bluuebunny/crossref_metadata_2025 huggingface dataset:
Alfvén-wave heating of a high-beta plasma column
Authors: Cekic M. , Nelson B. A. , Ribe F. L.
Published: July 2002

Alfvén-wave heating experiments are performed on the High Beta Q Machine (HBQM) [Phys. Fluids 25, 262 (1982)] straight plasma column. An investigation is made of Alfvén-wave energy absorption by the Alfvén-resonant layer at which the local shear Alfvén velocity is equal to the phase velocity of the driven plasma standing wave. The radial position of the resonance in the plasma is defined by the equilibrium magnetic field diamagnetic profile and nearly constant electron number density profile. The experiment uses a 250 MW ringing LC circuit in the 615–720 kHz frequency range with the capacitors driving the HBQM compression coil segments that are connected to form a 2.0 m long m=±1 antenna configuration with axial wave number kz=15.7 m−1. Experiments were conducted on both hydrogen and deuterium plasmas. In the hydrogen case the Alfvén resonance is predicted to lie inside the plasma. Internal magnetic field probes were used to measure both the rf and equilibrium magnetic fields. The rf heating produces a larger change in the hydrogen plasma axial magnetic field profile that is centered around r=6 cm, near the Alfvén-resonance location. The rf B̃θ signal shows a phase reversal at the same radius. Other measurements show a 25% increase in plasma pressure for hydrogen. The antenna resistance increases from 0.06 Ω vacuum value to approximately 0.2 Ω in the presence of hydrogen plasma with a 31% absorption efficiency. The rf-induced effects in the deuterium plasma are 20%–50% smaller, agreeing with estimates based on collisional viscous damping, or collisonless model with finite ion cyclotron and Larmor radius effects. In the hydrogen case both these models predict insufficient energy absorption to explain observed heating. However, a magnetofluid calculation taking account of the singularity at the Alfvén-resonant layer reproduces the value of the antenna resistance for the hydrogen case.
Electrostatic and magnetic fluctuations in the proximity of the velocity shear layer in the TJ-I tokamak
Authors: García-Cortés I. , Pedrosa M. A. , Hidalgo C. , Brañas B. , Estrada T. , Balbín R. , de la Luna E. , Sánchez J. , Navarro A. P.
Published: July 2002

The structure of the electrostatic and magnetic turbulence changes in the proximity of the naturally occurring velocity shear layer in the TJ-I tokamak [Nucl. Fusion 30, 717 (1990)]. Both a modification in the fluctuation levels and autocorrelation times of the Langmuir probe ion saturation current and a decorrelation in the broadband magnetic fluctuations, have been observed in the proximity of the shear layer location. However, no direct correlation between the sheared poloidal flows and turbulence characteristics has been established.
Second-order toroidicity-induced Alfvén eigenmodes and mode splitting in a low-aspect-ratio tokamak
Authors: Turnbull A. D. , Chu M. S. , Chance M. S. , Greene J. M. , Lao L. L. , Strait E. J.
Published: July 2002

A variational ideal magnetohydrodynamic stability code is used to compute the toroidicity-induced Alfvén eigenmode (TAE) for toroidal mode number n=1 in full toroidal geometry with circular cross section. At finite aspect ratio, toroidicity also couples poloidal mode numbers m with m+2 to produce a higher frequency gap in the shear Alfvén spectrum, which is associated with a new, second-order TAE mode. It is also found that toroidal coupling between the TAE modes and continuum branches induces splitting of the TAE modes into two or more global modes at slightly different frequencies. Both the new modes and splitting of the TAE modes have important consequences for the identification of TAE modes in experiments.
Deviations from the frozen-in law in the presence of small (but nonzero) resistivity
Authors: Fruchtman A.
Published: July 2002

The freezing of magnetic field flux into an electron fluid is examined during fast magnetic field evolution that results from the nonlinear skin effect. First, fast magnetic field evolution, which does not involve flux penetration, is shown to follow naturally from the frozen-in law. Second, fast evolution is analyzed in which large deviations from the frozen-in law (large flux penetration) occur in the presence of small (but nonzero) resistivity. A direct relation is shown between the deviation from the frozen-in law and the energy dissipated per electron along its orbit.
Laser-driven hydrodynamic instability experiments*
Authors: Remington B. A. , Weber S. V. , Haan S. W. , Kilkenny J. D. , Glendinning S. G. , Wallace R. J. , Goldstein W. H. , Wilson B. G. , Nash J. K.
Published: July 2002

An extensive series of experiments has been conducted on the Nova laser to measure hydrodynamic instabilities in planar foils accelerated by x-ray ablation. Single-mode experiments allow a measurement of the fundamental growth rates from the linear well into the nonlinear regime. Two-mode foils give a first direct observation of mode coupling. Surface-finish experiments allow a measurement of the evolution of a broad spectrum of random initial modes.
Longitudinal instability of space-charge dominated beams in transport channels with complex wall impedances
Authors: Wang J. G. , Reiser M.
Published: July 2002

The longitudinal instability of space-charge dominated beams in transport channels with complex wall impedances is studied. The dispersion equation, derived from the one-dimensional Vlasov equation, characterizes the instability with all the relevant parameters including the space charge, the beam energy spread, the real and imaginary parts of the complex wall impedances. For a beam without energy spread, the growth rates of the instability in the different parameter ranges are discussed. The application of the theory to a resistive-wall channel and to a channel with induction gaps is presented. For a beam with energy spread, Lorentzian and Gaussian distributions are treated and the results presented illustrate the dependence of the stable regions on the beam perveance, the energy spread and the wall properties.
Modeling and simultaneous sawtooth measurements of the thermal and energetic electron diffusivities from the Texas Experimental Tokamak
Authors: Catto Peter J. , Myra J. R. , Bengtson Roger D. , Wootton A. J.
Published: July 2002

Convenient mathematical models appropriate for simultaneously measuring the thermal and energetic electron diffusivities by their responses to internal disruptions are presented and compared to simultaneous soft and hard x-ray sawtooth data from the Texas Experimental Tokamak (TEXT) [Nucl. Technol./Fusion 1, 479 (1981)]. The eigenfunction expansion technique employed is first illustrated on a constant diffusivity, single sawtooth crash model that is unable to give a satisfactory fit to the TEXT soft x-ray data at disparate radii for an initial condition of a flattened temperature profile out to the mixing radius. A more sophisticated single crash model having a parabolic radial dependence for the thermal diffusivity and a flattened profile initial condition substantially improves the fit to the TEXT soft x-ray data. Simultaneous measurements of the sawtooth oscillations on the hard x-ray signal caused by runaway electrons hitting the limiter are interpreted to obtain a measure of the diffusivity of the energetic electrons in TEXT following the same crash used to measure the thermal diffusivity. A single sawtooth crash model is no longer adequate because the diffusivity of the energetic electrons may be substantially less than the thermal value. Therefore the eigenfunction expansion technique is extended to a periodic sawtooth crash model. Measurements of diffusivities far smaller than those that can be measured by a single crash model are possible and more than one value of diffusivity can fit the simultaneous TEXT hard x-ray data. The lower values of diffusivity could not have been obtained by a single crash model. Additional hard x-ray measurements having differing sawtooth periods are needed to remove the ambiguity. As more extensive hard x-ray data become available on TEXT, it may become possible to simultaneously and unambiguously measure the thermal and energetic electron diffusivities during the same crash.
Measurement and analysis of Rayleigh–Taylor instability in targets driven by incoherent laser radiation
Authors: Desselberger M. , Willi O.
Published: July 2002

The effectiveness of incoherence-induced optical smoothing techniques is studied experimentally and good agreement with theory is found. Using incoherent laser-drive radiation, the Rayleigh–Taylor instability is investigated in planar foil targets with imposed sinusoidal modulations. Growth rates of up to 60% of the classical value are inferred from the measurements and the nonlinear behavior of the instability is investigated. Growth is observed at all wavelengths tested between 30 and 100 μm.
Asymptotic laws for transversely homogeneous turbulent shear flows
Authors: Tavoularis S.
Published: July 2002

An analysis predicts an exponential downstream development of turbulent kinetic energy in the asymptotic development range of transversely homogeneous, uniformly sheared, turbulent flows. The analysis is found to be in good agreement with available experimental results and numerical simulations.
Nonlinear evolution of drift-tearing modes
Authors: Scott Bruce D. , Hassam A. B. , Drake J. F.
Published: July 2002

It is shown that diamagnetic propagation of drift-tearing modes ceases at small island widths as the local density profile is flattened by sound waves. The critical width for density flattening is small compared to island widths typically inferred from observed magnetic fluctuations of m=2 in tokamak discharges. The rotation of these fluctuations must therefore result from radial electric fields.
Attractive potential between resonant electrons
Authors: Nambu M. , Akama H.
Published: July 2002

A test charge potential around a resonant electron is studied. An attractive potential (wake potential) for ion sound waves is obtained. The result shows that the attractive potential energy of the resonant electron is greater than its kinetic energy. There is a strong tendency for resonant electrons to bind together into macroparticles.
High-efficiency targets for high-gain inertial confinement fusion
Authors: Gardner John H. , Bodner Stephen E.
Published: July 2002

Rocket efficiencies as high as 15% are possible using short wavelength lasers and moderately high aspect ratio pellet designs. These designs are made possible by two recent breakthroughs in physics constraints. First is the development of the induced spatial incoherence (ISI) technique, which allows uniform illumination of the pellet and relaxes the constraint of thermal smoothing, permitting the use of short wavelength laser light. Second is the discovery that the Rayleigh–Taylor growth rate is considerably reduced at short laser wavelengths. By taking advantage of the reduced constraints imposed by nonuniform laser illumination and Rayleigh–Taylor instability, pellets using (1)/(4) μm laser light and initial aspect ratios of about 10 (with in flight aspect ratios of about 150–200) may produce energy gains as high as 200–250.
Three-dimensional stellarator equilibrium as an Ohmic steady state
Authors: Park W. , Monticello D. A. , Strauss H. , Manickam J.
Published: July 2002

A stable three-dimensional stellarator equilibrium can be obtained numerically by a time-dependent relaxation method using small values of dissipation. The final state is an Ohmic steady state which approaches an Ohmic equilibrium in the limit of small dissipation coefficients. A method to speed up the relaxation process and a method to implement the B⋅∇p=0 condition are described. These methods are applied to obtain three-dimensional heliac equilibria using the reduced heliac equations.
Solitary vortices in a rotating plasma
Authors: Horton W. , Liu J. , Meiss J. D. , Sedlak J. E.
Published: July 2002

Nonlinear equations describing the flute dynamics of rotating plasma are derived and solitary vortex solutions are obtained. The solution takes the form of a shielded dipole vortex, similar to that found for nonlinear Rossby waves. The nonlinear dispersion relation, relating propagation speed to vortex radius, is obtained. Vortex speeds are shown to take values complementary to the phase velocities of the linear modes of the system. The E×B circulation velocity of the plasma trapped in the vortex is comparable to the diamagnetic drift velocity in the equilibrium plasma.
The sound field of a Tollmien–Schlichting wave
Authors: Akylas T. R. , Toplosky N.
Published: July 2002

An asymptotic theory is presented for predicting the sound field generated by small-amplitude instability waves in low-speed boundary-layer flows. A Tollmien–Schlichting wave of fixed frequency, which evolves spatially in a Blasius boundary layer, is considered, and the associated far-field acoustic radiation is calculated. It is found that the directional dependence of the radiation intensity is of the dipole type. This result is discussed in connection with the superdirective acoustic fields caused by instability waves, which were recently observed in low-Mach-number jets.
Spontaneous formation of magnetohydrodynamic equilibrium profiles: Tokamaks, reversed-field pinches, and their intermediates
Authors: Yoshida Z. , Kusano K. , Inoue N.
Published: July 2002

The spontaneous formation of magnetohydrodynamic (MHD) equilibrium profiles in toroidal current-carrying plasmas has been discussed for tokamaks, reversed-field pinches (RFP’s), and also for their intermediate states ultra-low q’s (ULQ’s). The significance of m (poloidal mode number)=1 fluctuations causing a shift of the magnetic axis is stressed. The characteristics of MHD relaxation have been studied in comparison with classical diffusion. The dynamics of ULQ’s is shown to illustrate the characteristics of both dissipative processes. A global-mode stable ULQ is observed to be produced through MHD relaxation, and then to be deformed, in a longer time scale, by classical diffusion which destabilizes the stable profile.
Ablation parameters in KrF laser/plasma interaction: An experimental study
Authors: Gupta P. D. , Tsui Y. Y. , Popil R. , Fedosejevs R. , Offenberger A. A.
Published: July 2002

Experimental measurements of ablation parameters in a KrF laser produced aluminum plasma are presented for the laser intensity range of 1011–1013 W/cm2. Plasma created from large focal spots shows single broad ion current pulses as compared to two groups of ions observed from small focal spots. Detailed analysis of the velocity distributions of emitted ions is carried out to understand this behavior and its effect on measurements of the ablation parameters. Scalings with laser intensity of the mass ablation rate (ṁ∼I0.42L) and of the ablation pressure (Pa∼I0.81L) giving ṁ=1.5×105 g cm−2 sec−1 and Pa=5.5 Mbar at a laser intensity of 1013 W/cm2 are obtained. The results are found to be in good agreement with the predictions of a plasma ablation model based on inverse bremsstrahlung dominated absorption.
Linear waves and stability in ideal magnetohydrodynamics
Authors: Eckhoff Knut S.
Published: July 2002

Linear waves superimposed on an arbitrary basic state in ideal magnetohydrodynamics (MHD) are studied by an asymptotic expansion valid for short wavelengths. It has not been necessary to introduce any assumption beyond the usual regularity assumptions on the arbitrarily given solution which represents the basic state in this paper; it may even be time dependent. The theory also allows for a gravitational potential; it may therefore be applied both in astrophysics and in problems related to thermonuclear fusion. The linearized equations for the perturbations of the basic state are found in the form of a symmetric hyperbolic system. This symmetric hyperbolic system is shown to possess characteristics of nonuniform multiplicity, which implies that waves of different types may interact. In particular, it is shown that mass, Alfvén, and slow magnetoacoustic waves will persistently interact in the exceptional case where the local wavenumber vector is perpendicular to the magnetic field. The equations describing this interaction are found in the form of a weakly coupled hyperbolic system. This weakly coupled hyperbolic system is studied in a number of special cases and detailed analytic results are obtained for some such cases. The results show that the interaction of the waves may be one of the major causes of instability of the basic state. It seems beyond doubt that the interacting waves contain the physically relevant parts of the waves, which often are referred to as ballooning modes, including Suydam and Mercier modes.
Magnetic drift vortex instabilities in an inhomogeneous plasma
Authors: Yu M. Y. , Chijin Xiao
Published: July 2002

The relation between several magnetic waves and instabilities is clarified. These modes involve only electron dynamics. It is shown that in the parameter regime of interest there are no propagating unstable waves driven by the first order baroclinic effect.
Linear spectrum of magnetostatic and thermally conducting planar plasmas
Authors: Hermans D. , Goossens M. , Kerner W. , Lerbinger K.
Published: July 2002

The linear spectrum of 1-D magnetostatic and thermally conducting equilibrium plasmas is analyzed. It is found that the influence of thermal conduction is fundamentally different on the various parts of the linear spectrum of ideal magnetohydrodynamics and that it is by far the most profound for the slow magnetoacoustic part. The ideal Alfvén continuous spectrum is unaffected by thermal conduction. However, the ideal slow continuous spectrum is replaced by the isothermal slow continuous spectrum. This new continuous spectrum owes its existence to thermal conduction but is independent of κ and involves a different range of continuum frequencies. In addition to these two continuous parts, the spectrum consists of discrete slow and fast magnetoacoustic modes and thermal modes. The point eigenvalues of the fast magnetoacoustic modes are slightly distorted in proportion to κ. However, the point eigenvalues of the slow magnetoacoustic modes lie on well-defined curves in the complex plane that are independent of κ and controlled by the ideal slow and isothermal slow continua. The discrete slow magnetoacoustic spectrum hangs, as it were, on the ideal slow and isothermal slow continua and is determined by the nonuniformity of the equilibrium. The thermal modes are the result of the inclusion of the nonideal effect of thermal conduction in the energy equation.
The extensional viscosity and effective thermal conductivity of a dispersion of aligned disks
Authors: Sundararajakumar R. R. , Koch Donald L. , Shaqfeh Eric S. G.
Published: July 2002

This paper discusses the properties of a semidilute suspension of disks, for which nl3≫1 and φ≪1, where n is the number of disks per unit volume, l is their large dimension, and φ is their volume fraction. The effective conductivity of a dispersion of aligned, highly conducting disks is shown to be O[k(nl3)2], where k is the conductivity of the matrix. The extensional viscosity is shown to be O[μ(nl3)2], where μ is the viscosity of the fluid. In addition, similar scaling results are shown to hold for the case of a semidilute suspension of aligned, two-dimensional slabs which are of infinite extent in the direction perpendicular to their plane of cross section. Specifically, for nl2≫1, the effective conductivity and the extensional viscosity are shown to be O[k(nl2)2] and O[μ(nl2)2] respectively, where n is now the number of slabs per unit area and l is the width of the slab. Planar extensional flow simulations of a periodic array of aligned slabs confirm the quadratic scaling for stress in the semidilute regime. The simulations also show the crossover from a linear dependence of the stress on particle concentration in the dilute regime to the quadratic, semidilute scaling.
Note on the conditional moment closure in turbulent shear flows
Authors: Klimenko A. Yu.
Published: July 2002

The general equation for the conditional expectation of a reactive scalar can be simplified for turbulent shear flows. The weak dependence of the conditional expectation on a coordinate across the shear flow, which was assumed earlier, is proved by asymptotic analysis. This analysis is an analog of the traditional boundary layer methods for the conventional (unconditional) expectations in turbulent flows, but it is carried out for the conditional expectation and the conserved scalar probability density function.
Non-isothermal rarefied gas flow through a slit
Authors: Sharipov Felix
Published: July 2002

A rarefied gas flow through a thin slit caused by small drops of the pressure and temperature is studied on the basis of the kinetic equation. The linearized S-model equation is solved numerically by the discrete velocity method. The gas–surface interaction is assumed diffuse-specular. As a result four kinetic coefficients have been calculated: the mass flow rate caused by the pressure drop WP, the mass flow rate caused by the temperature drop WT, the heat flux caused by the pressure drop QP, and the heat flux caused by the temperature drop QT. It is shown that only the coefficient WP very weakly depends on the gas–surface interaction parameter α. The rest of the coefficients, i.e., WT, QP, and QT, significantly depend on the parameter α. The Onsager relation WT=QP is numerically confirmed. An analysis of the thermomolecular pressure effect on the slit is given.
Strain, vortices, and the enstrophy inertial range in two-dimensional turbulence
Authors: Oetzel Kenneth G. , Vallis Geoffrey K.
Published: July 2002

The properties of vortices in a strain field are used to construct a phenomenological theory of the enstrophy inertial range in two-dimensional incompressible turbulence. The theory, based in part on the results and behavior of numerical simulations, attempts to combine spectral inertial range theories of the Kolmogorov type with the dynamics of vortex interactions in physical space. It is based on the assumptions that coherent vortices can survive in a turbulent flow if of sufficient strength compared to the background straining field, and that coherent structures feel a mean strain field, independent of their scale. The first assumption is suggested by a result in the theory of uniform elliptic vortices, while the second comes from numerical simulations. The theory employs a single non-dimensional parameter, essentially the ratio between the enstrophy flux and the mean strain, which then characterizes flows from extremely intermittent decaying turbulence to nearly Gaussian passive scalar dynamics. The theory predicts that in forced two-dimensional turbulence, coherent structures reside in a “background” straining field. The coherent vortices will dominate the flow at a sufficiently large scale, with a fairly abrupt transition at a small scale to a flow in which the classical k−1 enstrophy spectrum holds. In this classical region small amplitude vortices do not survive because the (large-scale) straining field is of larger amplitude than the (small-scale) vorticity. The vorticity itself is passively advected in this regime. If the enstrophy flux is very small compared to the enstrophy itself, then the dynamics will be highly intermittent, with a spectrum determined by the spectrum of the vortices themselves, rather than by the dynamics of the enstrophy flux. The theory predicts that at small scales in forced-dissipative two-dimensional turbulence the energy spectrum will obey the classical enstrophy inertial range predictions even though the non-linear interactions remain spectrally non-local. Passive scalar dynamics are predicted to be similar to vortex dynamics, at small scales. Available numerical simulations are consistent with these suggestions.
A model for universal time scale of vortex ring formation
Authors: Mohseni Kamran , Gharib Morteza
Published: July 2002

An analytical model for predicting the universal time scale for formation of vortex rings generated through impulsively started jets is considered. The model is based on two assumptions, namely the validity of the slug model in simulating the discharge process of the fluid out of the cylinder and the approximation of the vortex at the pinch off moment by a vortex in the Norbury family. The nondimensional stroke length L/D (referred to as “formation number,” following Gharib et al. [J. Fluid Mech. 360, 121 (1998)]) predicted by the model satisfactorily matches the experimental observation of Gharib et al. The model introduces two nondimensional parameters that govern the limiting formation number: nondimensional energy End and circulation Γnd. The predicted value of End matches very well with the experimental data. It is also predicted that there is a limiting value for the nondimensional circulation in the range 1.77≲Γnd≲2.07.
Mode coalescence in a two-fluid boundary-layer stability problem
Authors: Timoshin S. N. , Hooper A. P.
Published: July 2002

The triple-deck analysis of a two-fluid boundary layer stability at high Reynolds numbers in Timoshin [J. Fluid Mech. 353, 163 (1997)] is extended to include broader parameter variations, most notably variations in the scaled film thickness. The two underlying instabilities, the Tollmien–Schlichting and interfacial waves, are shown to have points of mode coalescence located in the stable or unstable part of the spectrum. An interpretation of the instability mechanisms operational in two-fluid triple-deck flows is proposed in terms of the disturbance vorticity equation.
Effect of quadrupole fields on local three-dimensional (3-D) ideal magnetohydrodynamic (MHD) stability in torsatrons
Authors: Moeckli R. , Cooper W. A.
Published: July 2002

The local three-dimensional (3-D) ideal magnetohydrodynamic (MHD) stability properties of a 12-field period torsatron device of moderate aspect ratio are investigated as a function of an externally applied quadrupole magnetic field. It is demonstrated that with a parabolic pressure profile the volume-averaged β limit due to the ballooning instabilities decreases from β≂2% to β≂1.5% as the current Imid in the quadrupole field coils normalized to that in the main helical coils increases from 0 to 0.19.
Small-amplitude waves in a nonuniform plasma
Authors: Auer P. L. , Mason R. J.
Published: July 2002

Linear waves in a magnetized cold electron plasma are studied as a means of further understanding the mechanism of field penetration in such devices as plasma opening switches. The one-dimensional wave penetration into a nonuniform plasma, discovered by others in earlier work, has been extended to two dimensions by the linear analysis. An exact solution is presented for waves propagating along a density discontinuity.
Can inertial electrostatic confinement work beyond the ion–ion collisional time scale?
Authors: Nevins W. M.
Published: July 2002

Inertial electrostatic confinement (IEC) systems are predicated on a nonequilibrium ion distribution function. Coulomb collisions between ions cause this distribution to relax to a Maxwellian on the ion–ion collisional time scale. The power required to prevent this relaxation and maintain the IEC configuration for times beyond the ion–ion collisional time scale is shown to be greater than the fusion power produced. It is concluded that IEC systems show little promise as a basis for the development of commercial electric power plants.
Radially resolved spectral observations of polyacetal capillary discharge plasmas
Authors: Morgan C. A. , Griem H. R. , Elton R. C.
Published: July 2002

The temporal evolution of the radial profile of lithium-like oxygen (OVI or O5+) 3d-4f, 52.0 nm emission from 80 ns rise time, 6–16 kA peak current, pulsed capillary discharges in 1 mm diam, 1 cm long polyacetal capillaries, has been studied. Evidence was seen for a central ‘‘flattening’’ in this emission. Extreme-ultraviolet, time-integrated, pinhole transmission grating spectra were taken of discharges at 16 and 23 kA peak currents in 0.5 mm diam, 1 cm long capillaries. Spectra obtained at the higher peak current (23 kA) showed evidence of helium-like (CV or C4+) and/or hydrogen-like carbon (CV or C5+) resonance line emission (4.0 and 3.4 nm wavelength) filling the capillary diameter.
Effects of scattering upon energetic ion energy loss in plasmas
Authors: Li Chikang , Petrasso Richard D.
Published: July 2002

The effects of scattering are assessed for energetic ions of interest as ρR, yield, and/or implosion symmetry diagnostics. Because the Coulomb logarithm between such energetic ions (≳1 MeV) and the ‘‘field’’ ions is always of order 10 or larger, scattering effects are shown to be small. In contrast the Coulomb logarithm between energetic ions and ‘‘field’’ electrons is typically of order 5; but here the large mass ratio (104) between ion and electron precludes the possibility of ion scattering. Consequently the continuous-slowing-down stopping power and the linear-energy-transfer (LET) stopping power are very close. The consequences of this near identity in the stopping powers is exploited in order to calculate ρR over a range of plasma parameters of relevance to inertial confinement fusion (ICF) plasmas.
Linear response of the two-dimensional pure electron plasma: Quasimodes for some model profiles
Authors: Corngold Noel R.
Published: July 2002

After examining the initial value problem for the linear, diocotron response of a long cylinder of pure-electron plasma, the ‘‘quasimodes’’ associated with convex, power-law density profiles are studied. For these profiles, exact, analytic results are available. The ‘‘quasimodes,’’ which are damped by phase mixing, may be characterized by their angular variation, flatness, and the magnitude of the gap separating the plasma from the containing wall.
Ponderomotive feedback stabilization of external kinks and disruptions in tokamaks
Authors: D’Ippolito D. A. , Myra J. R. , Jardin S. C. , Chance M. S. , Valeo E. J.
Published: July 2002

A novel mechanism for feedback stabilization of external kink modes and disruptions is proposed in which modulated radio-frequency (RF) antennas apply a stabilizing ponderomotive force (PF) to the plasma. The RF power required for n=1 kink stabilization in the Princeton Beta Experiment-Modified (PBX-M) [Phys. Fluids B 2, 1271 (1990)] is estimated, and it is shown that the idea can be tested with the existing antenna system. A simulation of a preliminary n=0 modulation experiment on PBX-M also shows that the PF can balance applied vertical field oscillations.
Three-dimensional collisional drift-wave turbulence: Role of magnetic shear
Authors: Zeiler A. , Biskamp D. , Drake J. F.
Published: July 2002

Three-dimensional (3-D) nonlinear simulations of collisional drift-wave turbulence are presented. Results for the Hasegawa–Wakatani equations (without magnetic shear) in 3-D are compared to former two-dimensional (2-D) simulations. In contrast to the 2-D system the 3-D situation is completely dominated by a nonlinear drive mechanism. The final state of the system is sensitive to the configuration of the computational grid since the sheared flow develops at the longest scales of the system. When magnetic shear is included, the system is linearly stable but the turbulence is self-sustained by basically the same nonlinear mechanism. Magnetic shear limits the size of the dominant eddies, so the system evolves to a stationary turbulent state independent of the computational box. Finally, it is shown that the level of turbulence in the system with magnetic shear depends sensitively on the size of the effective Larmor radius ρs compared with the characteristic transverse scale length of the eddies.
Nonlinear growth of the double tearing mode
Authors: Yu Qingquan
Published: July 2002

A theory for the growth of the double tearing mode in the early nonlinear stage is presented. The equations governing the nonlinear evolution of double tearing modes are formulated and are solved in different parameter regimes. When the phase difference between the two islands is π, the coupling of the two islands plays a destabilizing role. The island growth is of the form w∼t in the early nonlinear stage. The instability condition for the nonlinear double tearing mode is found, and is the same as that for the linear double tearing mode. When current profile is flattened near the two rational surfaces, the coupling plays the dominant role in determining the nonlinear growth.
Temperature fluctuations and plasma edge turbulence in the Brazilian tokamak TBR
Authors: Castro R. M. , Heller M. V. A. P. , Caldas I. L. , da Silva R. P. , Brasilio Z. A. , Nascimento I. C.
Published: July 2002

To investigate the tokamak turbulence, a set of Langmuir probes and a triple probe have been designed and used in the TBR [J. Fusion Energy 12, 529 (1993)] to measure average and fluctuating values of density, potential, and temperature of the plasma edge. The obtained results showed a significant influence of the temperature fluctuations in the transport parameters. Namely, taking into account this influence, the density and plasma potential power spectra were obtained, and the turbulence parameters reevaluated. Furthermore, the computed cross-power-spectra showed appreciable linear correlation, and the cross-bispectra showed a quadratic mode coupling between temperature fluctuations and other quantities. Significant bicoherence between these fluctuations was observed. Finally, for a fluctuation monitored at two probe points, no preferential direction for energy cascading was detected.
Parametric coupling of a high-frequency electromagnetic wave to whistler and lower-hybrid waves in the ionosphere
Authors: Sawhney B. K. , Tripathi V. K. , Singh S. V.
Published: July 2002

Parametric coupling of a high-frequency electromagnetic wave to an obliquely propagating whistler or lower-hybrid wave and a high-frequency sideband electromagnetic wave have been studied in the ionosphere. The growth rate of the three-wave resonant interaction process decreases with the frequency of the lower-hybrid wave and increases with whistler wave frequency. Growth rate is greater for the whistler wave than the lower-hybrid wave. The electromagnetic wave is scattered at a larger angle in the case of the lower-hybrid wave than the whistler wave.
Physical mechanism of enhanced stability from negative shear in tokamaks: Implications for edge transport and the L-H transition
Authors: Antonsen T. M. , Drake J. F. , Guzdar P. N. , Hassam A. B. , Lau Y. T. , Liu C. S. , Novakovskii S. V.
Published: July 2002

The enhancement of stability to ballooning modes from negative shear in tokamaks is shown to be a simple consequence of the orientation of the convective cell with respect to the toroidally outward effective gravity, g↘. For modest positive shear, convective cells remain oriented along g↘ as they map along field lines. In contrast, for negative shear or very positive shear convective cells twist strongly away from g↘ and are less strongly driven. The twist of convection cells is controlled by the profile of the vertical magnetic field along the outer midplane, Bz. Twist is a minimum in regions where Bz is independent of the major radius. Transport should be highest in such locations. Resistive ballooning modes in the tokamak edge are strongly stabilized by modest values of negative shear. Tokamak discharges with finite values of βp develop regions of local negative shear on the outside midplane of the plasma torus. This local negative shear should self-stabilize resistive ballooning modes at finite values of the poloidal beta. This effect may impact the transition to high confinement operation (H-mode).
Velocity shear effect on Rayleigh–Taylor vortices in nonuniform magnetized plasmas
Authors: Chakrabarti Nikhil , Kaw P. K.
Published: July 2002

In this paper, the effect of velocity shear on Rayleigh–Taylor vortices has been demonstrated. An inhomogeneous plasma is considered with a density profile such that the diamagnetic drift velocity Vn=(cTe/eB)dn0/dx is a constant and includes the effect of an ambient poloidal shear flow Veq(x)=V⊥0′(x−x0)y. The final equation describing the stationary Rayleigh–Taylor vortex is shown to have the structure of a nonlinear Poisson equation, where the nonlinearity arises essentially because of the velocity shear term. This equation has been solved numerically and it has been shown that qualitatively new two-dimensional monopole vortex solutions may be obtained in the appropriate parameter space. Therefore, a new important nonlinear effect related to equilibrium shear flow has been identified in the calculations of Rayleigh–Taylor vortices which results in monopole-like solutions in plasmas.
Properties of drift waves in a filamentary density depletion
Authors: Peñano J. R. , Morales G. J. , Maggs J. E.
Published: July 2002

This analytical and numerical study explores the properties of electrostatic, drift-wave eigenmodes trapped within a magnetic field-aligned depletion in plasma density and temperature whose transverse dimension is on the order of the electron skin depth. The dependence of the complex eigenfrequencies on key parameters is investigated for collisionless and collisional plasma. The collisional description is based on the Lorentz model of electron pitch-angle scattering. The separate roles of the gradients in density and temperature are illustrated for the collisional and collisionless regimes. The predictions are compared to experimental observations [J. E. Maggs and G. J. Morales, Geophys. Res. Lett. 23, 633 (1996); Phys. Plasmas 4, 290 (1997)] of a controlled striation in the laboratory.
Effects of ion temperature fluctuations on the stability of resistive ballooning modes
Authors: Singh R. , Nordman H. , Jarmén A. , Weiland J.
Published: July 2002

The influence of ion temperature fluctuations on the stability of resistive drift and ballooning modes is investigated using a two-fluid model. The eigenmode equations are derived and solved analytically in a low beta model equilibrium. Parameters relevant to low-confinement-mode (L-mode) edge plasmas from the Texas Experimental Tokamak (TEXT) [Phys. Fluids B 2, 2879 (1990)] are used. The resistive modes are found to be destabilized by ion temperature fluctuations over a broad range of mode numbers. The scaling of the growth rate with magnetic shear, ηi(Ln/LTi) and the mode number is elucidated.
Kinetic analysis of the ballooning modes of comparable frequency regime in rotating plasmas
Authors: Zheng L.-J. , Tessarotto M.
Published: July 2002

An analytical method based on two-scale analysis is developed to solve the combined boundary and initial value problem describing the ballooning modes with frequency comparable to the parallel ion acoustic one in toroidally rotating plasmas. A sonic rotation is shown possibly to be harmful for the stability. A subsonic rotation is anticipated to always play a role of stabilizing the strong ballooning modes. But, the stabilizing effect of a subsonic rotation on the weak ballooning modes is negligible. A comparison with the ideal magnetohydrodynamic (MHD) analysis is presented. The scheme for including the kinetic effects into the MHD analysis is given. The different features of the ion temperature gradient modes [L.-J. Zheng and M. Tessarotto, Phys. Plasmas 3, 4610 (1996)] and the ballooning modes in the presence of toroidal rotation are also discussed.
Nonlinear theory of collective effects in helix traveling wave tubes
Authors: Freund H. P. , Zaidman E. G.
Published: July 2002

A time-dependent collective nonlinear analysis of a helix traveling wave tube including fluctuating (ac) space-charge effects is presented for a configuration where an electron beam propagates through a sheath helix surrounded by a conducting wall. The effects of dielectric and vane loading of the helix are included, as is efficiency enhancement by tapering the helix pitch, and external focusing by means of either a uniform solenoidal magnetic field or a periodic field produced by a periodic permanent magnet stack. Dielectric loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. Vane loading describes the insertion of an arbitrary number of vanes running the length of the helix. The electromagnetic field is represented as a superposition of azimuthally symmetric waves in a vacuum sheath helix. The propagation of each wave in vacuo, as well as the interaction of each wave with the electron beam, is included by allowing the amplitudes of the waves to vary in z and t. The dynamical equation for the field is solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Collective effects from the fluctuating rf beam space-charge waves are also included in the analysis by means of a superposition of solutions of the Helmholtz equation. The simulation is compared with a linear theory of the interaction, and an example is described corresponding to a tube built at Northrop–Grumman Corp.
Three-dimensional analysis of the power transfer between crossed laser beams
Authors: McKinstrie C. J. , Kanaev A. V. , Tikhonchuk V. T. , Giacone R. E. , Vu H. X.
Published: July 2002

The power transfer between crossed laser beams made possible by an ion-acoustic wave is studied. The power transfer between beams with Gaussian intensity profiles shifts the beam centroids, and alters the beam directions and focal lengths. The power transfer between beams with hot spots in their intensity profiles depends sensitively on whether the associated beam filaments intersect.
Enhanced thermal dissipation and the Rayleigh–Taylor instability in emulsion-like media
Authors: Ryutov D. , Toor A.
Published: July 2002

Rayleigh–Taylor instability in a finely structured emulsion-like medium consisting of the two components of different compressibility is considered. Although the term “emulsion” is used to describe the structure of the medium, under typical conditions of pulsed fusion devices the medium is actually a plasma. The two components are chosen in such a way that their densities in the unperturbed state are approximately equal. Specific emphasis has been made on the analysis of perturbations with the scale ƛ considerably exceeding the size of the grains a. Average equations describing such perturbations are derived. The difference in compressibility of the two components leads to the formation of temperature variations at the scale a, and increases the rate of the thermal dissipation by a factor (ƛ/a)2. The strongest stabilizing effect of the thermal dissipation takes place when the heat conduction time is comparable with the instability growth rate.
Ion cyclotron range of frequencies heating and flow generation in deuterium–tritium plasmas
Authors: Wilson J. R. , Bell R. E. , Bernabei S. , Hill K. , Hosea J. C. , LeBlanc B. , Majeski R. , Nazikian R. , Ono M. , Phillips C. K. , Schilling G. , von Goeler S. , Bush C. E. , Hanson G. R.
Published: July 2002

Recent radio-frequency heating experiments on the Tokamak Fusion Test Reactor (TFTR) [Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] have focused on developing tools for both pressure and current profile control in deuterium–tritium (DT) plasmas. A new antenna was added to investigate pressure profile control utilizing direct ion Bernstein wave (IBW) heating. This was the first time direct IBW heating was explored on TFTR. Plasma heating and driven poloidal flows are observed. Previously heating and current drive via mode-converted IBW waves had been demonstrated in non-DT plasmas but efforts in DT plasmas had been unsuccessful. This lack of success had been ascribed to the presence of a small Li7 minority ion population. In the most recent experiments Li6 was used exclusively for machine conditioning and mode-conversion heating consistent with theory is now observed in DT plasmas.
Plasma rotation and finite Larmor radius losses in a reversed field pinch
Authors: Bartiromo Rosario
Published: July 2002

A model is proposed to calculate the effect of particle and momentum losses due to unconfined ion orbits at the edge of the plasma in a reversed field pinch device. The value of the shearing rate can be obtained, requiring that the electromagnetic force due to the return current in the confined plasma is equilibrated by the reaction force due to direct ion momentum losses. A natural scaling of the shearing rate with the ion cyclotron frequency is obtained by assuming that this equilibrium is achieved through the effect of orbit squeezing caused by the finite value of the radial electric field gradient. The extension of the shear layer has been obtained requiring that the contribution of charge exchange viscosity to the perpendicular momentum balance is negligible. A shear layer width scaling as the Larmor radius is obtained. The scaling relations derived account for recent observations obtained in the reversed field experiment, RFX [G. Rostagni, Fusion Eng. Des. 25, 301 (1995)]. A quantitative comparison with experimental data indicates that impurities in low ionization stages can play an important role in the shear layer dynamics.
Quasi-periodic, explosive events and singularities in relevant nonlinear models
Authors: Coppi A. C. , Coppi B.
Published: July 2002

A nonlinear analytical model is presented to describe sequences of quasi-periodic explosive events (bursts) involving a primary explosive instability which can be moderated by a secondary mode driven by the primary instability when this exceeds an appropriate threshold. The relevant time dependent nonlinear equations, involving the primary instability and the amplitude of the secondary mode, have a characteristic singularity of which two main types are identified. Only one type is shown to lead to explosive events. The considered bursts refer, for instance, to heated or accelerated particle populations, the case of electrons heated by the slowing down of charged nuclei produced by fusion reactions being a particularly significant one.
Transient and time periodic self focusing of a laser beam in a plasma
Authors: Bharuthram R. , Parashar J. , Tripathi V. K.
Published: July 2002

A Gaussian laser beam with step function temporal profile, propagating through a plasma exerts radial ponderomotive force on the electrons and creates a plasma channel. The channel evolves on a time scale ∼r0/cs, where r0 is the beam spot size and cs is the sound speed, accompanied by time-dependent self focusing of the laser. In the case of an amplitude modulated laser, strong self distortion occurs when modulation frequency Ωr0/cs, self focusing occurs at a higher laser power and the beam width parameter has a small time-dependent ripple.
Particle-in-cell simulations of ultra intense laser pulses propagating through overdense plasma for fast-ignitor and radiography applications
Authors: Lasinski Barbara F. , Langdon A. Bruce , Hatchett Stephen P. , Key Michael H. , Tabak Max
Published: July 2002

Zohar (two-dimensions, particle-in-cell) [C. K. Birdsall and A. B. Langdon, Plasma Physics via Computer Simulation (McGraw–Hill, New York, 1985)] simulations of ultra intense laser beams boring into overdense plasmas whose parameters are guided by the fast-ignitor concept and radiography applications are presented. Complex low frequency magnetic field structures, narrow channel formation, and beam deflection are all evident. Particle tracking diagnostics elucidate the nature of the currents that produce and interact with these static magnetic fields which are larger than 109 G for simulations at 1021 W/cm2 in a 50nc plasma. Tracking electron orbits provides a more complete understanding of the hot electron generation as the short pulse, high intensity laser penetrates overdense plasma. Particles which constitute the current in the narrow channel are partially confined by the low frequency magnetic field. In contrast, the return current particles on the outside of the channel are defocused by the high magnetic field and move away from the channel.