertia, and collisions. By the use of quadratic forms, it is proven that the roots of the recursion relation are stable and it is shown how perturbation theory can be applied to include frequency shifts due to other kinetic effects. Analytic forms are derived which display the competition between the resistive and radiative damping, where the radiation is carried by kinetic Alfvén waves. When the nonideal parameter is small, the KTAE modes appear in pairs. When this parameter is large, previously found scaling for the single gap case is reproduced analytically.
The small amplitude magnetohydrodynamic Riemann problem
Authors: Wu C. C. , Kennel C. F.
Published: July 2002

The small amplitude magnetohydrodynamic Riemann problem is studied using the Cohen–Kulsrud–Burgers equations. Unlike the coplanar Riemann problem, the evolution of noncoplanar Riemann problems is not self-similar and its flow structures could change in time. But its large-time behavior is very simple and a time-dependent 2→3 intermediate shock is always involved for the noncoplanar field rotations. The time-dependent 2→3 intermediate shock has a well-defined structure and exists for any degree of field rotation.
Improved confinement and stability in the DIII-D tokamak obtained through modification of the current profile*
Authors: Ferron J. R. , Lao L. L. , Taylor T. S. , Kim Y. B. , Strait E. J. , Wroblewski D.
Published: July 2002

Improvement in both the energy confinement time and the achievable value of normalized beta is obtained by modifying the current density profile from the relatively broad shape obtained in standard tokamak discharges to a more peaked shape. The peaked current profile is produced with either a rapid negative ramp in the total plasma current or a rapid increase in the discharge elongation. Discharges have been obtained with βN=β/(I/aB)=6% mT/MA simultaneously with total energy confinement time two times the value predicted by L-mode scaling relations. Up to a factor of 1.8 improvement in the normalized thermal energy confinement time, τth/Ip, has been obtained in both L-mode and H-mode discharges. It is shown that the increase in confinement can be attributed to a local decrease in the thermal diffusivity that is correlated with a local increase in the poloidal magnetic field and the magnetic shear.
Probability of orbit transition in asymmetric toroidal plasma
Authors: Cary John R. , Shasharina S. G.
Published: July 2002

Trapped particles in stellarators and tokamaks can be either locally trapped in the ripple wells or toroidally trapped by the 1/R variation of the magnetic field strength. Transitioning particles make transitions back and forth between these two states. The transition probability gives the relative rate of toroidally trapped particles to become locally trapped. In this paper the transition probably is uniquely defined, and an improved analytical calculation of the transition probability is given. It is then calculated numerically by following the full guiding-center dynamics. Comparison with the numerical results shows that the present calculation is more accurate than those previously presented.
Control of toroidal-like asymmetries in a heliac
Authors: Spanjers G. G. , Nelson B. A. , Ribe F. L. , Jarboe T. R. , Barnes D. C.
Published: July 2002

A simple modification to the heliac coil configuration [A. H. Boozer et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1966), Vol. 1, p. 103] is described that reduces the toroidal perturbation to the magnetic field structure and significantly restores the helical symmetry. This is accomplished by shifting the internal current-carrying conductor (hardcore) of the heliac radially inward from its normal position at the geometric center of the external l=1 coils. It is shown by computations that a large-aspect-ratio toroidal heliac is similar to a helically symmetric linear heliac in which the hardcore has been shifted. This similarity is seen in the analytic form of the magnetic potential, the flux surface shape, and in the harmonic spectra of the field lines. It is then shown that the toroidicity perturbations can be reduced in a toroidal heliac by combining these effects—using a radially inward hardcore shift in the toroidal configuration. In the experiments reported here the toroidal-like effects are created in the linear High Beta Q Machine (HBQM) heliac [C. M. Greenfield et al., Phys. Fluids B 2, 133 (1990)] by shifting the hardcore off the geometric center of the l=1 coils. The induced toroidal-like effects are seen from an asymmetry in the axial excluded flux; however, no decrease in global beta is seen as compared to the unshifted symmetric case (G. G. Spanjers, Ph.D. dissertation, University of Washington, 1992) indicating the absence of large-scale magnetohydrodynamic instabilities at high beta.
A study of compressional Alfvén waves in a toroidal plasma
Authors: Takamura S. , Yoshida Y. , Okuda T.
Published: July 2002

A pair of loop antenna in a toroidal plasma device is found to excite compressional Alfvén waves in the frequency range of a few times the ion-cyclotron frequency. Two-dimensional profiles of the wave field and phase are obtained in a poloidal cross section as well as one-dimensional radial profiles. The main wave excited is found to have the fundamental poloidal mode number. However the contribution from higher poloidal modes increases at higher wave frequencies and plasma densities. Comparison with theoretical wave profiles shows deviation from a simple cylindrical plasma model.
Direct interaction approximation for Vlasov turbulence from the Kadomtsev weak coupling approximation
Authors: DuBois D. F. , Pesme D.
Published: July 2002

The weak coupling approximation of Kadomtsev is applied to Vlasov turbulence and shown to lead straightforwardly to the equations of the direct interaction approximation. This application of the weak coupling approximation was apparently incorrectly carried out in Kadomtsev’s book. The method has the advantage of simplicity and makes contact with other more conventional approximations.
Reduced DIA equations for the weak warm beam instability in the strong mode-coupling limit
Authors: Pesme D. , DuBois D. F.
Published: July 2002

The direct interaction approximate (DIA) is considered for a one-component, one-dimensional turbulent plasma excited by wave–particle interactions in the limit of a weak warm beam instability. The situation where strong mode coupling effects take place is explicitly studied. In this limit the DIA equations for the nonlinear evolution of the instability are reduced to a dimensionless set of equations for the spectral harmonics of the correlation functions in which all time dependence has been scaled out. The distinctive polarization terms and mean field coupling renormalization effects of the DIA are seen to be of the same order as the usual quasi-Gaussian or ‘‘test particle’’ renormalization effects. From these equations it is shown that the true growth rate γk is given by γk =A[k]γqlk, where γqlk is the quasilinear growth rate; the renormalizing function A[k] is shown to be larger than unity with A[k] =O(1), corresponding to a net increase of energy transfer between the waves and the particles as compared with the quasilinear predictions. The function A[k] may be a slowly varying function of k, but is independent of beam parameters. The basic physical effects are the promotion of mode-coupling effects to the same order as the quasilinear terms by the intermediation of resonant particles in the mode-coupling matrix elements, plus the consideration of the non-Gaussian character of the self-consistent electric field fluctuations. Arguments are presented that the reduced DIA equations are exact in this limit.
An experimental study of magnetic fields in plasmas created by high intensity one micron laser radiation
Authors: Burgess M. D. J. , Luther-Davies B. , Nugent K. A.
Published: July 2002

Magnetic fields generated in plasmas formed by focusing intense picosecond (∼20 psec) duration Nd: laser pulses onto disk targets have been studied using simultaneous polarimetry and interferometry for a wide range of laser and plasma parameters. Megagauss fields, which extend throughout the plasma corona, have been observed during the pulse, but subsequently these decay rapidly and are unobservable only 100 psec after the pulse peak. The general morphology of the field together with this rapid decay suggests a role for the superthermal particles in magnetic field generation and saturation (via convection). Hard x-ray (∼20 keV) imaging has been used to relate the regions of energy deposition by superthermal electrons to the magnetic field structure. The results are compared with the predictions of recent two-dimensional (2-D) computer simulations.
Theory of semicollisional kinetic Alfvén modes in sheared magnetic fields
Authors: Hahm T. S. , Chen Liu
Published: July 2002

The spectra of the semicollisional kinetic Alfvén modes in a sheared slab geometry are investigated, including the effects of finite ion Larmor radius and diamagnetic drift frequencies. The eigenfrequencies of the damped modes are derived analytically via asymptotic analyses. In particular, as one reduces the resistivity, it is found that, because of finite ion Larmor radius effects, the damped mode frequencies asymptotically approach finite real values corresponding to the end points of the kinetic Alfvén continuum.
Instabilities of baroclinically driven magnetic and acoustic waves
Authors: Stenflo L. , Yu M. Y.
Published: July 2002

It is shown that gradient-driven electron-acoustic waves are subtly related to the newly discovered magnetic electron modes. The two types of waves, which are physically quite different, have similar instability conditions, and can be strongly coupled.
Effect of field-aligned current on the ion-temperature-gradient-driven drift mode
Authors: Gladd N. T.
Published: July 2002

The effect of field-aligned current on the ion-temperature-gradient-driven drift mode is examined with nonlocal kinetic theory. It is found that current couples otherwise distinct eigenmodes and strongly alters the character of the unstable spectrum. The growth rate is enhanced over the current-free situation, but the maximal growing modes shift from long to short wavelengths.
Viscosity and density stratification in vertical Poiseuille flow
Authors: Renardy Yuriko
Published: July 2002

The linear stability of plane three-layer vertical Poiseuille flow is considered. The layers are composed of two immiscible fluids, one next to the walls and one centrally located. The fluids have different viscosities and densities and surface tension effects are included. Intuitively, an analogy with the concentric Hagen–Poiseuille flow is expected and the similarities and differences are investigated. The ability of heuristic reasoning to predict which arrangements are more likely to be observed is tested. Numerical results concerning low Reynolds numbers, as well as the behavior of eigenvalues close to the first criticality of the one-fluid problem, are presented.
Nearly two-dimensional solutions of Euler’s equations
Authors: Childress Stephen
Published: July 2002

Motivated by a stretched version of the Taylor–Green initial-value problem for the Euler and Navier–Stokes equations, nearly two-dimensional incompressible flows are considered involving a single direction of slow spatial variation. A multiple-scale formulation of the problem leads by contour averaging to a system that determines the slow development in space and time. It is shown that the latter system is equivalent to an axisymmetric problem with nonstandard connection between circulation, cylindrical radius, and angular velocity component. Special solutions of the system in the exact axisymmetric case suggest that near-two-dimensionality is lost in finite time.
Raman scattering in a nearly resonant density ripple
Authors: Barr Hugh C. , Chen Francis F.
Published: July 2002

Stimulated Raman scattering of light waves by an underdense plasma is affected by the presence of a density ripple caused by a simultaneously occurring stimulated Brillouin instability. The problem is treated kinetically for the particularly interesting case where the ripple has nearly the same wavelength as the plasma wave. The ripple is found to reduce the growth rate of the usual Raman instability but allows other decay modes to occur. Numerical results for the frequencies, growth rates, and k spectra of these modes are obtained. A physical explanation is given for a baffling result of the calculation. The physical picture is also of interest to particle acceleration by plasma waves.
Bounds on the permeability of a random array of partially penetrable spheres
Authors: Torquato S. , Beasley J. D.
Published: July 2002

A bound on the fluid permeability k for viscous flow through a random array of N identical spherical particles (distributed with arbitrary degree of impenetrability) due to Weissberg and Prager [Phys. Fluids 13, 2958 (1970)] is rederived by averaging flow quantities with respect to the ensemble of particle configurations. The ensemble-averaging technique enables one to obtain series representations of certain n-point distribution functions that arise in terms of probability density functions which characterize a particular configuration of n(<N) spheres. The Weissberg–Prager bound on k is computed exactly through second order in the sphere volume fraction for arbitrary λ (where λ is the impenetrability parameter, 0≤λ≤1) for two different interpenetrable-sphere models. It is found that, at the same sphere volume fraction, the permeability of an assembly of partially overlapping spheres is greater than that of one characterized by a higher degree of impenetrability. The results of this study indicate that the Weissberg–Prager bound will not only yield the best available bound on k for an assemblage of totally impenetrable spheres (λ=1) but will provide a useful estimate of k for this model for a wide range of sphere volume fractions. It is also demonstrated that bounds which incorporate a certain level of statistical information on the medium are not always necessarily sharper than bounds which involve less information.
Theory of the presheath in a weakly ionized plasma with hot neutrals
Authors: Biehler S. , Ecker G. , Riemann K.-U.
Published: July 2002

In this paper a kinetic theory of the boundary layer of a weakly ionized plasma in contact with a perfectly absorbing wall is presented. The ion kinetics is governed by charge exchange collisions with neutrals of finite temperature; the collision frequency is constant. Based on the assumption that the Debye length is small compared with all other lengths of the system, a self-consistent two scale (presheath–sheath) analysis is performed. In contrast to Emmert’s analysis [Phys. Fluids 23, 803 (1980)] of the corresponding collision-free system, the sheath edge shows the usual field singularity. It is shown that in the case where the electron and neutral temperatures are of the same order a large fraction of ions enters the sheath with a multiple of the electron thermal energy.
A theory of fast-wave absorption, transmission, and reflection in the ion cyclotron range of frequencies
Authors: Lashmore-Davies C. N. , Fuchs V. , Francis G. , Ram A. K. , Bers A. , Gauthier L.
Published: July 2002

A second-order differential equation for the fast wave propagating in a hot, two-ion species plasma is obtained. This second-order approximation is obtained unambiguously and allows the wave amplitude to be identified with one of the electric field components. The approximation is based on replacing the coupling to the ion-Bernstein wave by a localized perturbation of the fast wave. For the case of perpendicular propagation, the second-order equation reduces to Budden’s equation giving the well-known transmission coefficient for both two-ion hybrid and second-harmonic resonance. The equation includes the effect of simultaneous minority fundamental and majority second-harmonic cyclotron damping. The solutions of the second-order equation as a function of n∥ give absorption transmission and reflection coefficients that agree well with the results based on models giving higher-order differential equations and solved by means of much more complex numerical codes.
Flux coordinates, self-consistent equilibria, and variational principle
Authors: Riedel K. S.
Published: July 2002

The magnetohydrodynamic (MHD) equations are presented in an invariant form in flux variables and variational principles are formulated for these variables. These equations are then solved in Boozer coordinates in the small curvature expansion.
Magnetohydrodynamic equations for systems with large Larmor radius
Authors: Hassam A. B. , Huba J. D.
Published: July 2002

A set of one-fluid, modified magnetohydrodynamic (MHD) equations is developed that describes magnetized plasmas for which the relevant scale lengths are intermediate between the electron and ion Larmor radii, and the relevant time scales are intermediate between the electron and ion cyclotron frequencies. It is shown that the momentum equation is the same as that of conventional MHD but that the evolution of the magnetic field is strongly affected. The implications of the modified MHD system on the ‘‘frozen-in’’ theorem, magnetosonic waves, Alfvén waves, and the interchange instability are examined.
A semiempirical formation model for field-reversed configurations
Authors: Tuszewski M.
Published: July 2002

Recent data from the FRX-C [Proceedings of the 7th Symposium on the Physics and Technology of Compact Toroids (Los Alamos National Laboratory, Los Alamos, NM, 1986), p.142] and FRX-C/LSM [Proceedings of the 14th European Conference on Controlled Fusion and Plasma Physics (European Physical Society, Madrid, 1987), p. 485] field-reversed theta pinches are presented concerning heating, internal flux, and particle retention after formation. These results extend the data base of field-reversed configuration plasmas to larger dimensions. Good agreement is found for collisionless plasmas with some existing formation laws [Fusion Technol. 9, 48 (1986)] and the main result is confirmation of improved internal flux retention for plasmas of larger radius. Some simple semiempirical laws are proposed, which allow a global analytical formation model. This model should provide reasonable estimates of the equilibrium parameters of field-reversed configurations formed in theta pinches.
A whistler-pumped free electron laser
Authors: Sharma Anamika , Tripathi V. K.
Published: July 2002

A relativistic electron beam in the presence of a whistler wave excites a negative-energy beam space charge mode and a high frequency laser radiation (ω1≫ωc). The instability is explosive for a copropagating beam and the whistler pump, resulting in the excitation of an intense laser in a short pulse. The explosion time τ={(4/kv0)[(ω1−k1vb−ωc) γ0ω1/(ω1−k1vb)ω′pb]1/2}. For both the counterpropagating beam and the whistler wave, the instability is nonexplosive. The laser frequency in this case is somewhat larger, whereas the exponentiation time is comparable to the explosion time of the earlier case.
Feedback control of rotating resistive modes
Authors: Lazzaro E. , Nave M. F. F.
Published: July 2002

To gain information for the design of practical feedback schemes, the response of nonlinear resistive modes to boundary conditions imposed by external conductors and resistive vessels is analyzed in the time domain. Newly derived explicit dynamical equations describe the nonlinear evolution of the tearing mode island width and frequency of rotation. These equations are appropriate to explain previous experimental and theoretical results quantitatively.
An experimental study of enhanced fluid transfer mechanisms between a channel and a cavity
Authors: Garrison Brevard , Rogers Chris
Published: July 2002

The fluid transfer mechanisms between a channel and a cavity are investigated by laser-induced fluorescence for a Reynolds number based on cavity height of 600. In particular, this work examines the effect of upstream geometry on the flushing of the cavity. Oscillation of the shear-layer and Ekman-layer pumping are the dominant mechanism for fluid transfer in most cases. The flapping shear layer impinging on the trailing edge of the cavity creates a net inflow of channel fluid into the central portion of the cavity. A resulting outflow of cavity fluid is observed at the cavity end walls. Geometrical modifications investigated include shortening the cavity length to isolate the Ekman layer from the channel wall boundary layer, placing a cylinder directly upstream of the cavity leading edge, and serrating the upstream edge of the cavity. A cylinder placed upstream increases the flushing rate a maximum of 400%. A serrated leading edge increases the flushing rate 200%. The shorter cavity illustrates the effect of the channel side walls on the cavity flow without affecting the mean flushing rate. Finally, three-dimensional (3-D) effects are shown to have a dominant influence on the flushing rate.
Effects of the forcing function spectrum on the energy spectrum in 2-D turbulence
Authors: Constantin Peter , Foias Ciprian , Manley Oscar P.
Published: July 2002

The response of the two-dimensional velocity field to a single eigenmode driving a fluid flow is analyzed. It is shown that independent of its amplitude such a driving force cannot lead to Kraichnan’s inertial range spectrum. At least a pair of eigenmodes, one acting as a power source, and the other as a power sink, is necessary to obtain a separation of length scales and an accompanying statistically steady inertial spectral range.
Distributed excitation of Tollmien–Schlichting waves by vortical free-stream disturbances
Authors: Crouch J. D.
Published: July 2002

Nonlocalized receptivity to vortical free-stream disturbances is analyzed for a Blasius boundary layer over a wavy surface. Vortical disturbances are rescaled through an interaction with steady disturbances generated by the surface waviness. The rescaled disturbances provide a distributed energy transfer to the natural eigenmodes when conditions are close to resonance; this occurs in the neighborhood of the lower branch of neutral stability. Downstream of the energy transfer, the traveling wave solution is dominated by the natural eigenmode. Forcing from the vortical disturbances is concentrated in the outer region of the boundary layer. This leads to a relatively weak receptivity as compared to the acoustic problem. Total receptivity amplitudes are approximately ten times larger than for localized vortical receptivity, but approximately fifty times smaller than for nonlocalized acoustic receptivity.
A perturbation approach for suction laminar flow control applications
Authors: Masad Jamal A. , Malik Mujeeb R.
Published: July 2002

A new method for computing the hydrodynamic stability eigenvalue problem in the presence of wall suction by using the eigenvalue and eigenfunctions of the no-suction problem is presented. The perturbation approach is computationally efficient in comparison with the direct approach; it is potentially applicable in various ways to hydrodynamic stability, particularly in laminar flow control.
Phase front analysis of vortex streets
Authors: Lefrançois Marcel , Ahlborn Boye
Published: July 2002

The continuous formation and development of a laminar vortex street behind a circular cylinder of radius D in a flow of velocity U∞ has been modeled as a Huygens-type wave process, where the transverse velocity Uy and the vorticity ω in the near wake oscillate at the vortex shedding frequency f. Starting from the Biot–Savart law for fluids a phase front propagation integral is derived. This formalism is used to calculate for each point along the span the phase of vortex shedding as a function of the phase of the previously shed vortex generation and the shedding frequency. The amplitude is determined by a simple renormalization calculation. In good agreement with experiments, the model predicts the propagation of spanwise phase perturbations into subsequent vortex generations for two-dimensional (2-D) flow geometries and the cell formation in three-dimensional flows around tapered cylinders.
Conditional Reynolds stress on a V-grooved surface
Authors: Benhalilou M. , Anselmet F. , Fulachier L.
Published: July 2002

Measurements of both mean and turbulent dynamic fields over a ribbed wall show the existence of a minute zone in which the boundary layer characteristics are strongly altered. The turbulent kinetic energy (k+), deduced from the three velocity standard deviations, is strongly increased above the ridge, and reduced over the valley. A similar effect is also visible on the Reynolds stress (−〈uv〉+) profiles, but the relative variations of k+ and −〈uv〉+, with respect to the flat plate case, are not of the same magnitude. Close to the ridge plane, contributions to the Reynolds shear stress from ejection and sweep types of motions are seen to be larger and more frequent above the ridge than over the smooth wall, whereas they are reduced above the valley. Comparison of the respective productions of turbulence above a riblet and above smooth walls, inferred at the distance y+=9 above the ridge plane, indicates a reduction of about 5% in the manipulated flow.
Kinetic model for heat and momentum transport
Authors: Garzó V. , López de Haro M.
Published: July 2002

Expressions for the heat and momentum transport are obtained for a dilute gas in a steady state with both temperature and velocity gradients. The results are derived from a kinetic model recently proposed: the Liu model [Phys. Fluids A 2, 277 (1990)]. This model improves the well-known Bhatnagar–Gross–Krook (BGK) model equation. At a hydrodynamic level, the solution is characterized by a constant pressure, a linear profile of the flow velocity with respect to a space variable scaled with the local collision frequency, and a parabolic profile of the temperature with respect to the same variable. The shear viscosity and thermal conductivity coefficients are explicitly obtained. They are nonlinear functions of the shear rate and their only dependence on the temperature gradient is through their zero shear-rate values. In addition, the shear-rate dependence of the viscometric functions is also analyzed. A comparison with previous results derived from the BGK equation is carried out.