bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma collection by an obstacle
Authors: Vu H. X. , Gould R. W.
Published: July 2002

Plasma flow to an obstacle is examined using the two-fluid equations. In this model the obstacle is assumed to be a two-dimensional strip that extends to infinity in the y direction (slab geometry). An obstacle inserted into a magnetized plasma will cast a ‘‘shadow’’ along the magnetic field lines. The natural collection length of such an obstacle is a measure of the length of its shadow. This study shows that in a typical fusion tokamak, where cs/Ωcid≪1 (Ωci, cs are the ion cyclotron frequency and the ion acoustic speed, respectively, d is the half-width of the strip), the particle collection length of an obstacle can be approximated as L∥=0.23csd2/ D⊥, if D⊥/ csd ≪1; or 0.30d, if D⊥/ csd ≫1. For the cases examined in this study, the inclusion of the electron–ion collisional drag parallel to B0 changes the solution only by an insignificant amount.
Investigation of (3/2)ω emission from an underdense CO2-laser-irradiated plasma
Authors: Meyer J. , Zhu Y. , Curzon F. L.
Published: July 2002

The angular and spectral distribution of (3/2)ω0 light emitted from a CO2-laser-irradiated underdense plasma has been measured. The results are interpreted with the help of Thomson scattering measurements of two-plasmon decay plasma wave k spectra and theoretical predictions based on the process in which pump photons are scattered off two-plasmon decay plasmons. If pump, (3/2)ω0-photon, and decay plasmon satisfy their wave vector matching condition, then the spectra provide good temperature diagnostics. Spectra of light scattered off plasmons having propagated up or down the density gradient are severely broadened.
Damping rate of the ion-sound wave by the plasma–maser effect for a magnetized plasma
Authors: Sarma S. N. , Bujarbarua S. , Nambu M. , Fujiyama H.
Published: July 2002

The damping of the ion-sound wave through the plasma–maser effect is considered for strongly magnetized plasmas. The damping of the ion-sound wave associated with the energy up-conversion from the ion-sound wave to the Langmuir wave comes from two mode coupling terms, viz., the direct and polarization terms. The total wave energy diminishes through the plasma–maser interaction.
Numerical simulations of induced spatial incoherence laser light self-focusing
Authors: Rankin R. , Capjack C. E. , James C. R.
Published: July 2002

The effect of induced spatial incoherence (ISI) on laser light self-focusing is investigated using a two-dimensional Eulerian hydrodynamic plasma simulation code. In homogeneous low-density plasmas (one-tenth of the critical density for radiation of wavelength λL=0.25 μm) it is found that ISI can effectively eliminate thermal and ponderomotive self-focusing for a wide range of intensities. In plasma containing an initial linear gradient in density, strong self-focusing occurs when the maximum intensity in the incident light I0 (where I0 represents the intensity that would be achieved for a perfectly coherent beam) is allowed to approach 1017 W cm−2. At lower intensities, I0∼1016 W cm−2, thermal self-focusing is eliminated and ponderomotive self-focusing is significantly reduced. The dwell time of the filamented light varies from the laser light coherence time to a few tens of picoseconds.
Anomalous electron thermal conduction from magnetic turbulence
Authors: Hong B. G. , Horton W.
Published: July 2002

The electron thermal balance equation from the Braginskii equations with the finite Larmor radius heat flux is analyzed for the space-time-average power balance in the presence of electromagnetic fluctuations. Formulas for the anomalous thermal flux associated with the E×B motion and the magnetic δB⊥ fluctuations are derived and evaluated for the c/ωpe scale electromagnetic turbulence typical of the ∇Te-driven short wavelength drift modes. The result is compared with several Ohkawa-type formulas for the anomalous electron thermal energy transport.
Final focusing of intense ion beams with radially nonuniform current density z discharges
Authors: Watrous J. J. , Ottinger P. F.
Published: July 2002

The spot size and focal length of a one-eighth betatron wavelength final focusing cell with a nonuniform current density distribution are predicted. The final focusing cell is modeled with an azimuthal magnetic field distribution that varies as rN. A Lie transform method is used to determine the behavior of the ion beam as it passes through the focusing cell. The analysis indicates that a final focusing cell with a current density distribution that is strongly concentrated at the channel edge focuses the beam much less efficiently than a channel with a uniform current density distribution, and provides a scaling relation for the focused beam radius: rfoc ∝rc (Ic/ If)1/2N, where rc is the unfocused beam radius, If is the discharge current in the focusing cell, and Ic is the discharge current in the transport channel.
Resistive wall modes in the setting-up phase of a reversed field pinch
Authors: Greene Philip , Barrick Greg , Robertson Scott
Published: July 2002

Resistive wall modes during the setting-up phase of a reversed field pinch have been investigated in an experiment (Reversatron II [IEEE Trans. Plasma Sci. PS-16, 667 (1988)]) operated without a conducting shell. The time for the vertical field to penetrate the vacuum chamber (4 μsec) is much shorter than the rise time of the plasma current (100 μsec). The discharges, when compared to discharges in the same device with a conducting shell, have a greater resistance and lower plasma current. Magnetic probes indicate the growth of a strong ‖n‖=6 helical kink resonant with the field on axis.
Anomalous thermal electron heating and heat transport inhibition due to parametric instabilities
Authors: Mizuno K. , De Groot J. S.
Published: July 2002

Measurements are presented of strong thermal electron heating and the heat transport inhibition. An electron plasma wave heats hot electrons near the critical density. A return current is induced to keep charge neutrality. Thermal electrons are heated strongly by the resistivity of parametrically excited isotropic ion wave turbulence (anomalous joule heating). The heat transport of thermal electrons is also inhibited by the resistivity. The experimental results agree reasonably well with theory.
Three-halves harmonic emission from two-color experiments
Authors: Young P. E. , Baldis H. A. , Remington B. A. , Estabrook K. G.
Published: July 2002

Three-halves harmonic emission from a plasma in which the peak plasma density is nc>n>nc/4 has been temporally and spectrally resolved. This is achieved by using 0.53 μm wavelength laser light to irradiate a plasma preformed by 0.35 μm wavelength laser light. It is observed that both the backscattered red-shifted and blue-shifted peaks of the 3ω0/2 emission from the 0.53 μm laser disappear at approximately the same time. This result constrains the possible production mechanism for backscattered blue-shifted photons to scattering of the blue-shifted photons or plasmons close to nc/4.
Auxiliary heated detached plasmas in tokamaks
Authors: Guzdar P. N. , Liu C. S.
Published: July 2002

Earlier studies on Ohmically heated plasmas have been extended to auxiliary heated detached plasmas in tokamaks. Assuming the electron energy loss channel to be the dominant one and caused by ηe modes, the effect of auxiliary power in determining the thermal equilibria of detached plasmas is examined. Specific predictions for central electron temperature, core plasma radius, and electron energy confinement time as a function of the auxiliary power for the tokamak fusion test reactor (TFTR) [J. Nucl. Mater. 145, 186 (1987)] parameters are obtained. Density limit enhancement by auxiliary power is also studied.
Electromagnetic beam stability in a betatron with helical quadrupole magnetic field
Authors: Tsang Kang T.
Published: July 2002

A Vlasov treatment is developed to investigate the beam stability in a betatron with stellarator windings. The general formulation includes effects of finite beam temperature and radius. An analytic dispersion relation is derived. In the limit of cold beam and small beam radius, the general dispersion relation reduces to simpler form, which exhibits instability due to frequency matching between the transverse electric (TE) beam pipe cavity modes and the Doppler shifted electron oscillations in a combined longitudinal and helical quadrupole magnetic field. In particular, an internal beam mode with azimuthal mode number ν=0 is found to be unstable for typical parameters of the Spiral Line Induction Accelerator design [Bull. Am. Phys. Soc. 34, 2160 (1989)]
Transition from moving to stationary double layers in a single-ended Q machine
Authors: Song Bin , Merlino R. L. , D’Angelo N.
Published: July 2002

Large-amplitude (≲100%) relaxation oscillations in the plasma potential are known to be generated when the cold endplate of a single-ended Q machine is biased positively. These oscillations are associated with double layers that form near the hot plate (plasma source) and travel toward the endplate at about the ion-acoustic velocity. At the endplate they dissolve and then form again near the hot plate, the entire process repeating itself in a regular manner. By admitting a sufficient amount of neutral gas into the system, the moving double layers were slowed down and eventually stopped. The production of stationary double layers requires an ion source on the high-potential side of the double layers. These ions are provided by ionization of the neutral gas by electrons that are accelerated through the double layer. The dependence of the critical neutral gas pressure required for stationary double-layer formation on endplate voltage, magnetic field strength, and neutral atom mass has been examined. These results are discussed in terms of a simple model of ion production and loss, including ion losses across the magnetic field.
Plasma transport in the scrape-off layer with a stochastic magnetic field
Authors: Takamura Shuichi
Published: July 2002

Based on an ergodic magnetic limiter experiment, a new physical mechanism of cross-field transport in the scrape-off layer with an applied helical magnetic field is examined, taking the effect of the edge electric field into account.
Local stability of fluid plasmas in the presence of flows
Authors: Chun S. T. , Hameiri Eliezer
Published: July 2002

The Mercier and ballooning instabilities are considered for plasmas with toroidal and poloidal flows in an axisymmetric torus. The stability criteria are derived and compared for the two instabilities. It is found that ballooning modes impose more severe requirements for plasma stability than the Mercier condition.
Gas-loaded free-electron lasers
Authors: Pantell R. H. , Özcan M.
Published: July 2002

Gas-loaded free-electron lasers offer the prospect of broad wavelength tuning and short wavelength operation without the need for high electron beam energy. Experiments that were performed using hydrogen gas in the infrared confirm the theory, and new experiments are described for the extreme ultraviolet (XUV) region using helium gas.
Measurement of suprathermal electron confinement by cyclotron transmission
Authors: Kirkwood Robert , Hutchinson I. H. , Luckhardt S. C. , Porkolab M. , Squire J. P.
Published: July 2002

The confinement time of suprathermal electrons is determined experimentally from the distribution function determined via wave transmission measurements. Measurements of the lowest moment of the distribution perpendicular to the B field as a function of the parallel electron momentum as well as the global input power allow the suprathermal electron confinement time (τse ) to be calculated during lower-hybrid and inductive current drive. Finite particle confinement is found to be the dominant energy loss term for the suprathermals and improves with plasma current and density.
The Bohm criterion for rf discharges
Authors: Meijer P. M. , Goedheer W. J.
Published: July 2002

The well-known dc Bohm criterion is extended to rf discharges. Both low- (ωrf≪ωpi) and high-(ωpi ≪ ωrf) frequency regimes are considered. For low frequencies, the dc Bohm criterion holds. This criterion states that the initial energy of the ions entering the sheath must exceed a limit in order to obtain a stable sheath. For high frequencies, a modified limit is derived, which is somewhat lower than that of the dc Bohm criterion. The resulting ion current density in a high-frequency sheath is only a few percent lower than that for the dc case.
Competing instabilities in the circular free-electron laser
Authors: Kawai Yasushi , Saito Hirobumi , Wurtele Jonathan S.
Published: July 2002

A small signal theory of the circular free-electron laser (FEL) is developed. A matrix dispersion relation, which includes coupling between the transverse magnetic (TM) and transverse electric (TE) waveguide modes, is derived from a Eulerian fluid model. The full dispersion equation is then expanded around the TM and TE mode resonant frequencies of the circular coaxial waveguide. The growth rate for frequencies near the TM mode resonance agrees with previous results obtained from a nonlinear pendulum model of the circular FEL, and becomes the negative mass growth rate as the wiggler field strength approaches to zero. It is shown that the dispersion relation expanded near the TE mode resonance has a coupling with the wiggler field that is different from the usual FEL mechanism. In the limit of a weak wiggler field, the dispersion relation for frequencies near a TE resonance reduces to that of the cyclotron maser. Numerical calculations of the growth rate and the ratio of the amplitudes of TE and TM modes are presented.
Increased x-ray conversion efficiency from gold plasmas irradiated with spatially and temporally incoherent light
Authors: Kania D. R. , Kruer W. L. , Bell P. , Obenschain S. P. , Kearney K. , Mostovych A.
Published: July 2002

The efficiency with which 1.05 μm laser light is converted into x rays with energy less than 1.5 keV in high Z materials (gold) is increased by approximately 20% with the use of spatially and temporally incoherent light when compared to a nominal, high-powered laser beam with an identical average irradiance of 1014 W/cm2. This effect is not a result of increased laser light absorption, laser bandwidth, nor reduced hot electron generation for the incoherent light, but is probably due to the change in the short scale length (∼100 μm) laser light intensity distribution in the target plane. The absolute levels of stimulated scattering and hot electron generation were small in all cases. Incoherent illumination reduced stimulated Brillouin scattering levels from 1.1% to 0.2% and, similarly, hot electron generation was decreased from 0.1% to 0.07%.
Reply to the comments of Sinha and Gupta
Authors: Estabrook Kent , Kruer W. L.
Published: July 2002

In a recent Comment, theoretical estimates of the density scale length in a laser-produced plasma were compared with some experimental observations. It is noted that many assumptions of the ideal model are not satisfied for the experiments with 5 nsec pulses of 1.06 μm light. It is also noted that the model predicts a local profile modification near the critical density which can be difficult to resolve in some experiments.
Simulation of the Rayleigh–Taylor instability in colliding, ablatively driven laser foils
Authors: Dahlburg J. P. , Gardner J. H. , Emery M. H.
Published: July 2002

This paper reports results from a series of numerical simulations of a pair of independently accelerated rectilinear foils in the presence of the ablative Rayleigh–Taylor (RT) instability on the laser-side surfaces. The foil thickness and laser intensity are chosen to be in the range relevant to high-gain inertial fusion pellets, with 80 μm thick plastic (CH) foils accelerated toward each other from a separation distance of 650 μm by a 1/4 μm laser beam with an intensity of 3×1014 W/cm2. At early times the foils are physically well separated from one another, and evolve independently in a way that is fully consistent with the previously studied evolution of ablatively RT unstable planar targets [Gardner et al., Phys. Fluids B 3, 1070 (1991)]. Subsequently, pressure builds up in the region between the foils, causing them to decelerate. This stabilizes the RT growth on the laser sides, while driving the RT instability on the inner sides. For thin foils, laser-side RT bubbles become rapidly growing inner surface RT spikes which mix and coalesce as the foils are pressed together.
Observation of wave-packet propagation in the ion cyclotron range of frequencies in a tokamak plasma
Authors: Greene G. J. , Gould R. W.
Published: July 2002

Experimental observation of wave-packet propagation in the ion cyclotron range of frequencies in a tokamak plasma is reported. Studies were carried out in the Caltech Research Tokamak [Phys. Fluids 23, 614 (1980)] in a pure hydrogen plasma and in a regime where fast-wave damping was sufficiently small to permit multiple toroidal transits of the wave packet. Waves were launched by exciting a small loop antenna with a short burst of radio-frequency current and were detected with shielded magnetic probes. Probe scans revealed a large increase in wave-packet amplitude at smaller minor radii, and the packet velocity was found to be independent of radial position. Measurement of the packet transit time yielded direct information about the wave group velocity. Packet velocity was investigated as a function of the fundamental excitation frequency, plasma density, and toroidal magnetic field. Results are compared with the predictions of a cold plasma model that includes a vacuum layer at the edge.
Stability of the gas dynamic trap
Authors: Berk H. L. , Stupakov G. V.
Published: July 2002

The description of stability of the gas dynamic trap is shown to be extremely sensitive to the nature of the boundary conditions. Two model boundary conditions in a moderately long mean-free-path limit are considered: insulating boundary conditions and conducting boundary conditions. The former boundary condition reproduces the magnetohydrodynamic (MHD) results of previous studies, where the outflow of ions contribute to the MHD stabilizing properties of the system. However, with conducting boundary conditions it is shown that the outflowing ions do not contribute to the system’s stability. In this case, which is likely to be physically relevant, the MHD stabilizing term only comes from the electron pressure in the expansion region, and the gas dynamic trap would not be as stable as previously envisioned. The physical difference in these two boundary conditions is attributed to a passive feedback mechanism from the oscillating edge potentials that is only present in the insulating boundary case.
Kinetic high-beta equilibria in a plasma slab
Authors: Li Jin , Scheffel Jan
Published: July 2002

Self-consistent, high-beta kinetic equilibria are derived from the Vlasov equation for a plasma slab in a nonzero magnetic field with a density gradient and a gravitational field. It is shown that a magnetic field inhomogeneity and an electric field arise as necessary consequences of the high-beta state. Hence the corresponding equilibrium forces should not be neglected in calculations of individual particle motion. Explicit expressions are derived for equilibrium quantities, useful, e.g., for large Larmor radius (LLR) studies of the Rayleigh–Taylor instability.
Radiative cooling instabilities in the low dense plasma corona of laser-irradiated solid targets
Authors: Dhareshwar L. J. , Naik P. A. , Sarkar Susmita , Khan Manoranjan , Chakraborty B.
Published: July 2002

Plasma jets produced from targets with atomic numbers ranging from 5 to 82 have been investigated in detail. Theoretical modeling based on a radiative cooling instability has been used to successfully explain the experimental results. Optical shadowgrams and x-ray pinhole pictures strongly indicate a correlation between the jets and x-ray emission from plasma. Plasma jets are intense in materials such as copper, molybdenum, tungsten, and gold which are characterized by strong x-ray emission. Interferograms indicate a large increase in electron density in the region of plasma jets.
Large anomaly of the perpendicular resistivity of a tokamak
Authors: Jensen T. H. , Greene J. M. , Politzer P. A.
Published: July 2002

Zarnstorff et al. have shown that the electrical resistivity in the direction of the magnetic field, the parallel resistivity, of Ohmic TFTR plasmas is nearly neoclassical [Phys. Fluids B 2, 1852 (1990)]. Using the same experimental results, it is shown that the perpendicular resistivity may exceed the parallel resistivity by more than three orders of magnitude. This large deviation from deductions from neoclassical theory is ‘‘well known’’ [A. H. Boozer, Phys. Fluids B 2, 2300 (1990)], but to the authors’ knowledge not well documented.
The snowplow in plasmas of nonuniform density
Authors: Fruchtman Amnon
Published: July 2002

The snowplow motion and the convective skin effect are shown to be the two opposite limits in a unified model of plasma pushing by magnetic pressure. During the snowplow motion the plasma is compressed to a high density in a thin layer and the ion velocity equals the shock velocity. If, on the other hand, the spatial scale of the density gradient is smaller than the ion skin depth, the magnetic field penetrates with a velocity much higher than the ion velocity and the plasma compression is small.
Electron trapping and acceleration in a modified elongated betatron
Authors: Song Y. , Fisher A. , Prohaska R. , Rostoker N.
Published: July 2002

Electron trapping and acceleration have been successfully accomplished in the modified elongated betatron at University of California, Irvine. About 150 nC of electrons have been trapped and accelerated for ∼900 μsec until the betatron field reached its maximum, establishing an electron layer with ∼80 A of circulating current and ∼1.6 MeV energy in the cylindrical chamber. No minimum current is required to start beam trapping in the betatron. There are essentially no electron losses during the acceleration at low injection currents; the electron losses at high injection currents are probably caused by the space charge effects, resistive chamber walls, and betatron field ripple. By filling the chamber with plasma, an electron beam of ∼120 A current and ∼1.6 MeV energy has been observed. No instabilities have been found during the acceleration except the precessional instability, which has been effectively controlled by a toroidal magnetic field. An electron orbit simulation has been carried out and it has shown that practically no resonance instabilities can be developed in the stretched betatron because of its unique geometry and field configuration, which has been confirmed by the experiment.
Energy conservation law for low-frequency plasma oscillations
Authors: Sosenko Petro P.
Published: July 2002

A basic description of low-frequency plasma oscillations within the context of renormalized plasma electrodynamics is applied to derive a reduced energy conservation law with regard to finite Larmor radius (FLR) and nonlinear effects. The energy density for low-frequency flute oscillations in inhomogeneous plasmas is calculated as a quadratic functional of the electric field strength for arbitrary regimes of nonlinear interaction (either weak or strong). The nonlinear equilibrium spectrum of low-frequency FLR oscillations is derived.
The shear Alfvén wave as a magnetostatic mode
Authors: Ryu Chang-Mo
Published: July 2002

It is proposed that the shear Alfvén wave could appear as a purely damped magnetostatic mode at zero frequency. Supporting arguments are provided, first, by a linear analysis, and, second, by applying the equipartition theorem to the allowable magnetic energy for the modes at zero frequency.
Movable limiter insertion and first pump limiter experiments in a reversed-field pinch
Authors: Shinohara Shunjiro , Ohdachi Satoshi , Toyama Hiroshi , Miyamoto Kenro , Banno Tatsuya
Published: July 2002

Various kinds of movable limiters are inserted into the plasma and effects on plasma performance are studied in the REPUTE-1 reversed-field pinch (RFP) device [Plasma Phys. Controlled Fusion 28, 805 (1986)]. The increases in one-turn loop voltage Vl and magnetic fluctuations with nearly constant value of ion temperature are found with an advance of movable limiters, and drastic increases in Vl are observed when the limiter is beyond a certain position. This Vl increase is discussed comparing with theories. The first pump limiter experiments in RFP are tried. Favorable effects such as reductions of Vl and impurity intensities are observed when the limiter head is inserted by ∼2 cm from the plasma edge.
Magnetic trapping effects on electron cyclotron maser in magnetic mirrors
Authors: Lin J. , Bian B. D.
Published: July 2002

The magnetic trapping effects of weakly relativistic electron on electron cyclotron maser instability in the magnetic mirror fusion devices are studied. The integrodifferential eigenmode equation is solved perturbatively with a small part of weakly relativistic electron population. Compared with the previous local model, the results indicate that the trapped effects provide stabilization to the electron cyclotron maser propagating along the magnetic field.
Applications of fractal geometry to dynamical evolution of sunspots*
Authors: Milovanov A. V. , Zelenyi L. M.
Published: July 2002

A fractal model for sunspot dynamics is presented. Formation of a sunspot in the solar photosphere is considered from the viewpoint of aggregation of magnetic flux tubes on a fractal geometry. Fine structure of the magnetic flux tubes is analyzed for a broad class of non-Maxwellian plasma distribution functions. The sunspot fractal dimension is proved to depend on the parameters of the plasma distribution function, enabling one to investigate intrinsic properties of the solar plasma by means of powerful geometrical methods. Magnetic field dissipation in the tubes is shown to result in effective sunspot decay. Sunspot formation and decay times as well as the diffusion constant K deduced by using the fractal model, are in a good agreement with observational data. Disappearance of umbras in decaying sunspots is interpreted as a second-order phase transition reminiscent of the transition through the Curie point in ferromagnetics.
Nonlinear hydromagnetic waves in a thermally stratified cylindrical fluid: Exact helically symmetric solutions
Authors: Hamabata Hiromitsu
Published: July 2002

The propagation of nonlinear hydromagnetic waves in a highly conducting, self-gravitating fluid in a cylindrical geometry, subject to the convective forces produced by a radial temperature gradient, is treated in a Boussinesq approximation. Exact wave solutions of the nonlinear magnetohydrodynamic equations (in the Boussinesq approximation) in the presence of convective forces are obtained for the case when the physical quantities are helically symmetric in cylindrical coordinates. The solutions represent waves propagating helically on the cylindrical surfaces, under the influence of the helical magnetic and velocity fields and the convective forces. The solutions may be applicable to the hydromagnetic waves in the Earth’s fluid core and the solar convection zone with suitable modifications to account for spherical geometry.
Distribution of particle velocity perpendicular to magnetic field in turbulent plasma
Authors: Fujisawa A.
Published: July 2002

In a turbulent magnetized plasma a relationship between velocity distribution of particles and its stochastic electric field is studied by solving the equation of motion both analytically and numerically. The distribution of velocity perpendicular to magnetic field direction is shown to be determined by the power spectral density of the stochastic electric field at the cyclotron frequency and the energy dissipation rate. This result will present a phenomenological understanding of high ion temperatures in several kinds of turbulent magnetized plasmas.
Implosion of reactor-size, gas-filled spherical shell targets driven by shaped pressure pulses
Authors: Piriz A. R. , Atzeni S.
Published: July 2002

The implosion of a family of reactor-size targets for inertial confinement fusion (ICF) is studied analytically and numerically. The targets consist of a deuterium–tritium (D–T) shell filled with D–T vapor and they are imploded by a multistep pressure pulse designed in such a way that the final hot spot is formed mainly from the initially gaseous fuel. The formation of the hot spot is described by means of a relatively simple model, and scaling laws for the quantities that characterize the state of the initially gaseous part of the fuel prior to ignition are derived. The results of the model are compared with one-dimensional fluid simulations, and good agreement is found. A parametric study of the fuel energy gain is then presented; the dependence of the gain and of the hot spot convergence ratio on the pulse parameters and on the filling gas density is analyzed. It is also shown that a substantial increase in the gain (for a given target and pulse energy) can be achieved by replacing the last step of the pulse with an exponential ramp.
Collisional and detuning effects on wave enhancement caused by a dc electric field
Authors: Fukumasa O. , Saeki S.
Published: July 2002

The effect of a dc electric field on the spatial evolution of an external wave in a small cold beam–plasma system is investigated numerically. The wave enhancement is strongly dependent on the sign of detuning δω(=ω−ω0) (ω is the wave frequency and ω0 is the frequency of the most unstable mode). Negative detuning, i.e., negative δω, leads to an anomalous increase in the wave power as compared with the wave enhancement for zero or positive detuning. Weak collisions also can play an important role in limiting the wave enhancement.
The production of (3/2) ω0 light by the two-plasmon decay instability
Authors: Villeneuve D. M. , Baldis H. A. , Walsh C. J.
Published: July 2002

Measurements of the emission of (3/2) ω0 light from CO2 laser plasma interactions are described. The target was an underdense preformed plasma with a relatively long density scalelength. Comparisons are made between the (3/2) ω0 emission and the spectrum of plasma waves driven by the two-plasmon decay instability as measured by Thomson scattering techniques. The spectrum of (3/2) ω0 light is shown as a function of scattering angle and compared with the relevant theories. Strong modulation of the spectrum was seen and is ascribed to the production of short-wavelength ion waves by the plasma waves.
Experimental investigation of the two-plasmon decay instability in a CO2-laser-produced plasma
Authors: Meyer J. , Houtman H.
Published: July 2002

The two-plasmon decay instability in a CO2-laser-irradiated gas jet target is investigated by a variety of diagnostic techniques. Growth rates and wavenumber spectrum measured by picosecond resolution Thomson scattering of probe ruby laser light are found to be in agreement with linear theory. The instability is saturated by coupling to ion-acoustic fluctuations of twice the wavenumber of the fastest growing wave and is quenched by radial profile modification caused by decay wave ponderomotive forces. The instability is seen to reappear at periodic intervals. The number of hot electrons measured with an electron spectrometer scales exponentially with the duration of the instability, and the energy distribution is seen to evolve from a one-dimensional (1-D) to a three-dimensional (3-D) Maxwellian, indicating the importance of space-charge potentials. Finally, the observed (3/2)ω0 radiation is shown to be consistent with scattering of CO2-laser radiation of two-plasmon decay fluctuations.
Dynamical properties of drift vortices
Authors: Laedke E. W. , Spatschek K. H.
Published: July 2002

Drift vortices are described by the Hasegawa–Mima equation in the case of density gradient drift waves. From computer results, it is known that the modon solutions of the Hasegawa–Mima equation are quite stable. Here the linear stability of ground state modon solutions is proved analytically by an indirect technique.
Light pressure of time-dependent fields in plasmas
Authors: Zeidler A. , Schnabl H. , Mulser P.
Published: July 2002

An expression of the light pressure Π is derived for the case of a nearly monochromatic electromagnetic wave with arbitrarily time-dependent amplitude. Thereby Π is defined as the time-averaged force density exerted on a plasma by the wave. The resulting equations are valid for both transverse and longitudinal waves. The light pressure turns out to consist of two components: the well-known gradient-type term and a new nonstationary solenoidal term. This is true for warm as well as cold plasmas. The importance of the new term for the generation of static magnetic fields is shown, and a model in which shear forces may result is given. Formulas for the nonstationary light pressure developed previously are discussed.
Two-dimensional ray-trace calculations of thermal whole beam self-focusing
Authors: Estabrook Kent , Kruer W. L. , Bailey D. S.
Published: July 2002

Thermal self-focusing of laser light may be significant when a plasma is irradiated with short-wavelength laser light. Self-focusing magnifies the light intensity which can increase absorption by plasma waves (producing hot electrons which may cause preheat), could increase scattering, and could be a perturbation source for the Rayleigh–Taylor instability. We use two-dimensional hydrodynamic simulations to characterize thermal self-focusing for parameters of interest to laser fusion applications, and present a simple model. A diverging beam is shown to reduce the self-focusing.
Particle simulation of the resistive g mode in a sheared magnetic field
Authors: Sydora R. D. , Leboeuf J. N. , Diamond P. H. , An Z. G. , Tajima T.
Published: July 2002

The linear and nonlinear evolution of the interchange instability of a resistive plasma (the resistive g mode) in a sheared magnetic field is studied using electrostatic particle simulation methods. Both the fast and slow interchange mode regimes are considered. In both cases the linear growth rates of the modes scale well with the theoretical values. The saturation of the instabilities is caused primarily by convective mixing of pressure over the width of the eigenmode. The saturation levels predicted by mixing length theory are in reasonable agreement with the simulation results.
A new Hamiltonian method in nonlinear gyrokinetic theory
Authors: Hagan W. K. , Frieman E. A.
Published: July 2002

Nonlinear gyrokinetic theory for a general magnetic field in the low-beta approximation is derived using a new Hamiltonian method. Gyroaveraged equations resulting from Hamiltonian procedures based on Darboux and Lie transform techniques [Phys. Fluids 24, 1730 (1981)] exhibit an unphysical dependence on the unit vectors used to describe the velocity perpendicular to the local magnetic field. The new method removes this unphysical dependence by modifying the Darboux algorithm and establishes the relationship between Hamiltonian gyrokinetic theory and the standard explicitly gyroaveraged theory.
Tilt stability of rotating current rings with resistive conductors
Authors: Zweibel Ellen G. , Pomphrey Neil
Published: July 2002

The combined effects of rotation and resistive passive conductors on the stability of a rigid current ring in an external magnetic field are studied. Numerical and approximate analytical solutions to the equations of motion are presented, which show that the ring is always tilt unstable on the resistive decay time scale of the conductors, although rotation and eddy currents may stabilize it over short times. Possible applications of our model include spheromaks which rotate or are encircled by energetic particle rings.
Visualization of the structure of a pulsed methane–air diffusion flame
Authors: Strawa Anthony W. , Cantwell Brian J.
Published: July 2002

Experiments have been carried out in a variable pressure flow facility with the objective of studying the structure of a co-flowing jet diffusion flame. The flow is visualized using an optical scheme which superimposes the luminous image of the flame on its Schlieren image. This gives a useful picture of the relationship between the bright, yellow-orange, soot-laden core flow and the edge of the surrounding hot gas envelope. A loudspeaker is used to force the central fuel jet at several frequencies. In the unforced flow and over most of the driving frequency range in the forced flow, a double structure is observed with two distinct wavelengths: a long wavelength associated with the luminous, buoyancy-driven core flow and a short wavelength associated with the shear-driven outer flow. Excitation at the proper frequency causes strong coupling to occur. In this case the core flow pinches off and the flame breaks up into a series of flamelets moving with a single wavelength.
Current enhancement in a conducting channel
Authors: Lau Y. Y. , Lampe M. , Fernsler R. F. , Hui B.
Published: July 2002

By the use of a simple model, it is shown that the transverse motion of a current-carrying filament in an electrically conducting medium may inductively produce a total current three times as large as that of the filament alone. An explanation of this phenomenon is given, and the implications are explored.
Interaction between matter and radiation at very high temperatures: I. Compton scattering
Authors: Herrera M. A. , Hacyan S.
Published: July 2002

The interaction between the two components of a relativistic binary gas is investigated. Starting from the relativistic Boltzmann equation, general expressions for the rate of energy exchange, the relaxation time, and the coefficient of bulk viscosity are obtained. As a particular example, the formalism is applied to the study of a gas at very high temperatures, T<∼6×109 °K, assuming that the energy exchange between matter and radiation is caused by Compton scattering; relativistic effects are fully taken into account.
Betatron-synchrotron detrapping in a tapered wiggler free-electron laser
Authors: Sprangle P. , Tang C. M.
Published: July 2002

Betatron-synchrotron resonance detrapping is shown to take place when the wiggler magnetic field amplitude is tapered. This resonance exists even if the radiation wave fronts are not curved and is dependent on the transverse gradient of the tapered wiggler field.
Stimulated Compton conversion of Langmuir waves by relativistic electron beams
Authors: Newman D. L.
Published: July 2002

Stimulated inverse Compton conversion in which a Langmuir wave scatters off a relativistic electron while converting into a transversely polarized electromagnetic wave is considered as a means for producing amplified electromagnetic radiation from a beam-plasma system at frequencies well above the electron plasma frequency. The stimulated emission growth rates of the radiation produced by a monoenergetic ultrarelativistic electron beam are determined as a functional of the Langmuir turbulence spectrum in the background plasma and are numerically evaluated for a range of model Langmuir spectra. Stimulated inverse Compton conversion can play a role in emission from astrophysical beam-plasma systems if the electron beam is highly relativistic and sufficiently narrow. However, it is found that the growth rates for this process are too small in all cases studied to account for the intense high-frequency radiation observed in the laboratory experiments of G. Benford et al. [G. Benford, D. Tzach, K. Kato, and D. F. Smith, Phys. Rev. Lett. 45, 1182 (1980); K. G. Kato, G. Benford, and D. Tzach, Phys. Rev. Lett. 50, 1587 (1983); K. G. Kato, G. Benford, and D. Tzach, Phys. Fluids 26, 3636 (1983)].
Spatial localization of bursts of the drift cyclotron loss cone instability
Authors: Leikind Bernard J. , Zwi Helio , Dimonte Guy , Wong Alfred Y.
Published: July 2002

Spatial localization of bursts of the drift cyclotron loss cone instability in a mirror machine is caused by asymmetric motion of the background plasma. The dynamic behavior of the plasma determines the appearance of the instability
Observations of a low-frequency mode in a linear multiple mirror
Authors: Makowski M. A. , Emmert G. A.
Published: July 2002

Two drift waves with similar frequencies are observed to be simultaneously unstable under the same plasma parameters. The modes are distinct below a critical magnetic field strength. By varying the mirror ratio and the magnetic field strength the frequencies of the two modes can be made to coalesce. As the critical magnetic field is exceeded the two modes again separate in frequency, but in addition a large-amplitude, low-frequency (<1 kHz) flute-type mode is also observed.
Electron heating caused by parametrically driven turbulence near the critical density
Authors: Mizuno K. , DeGroot J. S. , Estabrook K. G.
Published: July 2002

Microwave-driven experiments and particle simulation calculations are presented that model s-polarized laser light incident on a pellet. In the microwave experiments, the incident microwaves are observed to decay into ion and electron waves near the critical density if the microwave power is above a well-defined threshold. Significant absorption, thermal electron heating, and hot electron generation are observed for microwave powers above a few times threshold. Strong absorption, strong profile modification, strongly heated hot electrons with a Maxwellian distribution, a hot-electron temperature that increases slowly with power, and a hot-electron density that is almost constant, are all observed in both the microwave experiments and simulation calculations for high powers. In addition, the thermal electrons are strongly heated for high powers in the microwave experiments.
Fluid flow in a region bounded by a porous cylindrical pipe
Authors: Terrill Robert M.
Published: July 2002

The simplest experiments for flows in porous pipes occur when either the pressure or the pressure gradient outside the pipe is constant. Several classes of analytical solutions corresponding to some of these flows are presented.
The Korteweg–de Vries equation in Lagrangian coordinates
Authors: Osborne A. R. , Kirwan A. D. , Provenzale A. , Bergamasco L.
Published: July 2002

A Lagrangian form of the KdV equation (LKdV) is given which governs the evolution of nonlinear shallow water waves. Wave motion described by this equation in the context of the inverse scattering transform (IST) is discussed. For a single soliton it is shown that LKdV allows for an orthogonal decomposition of the particle motion into a kink (tanh) soliton in the horizontal and a pulse (sech2) soliton in the vertical. Generalizations to the orthogonal decomposition of N-soliton motion and to motion with periodic boundary conditions are also discussed. Implications of this approach on experimental measurements and on other integrable (soliton) systems are briefly explored.
Comparison of various thermal conduction models on the prediction of density limits in Ohmic tokamak discharges
Authors: Dong J. Q. , Guzdar P. N.
Published: July 2002

Four different thermal conduction models have been used to calculate the critical density limits in Ohmically heated tokamaks using the electron and ion energy balance equations. The results are compared to experimental density limits for a variety of Ohmically heated tokamaks.
Dynamics and fluctuation spectra of electrostatic resistive interchange turbulence
Authors: Sydora R. D. , Leboeuf J. N. , An Z. G. , Diamond P. H. , Lee G. S. , Hahm T. S.
Published: July 2002

The saturation mechanism for density and potential fluctuation spectra that evolve from linearly unstable electrostatic resistive interchange modes are investigated using particle simulations. Detailed comparisons of the nonlinear evolution, saturation levels, and resultant spectra between two- and three-dimensional sheared magnetic field configurations are made. Significant differences appear. The single rational surface, quasilinear-dominated evolution, fluctuation spectrum is adequately described using a density convection model. For the multiple rational surface case, the potential fluctuations are adequately represented by a balance between the nonlinearly modified source (curvature drive) and linear sink (parallel resistive field line diffusion). An accurate description of the density spectrum requires a mode coupling theory based on the two-point density correlation evolution equation.
Particle confinement and the anomalous Doppler instability during combined inductive and lower-hybrid current drive
Authors: Luckhardt S. C. , Chen K-I. , Mayberry M. J. , Porkolab M. , Terumichi Y. , Bekefi G. , McDermott F. S. , Rohatgi R.
Published: July 2002

Increases in average density have been observed during lower-hybrid current-drive experiments (LHCD) on the Versator II tokamak [Phys. Rev. Lett. 48, 152 (1982)], and on other LHCD experiments where inductive-Ohmic and lower-hybrid current drive are combined. In the present experiment it is found that this density increase is the result of an improvement in global particle confinement time τp in comparison to purely Ohmic discharges with similar plasma parameters. Furthermore, it is found that the improved confinement deteriorates when the anomalous Doppler relaxation instability of the anisotropic high-energy electron tail becomes unstable during LHCD, and the loss rate of tail electrons increases. However, energetic electron losses alone are at least an order of magnitude too small to explain this deterioration in particle confinement.
Electromagnetic instability in a quadrupole-focusing accelerator
Authors: Hughes Thomas P. , Godfrey Brendan B.
Published: July 2002

The addition of helical quadrupole focusing to a modified betatron configuration is shown to give rise to an electromagnetic instability under certain conditions. The instability arises from a three-wave coupling between the helical field, a transverse mode on the beam, and a transverse-electric waveguide mode. An analytic dispersion relation is derived. Several features of the instability are confirmed using three-dimensional computer simulations.
Influence of resistivity on energetic trapped particle-induced internal kink modes
Authors: Biglari Hamid , Chen Liu
Published: July 2002

The influence of resistivity on energetic trapped particle-induced internal kink modes, dubbed ‘‘fishbones’’ in the literature, is explored. A general dispersion relation, which recovers the ideal theory in its appropriate limit, is derived and analyzed. An important implication of the theory for present generation fusion devices such as the Joint European Torus [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, London, 1984), Vol I, p.11] is that they will be stable to fishbone activity.
Electrostatic interchange modes with finite parallel wavelengths
Authors: Shukla P. K. , Spatschek K. H.
Published: July 2002

A set of nonlinear equations is derived that includes the parallel electron dynamics in the study of the electrostatic interchange instability. In the linear limit, interchange modes are coupled with drift-modified convective cell modes. It is shown that a pressure gradient as well as a nonuniform magnetic field can induce a new class of instabilities.
Shift in plasma column position produced by azimuthally asymmetric electron heating in a tandem mirror
Authors: Sing D. C. , Hershkowitz N. , Scharer J. E.
Published: July 2002

Large shifts in the equilibrium position of the plasma column in the Phaedrus tandem mirror [Nucl. Fusion 25, 1213 (1985)] are observed when a 25 kW, 1 msec, 15.5 GHz microwave heating pulse (ω∼2ωce) is injected into one end cell of the device. The shift in the plasma column axial position occurs in all regions of the device, and is comparable in size to the initial plasma radius. The change in plasma position is a result of nonuniform electron heating produced by azimuthally asymmetric illumination of the plasma column by the microwave beam.