Volume 11, Issue 7, July 2004
Index of content:
 LETTERS


The penetration of plasma clouds across magnetic boundaries: The role of high frequency oscillations
View Description Hide DescriptionExperiments are reported where a collisionfree plasma cloud penetrates a magnetic barrier by selfpolarization. Two closely related effects, both fundamental for the penetration mechanism, are studied quantitatively: anomalous fast magnetic field penetration (two orders of magnitude faster than classical), and anomalous fast electron transport (three orders of magnitude faster than classical and two orders of magnitude faster than Bohm diffusion). It is concluded that they are both mediated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop which is set up in the plasma in the penetration process.

Application of stereoscopic particle image velocimetry to studies of transport in a dusty (complex) plasma
View Description Hide DescriptionOver the past 5 years, twodimensional particle image velocimetry(PIV) techniques [E. Thomas, Jr., Phys. Plasmas6, 2672 (1999)] have been used to obtain detailed measurements of microparticle transport in dusty plasmas. This Letter reports on an extension of these techniques to a threedimensional velocity vector measurement approach using stereoscopic PIV. Initial measurements using the stereoscopic PIVdiagnostic are presented.

 ARTICLES


Lasers, Particle Beams, Accelerators, Radiation Generation

Spot size effects on the stationary envelope of intense laser pulses in plasma
View Description Hide DescriptionNonlinear interactions between ultraintense laser pulses and plasmas reveals a great variety of important collective phenomena. Advances in laser technology have made possible compact terawatt lasers capable of generating subfemtosecond pulses at ultrahigh intensities. These ultrahigh intensities result in highly relativistic nonlinear electron dynamics. This paper addresses stationary stage of the interaction of an ultraintense short laser pulse with a plasma including finite spot size effects. The study is based on the approach that accounts for the transversal effects in an average manner. Following the standard method, a reduced wave equation for the slowly varying laser envelope is derived, including nonlinear effects associated with relativistic electron mass variation and relativistic ponderomotive force. Localized and periodic solutions of the envelope equation are presented and compared with the case when the transversal distribution is infinite. It is found that the finite transverse size of laser spot has significant effects on the propagation characteristics of the stationary profiles.

Harmonic generation in clusters
View Description Hide DescriptionA model is presented for the nonlinear response of a small cluster, with a size much smaller than the wavelength, at the third harmonic of the laser frequency. The model involves collective modes of a cold electron core confined within a positively charged ion background. The response of the electron core to the laser field is similar to that of a weakly nonlinear oscillator driven by an external force. In particular, there is a resonant enhancement of the third harmonic when the frequency of the applied field is close to one third of the core eigenfrequency. It is shown that density nonuniformity or nonspherical shape of the ion background is necessary for harmonic generation.Particleincell simulations have been performed to model the time evolution of the third harmonic response as the ion density profile changes due to cluster expansion. The simulation results are consistent with the predictions of the cold electron core model. In addition, the code quantifies the role of stochastic electron heating, an alternative harmonic generation mechanism.

Nonlinear Phenomena, Turbulence, Transport

Loss rate of an electrically trapped nonneutral plasma
View Description Hide DescriptionA nonneutral plasma in a Malmberg–Penning trap can be allowed to escape confinement in a controllable way, because the depth of the applied electric potential well used to confine the plasma axially is adjustable. A theoretical approach is presented for predicting the collisionbased axial plasma loss rate, just after the well depth in the trap is made shallower. The approach can also be used for predicting an upper limit for the loss rate of a nearthermal plasma from other types of traps, which may employ one, two, or threedimensional electric potential wells. Example predictions are provided, and comparisons with existing theories are made.

Basic Plasma Phenomena, Waves, Instabilities

A model for the condensation of a dusty plasma
View Description Hide DescriptionA model for the condensation of a dusty plasma is constructed by considering the spherical shielding layers surrounding a dust grain test particle. The collisionless region less than a collision mean free path from the test particle is shown to separate into three concentric layers, each having distinct physics. The method of matched asymptotic expansions is invoked at the interfaces between these layers and provides equations which determine the radii of the interfaces. Despite being much smaller than the Wigner–Seitz radius, the dust Debye length is found to be physically significant because it gives the scale length of a precipitous cutoff of the shielded electrostatic potential at the interface between the second and third layers. Condensation is predicted to occur when the ratio of this cutoff radius to the Wigner–Seitz radius exceeds unity and this prediction is shown to be in good agreement with experiments.

Lasers, Particle Beams, Accelerators, Radiation Generation

Design of the Nilike Ag xray laser at 13.9 nm
View Description Hide DescriptionThe Nilike Ag xray laser at 13.9 nm is numerically studied, using a onedimensional hydrodynamic code coupled with an atomic physics code. The effects of the drive laser intensity, pulse duration, and delay time on the performance of the Nilike Ag xray laser are investigated. A saturated xray laser output could possibly be generated for a short plasma length under the optimized drive pulse configuration. The analysis of the plasma conditions, including the evolution of gain in time and space, the electron temperature and the electron density, contribute to further understanding of the process of Nilike xray lasers.

Ionospheric, Solarsystem, and Astrophysical Plasmas

Experiment on collisionless plasma interaction with applications to supernova remnant physics
View Description Hide DescriptionResults from an experimental study of the collisionless interaction of two laserproduced plasmas in a magnetic field with applications to supernova remnant shock physics are presented. The dynamics of the two plasmas and their interaction are studied with and without magnetic field through spatially and temporally resolved measurements of the electron density. Experimental results show that counterpropagating collisionless plasmas interpenetrate when no magnetic field is present. In contrast, results obtained with the addition of a 7.5 T magnetic field perpendicular to plasma flow show density features in the interaction area that only occur when the field is present. The reason for this remains uncertain. It is suggested that this results from an increase in the effective collisionality as the magnetic field reduces the ion and electron gyroradius below the size of the experiment.

Inertially Confined Plasmas, Dense Plasmas, Equations of State

Modeling stimulated Brillouin scattering in the underdense corona of a direct drive inertial confinement fusion target
View Description Hide DescriptionMotivated by recent long scalelength single and multiplebeam experiments conducted on LLE’s OMEGA laser system [W. Seka et al., Phys. Rev. Lett. 89, 175002 (2002)], we have simulated backward stimulated Brillouin scattering(SBS) in inhomogeneous direct drive plasmas where a critical surface is present at all times. This was achieved by adapting pF3D, a parallel, threedimensional laser–plasma interaction code developed at the Lawrence Livermore National Laboratory [R. L. Berger et al., Phys. Plasmas 5, 4337 (1998)]. The shortcomings of the paraxial approximation, inherent in pF3D, were identified and circumvented via a practical approach that has led to an improved understanding of the experiments. The results show a qualitative agreement between the simulated and experimental SBSbackscattered power reflectivity, with the conclusion that the time dependence of the SBSreflectivity is governed by target hydrodynamics and a quantitative agreement in the blueshift in the frequency of reflected light. The simulations have also shown a sensitivity of the backscatteredreflectivity to the amplitude of the specularly reflected signal of an “opposing” beam.

Characterization of neutron yields from laser produced ion beams for fast neutron radiography
View Description Hide DescriptionInvestigations of reactions using Cu and CH primary and LiF secondary targets were performed using the VULCAN laser [C.N. Danson et al., J. Mod. Opt. 45, 1653 (1997)] with intensities up to The neutron yield was measured using CR39 plastic track detector and the yield was up to for CH primary targets and up to for Cu primary targets. The angular distribution of neutrons was measured at various angles and revealed a relatively anisotropicneutron distribution over that was greater than the error of measurement. It may be possible to exploit such reactions on high repetition, tabletop lasers for neutronradiography.

Nonlinear Phenomena, Turbulence, Transport

Observation of parametric decay spectrum in a cylindrical electron cyclotron resonance plasma system
View Description Hide DescriptionNonlinear wave coupling phenomena in an electron cyclotron resonance (ECR) produced plasma are observed. Experiments are carried out in a small linear cylindrical system with hydrogen plasma.Microwave power in the system is delivered by a magnetron at 2.45 GHz in mode and is launched radially to have extraordinary wave in plasma. The axial magnetic field configuration, required for ECR to occur in the system, is such that the first ECR surface resides at the center of the system. Parametric decay spectrum of the launched electromagnetic wave is observed at the upper hybrid resonance (UHR) layer with the appearance of sidebands on the higher and lower side of the launched frequency. Experiments show that the appearance of parametric decay spectrum has a threshold on input microwave power. The decay spectrum is observed only at the UHR surface indicating strong spatial dependence. The parametric decay is also verified in argon and helium plasmas.

Inertially Confined Plasmas, Dense Plasmas, Equations of State

Quantumshell corrections to Thomas–Fermi–Dirac equationofstate theory
View Description Hide DescriptionQuantumshell corrections are made directly to the finitetemperature Thomas–Fermi–Dirac (TFD) statistical model of the atom by a partition of the electronic density into bound and free parts. The bound part is calculated using analytic basis functions whose parameters are chosen to minimize the energy and pressure. Poisson’s equation is solved for the modified density. The shock Hugoniot is calculated for aluminum. Shell effects characteristic of quantum selfconsistent field (QSCF) models are fully captured by the present theory. The use of a quantum decription of the bound density removes the physically spurious singularity at the origin which is present in TFD theory.

Basic Plasma Phenomena, Waves, Instabilities

The complete set of Casimir constants of the motion in magnetohydrodynamics
View Description Hide DescriptionA set of functional equations that all Casimir constants of the motion must satisfy is derived. A simple technique is developed that can be used to obtain partial information on the functional dependence of the Casimirs on the state variables. Together with additional known information, this is sufficient to determine all the Casimirs for a magnetohydrodynamic plasma with nested magnetic fluxsurfaces, as well as for some other magnetic configurations.

Magnetically Confined Plasmas, Heating, Confinement

A driftkinetic approach to neoclassical transport theory for plasmas with large toroidal rotation
View Description Hide DescriptionA formulation of the neoclassical transporttheory for tokamak plasmas with large toroidal velocities that can be comparable to the ion thermal velocity is presented using the drift kinetic equation. In this formulation, the motion of the guiding centers is the same as in the small rotation theory, but the radial electric field is considered stronger: the ordering is adopted where is the poloidal magnteic field and the ion thermal velocity. A complete set of transport coefficients for both electrons and ions is calculated in the largeaspectratio limit in the banana regime for flux surfaces of arbitrary shape. The calculation utilizes a recently developed method of matched asymptotic expansions for particle and energy fluxes and a regular perturbation for the angular momentum flux. The consistent application of perturbation for largeaspectratio provides a satisfactory explanation why the angular momentum flux is much smaller than the ion energy flux.

Lasers, Particle Beams, Accelerators, Radiation Generation

Influence of magnetic field on the electrical breakdown characteristics in cylindrical diode
View Description Hide DescriptionThe influence of magnetic field on the electrical breakdownproperties is investigated by applying a magnetic field along the longitudinal direction in a cylindrical diode for two electrical polarities. Breakdown characteristics in a crossed magnetic field are analyzed with the equivalentreducedelectricfield concept and Townsend criterion. The discharge experiment at reduced pressure is carried out in the moderate magnetic field. Experimental investigation is concentrated on the magnetic dependent behavior of the electrical breakdown in the lower pressure side of Paschen’s minimum. It is found that the electrical breakdowncharacteristics with respect to the magnetic field depend on electrical polarity of the cylindrical diode, which is interpreted by taking the gyromotion of the individual electrons in the diode into accounts under the moderate magnetic field in the lower pressure side of Paschen’s minimum.

Magnetically Confined Plasmas, Heating, Confinement

Theory of synergy between electron cyclotron and lower hybrid waves
View Description Hide DescriptionA theoretical study of the improvement of the electron cyclotron current drive (ECCD) efficiency in regimes in which most of the current is driven by lower hybrid (LH) waves is presented. A perturbation technique is employed to solve the adjoint equation and derive the response function including both collisional and LH effects in the limit where the former dominate. An alternative treatment of the problem, involving a numerical solution of the Langevin equations, is proposed to gain insight into the current drive mechanism and to confirm the obtained results. The existence of a crosseffect between the two waves is demonstrated and the conditions for the synergy (i.e., significant enhancement of the ECCD efficiency in the presence of LH power) are identified.

Simulation of saturated tearing modes in tokamaks
View Description Hide DescriptionA quasilinear model, which includes the effect of the neoclassical bootstrap current, is developed for saturated tearing modes in order to compute magnetic island widths in axisymmetric toroidalplasmas with arbitrary aspect ratio and crosssectional shape. The model is tested in a simple standalone code and is implemented in the BALDUR [C. E. Singer et al., Comput. Phys. Commun. 49, 275 (1982)] predictive modeling code. It is found that the widths of tearing modeislands increase with decreasing aspect ratio and with increasing elongation. Also, the island widths increase when the gradient of the current density increases at the edge of the islands and when the current density inside the islands is suppressed, such as the suppression caused by the near absence of the bootstrap current within the islands. In simulations of tokamak discharges, it is found that tearing modeisland widths oscillate in time in response to periodic sawtooth crashes. The local enhancements in the transport produced by magnetic islands have a noticeable effect on global plasma confinement in simulations of low aspect ratio, high beta tokamaks, where saturated tearing modeislands can occur with widths that are greater than 15% of the plasma minor radius.

Størmer regions for axisymmetric magnetic multipole fields
View Description Hide DescriptionThe motion of a single charged particle in the space outside of a compact region of steady currents is investigated. The charged particle is assumed to produce negligible electromagnetic radiation, so that its energy is conserved. The source of the magnetic field is represented as a point multipole. After a general description, attention is focused on magnetic fields with axial symmetry. Lagrangian dynamical theory is utilized to identify constants of the motion as well as the equations of motion themselves. In particular, the energy and the canonical angular momentum conjugate to the azimuthal angle are conserved for axial symmetry. Following the precedent of some authors, and for brevity, the constant canonical angular momentum conjugate to the azimuthal angle will be called Størmer’s integral. In the present work, the magnetic vector potential equivalent to a given spherical harmonic scalar potential is also found. The qualitative method of Størmer is then used to examine charged particle motion in axisymmetric multipole fields of all orders, and while the treatment is fully relativistic, it also encompasses nonrelativistic motion. This qualitative method divides the configuration space of the charged particle into sets in which motion is either forbidden or allowed, i.e., it permits a topological examination of dynamical motion. The results obtained here allow for the determination of trappedparticle regions around those astrophysical objects and spacecraftplasma propulsion systems that have an associated axisymmetric magnetic field described by a multipole of given order.

Inertially Confined Plasmas, Dense Plasmas, Equations of State

Subignition fusion yields generated by fast heating of compressed deuterium–tritium and breakeven scaling
View Description Hide DescriptionA simple model is presented to calculate the fusion yield from the fast heating of compressed deuterium–tritium (DT). The model is applicable when the fusion yield is small enough to neglect selfheating. Since 80% of the fusion yield escapes as 14 MeV neutrons, selfheating is small as long as the fusion yield is less than the deposited energy, i.e., the fusion gain, We show the model is in good agreement with detailed numerical simulations when this condition is satisfied, as will be the case for fast ignitionexperiments in the near future. The model is used to calculate the fast heating fusion yields as a function of the important parameters such as the fuel density, deposited energy, and pulse length. The model is also used to obtain the minimum energy, and the fuel diameter, d, necessary to obtain which is given approximately by the scaling laws and where ρ is the DT density. These scaling laws should help the design of experiments.

Radiation: Emission, Absorption, Transport

Ultra hard x rays from krypton clusters heated by intense laser fields
View Description Hide DescriptionThe interaction of ultrashort laser pulses with krypton clusters at intensity up to has been investigated. Intense Kα and Kβ emission from krypton at 12.66 and 14.1 keV, respectively, has been observed using conventional solid state xray detectors. The measured xray spectra have broad bremsstrahlung continuum reaching to photon energies up to 45 keV, with evidence that approximately 10% of electrons that are heated to very high electron temperatures, which is consistent with a twotemperature electron distribution. This is ascribed to the presence of a hot electron population, similar to that found in laser–solid interactions. The highest laser energy to xray conversion efficiency observed is which is equivalent to 45 nJ xray pulse energy from the 12.66 keV krypton Kα transition.
