bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanisms for collisionless absorption of light waves obliquely incident on overdense plasmas with steep density gradients
Authors: Yang T.-Y. Brian , Kruer William L. , Langdon A. Bruce , Johnston Tudor W.
Published: July 2002

For p-polarized laser light obliquely incident on overdense plasmas with steep density gradients, a new collisionless absorption mechanism (sheath-transit absorption) is studied analytically and numerically. Complementary to Brunel’s ‘‘not-so-resonant’’ resonant absorption, and to the conventional resonant absorption, the sheath-transit absorption is most effective for steep density gradients and when the light pressure is less than the plasma pressure. It is also shown that the assumption of instantaneous particle reflection, usually a reasonable assumption for the normal incidence case, is invalid for the p-polarized oblique incident case. A test-particle model which provides a simple physical picture of the sheath-transit absorption is presented. Absorption coefficients obtained from the test-particle model agree reasonably well with those from particle-in-cell (PIC) simulations. The transition from the resonant absorption to the sheath-transit absorption as the density gradient steepens is demonstrated by PIC simulations with a wide range of density gradients.
Ionization equilibrium of hot hydrogen plasma
Authors: Potekhin Alexander Y.
Published: July 2002

The hydrogen plasma is studied at temperatures T∼104−106 K using the free energy minimization method. A simple analytic free energy model is proposed which is accurate at densities ρ≲1 g cm−3 and yields the convergent internal partition function of atoms. The occupation probability formalism is modified for solving the ionization equilibrium problem. The ionization degree and equation of state are calculated and compared with the results of other models.
Response to ‘‘Comment on ‘Three-dimensional stability of drift vortices’ ’’ [Phys. Plasmas 3, 160 (1996)]
Authors: Nycander J. , Pavlenko V. P. , Åkerstedt H. O.
Published: July 2002

The three-dimensional stability of drift vortices is studied in the framework of the Hasegawa-Mima equation. The coupling of the vortex to ion-acoustic waves along the magnetic fields causes the three-dimensionality. It is concluded that this coupling does not significantly destabilize the vortex. (AIP)
Radially localized measurements of superthermal electrons using oblique electron cyclotron emission
Authors: Preische S. , Efthimion P. C. , Kaye S. M.
Published: July 2002

It is shown that radial localization of optically thin electron cyclotron emission from superthermal electrons can be imposed by observation of emission upshifted from the thermal cyclotron resonance in the horizontal midplane of a tokamak. A new and unique diagnostic has been proposed and operated to make radially localized measurements of superthermal electrons during lower hybrid current drive on the Princeton Beta Experiment-Modified (PBX-M) tokamak [Bernabei, et al., Phys. Fluids B 5, 2562 (1993)]. The superthermal electron density profile as well as moments of the electron energy distribution as a function of radius are measured during lower hybrid current drive. The time evolution of these measurements after the lower hybrid power is turned off are given and the observed behavior reflects the collisional isotropization of the energy distribution and radial diffusion of the spatial profile.
Dynamics of relativistic electron-positron plasma cloud moving across a magnetic field
Authors: Kitanishi T. , Sakai J. I.
Published: July 2002

Results from a three-dimensional electromagnetic and relativistic particle simulation of a relativistic electron-positron plasma cloud (Lorentz factor γ=5/3) moving perpendicular to an ambient magnetic field with background plasmas are presented. It is shown that, in addition to the charge sheaths created at both sides of the cloud, secondary charge structures are created in the central region of the cloud and many cloud particles expand along the magnetic field. The Alfvén waves with large amplitude (δB/B0≊0.03) and linear polarization are excited dominantly with wave number kzc/Ωc<1, while the electromagnetic waves are weakly excited because of the relativistic effect of the cloud particles.
Gas breakdown effects in the generation and transport of light ion beams for fusion
Authors: Welch D. R. , Cuneo M. E. , Olson C. L. , Mehlhorn T. A.
Published: July 2002

The efficiency of delivering an ion beam to an inertial confinement fusion target depends on the ability to control the breakdown of both unintended (in the ‘‘vacuum’’ diode region) and intended (in the transport region) gas. The desorption and breakdown of anode-surface contaminants in an ion diode complicates the generation of a pure, high-brightness ion beam. Beyond the accelerator, the gas in the reactor vessel must provide excellent charge neutralization and specified current neutralization to permit the beam transport and focusing to a <1 cm radius, spherical target. Two schemes, in which controlling gas breakdown is essential, are ‘‘ballistic’’ and ‘‘self-pinched’’ ion transport. Results are discussed from hybrid particle-fluid simulations of anode contaminant desorption and ion beam transport.
Slowing down of an ion beam in a background plasma
Authors: Newsham D. , Ross T. J. , Rynn N.
Published: July 2002

The slowing down of a barium ion beam into two different plasma backgrounds was measured using laser-induced fluorescence. The measurements were performed in a Q machine (Ti=Te=0.2 eV, 6×1010≤nback≤1.2×1010 cm−3), where a barium ion beam, with energy 0–40 eV, was injected, parallel to the confining magnetic field, into both a cesium and a lithium plasma. In order to treat the ion beam as a class of test particles, the ion beam density was maintained at approximately two orders of magnitude below the density of the background plasma. Measured changes in the velocity profile of the ion beam agrees well with the predictions of the Fokker–Planck for both nearly equal mass beam and background ions as well as for a background ion with approximately 1/20th the mass of the beam ion.
Similarity of stability characteristics of planar and coaxial crossed-field diodes
Authors: Gopinath V. P. , Verboncoeur J. P. , Birdsall C. K.
Published: July 2002

Simulations of cylindrical crossed-field diodes for anode/cathode radius ratios of 2 and 5 indicate that the limiting current curve in the region B<BH in coaxial diodes follows the planar theory and simulations very closely. Coaxial diodes also follow planar theory predicting transition to turbulence in the region B≳BH. Larger radius ratio (10, 20) diodes show somewhat larger limits. These results show little variation with respect to operating voltage or cathode radius. A possible explanation for this behavior is also examined.
Experimental observations and simulations on relativistic self-guiding of an ultra-intense laser pulse in underdense plasmas
Authors: Chiron A. , Bonnaud G. , Dulieu A. , Miquel J. L. , Malka G. , Louis-Jacquet M. , Mainfray G.
Published: July 2002

The experimental images of the sidescattered light from a plasma, created by the multiterawatt laser pulse propagating in a hydrogen gas jet, exhibit clear dependence on both gas jet pressure and laser power. Two- and three-dimensional simulations of wave propagation, in presence of the relativistic electron mass increase and the ponderomotive expel of electrons, have been performed to reproduce the Thomson radiation from the plasma electrons. They show electron cavitation induced by the beam focusing, self-focusing, self-guiding, smoothing of the beam nonuniformities and, at larger power, beam filamentation. A bremsstrahlung model with account of the ionization, heating, expansion, and recombination dynamics of the gas, provides the plasma emission background. Both Thomson emission and bremsstrahlung are required to recover the experimental emission patterns. Among the interpretations, a scenario of laser self-guiding over five Rayleigh lengths can be found for 10 TW laser power and 5×1018 cm−3 electron density, which surprisingly disappears at larger powers and densities.
Relativistic Raman instability shifted by half-plasma frequency
Authors: Shvets G. , Fisch N. J. , Rax J.-M.
Published: July 2002

A new nonlinear Raman instability in underdense plasma is investigated theoretically. Unlike the usual linear Raman instabilities which grow exponentially in time, this instability takes a finite amount of time to diverge. The explosion time t∞ depends on the initial level of the perturbation. A general set of equations for spatio-temporal evolution of the forward nonlinear Raman scattering is derived and its temporal evolution is studied in detail. This new instability results in the generation of forward Raman radiation shifted by half the plasma frequency for laser intensities of order or exceeding 1018 W/cm2, something that has been recently observed [A. Modena (private communication, 1995)].
E×B flow shear effects on radial correlation length of turbulence and gyroradius scaling of confinement
Authors: Hahm T. S. , Burrell K. H.
Published: July 2002

The radial correlation length of the turbulence responsible for transport can have a different gyroradius scaling in low (L)-mode and high (H)-mode plasmas due to E×B flow shear effects, as predicted by the two-point nonlinear analysis in general tokamak geometry [Phys. Plasmas 2, 1648 (1995)]. This difference offers a possible understanding of the recent ρ*-scan experiment results on DIII-D [Fusion Technol. 8, 441 (1985)] L-mode and H-mode plasmas [Phys. Plasmas 2, 2342 (1995)]. Within our model, thermal diffusivity in H-mode plasmas scales like gyro-Bohm, independent of the scaling in L-mode plasmas.
Statistical mechanics of charged rings
Authors: Ganesh R. , Avinash K.
Published: July 2002

Statistical mechanics of massless thin rings carrying electric charge is formulated. The rings are circular, concentric around a common axis, and can undergo axisymmetric deformations. The model is applied to guiding center plasmas confined in a toroidal magnetic field, where charged rings are identified with flux tubes. It is argued that the steady state of such systems can be viewed in terms of the dynamics of these rings that slide on equipotentials and go through each other. A partial differential equation describing the most probable distribution of charges on the rings is obtained and solved numerically. Solutions corresponding to an ensemble with charges of single species and two species are obtained. The results show that the statistical mechanics of these rings differ in interesting ways from that of line charges.
Calculation and interpretation of analytic high-beta poloidal equilibria in finite aspect ratio tokamaks
Authors: Hsu S. C. , Artun M. , Cowley S. C.
Published: July 2002

The validity of the analytic large aspect ratio, high-β equilibria developed by Cowley et al. [Phys. Fluids B 3, 2066 (1991)] is extended to include finite aspect ratio equilibria with q2≫1, where q is the safety factor. These high-β equilibria have two regions. Most of the volume lies in the ‘‘core region,’’ where ψ=ψ(R). The flux surfaces close in the ‘‘boundary layer region,’’ which has thickness δ. The solutions are valid when δ/a∼𝒪 (√ε/βq2) is small, where a is the minor radius. Thus, finite ε is allowed when q2 is large. The equilibria are completely specified by the midplane profiles of pressure p(R) and poloidal magnetic field BP(R) and the shape of the plasma boundary, all of which can be measured experimentally. Note the departure from customary specification of p(ψ), q(ψ), or F(ψ). A fast numerical code, requiring a few seconds to execute, has been written to compute and illustrate the analytic high-β equilibria. The qualitative features of high-βP tokamaks are discussed in detail.
The role of two-step ionization in numerical predictions of electron energy distribution functions
Authors: Gordon M. H. , Kelkar U.
Published: July 2002

Spectroscopic emission measurements have been made in a microwave plasma and have been compared with results from a collisional-radiative model—one that accounts for a non-Maxwellian electron energy distribution function (EEDF)—to assess the effects of nonequilibrium. Typical operating conditions are 40 Torr pressure, 300 sccm hydrogen, and 3 sccm methane flow rates, and 1.6 kW deposited power. Optical emission measurements of atomic hydrogen’s excited state number densities indicate values that are inconsistent, by orders of magnitude, with those predicted by an atomic hydrogen collisional-radiative model if numerically estimated EEDFs (average energy near 2.5 eV) taken from the published literature are used as input. Satisfactory agreement between the experimental and numerical results, however, is obtained if two-step ionization is accounted for in a self-consistent coupled numerical solution for the free and bound–excited electron densities, the electric field, and the EEDF. Two-step ionization is the dominant electron production mechanism and thereby greatly impacts numerical predictions of the electric field, the electron average energy, and thus the EEDF. Self-consistently accounting for two-step ionization reduces the predicted sustaining electric field by 55% and thereby results in a predicted EEDF with an average energy near 1.1 eV.
On self-consistent stationary propagation of relativistically coupled electromagnetic and electrostatic waves in cold electron-ion plasma
Authors: Tsintsadze Levan N. , Nishikawa Kyoji
Published: July 2002

A general self-consistent theory is presented for one-dimensional stationary propagation of relativistically coupled electromagnetic and electrostatic waves. A finite amplitude plane wave solution is found to be always unstable against modulation. A modulated finite amplitude plane wave solution is found in a narrow region of the frequency, and the possibility for a solitary wave solution and its properties are discussed.
Detection of fusion neutrons from a deuterium-proton mixed plasma in the GAMMA 10 Tandem Mirror
Authors: Kiwamoto Y. , Tatematsu Y. , Saito T. , Abe H. , Ichimura M. , Inutake M. , Yamaguchi N. , Tamano T. , Nakashima Y. , Shoji M. , Cho T. , Hirata M. , Hojo H. , Ikeda K. , Ishii K. , Itakura A. , Katanuma I. , Mase A. , Nagayama Y. , Yatsu K.
Published: July 2002

Neutron diagnostics are applied to a deuterium plasma mixed with protons in the central cell of the GAMMA 10 tandem mirror [Inutake et al., Phys. Rev. Lett. 55, 939 (1985)]. The deuterium ions are heated with a slow ion cyclotron wave tuned to the fundamental deuterium resonance near the mid-plane of the central cell while the plasma is sustained with the fundamental resonance heating of protons in the minimum-B anchor cells. The measurement is based on in situ calibration to determine the transmission efficiency through machine walls and the counting efficiency of the detection system for neutrons emitted from 252Cf with energy close to that in the deuterium–deuterium (D–D) fusion reaction. The observation shows that the count rate of neutrons increases with diamagnetism, and this relation is accounted for in terms of fusion reaction between deuterium ions with a transverse temperature exceeding 10 keV. Discrepancies among ion temperatures determined with different diagnostics are mostly attributed to insufficient knowledge of the profiles of plasma parameters. The results indicate that the neutron measurement can be added as a powerful diagnostic tool for hot ions if combined with more detailed profile measurements.
Poloidal shear flow effect on toroidal ion temperature gradient mode: A theory and simulation
Authors: Kim J. Y. , Kishimoto Y. , Wakatani M. , Tajima T.
Published: July 2002

The effect of poloidal shear flow on the global structure and stability of toroidal ion temperature gradient mode is studied theoretically and also numerically using a toroidal particle simulation code. Reasonable agreements are found between the theoretical and simulation results; both indicating that with increasing shear flow the toroidal ion temperature gradient mode is stabilized in the increase of poloidal asymmetry and in the reduction of radial mode width.
Eikonal differential scattering cross sections for elastic electron–ion collisions in strongly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

The eikonal approximation is applied to investigate the elastic electron–ion collision processes in strongly coupled plasmas. Plasma-screening effects are investigated for the eikonal differential elastic scattering cross sections. The electron–ion interaction potential in strongly coupled plasmas has been approximated by the ion-sphere model potential. The classical straight-line trajectory approximation is applied to the motion of the projectile electron in order to investigate the variation of the eikonal differential elastic scattering cross section as a function of the impact parameter and ion-sphere radius. A modified eikonal approximation called the Wallace correction is also considered. The eikonal differential elastic scattering cross sections substantially decrease with an increase of the energy of the projectile electron and increase as the plasma-screening effect decreases through the ion-sphere radius.
Response to ‘‘Comment on ‘Bohm criterion for the collisional sheath’ ’’ [Phys. Plasmas 3, 1459 (1996)]
Authors: Valentini H.-B.
Published: July 2002

A plasma sheath concept including both the collisionless and collisional sheaths is explained. References to results of this concept are given. The plasma sheath concept used by Riemann and Meyer in their comment is not relevant for the thick collisional sheath and leads to conclusions which are in obvious contrast to relevant numerical results. Thus, the general remarks of the comment on the paper by Valentini [Phys. Plasmas 3, 1459 (1996)] are not relevant. The criticisms of a few details are justified. A slight quantitative modification of the previous results has been obtained (H.-B. Valentini, submitted to Phys. Plasmas).
Ionization rate in the presence of runaway electrons
Authors: Bulanov S. V. , Lontano M. , Sasorov P. V.
Published: July 2002

The role of a fast electron population, produced by the strong electric field driving the axial current in a capillary discharge, can be essential in order to justify the appreciable concentration of highly charged C ions (C5+, C6+) as observed in many experiments with low electron temperature. Under the assumption that a quasistationary state is established during the discharge, the runaway electron distribution function is determined analytically for Zeff>1, following the approximated method developed by Lebedev [A. N. Lebedev, Sov. Phys. JETP 21, 931 (1965)]. The strong enhancement of the ionization rate, due to the presence of an electron tail in the energy range of the ionization potential, is then demonstrated.
Ponderomotive modification of drift tearing modes
Authors: Urquijo G. , Singh R. , Sen A.
Published: July 2002

The linear characteristics of drift tearing modes are investigated in the presence of a significant background of radio-frequency waves in the ion cyclotron range of frequencies. The ponderomotive force, arising from the radial gradients in the rf field energy, is found to significantly modify the inner layer solutions of the drift tearing modes. It can have a stabilizing influence, even at moderate rf powers, provided the field energy has a decreasing radial profile at the mode rational surface.
Inertial confinement fusion ablator physics experiments on Saturn and Nova
Authors: Olson R. E. , Porter J. L. , Chandler G. A. , Fehl D. L. , Jobe D. O. , Leeper R. J. , Matzen M. K. , McGurn J. S. , Noack D. D. , Ruggles L. E. , Sawyer P. , Torres J. A. , Vargas M. , Zagar D. M. , Kornblum H. N. , Orzechowski T. J. , Phillion D. W. , Suter L. J. , Thiessen A. R. , Wallace R. J.
Published: July 2002

The Saturn pulsed power accelerator [R. B. Spielman et al., in Proceedings of the 2nd International Conference on Dense Z-pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3] at Sandia National Laboratories (SNL) and the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] at Lawrence Livermore National Laboratory (LLNL) have been used to explore techniques for studying the behavior of ablator material in x-ray radiation environments comparable in magnitude, spectrum, and duration to those that would be experienced in National Ignition Facility (NIF) hohlraums [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)]. The large x-ray outputs available from the Saturn pulsed-power-driven z pinch have enabled us to drive hohlraums of full NIF ignition scale size at radiation temperatures and time scales comparable to those required for the low-power foot pulse of an ignition capsule. The high-intensity drives available in the Nova laser have allowed us to study capsule ablator physics in smaller-scale hohlraums at radiation temperatures and time scales relevant to the peak power pulse for an ignition capsule. Taken together, these experiments have pointed the way to possible techniques for testing radiation-hydrodynamics code predictions of radiation flow, opacity, equation of state, and ablator shock velocity over the range of radiation environments that will be encountered in a NIF hohlraum.
Simulations of laser imprint for Nova experiments and for ignition capsules
Authors: Weber S. V. , Glendinning S. G. , Kalantar D. H. , Key M. H. , Remington B. A. , Rothenberg J. E. , Wolfrum E. , Verdon C. P. , Knauer J. P.
Published: July 2002

In direct drive inertial confinement fusion (ICF), nonuniformities in laser illumination seed ripples at the ablation front in a process called “imprint.” These nonuniformities grow during the capsule implosion and, if initially large enough, can penetrate the capsule shell, impede ignition, or degrade burn. Imprint has been simulated for recent experiments performed on the Nova laser [Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] examining a variety of beam smoothing conditions. Most used laser intensities similar to the early part of an ignition capsule pulse shape, I≅1013 W/cm2. The simulations matched most of the measurements of imprint modulation. The effect of imprint upon National Ignition Facility (NIF) direct drive ignition capsules has also been simulated. Imprint is predicted to give modulation comparable to an intrinsic surface finish of ∼10 nm rms. Modulation growth was examined using the Haan [Phys. Rev. A 39, 5812 (1989)] model, with linear growth factors as a function of spherical harmonic mode number obtained from an analytic dispersion relation. Ablation front amplitudes are predicted to become substantially nonlinear, so that saturation corrections are large. Direct numerical simulations of two-dimensional multimode growth were also performed. The capsule shell is predicted to remain intact, which gives a basis for believing that ignition can be achieved.
Test particle transport from long-range collisions
Authors: Anderegg F. , Huang X.-P. , Hollmann E. M. , Driscoll C. F. , O’Neil T. M. , Dubin D. H. E.
Published: July 2002

Enhanced cross-magnetic-field diffusion of test particles in pure ion plasmas has been measured. The ion plasma is contained in a Penning-Malmberg trap for weeks near thermodynamic equilibrium, characterized by rigid rotation and uniform density and temperature. Plasma expansion and loss is suppressed by a “rotating wall” technique, i.e., a weak electrostatic potential rotating faster than the plasma. Test particle transport is then measured even though there is zero net transport, in a regime where neutral collisions are negligible. The observed test particle transport is diffusive, i.e., proportional to the gradient of the test particle concentration. The measured diffusion coefficients scale as nT−1/2B−2 over a range of 40 in density, 50 in temperature, and 5 in magnetic field. This diffusion is about ten times greater than predicted by classical collisional theory, which describes velocity-scattering collisions with impact parameters ρ≲rc. The enhanced transport is thought to be due to non-velocity-scattering “E×B drift” collisions with rc<ρ≲λD. Initial estimates of diffusion due to these long-range collisions are three times less than the measurements, and substantial theory questions remain.
Dynamics of a high-power aluminum-wire array Z-pinch implosion
Authors: Sanford T. W. L. , Nash T. J. , Mock R. C. , Spielman R. B. , Struve K. W. , Hammer J. H. , De Groot J. S. , Whitney K. G. , Apruzese J. P.
Published: July 2002

Annular Al-wire Z-pinch implosions on the Saturn accelerator [D. D. Bloomquist et al., Proceedings, 6th Pulsed Power Conference (Institute of Electrical and Electronics Engineers, New York, 1987), p. 310] that have high azimuthal symmetry exhibit both a strong first and weaker second x-ray burst that correlate with strong and weaker radial compressions, respectively. Measurements suggest that the observed magnetic Rayleigh–Taylor (RT) instability prior to the first compression seeds an m=0 instability observed later. Analyses of axially averaged spectral data imply that, during the first compression, the plasma is composed of a hot core surrounded by a cooler plasma halo. Two-dimensional (2-D) radiation magnetohydrodynamic computer simulations show that a RT instability grows to the classic bubble and spike structure during the course of the implosion. The main radiation pulse begins when the bubble reaches the axis and ends when the spike finishes stagnating on axis and the first compression ends. These simulations agree qualitatively with the measured characteristics of the first x-ray pulse and the overall energetics, and they provide a 2-D view into the plasma hydrodynamics of the implosion.
Structures, wavelet and intermittency in tokamak edge turbulence
Authors: Jha R. , Mattoo S. K. , Saxena Y. C.
Published: July 2002

The wavelet bicoherence technique is used to examine floating potential fluctuations in ADITYA tokamak edge plasma, which have shown vortex-like coherent structures on statistical basis [Proceedings of the 15th International Conference on Plasma Physics and Controlled Nuclear Fusion, Seville (International Atomic Energy Agency, Vienna, 1995), Vol. 1, p. 583]. On a single discharge basis, the data exhibit intermittent episodes of quadratic interaction in which low and high frequency components are coupled. The broadband nonlinear phase coupling is used as a diagnostic signature of localized coherent structure. It is observed that the wavelet bicoherence is high in the presence of sparse structures and small when the structures are closely packed. The turbulent edge plasma has regions of packed Gaussian structures and sparse non-Gaussian structures.
Nonlinear δf simulation studies of intense ion beam propagation through an alternating-gradient quadrupole focusing field
Authors: Qian Qian , Lee W. Wei-li , Davidson Ronald C.
Published: July 2002

Newly-developed perturbative (δf) particle simulation techniques are used to investigate the detailed stability and transport properties of an intense ion beam propagating through an alternating-gradient focusing lattice with initial Kapchinskij-Vladimirskij (KV) distribution. Stability properties are investigated over a wide range of beam current and focusing field strength. In the unstable region, large-amplitude density perturbations with low azimuthal harmonic numbers, concentrated near the beam surface, are observed, and their nonlinear consequences are discussed. In the stable region, it is shown that the KV beam distribution propagates quiescently over large distances (up to 500 lattice periods in the simulations). The δf-scheme used in the present numerical studies exhibits minimal noise and accuracy problems in comparison with standard particle-in-cell simulations.
Measurements of laser-imprinted perturbations and Rayleigh–Taylor growth with the Nike KrF laser
Authors: Pawley C. J. , Gerber K. , Lehmberg R. H. , McLean E. A. , Mostovych A. N. , Obenschain S. P. , Sethian J. D. , Serlin V. , Stamper J. A. , Sullivan C. A. , Bodner S. E. , Colombant D. , Dahlburg J. P. , Schmitt A. J. , Gardner J. H. , Brown C. , Seely J. F. , Lehecka T. , Aglitskiy Y. , Deniz A. V. , Chan Y. , Metzler N. , Klapisch M.
Published: July 2002

Nike is a 56 beam Krypton Fluoride (KrF) laser system using Induced Spatial Incoherence (ISI) beam smoothing with a measured focal nonuniformity 〈ΔI/I〉 of 1% rms in a single beam [S. Obenschain et al., Phys. Plasmas 3, 1996 (2098)]. When 37 of these beams are overlapped on the target, we estimate that the beam nonuniformity is reduced by 37, to (ΔI/I)≅0.15% (excluding short-wavelength beam-to-beam interference). The extraordinary uniformity of the laser drive, along with a newly developed x-ray framing diagnostic, has provided a unique facility for the accurate measurements of Rayleigh–Taylor amplified laser-imprinted mass perturbations under conditions relevant to direct-drive laser fusion. Data from targets with smooth surfaces as well as those with impressed sine wave perturbations agree with our two-dimensional (2-D) radiation hydrodynamics code that includes the time-dependent ISI beam modulations. A 2-D simulation of a target with a 100 Å rms randomly rough surface finish driven by a completely uniform beam gives final perturbation amplitudes similar to the experimental data for the smoothest laser profile. These results are promising for direct-drive laser fusion.
Plasma compressibility induced toroidal Alfvén eigenmode
Authors: Zheng L.-J. , Chen Liu
Published: July 2002

For circular tokamaks, it is demonstrated, both analytically and numerically, that there exists a new frequency gap within the shear Alfvén continuous spectrum around the Alfvén frequency, ωA=vA/qR, due to the ion compressibility effect. Here, vA denotes the Alfvén speed, q is the safety factor, and R the major radius. The frequency gap width, meanwhile, is proportional to the core ion beta, βi (ratio between core ion and magnetic pressures), and, correspondingly, the new discrete toroidal Alfvén eigenmode (TAE) inside the gap is referred to as βTAE. Collective excitations of the βTAE by energetic ions are also analyzed.
Measurements of core and pusher conditions in surrogate capsule implosions on the OMEGA laser system
Authors: Bradley D. K. , Delettrez J. A. , Epstein R. , Town R. P. J. , Verdon C. P. , Yaakobi B. , Regan S. , Marshall F. J. , Boehly T. R. , Knauer J. P. , Meyerhofer D. D. , Smalyuk V. A. , Seka W. , Haynes D. A. , Gunderson M. , Junkel G. , Hooper C. F. , Bell P. M. , Ognibene T. J. , Lerche R. A.
Published: July 2002

Experiments have been carried out on the 60-beam, 30 kJ OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] as part of an integrated program to diagnose all phases of direct-drive capsule implosions. Laser-imprint levels and Rayleigh–Taylor growth rates associated with the spherical implosions have been inferred from planar-foil radiography experiments. In spherical targets, measurements of the combined effects of imprint and unstable growth at the ablation surface have been carried out using the burnthrough technique [J. Delettrez et al., Phys. Plasmas 1, 2342 (1994)]. Target behavior during the deceleration phase has been investigated using a series of surrogate cryogenic capsules in which the main fuel layer is represented by a Ti-doped CH shell and the hot spot is represented by an Ar-doped deuterium fill gas.
Modulational instability of random phase plasmons in collisional plasmas
Authors: Shukla P. K. , Stenflo L. , Faria R. T.
Published: July 2002

The modulational instability of random phase Langmuir waves in an unmagnetized collisional plasma is investigated. The growth rate of the instability is presented in several interesting limiting cases. The relevance of this investigation to space and laboratory plasmas is pointed out.
Surface waves in a magnetized plasma with mobile dust grains
Authors: Cramer N. F. , Yeung L. K. , Vladimirov S. V.
Published: July 2002

Wave propagation in a dusty, magnetized, strongly inhomogeneous plasma at frequencies below the ion–cyclotron frequency, and near the dust cyclotron frequency, is considered. The dust grains are assumed to carry a proportion of the negative charge of the plasma. Wave resonances and cutoffs in a homogeneous plasma are discussed. The dispersion relation for surface waves propagating on an interface between a dusty plasma and a vacuum is derived and studied, and their connection to surface waves in general two-ion-species plasmas is analyzed. The damping of the waves due to Alfvén resonance absorption in a narrow but nonzero width interface is derived, and it is shown that for a range of frequencies above the dust cyclotron frequency the surface wave can propagate undamped by resonance absorption.
Phenomenology of major and minor disruptions in high β deuterium and tritium tokamak fusion test reactor plasma
Authors: Mirnov S. , Semenov I. , Fredrickson E. , Budny R. , Chang Z. , McGuire K. , Park H. , Takahashi H. , Taylor G. , Von Goeler S. , Zakharov L. , Zweben S.
Published: July 2002

The main subject of this work is the experimental study of the low m and n magnetohydrodynamics (MHD) perturbations during disruptive instability. This work presents the magnetic probe data, electron cyclotron emission (ECE), α-particle losses, and neutron flux data measured during the disruptive instability in high β tokamak fusion test reactor (TFTR) [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)] plasmas. The major disruptions in high β regimes go through several phases. The first phase is the fast (150–250 μs) minor disruption (predisruption), causing a drop of the central temperature (and possibly, density). In this phase a powerful central m=1/n=1 mode initiates the sequential development of m=4/n=1, 3/1, 2/1, 3/2 peripheral modes, which lead to a 3/1 locked mode. The second phase is the slow thermal quench (2 ms) in the presence of a locked mode. The third phase is a fast positive current spike generation (5%–10% increase in Ip in less than 0.5 ms) and finally, the current quench occurs with a loss of 2.5 MA in 5 ms.
Modulation of ion acoustic surface wave
Authors: Lee Hee J. , Hong Hee Sook
Published: July 2002

A nonlinear Schrödinger equation is derived for an ion acoustic surface wave propagating on the interface between a plasma and vacuum, taking into account all the nonlinearities, and especially the nonlinear surface currents. The nonlinear frequency shift is obtained in terms of the plasma dielectric constant. The effect of the nonlinear surface currents on the nonlinear behavior of the surface wave is assessed.
The effect of dust charge fluctuations on ion cyclotron wave instability in the presence of an ion beam in a plasma cylinder
Authors: Sharma Suresh C. , Sugawa M.
Published: July 2002

An ion beam propagating through a magnetized dusty plasma cylinder drives electrostatic dust ion cyclotron waves to instability via Cerenkov interaction. The growth rate of the instability increases with the relative density δ (=n0i/n0e) of negatively charged dust. The growth rate scales as the one-third power of the beam density. The frequency of the unstable wave also increases with the relative density δ of negatively charged dust. The results of the theory are applied to explain some of the experimental observations of Barkan et al. [Planet. Space Sci. 43, 905 (1995)].
Spatiotemporal structure of resistive pressure-gradient-driven turbulence
Authors: Garcia L. , Carreras B. A. , Lynch V. E.
Published: July 2002

High-resolution turbulence calculations are needed to characterize the resistive pressuregradient-driven turbulence in toroidal geometry because the wave number spectrum is very broad with a low decay index. Here, two main issues are addressed: the spatiotemporal structure of the turbulence and the validity of the mixing-length approach for toroidal geometry. Well above the instability threshold, steady-state turbulence is established. In this case, the relevant spatial scales are the width of the individual poloidal components of the ballooning mode, instead of the toroidal scale lengths. The situation is reversed near the instability threshold, where the stationary state can be dominated by one toroidal mode.
Stability of highly asymmetric non-neutral plasmas
Authors: Backhaus E. Yu. , Fajans J. , Wurtele J. S.
Published: July 2002

The stability of non-neutral plasma equilibria subjected to external, azimuthal–asymmetric potentials is studied. The system exhibits a wide range of complex, symmetry-dependent, bifurcation phenomena.
Bremsstrahlung radiation from electron–ion polarization scatterings in strongly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

Low energy bremsstrahlung processes in electron–ion polarization scatterings in strongly coupled plasmas described by the ion-sphere model are investigated using the classical trajectory method. The interaction between the projectile electron and the screened target ion is given by the attractive polarization interaction between the projectile electron and the induced dipole. The classical straight-line trajectory method is applied to represent the differential bremsstrahlung radiation cross section as a function of the impact parameter and ion-sphere radius. The results show that the bremsstrahlung radiation due to the component of the acceleration perpendicular to the projectile velocity is dominant for small impact parameters, especially, for soft photon radiation. However, for large impact parameters, the contribution from the component of the acceleration parallel to the velocity is almost comparable to that from the component of the acceleration perpendicular to the velocity.
On Alfvén waves in a flowing atmosphere
Authors: Campos L. M. B. C. , Gil P. J. S.
Published: July 2002

The present paper considers Alfvén waves in an inhomogeneous medium, under a nonuniform external magnetic field and in the presence of a nonuniform flow, to obtain wave equations for the velocity and magnetic field perturbations, in some particular cases. The case which is chosen for solution is that of propagation along a vertical uniform external magnetic field, in an atmosphere with a vertical mean flow velocity, satisfying mass conservation. The wave equation has a critical layer where the mean flow velocity equals the Alfvén speed, and in the case of an isothermal atmosphere the wave equation for the magnetic field perturbation can be reduced to a Bessel type with complex order and imaginary variable. Thus it is possible to obtain exact solutions for all frequencies and distances, discussing the cases in which there are propagating or evanescent waves far below or far above the critical layer, i.e., the latter acts as a reflecting layer. The solution in the vicinity of the critical layer allows the amplitude and phase of the wave field to be plotted as a function of distance from the critical layer (measured in scale heights) for different values of the dimensionless frequency and initial Alfvén number (ratio of flow velocity to Alfvén speed). One application is the solar atmosphere, for which the magnetohydrodynamic (MHD) equations are appropriate, and the wavelengths are large compared with the density scale height, thus excluding the ray approximation.
Electron capture in the Lorentzian distribution plasma
Authors: Jung Young-Dae , Yoon Jung-Sik
Published: July 2002

Electron captures from hydrogenic ions by protons in a generalized Lorentzian (kappa) velocity distribution plasma are investigated using the semiclassical Bohr–Lindhard model. The potential energy between the projectile ion and the released electron is obtained by introduction of the plasma dielectric function. The straight-line trajectory approximation is applied to the motion of the projectile ion in order to obtain the electron capture probability as a function of impact parameter, collision energy, and spectral index κ. It is found that the semiclassical capture probability including the dynamic plasma screening effect is increased with increasing the spectral index. The plasma screening effects on the electron capture probabilities are found to be more effective for low projectile velocities. It is also found that the dynamic plasma screening effect on the electron capture probability in the Lorentzian distribution plasma is stronger than that in the Maxwellian distribution plasma since the effective Debye length in the Lorentzian plasma is smaller than the Debye length in the Maxwellian plasma.
Theories of relativistic ion cyclotron instabilities
Authors: Chen K. R.
Published: July 2002

A perturbation theory and a kinetic theory are developed to investigate the novel physics of relativistic ion cyclotron instabilities. The existence of the instabilities is determined by the normalized mass deficits per nucleon of fast and slow ions (δmf and δms, respectively), and by their Lorentz factors (γf and γs, respectively); while the ion bunching is caused by the relativistic variation of ion mass. If δmf−δms−γf+γs>0, only a quadratic instability can occur at high cyclotron harmonics of the fast ion in the lower-hybrid frequency regime and above; the threshold on the harmonic number is determined by the dielectric constant of the slow ion. The peak growth rate is higher at the harmonics just above the threshold. If it is negative, both a cubic instability (or instead a coupled quadratic instability if the resonant slow ion cyclotron harmonic is the first harmonic) and the high harmonic quadratic instability can be driven. The cubic instability is due to the harmonic interaction of fast and slow ion cyclotron motions with the wave frequency in between. This introduces a novel instability concept, namely, a two-streaming process in gyrospace. Thus, the cubic instability is also called a two-gyro-stream instability even without beams in real space in contrast to conventional two-stream instability. Both theories show that, as compared to the conventional axial phase bunching mechanism, the importance of the inclusion of the relativistic mass variation effect (and the gyro-bunching mechanism) depends on the phase velocity of the wave along the external magnetic field, and is not related to the Lorentz factors (or kinetic energies); that is, if ω/kz>c (e.g., kz=0), the relativistic gyro-bunching mechanism always dominates. While the importance of this study in fundamental plasma physics is emphasized here, some issues (e.g., nonlinear saturation, wave polarization, and nonuniform magnetic field) related to its application are also discussed.
Coupled full-wave and ray-tracing numerical treatment of mode conversion in a tokamak plasma
Authors: Petrov Yu. , Bécoulet A. , Monakhov I.
Published: July 2002

A new approach for the numerical description of tokamak plasma waves in the ion cyclotron range of frequencies is discussed. It implies coupling of the full-wave and ray-tracing codes and is capable of unified treatment of waves of completely different scale and behavior. The method is applied for simulations of the electron heating scenario, based on fast wave (FW)–ion Bernstein wave (IBW) mode conversion near the ion–ion hybrid resonance in Tore Supra [B. Saoutic et al., Phys. Rev. Lett. 76, 1647 (1996)]. The two-dimensional full-wave “ALCYON” code [D. J. Gambier and A. Samain, Nucl. Fus. 25, 283 (1985)] is used to describe the global FW field pattern in the plasma volume. A small-scale-waves filter, introduced into the code, artificially damps the mode-converted power, which is further prescribed to IBW rays. Remnant small-scale fields are extracted from the global pattern to provide information necessary for IBW rays to start. Three-dimensional evolution of IBW rays is simulated by the “RAYS” ray-tracing code [Yu. Petrov, Nucl. Fus. 34, 63 (1994)], being unrestricted by finite mesh size and finite Larmor radius effects.
Direct-drive, hollow-shell implosion studies on the 60-beam, UV OMEGA laser system
Authors: Marshall F. J. , Delettrez J. A. , Glebov V. Yu. , Town R. P. J. , Yaakobi B. , Kremens R. L. , Cable M.
Published: July 2002

Direct-drive implosion experiments have been performed on the University of Rochester’s 60-beam, 30 kJ, UV (351 nm) OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system to investigate the conditions at maximum compression of polymer–shell targets with zero- or low-pressure (⩽3 atm) gas fills. By using deuterium-bearing shells (CD), the imploded-core conditions have been diagnosed with both x-ray and neutron spectral measurements. The core electron temperature (kTe) and shell areal density (ρRshell) are determined from the emergent x-ray spectrum, while independent inferences of ρRshell are obtained from the measured primary (DD) and secondary (DT) neutron yields. Target performance was investigated for a range of beam-smoothing conditions [none to 0.25 THz-bandwidth smoothing by spectral dispersion along two dimensions (2-D SSD)] and a set of pulse shapes (1 ns square pulse to a 2.5 ns pulse with a 1:40 foot-to-main-pulse power ratio). The results have conclusively demonstrated the ability to compress targets to shell areal densities in the range of ∼60 to 130 mg/cm2.
Cylindrical solitary envelope pulses in warm overdense plasmas
Authors: Das A. , Sen A. , Kaw P. K. , Miklaszewski R.
Published: July 2002

A generalized nonlinear Schrödinger equation with an exponentially saturating nonlinearity is solved numerically in two dimensions using a novel iterative procedure. A spectrum of cylindrical envelope solitary solutions is presented which may be useful for modeling the nonlinear propagation of intense laser pulses in warm overdense plasmas.
Renormalized dissipation in the nonconservatively forced Burgers equation
Authors: Krommes John A.
Published: July 2002

A previous calculation [P. H. Diamond and T.-S. Hahm, Phys. Plasmas 2, 3640 (1995)] of the renormalized dissipation in the nonconservatively forced one-dimensional Burgers equation, which encountered a catastrophic long-wavelength divergence ∼kmin−3, is reconsidered. In the absence of velocity shear, analysis of the eddy-damped quasi-normal Markovian closure predicts only a benign logarithmic dependence on kmin. The original divergence is traced to an inconsistent resonance-broadening type of diffusive approximation, which fails in the present problem. Ballistic scaling of renormalized pulses is retained, but such scaling does not, by itself, imply a paradigm of self-organized criticality. An improved scaling formula for a model with velocity shear is also given.
Nonlinear Langmuir oscillations under nonequilibrium conditions
Authors: Karimov A. R. , Shcheglov V. A.
Published: July 2002

The influence of external ionization sources on the characteristics of Langmuir oscillations in a partially ionized plasma is considered. It is demonstrated that in the case of continuous sources of ionization the nonlinear Langmuir oscillations are nonharmonic and are determined by the Airy equation.
Surface wave enhanced collisionless transport in a bounded crossed-field non-neutral plasma
Authors: Cartwright K. L. , Christenson P. J. , Verboncoeur J. P. , Birdsall C. K.
Published: July 2002

Electron transport across the magnetic field in a cutoff planar smooth-bore diode is described on the basis of surface waves perpendicular to the magnetic field and along the cathode. A self-consistent (two spatial dimensions and three velocity components) electrostatic particle-in-cell (PIC) simulation of a crossed-field diode produces a near-Brillouin flow which slowly expands across the diode, punctuated by sudden transport across the diode. The theory of slow transport across the diode is explained by the addition of perturbed orbits to the Brillouin shear flow motion of the plasma in the diode. The wave results from the sheared flow instability, so that a definite mechanism behind the surface waves is established. The growth and wavelength of this wave are compared to PIC simulations. A slow drift compared to the shear flow is described which results from an electrostatic ponderomotive-like force in a dc external magnetic field. The slow drift obtained from single particle motion compares well to the expansion of the hub in PIC simulations. The slow expansion of the Brillouin hub leads to configurations that are not stable and quickly decay via a large transport of charge to the anode. Both the slow transport of electrons and current spikes have been observed in experiments.