bluuebunny/crossref_metadata_2025 huggingface dataset:
Conversion of laser light into soft x rays. Part II: Numerical results
Authors: Eidmann K. , Schmalz R. F. , Sigel R.
Published: July 2002

The conversion of laser light into soft x rays during interaction of intense laser light with a planar gold target was investigated numerically with the help of the multi code [Comput. Phys. Commun. 49, 475 (1988)]. It solves one-dimensional hydrodynamics including flux-limited electron heat conduction, multigroup radiation diffusion, and steady-state nonlocal thermodynamic equilibrium radiation physics. The influence of various parameters such as the laser intensity, wavelength, and pulse duration on the conversion efficiency of laser light into x rays was studied. Particular emphasis was placed on comparing the numerical results with the model presented in the preceding paper (Part I) [Phys. Fluids B 2, 199 (1990)]. According to this model the radiating plasma can be divided into a conversion layer and a reemission zone. Its essential features are confirmed by the numerical results.
Radial transport induced by externally excited waves in a magnetic mirror
Authors: Katanuma I. , Kiwamoto Y. , Ichimura M. , Saito T. , Miyoshi S.
Published: July 2002

Analytical investigations are made of perturbed orbits of particles under externally applied wave fields. An equation for radial transport associated with orbit perturbations is derived by assuming stochastic wave–particle interactions in a magnetic mirror field geometry. A Monte Carlo simulation code is developed in flux coordinates in order to study the radial transport numerically. It is found that the results of the analytical and numerical studies are consistent with each other. An important conclusion is that the radial transport resulting from orbit perturbations is not large in present-day tandem mirror experiments.
Plasma-aided manufacturing
Authors: Shohet J. L.
Published: July 2002

Plasma-aided manufacturing is used for producing new materials with unusual and superior properties, for developing new chemical compounds and processes, for machining, and for altering and refining materials and surfaces. Plasma-aided manufacturing has direct applications to semiconductor fabrication, materials synthesis, welding, lighting, polymers, anticorrosion coatings, machine tools, metallurgy, electrical and electronics devices, hazardous waste removal, high-performance ceramics, and many other items in both the high-technology and the more traditional industries.
Sawtooth stabilization by electron cyclotron heating at the q=1 surface in the WT-3 tokamak
Authors: Tanaka S. , Hanada K. , Tanaka H. , Iida M. , Ide S. , Minami T. , Nakamura M. , Maekawa T. , Terumichi Y. , Yamada M. , Manickam J. , White R. B.
Published: July 2002

Sawtooth oscillations (STO) in the Ohmically heated (OH) WT-3 tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1988 (IAEA, Vienna, 1989), Vol. 1, p. 563] are strongly modified or suppressed by localized electron cyclotron resonance heating (ECH) near the q=1 surface, where q refers to the safety factor. The effect of ECH is much stronger when it is applied on the high-field side, as compared to the low-field side. Complete suppression of the STO is achieved for the duration of the ECH when it is applied on the high-field side, in a low-density plasma, provided the ECH power exceeds a threshold value. This threshold decreases with the increasing safety factor at the limiter qL and the complete stabilization of STO can be obtained even when ECH is applied on the low-field side in the high-qL region. It is noticeable that these complete stabilizations can be obtained only by the localized ECH with the elliptic antenna. The STO stabilization is attributed to a modification of the current density profile by high-energy electrons generated by ECH, which reduces the shear in the q=1 region.
The expansion of polarization charge layers into a magnetized vacuum: Theory and computer simulations
Authors: Galvez Miguel , Borovsky Joseph E.
Published: July 2002

When a sufficiently dense plasma stream moves across a magnetic field, the stream will form polarization charge layers and will E×B drift across the field. One charge layer is composed of electrons and the other is composed of ions. The phenomena associated with the expansion of these polarization charge layers along the magnetic field away from the stream is investigated by means of analytic theory and is confirmed by two-dimensional electrostatic particle-in-cell computer simulations. At very early times, the expansion of the electron charge layer is described by single-particle motion in a dipolelike electric field. Eventually, the electron expansion is halted by the net positive charge left behind at the stream. Then an ambipolar expansion of the electron and ion charge layers forms, with the expansion velocity set by the stream voltage rather than by plasma temperatures. Steady decreases in the cross-field-propagation velocities of the streams are observed, owing to the steady losses of charge from the stream edges.
An alpha particle diagnostic based on measurements of lower-hybrid wave fluctuations
Authors: Wong K. L.
Published: July 2002

It is shown that the one-dimensional alpha particle velocity distribution function can be determined from a first-order differential equation based on measurements of lower-hybrid wave fluctuations in an equilibrium plasma. This method uses collective Thomson scattering data from an array of detectors placed on a prescribed curve. It is applicable only when the alpha particles have an isotropic velocity distribution.
Electrostatic convective cell and Alfvén waves
Authors: Ryu Chang-Mo
Published: July 2002

A derivation leading to the dispersion relation for the electrostatic convective cell has been extended to electromagnetic fluctuations. As a result, the dispersion relations for Alfvén waves with the damping rates similar to that of the convective cell are obtained. Furthermore, it is noticed that a dispersion relation for the shear Alfvén wave can be reduced to that of the convective cell, in the limit of k∥=0.
Nonlinear behavior of multiple-helicity resistive interchange modes near marginally stable states
Authors: Sugama Hideo , Nakajima Noriyoshi , Wakatani Masahiro
Published: July 2002

Nonlinear behavior of resistive interchange modes near marginally stable states is theoretically studied under the multiple-helicity condition. Reduced fluid equations in the sheared slab configuration are used in order to treat a local transport problem. With the use of the invariance property of local reduced fluid model equations under a transformation between the modes with different rational surfaces, weakly nonlinear theories for single-helicity modes by Hamaguchi [Phys. Fluids B 1, 1416 (1989)] and Nakajima [Phys. Fluids B 2, 1170 (1990)] are extended to the multiple-helicity case and applied to the resistive interchange modes. Nonlinear amplitude equations of the multiple-helicity modes are derived, from which the convective transport in the saturated state is obtained. It is shown how the convective transport is enhanced by nonlinear interaction between modes with different rational surfaces compared with the single-helicity case. We confirm that theoretical results are in good agreement with direct numerical simulations.
Three-dimensional structure of finite-amplitude ionization wave packets excited in He positive columns
Authors: Ohe K. , Naito A. , Kimura T. , Iwama N.
Published: July 2002

Spatiotemporal structures of large-amplitude ionization wave packets excited in a He positive column are reconstructed by the technique of computerized tomography (CT). The CT approach allows the use of sophisticated techniques to obtain radially resolved spectral images far superior to those from the more usual Abel inversion. Phenomena such as complicated ionization front interactions of the type modeled by Grabec [Phys. Fluids 17, 1834 (1974)] from the Pekarek equations are clearly seen.
Equivalence of resistive evolution and kinetic dynamo models for stochastic magnetic fields
Authors: Miller Guthrie
Published: July 2002

The kinetic dynamo model of Jacobson and Moses [Phys. Rev. A 29, 3335 (1984)] and the resistive evolution model of Miller [Phys. Fluids 31, 1133 (1988)] are compared. Fourier analyzing the x dependence in slab geometry, both models reduce to an Ohm’s law for Fourier components of the form jk=F(k)Ek/η, where jk and Ek are the Fourier components of j∥ and E∥, η is the parallel resistivity, and F(k) is some function of k. The field line distribution function in the resistive evolution model can be chosen to make both models exactly equivalent, and implies that the characteristic field line length over which j∥/B is constant is the electron mean-free path. The time scale for validity of both models is t>τ, where τ is the electron collision time. The stochastic current spreading implied by these models is in rough quantitative agreement with recent measurements.
Unified theory and comparative study of cyclotron masers, ion-channel lasers, and free electron lasers
Authors: Chen K. R. , Dawson J. M. , Lin A. T. , Katsouleas T.
Published: July 2002

A unified small-signal amplification theory is developed to compare growth mechanisms responsible for a number of relativistic radiation generators. The theory is formulated from the basis that the electron resonance frequency produced by the external fields of the devices depends on γ−q, where γ is the beam Lorentz factor and q is a constant (q=1 for cyclotron masers, q=1/2 for ion-channel lasers, and q=0 for free electron lasers). It is concluded that for wave amplification, the sign of the electron mismatch frequency is required to be the same as the sign of bunching parameter that is determined by the total bunching both axial and azimuthal; this depends on the q value. The two bunching mechanisms exist, not only in the single electron resonance regime, but also in the collective gain regime. Competition or reinforcement between the two bunching mechanisms is determined by the q value, the electron axial velocity, and the wave phase velocity.
A sufficient condition for the ideal instability of shear flow with parallel magnetic field
Authors: Chen X. L. , Morrison P. J.
Published: July 2002

A simple sufficient condition is given for the linear ideal instability of plane parallel equilibria with antisymmetric shear flow and symmetric or antisymmetric magnetic field. Application of this condition shows that plane Couette flow, which is stable in the absence of a magnetic field, can be driven unstable by a symmetric magnetic field. Also, although strong magnetic shear can stabilize shear flow with a hyperbolic tangent profile, there exists a range of magnetic shear that causes destabilization.
Nonlinear evolution of resistive modes for q<1 in tokamaks
Authors: Holmes J. A.
Published: July 2002

The nonlinear interaction of tearing instabilities resonant inside the q=1 surface can lead to stochastization of the plasma core without involving the n=1 mode. For certain profiles, the q=1 surface can survive this disruption, so that q<1 remains valid inside. These results are consistent with some experimental sawtooth observations in which q<1 occurs in the core even after the disruption. The competition between this process and the usual resistive reconnection of the (m=1)/(n=1) magnetic island can be sensitive to the initial perturbation as well as to the plasma configuration and profiles.
The connected presheath: One-dimensional models of neighboring objects in magnetized plasmas
Authors: Hutchinson I. H.
Published: July 2002

One-dimensional calculations are presented of plasma flow to multiple objects larger than the ion Larmor radius, when their ion collection presheaths overlap. Situations modeled include the effects of one Langmuir probe on another probe lying on the same field line, and Langmuir probes operated close to a limiter or divertor plate in the edges of magnetic confinement experiments. The results allow quantitative analysis of the operation of Langmuir probes and especially Mach probes for velocity measurements, even when they are close to other structures. New analytic results for inviscid plasmas, and numerical results incorporating shear viscosity are presented. They show that Mach probe measurements in inviscid plasmas experience strong perturbation by connection effects especially for Mach numbers greater than about 0.5. Viscous plasmas are relatively more weakly perturbed, although some distortions occur, especially for large probes. Taking into account these distortions, the viscous model is found to be able to fit recent experimental results.
Non-Maxwellian electron distributions in ionizing plasmas
Authors: Marchand R. , Matte J. P. , Parbhakar K.
Published: July 2002

Electron kinetics is considered in a plasma in which the distribution of ion charge stages is far from the coronal equilibrium. In rapidly ionizing plasmas, radiative cooling and ionization are found to cause the electron distribution to deviate significantly from a Maxwellian. The relevance of such distribution functions to divertor plasmas near the neutralizer plate is discussed.
Comment on ‘‘Gyrokinetics of transverse-magnetic-mode gyrotron, gyropeniotron, cyclotron autoresonance maser, and nonwiggler free-electron laser amplifiers’’ [Phys. Fluids B 1, 2502 (1989)].
Authors: Wang Changbiao
Published: July 2002

In this Comment, it is shown that the comprehensive kinetic theory, gyrokinetics, presented by Zhang, is incorrect. The introduction of the guiding-center shift-caused dipole current, which brings about the so-called gyropeniotron interaction in the transverse magnetic modes, is not justified at all.
Current limit in Ohmically heated high-beta plasmas: Application to field reversed configurations
Authors: Farengo Ricardo , Brooks Robert D.
Published: July 2002

The power balance in slowly formed, Ohmically heated, field reversed configurations is considered. It is shown that a critical current exists above which the radiated power (from impurities) exceeds the Ohmic dissipation. The basic physics is introduced by analyzing the case of a linear Z pinch. Two-dimensional equilibria corresponding to a conventional field reversed configuration and the coaxial slow source [Nucl. Fusion 27, 1478 (1987)] are also considered.
Dynamical picture of rf current drive phenomena in tokamaks
Authors: Uehara Kazuya
Published: July 2002

A dynamical picture of the rf current drive phenomenon is derived according to a Dawson-like treatment of Landau damping instead of the quasilinear theory, in which the parameter dependence of induced current drive efficiency corresponding to the experimental results is estimated. The N∥ dependence indicates that the spectrum gap can be made small for larger temperature, lower density, and higher injected frequency. The drive efficiency is usually proportional to the average electron temperature at the optimized condition. The efficiency increases with density at the lower-density region and decreases with density at the higher-density region. The optimum density for maximum efficiency shifts to the higher-density region with increasing N∥, frequency, and electron temperature. This picture may provide some insight in the understanding of the current drive phenomenon in tokamaks as a zeroth-order approximation.
Beat wave excitation of electron plasma waves in a toroidal magnetized plasma
Authors: Rogers J. H. , Hwang D. Q. , Thomas J. C. , Horton R. L. , Killeen J. , Dimonte G.
Published: July 2002

Electrostatic waves are driven in a toroidal plasma by counterpropagating microwave beams with a difference in frequency approximately equal to the electron plasma frequency. Energetic electrons are detected when the phase velocity of the electrostatic waves are 3ve< vph < 7ve, where ve is the electron thermal velocity. Experiments are performed in the Davis Diverted Torus (DDT) [Bull. Am. Phys. Soc. 33, 2049 (1988)] operating in a high repetition rate (15 Hz), low-density (7×107–2×109 cm−3) mode with only a toroidal magnetic field (∼110 G). The microwaves are triggered 30 μsec after the pulsed discharge ends. At this time the energetic electrons have left the system and the velocity distribution is Maxwellian (Te ∼ 1 eV). The microwaves have tunable frequencies over the range 8.5–9.5 GHz, and peak powers ∼180 kW (400 nsec). Bounded plasma modes are excited when the electron cyclotron frequency is larger than the electron plasma frequency. Direct measurements of the wave vector have been made with a double probe antenna, from which the dispersion relation of the electrostatic wave can be obtained. The electron distribution is measured with an electrostatic energy analyzer. The electron velocity distribution function is found to be constant over an interval that extends well beyond the phase velocity of the wave.
Hermitian description of interacting inhomogeneous electron beams
Authors: Friedland L. , Bers A.
Published: July 2002

Recently developed techniques for finding embedded pairwise mode couplings in high-order systems and calculating the transmission coefficient in linear mode conversion for passive, stable plasmas are extended to the treatment of coupling of positive and negative energy waves in nonuniform plasmas. In particular, the coupling of positive and negative energy waves in costreaming, nonuniform electron beams is treated in detail and the ensuing spatial amplification is determined.
Convection in a cylindrical plasma column with a free boundary
Authors: Gomberoff L. , Gomberoff K.
Published: July 2002

It is shown that in a current-carrying cylindrical plasma with a free boundary, resistivity and thermal conductivity can lead to large-scale steady convection. Convection can take place for any azimuthal wave number m, provided that ‖k‖a≫1 or ‖k‖a≪1, where k is the longitudinal wave number and a is the radius of the plasma column. Otherwise, convection occurs only for large m values. The states that can trigger large-scale stationary convection are incompressible and satisfy the heat conduction equation for arbitrary finite adiabaticity coefficient, γ, provided that η/κ=8π/3, where η is the resistivity and κ is the thermal conductivity. The convection cells are helically twisted tubes, and the number of convection cells is equal to 2m. The situation is very similar to the case when the plasma column is limited by perfectly conducting walls.
Autophase cyclotron autoresonance maser amplifiers
Authors: Menninger W. L. , Danly B. G. , Temkin R. J.
Published: July 2002

The phase stability of the output power from high-power microwave tubes is a critical issue in applications such as electron acceleration and radar. The radio frequency (rf) phase stability of cyclotron autoresonance maser (CARM) amplifiers is investigated, and the phase stability of a CARM amplifier with a suitable correlation between perpendicular momentum and beam energy is shown to result in an order-of-magnitude improvement in phase stability over other rf sources. A technique for producing such a correlation, termed autophasing, is identified.
Time-resolved absorption spectroscopy as a diagnostic of the thermal front zone of laser-illuminated thin plastic foils
Authors: Benattar R. , Sezen A. , Hüller S.
Published: July 2002

A new diagnostic method of the thermal transport zone of a laser-created plasma at short wavelength (0.26 μm) is presented. This method uses soft x-ray spectroscopy and allows one to deduce the temperature of the propagating thermal front during the laser interaction. Time-resolved X–UV spectra of the rear side of thin plastic foils coated on their front side with a thin layer of gold of different thicknesses (50–300 Å) are presented. The layers of gold must be sufficiently thin in order to avoid the fact that their spectrum perturbs the heating of the foil by radiative effects. The plastic thicknesses are chosen less or equal to the ablation thickness of the material for the laser illumination conditions, i.e., about 6 μm. The spectrum emitted by the gold layer plays the role of a backlighter that probes the plastic heated by the laser. Computer simulations including radiation (multi) confirm the weak effect of radiation heating particularly for the thinner layers of gold.
Experimental and theoretical study of the properties of plasma containing a hemispherically focused ion beam
Authors: Peterson J. M. , Oleson N. L.
Published: July 2002

Argon plasmas were produced in a modified multipole plasma device in which the electrons in the two plasmas were isolated by a hemispherical center grid biased negatively. Whenever ion beams are caused to stream from the driver plasma into the target plasma, an additional cold group of Maxwellian electrons were observed in the target plasma. The objective of this paper is to explore the properties of this double-beam plasma (DP) device. Hopefully, this information would provide a reason for the appearance of the cold electron group. To this end, measurements were made of the ion beam densities, energies, and ion temperatures at various axial distances from the hemispheric’s separating grid. Similarly, movable Langmuir probes and hot emissive probes were used to measure the corresponding electron temperatures, densities, and plasma potentials. All that can be said is that these data would suggest that the additional cold group of electrons could result from the interaction of the ion beam with the target electrons in the path of the ion beam. These cold electrons appear to ride along with the ion beam since they are detected throughout the target plasma, since the cold group of electrons are found everywhere in the target plasma.
Evolution of a magnetic field and plasma pushing in the presence of a parallel magnetic field
Authors: Fruchtman A. , Gomberoff K.
Published: July 2002

The evolution in the plasma of a magnetic field that is fast-rising at the plasma boundary, and the simultaneous pushing of the plasma by that magnetic field, are studied for the case that a parallel magnetic field is present in the plasma. It is shown that initially the magnetic field propagates into the plasma in the form of a whistler wave. The magnetic field evolution is then governed by the Schrödinger equation for a free particle, as described in the previous simplified analysis [Phys. Fluids B 3, 1546 (1991)]. Later, the gas-dynamics shock propagation exceeds the magnetic field propagation. If the magnetic field in the plasma is initially oblique and not parallel, a quasiperpendicular fast (super-Alfvénic) shock propagates in the plasma, following a whistler precursor. The width of the current channel is on the scale of the ion skin depth.
Axial magnetic field generation by ponderomotive force in a laser-produced plasma
Authors: Srivastava M. K. , Lawande S. V. , Khan Manoranjan , Das Chandra , Chakraborty B.
Published: July 2002

Axial magnetic field generation in a plasma due to ponderomotive action of an obliquely incident spatially inhomogeneous laser beam is discussed. The theoretical analysis shows that the field can be excited by the collisional as well as the noncollisional processes via ponderomotive action. The collisional contribution, estimated to be in the range of kilogauss, is dominant for lower intensity (I) and shorter wavelength (λ) lasers and scales as λ−2 for both the s- and p-polarized beams. The noncollisional process can excite the field to 0.5–0.6 MG for a p-polarized intense and longer wavelength laser beam which scales as I4/3λ14/3. Faraday rotation is estimated theoretically to highlight the significance of ponderomotive effect in generating the axial field. Finally, implications of the model are discussed along with its limitations in the context of laser–plasma experiments.
A self-consistent theory of collective alpha particle losses induced by Alfvénic turbulence
Authors: Biglari H. , Diamond P. H.
Published: July 2002

The nonlinear dynamics of kinetic Alfvén waves, resonantly excited by energetic ions/alpha particles, is investigated. It is shown that α particles govern both linear instability and nonlinear saturation dynamics, while the background magnetohydrodynamic turbulence results only in a nonlinear real frequency shift. The most efficient saturation mechanism is found to be self-induced profile modification. Expressions for the fluctuation amplitudes and the α-particle radial flux are self-consistently derived. The work represents the first self-consistent, analytical, turbulent treatment of collective α-particle losses by Alfvénic fluctuations.
On the fast penetration of magnetic field into plasma
Authors: Kalda Jaan
Published: July 2002

The fast penetration of a strong magnetic field into a plasma, caused by the Hall effect, is studied. The phenomenon is likely to govern the early evolution of the magnetic field not only in two-dimensional geometry, to which previous studies were restricted, but also in three dimensions. This penetration mechanism is suppressed, if both the plasma and electrodes are collisionless, vAeτe≳a, where vAe is the electron Alfvén velocity, τe−1 the collision frequency, and a the characteristic space-scale. However, the collisionless penetration can occur in the case of a resistive cathode. The self-consistent effect of ion motion is shown to be of importance only in a very special geometry. Then the penetration speed is limited by the value VA(aωpi/c)−1/3, where VA is the Alfvén velocity and c/ωpi the ion skin depth.
Neoclassical viscosity of a tokamak plasma with large mass flow
Authors: Chang C. S.
Published: July 2002

Neoclassical ion viscosity of a steady tokamak plasma with large mass flow is calculated in a general geometry. In addition to the usual ‘‘resonant’’ contribution, the ‘‘nonresonant’’ contribution has been evaluated. It is found that the nonresonant contribution is not small compared to the resonant contribution when the poloidal rotation speed is large. The viscosity evaluation has been reduced to one-dimensional integrals. Furthermore, simple analytic formula for the viscosity coefficients are suggested. The velocity strain, which is the source of the viscosity, is also analyzed in detail.
Correlation between backscattered 3ω/2 emission and stimulated Brillouin scattered light in laser-produced plasmas
Authors: Young P. E.
Published: July 2002

Evidence is presented that ion-acoustic waves associated with backscattered laser light are responsible for the observation of the blue-shifted feature in the backscatter direction. Backscattered light near the incident laser frequency, ω0, and near three-halves of the incident laser frequency has been spectrally and temporally resolved. The ratio of intensities of the red-shifted and blue-shifted 3ω0/2 features associated with the two-plasmon decay instability is a function of the incident laser intensity and the laser spot size. The intensity of the blue-shifted 3ω0/2 feature correlates well with the intensity of the backscattered light near ω0.
The nonlinear analysis of self-field effects in free-electron lasers
Authors: Freund H. P. , Jackson R. H. , Pershing D. E.
Published: July 2002

A model of the self-fields associated with the charge density and current of the electron beam is incorporated into three-dimensional nonlinear formulations of the interaction in free-electron lasers for both planar and helical wiggler configurations. The model assumes the existence of a cylindrically symmetric electron beam with a flat-top density profile and a uniform axial velocity, and the self-electric and self-magnetic fields are determined from Poisson’s equation and Ampère’s law. Diamagnetic and paramagnetic effects due the electron beam interaction with the wiggler field are neglected; hence, the model breaks down when the wiggler-induced transverse displacement is comparable to the beam radius. The nonlinear formulations are based upon the arachne and wigglin codes, which represent slow-time-scale formulations for the evolution of the amplitudes and phases of a multimode superposition of vacuum waveguide modes. The electron dynamics in these codes are treated by means of the complete three-dimensional Lorentz force equations, and the representations for the self-fields are incorporated directly into this formulation. The results of the simulations are compared directly with an experiment at Lawrence Livermore National Laboratory based upon a planar wiggler and experiments at the Massachusetts Institute of Technology and the Naval Research Laboratory, which employed helical wigglers. These experiments employed intense electron beams with current densities of 200–1200 A/cm2 and comparable space-charge depressions of Δγself/γ0≊0.53%–0.78% across the beam. The simulations are in reasonable agreement with the experiments, and indicate that the self-fields tend to (1) reduce saturation efficiencies and (2) enhance beam spreading depending upon the magnitude of external beam focusing.
Observation of coherent radiation at the electron plasma frequency from a tokamak
Authors: Gandy R. F. , Yates Daniel H.
Published: July 2002

High-resolution spectral measurements have been made of the fluctuating electron plasma frequency (ωpe) radiation from Alcator C. Results of spectral measurements with a dispersive delay line indicate that one or more very narrow, irregularly spaced components are present in each ‘‘burst’’ of ωpe emission. A direct-sampling technique with an instrumental resolution of 200 kHz has been used to resolve the individual features of a burst. Features as narrow as 350 kHz have been observed (Δf/f=6×10−6). This result suggests that a highly coherent process is responsible for the emission.
Radio-frequency current drive using α particles
Authors: Hamnén Håkan
Published: July 2002

In this brief communication, a recently proposed method for current drive in tokamak fusion reactors is analyzed. The method depends on Landau or transit-time damping of fusion-generated alpha particles. Assuming a narrow spectrum of parallel wave phase velocities, it is shown that the efficiency of this current drive scheme scales as I/P∼αT3/2/nRVph. Typical reactor parameters yield I/P≂0.2 A/W.
Long-wave instability at the interface between two viscous fluids: Thin layer effects
Authors: Hooper A. P.
Published: July 2002

The stability of the interface between two viscous fluids is considered when the depth of the lower fluid is much less than the depth of the upper fluid. A long wavelength perturbation scheme is used to solve the linear stability problem and the equation governing the nonlinear evolution of the interface is deduced. The exact dispersion relation is derived for arbitrary values of wavelength and then simplified for large wavelength values. It is found that the flow is always linearly unstable when the lower fluid is also the more viscous fluid.
Motion of a strong shock wave in an exponential medium
Authors: Yousaf M.
Published: July 2002

In this paper, corrections to the Chester, Chisnell, and Whitham characteristic exponent have been computed for the motion of a strong shock wave in a medium with exponentially varying density and ray-tube area. The similarity solution of Hayes is used to find the various interaction terms at different points of the flow.
Comments on ‘‘Transport associated with the collisionless detrapping/retrapping orbits in a nonaxisymmetric torus’’ [Phys. Fluids 2
7, 1 (1984)] Fluids 2
7, 1 (1984)]
Authors: Mynick Harry E. , Hitchon W. N. G.
Published: July 2002

A comment on charged particle transport associated with the collisionless detrapping/retrapping orbits in a nonaxisymmetric torus is presented. (AIP)
A nonlinear study of Kelvin–Helmholtz stability
Authors: Hsieh D. Y. , Chen Fengsu
Published: July 2002

The Kelvin–Helmholtz stability problem is studied by employing the variational method. With the restriction to a single dominant spatial mode, a set of fully nonlinear evolution equations is derived. The limiting states of these evolution equations are discussed and their stability is analyzed. It is found that at the critical point for the linear stability problem, another stable limiting state with finite amplitude appears. A nonlinear sinusoidal wave state is also found. The nonlinear dispersion relation for this wave is derived. The wavenumber of the dominant spatial mode and its evolution with time can also be determined with the aid of the variational method.
Electrostatic confinement in a bumpy torus
Authors: El Nadi A. M.
Published: July 2002

In a closed-field-line device such as a bumpy torus, the combined E×B and ∇B drifts lead to charge separation that is balanced by the ion polarization drift. In this work, we determine self-consistent potential and density profiles and the condition for electric island formation.
The influence of initial condition on the linear stability of time-dependent circular Couette flow
Authors: Chen J.-C. , Neitzel G. P. , Jankowski D. F.
Published: July 2002

The effect of the starting condition on the linear stability properties of circular Couette flow with a time-dependent inner-cylinder motion is investigated. In addition to the WKBJ approach employed previously by Eagles [Proc. R. Soc. London, Ser. A 355, 209 (1977)] for slowly varying flow, an initial-value method is also used. Results are presented for different stability criteria.
Interaction of the precessional wave with free-boundary Alfvén surface waves in tandem mirrors
Authors: Berk H. L. , Kaiser T. B.
Published: July 2002

A symmetric tandem mirror plugging a long central cell, with plugs stabilized by a hot-component plasma is considered. The system is taken to have a flat pressure profile with a steep edge gradient. The interaction of the precessional mode with Alfvén waves generated in the central cell is then considered. This analysis is noneikonal and is valid when mΔ/r<1 (m is the azimuthal mode number, r the plasma radius, and Δ the radial gradient scale length) for long-wavelength radial modes. Without finite-Larmor-radius (FLR) effects the precessional mode is always destabilized by the excitation of the Alfvén waves for m≥2. For m=1, it is possible to achieve stabilization with conducting walls. A discussion is given of how FLR affects stabilization of the m≥2 long-wavelength modes and of FLR stabilization of modes described in the eikonal approximation.
Gallery of fluid motion
Authors: Reed Helen L.
Published: July 2002

An annual fluid-mechanics photography contest has been established for the November meetings of the American Physical Society, Division of Fluid Dynamics. This article displays the winning photographs from the past two years.
Nonlinear resistive helical states in reversed-field pinches
Authors: Dagazian R. Y.
Published: July 2002

Time-independent nonlinear states resulting from resistive modes with helical symmetry are shown to exist in reversed-field pinch configurations. The importance of flow effects in such states is demonstrated, and the nonlinear redistribution of magnetic flux caused by the presence of the deformation is discussed.
Stability of rotating liquid drops: II. Homogeneously charged or self-gravitating drops
Authors: Luyten P. J. , Callebaut D. K.
Published: July 2002

The oscillations and instabilities of a uniformly rotating and homogeneously charged or self-gravitating liquid spheroid with surface tension are investigated. The dispersion relation is obtained by means of an appropriate energy integral better suited to yield the higher modes than the virial method. Criteria are derived for the onset of secular instability that indicate bifurcation toward nonrotational symmetric equilibrium configurations, and of dynamical stability. In the oblate case the validity of the spheroidal approximation can be verified both quantitatively and qualitatively by comparison with two limiting cases: an uncharged or nongravitating rotating drop and a rotating drop without surface tension (astronomical limit). For the prolate spheroids there exists a clear difference between rotating and nonrotating configurations, for the equilibrium as well as for the stability.
A helical axis stellarator equilibrium model
Authors: Koniges A. E. , Johnson J. L.
Published: July 2002

An asymptotic model is developed to study magnetohydrodynamic equilibria in toroidal systems with a helical magnetic axis. Using a characteristic coordinate system based on the vacuum field lines, the equilibrium problem is reduced to a two-dimensional generalized partial differential equation of the Grad–Shafranov type. A stellarator-expansion free-boundary equilibrium code is modified to solve the helical-axis equations. The expansion model is used to predict the equilibrium properties of toroidal asperator devices. Numerically determined flux surfaces, magnetic well, transform, and shear are presented. The equilibria show a toroidal Shafranov shift.
Control of tearing mode instabilities by localized electron-cyclotron wave heating
Authors: De Luca F. , Jacchia A. , Lazzaro E.
Published: July 2002

Resistive tearing mode instabilities are driven by unfavorable current gradients at the position where the safety factor q is rational. Their dynamics in the presence of radio-frequency (rf) power absorption in the electron-cyclotron wavelength range is investigated by means of a hybrid transport-magnetohydrodynamic (MHD) code and analytic considerations. A panorama of possibilities of stabilization and control of disruptions in tokamaks is presented.
Three-wave resonant interactions and multiple resonances in two-layer and three-layer flows
Authors: Tsutahara Michihisa , Hashimoto Kiyoshi
Published: July 2002

The nonlinear interactions of waves on the interfaces of two- and three-layer flows are studied. It is shown that in both flows there are resonant triads in which three waves have the resonant interactions, and that these triads have interactions through their common waves. This is called the multiple resonance. The total energy of the waves which take part in the multiple resonance is conserved; however, it is not conserved in each triad, which is not the case when the triad does not have interaction with other triads, and the energy of a triad is transferred to others. The solutions of the evolution equations of the amplitudes are studied analytically and numerically. It is shown that soliton solutions exist, and that in some cases the evolution of the amplitudes becomes irregular and chaotic. In the three-layer flow the interactions are either of the ‘‘explosive type’’ or of the ‘‘decay type.’’ These types can be switched by time from one to the other. On the other hand, all of the interactions in the two-layer flow are of the ‘‘decay type.’’
Radiation by electron bunching in large-amplitude Langmuir waves
Authors: Weatherall James C. , Hobbs Willard E.
Published: July 2002

Strong plasma turbulence is produced by electron beams in plasmas and can result in intense broadband radiation above the plasma frequency. A possible mechanism for producing plasma frequency harmonics is the high-frequency convective bunching of electrons in large-amplitude Langmuir waves. Using a model based on a localized, monochromatic wave solution, results for the spectrum of the radiation and its spatial distribution are derived.
Magnetic field compression and evolution in laser-produced plasma expansions
Authors: Kacenjar S. , Hausman M. , Keskinen M. , Ali A. W. , Grun J. , Manka C. K. , McLean E. A. , Ripin B. H.
Published: July 2002

The evolution of a magnetic bubble resulting from the expansion of Nd-laser-generated plasma into a photoionized magnetized background plasma is examined experimentally and is compared with theory and computer simulations. The initial laser-produced plasma speed is greater than the plasma sound and Alfvén speeds and is energetic enough to be unmagnetized; the background plasma is effectively magnetized and its density is varied from the collisionless to the collisional regimes. The data support theoretical predictions that the initial expansion of the magnetic bubble is dominated by the uncoupled laser-produced plasma.
Spatial viscous instability of the incompressible Falkner–Skan similarity profile at separation
Authors: Wazzan A. R. , Taghavi H. , Pafford D.
Published: July 2002

The spatial viscous instability of the Falkner–Skan similarity profile at separation to Tollmien–Schlichting waves is computed numerically. The computed viscous eigenvalues tend, with increasing Reynolds number R, to the corresponding inviscid values. For (R=finite, αc→ 0), computations show the real αr, and imaginary αi, parts of the wavenumber approach a constant nonzero value but that the real cr, and imaginary ci, parts of the wave speed approach zero. For (R→∞ and αc→ 0), however, (αi, αr, cr, and ci)→ 0.
Laser-induced volume gratings
Authors: Marlow W. C.
Published: July 2002

The equations governing the motion and thermodynamic state of a gas that absorbs energy from an electromagnetic field, interfering with itself within that gas, are linearized and solved in closed form. The dependence of the solution on the geometrical configuration of the gas enclosure, on the polarization state of the incident electromagnetic field, and on the various time constants of the system are derived. The relevant time constants of the system include the pulse duration of the electromagnetic field, the characteristic times for photon absorption, and for transfer of the absorbed energy into translational energy modes, the times associated with the diffusion of heat and vorticity, and the time required for pressure pulses to transit the distance between the laser-induced grating planes. The space and time variations of the mass density of a mixture of gaseous ammonia and nitrogen resulting from energy absorbed from a pulsed carbon dioxide laser are calculated to illustrate the characteristics of the solution.
Confinement properties of a low-beta discharge in a spindle cusp magnetic field
Authors: Bosch Robert A. , Merlino Robert L.
Published: July 2002

An experimental study of the confinement properties of a low-(average) beta discharge plasma in a spindle cusp magnetic field is described. Electron and ion densities, space potentials, and plasma flow velocities were measured in the ring and point cusps. The leak width of the escaping plasma was measured over a large range of magnetic field strengths and neutral pressures. Leak widths varying between the hybrid gyrodiameter and the ion gyrodiameter were observed. The dependence of the leak width on neutral pressure and magnetic field is accounted for by a simple model in which plasma diffuses across the magnetic field as a result of neutral particle collisions while streaming out of the cusps along the magnetic field lines. At the lowest neutral pressures investigated, the scaling of the leak width with magnetic field strength suggests that noncollisional mechanisms (e.g., Bohm diffusion) determine the leak width.
Second-order modeling of near-wall turbulence
Authors: Shih T-H. , Lumley J. L.
Published: July 2002

A new model form of the wall-proximity effect on the pressure-strain correlation term is carefully derived based on realizability. All other models used in this paper also satisfy realizability. As a test flow for a complete set of second-order realizable models, the turbulent boundary layer with zero-pressure gradient was calculated. Numerical solutions of the mean velocity, component energies, shear stress, and dissipation of the turbulence energy are obtained, which are in excellent agreement with experiment.
Stability of centrifugal separation
Authors: Sulsky D.
Published: July 2002

A fluid-particle mixture occupying the interior of a long cylindrical centrifuge, initially in solid-body rotation with the container, undergoes a transient process which leads to a final state consisting of separated fluid and particles in rigid rotation. The stability of this transient process is examined. It is found to be unstable to infinitesimal disturbances unless a diffusion term proportional to the gradient of volume fraction is added to the equations.
Radial potential formation with unstable waves in magnetized coaxial cylindrical double plasmas
Authors: Fujita Hiroharu , Handa Hiroshi , Yagura Shinya
Published: July 2002

A potential jump or double layer in the radial direction is formed by applying a potential difference between coaxial cylindrical double plasmas in a magnetic field. The potential profile is easily controlled with both hill- and well-type structures. The radial electric field E creates increases up to about 100 V/cm in the plasma. The unstable waves caused by drift motion of the potential and density gradients are observed to propagate in the E×B or ∇n×B direction. The calculation considering collisions of electrons and ions with neutral particles is found to fully account for the observed waves and reveals that the growth rate of the instability maximizes when the parallel wavenumber is equal to zero and the electric field affects the instability more strongly than the density gradient.
Quasilinear evolution of the self-filamentation instability
Authors: Hassam A. B.
Published: July 2002

A thermally inhomogeneous plasma immersed in a strong uniform magnetic field may be unstable to the formation of current filaments aligned to the field propagating at the diamagnetic drift frequency. The quasilinear evolution of this self-filamentation is studied. Transverse magnetic fields generated by the currents bring into thermal contact unequal temperature surfaces leading to a relaxation of the thermal gradient. For weak gradients, the instability is shown to saturate at a constant amplitude while, for stronger gradients, periodic behavior is exhibited. The transition from saturation to periodic behavior is predicted analytically from a three-wave model. The anomalous heat transport resulting from the instability is assessed.
Diffusion and transport of a fully collisional plasma
Authors: Rosenau Philip , Turkel Eli
Published: July 2002

The equations governing the diffusion and transport of a fully collisional plasma across a strong magnetic field in a bounded domain are analyzed. Following a relatively short relaxation time, the diffusion exhibits universal properties independent of the choice of initial data. Mathematically this appears as a time asymptotic solution which is space-time separable. The temporal decay rate is a nonlinear eigenvalue which is found via the solution of a related eigenvalue problem. This determines the spatial distribution of both the particle density and the pressure. Some of the transport effects caused by Bremsstrahlung radiation, particle, and heat injection are considered, and the conditions under which the system evolves into an equilibrium are examined.
Uniformity and isotropy of the fast ion emission at a target surface irradiated by a CO2 laser
Authors: Decoste R. , Pascale D. , Kieffer J-C. , Pépin H. , Lavigne P.
Published: July 2002

Filtered pinhole imaging is used to characterize spatially the fast ion emission at the target surface irradiated by a CO2 laser in a planar geometry. The bulk of the fast ions is found to be fairly well correlated with the lateral transport of hot electrons in the target plane, i.e., a large concentration of rather low-energy ions near the focal spot with total ion emission over a very large area. Higher-energy ions show large emission nonuniformities and very small local cone angles somewhat difficult to explain.
Stability of high-beta plasmas with helical magnetic axis
Authors: Mond Michael
Published: July 2002

The stability of high-beta plasmas with helical magnetic axis is studied by investigating the linearized magnetohydrodynamic equations. It is shown that the stability of shearless configurations is determined by an ordinary differential equation that describes modes with large variations across the flux surfaces.
Melting and freezing at constant speed
Authors: Keller Joseph B.
Published: July 2002

A solution of a one-dimensional melting or freezing problem (i.e., Stefan problem) is found in which the front moves at constant speed.