bluuebunny/crossref_metadata_2025 huggingface dataset:
Trkal flows in magnetohydrodynamics
Authors: Tasso H.
Published: July 2002

Force free fields can be generalized to Trkal flows in magnetohydrodynamics if a special velocity field parallel to the magnetic field is introduced. Such flows decay exponentially in time in case a constant viscosity and a constant resistivity are added. The stability of generalized Trkal flows is investigated and a sufficient condition for nonlinear stability is derived. For low velocities, this condition reduces essentially to the nonlinear sufficient stability condition for force free fields, previously found by the author.
A self-consistent quasistatic equilibrium for non-neutral diamagnetic electron vortices
Authors: Church B. W. , Sudan R. N.
Published: July 2002

A self-consistent quasistatic equilibrium for a non-neutral cylindrical electron vortex has been found using the two-dimensional relativistic electron fluid equations. While other work on electron vortices considered a regime where the vortex radius is much smaller than the collisionless skin depth λ=c/ωp, this equilibrium is valid for large-radius, diamagnetic vortices and predicts a maximum radius of 23/2λ for a highly relativistic electron vortex. The vortex model shows good agreement with observations of diamagnetic electron vortices in two-dimensional electromagnetic particle-in-cell simulations of magnetically insulated transmission lines.
Optimization studies of magnicon efficiency
Authors: Hafizi B. , Gold S. H.
Published: July 2002

Results from a time-dependent analysis of the electron–wave interaction in the output cavity of a frequency-doubling magnicon amplifier operating in X band are presented. The multiregion cavity contains a single rotating transverse magnetic, TM210, or transverse electric, TE211, eigenmode. The analysis models the synchronous interaction of electrons with the mode, allowing for slow-time evolution of the radio-frequency field due to a gyroresonant mechanism. The electrons entering the output cavity represent a beam that has been spun up by the gain cavities. For the case of an ideal beam in a simple, closed cavity, the optimized efficiencies for TM210 or TE211 operation are nearly the same (≊50% for a 1/2 MV, 180 A, α=1 beam, where α is the ratio of transverse to parallel velocity). For an ideal beam in the realistic field of the complex cavity system, the optimized efficiency for TM mode operation increases to ≊57%, while for TE operation it is reduced to ≊37%. With a finite beam radius of 2.75 mm, the TM210 mode efficiency is reduced, dropping to a value nearly the same as for an ideal beam in a closed cavity. For the TE211 mode, the finite radius has a minor effect on the efficiency obtained for the realistic cavity. The sensitivity of the TM210 mode efficiency to spreads in α, energy, and gyroangle is displayed graphically.
Coupled transverse modulational instability of electron cyclotron waves
Authors: Yang Kwang-Sup
Published: July 2002

Transverse modulation instabilities for single and double electron cyclotron waves in finite and infinite media are investigated. This research shows that there is a intensity threshold that depends on the strength of external magnetic field. The growth rate of instability decreases as the external magnetic field strength increases. In finite media, the threshold for absolute transverse modulational instability of counterpropagating waves is increased by the external magnetic field in the small transverse wave number region. However, in large wave number region, the threshold depends scarcely on the external magnetic field.
Fluid simulations of nonlocal dissipative drift-wave turbulence
Authors: Xu X. Q. , Cohen R. H. , Crotinger J. A. , Shestakov A. I.
Published: July 2002

A two-dimensional [2d(x,y)] fluid code has been developed to explore nonlocal dissipative drift-wave turbulence and anomalous transport. In order to obtain steady-state turbulence, the y-averaged fluctuating density 〈n〉 has been forced to be zero in simulations, thus the difficulty of choosing proper sources and sinks in turbulence simulation codes has been avoided. If Ln≫Lc or Lαlc≫Lc, where Ln is the density gradient scale length, Lc the turbulence correlation length Lc, and Lαlc the adiabaticity-layer width, it has been shown that ‘‘local’’ turbulence simulations give reasonable results. However, for Ln∼Lc, or Lαlc∼Lc ‘‘local’’ turbulence codes are found to overestimate the flux. For a family of hyperbolic tangent background density profiles, n0(x)=nm−n1 tanh[(2x−Lx)/2Δn] with n1<0.5nm, it has been demonstrated that the nonlocality of the turbulence leads to a transition from local gyro-Bohm (Dlocal≂7.6(Te/eB)[ρs/Ln(x)] [αlc(x)/0.01]−1/3), where αlc(x)=α(x)/κ(x)<1, to nonlocal gyro-Bohm transport scaling [Dnonlocal≂7.6(Te/eB)(n1ρs/nmΔn) (αnlc/0.01)−1/3(Δn/40ρs)2/5 for αnlc(x)=α/κmax<1, κ(x)=ρs/Ln(x) and α=k2∥χe]. For the case Φ0(x)=−n0(x) with the model hyperbolic tangent density profiles n0(x), velocity shear increases the turbulence flux by 230% and the root-mean-square (RMS) fluctuating density by 36%. Otherwise, for Φ0(x)=n0(x), the turbulence flux is reduced by 71% and the RMS value of fluctuating density is decreased by 31% by velocity shear effects.
Millimeter wave scattering off a whistler and lower hybrid waves in a tokamak
Authors: Sawhney B. K. , Tripathi V. K. , Singh S. V.
Published: July 2002

Obliquely propagating whistler and lower hybrid waves through a plasma cause density perturbations. A high-frequency electromagnetic wave sent into such a perturbed region suffers scattering. Theory of electromagnetic wave scattering has been developed in a tokamak where the density profile is taken as parabolic. The wave equation has been solved to evaluate the expression for the amplitude of a scattered wave. For typical parameters of the Princeton Large Torus (PLT) [Bernabei et al., Bull. Am. Phys. Soc. 26, 975 (1981)], the ratio of the power of the scattered to the incident electromagnetic wave comes out to be of the order of 10−4 and 10−5 in cases of scattering off whistler and off lower hybrid waves, respectively, which can be detected; the method can, hence, be used for the diagnostics of the waves.
Stabilization and saturation of the ideal tilt mode in a driven annular field-reversed configuration
Authors: Pierce W. F. , Jarboe T. R. , Brooks R. D.
Published: July 2002

The coaxial slow source (CSS) is a device that was designed as a means of forming annular field-reversed configurations (AFRCs) for magnetic fusion on relatively slow (50–100 μs) time scales. Such configurations are predicted to be ideally unstable to magnetohydrodynamic tilting modes in which one-half of the torus shifts in the positive axial direction and the other half in the negative direction. These instabilities have been observed to grow and saturate at finite amplitude in the CSS as formation progresses. Stable operating regimes of fill pressure/loop voltage space have also been found. The relatively cold temperature (∼2 eV) of the plasma gives rise to a high classical resistivity, placing the Lundquist number (ratio of resistive to Alfvén time scales) between 0.5 and 10.0. It is proposed that resistivity, along with other dissipation mechanisms such as viscosity and ion–neutral friction, are responsible for the observed stability. Experiments were performed to explore these stability issues. A nonlinear, dynamical systems stability model that accounts for the effects of inductive current drive and dissipation has also been developed, and shows some agreement with the experimental results. The findings of this study are significant for two reasons: (1) It is one of the few clear observations of tilting modes in a FRC-like geometry; and (2) it provides an experimental study of ideally unstable modes in a configuration that does not escape the low Lundquist number regime, finding that the configuration appears stable below a Lundquist number of 0.5.
Stabilization of sausage and kink instability modes of a plasma pinch by radial oscillations
Authors: Bud’ko A. B. , Kravchenko Yu. P. , Liberman M. A.
Published: July 2002

The growth of the global sausage (m=0) and kink (m=1) perturbations of a Z-pinch subject to radial oscillations is considered. It is demonstrated that the oscillations result in significant reduction of the growth rate of both kink and sausage instability modes with wavelengths long compared to the pinch radius. The analysis of stability is carried out in two ways. The first method is based on the averaging magnetohydrodynamic equations over the period of radial oscillations. The second one consists in the analysis of the growth of Fourier-components of perturbations. Numerical simulation demonstrates that even moderate radial oscillations cause reduction of the growth rate of long-wavelength sausage instabilities and complete stabilization of long kinks. This can be understood as a result of the effective gravitational field produced in the pinch by the oscillations. The effect in question can explain the anomalous stability of pinches with respect to the kink perturbations observed in experiments.
X-ray spectra from high-intensity subpicosecond laser produced plasmas
Authors: Teubner U. , Wülker C. , Theobald W. , Förster E.
Published: July 2002

The interaction of high-intensity subpicosecond KrF*-laser pulses with aluminum plasmas has been investigated. The laser pulse absorption and the x-ray production have been related at a series of intensities up to 1017 W/cm2 and different angles of incidence. Soft x-ray spectra were measured in the wavelength range from 7 to 150 Å with various spectrographs. The spectra show distinct dependences on the laser intensity, the polarization, and the angle of incidence. The observed dependence of the spectra on the intensity and on the angle of incidence is the result of both the angular dependence of the laser pulse absorption and that of the conversion efficiency of the absorbed energy into soft x-ray emission. The spectra are influenced by the size of the absorption volume, the electron density where the absorption mainly takes place, and other parameters, such as the electron temperature.
Modeling of sawtooth destabilization during radio-frequency heating experiments in tokamak plasmas
Authors: McClements K. G. , Dendy R. O. , Hastie R. J. , Martin T. J.
Published: July 2002

Sawtooth oscillations in tokamaks have been stabilized using ion cyclotron resonance heating (ICRH), but often reappear while ICRH continues. It is shown that the reappearance of sawteeth during one particular ICRH discharge in the Joint European Torus (JET) [Campbell et al., Phys. Rev. Lett. 60, 2148 (1988)] was correlated with a change of sign in the energy δW associated with m=1 internal kink displacements. To compute δW, a new analytical model is used for the distribution function of heated minority ions, which is consistent with Fokker–Planck simulations of ICRH. Minority ions have a stabilizing influence, arising from third adiabatic invariant conservation, but also contribute to a destabilizing shift of magnetic flux surfaces. As the minor radius of the q=1 surface rises, the stabilizing influence of minority ions diminishes, and the shape of the plasma cross section becomes increasingly important. It is shown that an increase in ICRH power can destabilize the kink mode: this is consistent with observations of sawteeth in JET discharges with varying levels of ICRH. It is suggested that the sawtooth-free period could be prolonged by minimizing the vertical extent of the ICRH power deposition profile.1996 American Institute of Physics
Stability of intense laser propagation in an underdense hollow channel plasma
Authors: Chiou T. C. , Katsouleas T. , Mori W. B.
Published: July 2002

The stability of very intense laser pulses (I≲1018 W/cm2) as they propagate through and are guided by a hollow channel in an underdense plasma is investigated. It is shown analytically and in two-dimensional (2-D) particle-in-cell (PIC) simulations that instabilities [Raman forward scatter (RFS)] would completely destroy the beam in homogeneous plasmas or parabolic channel plasmas. However, in hollow channel plasmas these instabilities can be completely suppressed. The results suggest that hollow plasma channels may be effective in transporting ultra-intense lasers over many Rayleigh lengths.
Momentum and energy exchange between species of a multicomponent gas mixture due to inelastic and reactive collisions
Authors: Benilov M. S.
Published: July 2002

Formulas are derived for the rates of change of the number density, momentum, and energy of species due to inelastic and reactive collisions, valid for finite values of the Mach number of the relative motion of the reacting species and for different temperatures of the species. The Wang Chang-Uhlenbeck-de Boer approach is employed. With the aim to make the obtained results more practicable, explicit formulas are given relating the introduced kinetic coefficients to the reaction rate constant, rather than to the reaction cross section.
Collisional effect on the magnetohydrodynamic modes of low frequency
Authors: Zheng L.-J. , Tessarotto M.
Published: July 2002

The kinetic singular layer equation is derived from the ballooning representation formalism. The mode frequency and the ion-ion collision frequency are assumed to be lower than the ion transit frequency. It is proven that the balance between the instability drives and the small-parallel-ion-velocity (SPIV) effect, instead of inertia effect, has to be imposed on determining the eigenfrequency. Consequently, it is found that the resistive effect is ignorable, while the dominant collisional effect appears to be the collisional contribution to the SPIV effect, leading to the conclusion that the customary resistive magnetohydrodynamic modes, such as the resistive interchange, resistive ballooning, and tearing modes, are nonexistent in this description.
Magnetohydrodynamic simulations of direct current helicity injection for current drive in tokamaks
Authors: Sovinec C. R. , Prager S. C.
Published: July 2002

Magnetohydrodynamic (MHD) computations of direct current (DC) helicity injection for tokamak-like configurations show current drive with no ‘‘loop voltage’’ in a resistive, pressureless plasma. Self-consistently induced, hollow current profiles are unstable to resistive modes that partially relax the profile through the MHD dynamo mechanism. The resulting current profiles remain quite hollow, however, and tokamaks are not generated. The current driven by the fluctuations leads to closed contours of average poloidal flux, but the 1% fluctuation level is large enough to produce a region of stochastic magnetic field. A limited Lundquist number (S) scan from 2.5×103 to 2×104 indicates that both the fluctuation level and current profile relaxation increase with S. A simple quasilinear power scaling is consistent with these results at low S and suggests that at larger S, the fluctuation level decreases.
Whistler waves produced by a modulated spiraling electron beam: Linear approach
Authors: Krafft C. , Volokitin A. , Matthieussent G.
Published: July 2002

The theory of whistler wave interaction with a modulated electron beam injected obliquely to the magnetic field in an unbounded space plasma is considered. The study of the energy transfer between the spiraling beam and the whistler wave is done in the case of sheared whistlers, when the parallel wave number is very small compared to the perpendicular one. In the linear regime, structures of potentials and electromagnetic fields in the beam vicinity and in the wave zone are obtained analytically: Simple expressions for field components are provided for Čerenkov and Doppler resonances, in double and single pole cases. Emitted power is calculated as a function of beam parameters.
Effects of applied rotating magnetic perturbations on ion drift orbits in the Compact Auburn Torsatron
Authors: Pritchard D. C. , Gandy R. F. , Hanson J. D. , Knowlton S. F. , Lin H. , Sasser G. E. , Thomas, Jr. E. , Cooney J.
Published: July 2002

Ion drift surfaces in the presence of applied rotating magnetic perturbations in the Compact Auburn Torsatron (CAT) [Fusion Technol. 18, 281 (1990)] have been studied. A compact, monoenergetic ion gun was used to launch a continuous beam of 6Li+ ions in the vacuum magnetic fields of the CAT. The ions are detected on a paddle probe that is swept though a poloidal cross section of the torsatron. The applied magnetic perturbation was produced using helical magnetic perturbation coils capable of generating rotating magnetic islands. Using a fixed energy ion beam the effects of constant frequency perturbations on ion drift orbits were studied as a function of the direction of rotation, amplitude, and frequency of the perturbation. A drift Hamiltonian model was developed to simulate ion trajectories in the presence of constant frequency rotating islands. The results of this experiment agree well with the predictions of the Hamiltonian model. Preliminary results of ion orbit modification by swept-frequency, rotating magnetic perturbations are also presented.
Growth of the Rayleigh–Taylor instability in an imploding Z-pinch
Authors: De Groot J. S. , Toor A. , Golberg S. M. , Liberman M. A.
Published: July 2002

Rayleigh–Taylor (RT) instability of cylindrical, imploding plasma liners in a Z-pinch is analyzed. The reduction in total perturbation growth for multicascade systems (multiple imploding shells) is presented. This reduction is effective if the pressure produced by the impacting shell exceeds the magnetic pressure at the time of impact. Analytical and numerical solutions are also obtained for the RT instability of an imploding plasma liner accelerated into undisturbed plasma. The snowplow model is used in which the mass encountered by the imploding magnetic piston is swept into an infinitely thin sheath. A shock front launched ahead of the liner is shown to reduce the growth rate. It is also shown that accumulating the mass increases the growth rate. However, the total perturbation growth can be reduced if the liner accumulates the mass during the implosion compared to a liner with the same mass imploding into vacuum. Finally, it is shown that the final kinetic energy density for a given shell nonuniformity is largest if the final liner mass is accumulated during the implosion.
A variational approach to parametric instabilities in inhomogeneous plasmas I: Two model problems
Authors: Afeyan Bedros B. , Williams Edward A.
Published: July 2002

A variational formalism is introduced in the theory of three-wave parametric instabilities in inhomogeneous plasmas. This minimum pump strength principle (MPSP) is then applied to two model problems, the first being the Rosenbluth model equations [Phys. Rev. Lett. 29, 565 (1972)]. By choosing appropriate trial functions, the MPSP is used to solve for the complex eigenfrequency of the most unstable mode. The wave vector mismatch is assumed to be of the form κ(x)=κ(n)(0)xn/n!, where n is any positive integer. The results are compared to numerical solutions of the same eigenvalue problem. The second problem is the Liu, Rosenbluth, and White Raman sidescattering model [Phys. Fluids 17, 1211 (1974)], which is treated for any positive-integer power law density profile. The choice of trial functions, the role of symmetry, and various useful approximations are discussed.
Corrugation instability of radiative shock waves in a relativistically hot plasma
Authors: Tsintsadze L. N. , Chilashvili M. G. , Shukla P. K. , Tsintsadze N. L.
Published: July 2002

The corrugation instability of magnetic shock waves in a relativistically hot plasma with photon gas has been investigated. Conditions under which spontaneous amplification of surface corrugations on the discontinuity occurs are obtained. The corrugation instability may cause the condensation of the plasma number density, as well as the magnetic field behind the shock wave.
Backward traveling wave amplification in the gyrotron
Authors: Kou C. S.
Published: July 2002

The gyrotron backward traveling wave amplifier (gyro-BTWA) is introduced and characterized by a linear theory in this study. The gain mechanism, instead of a convective instability, is due to the interference among three backward waves. As a result, a high gain, tunable amplifier can be realized with a short interaction length for lower beam voltage and current. The theoretical results show that a gain of 25 dB at 39.47 GHz can be achieved with an interaction length of 4 cm for a 10-kV, 0.554-A electron beam. These features could make this device as a promising coherent radiation source in the microwave and millimeter wave regimes.
Turbulence modeling of the toroidal wall heat load due to shear flows over cavities in the neutral gas blanket divertor regime
Authors: Vahala George , Vahala Linda , Morrison Joseph
Published: July 2002

Heat loads to the target plate in reactor tokamaks are estimated to be orders of magnitude higher than those that can be withstood by known materials. In regimes of plasma detachment, there is strong evidence that plasma recombination occurs near the divertor plate, leading to a cold neutral gas blanket. Because of the strong coupling between the plasma and the neutrals within the divertor region, there is significant neutral flows along field lines up to Mach 1.2 and Reynolds numbers over 1000. Here the effects of three dimensional (3D) neutral turbulence within the gas blanket on heat deposition to the toroidal wall are examined. Both two dimensional (2D) mean shear flows over toroidal cavities as well as a fully 3D initial value problem of heat pulse propagation are considered. The results for algebraic stress model, K-ε and laminar flows are compared. It is found that 3D velocity shear turbulence has profound effects on the heat loads, indicating that simple (linear) Reynolds stress closure schemes are inadequate.
Foam-buffered spherical implosions at 527 nm
Authors: Watt R. G. , Wilson D. C. , Chrien R. E. , Hollis R. V. , Gobby P. L. , Mason R. J. , Kopp R. A. , Lerche R. A. , Kalantar D. H. , MacGowan B. , Nelson M. B. , Phillips T. , McKenty P. W. , Willi O.
Published: July 2002

Creation of a low density, high temperature plasma buffer between the absorption and ablation layers of a directly driven inertial confinement fusion implosion capsule has been proposed as a means to reduce “early time” imprint from laser nonuniformities. This thermal smoothing blanket might be created from a low density foam layer wrapped around the deuterium–tritium filled microballoon. Preliminary spherical implosion tests of this concept using a polystyrene foam layer surrounding a glass microballoon were performed at the Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)], using a 527 nm drive wavelength. Comparison of capsule yield and imploded core symmetry showed promising improvements in overall target performance, relative to one-dimensional undegraded hydrodynamic simulations, when the foam-buffer layer was present.
Three-dimensional magnetic reconnection in the solar corona
Authors: Priest E. R.
Published: July 2002

In two dimensions the notions of magnetic topology and null-point bifurcations are straightforward. In addition, the nature of magnetic reconnection is fairly well understood and can be described by a new generation of fast reconnection mechanisms known as almost-uniform reconnection and nonuniform reconnection. However, in complex three-dimensional (3-D) magnetic fields, such as exist in the solar corona, these phenomena are only just beginning to be explored and are considerably more complex. The structural properties of the magnetic field created in turn by two, three and more magnetic sources at the photosphere have been recently studied. Passing through each 3-D magnetic null there is an isolated spine field line and a flux surface known as a fan. The fans form separatrix surfaces that separate the volume into topologically distinct regions, and the fan of one null can terminate at the spine of another null, while the spine terminates either at a source or at infinity. The skeleton of complex 3-D fields in the corona, therefore, comprises the magnetic-null points and a network of spine curves and separatrix fan surfaces. Magnetic reconnection can occur in these fields by a variety of mechanisms, including spine reconnection, fan reconnection, separator reconnection, and quasiseparatrix layer reconnection. These types of reconnection and the bifurcations of null points are described and found to be much richer than in the relatively simple two-dimensional fields.
The Hubble constant, supernova light curves and spectra, and radiation transport
Authors: Branch David , Baron E. , Nugent Peter , Hauschildt Peter H.
Published: July 2002

As luminous events that can be physically modeled, supernovae provide an attractive route to the value of the Hubble constant. The modeling involves radiation transport through matter undergoing homologous expansion with velocity gradient on the order of 10−6 s−1. For supernovae of type Ia, which are thermonuclear disruptions of mass accreting or coalescing carbon–oxygen white dwarfs, one wants to be able to calculate the light curve (luminosity in some optical passband versus time), which is powered by the radioactivity decay chain Ni56→Co56→Fe56. For all kinds of supernovae, including those of types II, Ib, and Ic, which result from the gravitational collapse of the cores of massive stars, the goal is to accurately calculate the emergent ultraviolet–optical–infrared spectra, as a function of time. Local-thermodynamic-equilibrium (LTE) light-curve calculations for type Ia supernovae by Höflich and co-workers, and our spectrum calculations based on a fully relativistic non-LTE radiative transfer code, are described. The associated radiative transport needs are discussed.
Injection into the wiggler in free electron laser using tapered axial magnetic field
Authors: Maraghechi B. , Mirzanejhad S.
Published: July 2002

Injection of electrons into the one-dimensional magnetostatic wiggler is studied numerically. Considerable reduction in the amplitude of oscillation about the steady-state orbits is found when the axial magnetic field is changed adiabatically.
Variational principles for equilibrium states with plasma flow
Authors: Hameiri Eliezer
Published: July 2002

A new constant of the motion is utilized to formulate a variational principle for plasma equilibria with general flow fields. Two additional variational principles are derived from the original one. None of these formulations leads to a stability criterion if the velocity is not parallel to the magnetic field since the functionals used, the first of which being the energy, do not possess in this case a minimum but only stationary points. It is shown that other stability criteria already reported in the literature also suffer from the same deficiency. It is suggested that the lack of a minimum is due to the presence of ballooning modes.
Global drift modes in rotating tokamak plasmas
Authors: Pavlenko Vladimir P. , Revenchuk Sergei M.
Published: July 2002

Drift waves in a rotating toroidal plasma are considered for an axisymmetric, large aspect-ratio tokamak with concentric, circular magnetic surfaces. The analysis performed is based on rigorously derived eigenmode equations, coupled in poloidal mode numbers through the toroidal effects. Analytical dispersion relations for both global and propagating drift waves are obtained and their simple solutions are found for some limiting cases. The principal effect of plasma rotation is that the global drift mode existence criterion can be satisfied for large values of the magnetic shear parameter, s>1/2, in the case of sufficiently high rotation velocity, when the Doppler shifted eigenfrequency changes sign. Such plasma rotation velocities have been observed in the Doublett III-D (DIII-D) tokamak in the high (H)-mode confinement regime [Kim et al., Phys. Rev. Lett. 72, 2199 (1994)].
HINST: A two-dimensional code for high-n toroidicity induced Alfvén eigenmodes stability
Authors: Gorelenkov N. N. , Cheng C. Z. , Tang W. M.
Published: July 2002

A high-n stability code, HINST, has been developed to study the stability of TAE (toroidicity induced Alfvén eigenmodes) in large tokamaks such as the International Thermonuclear Experimental Reactor (ITER) [D. E. Post, Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239] where the spectrum of unstable TAE modes is shifted toward medium to high-n modes. The code solves the two-dimensional (2-D) eigenmode problem by expanding the eigenfunction in terms of basis functions. Based on the Fourier-ballooning formalism the eigenmode problem is reduced to a system of coupled one-dimensional equations, which is solved numerically by using the finite element method. The numerical method allows one to include nonperturbatively nonideal effects such as: finite ion Larmor radius, trapped electron collisional damping, etc. The 2-D numerical results of TAE and resonance TAE (RTAE) modes are compared with those from local ballooning calculations and the global magnetohydrodynamic nonvariational code NOVA [C. Z. Cheng and M. S. Chance, J. Comput. Phys. 71, 124 (1987)]. The results show that for ITER-like plasma parameters, TAE and RTAE modes can be driven unstable by alpha particles for n=10–20. The growth rate for the most unstable mode is within the range γ/ωA≃0.3%–1.5%. The most unstable modes are localized near r/a≃0.5 and have a broad radial mode envelope width.
Solitary radial electric field structure in tokamak plasmas
Authors: Itoh K. , Itoh S.-I. , Yagi M. , Fukuyama A.
Published: July 2002

The solitary structure solution of the radial electric field Er in the tokamak plasmas is obtained. It is shown to be stable under an external power supply, like a biased electrode at the edge. The radial gradient is governed by the ion viscosity and the nonlinearlity of the perpendicular conductivity. The radial structure of Er and reduction of turbulent transport, which belong to key issues of the high confinement mode (H-Mode) [F. Wagner et al., Phys. Rev. Lett. 49, 1408 (1982)], are self-consistently determined. A bifurcation from a radially-uniform one to a solitary one occurs at a certain applied voltage, and a hysteresis is associated.
Resistive ballooning modes near the edge of toroidal configurations
Authors: Correa-Restrepo Darı́o
Published: July 2002

The resistive ballooning mode equations are cast in a new form appropriate for evaluation near the plasma edge of toroidal (axisymmetric as well as three-dimensional) configurations, where the resistive ballooning effects outweigh the diamagnetic effects. Explicit evaluation is carried out for cylindrically symmetric plasmas and for a tokamak model with circular cross sections. Owing to the large electric resistivity of the regions considered, resistive ballooning modes with growth rates comparable to the characteristic growth rate of ideal ballooning modes are possible. A general feature is that modes with large growth rates are localized around the regions of bad curvature and become less unstable with increasing shear, while those with smaller growth rates are extended along the magnetic field lines and are insensitive to shear.
Electron production in low pressure gas ionized by an intense proton beam
Authors: Oliver B. V. , Ottinger P. F. , Rose D. V. , Hinshelwood D. D. , Neri J. M. , Young F. C.
Published: July 2002

Electron density measurements from previous ion-beam-induced gas ionization experiments [F. C. Young et al., Phys. Plasmas 1, 1700 (1994)] are re-analyzed and compared with a recent theoretical model [B. V. Oliver et al., Phys. Plasmas 3, 3267 (1996)]. Ionization is produced by a 1 MeV, 3.5 kA, 55 ns pulse-duration, proton beam, injected into He, Ne, or Ar gas in the 1 Torr pressure regime. Theoretical and numerical analysis indicates that, after an initial electron population is produced by ion beam impact, ionization is dominated by the background plasma electrons and is proportional to the beam stopping power. The predicted electron density agrees with the measured electron densities within the factor of 2 uncertainty in the measurement. However, in the case of Ar, the theoretically predicted electron densities are systematically greater than the measured values. The assumptions of a Maxwellian distribution for the background electrons and neglect of beam energy loss to discrete excitation and inner shell ionization in the model equations are considered as explanations for the discrepancy.
Formalism for multi-fluid equilibria with flow
Authors: Steinhauer Loren C.
Published: July 2002

A formalism is developed for flowing multifluid equilibria. In the standard reduced case (massless electrons, quasineutrality) this system simplifies to a pair of second-order partial differential equations for the magnetic and ion flow stream functions plus a Bernoulli equation giving the density. Each species has its own characteristic surfaces, which are the drift surfaces, and three arbitrary surface functions associated with each species. In the case of minimum energy equilibria, the surface functions are no longer arbitrary. The flowing equilibrium system is a generalization of the familiar Grad–Shafranov system for magnetostatic equilibria.
Collisionless fluid equations: Lagrangian, Eulerian, and local forms
Authors: Mattor Nathan
Published: July 2002

A recent paper [N. Mattor, Phys. Plasmas 5, 1822 (1998)] derived closed fluid moment equations from a collisionless kinetic equation. These fluid equations retain the effects of linear and nonlinear wave-particle resonances, traditionally thought to require kinetic equations. This paper gives three methods to calculate this term. In the Lagrangian form, the components making up the closure term are constant along phase rays, and so can be traced from the initial time. The Eulerian form evolves the components on a constant spatial grid and so does not require explicit calculation of phase rays. The local approximation is a heuristic method of approximating global integrals based on local gradients, and offers significant simplification over the other two methods.
Reconnection layers in two-dimensional magnetohydrodynamics and comparison with the one-dimensional Riemann problem
Authors: Lin Y. , Lee L. C.
Published: July 2002

The outflow region of fast magnetic reconnection is studied by using a two-dimensional (2-D) resistive magnetohydrodynamic (MHD) model. The effects of the guide magnetic field By on the 2-D quasi-steady reconnection are investigated. The one-dimensional (1-D) MHD Riemann problem associated with the magnetic reconnection is also simulated for comparison. In the 2-D reconnection a trailing reconnection layer is formed behind a leading bulge. In the cases with By=0, intermediate shocks, slow shocks, slow expansion waves, and a contact discontinuity may be present in the quasi-steady reconnection layer. In general cases with By≠0, the intermediate shocks are replaced by time-dependent intermediate shocks (TDISs). The structure of the reconnection layer as a function of the distance z from the X line in the 2-D reconnection is found to be related to the 1-D simulation results as a function of time t.
Analysis of a nonlinear diffusion equation describing the formation of transport barriers in fusion plasmas
Authors: Ödblom A. , Anderson D. , Lisak M. , Tendler M.
Published: July 2002

In tokamak fusion plasmas, transport barriers may form in the region between the core and the scrape-off layer due to E×B shear suppression. The properties of the corresponding density transport can be modeled in terms of a diffusion equation with a strongly nonlinear diffusion coefficient, which depends on the first as well as on the second-order density gradients. An investigation of the steady state, as well as dynamical properties of this nonlinear diffusion, has been carried out, aiming at a simplified analytical description. In particular, different forms of steady-state solutions and their stability are discussed, hysteresis effects are shown to be a characteristic feature of the model equation, and the dynamical evolution between different stationary states is also described in terms of explicit analytical expressions.
Multi-faceted asymmetric radiation from the edge formation in DIII-D high-confinement mode discharges with continuous gas puffing
Authors: Stacey W. M. , Mahdavi M. A. , Maingi R. , Petrie T. W.
Published: July 2002

A set of continuously gas fueled DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986) p. 159] high confinement mode (H-mode) shots at three neutral beam injection powers and two values of magnetic field (q95) have been analyzed to investigate the evolution of plasma edge conditions leading to the formation of X-point MARFEs (multifaceted asymmetric radiation from edge), which are followed by an H–L transition that constitutes a practical density limit for this class of shots in DIII-D. The MARFE initiation is found to be caused by a combination of a sharply increasing concentration of recycling neutrals in the plasma edge, decreasing edge inverse temperature and density gradient scale lengths, and increasing edge density. The line-average density at which MARFEs precipitates increases with increasing injection power and with decreasing q95. It is not clear whether the formation of the X-point MARFE causes or merely precedes the H–L transition.
A self-consistent analysis of a collisional presheath
Authors: Goswami Monojoy , Ramachandran H.
Published: July 2002

A one-dimensional plasma discharge is analyzed under steady state conditions. Using simple models for source and collisions, a first-order differential equation is obtained that simultaneously describes both the bulk and the presheath of the plasma. This equation is numerically solved in various regimes and physically interesting quantities such as the ratio of bulk to edge density and the size of the inertial terms in the bulk region are presented. Analytic expressions are obtained for profiles when collision frequency is assumed constant. Findings include nB/nSE∼2.5 for highly collisional systems, significant flow in the bulk plasma, and modified I–V characteristics.
Whistlerization and anisotropy in two-dimensional electron magnetohydrodynamic turbulence
Authors: Dastgeer Sheikh , Das Amita , Kaw Predhiman , Diamond P. H.
Published: July 2002

A detailed numerical simulation to understand the turbulent state of the decaying two-dimensional electron magnetohydrodynamics is presented. It is observed that the evolved spectrum is comprised of a collection of random eddies and a gas of whistler waves, the latter constituting the normal oscillatory modes of such a model. The whistlerization of the turbulent spectra has been quantified by novel diagnostics. In this work, results are presented only in the regime where the spatial excitation scales are longer than the electron skin depth. Simulations suggest that spectra at short scales are comparatively more whistlerized. The long scale field merely acts as the ambient field along which whistler waves propagate. It is also observed that, in the presence of an external magnetic field, the power spectrum acquires a distinct directional dependence. This anisotropy is dominant at short scales. It is shown that such an anisotropy at short scales results from a cascade mechanism governed by the interacting whistlers waves.
Optimization of the current potential for stellarator coils
Authors: Boozer Allen H.
Published: July 2002

Stellarator plasma confinement devices have no continuous symmetries, which makes the design of appropriate coils far more subtle than for axisymmetric devices such as tokamaks. The modern method for designing coils for stellarators was developed by Peter Merkel [P. Merkel, Nucl. Fusion 27, 867 (1987)]. Although his method has yielded a number of successful stellarator designs, Merkel’s method has a systematic tendency to give coils with a larger current than that required to produce a stellarator plasma with certain properties. In addition, Merkel’s method does not naturally lead to a coil set with the flexibility to produce a number of interesting plasma configurations. The issues of coil efficiency and flexibility are addressed in this paper by a new method of optimizing the current potential, the first step in Merkel’s method. The new method also allows the coil design to be based on a freer choice for the plasma–coil separation and to be constrained so space is preserved for plasma access.
Effects of background gas pressure on the dynamics of a nonneutral electron plasma confined in a Malmberg–Penning trap
Authors: Chao Edward H. , Davidson Ronald C. , Paul Stephen F. , Morrison Kyle A.
Published: July 2002

The effects of electron–neutral collisions on plasma expansion properties and the evolution of the m=1 diocotron mode are investigated in the Electron Diffusion Gauge (EDG) experiment, a Malmberg–Penning trap with plasma length Lp≃15 cm, plasma radius Rp≃1.3 cm, and characteristic electron density 5×106 cm−3<n<3×107 cm−3. Essential features of the m=1 diocotron mode dynamics in the absence of electron–neutral collisions are verified to behave as expected. The mode frequency, the growth rate of the resistive-wall instability, and the frequency shift at nonlinearly large amplitudes are all in good agreement with theoretical predictions. When helium gas is injected into the trap, the evolution of the mode amplitude is found to be very sensitive to the background gas pressure down to pressures of 5×10−10 Torr, the lowest base pressure achieved in the EDG device. The characteristic time scale τ for nonlinear damping of the m=1 diocotron mode is observed to scale as P−1/2 over two orders-of-magnitude variation in the background gas pressure P. The evolution of the plasma density profile has also been monitored in order to examine the shape of the evolving density profile n(r,t) and to measure the expansion rate. The density profile is observed to expand radially while maintaining a thermal equilibrium profile shape, as predicted theoretically. While the expansion rate is sensitive to background gas pressure at pressures exceeding 10−8 Torr, at lower pressures the cross-field transport appears to be dominated by other processes, e.g., asymmetry-induced transport. Finally, the expansion rate is observed to scale approximately as B−3/2 for confining fields ranging from 100 to 600 G.
Mode interaction in backward-wave oscillators with strong end reflections
Authors: Nusinovich G. S. , Bliokh Yu. P.
Published: July 2002

In backward-wave oscillators (BWOs) with strong end reflections a number of standing waves with different axial indices can be excited. The theory describing the interaction of such modes is developed. The region of stable single-mode oscillations is determined and various hysteresis effects which may occur during the voltage rise and fall times are analyzed.
Asymptotic modeling of short plane-parallel high-pressure glow discharge
Authors: Bektursunova Rimma M. , Graham William G.
Published: July 2002

Here a self-consistent continuum model is presented for a narrow gap plane-parallel dc glow discharge. The set of governing equations consisting of continuity and momentum equations for positive ions, fast (emitted by the cathode) and slow electrons (generated by fast electron impact ionization) coupled with Poisson’s equation is treated by the technique of matched asymptotic expansions. Explicit results are obtained in the asymptotic limit: (χδ)≪1, where χ=eΦa/kT,δ=(rD/L)2 (Φa is the applied voltage, rD is the Debye radius) and pL≫1(Hg mm cm), where p is the gas pressure and L is the gap length. In the case of high pressure, the electron energy relaxation length is much smaller than the gap length, and so the local field approximation is valid. The discharge space divides naturally into a cathode fall sheath, a quasineutral plasma region, and an anode fall sheath. The electric potential distribution obtained for each region in a (semi)analytical form is asymptotically matched to the adjoining regions in the region of overlap. The effects of the gas pressure, gap length, and applied voltage on the length of each region are investigated.
Laser experiments to simulate supernova remnants
Authors: Drake R. P. , Carroll J. J. , Smith T. B. , Keiter P. , Glendinning S. Gail , Hurricane Omar , Estabrook Kent , Ryutov D. D. , Remington B. A. , Wallace R. J. , Michael Eli , McCray R.
Published: July 2002

An experiment using a large laser facility to simulate young supernova remnants (SNRs) is discussed. By analogy to the SNR, the laboratory system includes dense matter that explodes, expansion and cooling to produce energetic, flowing plasma, and the production of shock waves in lower-density surrounding matter. The scaling to SNRs in general and to SN1987A in particular is reviewed. The methods and results of x-ray radiography, by which the system in diagnosed, are discussed. The data show that the hohlraum used to provide the energy for explosion does so in two ways—first, through its radiation pulse, and second, through an additional impulse that is attributed to stagnation pressure. Attempts to model these dynamics are discussed.
Time-evolution and guiding regimes of the laser-produced plasma waveguide
Authors: Clark T. R. , Milchberg H. M.
Published: July 2002

A brief review of experiments investigating the hydrodynamics and linear optical properties of laser-produced plasma fibers is presented. Time- and space-resolved interferometry measurements yield plasma density profiles, expansion speeds and temperature estimates, and the physical mechanisms behind their evolution are discussed. Based on the measured profiles and the results of guiding experiments, two distinct optical guiding and coupling regimes are identified.
Shocks associated with the Kelvin–Helmholtz-resistive instability
Authors: Shen C. , Liu Z. X. , Huang T. S.
Published: July 2002

In this paper, a new type of shock associated with magnetic reconnection processes has been explored using a compressible magnetohydrodynamics simulation method. The simulations have shown that, when there are strong field-aligned shear flows at the two sides of a current sheet, the coupling mode between Kelvin–Helmholtz and resistive instabilities will appear; further, reflected shocks and incident shocks can be produced at both sides of the boundary layer. Both the reflected shocks and incident shocks are fast shocks, through which the magnetic field strength, density, and temperature all increase sharply, while the plasma velocity decreases steeply. It is expected that some inhomogeneous structures can be formed at plasma boundary layer regions due to the existence of fast field-aligned shear flow driven shocks.
Switching of low density diode plasmas
Authors: Waisman E. M. , Steen P. G. , Parks D. E. , Wilson A.
Published: July 2002

The switching action of low density injected carbon plasmas is investigated by a numerical collisionless two-dimensional electromagnetic particle simulation. The plasma switch is considered in a planar vacuum transmission line with input current per unit length ramped at 240 A/cm ns during 10 ns, and short circuit output. The calculations, done with C+ ions at 4×1012/cm3, exhibit switching of the current from the plasma switch to the short circuit load. At early times (1–2 ns) a sheath of positive charge forms at the cathode by depletion of the plasma electrons. The electric fields at the cathode are large enough to explain the formation of a denser cathode plasma. The mechanism of switching appears to be, for this density and load conditions, the magnetic limiting of drifting electrons. The ion motion seen plays an insignificant role in this particular case.