bluuebunny/crossref_metadata_2025 huggingface dataset:
A theory to describe heat transfer during laminar incompressible flow of a fluid in periodic porous media
Authors: Hsiao Kuang-Ting , Advani Suresh G.
Published: July 2002

In this paper, a theory is formulated that describes the heat transfer during the laminar flow of an incompressible fluid within a rigid periodic porous medium. To describe the macroscopic temperature distribution in porous media, the volume average concept is employed to derive the volume averaged energy equation. Previous researchers have used either the one-equation model with the local thermal equilibrium assumption between the various phases or the complex two-equation model to deal with the average temperatures of the solid phase and the fluid phase. Both types of models are derived and applied in a stationary observation frame. In this paper, we propose a generalized one-equation model by relaxing the local thermal equilibrium assumption between the phases and invoking the frame invariant principle. The three new terms arise due to the microscopic transient, convective, and conductive effects. By taking advantage of the objectivity of the generalized model, it is shown that the microscopic temperature distribution in a periodic unit cell results in a periodic heat flux boundary condition. The direct temperature solution is obtained in the periodic unit cell by solving the generalized volume averaged energy equation. A modified effective thermal conductivity tensor is calculated which encompasses the micro-convective effects encountered in porous media. An analytic study of a non-isothermal flow through two parallel thick plates is performed to explore the role of each new term in the generalized model.
On the stability of a stationary solution of the Tchen’s equation
Authors: Druzhinin O. A.
Published: July 2002

This note examines the stability of a stationary velocity solution of the Tchen’s equation, describing the motion of a spherical particle with a Reynolds number much less than unity in a uniform unsteady fluid flow. The results show that the stationary solution is unstable if the ratio of the particle and fluid densities ρp/ρf<7/4. Therefore, applying the stationary solution is not justified in this case.
Frequency shifting in free-electron lasers
Authors: Shvets G. , Wurtele J. S.
Published: July 2002

Frequency shifting in free-electron laser (FEL) oscillators and amplifiers is investigated theoretically and numerically. The analysis includes frequency shifts from the resonant FEL interaction and the nonresonant beam dielectric. Expressions for the frequency shift in a microwave amplifier with time-dependent beam energy and current are derived and found to be in good agreement with experiments. The theory shows that temporal changes in the detuning are the dominant factor in determining the frequency shift. Electron energy fluctuations produce frequency shifts in the Compton regime, while both current and energy variations are significant in the Raman regime. The effect is particularly important for high-power microwave drivers proposed for high gradient accelerators, where the phase of the radio-frequency radiation is subject to significant constraints. FEL oscillator response to variations in beam energy is examined. It is shown that in a low-gain oscillator which experiences a sudden jump in beam energy the FEL creates spikes at the head and tail of the beam which are at the shifted frequency. The shifting is generated by time dependence in dielectric function which arises from slippage and finite lengths of the electron or optical pulse. The propagation diffusion equation is shown to describe the propagation of the spikes into the main body of the pulse.
Investigation of the second harmonic electron cyclotron current drive efficiency on the T-10 tokamak*
Authors: Razumova K. A. , Alikaev V. V. , Dremin M. M. , Esipchuk Yu. V. , Kislov A. Ya. , Notkin G. E. , Pavlov Yu. D. , Forest C. B. , Lohr John , Luce T. C. , Harvey R. W.
Published: July 2002

Experiments on second harmonic electron cyclotron current drive were done on the T-10 tokamak using four gyrotrons. Total powers up to 1.2 MW at a frequency of 140 GHz were injected. Current generation by electron cyclotron (EC) waves was demonstrated in the experiments. The efficiency η of current generation and its dependence on plasma parameters were measured and it was shown that the efficiency is a nonlinear function of input power, more closely predicted by Fokker–Planck calculations than by linear theory. The interaction of EC waves with the tail of the electron distribution was shown to be important. It was also found that current density profile redistribution played an important role in the plasma behavior.
Effect of negative ions on electrical breakdown in a nonuniform air gap between a wire and a plane
Authors: Ramakrishna K. , Cohen I. M. , Ayyaswamy P. S.
Published: July 2002

Electrical breakdown of an axisymmetric, atmospheric pressure air gap between a wire and a plane has been investigated for a gap length of 0.5 mm. O− and O−2 have been identified as the negative ions affecting the discharge development in air, besides electrons and positive ions, and have been included in the electrical breakdown model. Five coupled two-dimensional transient partial differential equations describing the discharge evolution in the air gap have been solved using a finite difference algorithm developed earlier. Temporal development of the charged particle number densities, electrostatic potential, electric field, and current at both the electrodes is presented when the wire is negatively biased at 2500 V. The impact of negative ions on gap breakdown has been assessed by comparing the results of analyses with and without negative ions. It is concluded that the negative ions have negligible effect during the early stages of the discharge development. However, as the discharge evolves, the negative ions cause a net loss of electrons from the discharge. The effect is most pronounced away from the discharge axis, where peaks in the electron density occur as breakdown proceeds. Radial spread of discharge and current growth rate are relatively unaffected by the presence of negative ions, but the magnitude of total current at the electrodes has been found to decrease by a decade when the negative ions are present.
On the interpretation of diamagnetic loop measurements for a current-carrying plasma column in a conducting chamber
Authors: Aleynikov A. N. , Breizman B. N. , Cherkassky V. S. , Knyazev B. A.
Published: July 2002

An expression is derived for the signal of a magnetic loop encircling a plasma column inside a conducting chamber with nonuniform current distribution over the plasma cross section. The plasma with radius much smaller than the length of the column is assumed to be in a quasistationary force-balanced state. The ratio of the paramagnetic component to the diamagnetic component of the signal is shown to be independent of the loop radius. Both components increase as the loop radius decreases from the chamber radius to the plasma radius. From the derived expressions, the paramagnetic component of the signal is calculated numerically for several current distributions including those of interest for the experiments. At a given total current, the paramagnetic component of the signal may vary considerably, which generally has to be taken into account in interpreting experimental data. The results of the calculations are used to process the data obtained in the experiments on the SPIN plasma device [Knyazev et al., ‘‘Characteristics of plasma produced by a linear discharge in the metal chamber of the SPIN device,’’ J. Tech. Phys. 63, 49 (1993) (in Russian)].
Transient behavior of cavity excitation driven by a modulated electron beam
Authors: Uhm Han S.
Published: July 2002

A theoretical model of cavity excitation driven by a modulated electron beam is developed in connection with application to relativistic klystron amplifiers. Based on an equivalent circuit representation of a cavity impedance, the equations which govern the phase and amplitude of the excitation voltage appearing on the cavity opening are obtained in terms of time t and current modulation strength. Several points are noteworthy from the theoretical analysis. First, amplitude and phase shift of the induced voltage indicate a damping oscillation, whose frequency is proportional to the mismatch ‖Δω‖ between the modulation frequency ω and the cavity resonance frequency ω0. Second, rise time of the cavity excitation amplitude decreases as the value of the frequency mismatch increases. Third, for a large value of the frequency mismatch ‖Δω‖, the power transfer from the modulated beam to the cavity occurs at the beginning of the beam pulse. Finally, it is observed that the absolute amount of energy delivered from the beam to the cavity decreases drastically as the frequency mismatch increases.
Scaling of Ohmic energy confinement with major radius in the Tokamak Fusion Test Reactor
Authors: Grisham L. R. , Scott S. D. , Goldston R. J. , Bell M. G. , Bretz N. L. , Grek B. , Johnson D. , Mansfield D. , Mueller D. , Park H. K. , Ramsey A. , Taylor G.
Published: July 2002

Ohmic plasma size scans have been carried out in the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)] to measure the influence of the major radius upon energy confinement. The major radius, minor radius, and aspect ratio were varied over wide ranges (R=2.08–3.2 m, a=0.4–0.9 m, and R/a=2.9–8.0) at constant qc. The energy confinement determined from kinetic diagnostics varies strongly with major radius. The data set is less well suited to determine minor radius scaling, but it appears to be distinctly weaker than the major radius scaling. The anomaly in ion thermal conductivity over neoclassical predictions appears to decline with increasing aspect ratio, which is a better ordering parameter for the magnitude of the anomaly than either the minor radius or the major radius.
Low noise asymptotic approach to the laser cooling of trapped ions
Authors: Soběhart J. R.
Published: July 2002

The probability distributions of dilute ion systems and two-ion systems trapped in time-dependent potential wells and driven by a radiation field are studied. Ions are excited by electromagnetic radiation and relax back due to either spontaneous or stimulated emissions. The photon statistics are considered Poissonian and the momentum transfer between the electromagnetic field, and the ions are assumed discrete. In the present classical treatment, the photon–ion interaction is simulated, with an effective noise acting on the ions, which produces a stochastic transfer of momentum. The present results are closely related to the quantum treatment in the heavy particle limit as well as to those derived from previous semiclassical models.
Multipole stability revisited
Authors: Pellat R. , Hurricane O. A. , Coroniti F. V.
Published: July 2002

Multipole stability with respect to quasielectrostatic low frequency modes (frequencies smaller than the average ion bounce frequency) is revisited. Aside from localized modes found previously by M. N. Rosenbluth [Phys. Fluids 11, 869 (1968)], it is shown that convective type modes that are equipotentials along field lines are likely to be more unstable, when the average curvature drift is positive (v″<0) and smaller than the local curvature drift.
Fluid description of kinetic modes
Authors: Lau Y. Y.
Published: July 2002

The classical kinetic modes, such as the Bernstein mode, the loss cone modes, and the Harris dispersion relation are reconstructed from a fluid-like analysis. The analysis begins with a delta function in the equilibrium distribution. By simply calculating the displacement of a single electron exposed to a small signal electric field, the charge perturbation density, and the dispersion relation, immediately follow. The effect of a velocity distribution enters only through a trivial integration of the dispersion relation thus obtained for the monoenergetic plasma. The entire analysis is done in the configuration space. Thus, without explicitly performing the customary integration over the unperturbed orbits in phase space, finite Larmor radius effects to all order have been retained. Possible extensions to nonuniform plasma and to nonuniform magnetic fields are indicated.
Enhanced phase stability for a Raman free-electron laser amplifier in the exponential growth regime
Authors: Carlsten Bruce E.
Published: July 2002

The wave propagation number for the growing wave of a free-electron laser in the exponential growth regime is a complicated function of electron beam current and voltage. As a result if the electron beam voltage drifts the imaginary part of the propagation number in general will change too, leading to a change in the radio-frequency phase of the output power. For typical gains, the two largest contributions to the shift in the imaginary component of the propagation constant are due to the change in the beam’s reduced plasma wave number and the change in the detuning from resonance. This paper demonstrates that these changes can be made to cancel each other with the correct selection of plasma reduction factor and initial detuning from resonance if the interaction strength is weakly dependent on beam velocity, leading to a device that is first-order phase and gain stable under voltage fluctuations. The analysis is performed for the axial free-electron laser interaction and the results also apply for transverse-coupling free-electron lasers.
Gain in the non-steady-state free-electron laser
Authors: Wu Ding , Min Yu
Published: July 2002

The non-steady-state self-consistent equation in the linear regime of the free-electron laser (FEL) and the low gain formulas in the non-steady-state FEL are derived in this paper. It is found that due to slippage the nonuniformity effect in the longitudinal distribution of the electron beam density is dominant in the influence of the electron pulse length on the gain of the FEL.
Response to ‘‘Comment on ‘Nonlocal theory of a whistler pumped free electron laser’ ’’ [Phys. Plasmas 1, 1025 (1994)]
Authors: Tripathi V. K. , Pant K. K.
Published: July 2002

Contrary to Bellan’s (Ref. 1) view that Alfven resonance does not exist in magnetized plasma, it is shown that the Alfven resonance remain a valid concept. (AIP)
Potential formation due to plasma injection along converging magnetic field lines
Authors: Ishiguro Seiji , Kishi Yoshizo , Sato Noriyoshi
Published: July 2002

Using a two-and-half-dimensional electrostatic particle simulation code, potential formation in a high-speed plasma flow along converging magnetic mirror field lines has been investigated. When a plasma flows into the converging magnetic field region, a large potential jump with φ≳Ts/e (Ts: electron temperature, e: electronic charge) is formed along the magnetic field lines, increasing with an increase in the mirror ratio or the plasma flow speed. The results are compared with those in the previous theoretical analyses and particle simulations for a one-dimensional configuration. The simulation is found to yield a good agreement with the results observed in the Q-machine experiment [N. Sato et al., Phys. Rev. Lett. 61, 1615 (1988)].
Theory of optical phase conjugation via four-wave mixing in laser plasmas
Authors: Lahiri Joydev , Sinha B. K.
Published: July 2002

Theoretical studies of optical phase conjugation via four-wave mixing in a two-temperature laser produced carbon plasma are reported. Starting from Maxwell equations and using the theory of parametric decay instability, analytical expressions of the phase conjugate reflectivity for a steady-state probe have been obtained and numerically evaluated for the case of the laser plasma formed by irradiating a carbon slab target with a Nd:Glass laser operating at λ0=1.06 μ. The variation of reflectivity as a function of frequency and angular mismatch between the pump and probe waves has been considered. It is observed that the reflectivity peaks occur under the situation of resonance when the frequency mismatch equals the ion-acoustic frequency of the plasma. The detailed numerical results are graphically reported and discussed.
Response to ‘‘Comment on ‘Free electron laser with longitudinal wiggler and finite magnetic field in a partially filled waveguide’ ’’ [Phys. Plasmas 3, 1130 (1996)]
Authors: Willett J. E.
Published: July 2002

Shoucri's [Phys. Plasmas 3, 3864 (1996)] comments concerning use of a biquadratic equation for the radial wave numbers [Eq. (3) in his comment], are regarded as inappropriate. Results are presented based on correct biquadratic radial-wave number equation derived by using the approach of Uddholm et al. [J. Phys. D: Appl. Phys. 24, 1278 (1991)] (AIP)
Pressure driven tokamaks
Authors: Jensen T. H. , Miller R. L. , Lin-Liu Y. R.
Published: July 2002

‘‘Pressure driven tokamaks’’ are special tokamaks for which the rate of injection of energy and mass (e.g., by neutral beams) is so large that no drive for the toroidal current is needed. Examples of pressure driven tokamak equilibria are found numerically; for these examples, both the poloidal and the toroidal magnetic fields vanish in a region around the plasma center. Thus, the ratio between the plasma pressure and the magnetic field pressure is large, namely of order unity. Therefore, pressure driven tokamaks appear attractive for fusion reactors; it is, however, an open question whether there exist magnetohydrodynamically stable pressure driven equilibria.
Excitation of large-amplitude Alfvén waves in a nonrelativistic electron–positron plasma with a temperature anisotropy T⊥>T∥
Authors: Zhao J. , Ohi T. , Sakai J. I. , Neubert T.
Published: July 2002

Excitation of large-amplitude Alfvén waves in an anisotropic electron–positron plasma is investigated with a two-and-one-half-dimensional (212-D), electromagnetic particle code. If the plasma distribution is a bi-Maxwellian with T⊥/T∥>1, where the subscript symbols denote directions perpendicular and parallel to the ambient magnetic field, Alfvén waves with arbitrary polarization are generated. Simulations show that the amplitude of the waves increases both with increasing temperature anisotropy and parallel plasma β∥. The stabilization of individual wave modes occurs sequentially, from higher values of wave number down to lower ones, in the evolution of the temperature anisotropy instability. Wave–particle scattering by the enhanced waves maintains the initially bi-Maxwellian character throughout the simulation, even as it reduces T⊥ and increases T∥. After saturation of wave magnetic energy a large residual temperature anisotropy still remains and persists through the end of the simulation.
Transverse distribution of density in charged-particle beams at later times
Authors: Karimov A. R.
Published: July 2002

The structural dynamics of a cold, non-neutralized charged-particle beam within limited spaces is examined with reference to Lagrangian coordinates. The formation of density distribution in a transverse direction that is perpendicular to the beam’s propagation directions is considered. Conditions leading to the formation of various inhomogeneous structures in beams are found.
Confinement of test particles in a Malmberg–Penning trap with a biased axial wire
Authors: Eggleston D. L.
Published: July 2002

A non-neutral plasma trap has been constructed in which the radial electric field of a non-neutral plasma column is simulated by a biased wire stretched along the axis of the device. The confinement time of test electrons in this device is found to be comparable in magnitude and scaling with that found in pure electron plasma experiments, in spite of the fact that the test electron density is 102 times smaller than in a typical pure electron plasma. The confinement time is only weakly dependent on the central wire bias. These results may provide useful input to theoretical efforts to explain transport in these traps.
X-ray lasing in colliding plasmas
Authors: Clark R. W. , Davis J. , Velikovich A. L. , Whitney K. G.
Published: July 2002

Conditions favorable for the achievement of population inversion and large gains in short-pulse laser-heated selenium have been reported on previously [K. G. Whitney et al., Phys. Rev. E50, 468 (1994)]. However, the required density profiles to minimize refraction and amplification losses can be difficult to achieve in conventional laser heated blowoff plasmas. The feasibility of accelerating plasma with a laser, and letting it collide with a solid density wall plasma has been explored. The density of the resulting shocked plasma can be controlled and refraction can be reduced in this design. A radiation hydrodynamics model is used to simulate the collision of the laser produced selenium plasma with the wall plasma. The heating of the stagnated plasma with a short-pulse laser is then simulated, providing the hydrodynamic response of the selenium plasma and detailed configuration nonequilibrium atomic populations. From the results of these calculations, it appears feasible to create an x-ray lasing selenium plasma with gains in the J=0–1 line at 182 Å in excess of 100 cm−1.
Evidence of magnetic fields imprinted in ferromagnetic targets by the spontaneous magnetic field of a laser-produced plasma demonstrated by after-pulse pulsations of Kα emission
Authors: Rode A. V. , Gamaly E. G. , Luther-Davies B.
Published: July 2002

The observation of an unusually long sequence of Kα pulses from planar Fe foil targets irradiated by a 180 ps laser pulse at intensities of (0.1–3)×1016 W/cm2 is presented. The duration of the Kα emission was up to 5 ns: approximately 30 times longer than the laser pulse. A proposed interpretation of these observations is that suprathermal electrons generated in the laser-produced plasma survive after the laser pulse and are gradually decelerated as they circulate in the magnetic field imprinted during the laser pulse in the ferromagnetic target material. Short bursts of Kαemission occur as these electrons pass through the target. The magnetic field in the cold part of the ferromagnetic target with temperature below the Curie temperature is an integral imprint of the spontaneous magnetic field generated in the laser-produced plasma. A model of this process provides a good fit to the experimental data. The effect could be useful as a diagnostic tool for measuring the spontaneous magnetic field and the energy of fast electrons. These results also open an opportunity for studies of magnetization processes in a ferromagnetic at an extremely high magnetic field: on the megagauss scale.
Dynamo action in the Taylor–Green vortex near threshold
Authors: Nore C. , Brachet M. E. , Politano H. , Pouquet A.
Published: July 2002

Dynamo action is demonstrated numerically in the forced Taylor–Green (TG) vortex made up of a confined swirling flow composed of a shear layer between two counter-rotating eddies, corresponding to a standard experimental setup in the study of turbulence. The critical magnetic Reynolds number above which the dynamo sets in depends crucially on the fundamental symmetries of the TG vortex. These symmetries can be broken by introducing a scale separation in the flow, or by letting develop a small non-symmetric perturbation which can be either kinetic and magnetic, or only magnetic. The nature of the boundary conditions for the magnetic field (either conducting or insulating) is essential in selecting the fastest growing mode; implications of these results to a planned laboratory experiment are briefly discussed.
Collisionless pitch angle scattering of plasma ions at the edge region of a field-reversed configuration
Authors: Takahashi Toshiki , Tomita Yukihiro , Momota Hiromu , Shabrov Nikita V.
Published: July 2002

The motion of a plasma ion gyrating around the separatrix of a field-reversed configuration is studied. Numerical studies showed that the action integral of a particle changes abruptly when a particle passes through the vicinity of a field null x point. This phenomena is understood as collisionless stochastic scattering of the pitch angle. In the case of a particle with positive canonical angular momentum Pθ, the resultant correlation coefficients of the action integral between before and after the scattering appear to be stochastic in some cases. As the action integral increases for a particle with negative Pθ, its motion tends to be adiabatic. If the negative Pθ of a particle approaches zero, a stochastic motion is observed.
Relativistic guiding-center dynamics in the presence of strong flows
Authors: Pozzo Monica , Tessarotto Massimo
Published: July 2002

A Lagrangian approach to relativistic particle guiding-center dynamics in strongly magnetized plasmas, based on a covariant formulation, is presented, proving that a four-dimensional transformation is required in order to define the correct relativistic gyrophase-averaged Lagrangian. Comparisons with previous approaches show that these become potentially inconsistent, particularly for magnetoplasmas exhibiting relativistic mass-flow velocities and/or deeply relativistic particles, causing the paradox of hyper-relativistic guiding-center velocities. To avoid this inconvenience, the relativistic velocity addition law is adopted to describe the particle motion with respect to the reference frame locally at rest with the fluid. Based on the adoption of the relativistic Hamilton principle, new expressions are determined for the relativistic guiding-center velocity transformation, the relativistic magnetic moment and the relativistic gyrokinetic equations of motion.
Interaction of sound waves with slow dissipative layers in anisotropic plasmas in the approximation of weak nonlinearity
Authors: Ballai I. , Erdélyi R. , Ruderman M. S.
Published: July 2002

Nonlinear theory of driven magnetohydrodynamics waves in the slow dissipative layer in anisotropic plasmas developed by Ballai, Ruderman, and Erdélyi [Phys. Plasmas 5, 252 (1998)] is used to study the nonlinear interaction of sound waves with static one-dimensional anisotropic plasmas. The magnetic configuration consists of a nonhomogeneous magnetic slab sandwiched by a homogeneous magnetic-free plasma and by a homogeneous magnetic plasma. Sound waves launched from the magnetic-free plasma propagate into the inhomogeneous region interacting with the localized slow dissipative layer and are partially reflected, dissipated or transmitted by this region. The nonlinearity parameter, introduced by Ballai, Ruderman, and Erdélyi, is assumed to be small and a regular perturbation method is used to obtain analytical wave solutions. The main effects caused by nonlinearity in the dissipative layer are the decrease in the absorption coefficient and the generation of higher harmonics in the outgoing wave in addition to the fundamental one. We also found that the nonlinear coefficient of wave energy absorption does depend on the particular type of dissipation operating in the slow dissipative layer.
External inductance of large to ultralow aspect-ratio tokamak plasmas
Authors: Ludwig G. O. , Andrade M. C. R.
Published: July 2002

This paper presents a method for calculating the external inductance and mutual inductance coefficients of tokamak plasma configurations in a consistent way. The method actually solves the external equilibrium problem, linking the poloidal equilibrium fields with the value of the total plasma current and the geometric parameters that describe the plasma cross section. This link imposes constraints upon the values of the inductance for superconducting tori obtained by a previous method described by S. P. Hirshman and G. H. Neilson [Phys. Fluids 29, 790 (1986)]. Only if these constraints are properly taken into account do their results correspond to real tokamak equilibrium configurations. The present method is illustrated by calculating the external equilibrium parameters for a wide range of values of the tokamak aspect ratio. Of particular interest are the results for the external inductance, the elongation, and the vertical equilibrium field in the low aspect-ratio range A<1.5.
Ion Bernstein waves driven by two transverse flow layers
Authors: Reynolds M. A. , Ganguli G.
Published: July 2002

The interaction between two narrow layers of E×B flow is investigated, along with their stability properties. The mode frequencies, growth rates, and eigenfunctions are calculated. It is found that the instability due to a single layer is robust to the inclusion of a second layer. Specifically, when the separation between the layers is on the order of the ion-cyclotron radius, there is strong coupling between the two layers and the second layer is destabilizing. In addition, when the flow velocities are in opposite directions a wide variety of modes is possible, including near-zero-frequency modes, resulting in broadband structure in both the frequency spectrum and the wave number spectrum. These results may have implications for the understanding of the auroral ionosphere, where such spatial structure in the transverse electric field is often observed.
Initial condition sensitivity of global quantities in magnetohydrodynamic turbulence
Authors: Dar Gaurav , Verma Mahendra K. , Eswaran V.
Published: July 2002

The effect of subtle changes in initial conditions on the evolution of global quantities in two-dimensional magnetohydrodynamic (MHD) turbulence is studied. It is found that a change in the initial phases of complex Fourier modes of the Elsässer variables, while keeping the initial values of total energy, cross helicity, and Alfvén ratio unchanged, has a significant effect on the evolution of cross helicity. On the contrary, the total energy and Alfvén ratio are insensitive to the initial phases. The simulations are based on direct numerical simulation using the pseudospectral method.
Collective single pass gain in a tunable rectangular grating amplifier
Authors: Louis L. J. , Scharer J. E. , Booske J. H.
Published: July 2002

The shape of the gain curve and physics of interaction between microwave fields and the electron beam for a periodic rectangular grating slow-wave Traveling Wave Tube (TWT) amplifier are investigated. This research focuses on the collective (ωpτ≫1) low-voltage (⩽10 kV) interaction in the backward wave regime for the Ku-band (12.5–17.5 GHz) range of frequencies. For amplifier experiments, a round “probe” beam (10 kV, 0.25 A, 1 mm diameter) confined by a 1 kG solenoidal focusing magnetic field was utilized. Simultaneous single-pass collective gain and electron-beam velocity spread measurements are performed by means of a Faraday cup repeller energy analyzer configured as the beam collector. Experiments near 13.0 GHz show the maximum normalized single-pass gain to be in agreement with the theoretical prediction for several axial velocity spreads (δv‖/v‖). Collective single-pass experimental results are presented and compared with theory. The effect of an increased interaction length which is accomplished by partial end reflections on the enhanced convective growth is measured and discussed. Narrowing of the gain curve as a function of beam voltage and frequency is shown to result from critical details of the electron-beam radial current density distribution and is verified experimentally by varying the beam-grating separation. Experimental results for several beam-grating separations and the effect of a nonuniform peaked radial current density on the interaction physics are also discussed.
Studies of plasma breakdown and decay in a capacitive radiofrequency argon discharge
Authors: Smith H. B.
Published: July 2002

A one-dimensional particle-in-cell simulation with Monte-Carlo collisions (PIC-MCC) has been used to study the breakdown, evolution and decay of a capacitive, radiofrequency (rf) argon discharge with parallel plate electrodes. The simulation uses spherical discharge geometry in order to represent a system with different area electrodes. Simple analytic and semi-analytic expressions have been determined to describe the temporal development of the average electron energy and the plasma density during these non-equilibrium periods. The expressions give quantities which are averaged over space and the rf cycle, and the results are compared with the simulated conditions. This simple model can be used to determine general behavioral trends relevant to time-modulated, or pulsed, rf systems.
Bounce-averaged kinetic equations and neoclassical polarization density
Authors: Fong B. H. , Hahm T. S.
Published: July 2002

The rigorous formulation of the bounce-averaged equations is presented based upon the Poincaré–Cartan one-form and Lie perturbation methods. The resulting bounce-averaged Vlasov equation is Hamiltonian, and is thus suitable for the self-consistent simulation of low frequency electrostatic turbulence in the trapped ion mode regime. In the bounce-kinetic Poisson equation, the “neoclassical polarization density” arises from the difference between the bounce-averaged banana center and real trapped particle densities across a field line. This representation of the neoclassical polarization drift as a shielding term provides a systematic way to study the long term behavior of the turbulence driven E×B flow.
Finite Larmor radius effect on thermosolutal instability of a compressible Hall plasma in porous medium
Authors: Sunil
Published: July 2002

The thermosolutal instability of a compressible plasma in porous medium is considered in the presence of a uniform vertical magnetic field to include the finite Larmor radius and Hall effects. The dispersion relation has been obtained. The system is found to be stable for (Cp/g)β<1, where Cp, β, and g stand for specific heat at constant pressure, uniform adverse temperature gradient, and acceleration due to gravity, respectively. The uniform vertical magnetic field, finite Larmor radius (FLR), Hall current, and stable solute gradient effects introduce oscillatory modes in the system for (Cp/g)β>1, which were nonexistent in their absence. For the stationary convection, the effect of compressibility is found to postpone the onset of thermosolutal instability. The effect of stable solute gradient, finite Larmor radius, Hall currents, magnetic field, and medium permeability have been investigated analytically.
Cathode multipactor induced by traveling waves
Authors: Riyopoulos Spilios
Published: July 2002

While ac multipactors between gaps have been extensively studied, traveling wave multipactors, widely thought responsible for hub formation in cold-cathode crossed-field devices, have received much less attention. It is shown that a single surface multipactor is induced by a slow wave with two-dimensional space variation. Phase locking with the rf occurs when the distance traveled, and flight time between successive impacts correspond to a collisional drift velocity vc=Δy/Δt equal to the phase velocity vp. The flight time is not a multiple of the rf period, as in ac multipactor, but closer to the cyclotron period. A stable resonant orbit is an attractor in space, pulling nearby orbits; in a rf-synchronous moving window, particles cluster into a Larmor radius size ring, as the rest of the wavelength is swept clean. The bunched charge is later dispersed due to the mutual repulsion, creating a nonuniform hub. The parameter space where avalanching occurs for secondary yield above unity is found much more extended than for an ac multipactor. A crossed-field device based on secondary emitting (“cold”) cathode must operate in the same parameter range.
Kinetic theory of dusty plasmas. I. General approach
Authors: Tsytovich V. N. , de Angelis U.
Published: July 2002

The kinetic theory of dusty plasmas is formulated introducing the dust charge as an independent variable. The Bogoliubov–Klimontovich approach is generalized for the case where the discreetness of the dust grain distribution is described rigorously, while the electrons and ions are described by continuous kinetic equations which take into account their absorption on the highly charged dust grains. The theory is valid for dust densities larger than the critical value where the binary plasma particle collisions can be neglected with respect to the collisions with dust particles. This condition is fulfilled in most dust-plasma experiments and often in space plasmas. The discreetness in the dust distribution leads to both dust fluctuations and plasma particle fluctuations, the latter induced by the dust fluctuations. The dust charge fluctuations alter the interaction appreciably, leading to effective dust charges in interactions which depend on distance and deviate substantially from the equilibrium dust charges. New collision integrals describing the dust charge distribution and inelasticity of dust-plasma particle collisions are found and the problem of dust charging is formulated self-consistently.
Self-trapped electron acceleration from the nonlinear interplay between Raman forward scattering, self-focusing, and hosing
Authors: Tzeng K.-C. , Mori W. B. , Katsouleas T.
Published: July 2002

The generation of high current (>kA), relativistic beams from the wave breaking of plasma waves that result from a high-power (>5 TW), short-pulse (<ps) laser propagating through an underdense plasma is studied in detail using the fully explicit particle-in-cell model PEGASUS [K.-C. Tzeng et al., Phys. Rev. Lett. 76, 3332 (1996)]. The plasma waves and the self-trapped acceleration are due to a highly nonlinear interplay between Raman forward scattering, self-focusing, laser heating, hosing, and wave breaking. The resulting beams have a continuous energy spread with a maximum energy exceeding simple dephasing estimates. For a 5 J laser, a total of 2×1011 electrons are accelerated to relativistic energies with 2×108 of these at 50±1 MeV with a normalized emittance of 13π mm mrad. Details in the correlation of anti-Stokes generation and electron acceleration, the meaning of wave breaking, and the maximum electron energies are presented. A plasma wave accordion mechanism and multibunch beamloading can occur after wave breaking, and these are offered as an explanation for how higher than expected energies are observed. Comparisons to published experimental results are also given.
Numerical computation of collisionless drift wave turbulence
Authors: Jenko Frank , Scott Bruce D.
Published: July 2002

Collisionless drift waves are studied by means of nonlinear numerical simulations in a three-dimensional sheared slab geometry. The electron dynamics is described by a drift-kinetic equation, and the ions are treated as a cold fluid. The energy spectra of the turbulent fluctuations and the dependence of the resulting anomalous transport on various dimensionless plasma parameters are investigated. It is shown that this model resolves fundamental contradictions between experimental results and linear drift wave theory, especially the dependence of turbulent transport on radial position but also the scaling with ion mass.
The effect of sheared diamagnetic flow on turbulent structures generated by the Charney–Hasegawa–Mima equation
Authors: Botha G. J. J. , Haines M. G. , Hastie R. J.
Published: July 2002

The generation of electrostatic drift wave turbulence is modeled by the Charney–Hasegawa–Mima equation. The equilibrium density gradient n0=n0(x) is chosen so that dn0/dx is nonzero and spatially variable (i.e., v*e is sheared). It is shown that this sheared diamagnetic flow leads to localized turbulence which is concentrated at max(∇n0), with a large dv*e/dx inhibiting the spread of the turbulence in the x direction. Coherent structures form which propagate with the local v*e in the y direction. Movement in the x direction is accompanied by a change in their amplitudes. When the numerical code is initialized with a single wave, the plasma behavior is dominated by the initial mode and its harmonics.
Stimulated Raman scattering of water maser lines in astrophysical plasmas
Authors: Gangadhara R. T. , Deguchi Shuji , Lesch H.
Published: July 2002

Radiative transfer equations are derived and solved for the stimulated Raman scattering of water maser lines in the astrophysical plasmas with electrons density of about 106–107 cm−3. In stimulated Raman scattering, the energy of the water maser line is transferred to the side band modes: Stokes mode and anti-Stokes mode. The Stokes mode is easily produced by backward Raman scattering while the anti-Stokes mode is created by the interacting intersecting masers in the plasma. The intensity of the Stokes mode is higher than that of the anti-Stokes mode. These side band modes are proposed as explanation for the extreme velocity features observed in the galaxy NGC 4258. The threshold value of the brightness temperature for the Raman scattering is about 1016–1019 K, and it is satisfied in the case of NGC 4258.
Plasma heating in stellarators at the fundamental ion cyclotron frequency
Authors: Svidzinski V. A. , Swanson D. G.
Published: July 2002

The propagation and absorption of Alfvén waves at ω≈ωci is studied in a straight nonuniform helical plasma with elliptical cross section, placed in an inhomogeneous l=2 magnetic field created by helical windings. The model antenna with a spectrum of kz induces the m=+1 poloidal harmonic. In a uniform plasma in a uniform magnetic field the antenna excites the m=+1 fast wave fields. These fields are right-handed almost everywhere in the plasma and they are not absorbed by plasma ions. In the nonuniform helical l=2 magnetic field under certain conditions the antenna excites mostly the m=+1 fast wave and left-handed m=−1 slow wave fields. The coupling between these waves in the nonuniform magnetic field leads to the excitation of the left-handed wave field component near the plasma center and to the heating of plasma ions by this wave component. Collisionless absorption by plasma ions is assumed. The power deposition distribution in the plasma is examined for different plasma parameters. Results demonstrate that effective heating of the central part of the plasma at ω=ωci is possible for the low density plasmas in small scale devices. Because of the restrictions in the model, no firm conclusions can be made about the effectiveness of this method for high density plasmas in large scale devices.
The possibility of high amplitude driven contained modes during ion Bernstein wave experiments in the tokamak fusion test reactor
Authors: Clark Daniel S. , Fisch Nathaniel J.
Published: July 2002

Extremely high quasilinear diffusion rates for energetic beam ions can be deduced from mode conversion experiments in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire, H. Adler, P. Alling et al., Phys. Plasmas 2(6), 2176 (1995)]. A comparison of the experimental loss rates with the theoretical prediction for the interaction of energetic ions with mode converted ion Bernstein waves showed the theory to underpredict the diffusion coefficient by a factor of 30–70. An anomalously high diffusion coefficient might enhance the advantageous channeling of energetic alpha particle energy in a tokamak reactor. Resolving this discrepancy is thus of importance from the standpoint of practical interest in an improved tokamak reactor as well as from the standpoint of academic interest in basic wave–particle theory. A mechanism is proposed for this accelerated diffusion involving the excitation of a contained mode, possibly similar to that used in explaining the ICE (ion cyclotron emission) phenomenon, near the edge of a tokamak.
Comment on “Surface wave propagation in a magnetized dusty plasma with charge fluctuation” [Phys. Plasmas 6, 3765 (1999)]
Authors: Shoucri M.
Published: July 2002

The dispersion relation of surface waves at the interface of a semi-infinite magnetized drifting plasma was recently derived with the assumption that the tangential component of the wave magnetic field at the beam surface is continuous. It is the purpose of the present note to show that this assumption is incorrect.
Upper bound for the space-charge limited current of relativistic electron beams in finite-length coaxial drift tubes
Authors: Stephens K. F. , Ordonez C. A.
Published: July 2002

A two-dimensional theory for calculating an upper bound for the space-charge limited current of relativistic electron beams is extended to coaxial drift tubes. The theory applies to annular beams of arbitrary radius, thickness and length travelling in a grounded coaxial drift tube of circular cross section. Poisson’s equation for space-charge limited flow is replaced with a Sturm–Liouville-type eigenvalue problem. The problem reduces to numerically solving a determinantal equation rather than a partial differential equation. Under limiting cases, closed-form analytical expressions are derived.
Bias-dependent photoluminescence of InP in aqueous iodine solutions
Authors: Etcheberry A. , Gautron J. , Sculfort J. L.
Published: July 2002

I n situ photoluminescence measurements were used to explain the behavior of the n-InP/electrolyte junction when iodide-iodine redox couple is added to an acid aqueous medium. Adsorption of iodine atoms on the surface of InP drastically reduces the forward-bias luminescence intensity. Luminescence recovery occurs after a large reverse polarization is able, through a photodissolution, to eliminate the adsorbed iodine atoms.
Optical sampling using an all-fiber optical Kerr shutter
Authors: Kitayama Ken-ichi , Kimura Yasuo , Okamoto Katsunari , Seikai Shigeyuki
Published: July 2002

An all-fiber optical Kerr shutter consisting of a short single-mode fiber and a highly birefringent fiber which serves not only as a polarizer but also as a wavelength filter to reject pump light is successfully realized. By utilizing the Kerr shutter, optical sampling is demonstrated. Time-varying signal light from a laser diode at λ=0.84 μm is sampled into a train of pulses through the Kerr shutter which is driven by an intense light pulse at λ=1.064 μm. The applications to high-speed signal processing and the generation of picosecond optical pulse are promising because of the fiber’s fast response of optical Kerr effect.
Monolithic integrated receiver front end consisting of a photoconductive detector and a GaAs selectively doped heterostructure transistor
Authors: Chen C. Y. , Olsson N. A. , Tu C. W. , Garbinski P. A.
Published: July 2002

We report the first demonstration of a monolithically integrated receiver front end consisting of a GaAs selectively doped heterostructure transistor (SDHT) and a photoconductive detector. Due to its simplicity in the required epitaxial layers and device fabrication, the photoconductive detector/SDHT integration scheme is one of the simplest ever reported. The detector has a measured gain-bandwidth product of 5 GHz and the SDHT has a transconductance of 100–140 mS/mm. For an error rate of 10−9 at 90 MHz, the receiver has sensitivities at 0.82 μm of −36.2 dBm and −42.2 dBm for pseudorandom patterns and fixed bit patterns of 101010..., respectively, for an uncoated device.
Potential enhanced Sb and As doping in Si molecular beam epitaxy
Authors: Kubiak R. A. A. , Leong W. Y. , Parker E. H. C.
Published: July 2002

Potential enhanced doping (PED) is a method of enhancing the incorporation efficiency of certain low sticking coefficient dopants in Si molecular beam epitaxy, and obtaining precise control over profiles. The efficacy of PED is demonstrated for Sb, As (and Ga) doping, using elemental and III-V compound coevaporation sources. Enhanced doping by PED depends on increased incorporaton efficiency from the adsorbed dopant adlayer, and does not significantly affect adsorption/desorption processes.
Interface and precipitation effects in solid phase epitaxy of Sb implanted amorphous Si
Authors: Campisano S. U. , Gibson J. M. , Poate J. M.
Published: July 2002

The solid phase epitaxial growth of amorphous silicon implanted with Sb has been investigated by channeling and transmission electron microscopy over a wide Sb concentration range. The amorphous-crystal interface velocity increases with concentration up to about 5×1020 at/cm3 above which it stops increasing, apparently even slightly decreasing. At these high concentrations, amorphous agglomerates of apparently Sb-rich regions are formed in the amorphous layer and the amorphous-crystal interface appears to roughen. It is suggested that these agglomerates are responsible for the decrease in regrowth velocity and are precursors of Sb precipitates observed in the crystalline structure at higher concentration.