bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimal growth of small disturbances in pipe Poiseuille flow
Authors: Bergström Lars
Published: July 2002

A theoretical study is made of initial algebraic growth for small angular-dependent disturbances in pipe Poiseuille flow. The analysis is based on the homogeneous equation for the pressure for which the eigenvalue problem is solved numerically. In the limit of small streamwise wave numbers asymptotic results for the eigenvalues are derived. On the basis of the modes of the system, which are all damped, the initial value problem is considered and in particular the largest possible growth of the disturbance energy density is determined following the ideas of Butler and Farrell [Phys. Fluids A 4, 1637 (1992)]. The results show that a large amplification of the disturbance energy is possible. The largest amplification is obtained for disturbances with a small streamwise wave number and with an azimuthal wave number of one. The energy growth is then only due to the growth of the streamwise disturbance component. However, for disturbances of shorter wavelength, the energy growth is also substantial and not only concentrated to the streamwise velocity component. The wall shear corresponding to disturbances with the largest energy growth also shows a large amplification and the dependence of wave numbers and the Reynolds number is the same as for the energy. However, the wall pressure of a long wavelength disturbance of the largest growth just decays from its initial value, but for disturbances of shorter wavelength, it is also amplified.
Evolution of short gravity waves on long gravity waves
Authors: Naciri Mamoun , Mei Chiang C.
Published: July 2002

The evolution of short gravity waves on long gravity waves on the surface of deep water is studied. Both wave trains are assumed to be irrotational, mild in slope, and slowly modulated in space and time, but their scales are so different that the short wavelength is very much less than the long-wave amplitude. Here, it is shown that the use of Lagrangian instead of the usual Eulerian coordinates is advantageous for yielding analytical results. Linear and nonlinear evolutions of short waves over intermediate and very long distances are discussed.
Evidence for the modification of half-harmonic spectral shifts by plasma flow
Authors: Gabl E. F. , Berger R. L. , Busch Gar. E. , Campbell P. M. , Schroeder R. J. , Shepard C. L. , Tarvin J. A.
Published: July 2002

The (3)/(2) ω0 harmonic emission has been observed from a laser-produced plasma using exploding foil targets illuminated by 1.053 μm laser light with a 300 psec square pulse. The shift of the measured wavelength from (2)/(3) λ0 is found to depend markedly on the target thickness. Variation of the flow velocity and thus the Doppler shifts of the wave frequency with target thickness is found to be more important than variation of the electron temperature. Adjusting the measured wavelength by Doppler shifts calculated with flow velocities from simulations brings the measurements from different target thicknesses into agreement. These results indicate that previous predictions of electron temperatures from analyses of half-harmonic spectra from inertial confinement fusion plasmas are up to a factor of 2 too small.
Radiation transport effects in heavy-ion beam–target interaction studies: Measurement of target opacity and beam conversion efficiency
Authors: Tahir N. A. , Arnold R. C.
Published: July 2002

In this paper detailed simulations are presented of radiation-hydrodynamic response of gaseous cylindrical targets irradiated with heavy-ion beams that will be produced at the Gesellschaft für Schwerionenforschung, Darmstadt, using a heavy-ion synchrotron (SIS) [Heavy Ion Fusion, AIP Conference Proceedings No. 152 (AIP, New York, 1986), p. 23]. The purpose of this work is to explore material conditions for which the thermal radiation effects can be maximized. This is desirable in order to study a number of interesting and important effects including maximization of conversion efficiency of the ion beam energy to thermal radiation and measurement of the target opacity in the SIS experiments. It is expected that the SIS beams will produce a specific deposition power of 10 TW/g. The simulations in this paper show that a temperature of the order of 10 eV could be achieved by the SIS beams using homogeneous, cylindrical Xe targets. It has been shown that with the help of these computer simulations one should be able to measure the target opacity in these experiments within a factor of 3. Also these calculations show that in the SIS experiments one should be able to have a 50% conversion efficiency using a Xe target under optimum conditions. It has been found that the radiation effects will be optimized in the SIS experiments if the initial target density is of the order of 10−3 g/cm3. If the initial density is too high (of the order of 10−1 g/cm3 or more), hydrodynamic effects will dominate, while, on the other hand, if the initial density is too low (of the order of 10−4 g/cm3 or less), the electron thermal conductivity will take over.
Long-term development of elongated tokamak plasmas after failure of feedback stabilization
Authors: Jensen Torkil H. , Chu Ming S.
Published: July 2002

Tokamaks with cross sections elongated in the axial direction are subject to an instability that basically involves an axial displacement of the plasma. This instability can be stabilized by a feedback circuit. This Brief Communication deals with the long-term development of the plasma after a failure of the feedback circuit during an otherwise normal discharge. The short-term (linear) development of such a plasma was considered earlier by Yokomizo et al. [Nucl. Fusion 23, 1593 (1983)]. During the long-term development, the plasma makes intimate contact with the surrounding conducting shell (vacuum vessel). The timescale for this development is long compared to the Alfvèn time, so that it is appropriate to consider the plasma in a magnetohydrodynamic (MHD) equlibrium which evolves slowly. This equilibrium (assumed axisymmetric) is unusual in that currents and forces are exchanged between the shell and the plasma. The dynamics of the equilibrium is determined by Ohmic dissipation associated with the plasma currents and the toroidal and poloidal currents of the shell. For a ‘‘typical’’ large tokamak, it is found that dissipation in the shell may dominate. Modeling of such long-term development may be important because the forces acting on the shell due to the shell currents may be large. It may be important that dissipation in the plasma can be neglected or that it is small in such modeling since the uncertainty of the results, due to uncertainties associated with the plasma, is absent or may be small.
Depletion and diffraction of an electromagnetic wiggler field during the free electron laser interaction
Authors: Similon P. L. , Wurtele J. S.
Published: July 2002

The self-consistent interaction of an electron beam with an electromagnetic wiggler and a radiation field is analyzed. Equations are derived for the depletion and diffraction of the wiggler wave. The formalism yields local conservation laws for the system energy flux and photon flux. Using a one-dimensional approximation, pump depletion is examined for pulsed and steady-state electron beams. A three-dimensional analysis shows that the wiggler diffraction is dominated by the nonresonant interaction of the electrons with the wiggler wave. This is studied in detail for a specific electron pulse shape. It is concluded that wiggler depletion and diffraction should not substantially degrade the free electron laser interaction for experimentally obtainable system parameters.
The behavior of magneto-acoustic-gravity waves near the cusp resonance in a lossless, compressible, isothermal, stratified, electrically conducting, and uniformly magnetized atmosphere. I. Mode conversion approach
Authors: Kamp Leon P. J.
Published: July 2002

In the present paper the propagation of (internal) magneto-acoustic-gravity waves is analyzed in a compressible, isothermal, stratified, electrically conducting atmosphere that is permeated by a uniform, nearly horizontal magnetic field. The conversion, near the so-called cusp resonance of a long acoustic-gravity wave into a short slow magneto-acoustic wave, is demonstrated by means of boundary layer theory based on the smallness of the vertical component of the magnetic field. The magneto-acoustic wave subsequently carries the energy off upward, which in the limit of a horizontal magnetic field would be fed into the cusp resonance ad infinitum. This gives rise to singular fields. The scaling of the tendency toward singular behavior of a field quantity with the obliqueness of the magnetic field is discussed, as well as the partial reflection of the long acoustic-gravity wave.
Wave-dynamical analysis of conversion and absorption of oblique extraordinary and Bernstein modes near the second electron-cyclotron harmonic
Authors: Petrillo V. , Lampis G. , Maroli C. , Riccardi C.
Published: July 2002

The propagation and absorption of oblique second-harmonic electron-cyclotron waves in a plasma slab are studied by means of propagation equations directly deduced from the Vlasov–Maxwell system. An electromagnetic wave injected from the vacuum into the plasma can be transformed into a quasielectrostatic forward mode, which, in turn, is converted into a Bernstein backward wave. The occurrence of this double conversion process and the global absorption and reflection are analyzed, and the dependence of these phenomena on the plasma parameters and on the wave characteristics is investigated.
Onset of laser light filamentation in preformed plasmas
Authors: Havazelet D. , Capjack C. E. , Shvarts D. , Marchand R.
Published: July 2002

The onset of laser beam filamentation in a preformed nonuniform plasma is investigated by using a two-dimensional computer code in which time-dependent plasma hydrodynamics and heat transport effects are accounted for. Laser beam propagation is modeled using a paraxial wave approximation—refraction, diffraction, absorption, and ponderomotive force effects are accounted for. The simulations reveal details by which multifilamentary plasma and laser beam structures evolve in the region near the critical density surface. For the particular case of a 1.05 μm laser beam with an intensity of 1015 W/cm2 incident on a long scale length preformed plasma, a consistent treatment of the ponderomotive forces and of the plasma dynamics is found to be essential in determining the evolution of the filamentary structures.
X-ray line emission from laser-produced spherical plasma flows
Authors: Mostacci Domiziano , Montierth Leland M. , Dinguirard Jean-Pierre , Morse Richard L.
Published: July 2002

A numerical model has been developed for calculating x-ray line emission from spherical plasma flows and has been applied to a laser-heated, ablating aluminum target. The results are in good agreement with previous experimental work. The intensity ratios of the dominant lines are predicted in good agreement with experiment. A novel effect, not yet observed, is predicted in the interaction of nearby lines, here an Hα doublet: through Doppler shift caused by plasma flow one line absorbs the other. This line obscuring effect has potential applications to diagnosing details of ablative flows in x-ray lasing and inertial confinement fusion experiments. The main features of the model are plasma parameters obtained through a Lagrangian hydrodynamics model, radiative transport by multienergy and multi-impact parameter ray groups, and self-consistent, time-dependent computation of ion and ion energy-level populations. Approximately 100 energy groups per line resolve line structure and show Doppler effects. The scalings involved in extending this work to other plasma conditions and materials are discussed.
Global and local relaxations in magnetohydrodynamic plasmas
Authors: Sato Tetsuya , Horiuchi Ritoku , Kusano Kanya
Published: July 2002

Based on theoretical and computational considerations, it is confirmed that magnetic reconnection driven nonlinearly by a kink instability plays a key role in the magnetohydrodynamic relaxation. An important finding is that the relaxation takes the form of either global relaxation (e.g., the reversed field pinch relaxation) or local relaxation (e.g., the tokamak sawtooth relaxation), depending on whether the driven reconnection is total (antiparallel) or partial (partially antiparallel).
Observation of unstable waves around a dipole magnet in a weakly magnetized plasma
Authors: Yagura Shinya , Fujita Hiroharu
Published: July 2002

Spatially localized unstable waves are observed around a dipole magnet in a low-density plasma. The observed wave frequency (≂150–200 kHz) with a high fluctuation level (≂20%) is much higher than an ion-cyclotron frequency. Correlation measurements confirm azimuthal propagation of the unstable wave on the axis of the dipole magnet. The calculation using a fluid model is found to account qualitatively for the results, and predicts that the waves are excited by two stream instabilities ascribed to an electron drift caused by the density and the potential gradients perpendicular to the magnetic field.
Energy deposition and shock wave evolution from laser-generated plasma expansions
Authors: MacFarlane J. J. , Moses G. A. , Peterson R. R.
Published: July 2002

One-dimensional radiation-hydrodynamics calculations have been performed to study the structure and evolution of blast waves resulting from laser-generated plasma expansions. The results are compared with experimental data obtained at the Naval Research Laboratory [Laser Interaction and Related Plasma Phenomena (Plenum, New York, 1986), Vol. 7, pp. 857–877]. In these calculations, target ions expand into a stationary background gas with an initial pressure ranging from 25 mTorr–5 Torr. Because the stopping ranges of the target ions are comparable to the dimensions of the shock, the blast wave properties are sensitive to the ion stopping cross sections. In some calculations, the effects of radiation transport were neglected in order to assess its importance. The results presented in this paper are in generally good agreement with available experimental data, and indicate that the observed blast wave properties can be accounted for by using a classical collision model to describe the interaction between the target ions and the background plasma.
Role of neutrals in plasma expansion in ion diodes
Authors: Litwin Christof , Maron Yitzhak
Published: July 2002

An attempt to understand the mechanism of fast anode plasma motion across the magnetic field, which has been observed in recent experiments, is presented. For a fully ionized anode plasma it is seen that electron heating imposes an upper limit on the expansion rate. This limit is much lower than the observed rate of expansion. A model based on a hot, fast-expanding neutral layer is considered. This layer is created by charge exchange processes near the anode and is subsequently ionized by energetic ions and the ensuing electron avalanche. The resulting plasma parameters are calculated and found to be in reasonable agreement with the observations.
Emission of ion and electron cyclotron harmonic radiation from mode conversion layers
Authors: Cho Suwon , Swanson D. G.
Published: July 2002

The asymmetry of cyclotron radiation from a mode conversion layer is presented for harmonics of the ion cyclotron frequency and the second harmonic of the electron cyclotron frequency for weakly relativistic electrons. The same form of Kirchhoff’s law is found for all cases, relating the emission along each branch to the absorption of an incident wave along the corresponding branch. Results show that the fast wave radiation is more strongly asymmetric at the third harmonic than at the second harmonic of the ion cyclotron frequency, while the slow wave radiation ratio is about same. At the second cyclotron harmonic of weakly relativistic electrons, the asymmetry of radiation is found to be small at high temperature. The effect of equilibrium Bernstein wave radiation is also discussed.
A hyperbolic model for the one-dimensional electromagnetic pulse
Authors: Bampi F. , Zordan C.
Published: July 2002

An account of the current phenomenology relative to the electromagnetic pulse is obtained by having recourse to the model of a charged fluid. In particular, a differential system is deduced for the description of a one-dimensional electric field generated by the so-called Compton current. The system turns out to be hyperbolic, thus allowing for discontinuity waves. The evolution law for the amplitude of such waves is examined in detail. Mathematical problems resulting from vanishing of the electric field ahead of the wave front are also investigated.
Stable single mode operation of a quasioptical gyrotron
Authors: Antonsen T. M. , Levush B. , Manheimer W. M.
Published: July 2002

The regions in parameter space of stable single mode operation of a quasioptical gyrotron are determined theoretically for a number of configurations. A pencil beam is found to give rise to large operating regimes where single mode operation is stable. These regions shrink dramatically when an annular beam is considered. However, stable operation can be restored by slightly tilting the axis of the cavity.
Nonlinear hydromagnetic waves in a cylindrical current-carrying plasma. I. Exact translationally and axially symmetric solutions
Authors: Hamabata Hiromitsu
Published: July 2002

Exact wave solutions of the nonlinear magnetohydrodynamic equations for a highly conducting incompressible fluid within an axisymmetric container are obtained for the case when the physical quantities are independent of the axial coordinate or the azimuthal angle in the cylindrical coordinates. The solutions represent waves propagating in the azimuthal or axial direction under the influence of helical magnetic and velocity fields whose strength varies with radius. The solutions are applicable to astrophysical and laboratory plasmas as well as the Earth’s core.
The effect of viscosity on the resistive tearing mode with the presence of shear flow
Authors: Chen X. L. , Morrison P. J.
Published: July 2002

The effect of small viscosity on the ‘‘constant ψ’’ tearing mode in the presence of shear flow is analyzed using the boundary-layer approach. It is found that the influence of viscosity depends upon the parameter G′(0)/F′(0), where G′(0) and F′(0) denote the flow shear and magnetic field shear at the magnetic null plane, respectively. When ‖G′(0)/F′(0)‖≪1, the tearing mode growth rate is suppressed by the viscosity, but not completely stabilized. When ‖G′(0)/F′(0)‖∼𝒪(1) and the viscosity is comparable to the resistivity, the growth rate vanishes as [1−G′(0)2/F′(0)2]1/3, when G′(0)2→F′(0)2 from below. In the case where 1−[G′(0)2/F′(0)2]<0 matching cannot be achieved and the tearing mode vanishes.
Ion-end-loss rate for rf-plugged mirrors and its application to electrostatically plugged mirrors
Authors: Sadamoto Yoshiro
Published: July 2002

Improvement of plasma confinement attributed to both ion heating and quasipotential is experimentally investigated by imposing rf electric fields with the ion-cyclotron range of frequency near both ends and one end of a minimum-B mirror, respectively. An expression of the ion-end-loss rate for rf-plugged mirrors is compared with the experimental results for one-end plugging by the quasipotential. The expression agrees with a Pastukhov for large potentials. The same expression applied to end losses from electrostatically plugged mirrors shows a good fit to the Fokker–Planck and Monte Carlo code results within the accuracies for both plug potential and mirror ratio and is good for arbitrary values of plug potential and at least for mirror ratios larger than or equal to 2.
Results for resistive compressible flow ballooning modes in a large aspect ratio expansion
Authors: Grassie K. , Krech M.
Published: July 2002

Based on an eikonal approach and the corresponding expansion in the inverse toroidal mode number 1/n large aspect ratio expanded equations for linear resistive, compressible ballooning modes including toroidal, sheared flow are derived. Limitations of this formalism leading to a nonexponential time behavior of perturbed quantities are discussed and a procedure for the extraction of an effective growth rate is offered. The ballooning equations are solved numerically for parameter values corresponding to a typical ASDEX tokamak [Nucl. Fusion 25, 705 (1985)] high beta discharge. The importance of the flow shear on the stabilization of ballooning modes compared to purely rigid rotation is demonstrated. Furthermore, results for perturbed quantities in physical space are given and their dependence on flow is investigated.
Current drive by wave helicity injection
Authors: Chan V. S. , Miller R. L. , Ohkawa T.
Published: July 2002

It is shown that a steady-state current can be sustained by the injection of wave helicity. In generalizing to waves of arbitrary polarizations and damping directions, the wave helicity has to include both a magnetic and a kinetic component. A physical picture is presented describing competing effects on current generation, and a generalized helicity conservation equation is derived that encompasses all the results discovered.
Theory of energetic ion transport induced by waves of ion cyclotron range of frequencies in a tokamak plasma
Authors: Chang C. S. , Lee June-Yub , Weitzner Harold
Published: July 2002

A theoretical description of radial energetic-ion transport induced by electromagnetic waves of ion cyclotron range of frequencies (ICRF) is presented in a tokamak with general flux surface geometry. A known form of quasilinear operator is used to represent the ICRF wave-induced particle scattering in velocity space. The theory is applied to both nonflowing and flowing groups of energetic ions, represented by alpha particles born from deuterium–tritium reactions, and energetic ions born from unbalanced neutral beam injection, respectively. It is found that ICRF waves with reasonable power can be used for manipulation of the local radial energetic ion flow at practical level. A simple integral representation of the radial fluxes is suggested for implementation into a transport code. A brief discussion on the possibility of the strong negative electric field generation by ICRH at the plasma edge is also presented.
The properties of the frequency splitting of the axisymmetric magnetoacoustic wave and an application to current profile diagnosis
Authors: Borg G. G. , de Wit T. Dudok
Published: July 2002

In the presence of finite plasma current, the axisymmetric (m=0) magnetoacoustic wave resonance exhibits a frequency splitting for finite toroidal mode number (n≠0) between oppositely directed traveling waves. Calculations are presented that demonstrate that Δ(1/f), the difference between the inverse resonance frequencies, is closely proportional to known moments of the plasma current profile and, in the limit where the radial wave number is much larger than the parallel wave number, is independent of the plasma mass density and its profile. A discussion of the observability of the splitting for the purpose of current profile diagnosis, given the theoretical and experimental data available, is also presented.
Motion and equilibrium of a spheromak in a toroidal flux conserver
Authors: Brown M. R. , Cutrer D. M. , Bellan P. M.
Published: July 2002

A number of experiments have been performed on spheromaks injected into the empty vacuum vessel of the Caltech ENCORE tokamak (i.e., without tokamak plasma) [Phys. Rev. Lett. 64, 2144 (1990); Phys. Fluids B 2, 1306 (1990)]. Magnetic probe arrays (in a number of configurations) have been used to make single shot, unaveraged, in situ measurements of the spheromak equilibrium. These measurements are important because (i) they reveal for the first time the equilibrium structure of spheromaks in a toroidal geometry, (ii) they provide a reliable estimate of magnetic helicity and energy of spheromak plasmas used in injection experiments [Phys. Rev. Lett. 64, 2144 (1990)], and (iii) they constitute the first measurements of spheromak motion across and interaction with static magnetic fields (which are useful in corroborating recent theories). Probe measurements in the tokamak dc toroidal field show for the first time that the spheromak exhibits a ‘‘double tilt.’’

The spheromak first tilts while in the cylindrical entrance region, emerging into the tokamak vessel antialigned to the dc toroidal field, then expands into the tokamak vacuum vessel, and finally tilts again to form an oblate (nonaxisymmetric, m=1) configuration. In addition, the spheromak drifts vertically in the direction given by Jcenter×Btok, where Jcenter is the unbalanced poloidal current that threads the center of the spheromak torus. Probe arrays at different toroidal locations show that the spheromak shifts toroidally (horizontally left or right) in the direction opposite that of the static toroidal field. In the absence of toroidal flux, the m=1 object develops a helical pitch, the sense of the pitch depending on the sign of the spheromak helicity. The spheromak equilibrium in the toroidal vessel is well fit by a pressureless infinite cylindrical model; however, there is evidence of deviation from m=1 symmetry because of toroidal effects, nonuniform J/B profile, and finite β. Experiments performed in a test facility consisting of the spheromak gun and a replica of the entrance region (with a closed end) show that the spheromak is generated with its axis coaxial with that of the gun. Coherent, m=2 magnetic modes are observed during the formation stage rotating in the E×B direction at about 125 kHz (rotation velocity corresponding to 40% of the Alfvén speed).
Coherent structures in ηi-mode turbulence
Authors: Hong B. G. , Romanelli F. , Ottaviani M.
Published: July 2002

Numerical simulations of the ion-temperature-gradient-driven mode (ηi- mode) show that long-lived, large-scale coherent structures exist and play an important role in the study of the anomalous transport. In this work, the stationary solutions of the nonlinear equations describing the dynamics of the ηi -mode are investigated. The simplest solution is found to be a dipolar vortex in the shearless case. In the presence of the magnetic shear, a modified dipolar vortex solution is found through perturbation theory.
Stability regimes in a helical quadrupole focusing accelerator−theory and simulation
Authors: Krall J. , Tang C. M. , Joyce G. , Sprangle P.
Published: July 2002

The inclusion of helical quadrupole (stellarator) and axial guide fields in high current spiral line or recirculating accelerator configurations provides a high tolerance to energy mismatch and confines the beam against space charge forces. Such a configuration is studied via three-dimensional particle simulation. It is found that the electron beam centroid motion can interact with the external fields and with the TE11 waveguide mode to obtain (i) highly unstable particle orbits, (ii) the electromagnetic three-wave instability, or (iii) a fully stable beam. Theoretical stability conditions are presented in the limit of zero beam current and are found to be good predictors of simulation results. Minor departures from the stability conditions at high current are discussed in terms of the full linear dispersion relation. Saturation of the instability without beam loss is observed in some cases. This is the result of detuning in the relativistic cyclotron wave number as the beam energy and axial velocity decrease. The effectiveness of this detuning mechanism is evaluated for the various regimes.
Control of energetic ion confinement by ion cyclotron range of frequency waves
Authors: Chang C. S.
Published: July 2002

It is shown that ion cyclotron range of frequency (ICRF) waves can induce fast convective radial transport of energetic ions in a tokamak geometry without affecting the background ion transport. Poloidally inhomogeneous ICRF waves with directional parallel wave vectors are needed; otherwise, a new parallel flow of the energetic ions has to be present.
Reconnection of magnetic field lines through an electrostatic wave
Authors: Kleva Robert G.
Published: July 2002

The effect of a short-wavelength electrostatic wave on the rate of reconnection of magnetic field lines by a growing long-wavelength tearing mode is investigated. The influence of the electrostatic wave depends strongly on the parity of the wave potential about the resonant surface of the tearing mode and on the relative magnitude of the plasma resistivity η and viscosity μ, as well as on the amplitude of the wave. In general, by coupling the growing long-wavelength tearing mode with stable short-wavelength modes, the electrostatic wave has a stabilizing influence on the long-wavelength tearing mode. However, when the electrostatic wave potential is odd about the resonant surface, the same parity as the scalar potential associated with the tearing mode, and μ≪η, then there is a large increase in the growth rate of the tearing mode because of the presence of a large-amplitude electrostatic wave. Furthermore, a tearing mode that is stable in the absence of the wave can actually be destabilized by the wave. This increase in the growth rate appears to be due to a forced reconnection of opposing magnetic field lines by the flow associated with the electrostatic wave.
Tokamak magnetic islands in the presence of nonaxisymmetric perturbations
Authors: Reiman A. H.
Published: July 2002

The effects of a small, externally imposed, nonaxisymmetric magnetic field perturbation on magnetic islands are studied analytically, assuming zero β, tokamak ordering, and narrow islands. For the tearing stable case, the conditions under which the self-consistent plasma response is self-healing or amplifying are elucidated. For the tearing unstable case, the quasilinear theory of tearing modes is extended to a description of locked modes.
Spectral emission diagnostics using oxygen impurities in optically thin plasmas
Authors: Dunning M. J. , Cerjan C. , Molvik A. W. , Nash J. K. , Phillips T. W. , Vitello P. A.
Published: July 2002

Emission from oxygen in the vacuum ultraviolet (VUV) spectral range from 140 to 260 Å is evaluated as a diagnostic for optically thin plasmas. A one-dimensional Lagrangian, two-fluid hydrodynamic simulation code, which is self-consistently coupled to a comprehensive atomic rate equation model is used for the analysis. This model with the associated atomic rates is described and compared to recent data from laboratory experiments.
Tilt stability of a gyroviscous field-reversed configuration with realistic equilibria
Authors: Ishida A. , Kanno R. , Steinhauer L. C.
Published: July 2002

The gyroviscous fluid theory [L. C. Steinhauer and A. Ishida, Phys. Fluids B 2, 2422 (1990)] is applied to the tilting instability of field-reversed configurations (FRC) using realistic equilibria and a more complete basis set than in the previous treatment. This leads to two important new results. (1) Quantitative agreement is found for the first time between experiment and the theory of FRC tilting stability, i.e., the stability of nearly all FRCs can be explained by the gyroviscous theory. (2) Quantitative agreement (within 30%) is also found between the gyroviscous theory (with modifications to account approximately for parallel kinetics and the Hall effect) and the more complete—but harder to apply—Vlasov-fluid model.
Gyrokinetics in the neutral sheet geometry
Authors: Mu Jian-lin , Tsai Shih-tung
Published: July 2002

A derivation of the gyrokinetic equation is presented which employs a set of gyrokinetic variables suitable for the neutral sheet geometry where the usual theories are not valid. A closed set of equations is obtained for the study of linear electromagnetic stability problems. Since the present theory avoids treating the complicated particle orbit directly, it can be used to study the plasma behavior in the neutral sheet geometry in space and the laboratory, once the distribution function and magnetic geometry are given.
Effects of bounce-resonance damping on the harmonics of a plasma microinstability
Authors: Koepke Mark E.
Published: July 2002

Coupling is observed between harmonics of unstable drift cyclotron loss-cone waves in an axisymmetric mirror trap. The behavior of the nonlinear interaction is apparent from differences in average fluctuation amplitudes between discharges in which the fundamental frequency is varied and from the evolution in fluctuation amplitudes during a discharge. The coupling occurs only when the fundamental mode is suppressed by electron bounce-resonance Landau damping. The evolution in fluctuation amplitudes may be related to observed changes in this damping during the discharge.
Experimental and numerical study of the plasma drift effect on upper-hybrid resonance cones
Authors: Pierre Th. , Rohde V. , Piel A.
Published: July 2002

The experimental and numerical study of the radiation pattern around a point source antenna immersed in a hot drifting magnetoplasma (the so-called resonance cones) at frequencies lying inside the upper branch shows that the electron drift velocity can be detected through the determination of the cone angles shifted downstream by the Doppler effect. The easiest measurement is performed at the plasma frequency. The drift velocity is also compared to the value given by the electron saturation current of a rotatable plane Langmuir probe.
Dynamical evolution and structure of solar coronal magnetic fields
Authors: Longcope D. W. , Sudan R. N.
Published: July 2002

A simple dynamical system is presented which is analogous in many ways to a solar coronal loop. When this system is subject to slow, external driving, representing photospheric motion, it undergoes occasional impulsive relaxations. These relaxations release some fraction of the magnetic energy as Alfvén waves in the loop. When the model system is integrated for long times it reaches a statistical steady state in which relaxation events occur at random times with random amplitudes. The distributions of these quantities is compared to similar distributions for microflares occurring in the solar corona.
Ion energetics in collisionless sheaths of rf process plasmas
Authors: Hamaguchi S. , Farouki R. T. , Dalvie M.
Published: July 2002

Ion energy distribution functions in collisionless radio-frequency (rf) sheaths are discussed from the viewpoint of kinetic theory. Effects of rf fields on ion density and velocity profiles in plasma sheaths are also derived, based on ion fluid equations. It is shown that the ponderomotive force due to rf modulation of the magnitude of the sheath electric field exerts a retarding effect on the ion motion that counteracts the dc-bias field when the ratio of the ion transit frequency ωtr to rf modulation frequency ω is small but finite. Consequently, the time-averaged ion density is higher and the time-averaged ion fluid velocity is lower in rf sheaths by the order of (ωtr/ω)2 than those in corresponding dc sheaths. The influence of an oscillating plasma/sheath boundary on the ion energetics is also considered. Under suitable conditions, this induces rapid ‘‘quasiperiodic’’ variations in the ion energy distribution as the rf ω is increased.
On stability of strongly nonlinear plasma oscillations
Authors: Hada T. , Nambu M.
Published: July 2002

The modulational instability of finite-amplitude, longitudinal, nonlinear plasma oscillations in a cold plasma is discussed. Stable and unstable regimes appear alternatively as the wave nonlinear parameter ε varies. A possible laboratory experiment that may confirm this result is suggested.
Exact solutions for a system of nonlinear plasma fluid equations
Authors: Prahović M. G. , Hazeltine R. D. , Morrison P. J.
Published: July 2002

A method is presented for constructing exact solutions to a system of nonlinear plasma fluid equations that combines the physics of reduced magnetohydrodynamics and the electrostatic drift-wave description of the Charney–Hasegawa–Mima equation. The system has nonlinearities that take the form of Poisson brackets involving the fluid field variables. The method relies on modifying a class of simple equilibrium solutions, but no approximations are made. A distinguishing feature is that the original nonlinear problem is reduced to the solution of two linear partial differential equations, one fourth order and the other first order. The first-order equation has Hamiltonian characteristics and is easily integrated, supplying information about the propagation of solutions.
Multispecies transport theory for axisymmetric rotating plasmas
Authors: Tessarotto Massimo , White Roscoe B.
Published: July 2002

A reduced gyrokinetic equation is derived for a multispecies toroidal axisymmetric plasma with arbitrary toroidal differential rotation speeds and in the presence of a finite induced electric field. The kinetic equation obtained, extending previous results obtained by Hinton and Wong [Phys. Fluids 28, 3082 (1985)] and by Catto et al. [Phys. Fluids 30, 2784 (1987)], has a form suited for transport applications, via variational techniques; in particular it exhibits the feature that all source terms, including the Spitzer source term, carrying the contribution due to the inductive electric field, appear to be acted upon by the collision operator. Moreover, the equation displays a new contribution due to ‘‘explicit’’ velocity perturbations, here proven to be consistent with transport ordering, whose evaluation appears relevant for transport calculations. In addition, general expressions are obtained for the neoclassical fluxes in terms of a variational principle, as well as for the classical ones, retaining, in both cases, the contributions due to Spitzer’s inductive terms.
The scaling of relativistic double-layer widths: Poisson–Vlasov solutions and particle-in-cell simulations
Authors: Sulkanen Martin E. , Borovsky Joseph E.
Published: July 2002

In this paper the study of relativistic plasma double layers is described through the solution of the one-dimensional, unmagnetized, steady-state Poisson–Vlasov equations and by means of one-dimensional, unmagnetized, particle-in-cell simulations. The thickness versus potential-drop scaling law is extended to relativistic potential drops and relativistic plasma temperatures. The transition in the scaling law for ‘‘strong’’ double layers suggested by analytical two-beam models by Carlqvist [Astrophys. Space Sci. 87, 21 (1982)] is confirmed, and causality problems of standard double-layer simulation techniques applied to relativistic plasma systems are discussed.
Kolmogorov spectra of Langmuir and optical turbulence
Authors: Falkovich G. E. , Ryzhenkova I. V.
Published: July 2002

Weak developed turbulence in the framework of both scalar and vector nonlinear Schrödinger equations is considered. It corresponds to waves with a quadratic dispersion law ωk=ω0+βk2 and with a zero scaling exponent of the interaction coefficient. The consideration thus embraces the turbulence of envelopes (usually referred to as optical turbulence) as well as Langmuir turbulence in nonisothermal plasma and other examples. Steady spectra of turbulence are shown to be close to Kolmogorov-like cascade spectra with the fluxes of energy and wave action.
Electromagnetic instability of the ion-focused regime
Authors: Whittum David H.
Published: July 2002

A relativistic electron beam propagating through a wide plasma channel in the ion-focused regime exhibits an electromagnetic instability coupled to the ‘‘betatron’’ motion, with peak growth rate near a resonant angular frequency ω∼2γ2ωβ, where γ is the Lorentz factor for the beam, and ωβ is the angular frequency of transverse oscillations. An eikonal formalism is derived analogous to that for the free-electron laser and the dispersion relation is determined. Constraints due to energy spread and other sources of detuning are noted. Analytic scaling laws are compared to the results of numerical simulation for a practical example corresponding to a microwave amplifier. Constraints due to competing instabilities are noted and the extension to short wavelengths is discussed.
Quasilinear driven transport in a sheared flow field
Authors: Riedel K. S.
Published: July 2002

The evolution of a passive scalar field is considered for a slowly varying stratified medium, which is convected in an incompressible sheared flow with many overlapping static flux islands. Within the quasilinear/random phase approximation, a multiple scale expansion is made. Because of the rapid spatial variation of the temperature, the ‘‘ensemble’’ averaged slowly varying part of the solution is not described by the arithmetic average of the oscillatory evolution equation. The standard Markovian and continuum approximations are shown to be invalid. For times of order N, where there are O(N2) excited modes, most of the time-dependent perturbation phase mixes away and the fluid reaches a new saturated state with small time oscillations about the temperature. This saturated state has smaller resonance layers (corresponding to magnetic islands), than those that occur in the isolated resonant perturbation case. Thus the quasilinear response to the resonant interactions reduces the effective size of the perturbations. The temperature gradient of the saturated state vanishes at all the excited resonance surfaces but has a nonzero average. Thus either the quasilinear approximation ceases to be valid on long time scales or the fluid remains essentially in this modified equilibrium and does not evolve diffusively. Thus collisionless, driftless fast particles will not be lost rapidly in equilibria with many small islands.
Effect of magnetic and density fluctuations on the propagation of lower hybrid waves in tokamaks
Authors: Vahala George , Vahala Linda , Bonoli Paul T.
Published: July 2002

Lower hybrid waves have been used extensively for plasma heating, current drive, and ramp-up as well as sawteeth stabilization. The wave kinetic equation for lower hybrid wave propagation is extended to include the effects of both magnetic and density fluctuations. This integral equation is then solved by Monte Carlo procedures for a toroidal plasma. It is shown that even for magnetic/density fluctuation levels on the order of 10−4, there are significant magnetic fluctuation effects on the wave power deposition into the plasma. This effect is quite pronounced if the magnetic fluctuation spectrum is peaked within the plasma. For Alcator-C-Mod [I. H. Hutchinson and the Alcator Group, Proceedings of the IEEE 13th Symposium on Fusion Engineering (IEEE, New York, 1990), Cat. No. 89CH 2820-9, p. 13] parameters, it seems possible to be able to infer information on internal magnetic fluctuations from hard x-ray data—especially since the effects of fluctuations on electron power density can explain the hard x-ray data from the JT-60 tokamak [H. Kishimoto and JT-60 Team, in Plasma Physics and Controlled Fusion (International Atomic Energy Agency, Vienna, 1989), Vol. I, p. 67].
A model of beam–plasma discharge in a nonuniform plasma
Authors: Sotnikov V. I , Omelchenko Yu. A. , Shapiro V. D. , Shevchenko V. I. , Ashour-Abdalla M. , Schriver D.
Published: July 2002

This paper presents a model of beam–plasma discharge resulting from the quasilinear heating of plasma electrons by Langmuir waves which are excited by beam–plasma interactions. The heating is made possible by the spectral transformation of waves propagating radially from the central beam-occupied region toward the region of lower plasma density. In this paper equations describing the wave spectral density, the distribution function of a high-energy electron tail, and its stationary density profile are obtained and numerically solved; to do so a balanced diffusion and ionization is assumed. The possibility of significant plasma density enhancement in beam–plasma discharge is demonstrated.
Explicit expression for growth rates in free-electron lasers with helical and axial magnetic fields
Authors: Demokan O. , Kabak Y.
Published: July 2002

The dispersion relations of transverse and longitudinal modes in a free-electron laser with helical and axial magnetic fields are derived exactly for the one-dimensional case. The modes with positive and negative helicity are distinguished clearly. An analytically solvable expression for the growth rate of the modes, in resonance with the beam, is obtained. The dependence of the growth rate on system parameters is studied for broad ranges of values and comments on optimization are made.
Electron acceleration in propagating electron plasma waves
Authors: Aithal Shridhar , Pépin Henri
Published: July 2002

Electrons of 2.3 MeV energy and light in the wavelength range from 2λ0 to 1.28λ0 scattered in the direction of incident laser have been observed from the plasmas of exploding thin foils created by a carbon dioxide laser. The correlation measurements of electrons and light, performed independently, pointed to a common physical origin for these near the quarter-critical density region of the plasma where low phase velocity plasma waves are driven. This also indicated that low-density forward Raman scattering with high phase velocity plasma waves may not be principally responsible for the energetic electrons. In order to interpret the experimental results a simple computer calculation has been performed obtaining the final energies of electrons trapped in freely propagating electron plasma waves with increasing phase velocity.
Electromotive excitation of a plasma sheath
Authors: Carter M. D. , Batchelor D. B. , Jaeger E. F.
Published: July 2002

The heating of the electrons in a plasma by radio frequency (rf) fields results in an electron distribution function that can be highly non-Maxwellian. The response of these heated electrons can lead to a substantially larger total potential drop than that obtained by arbitrarily choosing a single-temperature parameter to describe the electron distribution in a traditional Boltzmann approximation. Electromotively driven rf fields can have scale lengths along a static magnetic field line that are much larger than a Debye length and produce electron–sheath interactions that are best described as collisions. Estimates of the floating potential of a plasma that is excited by electromotively driven rf plasma currents parallel to a strong static magnetic field have been made using a Fermi acceleration heating model and linear estimates for the rf fields with plasma. When this non-Maxwellian distribution function is used for estimates of the static potential, the results show that the total potential drop is proportional to the rf oscillation energy and the square root of the ion to electron mass ratio. Numerical solutions to Poisson’s equation are presented for non-Boltzmann electrons. Analytic estimates of parasitic rf power absorption and edge profile modification using a Bohm diffusion model are presented. Estimates for a purely electrostatic heating operator with rf fields that are assumed to scale as the Debye length along static magnetic field lines are also made for comparison with the electromagnetic result.
Efficiency of combined cyclotron–Čerenkov interaction between electrons and electromagnetic fields
Authors: Nusinovich G. S. , Vlasov A. N.
Published: July 2002

A theory is presented describing the electron cyclotron interaction at frequencies near cutoff, followed by a Čerenkov interaction region. In such a case, the cyclotron interaction withdraws only the orbital component of electron momentum, while in the Čerenkov interaction the electrons lose their axial momentum. It is shown that the addition of the Čerenkov interaction significantly enhances the total electronic efficiency. Since both kinds of operation are relatively insensitive to electron velocity spread, the efficiency of the combined interaction is also rather tolerant to velocity spread. Thus, rather efficient sources of electromagnetic radiation based on poor quality electron beams may be developed.