bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of compressibility on vortex pairing
Authors: Sandham N. D.
Published: July 2002

Direct numerical simulations are conducted to investigate in detail the effect of Mach number on vortex pairing in a mixing layer. The pairing process is found to be delayed at higher Mach numbers and the paths followed by the vortices change. To investigate the effect of the initial shape of the vortices a simple vortex dynamical model of pairing is constructed which accurately models pairing at low Mach numbers. Results from the model suggest that a variation in the initial shape of the vortices is not sufficient to explain the changes in the pairing process due to Mach number. Further simulations are conducted for isolated vortex pairs. There is little departure from the expected rotation rate as Mach number is increased, but strong core effects. Overall, changes in the pairing process reflect changes in the evolution of the primary instability, with vortex trajectories becoming more elongated as the Mach number is increased.
The dynamics of fibrous aerosols in a quiescent atmosphere
Authors: Newsom R. K. , Bruce C. W.
Published: July 2002

Measurements of the orientational properties of small fibrous graphite aerosols in free fall through air have been performed. The fibers have lengths on the order of millimeters and diameters on the order of microns. Estimates of the Reynolds number based on the fiber’s half-length and terminal velocity are on the order of 1. Measurements of the relative extinction in selective polarizations of microwave radiation upon transmission through a cloud of falling fibers in dilute concentration as well as direct photographic measurements indicate the fibers exhibit a tendency to assume a stable orientation with their long axes horizontal under quiescent conditions. Terminal velocities were found to range from 2.5 to 3 cm/s. Photographic measurements on single fibers were also made of the rate at which the fibers settle into their stable orientation given an initial arbitrary orientation. Orientational stabilization times, otherwise referred to as orientational relaxation time, are typically on the order of 3 to 5 s; this is mostly dependent on initial orientation. The measured transient angular motion of the fibers is compared with a proposed model of the motion. The model shows qualitative agreement with observations.
Short-scale convection and long-scale deformationally unstable Rossby wave in a rotating fluid layer heated from below
Authors: Tikhomolov Evgeniy
Published: July 2002

A rotating fluid layer, heated from below, with a deformable upper and nondeformable lower stress free surfaces is considered in the Boussinesq approximation. The system of the differential equations that governs the long-scale Rossby waves and short-scale convection is obtained in the rapid-rotation approximation. Long-scale flows are unstable due to heating and deformation of the upper surface. The neutral stability curves for Rossby waves and convection are obtained for linearized version of the equations. In a slightly supercritical regime the amplitude equations for convection and Rossby waves are derived by the use of the method of multiscale expansions. The properties of the amplitude equations are discussed. The existence of the two weakly supercritical stationary convection regimes is shown by numerical integration of the equations in the rapid-rotation approximation. In one of them, the amplitude of short-scale convection is modulated due to long-scale deformation of the upper surface associated with the excitation of the Rossby wave. In the other regime, the presence of deformation gives rise to alternating regions with and without convection.
Elimination of three-dimensional waves in a film flow
Authors: Lin S. P. , Chen J. N.
Published: July 2002

It is known that a vertical film flow is always unstable [T. B. Benjamin, J. Fluid Mech. 554, 505 (1957); C. S. Yih, Phys. Fluids 6, 321 (1963)] with respect to long surface wave disturbances. It has been shown [S. P. Lin et al., Phys. Fluids 8, 3247 (1996)] that the unstable two-dimensional waves in a vertical film can be suppressed by use of appropriate amplitudes and frequencies of the plate oscillation. It is shown here that the Squire theorem for the present problem does not exist. By use of the Chebyshev series solution with Floquet theory we show that three-dimensional unstable waves can also be suppressed, although they are not necessarily less dangerous than two-dimensional disturbances.
Stability of a liquid-metal layer between gas streams with and without a magnetic field
Authors: Witkowski Laurent Martin , Marty Philippe
Published: July 2002

The linear stability of a plane liquid sheet of metal flowing between parallel gas streams having different velocities is considered. The presence of a magnetic field only slightly modifies the neutral stability curves obtained in the hydrodynamic regime where Kelvin–Helmholtz instabilities are the dominant mechanism in the destabilization of the sheet. The most unstable wavelength is found to scale with We−1 where We is the Weber number. As expected, the growth rate of the instabilities is decreased when a transverse magnetic field is applied. However, the growth rate is increased when the applied magnetic field is parallel to the direction of the velocities. A possible explanation of this unusual phenomenon is presented. In an experimental air-blast atomizer with water and nitrogen, particle size and velocity measurements have been made with a laser-doppler analyzer. The results agree with the theoretical predictions for zero magnetic flux density and are also consistent with predictions based on a more global breakup mechanism.
Pressure-driven channel flows of a model liquid-crystalline polymer
Authors: Feng J. , Leal L. G.
Published: July 2002

Shear flows disrupt molecular orientation in liquid-crystalline polymers (LCPs) through director tumbling, and this causes difficulty in controlling the polymer structure and properties in injection molding and extrusion. In this paper we simulate LCP channel flows using the Doi theory. A Bingham closure is used to preserve director tumbling and wagging. The objective is to examine how contractions and expansions in a channel affect LCP orientation and to explore the possibility of using the channel geometry as a means of manipulating LCP order. A finite-element method is used to solve the coupled equations for fluid flow and polymer configuration. Results show that a contraction aligns the director with the streamline and improves molecular order, while an expansion drives the director away from the flow direction and reduces molecular order. If the expansion is strong enough, an instability develops downstream as disturbances in the flow and polymer configuration reinforce each other through the polymer stress. This instability generates a wave that spans roughly the central half of the channel and propagates downstream at the centerline velocity. For abrupt contractions or expansions, disclinations of ±1/2 strength arise in the corner vortex. The numerical results agree qualitatively with experiments when comparisons can be made. In particular, the wavy pattern following a sudden expansion is remarkably similar to previous experimental observations. The simulations suggest that using contractions and expansions may be a feasible strategy for controlling LCP order and morphology in processing.
Influence of energetics on the stability of viscoelastic Taylor–Couette flow
Authors: Al-Mubaiyedh U. A. , Sureshkumar R. , Khomami B.
Published: July 2002

Previously reported isothermal linear stability analyses of viscoelastic Taylor–Couette flow have predicted transitions to nonaxisymmetric and time-dependent secondary flows for elasticity numbers E≡De/Re>0.01. In contrast, recent experiments by Baumert and Muller using constant viscosity Boger fluids have shown that the primary flow transition leads to axisymmetric and stationary Taylor-type toroidal vortices. Moreover, experimentally observed onset Deborah number is an order of magnitude lower than that predicted by isothermal linear stability analyses. In this work, we explore the influence of energetics on the stability characteristics of the viscoelastic Taylor–Couette flow. Our analysis is based on a thermodynamically consistent reformulation of the Oldroyd-B constitutive model that takes into account the influence of thermal history on polymeric stress, and an energy equation that takes into account viscous dissipation effects. Our calculations reveal that for experimentally realizable values of Peclet and Brinkman numbers, the most dangerous eigenvalue is real, corresponding to a stationary and axisymmetric mode of instability. Moreover, the critical Deborah number associated with this eigenvalue is an order of magnitude lower than those associated with the nonisothermal extensions of the most dangerous eigenvalues of the isothermal flow. Eigenfunction analysis shows stratification of perturbation hoop stress across the gap width drives a radial secondary flow. The convection of base state temperature gradients by this radial velocity perturbation leads to this new mode of instability. The influence of geometric and kinematic parameters on this instability is also investigated.
Self-consistent study of the alpha-particle-driven toroidicity-induced Alfvén eigenmode
Authors: Wu Yanlin , White Roscoe B.
Published: July 2002

The interaction of high energy particles with an Alfvén eigenmode is investigated self-consistently by using a kinetic dispersion relation. All important poloidal mode numbers and their radial mode profiles as calculated with the nova−k code [C. Z. Cheng, Phys. Rep. 211, 1 (1992)] are included. A Hamiltonian guiding center code is used to simulate the alpha particle motion. The numerical simulations include particle orbit width, nonlinear particle dynamics, and the effects of the modes on the particles. Modification of the particle distribution leading to mode saturation is observed. Particle loss depends on the radial extent of the mode and the ratio of the alpha particle gyroradius to the minor radius.
Quadratic free energy for the linearized gyrokinetic Vlasov–Maxwell equations
Authors: Brizard Alain
Published: July 2002

The motivation for the present work resides in the search for a new formalism for investigating the stability of plasma equilibria perturbed by low-frequency electromagnetic field fluctuations. In this context, the free-energy method of Morrison and Pfirsch [Phys. Fluids B 2, 1105 (1990)] has been extended by deriving an expression for the gyrokinetic free energy. As a check on the gyrokinetic free-energy method, known formulas previously obtained in the electrostatic limit are recovered, while recent (drift-kinetic) formulas obtained by Betti and Freidberg [Phys. Fluids B 4, 1465 (1992)] in the electromagnetic limit are generalized.
One-dimensional simulations of Langmuir collapse in a radiation-driven plasma
Authors: Wang J. G. , Payne G. L. , DuBois D. F. , Rose H. A.
Published: July 2002

Langmuir collapse in a radiation-driven plasma is simulated in one dimension using both the hybrid Vlasov model and the modified Zakharov model. In the hybrid model, the electrons are described by the Vlasov equation and the ions are described by the fluid equations. Two numerical runs with different parameters are performed: (a) a weakly driven case, and (b) a strongly driven case. The comparisons of the results between the two models for each case are carried out in detail. It is found that the dynamic behavior of the plasma can be changed dramatically by varying the external driving field. The electron temperature increases only ∼10% in the weakly driven case, while it increases ∼200% in the strongly driven case during a single Langmuir collapse. The two models agree with each other during the first collapse after the saturation of modulational instability. Dynamic differences appear after the first collapse, caused by the electron heating in the hybrid model, which is not accounted for in the standard isothermal Zakharov model.
Nonlocal theory of a whistler pumped free electron laser
Authors: Pant K. K. , Tripathi V. K.
Published: July 2002

A cylindrical waveguide loaded with a magnetized plasma supports a short wavelength whistler mode. In the presence of a relativistic electron beam the whistler mode couples to a negative energy beam mode and a high-frequency electromagnetic mode giving coherent radiation. A nonlocal theory of the process reveals that larger beam cross section is preferable for better coupling. The phase matching conditions give the frequency of operation as ω1=(ck0z/η0)[(η0−η)/(η−η1)] where ω1 is the frequency of the free electron laser radiation, k0z is the wave number of the pump wave, c is the velocity of light in free space, and ηj=ckj/ωj, ηj=η0, η1, η correspond to the pump, radiation signal, and the beam mode, respectively. The frequency of radiation is maximum when it propagates in the transverse magnetic TM01 mode. As the frequency of the whistler approaches electron cyclotron frequency, the frequency of radiation is enhanced while the growth time also increases. The growth time scales as γ9/20k3/20zω1/2 [ω2c−γ20(ω0−k0zvb)2]1/2 /(ω2pbω1/21).
The dynamics of long wavelength electrostatic turbulence in tokamaks*
Authors: Newman D. E. , Terry P. W. , Diamond P. H. , Liang Y.-M. , Craddock G. G. , Koniges A. E. , Crottinger J. A.
Published: July 2002

From extensive simulation of simple local fluid models of long wavelength drift wave turbulence in tokamaks, it is found that conventional notions concerning directions of cascades, locality and isotropy of spectral transfer, frequencies of fluctuations, and stationarity of saturation do not hold for moderate to long wavelengths (kρs≤1). In particular, at long wavelengths, where spectral transfer of energy is dominated by the E×B nonlinearity, energy is carried to short scale (even in two dimensions) in a manner that is anisotropic and highly nonlocal (energy is efficiently passed between modes separated by the entire spectrum range in a correlation time). At short wavelengths, transfer is dominated by the polarization drift nonlinearity. While the standard dual cascade applies in this subrange, it is found that finite spectrum size can produce cascades that are reverse directed (i.e., energy to high k) and are nonconservative in enstrophy and energy similarity ranges (but conservative overall). In regions where both nonlinearities are important, cross-coupling between the nonlinearities gives rise to large nonlinear frequency shifts which profoundly affect the dynamics of saturation by modifying the growth rate and nonlinear transfer rates. These modifications produce a nonstationary saturated state with large amplitude, long period relaxation oscillations in the energy, spectrum shape, and transport rates. Methods of observing these effects are presented.
Whistler waves produced by a modulated electron beam: Electromagnetic fields in the linear approach
Authors: Volokitin A. , Krafft C. , Matthieussent G.
Published: July 2002

The theory of whistler wave interaction with a modulated electron beam of finite radius and injected parallel to the magnetic field in an unbounded space plasma is considered. The study of the energy transfer between the thin beam and the whistler wave is done in the case of sheared whistlers, when the parallel wave number is very small compared to the perpendicular one. In this case, and in the linear regime, structures of potentials and electromagnetic fields inside and outside the beam are determined analytically. In the vicinity of the beam, simple expressions for field components are provided in the case of Čerenkov resonance near the double pole.
Thermal effects on coupled self-focusing and Raman scattering of a laser in a self-consistent plasma channel
Authors: Liu C. S. , Tripathi V. K.
Published: July 2002

A self-ducted Gaussian laser beam, propagating through a plasma, is unstable to a perturbation in its spot size, r0. A reduction (or enhancement) in r0 leads to an enhancement (or reduction) in axial intensity, translating r0 modulation into intensity variation with axial distance and time, causing ponderomotive force on the electrons that drive a Langmuir wave. The density troughs/crests of the Langmuir wave, moving with the group velocity of the laser, cause self-focusing/defocusing of the laser, enhancing the original perturbation and leading to a combined instability of self-focusing, and Raman scattering of the laser beam. The thermal effects allow the Langmuir wave to be treated as a radially localized eigenmode with finite group velocity in the axial direction. The spatial growth rate of the instability decreases with electron temperature while it increases with plasma density.
Optimization of x-ray emission in Z-pinch implosions: Theory versus experiment
Authors: Whitney K. G. , Thornhill J. W. , Apruzese J. P. , Davis J. , Deeney C. , LePell P. D. , Failor B. H.
Published: July 2002

Three sets of Z-pinch experiments were performed in recent years [Phys. Rev. E 50, 2166 (1994)] that confirmed the existence of a theoretically predicted region in (load mass)-(implosion velocity) space where efficient conversion of implosion energy to kilovolt x rays was possible [J. Appl. Phys. 67, 1725 (1990)]; but they also raised questions about the validity of the theoretical models [Phys. Plasmas 1, 321 (1994)] and about the influence of the pulse-power generator and load design on the detailed behavior of the measured x-ray yields. Newly completed experiments suggest that some of these influences can be overcome and that K-shell yields can be increased in greater accord with theoretical expectations. In this paper, a brief description of these experiments is given, and some comparisons with recent theoretical findings are made along with comparisons with the above-mentioned earlier work. These new comparisons demonstrate the need for further improvements in the theoretical models and in the way experiments are designed.
Meeting the challenge of detecting ion plasma waves
Authors: Bauer Bruno S. , Drake R. P. , Estabrook Kent G. , Camacho J. F. , Watt Robert G. , Wilke Mark D. , Busch Garland E. , Caldwell Steven E. , Baker Stuart A.
Published: July 2002

Ion plasma waves—purely electrostatic ion waves with a wavelength of order of the electron Debye length and frequency of the order of the ion plasma frequency—have long been known in theory but have proven difficult to detect experimentally. The difficulties stemmed from the techniques used to produce the plasma and to drive and detect the waves. In the work reported here, these problems were overcome by using resonant laser scattering to detect ion plasma waves in a multiply ionized, laser-produced plasma. This nonetheless required careful experimental design to minimize frequency smearing of the scattered signal by plasma gradients. The plasma was extensively characterized, allowing comparison of the theoretical dispersion relation with the wave data. The agreement of these two provides conclusive proof of the detection of ion plasma waves.
Excitation of a transverse magnetic waveguide mode near cutoff in a free-electron laser
Authors: Willett Joseph E. , Aktas Yildirim , Mehdian Hassan
Published: July 2002

A free-electron laser with a helical wiggler field, waveguide, and axial guide field operating in the collective regime infinitesimally above cutoff of a transverse magnetic (TM) mode is considered. The waveguide is completely filled with a relativistic electron beam. Beam-frame analysis of the parametric decay of the wiggler wave into a forward scattered space-charge wave and a backscattered TM mode is presented. A simple, analytical formula is derived for the laboratory-frame spatial growth rate. Calculations show that this stimulated Raman scattering process can occur if the axial guide field is sufficiently strong, and that the growth rate is enhanced when the beam-frame wiggler frequency is near the cyclotron frequency.
Poloidal rotation spin-up in divertor tokamaks
Authors: Strauss H. R.
Published: July 2002

Poloidal spin-up can occur in divertor tokamaks as a consequence of magnetic field geometry. The presence of a divertor X point causes poloidal variation in the diffusion of particles and heat. The resulting pressure variation on flux surfaces produces a poloidal torque. The spin-up is opposed by sound wave propagation. The physical model consists of dissipative compressional reduced magnetohydrodynamics (CRMHD), including viscous stress. Numerical computations have been carried out using an adaptive unstructured mesh method capable of good resolution near a separatrix X point. The simulations show poloidal flow spin-up increasing with pressure diffusivity. A theory is derived that is consistent with the simulations. The equilibrium poloidal E×B/B2 drift velocity is a root of a fifth-order polynomial. Bifurcations from one rotation state to another can occur at critical parameter values.
Role of beam quality in free-electron lasers
Authors: Hafizi B. , Roberson C. W.
Published: July 2002

A kinetic theory of the scaling of free-electron lasers is presented. The results are based on a variational solution of an eigenvalue equation that is derived from the Vlasov–Maxwell equations and includes the effects of optical guiding, betatron oscillations and emittance, energy spread, and self-fields. The dependence of growth rate and filling factor on betatron oscillations and emittance is displayed in terms of scaled variables. A scaled thermal velocity, S, is defined as a measure of axial beam quality and shown to provide a quantitative distinction between the cold and the kinetic regimes of operation. The possibility of enhancing the growth rate by compressing the beam longitudinally is explored and discussed on the basis of the variation of S. Finally, we discuss a surprising result. It is shown that self-fields, by opposing the betatron oscillations due to wiggler gradients, tend to reduce the spread in the axial velocity of the electrons. That is, self-fields effectively cool the beam.
A magnetohydrodynamic stability study of reverse shear equilibria in the Tokamak Fusion Test Reactor
Authors: Phillips M. W. , Zarnstorff M. C. , Manickam J. , Levinton F. M. , Hughes M. H.
Published: July 2002

A study is presented of the low-n (n=1,2,3) magnetohydrodynamic stability of equilibria with reverse shear safety factor profiles. The low-n stability boundaries are found to be characterized by resonance structures due to internal so-called ‘‘infernal’’ mode types of instabilities. The parametric dependence of shear reversal width and depth, current, and pressure gradient on the beta limit are determined by using profile models that allow each parameter to be varied independently. Reverse magnetic shear is found to have a stabilizing influence for modes with toroidal mode numbers n≥2 leading to the possibility of improved β limits in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 26, 11 (1984)].
Coulomb crystallization in colloidal plasmas with streaming ions and dust grains
Authors: Shukla P. K. , Rao N. N.
Published: July 2002

The test charge potential of a dust particle in dusty plasmas with equilibrium ion and dust particle drifts is presented. The possibility of Coulomb crystallization due to an attractive force between like polarity particulates is discussed.
A Vlasov moment description of cyclotron wave–particle interactions
Authors: Siregar Edouard , Goldstein Melvyn L.
Published: July 2002

A quasifluid formalism designed to capture some effects of cyclotron interactions is presented. Starting from the contractions of exact moments of the Vlasov equation, a closure for cyclotron interactions is achieved by using kinetic information directly. This nonperturbative approach does not require a priori assumptions about zeroth-order particle velocity distributions. The nonlinear coupling between field-aligned particle thermal velocities and transverse cyclotron wave and thermal motions are described by off-diagonal elements of the pressure tensor. These elements are related to the growth and damping of cyclotron wave energy. A functional form for an effective wave–particle momentum transport coefficient is derived from the requirement of consistency between the energy and momentum moment equations, but its specific magnitude and sign, determined by threshold temperature anisotropy levels, must be input from kinetic theory. This effective transport coefficient has a nondefinite sign, reminiscent of the gyroviscous coefficients of classical transport, and is consistent with the time reversibility of the Vlasov equation. A coupled set of cyclotron equations of state for the evolution of the parallel and perpendicular pressures are derived. This formalism provides a connection between known kinetic solutions of cyclotron interactions and fluid plasma equations.
Transport of impurity ions along the field lines in magnetized plasma
Authors: Kálvin S. , Kocsis G. , Bakos J. S. , Könen L. , Mank G. , Pospieszczyk A.
Published: July 2002

A simple one-dimensional model describing the spread of laser blow-off injected ions along the magnetic field lines is presented. The model is based on the kinetic theory. A closed set of kinetic equations that contain a Coulomb collision term is solved numerically. For impurity density less than the plasma density, the effect of the ambipolar electric field is negligible, as shown. The results of the model calculation are compared with experimental results obtained on the Tokamak Experiment for Technically Oriented Research (TEXTOR) [H. Soltwisch et al., Plasma Phys. Controlled Fusion 26, 23 (1984)] tokamak by the use of injection of lithium atoms, and the agreement is satisfactory. The results of the model and possible applications for diagnostic purposes are discussed in detail.
Confinement and the safety factor profile
Authors: Batha S. H. , Levinton F. M. , Scott S. D. , Mikkelsen D. R. , Budny R. V. , Chang Z. , Park H. , Synakowski E. , Taylor G. , Zarnstorff M. C. , Sabbagh S. A.
Published: July 2002

The conjecture that the safety factor profile, q(r), controls the improvement in tokamak plasmas from poor confinement in the Low- (L-) mode regime to improved confinement in the supershot regime has been tested in two experiments on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 1, 51 (1987)]. First, helium was puffed into the beam-heated phase of a supershot discharge, which induced a degradation from supershot to L-mode confinement in about 100 ms, far less than the current relaxation time. The q and shear profiles measured by a motional Stark effect polarimeter showed little change during the confinement degradation. Second, rapid current ramps in supershot plasmas altered the q profile, but were observed not to change significantly the energy confinement. Thus, enhanced confinement in supershot plasmas is not due to a particular q profile, which has enhanced stability or transport properties. The discharges making a continuous transition between supershot and L-mode confinement were also used to test the critical-electron-temperature-gradient transport model. It was found that this model could not reproduce the large changes in electron and ion temperature caused by the change in confinement.
A three-circulation theorem for relativistic plasmas
Authors: Elsässer Klaus , Popel Sergey
Published: July 2002

It is shown that the sum of three circulations (hydrodynamic, magnetic, and thermal) is constant in time for each component of a perfect relativistic plasma. A constraint for the initial configuration (‘‘Weber/Ertel case,’’ the lines of canonical vorticity lie on surfaces of constant entropy) leads to zero potential vorticity also for later times; this is generally realized in inhomogeneous flow equilibria for which the explicit time-dependence of Clebsch potentials is given. Equilibrium states of warm relativistic plasmas with vanishing thermal circulation are discussed.
Comment on ‘‘Bohm criterion for the collisional sheath’’ [Phys. Plasmas 3, 1459 (1996)]
Authors: Riemann K. U. , Meyer P.
Published: July 2002

Recently, Valentini [Phys. Plasmas 3, 1459 (1996)] investigated the influence of collisions on the space charge formation and derived a modified Bohm criterion accounting for collisions in the sheath. It is shown that this derivation is wrong and is based on a misinterpretation of the plasma sheath concept.
Periodic nonlinear wave–wave interaction in a plasma discharge with no external oscillatory driving force
Authors: Koepke M. E. , Klinger T. , Seddighi F. , Piel A.
Published: July 2002

Experimental measurements of time series and frequency spectra characterize a periodic nonlinear interaction between pairs of self-excited, propagating, ionization waves simultaneously present in the positive column of a neon glow discharge. No periodically varying external driving force is applied. The interaction is the spatio-temporal extension of the previously established temporal periodic pulling process, the incomplete entrainment of a driven nonlinear oscillator. The particular mode playing the role of the driving perturbation in the interaction can be selected and is identified by inspection of the asymmetric, multisideband spectrum of light fluctuations, which reflect the system’s dynamics. A comparison between the spatio-temporal and temporal periodic pulling shows that the former is associated with a relatively strong driving force.
Time-resolved energy measurement of electron beams in fiber Z-pinch discharges
Authors: Robledo A. , Mitchell I. H. , Aliaga-Rossel R. , Chittenden J. P. , Dangor A. E. , Haines M. G.
Published: July 2002

The emission of hard x rays with energies much greater than the applied anode-cathode voltage is a common feature of Z-pinch plasmas. Here time-resolved measurements of such emission from fiber Z pinches at the mega-ampere current level are reported. The x-ray spectrum measured by an array of detectors is used to calculate the energy of the electron beam producing the emission. Pulses of between 20 and 100 ns duration were observed and electron-beam energy measurements of around 2 MeV obtained. It is thought that these high-energy beams are generated by the plasma becoming resistive at the time of x-ray emission.
Free electron laser in a partially filled waveguide
Authors: Maraghechi B. , Mirzanejhad S.
Published: July 2002

A linear theory for a free electron laser with a one-dimensional helical wiggler and axial magnetic field in the collective regime is presented. The configuration consists of a cylindrical waveguide with arbitrary ratio of electron beam radius a to waveguide inner radius R. Parametric decay of the wiggler pump wave, in the beam frame, into a space-charge wave and an electric–magnetic (EH) waveguide mode is analyzed in three dimensions. A nonlinear wave equation for the three-wave interaction is derived and employed to obtain a formula for the spatial growth rate of the excited eigenmodes. It was found that the relativistic treatment of the electron oscillations in the wiggler field destroys the cyclotron resonance which appears in the nonrelativistic case. Nevertheless, appreciable amplification was found. Numerical analysis is conducted to study the growth rate, radiation wavelength, and required relativistic factor as functions of axial magnetic field B0 and radius ratio a/R. The suitable value for a/R was found to be around 0.65 for our choice of parameters.
Dynamic plasma screening effects on electron capture process in hydrogenic ion–fully stripped ion collisions in dense plasmas
Authors: Jung Young-Dae
Published: July 2002

In dense plasmas, dynamic plasma screening effects are investigated on electron capture from hydrogenic ions by past fully stripped ions. The classical Bohr–Lindhard model has been applied to obtain the electron capture probability. The interaction potential in dense plasmas is represented in terms of the longitudinal dielectric function. The classical straight-line trajectory approximation is applied to the motion of the projectile ion in order to visualize the electron capture probability as a function of the impact parameter, projectile energy, and plasma parameters. The electron capture probability including the dynamic plasma screening effect is always greater than that including the static plasma screening effect. When the projectile velocity is smaller than the electron thermal velocity, the dynamic polarization screening effect becomes the static plasma screening effect. When the projectile velocity is greater than the plasma electron thermal velocity, the interaction potential is almost unshielded. The difference between the dynamic and static plasma screening effects is more significant for low energy projectiles. It is found that the static screening formula obtained by the Debye–Hückel model overestimates the plasma screening effects on the electron capture processes in dense plasmas.
A kinetic derivation of multifluid equations for multispecies nonequilibrium mixtures of reacting gases
Authors: Benilov M. S.
Published: July 2002

Derivation of the multifluid equations by means of the Chapman–Enskog method is considered. The forces of inter-species friction and the rates of energy exchange due to elastic collisions are calculated taking into account the effect of the relative motion of the species and of the non-equality of their temperatures on the frequency of inter-species collisions.
Numerical measurement of turbulent responses in drift-Alfvén turbulence
Authors: Fernandez E. , Terry P. W.
Published: July 2002

A drift-Alfvén magnetoturbulence model that augments reduced magnetohydrodynamics with evolution of electron density under parallel compression and fluid advection has been studied numerically. In the Alfvénic regime, measurement of spectral transfer rates, frequency spectra, energy partitions, and the ensemble-averaged turbulent response reveals both Alfvénic and hydrodynamic characteristics. The rms turbulent frequency is Alfvénic, the energies are equipartitioned, and there is a fast, Alfvén-time scale relaxation in the turbulent response. The mean frequency is hydrodynamic, with diamagnetic and eddy straining signatures, and there is an eddy straining decorrelation appearing as a distinct, long time scale branch in the turbulent response. The decay rates and relative fluctuation strengths associated with fast and slow time scale decorrelation are in good agreement with theoretical predictions that posit a Kolmogorov spectrum in the Alfvénic regime.
High-resolution x-ray imaging of planar foils irradiated by the Nike KrF laser
Authors: Brown C. , Seely J. , Feldman U. , Obenschain S. , Bodner S. , Pawley C. , Gerber K. , Sethian J. , Mostovych A. , Aglitskiy Y. , Lehecka T. , Holland G.
Published: July 2002

Thin plastic (CH) foils were irradiated by the Naval Research Laboratory Nike [Obenschain et al., Phys. Plasmas 3, 2098 (1996)] KrF laser and were imaged in the x-ray and extreme ultraviolet regions with two-dimensional spatial resolution in the 3–10 μm range. The CH foils were backlit by a silicon plasma. A spherically curved quartz crystal produced monochromatic images of the Si+12 resonance line radiation with energy 1865 eV that was transmitted by the CH foils. Instabilities that were seeded by linear ripple patterns on the irradiated sides of CH foils were observed. The ripple patterns had periods in the 31–125 μm range and amplitudes in the 0.25–5.0 μm range. The silicon backlighter emission was recorded by an x-ray spectrometer, and the 1865 eV resonance line emission was recorded by a fast x-ray diode. The multilayer mirror telescope recorded images of the C+3 1550 Å emission (energy 8.0 eV) from the backside of the CH foils.
Response to “Comments on ‘Dispersion relation of a plasma in the periodic magnetic field of a wiggler’ ” [Phys. Plasmas 3, 1773 (1996)]
Authors: Petrillo V. , Maroli C.
Published: July 2002

It is argued that a wiggler plasma may acquire a non-zero velocity in the equilibrium and that it should be somehow consistent with the conservation of energy. (AIP)
Comment on “Explanation for MARFE formation and subsequent evolution into a detached symmetric plasma edge” [Phys. Plasmas 3, 2673 (1996)], “MARFE impurity density limits in tokamaks with poloidal asymmetry and rotation” [Phys. Plasmas 3, 3032 (1996)], and “MARFE suppression by external edge heating” [Phys. Plasmas 4, 134 (1997)]
Authors: De Ploey A. , Van der Linden R. A. M. , Goossens M.
Published: July 2002

W. M. Stacey presents stability criteria for the thermal instability based on a linear one-dimensional (1D) slab model including the effects of ionization recombination [Phys. Plasmas 3, 2673 (1996)], large radial gradients and rotation [Phys. Plasmas 3, 3032 (1996)] and external edge heating [Phys. Plasmas 4, 134 (1997)]. The second paper is incorrectly presented as a two-dimensional (2D) analysis. The hypothesis for the poloidal asymmetry of the multifaceted asymmetric radiation from the edge (MARFE) is based on a misinterpretation of the linear results and contradicted by our own linear analyses in a 1D cylindrical [Phys. Plasmas 1, 2623 (1994)] and a 2D tokamak model [Plasma Phys. Controlled Fusion 39, 423 (1997)]. The explanation for the evolution into a detached plasma and for MARFE suppression by external edge heating are interesting hypotheses, supported by experimental observations. However, a 1D linear analysis does not offer the basis to support these hypotheses theoretically.
Nonlinear damping of electron plasma surface waves
Authors: Stenflo L. , Gradov O. M.
Published: July 2002

The damping rate, as a function of the wave amplitude, of an electron plasma surface wave is deduced and discussed.
Whistler waves emission by a modulated electron beam: Nonlinear theory
Authors: Volokitin A. , Krafft C. , Matthieussent G.
Published: July 2002

The nonlinear theory for the interaction of a thin modulated electron beam with a monochromatic whistler wave is considered. On the basis of wave field structures calculated previously in the linear approach, equations describing the wave amplitude evolution inside and outside the beam are obtained. These equations, together with the equations for particle motion in the drift approximation, form the self-consistent nonlinear model. Double and single pole resonance cases are considered. The full system of equations (taking into account the finite size of the beam) is resolved by a computer code which yields clear physical results. This code avoids the heavy structure and the long computer time inherent in the usual Particle-In-Cell simulation codes.
Quasistatic magnetic field generated by a short laser pulse in an underdense plasma
Authors: Gorbunov Leonid M. , Mora Patrick , Antonsen Thomas M.
Published: July 2002

The quasistatic magnetic field generated by short laser pulse in a uniform rarefied plasma is found analytically and compared to that of two-dimensional particle simulations. It is shown that an axisymmetric laser pulse generates an azimuthal magnetic field the structure of which is rather complicated inside the laser pulse body. Behind the pulse in the wake region the magnetic field contains a component that is homogeneous in the longitudinal direction and a component that is oscillating at the wave number 2kp, where kp is the wave number of the plasma wake. Particle simulations confirm the analytical results and are also used to treat the case of high intense laser pulses.
A high-resolution x-ray microscope for laser-driven planar-foil experiments
Authors: Marshall F. J. , Allen M. M. , Knauer J. P. , Oertel J. A. , Archuleta T.
Published: July 2002

A soft x-ray microscope (E≲3 keV) with high spatial resolution (∼3 μm) has been characterized at the University of Rochester’s Laboratory for Laser Energetics and used for initial experiments on the Omega laser system [Boehly et al., Opt. Commun. 133, 495 (1997)] to study the hydrodynamic stability of directly driven planar foils. The microscope, which is an optimized Kirkpatrick–Baez-type design, is used to obtain four x-ray radiographs of laser-driven foils. Time-resolved images are obtained with either custom-built framing cameras (time resolution ∼80 ps) or by using short-pulse backlighter beams (Δt≲200 ps). In the former case, a spatial resolution of ∼7 μm was obtained (limited by the framing camera), while in the latter case a resolution of ∼3 μm was obtained. This paper details the testing, calibration, and initial use of this microscope in the laboratory and on Omega.
On the nonlinear mechanics of magnetohydrodynamic stability
Authors: Dahlburg R. B.
Published: July 2002

The nonlinear stability of magnetized plane Poiseuille flow, the magnetized plane jet, and the plane current-vortex sheet is examined. If it is assumed that the perturbation retains the form of the linearly unstable eigenmode, and that the most significant nonlinear effect is the distortion of the mean flow and magnetic field, then a Landau nonlinear stability equation can be derived from the streamwise-averaged perturbation energy balance. Secondary equilibrium velocity and magnetic fields are predicted. The possibility of subcritical instabilities also is discussed.
Statistically averaged rate equations for intense non-neutral beam propagation through a periodic solenoidal focusing field based on the nonlinear Vlasov–Maxwell equations
Authors: Davidson Ronald C. , Lee W. Wei-li , Stoltz Peter
Published: July 2002

In this paper we present a detailed formulation and analysis of the rate equations for statistically averaged quantities for an intense non-neutral beam propagating through a periodic solenoidal focusing field Bsol(x) with axial periodicity length S=const. The analysis is based on the nonlinear Vlasov–Maxwell equations in the electrostatic approximation, assuming a thin beam with characteristic beam radius rb≪S, and small transverse momentum and axial momentum spread in comparison with the directed axial momentum pz=γbmβbc. The global rate equation is derived for the self-consistent nonlinear evolution of the statistical average 〈χ〉=Nb−1∫dXdYdX′dY′χFb, where χ(X,Y,X′,Y′,s) is a general phase function, and Fb(X,Y,X′,Y′,s) is the distribution function of the beam particles in the transverse phase space (X,Y,X′,Y′) appropriate to the Larmor frame. The results are applied to investigate the nonlinear evolution of the generalized entropy, mean canonical angular momentum 〈Pθ〉, center-of-mass motion for 〈X〉 and 〈Y〉, mean kinetic energy (1/2)〈X′2+Y′2〉, mean-square beam radius 〈X2+Y2〉, and coupled rate equations for the unnormalized transverse emittance ε(s) and root-mean-square beam radius Rb(s)=〈X2+Y2〉1/2. Most importantly, the present derivation of nonlinear rate equations for various statistical averages 〈χ〉 allows for general azimuthal variation (∂/∂θ≠0) of the distribution function and self-field potential, and therefore represents a major generalization of earlier calculations carried out for the case of axisymmetric beam propagation.
Role of ponderomotive density expulsion in ion Bernstein wave coupling to the core plasma
Authors: Russell D. A. , Myra J. R. , D’Ippolito D. A.
Published: July 2002

When the density at the ion Bernstein wave (IBW) antenna is relatively low, mode transformation of the electron plasma wave to the IBW is sensitive to the density gradient scale length, and hence to ponderomotive effects. A second-order nonlinear ordinary differential equation that describes mode transformation at the lower-hybrid layer, including self-consistent ponderomotive density profile modification, is solved for the rf electrostatic potential in front of the IBW antenna, for the particular case of heating just below the second harmonic of the deuterium cyclotron frequency. The complex antenna impedance and a local reflectivity are calculated, assuming vacuum within the antenna box. These calculations reveal diminished antenna coupling to the IBW with increasing ponderomotive density expulsion, as compared to the linear prediction. The ponderomotive force increases the density gradient in the edge plasma, thus enhancing reflection and lowering the loading resistance. The model also describes the direct launch of IBWs in high edge density regimes, lacking a lower-hybrid layer, where the impedance is found to be much smaller than in the low density regime.
Measurements of the high confinement mode pedestal region on Alcator C-Mod
Authors: Hubbard A. E. , Boivin R. L. , Granetz R. S. , Greenwald M. , Hutchinson I. H. , Irby J. H. , In Y. , Kesner J. , LaBombard B. , Lin Y. , Rice J. E. , Sunn Pedersen T. , Snipes J. A. , Stek P. C. , Takase Y. , Wolfe S. M. , Wukitch S.
Published: July 2002

Measurements of the steep transport barrier at the edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The parameters at the top of this barrier are in the range Te=150–750  eV and ne=0.5−3.3×1020 m−3, depending on the confinement regime. Type III edge localized modes (ELMs) have an upper temperature limit. Te pedestal profiles show a barrier width ΔT≃8 mm. Soft x-ray emissivity profiles are narrower, with Δ=2–4 mm. Edge currents are calculated to alter the ideal stability boundary favorably, leading to ideally stable pedestal profiles. High frequency, broadband, edge density fluctuations are sometimes observed in H-mode (high-confinement mode) and are associated with enhanced particle transport. Coherent magnetic fluctuations localized near the pedestal are also seen.
Generation of microwave pulses from the static electric field of a capacitor array by an underdense, relativistic ionization front
Authors: Muggli P. , Liou R. , Lai C. H. , Hoffman J. , Katsouleas T. C. , Joshi C.
Published: July 2002

The dc to ac radiation converter is a new device in which a relativistic ionization front directly converts the static electric field of an array of alternatively biased capacitors into a pulse of tunable radiation. In a proof-of-principle experiment frequencies between 6 and 21 GHz were generated with plasma densities in the 1012 cm−3 range and a capacitor period 2d=9.4 cm. In the present experiment, short pulses with frequencies between 39 and 84 GHz are generated in a structure with 2d=2 cm. The frequency spectra of these pulses are measured using a diffraction grating. The spectra are discrete, and their center frequency varies linearly with the gas pressure prior to ionization (or plasma density), as expected from theory. Their relative spectral width is around 18%, consistent with the expected number of cycles (six) contained in the pulses. An upper limit of 750 psec (bandwidth detection limited) is placed on the pulses length. The emitted frequency increases from 53 to 93 GHz when the capacitors are connected by pair to obtain a effective array period of 4 cm.
Stable, thermal equilibrium, large-amplitude, spherical plasma oscillations in electrostatic confinement devices
Authors: Barnes D. C. , Nebel R. A.
Published: July 2002

The problem of large-amplitude spherical oscillations of an ion cloud in an Inertial Electrostatic Confinement (IEC) device is examined. It is shown that ion fluctuations of a Gaussian profile in a spherical, harmonic well are stable to all hydrodynamic modes, and stable oscillations about the oscillating equilibrium state may be damped by continuum damping. It is also shown that the ion state forms a thermal equilibrium, in spite of the orders of magnitude, density, and temperature changes during the oscillation cycle. Finally, a brief discussion of how to experimentally realize the required electron distributions for these oscillations is presented.
Controlling chaotic states of a Pierce diode
Authors: Friedel Holger , Grauer Rainer , Spatschek Karl H.
Published: July 2002

A recently developed nonlinear approach to control chaos is applied to the Pierce diode. In the latter, both (kinetic) virtual cathode oscillations and (hydrodynamic) plasma oscillations appear. Via the period doubling route, the plasma oscillations can become chaotic. They are, however, usually superimposed by virtual cathode oscillations. Here it is shown that in the hydrodynamic, as well as in the kinetic regime, unstable periodic orbits can be stabilized. The results can be applied to bring the Pierce diode into a well-defined state of microwave oscillations.