bluuebunny/crossref_metadata_2025 huggingface dataset:
An approximate analytic expression for plasma viscosity in finite aspect ratio tokamaks and its applications
Authors: Shaing K. C. , Yokoyama M. , Wakatani M. , Hsu C. T.
Published: July 2002

An approximate analytic expression for plasma viscosity in finite aspect ratio tokamaks is constructed from all the asymptotic limits. The resultant viscosity coefficient is compared with the numerical results of the solution of the linearized drift kinetic equation. Neoclassical fluxes are reformulated in terms of the viscosity and friction coefficients. These fluxes can be employed to study the omnigeneous property of high-beta small, or large aspect ratio tokamaks.
Entropy production and Onsager symmetry in neoclassical transport processes of toroidal plasmas
Authors: Sugama H. , Horton W.
Published: July 2002

Entropy production and Onsager symmetry in neoclassical transport processes of magnetically confined plasmas are studied in detail for general toroidal systems, including nonaxisymmetric configurations. It is found that the flux surface average of the entropy production defined from the linearized collision operator and the gyroangle-averaged distribution function coincides with the sum of the inner products of the thermodynamic forces and the conjugate fluxes consisting of the Pfirsch–Schlüter, banana-plateau, nonaxisymmetric parts of the neoclassical radial fluxes and the parallel current. It is proved from the self-adjointness of the linearized collision operator that the Onsager symmetry is robustly valid for the neoclassical transport equations in the cases of general toroidal plasmas consisting of electrons and multi-species ions with arbitrary collision frequencies. It is shown that the Onsager symmetry holds whether or not the ambipolarity condition is used to reduce the number of the conjugate pairs of the transport fluxes and the thermodynamic forces. The full transport coefficients for the banana-plateau and nonaxisymmetric parts are separately derived, and their symmetry properties are investigated. The nonaxisymmetric transport equations are obtained for arbitrary collision frequencies in the Pfirsch–Schlüter and plateau regimes, and it is directly confirmed that the total banana-plateau and nonaxisymmetric transport equations satisfy the Onsager symmetry.
Transit-time scattering and heating of a relativistic electron beam in strong Langmuir turbulence
Authors: Melatos A. , Padden W. E. P. , Robinson P. A.
Published: July 2002

A Fokker–Planck theory is developed to describe the diffusion in momentum space of a beam of relativistic electrons due to multiple transit-time interactions with an ensemble of coherent Langmuir wave packets. The theory incorporates two ingredients: a perturbed-orbit calculation of the momentum change of a test particle during a single transit-time interaction, and an ensemble average of the resulting Fokker–Planck coefficients based on the statistical properties of strong Langmuir turbulence. An approximate analytic solution of the Fokker–Planck equation is obtained for the case of a strongly collimated beam, and is used to interpret measurements of energy and pitch-angle scattering in relativistic-electron-beam (REB) experiments. Fokker–Planck coefficients are also calculated for a weakly collimated beam. It is shown that the theory correctly predicts the amount of energy scattering in REB experiments, but underestimates the pitch-angle scattering regardless of the distribution of wave packet orientations and the degree of collimation of the beam. This discrepancy may be a product of the approximate wave-packet structure assumed in the analysis, or of systematic errors in the experimental data; alternatively, it may imply that a non-transit-time process is responsible for part of the pitch-angle scattering observed.
Ion cyclotron range of frequency experiments in the Tokamak Fusion Test Reactor with fast waves and mode converted ion Bernstein waves
Authors: Majeski R. , Rogers J. H. , Batha S. H. , Bers A. , Budny R. , Darrow D. , Duong H. H. , Fisher R. K. , Forest C. B. , Fredrickson E. , Grek B. , Hill K. , Hosea J. C. , Ignat D. , LeBlanc B. , Levinton F. , Medley S. S , Murakami M. , Petrov M. P. , Phillips C. K. , Ram A. , Ramsey A. T. , Schilling G. , Taylor G. , Wilson J. R. , Zarnstorff M. C.
Published: July 2002

Recent experiments in the ion cyclotron range of frequencies (ICRF) in the Tokamak Fusion Test Reactor [Fusion Technol. 21, 13 (1992)] are discussed. These experiments include mode conversion heating and current drive, fast wave current drive, and heating of low (L)- mode deuterium–tritium (D–T) plasmas in both the hydrogen minority and second harmonic tritium regimes. In mode conversion heating, a central electron temperature of 10 keV was attained with 3.3 MW of radio-frequency power. In mode conversion current drive experiments, up to 130 kA of current was noninductively driven, on and off axis, and the current profiles were modified. Fast wave current drive experiments have produced 70–80 kA of noninductively driven current. Heating of L-mode deuterium and D–T plasmas by hydrogen minority ICRF has been compared. Finally, heating of L-mode D–T plasmas at the second harmonic of the tritium cyclotron frequency has been demonstrated.
Dispersion properties of low-frequency waves in magnetized dusty plasmas with dust size distribution
Authors: Tripathi K. D. , Sharma S. K.
Published: July 2002

Dispersion properties of some low-frequency modes in magnetized dusty plasmas have been investigated theoretically, taking into account the dust size distribution. In contrast to earlier work on dispersion properties of different modes in magnetized dusty plasmas with a single dust size, it is assumed that the dust grains are distributed in size. Assuming that the dust size is given by a power law distribution, with a nonzero minimum and finite maximum grain size, the effect of dust size distribution on the dispersion properties of some electrostatic and electromagnetic modes in magnetized dusty plasmas is investigated. It is seen that in the very low-frequency (VLF) regime, taking into account the size distribution leads to a new kind of damping for an electrostatic dust-cyclotron and right-handed circularly polarized (RCP) electromagnetic Alfvén mode, whereas the left-handed circularly polarized (LCP) Alfvén mode remains undamped. In the low-frequency (LF) regime, size distribution results only in the modification of the dispersion relations for all the modes.
Normal curvature, local magnetic shear and parallel current density in tokamaks and torsatrons
Authors: Cooper W. A.
Published: July 2002

Reversed magnetic shear is associated with large negative surface averaged parallel current densities that increase in magnitude towards the plasma edge and guarantee stability at large β in tokamaks. The surface averaged parallel current density is smaller and decreases in magnitude radially in conventional shear tokamaks. This results in much lower magnetohydrodynamic (MHD) stable β. The parallel current density in torsatrons is dominated by the Pfirsch–Schlüter currents, the global shear contributes negligibly to the local shear and the β limits are comparable to that of a conventional tokamak. A simplistic evaluation of the local sign of the driving terms in the energy principle does not provide a useful guide to even qualitatively describe the MHD stability of a device. Ballooning instabilities concentrate in regions where the curvature is most destabilizing. Their localization does not appear to depend on the sign of the interaction between the parallel current density and the local magnetic shear contribution to the driving term.
The evidence for nonlocal transport in the Texas Experimental Tokamak
Authors: Gentle K. W. , Bravenec R. V. , Cima G. , Hallock G. A. , Phillips P. E. , Ross D. W. , Rowan W. L. , Wootton A. J. , Crowley T. P. , Ouroua A. , Schoch P. M. , Watts C.
Published: July 2002

The electron temperature response of a tokamak to rapid edge cooling has characteristics difficult to reconcile with local transport analysis. The initial observations in the Texas Experimental Tokamak [K. W. Gentle, Nucl. Tech. Fusion 1, 479 (1981)] have been extended to a wider range of plasma and perturbation parameters, including auxiliary heating, and the associated turbulence changes have been measured across the plasma radius. The fast edge temperature drops and core temperature increases are quantified by more extensive analysis. A perturbation complementary to edge cooling, edge heating by a fast current ramp, evokes a completely complementary plasma response.
The effect of a radial electric field on ripple-trapped ions observed by neutral particle fluxes
Authors: Heikkinen J. A. , Herrmann W. , Kurki-Suonio T.
Published: July 2002

The effect of a radial electric field on nonthermal ripple-trapped ions is investigated using toroidal Monte Carlo simulations for edge tokamak plasmas. The increase in the neutral particle flux from the ions trapped in local magnetic wells observed by the charge exchange (CX) detector at a low confinement to high confinement transition at ASDEX (Axially Symmetric Divertor Experiment). Upgrade tokamak [Proceedings of the 20th European Conference on Controlled Fusion and Plasma Physics, Lisbon (European Physical Society, Petit-Lancy, Switzerland, 1993), Vol. 17C, Part I, p. 267] is reproduced in the simulations by turning on a radial electric field near the plasma periphery. The poloidal and toroidal angles at which the CX detector signal is most sensitive to the radial electric field are determined. A fast response time of the signal in the range of 50–100 μs to the appearance of the electric field can be found in the simulations with a relatively large half-width of the negative electric field region.
Interplay between ion acoustic waves and electron plasma waves associated with stimulated Brillouin and Raman scattering
Authors: Labaune C. , Baldis H. A. , Renard N. , Schifano E. , Michard A.
Published: July 2002

Direct evidence is presented of the temporal and spatial interplay between ion acoustic waves (IAWs) associated with stimulated Brillouin scattering (SBS) and electron plasma waves (EPWs) associated with stimulated Raman scattering (SRS) in conditions of interest to inertial confinement fusion (ICF). The two types of waves grow over a limited region in the front part of the plasma, but at different times. Inhibition of the EPWs by the IAWs has been observed in the early part of the laser pulse, and this inhibition can be strengthened by increasing the level of the IAWs.
Theoretical and experimental studies of laser-produced plasmas driven by high-intensity femtosecond laser pulses
Authors: Gauthier J.-C. , Geindre J. P. , Audebert P. , Bastiani S. , Quoix C. , Grillon G. , Mysyrowicz A. , Antonetti A. , Mancini R. C.
Published: July 2002

A theoretical and experimental study of the dynamics of the electron density gradient in near-solid-density plasmas produced by the interaction of ultra-short laser pulses with solid targets at intensities between 1013 and 1016 W/cm2 and pulse duration between 0.12 and 2.5 ps is presented. X-ray spectroscopy of n=3 to n=1 resonance and dielectronic satellite lines is used to determine the range of electron densities in the plasma. Frequency-domain interferometry is employed to measure the expansion velocity and the electron density gradient scale length as a function of laser pulse duration and intensity. Quantitative agreement is noticed with one-dimensional hydrodynamic simulations which include the solution of the wave equation for the laser field.
Inductive electron heating revisited
Authors: Tuszewski M.
Published: July 2002

The induced rf magnetic fields of low-frequency inductively coupled plasmas are measured and modeled [M. Tuszewski, Phys. Rev. Lett. 77, 1286 (1996)]. The fields penetrate deep into the discharges, in contrast with existing predictions of field decay within a thin skin layer. Fluid calculations suggest that the enhanced rf penetration is due to a reduction of the plasma conductivity by the induced magnetic fields. In this paper, new oxygen and argon data concerning rf power absorption are reported and a general description of inductive electron heating is suggested.
Impurity screening in Ohmic and high confinement (H-mode) plasmas in the Alcator C-Mod tokamak
Authors: McCracken G. M. , Lipschultz B. , LaBombard B. , Goetz J. A. , Granetz R. S. , Jablonski D. , Lisgo S. , Ohkawa H. , Stangeby P. C. , Terry, and the Alcator Group J. L.
Published: July 2002

A series of experiments has been carried out to determine the relative screening of the low recycling gaseous impurities, nitrogen and methane, in Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] discharges. The impurity density in the core plasma is directly proportional to the rate of impurity injection. Screening was found to vary weakly with plasma density but to be markedly dependent on the poloidal position of injection. Detachment of the plasma results in significantly less screening both in Ohmic and in high confinement (H-mode) discharges, particularly when the gas is injected into the divertor or at the inner vessel wall. Evidence of impurity transport from the scrape-off layer into the divertor using bolometry and visible spectroscopy is presented and modeling of the transport using a two-dimensional Monte Carlo code is described.
Linear waves and instabilities in thermally anisotropic two-component magnetohydrodynamics
Authors: Ghildyal V. , Kalra G. L.
Published: July 2002

The paper analyzes low-frequency waves in a plasma model that is made up of two thermally anisotropic magnetohydrodynamic components, by means of wave-front diagrams, a useful tool that has not attracted the desired attention. The wave-front diagrams of the fast propagating suprathermal mode, besides the usual fast, slow, and Alfvén modes, have been plotted for a variety of situations. These diagrams are used to bring out the physical significance of the anisotropic model vis-à-vis the isotropic model. The question of stability that has been completely ignored so far in the plasma models based on two magnetohydrodynamic components has also been addressed. Analogues of the firehose and mirror instabilities, which are supported by this model, are examined. Their comparison with single-component anisotropic plasma results suggests the possibility of suppressing the mirror instability.
Particle acceleration by slow magnetosonic wave turbulence
Authors: Gallegos-Cruz A.
Published: July 2002

An important alternative for particle acceleration in a turbulent plasma is through magnetohydrodynamic (MHD) wave modes. However, particle acceleration by slow magnetosonic mode has been systematically disregarded for some nonquantitative considerations. Slow magnetosonic waves interchange energy with the medium through two basic processes: resonant (Landau and gyroresonant) and nonresonant (as viscosity and thermal conduction). The energized particles lose energy mainly through Coulombian collisions with the plasma particles. In this work, a quantitative study of the time scales involved in several energetic transfer wave–particle processes is done. Comparison of all these time scales allows one to determine the possibilities that particle acceleration by slow magnetosonic wave mode may occur within the solar corona. It is shown that physical conditions can hardly exist within the solar corona, where thermal particles can be energized and form an acceleration spectrum after crossing the collisional barrier.
Modification of plasma edge fluctuations using electrostatic probes in Keda Tokamak-5C
Authors: Kan Zhai , Yi-zhi Wen , Wen Yu , Shu-de Wan , Wan-dong Liu , Chang-xuan Yu , Zhi-zhan Xu
Published: July 2002

The experiments to characterize the modification of edge fluctuations using electrostatic probes have been performed on Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190]. The results show that the externally excited perturbation propagates in the direction of electron directional movement. The observed phenomena are mainly related to the electrons, and can be well explained by the proposed dynamic theory of the ballistic mode. The experimental correlation coefficient and the longitudinal dispersion relation are in good agreement with the proposed ballistic model.
The energy balance in the plasma of a coaxial plasma opening switch
Authors: Fruchtman A. , Ivanov A. A. , Kingsep A. S.
Published: July 2002

The two-dimensional energy flow in the plasma of a coaxial plasma opening switch (POS), during the Hall-induced shock penetration of a magnetic field, is analyzed. The electron collisionality is assumed to be high enough that the dissipated magnetic-field energy becomes electron thermal energy. It is shown that that part of the magnetic-field energy (a third) that is dissipated at the cathode at the generator side of the plasma, becomes an electron kinetic energy, that is convected along the current channel. It is also shown that in the magnetized plasma magnetic-field energy flows backwards towards the generator. The third new result is that inside the shock front, electron thermal energy is converted into magnetic-field energy, contrary to the usual situation in shock waves in which field energy is converted into particle thermal energy.
Convective amplification of wake field due to self-modulation of a laser pulse induced by field ionization
Authors: Kato S. , Kishimoto Y. , Koga J.
Published: July 2002

By using a particle-in-cell simulation code which includes tunneling ionization, it is found that strong wake fields are excited and amplified due to the modulation of an intense laser pulse induced by an ionizing plasma in the regime where the matching condition between laser pulse length (LLaser) and plasma wavelength (λp), i.e. LLaser⩽λp is not satisfied. The modulation is characterized by a gradual steepening of the pulse front where there is a copropagating density gradient which is due to the rapid plasma formation from ionization there. Accordingly, the wake field is convectively amplified to a significant level. This amplification of plasma waves always occurs, more or less, for laser propagation in ionizing plasmas.
Laser wakefield: Experimental study of nonlinear radial electron oscillations
Authors: Marquès J. R. , Dorchies F. , Amiranoff F. , Audebert P. , Gauthier J. C. , Geindre J. P. , Antonetti A. , Antonsen T. M. , Chessa P. , Mora P.
Published: July 2002

The plasma electron density oscillation produced in the wake of a narrow (beam waist≪plasma wavelength) ultrashort laser pulse is measured by frequency-domain interferometry with a temporal resolution much better than the electron plasma period, and a spatial resolution across the laser focal spot. The absolute density perturbation is observed to be maximum when the pulse duration equals half the plasma period. The relative density perturbation varies from a few percent at high density to 100% at low density. For nonlinear oscillations we measure the increase of the electron plasma frequency predicted for radial oscillations [J. M. Dawson, Phys. Rev. 113, 383 (1959)]. The damping of the oscillations is observed. It is very rapid (a few periods) when the oscillation is nonlinear. Comparison with the code WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Rev. E 53, R2068 (1996)] indicates that the gas ionization creates a steep radial density gradient near the edge of the focus and that the electrons oscillating near this density gradient are responsible for the damping.
Three-dimensional kinetic stability theorem for high-intensity charged particle beams
Authors: Davidson Ronald C.
Published: July 2002

Global conservation constraints obtained from the nonlinear Vlasov–Maxwell equations are used to derive a three-dimensional kinetic stability theorem for an intense non-neutral ion beam (or charge bunch) propagating in the z direction with average axial velocity vb=const and characteristic kinetic energy (γb−1)mc2 in the laboratory frame. Here, γb=(1−vb2/c2)−1/2 is the relativistic mass factor, and a perfectly conducting cylindrical wall is located at radius r=rw, where r=(x2+y2)1/2 is the radial distance from the beam axis. The particle motion in the beam frame (“primed” coordinates) is assumed to be nonrelativistic, and the beam is assumed to have sufficiently high directed axial velocity that vb≫|v′|. Space-charge effects and transverse electromagnetic effects are incorporated into the analysis in a fully self-consistent manner. The nonlinear Vlasov–Maxwell equations are Lorentz-transformed to the beam frame, and the applied focusing potential is assumed to have the (time-stationary) form ψsf′(x′)=(γbm/2)[ωβ⊥2(x′2+y′2)+ωβz2z′2], where ωβ⊥ and ωβz are constant focusing frequencies. It is shown that a sufficient condition for linear and nonlinear stability for perturbations with arbitrary polarization about a beam equilibrium distribution feq(x′,p′) is that feq be a monotonically decreasing function of the single-particle energy, i.e., ∂feq(H′)/∂H′⩽0. Here, H′=p′2/2m+ψsf′(x′)+φeq(x′), where φeq(x′) is the space-charge potential.
Reduced scale National Ignition Facility capsule design
Authors: Dittrich T. R. , Haan S. W. , Marinak M. M. , Pollaine S. M. , McEachern R.
Published: July 2002

In this article we describe the design and simulated performance characteristics of an indirectly-driven inertial confinement fusion capsule which utilizes only 900 kJ of laser energy and 250 TW of laser power from the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)]. This intentional reduction in laser performance from the nominal NIF specifications of 1.8 MJ and 500 TW results in lowering the hohlraum x-ray drive temperature from 300 eV to 250 eV. These energy and radiation temperature reductions are believed to define a “lower bound” on the successful implosion of an ignition capsule. This reduced scale capsule has a beryllium ablator containing a radially varying copper dopant, and a cryogenic solid deuterium–tritium fuel layer surrounding a cavity filled with equilibrium vapor pressure gaseous deuterium and tritium. Two-dimensional simulations predict ignition and propagated burn from this capsule when either Rayleigh–Taylor instability or time-dependent drive asymmetry effects are included.
Nonlinear interaction of whistler waves with a modulated thin electron beam
Authors: Krafft C. , Volokitin A.
Published: July 2002

The nonlinear theory of a thin modulated electron beam interaction with a monochromatic whistler wave is considered. The self-consistent set of differential equations describing the wave amplitude evolution and the beam particle motion has been solved by a computer code. Results issued from the numerical solution of the differential system are discussed, namely the physical features of the nonlinear beam–wave interaction (trapping, slowing down of the beam, wave damping, multiple bunching, beam focusing), as well as the influence of the physical parameters on the wave emission: beam energy and density, initial beam velocity distribution, and beam current modulation. It has been shown that the trapped particles are the source of the emission; they are decelerated in phase with the wave and remain in Cherenkov resonance with it owing to a nonlinear shift of the parallel wave number. No quasiperiodic exchange of energy between the wave and the particles has been observed. Time evolution of the wave amplitude and the particle energy has been explained by a simple model, as well as the multibunched structures appearing in the particle dynamics for certain physical parameters.
Observation of Rayleigh–Taylor growth to short wavelengths on Nike
Authors: Pawley C. J. , Bodner S. E. , Dahlburg J. P. , Obenschain S. P. , Schmitt A. J. , Sethian J. D. , Sullivan C. A. , Gardner J. H. , Aglitskiy Y. , Chan Y. , Lehecka T.
Published: July 2002

The uniform and smooth focal profile of the Nike KrF laser [S. Obenschain et al., Phys. Plasmas 3, 2098 (1996)] was used to ablatively accelerate 40 μm thick polystyrene planar targets with pulse shaping to minimize shock heating of the compressed material. The foils had imposed small-amplitude sinusoidal wave perturbations of 60, 30, 20, and 12.5 μm wavelength. The shortest wavelength is near the ablative stabilization cutoff for Rayleigh–Taylor growth. Modification of the saturated wave structure due to random laser imprint was observed. Excellent agreement was found between the two-dimensional simulations and experimental data for most cases where the laser imprint was not dominant.
Influence of chamber temperature on properties of the corona discharge system
Authors: Uhm Han S.
Published: July 2002

This paper investigates the influence of the chamber temperature on properties of the corona discharge system. It is found that the critical voltage Vc required for the corona discharge breakdown is inversely proportional to the chamber temperature T. The electrical energy wc required for corona discharge breakdown is inversely proportional to the square of the chamber temperature T. Thus, the electrical energy consumption for the corona discharge system decreases significantly as the temperature increases. The plasma generation by corona discharge in a hot chamber is much more efficient than that in a cold chamber.
Dust–dust and dust-plasma interactions of monolayer plasma crystals
Authors: Schollmeyer H. , Melzer A. , Homann A. , Piel A.
Published: July 2002

The horizontal and vertical oscillation modes of monolayer plasma crystal are investigated and are related to dust–dust and dust-plasma interactions. It is found that the dust particles in the plasma sheath excited by a biased wire show parametric resonances. This parametric resonance is identified as the reason for the observed “sublimation” transition of the plasma crystal from a solid to a gaslike state.
Resonant Cherenkov emission of whistlers by a modulated electron beam
Authors: Starodubtsev M. , Krafft C. , Lundin B. , Thévenet P.
Published: July 2002

Emission of whistler waves through the Cherenkov resonance mechanism has been studied in a laboratory experiment modeling the interaction of a modulated electron beam with a magnetized plasma under conditions relevant to space experiments involving electron beam injection. The characteristics of the emission and the propagation of the radiated whistlers, and namely the features of their directive radiation patterns, have been controlled as a function of the beam and plasma parameters. This beam radiation process can be distinguished from another mechanism of wave emission, the nonresonant transition radiation from the beam injection point, owing to adequate choices of the physical conditions.
Identifying charge-exchange collision products within the ion-energy distribution of electrostatically accelerated plasmas
Authors: King Lyon B. , Gallimore Alec D.
Published: July 2002

When used in flowing electrostatically accelerated plasmas, electrostatic energy analyzers, such as Retarding Potential Analyzers (RPA’s) or Cylindrical Mirror Analyzers (CMA’s), occasionally yield data which seem implausible: for a known applied plasma acceleration voltage, electrostatic analyzers may indicate populations of ions having voltages much greater than that available through the electrostatic discharge. The process responsible for this phenomenon is resonant charge-exchange (CEX) collisions between ions of different charge species. Through the transfer of electrons, an ion of charge n+ can appear in an electrostatic analyzer with equivalent voltage of n-times the available acceleration potential. This paper discusses the phenomena responsible for the appearance of such high-voltage ions in the measured voltage distribution function and presents evidence for such reactions in the flowing xenon plasma produced by a Hall-effect current accelerator designed for spacecraft propulsion. For a 300 V applied potential on the Hall accelerator electrodes, CEX collisions are shown to produce ions having voltages as high as 800 V when measured using an electrostatic analyzer.
On the viscous and resistive dissipation of magnetohydrodynamic waves
Authors: Campos L. M. B. C.
Published: July 2002

The dissipative magnetohydrodynamic (MHD) equations, in the presence of Ohmic electrical resistance and bulk and shear viscosities, are linearized relative to a mean state of rest, and eliminated for the velocity perturbation. The result is a vector dissipative MHD equation consisting of (i) nondissipative terms, involving second-order space and time derivatives; (ii) dissipative terms, linear or quadratic in the diffusivities, involving fourth- and fifth-order space–time derivatives. Considering plane wave solutions, the dispersion matrix shows that Alfvén waves decouple, and are damped by Ohmic electrical resistance and shear viscosity. The latter two, and the bulk viscosity, damp the coupled slow and fast modes, which satisfy a dispersion relation of degree five in the frequency; it reduces to degree three in the case of magnetosonic waves, i.e., wave vector transverse to the direction of propagation. The roots of these dispersion relations of degree three to five, are obtained by a novel method, which applies first to the case of weak damping, and then can be extended to strong damping, e.g., taking into account products of diffusivities. For most applications, e.g., MHD waves in the solar atmosphere, the second-order effects of products of diffusivities are small.
Nonlinear δ
F
simulation studies of high-intensity ion beam propagation in a periodic focusing field
Authors: Stoltz Peter H. , Davidson Ronald C. , Lee W. Wei-li
Published: July 2002

This paper makes use of the nonlinear Vlasov–Poisson equations to describe the propagation of an intense, non-neutral ion beam through a periodic focusing solenoidal field with coupling coefficient κz(s+S)=κz(s) in the thin-beam approximation (rb≪S). The nonlinear δF formalism is developed for numerical simulation applications by dividing the total distribution function Fb into a zero-order part (Fb0) that propagates through the average focusing field κ̄z=const, plus a perturbation (δFb) which evolves nonlinearly in the zero-order and perturbed field configurations. To illustrate the application of the technique to axisymmetric, matched-beam propagation, nonlinear δF-simulation results are presented for the case where Fb0 corresponds to a thermal equilibrium distribution, and the oscillatory component of the coupling coefficient, δκz(s)=κz(s)−κ̄z, turns on adiabatically over many periods S of the focusing lattice. For adiabatic turn-on of δκz(s) over 20–100 lattice periods, the amplitude of the mismatch oscillation is reduced by more than one order of magnitude compared to the case where the field oscillation is turned on suddenly. Quiescent, matched-beam propagation at high beam intensities is demonstrated over several hundred lattice periods.
Effect of dust charge fluctuations on energy loss of a test dust charged particulate in a dusty plasma
Authors: Nasim M. H. , Shukla P. K. , Murtaza G.
Published: July 2002

The energy loss of a test charged particulate in an unmagnetized dusty plasma is calculated by employing the dusty dielectric response function which accounts for the dust charge fluctuation. It is found that the dust charge fluctuation introduces a new contribution to the energy loss, which enhances it for fast charge relaxation rates and reduces it for slow rates. In the energy loss curves drawn against the test charge velocity, some peaks are observed. These peaks diminish for large charge relaxation rates. Possible explanation for these peaks is also discussed. These results may be useful for understanding coagulation of dust grains in space and laboratory plasmas and for transport of dust particulates in the planetary rings.
Modes and quasi-modes for m=1,2 in a gyrokinetic model for a non-neutral plasma
Authors: Neil Rasband S. , Spencer Ross L.
Published: July 2002

Modes and quasi-modes for m=1,2 are studied in a gyro-kinetic model for a pure-electron plasma. Only z-independent perturbations are considered. Numerical methods are used to solve the relevant differential equations for smooth, analytic density profiles. Different temperatures and representative profiles are considered and comparison is made with the familiar cold fluid model from which the results depart but little, except at higher temperatures. A continuum component to the spectrum, present in the cold-fluid model, remains in the gyro-kinetic model to the order considered.
Calculation of resistive magnetohydrodynamic spectra in tokamaks
Authors: van der Holst B. , Beliën A. J. C. , Goedbloed J. P. , Nool M. , van der Ploeg A.
Published: July 2002

Resistive magnetohydrodynamic spectra of toroidal plasmas are calculated using the recently developed Jacobi–Davidson eigenvalue solver. Poloidal mode coupling in finite aspect ratio tokamaks yields gaps in the ideal Alfvén continuous spectrum. If resistivity is included, the ideal continua disappear and are replaced by damped global waves located on specific curves in the complex frequency plane. The end points of these curves join the tips of the ideal continua and the boundaries of the ideal spectral gap. The eigenfunctions of the waves on these resistive curves are shown to have definite parity in the poloidal harmonics. It is shown that for very small toroidicity the topology of the resistive spectrum is completely different from the cylindrical one. Independent of the size of the inverse aspect ratio the ideal gap remains visible in the resistive spectrum.
Global mode analysis of ideal magnetohydrodynamic modes in a heliotron/torsatron system: I. Mercier-unstable equilibria
Authors: Chen J. , Nakajima N. , Okamoto M.
Published: July 2002

By means of a global mode analysis of ideal magnetohydrodynamic (MHD) modes for Mercier-unstable equilibria in a planar axis L=2/M=10 heliotron/torsatron system with an inherently large Shafranov shift, the conjecture for global mode in Mercier-unstable equilibria from local mode analysis [N. Nakajima, Phys. Plasmas 3, 4556 (1996)] has been confirmed and the properties of pressure-driven modes inherent to such three-dimensional systems have been clarified. The Mercier-unstable equilibria are categorized into toroidicity-dominant and helicity-dominant Mercier-unstable equilibria. In the toroidicity-dominant Mercier-unstable equilibria, the pressure-driven modes change from interchange modes for low toroidal mode numbers n<M, to tokamak-like ballooning modes for moderate toroidal mode numbers n∼M, and finally to ballooning modes purely inherent to three-dimensional systems for fairly high toroidal mode numbers n≫M. In the helicity-dominant Mercier-unstable equilibria, the pressure-driven modes change from interchange modes for n<M or n∼M, directly to ballooning modes purely inherent to three-dimensional systems for n≫M.
Magnetospheric dynamics from a low-dimensional nonlinear dynamics model
Authors: Doxas I. , Horton W.
Published: July 2002

A physics based model for the coupled solar WIND–Magnetosphere–Ionosphere system (WINDMI) is described. The model is based on truncated descriptions of the collisionless microscopic energy transfer processes occurring in the quasineutral layer, and includes a thermal flux limit neglected in the Magnetohydrodynamic (MHD) closure of the moment equations. All dynamically relevant parameters of the model can be computed analytically. The system is both Kirchhoffian and Hamiltonian, ensuring that the power input from the solar wind is divided into physically realizable energy sub-components, a property not shared by data-based filters. The model provides a consistent mathematical formalism in which different models of the solar wind driver, ionospheric dissipation, global field configuration, and substorm trigger mechanism can be inserted, and the coupling between the different parts of the system investigated.
Coupling between nonlinear Alfvén waves and reduced magnetohydrodynamics for compressible fluids
Authors: Gazol A. , Passot T. , Sulem P. L.
Published: July 2002

A reduced description of magnetohydrodynamic (MHD) turbulence in a plasma permeated by a strong uniform magnetic field is derived using a reductive perturbative expansion method. This leads to a coupled system of equations for the velocity and magnetic fields in the planes perpendicular to the ambient field, described by the usual reduced MHD equations [Zank and W. H. Matthaeus, J. Plasma Phys. 48, 85–100 (1992)] and for parallel propagating Alfvén waves, which on the characteristic time and length scale of the transverse flow, undergo cumulative nonlinear effects described by a generalized Derivative Nonlinear Schrödinger equation (DNLS). The cases β far from unity and β≈1 are addressed separately. In the latter case, the nonlinear Alfvén waves drive a mean longitudinal velocity whose magnitude is comparable to that of the transverse components. A modulational analysis for a quasi-monochromatic plane Alfvén wave at β far from unity illustrates the effect of the transverse MHD flow on the Alfvén wave filamentation.
The toroidal 1/1 internal kink mode
Authors: Sugiyama Linda E.
Published: July 2002

The m0/n=1/1 internal mode in toroidal plasma is shown to have an intrinsic dependence on a (global) epsilon beta-poloidal (εβp) parameter independent of the inverse aspect ratio ε, when the degree of compressibility is determined self-consistently. The εβp parameter introduces properties very different from those predicted from strict incompressibility, where the mode depends strongly on aspect ratio. An exact solution and scaling properties are presented numerically. A companion paper provides a supporting analysis based on small parameter solutions of the toroidal perturbation equations [L. E. Sugiyama, submitted to Phys. Plasmas (1998)]. The maximal ordering of the mode growth rate is γ=O(ε1/2) or (δq)1/2 at low beta, βp∼1. The compressible mode has two distinct regimes in εβp and ε=a/R. At moderate εβp≲ε<1, the growth rate varies linearly with εβp−εβpc and is almost independent of ε when δq=1−q within q⩽1 is small, δq∼ε. At very small ε<εβp, corresponding to large βp>1 and also to δq>ε, the growth rate varies as γ∝βp2. For the 1/1 mode, the small value of δq=1−q within the region q<1, satisfied by almost all tokamaks with q0<1, determines a number of basic properties of the toroidal mode. Small δq makes the m=1 component of the magnetic flux order δq, causing the m=2 component of the toroidal current density to be the same order of magnitude as m=1 on the q=1 surface and thereby giving the 1/1 mode a fundamental inboard/outboard asymmetry—on the outboard side of the torus the separatrix current sheet vanishes. The asymmetry strengthens with resistivity and in the nonlinear phase.
Drift-kink instability induced by beam ions in field-reversed configurations
Authors: Nishimura Kazumi , Horiuchi Ritoku , Sato Tetsuya
Published: July 2002

The drift-kink instability in field-reversed configurations with a beam component is investigated by means of a three-dimensional particle simulation. The unstable mode with the toroidal mode number n=4 grows with the rate γ∼0.1−1.0ωci for a strong beam current and deforms the plasma profile along the beam orbit in the vicinity of the field-null line. This mode is nonlinearly saturated as a result of the relaxation of current profile. Both the saturation level and the growth rate tend to increase as the ratio of the beam current to the plasma current Ib/Ip increases. It is also found that there is a threshold value of the beam velocity vb∼vTi (ion thermal velocity) for the excitation of the instability.
Energetic particle transport in compact quasi-axisymmetric stellarators
Authors: Redi M. H. , Mynick H. E. , Suewattana M. , White R. B. , Zarnstorff M. C.
Published: July 2002

Hamiltonian coordinate, guiding center code calculations of the confinement of suprathermal ions in quasi-axisymmetric stellarator (QAS) designs have been carried out to evaluate the attractiveness of compact configurations which are optimized for ballooning stability. A new stellarator particle following code is used to predict the confinement of thermal and neutral beam ions in a small experiment with R=145 cm, B=1–2 T and for alpha particles in a reactor size device. As for tokamaks, collisional pitch angle scattering drives ions into ripple wells and stochastic field regions, where they are quickly lost. In contrast, however, such losses are enhanced in QAS so that high edge poloidal flux has limited value in improving ion confinement. The necessity for reduced stellarator ripple fields is emphasized.
Effective statistical weights of Rydberg orbitals of dopant Ar in dense plasmas and their effect on higher order satellite lines
Authors: Kawamura Tohru , Mima Kunioki , Koike Fumihiro
Published: July 2002

Calculated are the effective statistical weights of Rydberg orbitals of dopant Ar ions in dense hydrogen plasmas. Energy levels are shifted due to the screening of the nuclear Coulomb potential and the Stark effect. The electric microfield distribution is evaluated by using the adjustable-parameter exponential approximation. The Ryberg orbitals are easily ionized not only by the continuum lowering but also by the plasma electric microfield. The statistical weights of high-lying orbitals are strongly influenced by surrounding charged particles. The results of present calculations are compared with More’s empirical formula, and it is found that the effective statistical weights of Rydberg orbitals remain higher than those predicted by More’s formula.
Intense on-axis plasma production and associated relaxation oscillations in a large volume helicon source
Authors: Degeling A. W. , Sheridan T. E. , Boswell R. W.
Published: July 2002

A helicon wave mode with a peak downstream density of greater than 1018 m−3 in argon that exhibits bright ArII emission along the axis has been characterized. The experimental conditions are: Ar gas pressure of 1–5 mTorr, external magnetic field of 70–150 G and radio frequency (rf) power input between 2 and 4 kW a 13.56 MHz using a double half-turn antenna into a source of 9 cm inner radius and 50 cm length that opens into a diffusion chamber 45 cm radius and 200 cm length. Radial profiles of the density in the source and downstream show that plasma production is strongly concentrated on axis. B-dot probe measurements indicate that the wave phase velocity in this discharge mode is between 2 and 2.5×106 m/s, which has been shown previously to be the optimum velocity for resonant wave heating of electrons to increase the ionization rate. An interesting property of the high-density mode is that it is unstable on timescales of a few milliseconds and that a relaxation oscillation occurs between the high- and low-density modes. It is believed that this is driven by the depletion of neutrals in the source region due to ionization and momentum exchange with ions leaving the source.
Theory of plasma–wall transition with account of variable ion temperature
Authors: Benilov M. S.
Published: July 2002

The fluid model describing a transition from a weakly ionized collision-dominated plasma to a negative wall is supplemented with the ion energy equation, accounting for thermal energy exchange with atoms, expansion/compression work, and heating due to conversion of kinetic energy of the ion flow into thermal energy in collisions of ions with atoms. An asymptotic solution is obtained for the limit case when the Debye length is much smaller than the mean free path for collisions ion-neutral. If the electron temperature exceeds the temperature of the atoms, the distribution of the ion temperature in the (quasineutral) Knudsen layer is nonmonotonic. In the case when the electron temperature exceeds the temperature of the atoms by two orders of magnitude or more, the ion temperature in the bulk of the Knudsen layer exceeds substantially the atom temperature, although it does not exceed about 8% of the electron temperature.
Effects of pressure anisotropy on plasma equilibrium in the magnetic field of a point dipole
Authors: Krasheninnikov Sergei I. , Catto Peter J.
Published: July 2002

A physically meaningful equilibrium is derived for a plasma of finite pressure anisotropy confined by the magnetic field of a point dipole. Analytic solutions are explicitly displayed for low plasma pressure with strong pressure anisotropy and for high plasma pressure with weak pressure anisotropy, and numerical results presented for intermediate cases.
Observation of MeV energy deuterons produced by knock-on collisions between deuterium–tritium fusion α-particles and plasma fuel ions
Authors: Korotkov A. A. , Gondhalekar A. , Akers R. J.
Published: July 2002

This paper reports first observations in a tokamak plasma of MeV energy ions produced by close elastic collisions (knock-on) between deuterium–tritium fusion α-particles and thermal plasma fuel ions. Measurements of flux of MeV energy helium and deuterium atoms from the plasma in the Joint European Torus [P.-H. Rebut, A. Gibson, M. Huget et al., Nucl. Fusion 32, 187 (1992)] were made from which line-integrated energy distribution function of knock-on deuterons, F̄d(E), was deduced. Neutralization of the deuterons and α-particles was effected by charge-exchange with one- and two-electron species of the main intrinsic plasma impurities, carbon, beryllium, and helium. Deduced F̄d(E) is shown to be in agreement with calculations using classical confinement and slowing-down of fusion α-particles and knock-on deuterons.
Exploding aluminum wire expansion rate with 1–4.5 kA per wire
Authors: Sinars D. B. , Shelkovenko T. A. , Pikuz S. A. , Greenly J. B. , Hammer D. A.
Published: July 2002

X-ray backlighter images (radiographs) of current-induced explosions of 12.7–25 μm diam Al wires have been used to determine the expansion rate and internal structure of the dense wire cores. The current rises to 1–4.5 kA per wire in 350 ns, but voltage and current measurements show that the energy driving the explosion is deposited resistively during the first 40–50 ns, when the current is only a few hundred amperes per wire. A voltage collapse then occurs as a result of plasma formation around the wire, effectively terminating the energy deposition in the wire core. High-resolution radiographs obtained over the next 150–200 ns show the expanding wire cores to have significant axial stratification and foamlike structures with ∼10 μm scale lengths over most of the wire length before they disappear in the expansion process. The expansion rate of the portion of the wire cores that is dense enough to be detected by radiography is 1.4–2 μm/ns commencing approximately 25 ns after the moment of the voltage collapse. (The sensitivity limit is equivalent to 0.2 μm of solid density Al.) By 250 ns after the start of the current pulse, the detectable wire core diameter is 250 μm, but it contains only about 30% of the initial wire material.
On threshold of radial detachment in tokamaks
Authors: Tokar M. Z.
Published: July 2002

The threshold for radial plasma detachment in limiter tokamaks is theoretically investigated. It is shown by taking into account of realistic boundary conditions at the last closed magnetic surface that detachment can start at a radiation level significantly lower than the input power as it occurs in experiments. The origin of contradictions between previous approaches to the problem is clarified.
Convective cell formation in a levitated dipole
Authors: Kesner J. , Garnier D. T.
Published: July 2002

A plasma that is confined in a closed field line geometry, such as a levitated dipole, may be subject to the formation of convective cells. Using a “drift” fluid model, it is shown that such flow patterns can represent an appropriate equilibrium in a dipole field that contains a plasma with a nonaxisymmetric pressure profile. This can provide a means of fueling and heating a dipole confinement device.