bluuebunny/crossref_metadata_2025 huggingface dataset:
Turbulent transport modeling in low Mach number flows
Authors: Shimomura Yutaka
Published: July 2002

Model representations for the Reynolds stress and the turbulent heat flux of flows at low Mach numbers have been theoretically derived by applying the two-scale direct-interaction approximation to their transport equations based on the assumption that the density variation is relatively small. The derived representations can be applied to general turbulent flows at low Mach numbers, whether the Boussinesq approximation holds in the flow or not. The model representation for the turbulent heat flux newly involves the differential effect of mean pressure gradients on hot, light fluids and on cold, heavy ones, in addition to the familiar effect of mean temperature diffusion. Certain a priori tests have shown that the model can reproduce the vertical turbulent heat flux in the natural convection along a heated vertical plate and the countergradient diffusion for the turbulent heat flux observed in a nonpremixed swirling flame, both of which can not be explained by the standard turbulence model of the gradient diffusion type.
Linewidth limits in free-electron lasers caused by sidebands
Authors: Riyopoulos Spilios
Published: July 2002

Sideband excitation near the carrier determines the minimum spectral width for steady-state free-electron laser oscillators fed by continuous electron beams. A sideband separated by δω from the carrier resonates with harmonics of the upshifted bounce frequency for trapped particles, δω=n2γ2zΩ. The analysis focuses on sidebands excited in the immediate vicinity of the carrier δω→0, in resonance with particles trapped near the separatrix, Ω→0. For electrons distributed uniformly around their orbits, the growth tends to zero as δω, Ω→0, despite the infinite number of contributing harmonics. However, the distributions produced by injected electron beams are nonuniform around the trapped orbits, yielding finite growth rates Γ. Stability depends on the nonlinear shift δk0(a0,ω0) of the carrier wave number from the empty cavity value, where the carrier amplitude a0 and frequency ω0 parametrize the free-electron laser (FEL) operation point. The curve δk0(a0,ω0)=0 divides the FEL parameter space into areas stable and unstable to sidebands. If δk0 is negative, near-the-carrier sidebands are stable, and the linewidth is limited only by quantum effects. If δk0 is positive an unstable frequency band can emerge around the carrier, of width Δω≂8γ2zv0 δk0, and maximum growth rate Γmax/k0≂(1/6)[2πN(δk0/k0)]2, where N is the number of wiggler periods. The minimum linewidth is Δω if the frequency separation between cavity modes is less than Δω. ‘‘Single mode’’ operation in the unstable region is still possible if the cavity mode separation exceeds the unstable bandwidth Δω. The above stability conclusions do not apply to sidebands ‘‘far’’ from the carrier Δω∼2γ2zΩ0. The latter poses less of a threat to FEL operation, since they are easier to filter out.
Observation of intermittent structures associated with the low-frequency turbulence in toroidal magnetized plasma
Authors: Singh A. K. , Saxena Y. C.
Published: July 2002

Two-dimensional structures in a plane perpendicular to the magnetic field, of the low-frequency turbulence in a toroidal plasma, have been studied. The probability distribution functions for the density and the potential fluctuations are found to deviate from the Gaussian distribution significantly indicating intermittency in the turbulence and formation and destruction of the coherent structures in the strongly turbulent state. The presence of such short-lived structures has been established, on a statistical basis, by means of conditional averaging of the fluctuation amplitudes.
Remote feedback stabilization of tokamak instabilities*
Authors: Sen A. K.
Published: July 2002

A novel remote suppressor consisting of an injected ion beam has been used for the stabilization of plasma instabilities. A collisionless curvature-driven trapped-particle instability, an E×B flute mode and an ion temperature gradient (ITG) instability have been successfully suppressed down to noise levels using this scheme. Furthermore, the first experimental demonstration of a multimode feedback stabilization with a single sensor–suppressor pair has been achieved. Two modes (an E×B flute and an ITG mode) were simultaneously stabilized with a simple state-feedback-type method where more ‘‘state’’ information was generated from a single-sensor Langmuir probe by appropriate signal processing. The above experiments may be considered as paradigms for controlling several important tokamak instabilities. First, feedback suppression of edge fluctuations in a tokamak with a suitable form of insulated segmented poloidal limiter sections used as Langmuir-probe-like suppressors is proposed. Other feedback control schemes are proposed for the suppression of electrostatic core fluctuations via appropriately phased ion density input from a modulated neutral beam. Most importantly, a scheme to control major disruptions in tokamaks via feedback suppression of kink (and possibly) tearing modes is discussed. This may be accomplished by using a modulated neutral beam suppressor in a feedback loop, which will supply a momentum input of appropriate phase and amplitude. Simple theoretical models predict modest levels of beam energy, current, and power.
Comment on ‘‘Validity of the field line resonance expansion’’ [Phys. Fluids B 4, 2713 (1992)]
Authors: Thompson M. J. , Wright A. N.
Published: July 2002

In a recent paper Hansen and Goertz (hereafter HG) [Phys. Fluids B 4, 2713 (1992)] considered the coupling between fast and Alfvén modes in a cold plasma containing a uniform magnetic field (B0x̂) extending between two perfectly reflecting plane boundaries at constant x. The equilibrium medium is invariant in only one direction (ŷ), and (importantly) the density may vary along the equilibrium field lines, ρ0(x,z). HG sought solutions of the coupled governing PDEs (partial differential equations) for linear perturbations of the form exp i(kyy−ωt). The solution has been studied previously [Planet. Space Sci. 22, 483 (1974); J. Geophys. Res. 79, 1024 (1974)] in the case when ρ0 does not vary along the background field lines, when each Fourier mode in x decouples from the others and may be considered separately—reducing the problem to an ODE (ordinary differential equation). In this case a logarithmic singularity exists at the resonant field line where ω2=k2xV2A(z), VA being the Alfvén speed (V2A=B20/4πρ0). HG claim the introduction of density variation along the equilibrium field causes the modes in x to become coupled resulting in the singular ODE solution becoming a nonsingular solution in the PDE case. If this conclusion is true it is of great importance for researchers in many areas such as solar corona and laboratory plasma heating, and magnetospheric pulsations. Indeed, it suggests that a large portion of the existing literature in these fields is wrong. Clearly it is important to decide whether the calculation of HG is correct or not. In this Comment the equations they set up are analyzed and are solved in a different fashion to HG. The solution found is different from that of HG and in agreement with the existing body of literature. Some sources of error in HG’s analysis are pointed out.
Analytic calculation of the nonzero fast wave reflection coefficient from the tunneling equation with absorption
Authors: Ng C. S. , Swanson D. G.
Published: July 2002

An analytic series expression of the nonzero fast wave reflection coefficient has been found from the tunneling equation which models the wave propagation in weakly inhomogeneous media near a back-to-back resonance-cutoff pair. Explicit calculations are done for both second ion cyclotron and second electron cyclotron harmonic cases. Numerical values of this expression are compared to those calculated by solving an integral equation iteratively. They agree well with each other whenever both the series and the iteration converge. This result is based on a new method of evaluating integrals derived from the integral equation without having to solve the equation itself. This new method may have more applications and deserves further research and development.
On the influence of atomic physics mechanisms on edge plasma turbulence in the TJ-I and Princeton Beta Experiment-Modified tokamaks
Authors: Pedrosa M. A. , García-Cortés I. , Brañas B. , Balbín R. , Hidalgo C. , Schmitz L. , Tynan G. , Post-Zwicker A.
Published: July 2002

The role of neutrals as a driving force of plasma turbulence was investigated in the TJ-I tokamak [Phys. Fluids B 5, 4051 (1993)]. No influence of the local neutral source strength on fluctuation levels was found, neither in the plasma bulk side nor in the scrape-off layer side of the velocity shear layer location. Helium puffing was used to study the influence of impurity radiation on turbulence in the Princeton Beta Experiment-Modified (PBX-M) [Plasma Physics and Controlled Nuclear Fusion Research 1988 (International Atomic Physics Agency, Nice, 1989), Vol. 1, p. 97]. Evidence of fluctuation levels modified increasing He-impurity radiation was obtained.
Two-dimensional modeling of x-ray output from switched foil implosions on Procyon
Authors: Bowers R. L. , Nakafuji G. , Greene A. E. , McLenithan K. D. , Peterson D. L. , Roderick N. F.
Published: July 2002

A series of two-dimensional radiation magnetohydrodynamic calculations are presented of a Z-pinch implosion using a plasma flow switch. Results from a recent experiment using the high explosive driven generator Procyon, which delivered 16.5 MA to a plasma flow switch and switched about 15 MA into a static load, are used to study the implosion of a 29 mg load foil [J. H. Goforth et al., ‘‘Review of the Procyon Explosive Pulsed Power System,’’ in Ninth IEEE Pulsed Power Conference, June 1993, Albuquerque, edited by K. R. Prestwich and W. L. Baker (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1993), p. 36]. The interaction of the switch with the load plasma and the effects of background plasma on the total radiation output is examined. Models which assume ideal switching are also included. Also included are the effects of perturbations in the load plasma which may be associated with initial vaporization of the load foil. If the background plasma density in the switch region and in the load region does not affect the dynamics, the pinch is predicted to produce a total radiation output of about 4 MJ. Including perturbations of the load plasma associated with switching and assuming a background plasma density after switching in excess of 10−7 g/cm3 results in a total output from the pinch of about 0.6 MJ.
Characterization of a partially-ionized, detached, divertor plasma
Authors: Knoll D. A. , Krasheninnikov S. I. , McHugh P. R. , Sigmar D. J.
Published: July 2002

A combined edge plasma/Navier–Stokes neutral transport model is used to characterize divertor plasma detachment, in the collisional limit for neutrals, on a simplified two-dimensional slab geometry with Alcator-C-MOD-like plasma conditions [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. The neutral model contains three momentum equations which are coupled to the plasma through ionization, recombination, and ion–neutral elastic collisions. The neutral transport coefficients are evaluated including both ion–neutral and neutral–neutral collisions. Detachment is brought about via impurity radiation using a fixed fraction impurity model. The transport model is shown to reproduce all salient features of experimentally observed detachment, such as large drops in ion saturation current and plasma heat flux at the divertor plate. The solutions are also shown to be sensitive to volume recombination. A region of relatively high toroidal neutral Mach number is observed upon detachment. Due to the high neutral densities, 75% of the Lyman α radiation is assumed trapped in the problem. A total edge radiative loss (neutrals and impurities) of approximately 75% of the power crossing the separatrix is needed to observe strong detachment on Alcator-C-MOD-like plasmas using the described model.
Effect of negative ions on filamentation instability in collisionless three-fluid theory of plasmas with cold ion species
Authors: Cho Guan Sik , Lee Jae Yong , Kong H. J.
Published: July 2002

The growth rate of filamentation is examined analytically, as a function of the number density and the particle mass of negative ions, in the collisionless three-fluid theory of a plasma with cold ions. The growth rate dependence on the number density shows a qualitative change according to whether the pump intensity is below or above a critical level. In all cases of the pump intensity, the higher particle mass of negative ions means the weaker growth of filamentation.
A vacuum sheath propagation along a cathode
Authors: Fruchtman A.
Published: July 2002

The velocity of propagation of a vacuum sheath along the cathode is calculated. A regime of parameters is identified in which, surprisingly, this velocity is lower for higher currents.
Electrical conductivity for nonideal partially ionized alkali plasmas
Authors: Starzynski J. , Redmer R. , Schlanges M.
Published: July 2002

The electrical conductivity is calculated for partially ionized Li, Na, K, Rb, and Cs plasmas starting from quantum kinetic equations which account for collision terms of free and bound particles including many body effects. The scattering cross sections entering the conductivity are calculated in T matrix approximation. The description of electron-atom scattering is based on the close coupling system of equations considered in the adiabatic exchange model. The plasma composition is determined from generalized mass action laws valid for nonideal plasmas.
Plasma-wall transition: The influence of the electron to ion current ratio on the magnetic presheath structure
Authors: Beilis I. I. , Keidar M. , Goldsmith S.
Published: July 2002

The effect of magnetic field on the plasma-wall transition layer is investigated using the two-fluid formulation. The quasi-neutral, near-sheath plasma (presheath) is examined, with the presence of neutral particles, and a magnetic field parallel to the confining wall. A general approach is used which takes into account the electron momentum equation, including the electric field force, the magnetic force, the pressure gradient, and the drag force. The influence of the electron to ion current ratio on the potential and velocity distribution in the near-sheath plasma is investigated. It is shown that the electron density distribution in the presheath may deviate from the Boltzmann distribution normally used in previous presheath models. Even when the plasma density dependence on the potential corresponds to the Boltzmann distribution, the presheath thickness deviates from that calculated with a model based on this distribution. The potential in the presheath with respect to the plasma–presheath interface can be negative or positive depending on the electron to the ion flux ratio η and Hall parameter βi. In the case of magnetized ions (βi>1) the potential distribution has a positive maximum and is always negative at the wall edge of the presheath. The value and position of the maximum depend on the parameter η. In the case of unmagnetized ions (βi≪1) the potential is positive for large η and is negative for η<100. With large βi the influence of the electrons is significant so that the presheath thickness decreases to the electron Larmor radius and has a strong dependence on the parameter η.
Nonsingular canonical coordinates for the drift Hamiltonian in a magnetic field with a separatrix
Authors: Boozer Allen H.
Published: July 2002

Magnetic coordinates are singular on a separatrix. A method is given for transforming to nonsingular coordinates that are closely related to magnetic coordinates and can be used in the canonical coordinates of the Hamiltonian for particle drifts. The case of a magnetic separatrix with a single X point is emphasized and approximate expressions are given for the functions that appear in the drift Hamiltonian.
Non-relativistic response function for higher harmonic electron cyclotron resonance heating
Authors: Tatematsu Y. , Kiwamoto Y. , Saito T. , Katanuma I. , Tamano T.
Published: July 2002

A response function [Y. Kiwamoto et al., Phys. Plasmas 1, 834 (1994)] for higher harmonic cyclotron resonance heating is constructed through the analysis of electron trajectory under harmonic heating in an inhomogeneous magnetic field. General expressions of orbital displacement and energy gain for higher harmonic resonance are obtained. Phase-averaged energy gain of electrons by n-th harmonic electron cyclotron resonance heating (ECRH) is larger by a factor of n, if the effect of the orbital displacement from the unperturbed orbit is correctly included, than that evaluated based on an unperturbed orbit assuming small k⊥ρ. The energy gain for finite k⊥ρ is also obtained. These results are confirmed with a numerical calculation for second harmonics.
The evolution of a layer of electron vortices produced by the interaction of an intense laser pulse and plasma
Authors: Venkatesan S. K. , Das Chandra , Roychoudhury R.
Published: July 2002

The instability of the large-scale motion of a thin layer of electron vortices produced by the interaction of intense laser pulse and a uniform plasma is studied here. The standard point vortex models and vortex sheet models of the flow field involve unphysical singularities in flow field, leading to ill-posed problems. To make the problem well posed a description in terms of vortex blobs is considered. If the vortex sheet has a smooth localized core then the linear growth rates rapidly decrease to zero at high wave numbers, thus providing a natural ultraviolet cutoff. Such a model can then be further used to study the development of small-scale inhomogeneity in the magnetic field produced by the ultraintense laser beam.
Particle-in-cell (PIC) simulations of beam instabilities in gyrotrons
Authors: Tran T. M. , Jost G. , Appert K. , Alberti S. , Pedrozzi M.
Published: July 2002

Extensive simulations are performed to investigate effects of electron cyclotron instabilities on the gyrotron beam quality, using two-dimensional axisymmetric particle-in-cell (PIC) codes. Both electrostatic and electromagnetic models, as well as realistic geometries of the gyrotron, are considered. It is found that a large beam density can lead to an electrostatic-instability-induced energy spread which substantially degrades the gyrotron efficiency.
Analysis of magneto-plasma dynamic transients in a combustion gas magnetohydrodynamic generator
Authors: Borghi C. A. , Cristofolini A. , Ribani P. L.
Published: July 2002

In the present work a model for the two-dimensional time dependent description of the magneto-fluid dynamics in the channel of an MHD (magnetohydrodynamic) generator is described. The numerical code obtained from it can be used for the analysis of the MHD channel operation, during the design procedure and for numerical experiments. The model is based on a time dependent two-dimensional description of the fluid dynamics iteratively coupled to a steady state two-dimensional description of the electrodynamics. In the plasma flow considered the magnetic field diffusion dominates over the magnetic field convection. A semi-implicit finite difference pressure scheme has been utilized for the solution of the fluid dynamic problem. The turbulence behavior is taken into account. The electrodynamics is described by a second order elliptic partial differential equation, obtained from the Maxwell equations and generalized Ohm’s law, and solved by means of an exponential fitting finite difference method. The model has been validated by the comparison of its results with experimental data. Two regime conditions and a transient caused by the interruption of the seeding injection, have been analyzed.
Velocity shear-induced effects on electrostatic ion perturbations
Authors: Rogava Andria D. , Chagelishvili George D. , Berezhiani Vazha I.
Published: July 2002

Linear evolution of electrostatic perturbations in an unmagnetized electron–ion plasma shear flow is studied. New physical effects, arising due to the non-normality of linear dynamics are disclosed. A new class of nonperiodic collective mode with vortical motion of ions, characterized by intense energy exchange with the mean flow, is found. It is also shown that the velocity shear induces extraction of the mean flow energy by ion-sound waves and that during the shear-induced evolution the ion-sound waves turn eventually into ion plasma oscillations.
Effect of finite magnetic moment on threshold amplitude of electrostatic waves in nonlinear dynamics
Authors: Mohanty J. N. , Naik Antaryami
Published: July 2002

An ensemble of electrostatic waves propagating obliquely to the external uniform magnetic field accelerates the plasma electrons, giving rise to a mechanical energy in terms of finite magnetic moment (μ). In nonlinear analysis, this magnetic moment modifies the resonances among the harmonics (l) and the overlapping parameter (K) leading to stochasticity. It is shown that the threshold amplitude (φk) of electrostatic waves at the onset of chaos increases slowly for l=0 harmonics and sharply for higher harmonics (l⩾1), owing to the effect of magnetic moment. A sharp decrease of threshold amplitude with wave vector (k) pertaining to the ensemble of waves for higher harmonics (l⩾1) is also delineated. The findings may have astrophysical and cosmological signatures in the high energy particle regimes.
Double-crossing mode conversion in nonuniform media
Authors: Brizard A. J. , Morehead J. J. , Kaufman A. N. , Tracy E. R.
Published: July 2002

A new type of mode conversion between two linear waves in a nonuniform medium is investigated. Single-crossing conversion occurs when a ray of one wave crosses transversely the dispersion manifold of another wave. Double-crossing mode conversion theory describes when the ray punctures the dispersion manifold twice due to ray curvature. To study this new process, a one dimensionally nonuniform background medium is considered, which gives rise to four scenarios. These scenarios are distinguished on the basis of whether the two waves have equal or opposite energy signs, and whether they are copropagating or counterpropagating. Using modular-eikonal theory (suitable for multiple crossings), each scenario is first studied analytically by constructing an S-matrix relation between the outgoing and incoming asymptotic wave amplitudes. The analytical results are then compared with numerical results and excellent agreement is found.
Population kinetics of dense hydrogen-like plasmas
Authors: Bornath Th. , Schlanges M. , Prenzel R.
Published: July 2002

The population kinetics of dense hydrogen-like plasmas is investigated within a quantum kinetic approach. Recombining plasmas are considered for the isothermal, the isoenergetic and the adiabatic case. In the latter cases, additionally to the rate equations, balance equations for the temperatures of the plasma species are used. The influence of many-body effects on the relaxation is discussed. Comparisons are given with a recent experiment on laser-produced carbon plasmas.
High-intensity scattering processes of relativistic electrons in vacuum
Authors: Hartemann F. V.
Published: July 2002

Recent advances in novel technologies such as chirped pulse amplification and high gradient rf photoinjectors make it possible to study experimentally the interaction of relativistic electrons with ultrahigh intensity photon fields. Femtosecond laser systems operating in the TW–PW range are now available, as well as synchronized relativistic electron bunches with subpicosecond durations and THz bandwidths. Ponderomotive scattering can accelerate these electrons with extremely high gradients in a three-dimensional vacuum laser focus. The nonlinear Doppler shift induced by relativistic radiation pressure in Compton backscattering is shown to yield complex nonlinear spectra which can be modified by using temporal laser pulse shaping techniques. Colliding lasers pulses, where ponderomotive acceleration and Compton backscattering are combined, could also yield extremely short wavelength photons. Finally, strong radiative corrections are expected when the Doppler-upshifted laser wavelength approaches the Compton scale. These are discussed within the context of high field classical electrodynamics, a new discipline borne out of the aforementioned innovations.
Feedback control of multimode magnetohydrodynamic instabilities via neutral beams
Authors: Sen A. K.
Published: July 2002

In most scenarios of confinement degradation due to MHD (magnetohydrodynamic) fluctuations in both tokamaks and reversed field pinches several MHD modes are involved. This is the motivation for the development of a multimode feedback scheme in the present paper, in contrast to the past work. The scheme is based on modal (state) feedback, where each mode is unscrambled out of the sensor signal, which is a superposition of all mode information and then individually acted upon by a unique gain and phase. Finally, all these individually processed mode signals are electronically summed and impressed on the accelerator grid of a neutral beam as a single control signal. It is shown that this process can lead to the stabilization of all unstable modes without destabilization of any stable modes, in contrast to previous feedback experiments.
Once more: The continuous spectrum of ideal magnetohydrodynamics
Authors: Goedbloed J. P.
Published: July 2002

A controversy on the existence of continuous spectra of ideal magnetohydrodynamics in addition to the well-known Alfvén and slow continua, dating back to a conjecture by Grad [Proc. Natl. Acad. Sci. USA 70, 3377 (1973)] and revived by Lashmore-Davies, Thyagaraja, and Cairns [Phys. Plasmas 4, 3243 (1997)], is once more resolved by demonstrating that the resolvent operator is bounded in the relevant domain: There are no additional continua. In addition, the solution of the initial value problem is constructed in terms of the three-dimensional Green's dyadic, which is free of apparent singularities and clearly exhibits the classical continua as δ functions on the diagonal. This construction provides the connection with the proper and improper normal modes and shows that the local dynamics on the magnetic surfaces is described by the classical continua.
Self-similarity of the plasma edge fluctuations
Authors: Carreras B. A. , van Milligen B. Ph. , Pedrosa M. A. , Balbı́n R. , Hidalgo C. , Newman D. E. , Sánchez E. , Frances M. , Garcı́a-Cortés I. , Bleuel J. , Endler M. , Riccardi C. , Davies S. , Matthews G. F. , Martines E. , Antoni V. , Latten A. , Klinger T.
Published: July 2002

The rescaled range analysis techniques are used to investigate long-range dependence in plasma edge fluctuations [Mandelbrot and Wallis, Water Resources Res. 4, 909 (1969)]. This technology has been applied to data from several confinement devices such as tokamaks, stellarators, and reversed-field pinch. The results reveal the self-similar character of the electrostatic fluctuations at the plasma edge with self-similarity parameters ranging from 0.62 to 0.72. These results show that the tail of the autocorrelation function decays as a power law for time lags longer than the decorrelation time and as long as times of the order of the confinement time. In cold plasma devices (Te<1 eV at the core), there is no evidence of algebraic tails in the autocorrelation function. Some other characteristic features of the autocorrelation function and power spectrum have been investigated. All of these features are consistent with plasma transport as characterized by self-organized criticality.
Comment on “Is dust acoustic wave a new plasma acoustic mode?” [Phys. Plasmas 4, 3427 (1997)]
Authors: Verheest Frank
Published: July 2002

Dwivedi’s claim [Phys. Plasmas 4, 3427 (1997)] that the dust-acoustic wave should not be viewed as a novel plasma wave is rebutted. The so-called acoustic-like mode introduced earlier by Dwivedi et al. [J. Plasma Phys. 41, 219 (1989)] rests physically on very dubious grounds, because it considers a plasma with two ion species in the presence of a fixed electron background. In addition, Dwivedi et al. have nowhere included heavy charged dust grains as potential applications of their (flawed) theoretical treatment. Hence Rao et al. (1990) were indisputably the first ones to point out the possibility of dust-acoustic modes, correctly so, as we argue, because a distinct name is fully justified by the enormous order-of-magnitude differences in the charge-to-mass ratios of the usual charged dust grains compared to “ordinary” ions and by the recent experimental verification.
Effect of finite ion temperature on large-amplitude solitary kinetic Alfvén waves
Authors: Roychoudhury Rajkumar , Chatterjee Prasanta
Published: July 2002

By using the Sagdeev pseudopotential method, solitary kinetic Alfvén waves (SKAW) are studied in a low-β plasma, taking into account the electron inertia and ion temperature. Solitary wave solutions, and the electric and magnetic fields, are obtained using the knowledge of the pseudopotential analysis in plasma dynamics. It is found that both hump and dip solitons exist for SKAWs, conforming to the results of Freja scientific satellite observations in space.
Transverse conductivity in a braided magnetic field
Authors: Kaganovich I. , Rozhansky V.
Published: July 2002

The cross-field current is calculated as a function of the applied electric field, density, and temperature gradients in a given perturbed magnetic field. The current is calculated both for the cases of regular and stochastic magnetic fields. The analysis is focused on the impact of local ambipolarity. It is demonstrated that the local ambipolarity requirement results in the significant suppression of the cross-field radial current with respect to the current of test particles.
Energy distribution normal to the liquid metal ion emission surface
Authors: Cho Guangsup , Kim Young-Guon , Choi Eun-Ha , Kang Seung-Oun , Uhm Han S.
Published: July 2002

The analytical energy distribution for the ions emitted at the protrusion apex surface of an anode is obtained from field evaporation theory. The distribution profiles are a kind of shifted exponential asymmetry function having a sharp increase and exponential decay. These asymmetric profiles are seen in many experiments. The full width at half maximum of ion energy distribution at the emission surface is about that of thermal energy (∼kT), while the measured values by the energy analyzer located beyond the diode are typically of the order of ∼eV. The influence of emission current and temperature at the emission surface on the energy-distribution profiles at the end of the diode will be investigated. It is found that the energy spread increases from the thermal energy of kT at the emission electrode to a value of the order of eV as the ions propagate from the anode to the cathode.
General linear theory of damped, beam-driven oscillations of a single mode
Authors: Stoltz Peter H. , Cary John R.
Published: July 2002

A single wave model of beam-cavity interactions is developed. The single wave model makes use of the spatial periodicity of a circular accelerator to Fourier expand the on-axis cavity vector potential. Combined with a harmonic time dependence, this Fourier expansion expresses the cavity vector potential as a sum of traveling waves. The single wave model keeps only the wave most resonant with the particles. The linearized dispersion relation is examined as a function of three parameters, damping rate, detuning, and beam temperature, and the analysis here is shown to span that parameter space (previous work has not given such a general treatment). The regions of validity within the parameter space for some familiar results, including a negative mass-like instability of accelerator physics, the cold-beam plasma result, the Keil-Schnell criterion, and Landau damping are discussed. A further result of this analysis is a clarification of the differences between Landau damping in plasma physics and Landau damping in accelerator physics. The two phenomena are shown to be from different regimes of parameter space with different physical mechanisms as their source. The mechanism of plasma Landau damping is the energetic exchange between the wave and resonant particles, while the mechanism behind accelerator Landau damping is phase mixing due to nonzero beam temperature. One manifestation of this difference is that plasma Landau damping can produce wave growth, whereas accelerator Landau damping cannot.
Nonlinear hydromagnetic waves in a nonideal magnetohydrodynamic plasma: Exact translationally symmetric solutions
Authors: Hamabata Hiromitsu
Published: July 2002

The time evolution of translationally symmetric hydromagnetic waves in a nonideal magnetohydrodynamic (MHD) plasma is investigated. Exact time-dependent wave solutions of the nonlinear resistive and dissipative MHD equations for the plasma within a cylindrical annulus are presented. It is shown that the magnetic field and the velocity have different damping time and that in the asymptotic limit t→∞ the plasma relaxes toward the equilibrium state involving uniform rotation and no magnetic field.
Electron temperature control with grid bias in inductively coupled argon plasma
Authors: Hong J. I. , Seo S. H. , Kim S. S. , Yoon N. S. , Chang C. S. , Chang H. Y.
Published: July 2002

The mechanism of controlling electron temperature with grid-biased voltage is studied experimentally and the relevant physics is discussed in an inductively coupled Ar discharge. To obtain the electron density and electron temperature, the electron energy distribution functions (EEDFs) are measured with a Langmuir probe. As the grid voltage decreases negatively, the effective electron temperature is controlled from 2.0 to 0.6 eV and the electron density changes from 3×1010 to 2×1010 cm−3 in the diffusion region, while the effective electron temperature and electron density are not changed in the source region. The dependence of such various parameters, as electron density, electron temperature, plasma potential in each region, and so on, on the applied voltage, is presented. The functional relations between the measured physical quantities are well explained based on a global particle and energy balance relations.
Magnetic reconnection with Sweet-Parker characteristics in two-dimensional laboratory plasmas
Authors: Ji Hantao , Yamada Masaaki , Hsu Scott , Kulsrud Russell , Carter Troy , Zaharia Sorin
Published: July 2002

Magnetic reconnection has been studied experimentally in a well-controlled, two-dimensional laboratory magnetohydrodynamic plasma. The observations are found to be both qualitatively and quantitatively consistent with a generalized Sweet-Parker model which incorporates compressibility, downstream pressure, and the effective resistivity. The latter is significantly enhanced over its classical values in the collisionless limit. This generalized Sweet-Parker model also applies to the case in which a unidirectional, sizable third magnetic component is present.
Laser-induced electron trapping in plasma-based accelerators
Authors: Esarey E. , Schroeder C. B. , Leemans W. P. , Hafizi B.
Published: July 2002

Trapping of plasma electrons in the self-modulated laser wakefield accelerator (LWFA) via the coupling of Raman backscatter to the wake is examined analytically and with three-dimensional (3-D) test particle simulations. The trapping threshold for linear polarization is much less than for circular and occurs for wake amplitudes of δn/n∼25%, which is well below wave breaking. Self-channeling provides continuous focusing of the accelerated electrons which, along with relativistic pump laser effects, can enhance the energy gain by a factor ⩾2. The colliding pulse method for injecting electrons in the standard LWFA is examined. Simulations of test electrons in 3-D fields indicate the production of relativistic (⩾25 MeV) high-quality electron bunches with ultrashort durations (a few femtoseconds), small energy spreads (a few percent), and low normalized emittances (1 mm mrad).
A class of stationary nonlinear dusty plasma equilibria
Authors: Faria R. T. , Farid Tahir , Shukla P. K. , Sakanaka P. H.
Published: July 2002

It is shown that nonparallel density and temperature gradients can produce magnetic fields in dusty plasmas. Spontaneously created magnetic fields can be maintained if there exists plasma vorticity. In order to understand this phenomena, a self-consistent dusty plasma equilibrium model is constructed by employing a kinetic description and invoking the Hamiltonian approach. Stationary nonlinear dusty plasma equilibria contain specific profiles for the plasma number density, the plasma current, and the magnetic field. The relevance of this investigation to low-temperature laboratory dusty and space plasmas is discussed.
Optical properties of cluster plasma
Authors: Tajima T. , Kishimoto Y. , Downer M. C.
Published: July 2002

It is shown that unlike a gas plasma or an electron plasma in a metal, an ionized clustered material (“cluster plasma”) permits propagation below the plasma cut-off of electromagnetic (EM) waves whose phase velocity is close to but below the speed of light. Its unique properties allow a variety of applications, including direct acceleration of particles with its EM fields and the phase matching of waves of high harmonic generation (HHG). The particle-in-cell simulation demonstrates the new optical mode, the cluster mode.
Radiation hydrodynamic effects in two beryllium plates with an idealized aluminum joint
Authors: Bel’kov S. A. , Mkhitarian L. S. , Vinokurov O. A. , Wilson D. C. , Goldman S. R.
Published: July 2002

A beryllium capsule formed from two hemispherical shells with a thin bond is one possible ignition target for the National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] Nonuniformities in density, opacity, and interface position at the joint between these hemishells will initiate two-dimensional (2-D) perturbations of the shock wave and material behind the shock as the shock passes through the shell perpendicular to the joint width. Rarefaction of material flow behind the shock front can cause the interface between the shell and joint material to oscillate in position. The amplitude of these oscillations may be comparable to the joint width. The evolution of these perturbations is studied by numerically simulating shock passage through flat beryllium plates containing aluminum joints. Using the MIMOSA-ND code [D. Sofronov et al., Vopr. At. Nauki Tekh., Ser: Mat. modelirovanie fizicheskih processov 2, 3 (1990)] two different cases are calculated—a wide (10 μm) and a narrow (1 μm) joint of aluminum between two 150 μm long semiinfinite beryllium plates. Both cases showed good agreement with an analytic representation of the oscillation behavior. For the narrow joint, a special technique allows the calculation of mixing between the joint and surrounding material caused by the Kelvin–Helmholtz instability.
Effect of field ionization on interaction of an intense subpicosecond laser pulse with foils
Authors: Zhidkov A. , Sasaki A.
Published: July 2002

The effect of transient ionization in an intense, short laser pulse irradiated foil is investigated via a simulation based on the collisional particle-in-cell method coupled with the atomic kinetics. A strong electric field induced by superthermal electrons causes significant plasma ionization at the rear surface of the target. The plasma induced field ionization is found to dominate the production of the multiple-charged ions at the rear surface of the foil, which are accelerated forward up to multi mega electron volts by irradiation of a subpicosecond laser pulse with intensities from 1019 to 1020 W/cm2.
Coaxial mode excitation and dissipation in ion Bernstein wave experiments
Authors: Myra J. R. , D’Ippolito D. A. , Russell D. A. , Rogers J. H. , Intrator T.
Published: July 2002

In recent ion Bernstein wave (IBW) heating experiments on the Tokamak Fusion Test Reactor (TFTR) [J. R. Wilson. R. E. Bell, S. Bernabei, K. Hill et al., Phys. Plasmas 5, 1721 (1998)] a velocity shear layer in the plasma core was obtained. The magnitude of velocity shear was believed to be too small to create an internal transport barrier, because of parasitic edge processes which reduced the power coupled to the core. In this paper we investigate these rf (radio frequency) edge processes by employing a model which includes both coaxial modes and their dissipation in rf plasma sheaths. The coaxial mode (here, an electron plasma wave trapped in the halo plasma between the lower hybrid layer and the vessel wall) can propagate at low poloidal wave numbers. This feature is shown to relate to the observed poloidal phasing dependence of the antenna loading. Results of analytical models and a three-dimensional antenna code are presented. The experimentally observed loading is also nonlinear, being larger at very low powers. This feature is explored using an rf sheath dissipation model. Loading into the coaxial mode is expected to maximize when the density gradient at the lower hybrid layer is steep, preventing efficient mode transformation to the IBW. The role of ponderomotive force in modifying the density profile is also discussed.
Simple theory of the line emission profile for the charge-exchange recombination spectroscopy method
Authors: Ryutov D. D.
Published: July 2002

A systematic derivation of the distribution function of excited ions formed by charge-exchange recombination is presented. The following effects have been taken into account: (1) Finite lifetime of the excited state; (2) energy dependence of the charge-exchange reactivity; (3) nonuniformity of the neutral beam. It is pointed out that the contribution of the terms related to the ion temperature gradient has a different structure compared to the other terms, thereby allowing a direct measurement of the ion temperature gradient. Possible ways for improving the accuracy of electric field measurements are discussed.
Nonlocal radiative forces in fully-ionized plasmas
Authors: Tahraoui A. , Bendib A. , Bendib K.
Published: July 2002

The solution of the Fokker–Planck equation which contains the contribution of high-frequency electric fields is presented. For this, the projection operators of the Krook collision operator and the semicollisional propagator computed with the continued fractions were used. Both elastic and inelastic momentum transfer from the high-frequency electric field to electrons have been considered. The contributions of such high-frequency electric fields to the stress tensor and to the mean change of momentum due to electron–ion collisions are derived in the whole collisionality range. As a result, in the collisionless range, these contributions led to a new force which is 1.6 (instead of 4/3) more important than the Miller force. In the collisional range, the radiative force due to the inverse bremsstrahlung absorption is recovered and a new contribution which arises from the mean change of momentum due to collisions is found.
Hot electrons generated by ultraintense laser plasma interaction
Authors: Zeng Guihua
Published: July 2002

Hot electron generation produced by a circularly polarized laser pulse propagating in plasma is investigated. Driven by the combination field of the laser field and the induced magnetic field, plasma electrons with initially longitudinal velocity component u0, make circular motion with cyclotron frequency Ω. At the proper condition, those plasma electrons driven by the combination field can absorb energy from the laser pulse.
Resistive axisymmetric equilibria with arbitrary flow
Authors: Bora M. P.
Published: July 2002

An analysis of axisymmetric equilibria with arbitrary incompressible flow and finite resistivity is presented. It is shown that with large aspect ratio approximation or vanishing poloidal current, a uniform conductivity profile is consistent with equilibrium flows. Also a comment is made on coexistence of both toroidal and poloidal flows in an axisymmetric field-reversed configuration.
Turbulent hydrodynamics experiments using a new plasma piston
Authors: Edwards J. , Glendinning S. G. , Suter L. J. , Remington B. A. , Landen O. , Turner R. E. , Shepard T. J. , Lasinski B. , Budil K. , Robey H. , Kane J. , Louis H. , Wallace R. , Graham P. , Dunne M. , Thomas B. R.
Published: July 2002

A new method for performing compressible hydrodynamic instability experiments using high-power lasers is presented. A plasma piston is created by supersonically heating a low-density carbon based foam with x-rays from a gold hohlraum heated to ∼200 eV by a ∼1 ns Nova laser pulse [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)]. The piston causes an almost shockless acceleration of a thin, higher-density payload consisting of a layer of gold, initially 1/2 μm thick, supported on 10 μm of solid plastic, at ∼45 μm/ns2. The payload is also heated by hohlraum x-rays to in excess of 150 eV so that the Au layer expands to ∼20 μm prior to the onset of instability growth. The Atwood number between foam and Au is ∼0.7. Rayleigh–Taylor instability, seeded by the random fibrous structure of the foam, causes a turbulent mixing region with a Reynolds number >105 to develop between piston and Au. The macroscopic width of the mixing region was inferred from the change in Au layer width, which was recorded via time resolved x-radiography. The mix width thus inferred is demonstrated to depend on the magnitude of the initial foam seed. For a small initial seed, the bubble front in the turbulent mixing region is estimated indirectly to grow as ∼0.036±0.19 [∫√(Ag)dt]2 which would imply for a constant acceleration 0.036±0.019 Agt2. More direct measurement techniques must be developed in larger scale experiments to remove potential complicating factors and reduce the error bar to a level that would permit the measurements to discriminate between various theories and models of turbulent mixing.
Trajectories of nonthermal electrons in the hot spot of a vacuum spark
Authors: Semyonov O. G.
Published: July 2002

The trajectories of high-energy electrons in the inner part of a micropinch (hot spot) are calculated. It is shown that the collisionless electrons (born, for example, in the tail of Maxwellian distribution) drift to the pinch axis in the current-induced magnetic field and the inductive electric field to form axisymmetric orbits with radial oscillations and a shift along the z-axis to the anode (modified run-away). On average, the time when the electron moves chiefly across the z-axis exceeds the period of its movement along the discharge axis, hence the probability of exciting the ions is higher when the electrons have a relatively larger perpendicular velocity component. This can explain the experimental observations of polarization of x-ray lines emitted from hot spots in a vacuum spark. Modified run-away can also play a crucial role in the energy balance of the micropinch plasma.
High-speed ellipsometry of arsenic-implanted Si during cw laser annealing
Authors: Moritani A. , Hamaguchi C.
Published: July 2002

Ellipsometric study of solid phase epitaxial growth during cw Ar-ion laser annealing in arsenic-implanted Si is reported for the first time. Use of a high-speed microscopic ellipsometer has made it possible to measure growth rates and growth temperature under laser irradiation.
Doping interface dipoles: Tunable heterojunction barrier heights and band-edge discontinuities by molecular beam epitaxy
Authors: Capasso Federico , Cho Alfred Y. , Mohammed Khalid , Foy Philip W.
Published: July 2002

We have succeeded for the first time in artificially tuning the conduction and valence-band barrier heights at an abrupt intrinsic semiconductor-semiconductor heterojunction via a doping interface dipole (DID). This is achieved by means of ultrathin ionized donor and acceptor sheets in situ grown within ≲100 Å from the heterointerface by molecular beam epitaxy. In the limit of a few atomic layers separation between the charge sheets this amounts to modify the effective band-edge discontinuities. A near one order of magnitude enhancement in the photocollection efficiency of an abrupt AlGaAs/GaAs heterojunction has been observed as result of the conduction-band barrier lowering induced by the DID.