bluuebunny/crossref_metadata_2025 huggingface dataset:
Study of indirectly driven implosion by x-ray spectroscopic measurements
Authors: Nishimura H. , Kiso T. , Shiraga H. , Endo T. , Fujita K. , Sunahara A. , Takabe H. , Kato Y. , Nakai S.
Published: July 2002

Fusion pellet implosion by laser-generated x rays was investigated by means of time-integrated spectroscopic measurements. Deuterium fuel was seeded with a small amount of Ar in order to determine the electron temperature and the density of the compressed fuel from, respectively, the emission intensity ratio and the broadening of the Lyβ (Ar17+1s-3p) and Heβ (Ar16+1s2-1s3p) lines. Comparison of the observed results with volume-averaged temperatures and densities obtained from one-dimensional (1-D) fluid-dynamic simulations showed large discrepancies at maximum compression. One possible explanation is that the fuel is stably compressed until the beginning of pusher deceleration by collision with a reflected shock wave from the pellet center, and that further compression during the deceleration phase is terminated in particular for heavy stagnation cases. Similar results were obtained for fusion output. Experimentally obtained neutron yields were close to those from the 1-D simulations at the beginning of the deceleration but a factor of 10–100 less than those expected at maximum compression. These results were examined in terms of shell disintegration in the acceleration phase and pusher–fuel mixing at the contact surface in the deceleration phase due to growth of initial perturbations of the pellet resulting from fluid instabilities.
Development of submillimeter wave gyrotron using 12 T superconducting magnet
Authors: Shimizu Y. , Makino S. , Ichikawa K. , Kanemaki T. , Tatsukawa T. , Idehara T. , Ogawa I.
Published: July 2002

The development of a submillimeter wave, high-harmonic gyrotron using a 12 T superconducting magnet is described. It has achieved frequencies as high as 636 GHz (corresponding to a wavelength of 472 μm) in second harmonic operation. Many resonances could be obtained as a single mode without competing modes. Typical output power is several hundred watts at both the fundamental and the second harmonic. At low beam currents (Ib∼0.5 A), third harmonic operation has been obtained.
Mode conversion and electron damping of the fast Alfvén wave in a tokamak at the ion–ion hybrid frequency
Authors: Fuchs V. , Ram A. K. , Schultz S. D. , Bers A. , Lashmore-Davies C. N.
Published: July 2002

Mode conversion of the fast Alfvén wave (FAW) at the ion-hybrid frequency in the ion cyclotron range of frequencies (ICRF) is studied in the presence of ion cyclotron absorption and direct electron damping in a tokamak plasma. The usual Budden model is extended to include the effect of electron damping and of the high-field-side cutoff, and is solved analytically and numerically. The mode-conversion efficiency is given as a function of the Budden transmission coefficient and of a phase integral, which describes interference between the incoming and outgoing waves. In incidence from the low-field side, a discrete spectrum of phases exists for which complete absorption (i.e., combined mode conversion and direct electron damping) of the FAW for a single transit of the resonance region can be achieved. This permits efficient electron heating and/or current drive via mode conversion of FAWs.
Influence of L-shell dynamics on K-shell yields for imploding krypton Z-pinch plasmas
Authors: Davis J. , Giuliani J. L. , Mulbrandon M.
Published: July 2002

The radiative performance of a Z-pinch krypton puff gas heated by a proposed multiterawatt generator is investigated with the aid of a one-dimensional (1-D) non-Local Thermodynamic Equilibrium (LTE) radiation hydrodynamic model self-consistently coupled to a circuit model. For stable loads configured either as cylindrical annular shells or uniform fills, predictions are made for the K- and L-shell soft x-ray emission as a function of the L-shell level structure. The results of numerical simulations show that both the thin annular shell and uniform fill are prolific K- and L-shell radiators. It is also predicted that the L-shell level structure profoundly affects the optimum K-shell soft x-ray yield, as well as the choices for load mass corresponding to optimum emission.
Electron and ion energies in plasmas generated by the electron-cyclotron-resonance mechanism
Authors: Uhm Han S. , Chang Hong Y. , Kim Jung H. , Song Sun K.
Published: July 2002

A theory is developed for the electron-cyclotron-resonance (ECR) plasmas, and an experiment is carried out to compare the theoretical predictions and experimental measurements. Several points are noteworthy from the theory and from experiment for electron temperature. First, electron temperature is inversely proportional to the square root of the chamber pressure. Second, electron temperature in a high-pressure (p≳1 mTorr) chamber is almost constant as the microwave power increases. Third, two components of electron temperature are observed in a low-pressure (p<1 mTorr) chamber. Fourth, energetic electrons are mostly concentrated around the ECR region. Fifth, overall electron temperature may display anisotropic behavior in a low-pressure chamber. A theoretical model of an ion acceleration mechanism deep inside ECR plasmas is developed, based on the ambipolar diffusion. It is found that the mean ion energy is proportional to the electron temperature and inversely proportional to the square of the density variation length. Therefore, the ion energy in a uniform plasma is negligibly small.
Measurements of the weak warm beam instability
Authors: Hartmann D. A. , Driscoll C. F. , O’Neil T. M. , Shapiro V. D.
Published: July 2002

Experiments are described on the interaction of a weak warm beam with a broad spectrum of unstable waves on a traveling wave tube. The wave–particle interactions are similar to those in beam–plasma systems, and are traditionally described by quasilinear theory. The precise wave evolution is obtained by launching a specified waveform, allowing it to interact with the beam, and analyzing the received waveform. Significant mode coupling is observed, resulting in saturated waves correlated less than 0.5 with their launch values. Experimentally, each wave is separated into a component proportional to the launch amplitude and a component due solely to mode coupling. The measured properties of these separate components agree quantitatively with a four-wave coupling model. Strongest coupling is observed between waves whose wave numbers match within about an inverse turbulent trapping length. In the linear growth regime, the measured ensemble-averaged wave growth rates and beam velocity diffusion rates agree reasonably with quasilinear and resonance-broadening theory; in the nonlinear regime near saturation, the discrepancies become larger.
Suppression of secondary electrons from an endplate of a tandem mirror by use of a biased mesh
Authors: Saito T. , Kiwamoto Y. , Tatematsu Y. , Yoshimura Y. , Takahashi T. , Inutake M. , Tamano T.
Published: July 2002

Negative bias is applied to a mesh placed in front of an endplate of the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985)]. A bias voltage of around 10 V results in an increase of the potential difference by an order of kV in the end region. The increment is about two times the temperature of end-loss electrons and most of that stems from the sheath potential drop in front of the endplate. Provided that almost all the secondary electrons emitted from endplate are reflected, these results agree with the prediction of a potential model for the open magnetic field configuration [J. Phys Soc. Jpn. 61, 3153 (1992)].
Indirect-drive radiation uniformity in tetrahedral hohlraums
Authors: Schnittman J. D. , Craxton R. S.
Published: July 2002

Tetrahedral hohlraums, by which are understood spherical hohlraums with four laser entrance holes (LEH’s) placed at or near the vertices of a tetrahedron, are proposed for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)] and the upgraded OMEGA laser [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)]. All but four of the 48 NIF beams can irradiate a tetrahedral hohlraum, assuming that 72 beam ports are provided to accommodate direct drive. On OMEGA, the target chamber provides an exact tetrahedral symmetry, permitting the irradiation of tetrahedral hohlraums with all 60 beams. Hohlraum designs are optimized using a new three-dimensional view-factor program called Buttercup, which traces all beam paths through the hohlraum and calculates the radiation flux on the capsule for different values of the albedo. Good irradiation uniformity (∼2% rms) can be obtained on the capsule at all times during the implosion, even with identical beam temporal histories, in contrast to the case of cylindrical hohlraums where ‘‘beam phasing’’ is needed.
Nonlaminar multicomponent models for electron flow in positive polarity multigap accelerators
Authors: Church B. W. , Sudan R. N.
Published: July 2002

Electron flow in multigap positive-polarity inductive accelerators is studied by numerical simulation and modeling. The objective of this work is to determine the operating principles of the electron flow such that an optimally efficient design of such machines can be achieved for intense ion beam generation. Because the electrons emitted in different gaps have different energies and canonical momenta, the theory of single-component magnetic insulation has to be extended in order to describe such multicomponent electron flows. A two-dimensional electromagnetic particle-in-cell code is used to simulate multicomponent electron flow in multigap accelerators with two, three, and four gaps. Observations from these simulations lead to new one-dimensional, time-independent models for these flows that incorporate the time-averaged effects of diamagnetic electron vortices. Equivalent circuits are constructed for simulated accelerators using voltage–current relations predicted by the models. These circuit models are incorporated into a software package to aid in the design of multigap inductive accelerators.
Kinetic effects on particle and heat fluxes in detached plasmas
Authors: Batishchev O. V. , Xu X. Q. , Byers J. A. , Cohen R. H. , Krasheninnikov S. I. , Rognlien T. D. , Sigmar D. J.
Published: July 2002

A model of high recycling scrape-off layer plasmas in tokamaks is presented where both ion and electron species are described by nonlinear kinetic equations. Coulomb and charged-neutral particle collisions are included. The ambipolar electric field and electrostatic sheath potential are evaluated self-consistently. Two models of fluid neutral transport are used to distinguish the neutral density variation for different tokamak divertor geometries. These models are incorporated into a comprehensive three-dimensional (1-D, 2 V) hybrid collisional particle-in-cell–Monte Carlo code W1 [Contrib. Plasma Phys. 34, 436 (1994)]. This code is used to investigate the effects of neutrals on divertor plasma detachment phenomena and on parallel heat and particle fluxes in the presence of strong gradients where fluid descriptions break down. Results are given for simulations of detached and attached divertor plasmas, and comparisons are made with solutions from a one-dimensional fluid model and with experimental observations.
Dispersion relation of a plasma in the periodic magnetic field of a wiggler
Authors: Petrillo V. , Maroli C.
Published: July 2002

The dispersion relation of a cold plasma in a periodic magnetic field is deduced and analyzed in both the presence and absence of plasma currents. In particular, the characteristics of the waves with phase velocity equal to the speed of light are shown.
Response to ‘‘Comment on ‘Bifurcations from periodic solution in a simplified model of two-dimensional magnetoconvection’ ’’ [Phys. Plasmas 2, 2945 (1995)]
Authors: Bekki N. , Karakisawa T.
Published: July 2002

In the above comment (Ref. 1), E. Knobloch, M. R. E. Proctor, A. M. Rucklidge, and N. O. Weiss have criticized a number of our results, in the following we present a detailed response to their criticism. (AIP).
Self-similar sheath expansion from a segmented planar electrode
Authors: Sheridan T. E.
Published: July 2002

The expansion of the pulsed plasma sheath away from a segmented planar electrode is studied using a collisionless particle-in-cell simulation. One segment of the electrode is biased negatively, while the other is fixed at ground potential. Cases where the grounded and biased segments are semi-infinite and where the width of the grounded segment is equal to the ion-matrix sheath width are considered. For the case of two half-planes, one grounded and one biased, it is found that the expanding sheath sweeps up ions from above the grounded segment and accelerates them to the biased segment, forming an arc of high ion density encircling a void region at the junction between the segments. Further, for this case sheath expansion is found to be self-similar as the problem lacks any length scale excepting the sheath width.
Fast switching with laser-generated continuous plasma columns
Authors: Polonskiy L. Ya. , Goltsov A. Yu. , Morozov A. V.
Published: July 2002

A novel approach to laser triggering of spark gaps is proposed. The method is based on switch initiation by Besselian laser beams. This results in the fast generation of a continuous conducting plasma column(s) and almost instantaneous electrical connection of electrodes. Experiments showed excellent performance of a gas-filled coaxial switch triggered with 2 ns laser pulses: 1.5–2.5 ns rise time, approximately zero delay, and 0.2–0.3 ns jitter obtained within a broad operational voltage range of three orders of magnitude. Depending on the type and pressure of the gas used in the spark gap, the experimentally determined laser energy required for triggering ranged from 40 to 500 mJ. It can be further reduced by factor of 5 by optimization of the focusing system. The unique feature of the proposed triggering method is independence of the required driving energy and switching parameters on the voltage applied to the gap in the range from nearly zero up to standoff voltage. Comparison of this approach with previously developed methods is presented. The switch in single- or multichannel modifications can be used as closing or crowbar devices.
Plasma discreteness effects in the presence of an intense, ultrashort laser pulse
Authors: Savchenko V. I. , Fisch N. J.
Published: July 2002

Discrete effects of the plasma irradiated by an ultrashort, intense laser pulse are investigated. Although, for most plasmas of interest, the damping of the laser pulse is due to collective plasma effects, in certain regimes the energy absorbed in the plasma microfields can be important. A scattering matrix is derived for an electron scattering off an ion in the presence of an intense laser field.
Self-consistent magnetohydrodynamic equilibria for the closure of field-aligned currents in the ionosphere
Authors: Becker U. , Neukirch T. , Birk G. T.
Published: July 2002

Results are presented of numerical calculations of self-consistent equilibria based on a simple fluid model for the interaction of plasma and neutral gas in the lower ionosphere (E and F layer). The model focuses on the aspect of the closure of field-aligned (Birkeland) currents which flow into and out of the lower ionosphere. The dependence of the current structure on the parameters of the model and on the boundary conditions is investigated. It is shown that the model is largely consistent with the observed large scale features of the ionospheric current system.
Fast ion orbits in spherical tokamaks
Authors: Solano Emilia R.
Published: July 2002

In a spherical tokamak, the 1/R variation of the toroidal field is extreme, and for a given value of the safety factor a relatively low average toroidal field can be used, together with large plasma current and large plasma minor radius and elongation. The poloidal and toroidal fields are then of similar size. In consequence, the orbits of fast ions depart considerably from the guiding center orbits because of gyromotion in the small magnetic fields on the low-field side.
Influence of trapped electrons on the time-independent states of a negatively biased single-ended Q machine
Authors: Oertl M. , Kuhn S. , Krumm P. , Schupfer N. , Crystal T. L.
Published: July 2002

Measurements of basic plasma parameters in a low-density single-ended Q machine with a negatively biased cold plate are compared with pertinent predictions of time-independent collisionless plane-diode theory. As was also found in particle simulations [Phys. Fluids B 3, 244 (1991)] and previous experiments, theory and experiment agree well if space-charge conditions near the hot-plate surface are electron rich, in which case the potential distributions are monotonically decreasing. For ion-rich conditions, however, where the potential distributions exhibit a single maximum located in front of the hot plate, the plasma properties are found to depend very sensitively on the amount and distribution of the electrons trapped in the region of positive potential. In this regime, the experimental findings agree very well with a theoretical model in which a certain amount of trapped electrons is assumed.
A physical mechanism for the onset of radial electric fields in magnetically confined plasmas
Authors: Moleti A.
Published: July 2002

A simple physical mechanism is described, which could trigger the Low-mode to High-mode (L–H) transition. The instantaneous ion density profile is significantly modified by a sudden temperature increase, because Larmor radii and banana orbit widths are proportional to thermal velocity. The electric fields that are observed in H-mode plasmas could be produced in the radial region where a large second derivative of the density profile exists, either by strong additional heating or by the heat pulse associated to a sawtooth crash. The L–H transition threshold for the time derivative of the ion temperature is of the order of magnitude of the values that are measured in the outer part of the plasma by electron temperature fast diagnostics at sawtooth crashes. This model agrees with the experimental evidence that L–H transitions are often triggered by a sawtooth crash, and the predicted dependence of the threshold on plasma parameters is fairly consistent with available data.
Gyroscopic stabilization of the internal kink mode
Authors: Waelbroeck F. L.
Published: July 2002

Toroidal rotation is shown to have a strongly stabilizing effect on the ideal internal kink mode. In the gyroscopically stabilized regime, the internal kink is a neutrally stable discrete eigenmode with a finite rotation velocity relative to the plasma at the resonant surface. In the unstable regime, the mode frequency is equal to the plasma rotation frequency at the resonant surface, so that reconnection will prevent nonlinear saturation.
Alfvén ion–ion hybrid wave heating in the Phaedrus-T tokamak
Authors: Intrator T. , Probert P. H. , Vukovic M. , Wukitch S. , Elfimov A. , Durst R. , Breun R. A. , Brouchous D. , Diebold D. , Doczy M. , Fonck R. , Hershkowitz N. , Kishinevsky M. , Litwin C. , Majeski R. , Nonn P. , Winz G.
Published: July 2002

In the Phaedrus-T tokamak [R. A. Breun et al., Fusion Technol. 19, 1327 (1991)], Alfvén waves are indirectly driven by a fast wave antenna array. Small fractions of minority ions are shown to have a large effect on the Alfvén spectrum, as measured at the edge. An ion–ion hybrid Alfvén mode has been identified by measuring dispersion properties. Landau damping is predicted to be large and spatially localized. These Alfvénic waves are experimentally shown to generate correlated electron heating and changes in density near the core of the tokamak plasma. Fast wave antenna fields can mode convert at a hybrid Alfvén resonance and provide a promising route to spatially localized tokamak heating and current drive, even for low effective ionic charge Zeff≊1.3–2.
Construction of the weakly-relativistic Fokker-Planck kinetic equation in the Darwin approximation
Authors: Pozzo Monica , Tessarotto Massimo
Published: July 2002

The explicit form of the weakly-relativistic Vlasov and the Fokker-Planck collision operators for a multispecies plasma are evaluated by using a systematic expansion in β_=v_/c and retaining corrections up to the second order. Properties of these operators are investigated.
Mode coupling effects on resistive wall instabilities
Authors: Finn John M. , Gerwin Richard A.
Published: July 2002

It is shown that destabilization of resistive wall magnetohydrodynamic (MHD) modes in the presence of rotation is a mode coupling phenomenon. Based on this observation, certain unanticipated effects are readily explained. These include the fact that resistive wall modes with rotation can be unstable, even for parameters for which the MHD modes are stable with the wall at infinity, and the fact that this destabilization depends critically on the plasma parameters.
Microturbulence reduction during counter neutral beam injection in the DIII-D tokamak
Authors: Rettig C. L. , Peebles W. A. , Doyle E. J. , Burrell K. H. , Groebner R. J.
Published: July 2002

When significant neutral beam power is injected in the direction counter to the plasma current in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159], a prompt reduction in electrostatic turbulence is observed, principally in shorter wavelength modes. The core radial electric field reverses polarity and becomes sheared as the fluctuations reduce. As the electric field continues evolving, a larger reduction is observed on a slower time scale. These observations support the possibility of controlling turbulence and its associated transport by controlling the internal radial electric field.
Electronmagnetohydrodynamic response of a plasma to an external current pulse
Authors: Zhou H. B. , Papadopoulos K. , Sharma A. S. , Chang C. L.
Published: July 2002

In this paper we examine the dynamic response of a magnetoplasma to an external time-dependent current source in the context of electronmagnetohydrodynamics (EMHD). A combined analytic and numerical technique is developed to address this problem. The set of cold electron plasma and Maxwell’s equations are first solved analytically in the (k,ω) space. Inverse Laplace and three-dimensional complex Fast Fourier Transform techniques are used subsequently to numerically transform the radiation fields and plasma currents from the (k,ω) space to the (r,t) space. The results show that the electron plasma responds to a time-varying current source imposed across the magnetic field by exciting whistler/helicon waves and forming an expanding local current loop, driven by field-aligned plasma currents. The current loop consists of two antiparallel field-aligned current channels concentrated at the ends of the imposed current and a cross-field Hall current region connecting these channels. The characteristics of the current closure region are determined by the background plasma density, the magnetic field, and the time scale of the current source. The results are applied to the ionospheric generation of extremely low-frequency (ELF) and very low-frequency (VLF) radiation using amplitude modulated high-frequency heating. It is found that contrary to previous suggestions the dominant radiating moment of the ELF/VLF ionospheric source is an equivalent horizontal magnetic dipole.
High-mode-number ballooning modes in a heliotron/torsatron system. I. Local magnetic shear
Authors: Nakajima N.
Published: July 2002

The characteristics of the local magnetic shear, a quantity associated with high-mode-number ballooning mode stability, are considered in heliotron/torsatron devices that have a large Shafranov shift. The local magnetic shear is shown to vanish even in the stellarator-like region in which the global magnetic shear s≡2(d ln ι//d ln ψ) (ι/ is the global rotational transform and 2πψ is the toroidal flux) is positive. The reason for this is that the degree of the local compression of the poloidal magnetic field on the outer side of the torus, which maintains the toroidal force balance, is reduced in the stellarator-like region of global magnetic shear because the global rotational transform in heliotron/torsatron systems is a radially increasing function. This vanishing of the local magnetic shear is a universal property in heliotron/torsatron systems with a large Shafranov shift since it results from toroidal force balance in the stellarator-like global shear regime that is inherent to such systems.
New method to analyze internal disruptions with tomographic reconstructions
Authors: Tanzi C. P. , de Blank H. J.
Published: July 2002

Sawtooth crashes have been investigated on the Rijnhuizen Tokamak Project (RTP) [N. J. Lopes Cardozo et al., Proceedings of the 14th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Würzburg, 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. 1, p. 271]. Internal disruptions in tokamak plasmas often exhibit an m=1 poloidal mode structure prior to the collapse which can be clearly identified by means of multicamera soft x-ray diagnostics. In this paper tomographic reconstructions of such m=1 modes are analyzed with a new method, based on magnetohydrodynamic (MHD) invariants computed from the two-dimensional emissivity profiles, which quantifies the amount of profile flattening not only after the crash but also during the precursor oscillations. The results are interpreted by comparing them with two models which simulate the measurements of the m=1 redistribution of soft x-ray emissivity prior to the sawtooth crash. One model is based on the magnetic reconnection model of Kadomtsev. The other involves ideal MHD motion only. In cases where differences in magnetic topology between the two models cannot be seen in the tomograms, the analysis of profile flattening has an advantage. The analysis shows that in RTP the clearly observed m=1 displacement of some sawteeth requires the presence of convective ideal MHD motion, whereas other precursors are consistent with magnetic reconnection of up to 75% of the magnetic flux within the q=1 surface. The possibility of ideal interchange combined with enhanced cross-field transport is not excluded.
Impulsive and transient excitation of Bohm–Gross waves in a dissipative plasma
Authors: Randriamboarison Orélien C.
Published: July 2002

Based on the resolution of the partial differential equation describing the external excitation of the Bohm–Gross longitudinal wave, analytical expressions for causal responses of a dissipative macroscopic plasma are derived. Both impulsive and harmonic solutions representing the spatial Green’s functions of the radiation problem are given. These exact responses of the plasma, expressed in terms of two-variable Lommel functions, are then used to gain some better understanding of the excitation and dynamics of the well-known thermal wave. Special attention is paid to the resonant excitation case. Intrinsic characteristics of the secular behavior of the radiated signal are illustrated and analyzed. It is shown that the proffered algebraic solutions constitute a generalization of previous results inferred from an asymptotic representation of the Green’s functions, or from the familiar steady state harmonic approach.
Suppression of Rayleigh–Taylor instability by flow curvature
Authors: Sen S. , Storer R. G.
Published: July 2002

The question of whether it is the shear [V(r)′] or the curvature [V(r)″] in the flow profile [V(r)] which plays the dominant role in suppressing the low-frequency (ω<ωi, where ωi is the ion gyrofrequency) instabilities and fluctuations is investigated. Rayleigh–Taylor (RT) instability is considered in this work. The role of flow curvature is found to have a robust effect on the stability and on the radial structure of the mode. For a positive value in the flow curvature RT is suppressed whereas a negative flow curvature excites the mode. Flow shear, on the other hand, plays an insignificant role in this matter. These theoretical findings are in agreement with the recent experimental results.
Large orbit neoclassical transport
Authors: Lin Z. , Tang W. M. , Lee W. W.
Published: July 2002

Neoclassical transport in the presence of large ion orbits is investigated. The study is motivated by the recent experimental results that ion thermal transport levels in enhanced confinement tokamak plasmas fall below the “irreducible minimum level” predicted by standard neoclassical theory. This apparent contradiction is resolved in the present analysis by relaxing the basic neoclassical assumption that the ions orbital excursions are much smaller than the local toroidal minor radius and the equilibrium scale lengths of the system. Analytical and simulation results are in agreement with trends from experiments. The development of a general formalism for neoclassical transport theory with finite orbit width is also discussed.
Greatly enhanced acoustic noise and the onset of stimulated Brillouin scattering
Authors: Drake R. P. , Estabrook Kent , Watt R. G.
Published: July 2002

Experiments using near-infrared to ultraviolet lasers offer the potential to study the acoustic noise in plasmas. As the onset of stimulated Brillouin scattering (SBS) has come to be closely examined, the evidence indicates that the acoustic noise may often or always be far above thermal levels. Evidence regarding the noise is reported here, from two recent experiments which confirmed the theoretically anticipated onset behavior for SBS. In one case, the noise appears to be greatly enhanced above thermal levels. In the other case, the data place an upper limit on the noise level. There is physical grounds to believe that enhanced acoustic noise may be ubiquitous in plasmas, even in the absence of plasma instabilities which drive turbulence.
Three-wave processes in a turbulent nonstationary plasma
Authors: Vladimirov S. V. , Stenflo L.
Published: July 2002

The contribution of plasma turbulence to the decay-type coupling of three regular waves is derived and discussed. The effect is due to the nonlinear interaction of the waves with the turbulence as well as the plasma nonstationarity induced by the turbulence. The conservation relations for the wave occupation numbers, energy, and momenta are considered. It is shown that for the interactions of three regular waves in a closed system, the Manley-Rowe relations are valid (i.e., there is no exchange of the number of quanta with the plasma turbulence). However, the turbulence causes phase shifts of the interacting waves, and affects the energy balance via the nonstationarity of the system.
Astrophysically relevant experiments on radiation transfer through plasmas with large velocity gradients
Authors: Wark J. S. , Rose S. J. , Patel P. K. , Djaoui A. , Renner O. , Hauer A.
Published: July 2002

The transfer of radiation through plasmas with large velocity gradients is of relevance to several astrophysical situations, such as supernova explosions, maser operation, and stellar winds. Similar conditions often prevail in laser-produced plasmas, with velocity gradients of order 109 s−1 significantly altering the effective optical depth and line shape. Some of the simplest cases to study experimentally are the hydrogenic resonance lines. Experiments performed in both planar and cylindrical geometry, comparing the observed line profiles with those modeled using a one-dimensional Lagrangian hydrocode, incorporating average-atom nonlocal thermodynamic equilibrium (non-LTE) atomic physics are described. The opacity effects on the ion populations are treated within the escape factor approximation, taking into account the effects of the velocity gradient. The hydrocode gives time- and space-dependent values of the electron and ion densities, excited state fractions, electron and ion temperatures, and velocities. The hydrodynamic output is post-processed with a radiative transfer routine to construct the simulated line shape. Details of the experiments and results are presented, and relevance to the astrophysical situations discussed.
Improvement of the highly energetic passing particle confinement in torsatrons and heliotrons
Authors: Smirnova M. S.
Published: July 2002

The configurational effects on the confinement of highly energetic passing particles, especially the helical-field ripple effects, are considered in torsatrons and heliotrons from the viewpoint of increasing the efficiency of tangential neutral beam injection (NBI) heating of plasma. The drift resonances of passing particles (ιΩφ=ΩD, where ι is the rotational transform, Ωφ is the particle toroidal transit frequency, and ΩD is the particle drift frequency) are investigated. It is discovered that the resonant passing particles oscillate poloidally through the variation of the field-line curvature. The passing particles with the energies located in the vicinity of the resonance separatrix are revealed to be as dangerous for confinement, as superbananas are. The existence of the highly poloidally localized resonant particles with lower radial deviations than the corresponding values for the nonresonant particles was found. The particles with the energies close to the state of the high poloidal localization are proposed as the most preferable beam particles for the tangential NBI heating of plasma.
Convective stimulated Brillouin scattering of obliquely incident laser light in laser plasmas
Authors: Gupta G. P. , Sinha B. K.
Published: July 2002

Convective stimulated Brillouin scattering (SBS) in the directly backward direction of the obliquely incident laser light in an expanding and inhomogeneous laser plasma is studied. Accounting for the thermal motion of both electrons and ions of the plasma in the dispersion relation of the ion-acoustic wave, the expression of the convective SBS gain coefficient is derived in the weak coupling limit. The behavior of the gain with the angle of incidence of the laser irradiation in a laser-produced CH plasma is investigated by considering various plasma and laser conditions in light of the reported laser-plasma experiment with the 0.35 μm laser irradiating the CH target. The results are observed to be significantly different from those for normal incidence.
Comments on “Dispersion relation of a plasma in the periodic magnetic field of a wiggler” [Phys. Plasmas 3, 1773 (1996)]
Authors: Tsui K. H. , Serbeto A.
Published: July 2002

It is argued that the equilibrium state of a wiggler background magnetoplasma corresponds to a zero momentum rather than to a plasma acquiring a non-zero velocity. (AIP)
An electrostatic magnetohydrodynamics theory for resistive-viscous helical instabilities of arc discharges
Authors: Wang Xiaogang , Liu Jinyuan , Gong Ye , Li Guobing , Ma Tengcai
Published: July 2002

A simplified linear analysis for resistive-viscous magnetic helical instabilities of arc discharges in a cylindrical plasma is developed. Based on a set of electrostatic magnetohydrodynamic (MHD) equations, resistive-viscous m=1 modes with an external axial magnetic field are studied. Explicit analytic results are obtained, from which the growth rate and the stability criterion can be shown, and the electrostatic assumption can be justified. In comparison with the previous channel model calculations, this analytic treatment can provide a simplified model for instability estimates, while avoiding artificial assumptions and misorderings in the energy equation.
Kinetic ballooning modes at the tokamak transport barrier with negative magnetic shear
Authors: Yamagiwa M. , Hirose A. , Elia M.
Published: July 2002

Stability of the kinetic ballooning modes is investigated for plasma parameters at the internal transport barrier in tokamak discharges with negative magnetic shear employing a kinetic shooting code with long shooting distance. It is found that the second stability regime with respect to the pressure gradient parameter, which was predicted for negative shear [A. Hirose and M. Elia, Phys. Rev. Lett. 76, 628 (1996)], can possibly disappear. The mode with comparatively low toroidal mode number and mode frequency below 100 kHz is found to be destabilized marginally only around the transport barrier characterized by steep pressure and density gradients.
Ion pitch-angle scattering by Alfvén waves
Authors: Li Y. , Yoon Peter H. , Wu C. S. , Weatherwax A. T. , Chao J. K. , Wu B. H.
Published: July 2002

Freshly created ions can be picked up by a moving plasma without relying on collisions. It is well known that such an ion pickup process can be accomplished via the interaction with Alfvén waves. However, it should be stressed that in general ion pickup is attributed to two distinctly different sub-processes, namely, pitch-angle diffusion and pitch-angle scattering. In this article their difference is discussed and furthermore, some new results from a recent theoretical study are reported. It is found that under some conditions the usual quasilinear theory which describes the pitch-angle diffusion process cannot be justified even when the turbulence level is low. Another significant finding is that in the presence of strong Alfvén turbulence, thermal ions can be intensely heated by a nonlinear damping of the waves, which does not depend upon the usual ion cyclotron resonance.
Calculation of electron transport in Ar/N2 and He/Kr gas mixtures—implications for validity of the Blanc’s law method
Authors: Wang Yicheng , Van Brunt R. J.
Published: July 2002

The electron drift velocities and corresponding mean energies have been calculated numerically using an approximate two-term solution of the Boltzmann transport equation for Ar/N2 gas mixtures at electric field-to-gas density ratios (E/N) below 2.0×10−20Vm2 (20 Td) and for He/Kr mixtures at E/N below 5.0×10−21 Vm2 (5.0 Td). The results are compared with predictions obtained from a method proposed by Chiflikian based on an “analog of Blanc’s law” [Phys. Plasmas 2, 3902 (1995)]. Large differences are found between the results derived from the Blanc’s law method and those found here from solutions of the transport equation that indicate serious errors and limitations associated with use of the Blanc’s law method to compute drift velocities in gas mixtures.
Space-charge waves in a coaxial plasma waveguide
Authors: Hwang Un-Hak , Willett Joseph E. , Mehdian Hassan
Published: July 2002

The propagation of space-charge waves through a coaxial waveguide containing an annular plasma in an axial magnetic field is investigated. Both plasma and cyclotron types of waves are analyzed in the electrostatic approximation. Equations for the determination of the dispersion relations are derived from the Poisson equation and the electron continuity and momentum transfer equations. A numerical study of the dispersion curves for azimuthally symmetrical waves is presented. A significant departure from the dispersion characteristics of a cylindrical plasma waveguide are found to occur unless the inner radius of the waveguide is small compared to the outer radius.
The neutrino electron accelerator
Authors: Shukla P. K. , Stenflo L. , Bingham R. , Bethe H. A. , Dawson J. M. , Mendonça J. T.
Published: July 2002

It is shown that a wake of electron plasma oscillations can be created by the nonlinear ponderomotive force of an intense neutrino flux. The electrons trapped in the plasma wakefield will be accelerated to high energies. Such processes may be important in supernovas and pulsars.
Rayleigh–Taylor instability: Comparison of hybrid and nonideal magnetohydrodynamic simulations
Authors: Huba J. D. , Winske D.
Published: July 2002

The evolution of the Rayleigh–Taylor instability in a low β, two-dimensional plasma is studied using a hybrid code and a nonideal magnetohydrodynamic (MHD) code. The Rayleigh–Taylor instability was chosen as a test case because it is an important mixing process at boundary layers, and because it exhibits different behaviors in the conventional and nonideal limits. The nonideal MHD effects considered are the Hall term and finite Larmor radius (FLR) corrections. Three cases are considered in detail: conventional MHD, weak nonideal effects, and strong nonideal effects. In the conventional MHD regime the usual “bubble and spike” behavior of the Rayleigh–Taylor instability is observed. In the weak nonideal MHD regime long wavelength modes, reminiscent of the Kelvin–Helmholtz instability, dominate nonlinearly but very short wavelength filaments develop at the boundary interface. In the strong nonideal MHD regime, small-scale structures dominate and the boundary layer relaxes via a diffusion-like process rather than a large-scale nonlinear mixing process. In general, the hybrid and fluid simulations are in good agreement. The differences, both physical and numerical, are discussed.
Statistical analysis of turbulent-transport suppression based on the magnetohydrodynamic approximation with electric-field effects incorporated
Authors: Yoshizawa Akira , Yokoi Nobumitsu
Published: July 2002

Suppression mechanism of turbulent thermal-energy transport is studied using the statistical method based on an extended magnetohydrodynamic (MHD) approximation. The one-fluid MHD system of equations is supplemented with effects of electric fields coming from the inhomogeneity of charge density. A Markovianized two-scale method is applied to the resulting system, and the turbulent transport rate of thermal energy is examined. In cylindrical geometry, the transport is shown to be suppressed through the combined effects of the radial electric field and the charge nonuniformity arising from its curvature. This finding is discussed in light of the formation of transport barriers observed in tokamak’s high-confinement modes and is confirmed to be consistent with observational results.
Kinetic formulation of neutrino–plasma interactions
Authors: Tsintsadze N. L. , Mendonca J. T. , Tsintsadze L. N.
Published: July 2002

A kinetic equation is derived for a neutrino gas interacting with a background plasma via the weak force. It is shown how the neutrino fluid equations can be derived from the kinetic equation. The plasma electrons are also linked with the neutrino population by a ponderomotive force term, and the neutrino–plasma interaction problem can be described as the coupling of two fluids: the electron and the neutrino fluids (the plasma ions being assumed at rest).
Particle end loss in the edge plasma of a field-reversed configuration
Authors: Ohtsuka T. , Okubo M. , Okada S. , Goto S.
Published: July 2002

Plasma parameters, particle end loss flux, flow velocity, and pressure are measured using a radial array of magnetic probes and directional electrostatic probes, in order to investigate particle loss processes in the edge layer of a field-reversed configuration (FRC). A plasma flow toward the end region is detected outside the separatrix between the axial midplane and the end region. The exhaust flow is also found in the end region. These results imply that particles are lost radially across the separatrix and then axially to the end. Measured flow velocity in the end region agrees within an error of 20% with the fluid-theory prediction, in which isentropy and axial momentum balance along magnetic flux tubes are assumed. The existence of the sonic condition in the end region is also suggested, analogous to ordinary fluid flow in a nozzle. The magnetic flux embedded in the edge layer of the confinement region and in the end region agrees within an error of 30%. These results indicate the applicability of the magnetohydrodynamics (MHD) theory for particle end loss. The end loss time along the open field agrees with the MHD prediction within an error of 20%. The measured particle loss flux from the end region is explained by the MHD theory within an error of 20%. The plasma outside the separatrix is considered to behave as hydrodynamic flow through the magnetic loss channel, contrary to the previous work [L. C. Steinhaur, Phys. Fluids 29, 3379 (1986)]. It seems that the magnetic mirror field improves the particle confinement in the edge plasma of the FRC and thus assist the FRC confinement as previously predicted [Slough et al., Nucl. Fusion 24, 1537 (1984)].
Suppression of drift-type instabilities by radio frequency waves
Authors: Sen S. , Cairns R. A.
Published: July 2002

The use of radio frequency waves is shown to stabilize drift-type waves in a tokamak if the radial profile of the radio frequency field energy is properly chosen. The estimate of the radio frequency power required for this stabilization is found to be rather modest. This result might have important implications as regards whether the use of the radio frequency waves can create a transport barrier to reduce the loss of particle and energy from the plasma.
Feedout coupling of Richtmyer–Meshkov and Rayleigh–Taylor instabilities in stratified, radiation-driven foils
Authors: Smitherman D. Palmer , Chrien Robert E. , Hoffman Nelson M. , Magelssen Glenn R.
Published: July 2002

Coupling between Richtmyer–Meshkov- and Rayleigh–Taylor-unstable surfaces of indirectly driven, stratified, planar foils composed of an aluminum ablator and a beryllium payload carrying a machined initial perturbation was studied using radiographic techniques. The beryllium was transparent to the x-ray backlighter, allowing clear observation of the perturbation transport into the aluminum. The maximum perturbation amplitude (areal–mass difference) was smaller in the aluminum/beryllium foils than in equivalent pure aluminum foils, which could have been the result of shorter growth time, a lower differential acceleration coupling, or the density discontinuity at the aluminum/beryllium interface. Second and third harmonics were not observed in the composite foils.
Geometrical aspects of a hollow-cathode planar magnetron
Authors: Wang Zhehui , Cohen Samuel A.
Published: July 2002

A hollow-cathode planar magnetron (HCPM), built by surrounding a planar sputtering-magnetron cathode with a hollow-cathode structure (HCS) [Z. Wang and S. A. Cohen, J. Vac. Sci. Technol. A 17, 77 (1999)], is operable at substantially lower pressures than its planar-magnetron counterpart. HCPM operational parameters depend on the inner diameter D and length L of its cylindrical HCS. Only when L is greater than L0, a critical length, is the HCPM operable in the new low-pressure regime. The critical length varies with HCS inner diameter D. Explanations of the lower operational pressure regime, critical length, and plasma shape are proposed and compared with a one-dimension diffusion model for energetic electron transport. At pressures above 1 mTorr, Bohm diffusion (temperature≅primary electron energy), with an ambipolar constraint, can explain the ion–electron pair creation required to sustain the discharge. At the lowest pressure, ∼0.3 mTorr, collision-limited diffusion creates fewer ion–electron pairs than required for steady state and therefore cannot explain the experimental data. The critical length L0 is consistent with the magnetization length of the primary electrons.
On the high-confinement-mode equilibrium in tokamak plasmas
Authors: Singh R. , Weiland J.
Published: July 2002

A new type of toroidal condensation instability is found to dominate transport in the high-confinement-mode (H-mode) edge of tokamak plasmas when the temperature gradient is steeper than the density gradient. A particle pinch develops at the edge in agreement with experimental observations and the transport can be dominated by the ion or electron channel.
Characterization of the frequency ranges of the plasma edge fluctuation spectra
Authors: Carreras B. A. , Balbin R. , van Milligen B. , Pedrosa M. A. , Garcia-Cortes I. , Sanchez E. , Hidalgo C. , Bleuel J. , Endler M. , Thomsen H. , Chankin A. , Davies S. , Erents K. , Matthews G. F.
Published: July 2002

Frequency spectra of fluctuations for the ion saturation current, floating potential, and turbulent transport measured in the plasma edge of plasma confinement experiments (tokamaks and stellarators) have been analyzed to identify the frequency ranges characterized by a power dependence. Three main regions can be identified. For the intermediate frequency region, the decay of the spectra is close to 1/f, as is expected in self-organized criticality systems. This region is particularly important for the role that it plays in plasma transport and the self-similarity of the fluctuations and fluxes. The effect of plasma rotation on the decay indices has also been studied.