bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic energy diffusion of electrons in a plasma by an electron cyclotron wave
Authors: Farina Daniela , Pozzoli Roberto , Mennella Aniello , Ronzio Dario
Published: July 2002

The investigation of the energy diffusion process of the electrons in a magnetized plasma due to an electron cyclotron wave in perpendicular propagation with respect to the magnetic field is performed. Starting from the description of the relativistic electron motion by means of an action-angle Hamiltonian H(I,θ,t), the Fokker–Planck–Kolmogorov (FPK) approach to the diffusion is considered for a globally stochastic regime of the system. In this regime, the phase correlation process is analyzed, and the characteristic decay time is estimated analytically. The action diffusion coefficient D(I) is derived and compared with a local quasilinear expression. With explicit reference to the low-density regime, where D increases with I, and is very close to the quasilinear coefficient, the solution of the diffusion equation is compared with the results obtained by direct numerical integration of the motion equations for a statistically significant ensemble of particles. A good agreement with the local quasilinear FPK diffusion is observed. In addition, when the amplitude of the perturbation is varied, oscillations of the average action values around the quasilinear results are found on the long time scale.
Stability criteria for edge flute modes in the two-fluid regime
Authors: Zheng L.-J.
Published: July 2002

Necessary and sufficient stability criteria for flute modes localized at the edge of toroidal plasmas are obtained from the Braginskii’s two-fluid equations without taking into account the collisional effects. It is assumed that the plasma pressure tends to vanish, but its gradient remains finite at the edge of the plasma. The results show that the free-boundary edge flute modes (namely, the peeling modes) are more dangerous than the fixed-boundary modes (namely, the Mercier modes). Numerical investigation of the criterion for peeling modes shows that the finite ion-gyroradius effect can substantially stabilize the modes, especially for the case Δ≥0, where the equilibrium quantity Δ≡1/2+S−1<j⋅B‖∇v‖−2≳, with S denoting the global shear, B the magnetic field, j the current density, v the volume inside the reference magnetic surface, and <...≳ denoting the average over the magnetic surface. Equilibria with Δ≥0 are shown to be more stable to the peeling modes than those with Δ≤0.
Finite beta stabilization of the kinetic ion mixing mode
Authors: Pu Yi-Kang , Migliuolo Stefano
Published: July 2002

The full finite-β local dispersion relation of the ion mixing mode is derived and analyzed, using kinetic theory. The physical mechanism of finite-β stabilization of a single (i.e., fixed wavenumber, kzrn and bi constant) ion mixing mode is discussed. The distortion of the resonant region, in velocity space (by ∇B drifts) and the coupling to the shear Alfvén wave are shown to be the important stabilization mechanisms. It is found that high values of β (β>βc, where βc =0.1→1.0, for relevant parameters) are necessary for stabilization.
A theoretical model for the annular jet instability
Authors: Lee C. P. , Wang T. G.
Published: July 2002

The instability of an annular gas-core liquid jet is modeled theoretically by treating the liquid layer as a membrane moving under the influences of its own inertia, surface tension, and the gaseous hydrostatic pressure difference between its two sides. Essential physical mechanisms are reconstructed without making any attempt to fit experimental data with model constants. The results compare favorably with those of experiment.
Island bootstrap current modification of the nonlinear dynamics of the tearing mode
Authors: Carrera R. , Hazeltine R. D. , Kotschenreuther M.
Published: July 2002

A kinetic theory for the nonlinear evolution of a magnetic island in a collisionless plasma confined in a toroidal magnetic system is presented. An asymptotic analysis of a Grad–Shafranov equation including neoclassical effects such as island bootstrap current defines an equation for the time dependence of the island width. Initially, the island bootstrap current strongly influences the island evolution. As the island surpasses a certain critical width the effect of the island bootstrap current diminishes and the island grows at the Rutherford rate. For current profiles such that Δ′<0 the island bootstrap current saturates the island.
A model for estimating transport quantities in two-phase materials
Authors: Acrivos Andreas , Chang Eric
Published: July 2002

A novel technique is presented for estimating the various transport quantities in two-phase materials of random structure by extending, in an approximate manner, the effective continuum description of such systems. The method is then applied to three representative examples and is found to give results in excellent agreement with those obtained from exact calculations.
Anomalous diffusion in heterogeneous porous media
Authors: Koch Donald L. , Brady John F.
Published: July 2002

The dispersion of a passive tracer resulting from flow through heterogeneous porous media is studied. When the correlation length for the permeability fluctuations is finite, a normal diffusive process, with the mean-square displacement of the tracer growing linearly with time, is obtained at long times. However, it is shown that, when the correlation length diverges, anomalous diffusion occurs in which the mean-square displacement grows faster than linearly with time. The space–time evolution of the tracer’s concentration is calculated and shown to be universal—uniquely related to the covariance of the permeability field—in the anomalous regime.
Experiments on the instabilities of a swirling jet
Authors: Panda J. , McLaughlin D. K.
Published: July 2002

Instabilities present in a swirling jet in the Reynolds number range from 20 000 to 60 000 and a nominal swirl number of 0.5 were studied experimentally, using smoke visualization, hot-wire anemometry and acoustic excitation. Flow visualization photographs of the natural jet show vortex breakdown at the core and rolling up of the shear layer around the jet into weak, irregular, large-scale organized structures. When forced by acoustic excitation these structures became energetic and periodic. Axisymmetric and helical instability waves in the Strouhal number range 0.75 to 1.5 were excited and their evolution along the axial direction were measured from velocity spectra and ensemble averaged measurements. Compared to a nonswirling jet, the overall growth of the instability waves is considerably smaller, and vortex pairing is suppressed in a swirling jet. However, the overall spread and mass entrainment rates are higher in the latter. Measurements of the mean velocity components and turbulence fluctuations show that the vortex breakdown affects the axial velocity distribution and rapidly replaces the potential core with a large amount of turbulence. Upon interacting with the vortex breakdown, the shear layer along the jet periphery loses its organized structure and, in general, ‘‘random turbulence’’ follows.
On the spectrum of a stretched spiral vortex
Authors: Pullin D. I. , Buntine James D. , Saffman P. G.
Published: July 2002

Corrections are found to the k−5/3 spectrum of Lundgren [Phys. Fluids 25, 2193 (1982)] for a stretched spiral vortex model (a is the stretching strain rate and k the scalar wave number) of turbulent fine scales. These take the form of additional terms arising from the early time evolution, when the stretching of vortex lines is small. For the special case when the spiral takes the form of a rolled-up shear layer, it is shown that the composite spectrum is divergent, thus requiring the introduction of a finite early cutoff time τ1 in the time integral for the nonaxisymmetric contribution. The identity νω2 = 2ν∫0∞k2E(k)dk which gives the dissipation is then satisfied self-consistently. Direct numerical calculation of the energy spectrum from the approximate vorticity field for a special choice of spiral structure nevertheless indicates that the one-term k−5/3-spectrum result is asymptotically valid in the inertial range provided aτ1 is O(1) but that the numerically calculated dissipation spectrum appears to lie somewhere between an exp(−B1k2) and an exp(−B2k) form. It is also shown that the stretched, rolled-up shear-layer model predicts asymptotic shell-summed spectra of the energy dissipation and of the square of the vorticity, each asymptotically constant, with no power-law dependence, for k smaller than the Kolmogorov wave number.

The corresponding one-dimensional spectra each show −log(k1) behavior for small k1. The extension of the model given by Pullin and Saffman [Phys. Fluids A 5, 126 (1993)] is reformulated by the introduction of a long-time cutoff in the vortex lifetime and an additional requirement that the vortex structures be approximately space filling. This gives a reduction in the number of model free-parameters but introduces a dependence of the calculated Kolmogorov constant and skewness on the ratio of the initial vortex radius to the equivalent Burgers-vortex radius. A scaling for this ratio in terms of the Taylor microscale Reynolds number is proposed in which the stretching strain is assumed to be provided by the large scales with spatial coherence limited to the maximum stretched length of the structures. Postdictions of the fourth-order flatness factor and of higher moments of the longitudinal velocity gradient statistics are compared with numerical simulation.
Correlation structure dependence of the effective permeability of heterogeneous porous media
Authors: De Wit A.
Published: July 2002

A theory is given in which the effective permeability tensor Keff of heterogeneous porous media is derived by a perturbation expansion of Darcy’s law in the variance σ2 of the log-permeability ln[κ(ub;;-45rubx)]. The only assumption is that the spatially varying permeability κ(ub;;-45rubx) is a expressed in terms of the moments of the distribution of ln[κ(ub;;-45rubx)], i.e. Keff can formally be computed for any given distribution of the fluctuations of the log-permeability. The explicit dependence of Keff on multi-point statistics is given for non-gaussian log-permeability fluctuations up to order σ6. As a special case of the theory, we examine Keff for a normal distribution function for both isotropic and anisotropic media. In the case of three-dimensional isotropic porous media, a conjecture has been made in the past according to which the scalar effective permeability κeff=KGexp[σ2/6] where KG is the geometric mean of the log-permeability. It is shown here that this conjecture is incorrect as the σ6-order term of κeff contains additional terms than those corresponding to the development of the above formula. Moreover, these additional terms depend on the structure of the two-point correlation function of ln[κ]. The resulting κeff computed for both Gaussian and exponentially decaying covariances lies below the exponential formula. This result might suggest the exponential formula as being an upper bound for κeff. For anisotropic systems, Keff is given up to the σ4-order for the general case where the mean flow is arbitrarily oriented with regard to the axes of stratification.
Direct numerical simulation of turbulent flow over a wavy wall
Authors: De Angelis V. , Lombardi P. , Banerjee S.
Published: July 2002

The Navier–Stokes equations have been solved, by a pseudospectral method, for pressure-driven flows between a no-slip wavy wall and a slip flat wall. Periodic boundary conditions were used in the streamwise and spanwise directions. The physical domain is mapped into a computational domain that is a rectangular parallelepiped using a nonorthogonal transformation. The pseudospectral solution procedure employed in previous studies, for example, Lam and Banerjee [Phys. Fluids A 4, 306 (1992)], eliminated the pressure and solved for the wall–normal velocity and vorticity. The other velocity components were calculated using the definition of vorticity, and the continuity equation. This procedure leads to oscillations in the pressure field when solutions were attempted in the mapped computational domain. To overcome the problem, the procedure had to be modified and the pressure solved for directly using a fractional time step technique. For the cases examined here, these modifications resulted in spectral accuracy being maintained. Flow over sinusoidal wave trains has been simulated and the results compare well with available experiments. The simulations show significant effects of the wavy boundary on the mean flow and the turbulence statistics. The mean velocity profile differs substantially from the profile for the flat-wall case, particularly in the buffer region where the fluid is under the influence of both the wavy wall and the slip boundary. The velocity fluctuations in the streamwise direction decrease in the buffer region. This effect becomes more pronounced when the wave amplitude increases. Most of the redistribution of energy, from the streamwise direction to the spanwise and wall–normal directions, occurs in a thin layer close to the boundary, downstream of the wave troughs. The energy primarily redistributes into spanwise fluctuations. High shear stress regions form downstream of the wave troughs, and streaky structures and quasi-streamwise vortices are also seen to initiate in these regions. The length of the streaks, and the extent of the quasi-streamwise vortices, scale with wave length for the two cases investigated.
A large-scale control strategy for drag reduction in turbulent boundary layers
Authors: Schoppa Wade , Hussain Fazle
Published: July 2002

Using direct numerical simulations of turbulent channel flow, we present a new method for skin friction reduction, enabling large-scale flow forcing without requiring instantaneous flow information. As proof-of-principle, x-independent forcing, with a z wavelength of 400 wall units and an amplitude of only 6% of the centerline velocity, produces a significant sustained drag reduction: 20% for imposed counterrotating streamwise vortices and 50% for colliding, z-directed wall jets. The drag reduction results from weakened longitudinal vortices near the wall, due to forcing-induced suppression of an underlying streak instability mechanism. In particular, the forcing significantly weakens the wall-normal vorticity ωy flanking lifted low-speed streaks, thereby arresting the streaks’ sinuous instability which directly generates new streamwise vortices in uncontrolled flows. These results suggest promising new drag reduction techniques, e.g., passive vortex generators or colliding spanwise jets from x-aligned slots, involving durable actuators and no wall sensors or control logic.
A note on swimming using internally generated traveling waves
Authors: Ajdari Armand , Stone Howard A.
Published: July 2002

In this note we bring together two ideas related to modeling the dynamics of swimming microorganisms: (1) as in Ajdari et al. [C. R. Acad. Sci. Paris, Ser. II 319, 861 (1994)], we consider fluid transport from one end of an organism to the other using an internal deformable canal along which a traveling peristaltic deformation propagates at speed V; (2) as described in Stone and Samuel [Phys. Rev. Lett. 77, 4102 (1996)], the swimming speed U(t) of an organism may be simply related to the velocity of the surface. Thus, we obtain an explicit relation between the average swimming velocity 〈U〉 and V, as well as a more detailed picture of the time-varying flow in the canal and of the instantaneous swimming speed of the organism.
Coherent structures in swirling flows and their role in acoustic combustion control
Authors: Paschereit Christian Oliver , Gutmark Ephraim , Weisenstein Wolfgang
Published: July 2002

Interaction between flow instabilities and acoustic resonant modes and their effect on heat release were investigated and controlled in an experimental low-emission swirl stabilized combustor. Acoustic boundary conditions of the combustor were modified to excite combustion instability at various axisymmetric and helical unstable modes in a fully premixed combustion. The combustion unstable modes were related to flow instabilities in the recirculating wakelike region on the combustor axis and the separating shear layer at the sudden expansion (dump plane). Flow field measurements were performed in a water tunnel using a simulated combustor configuration. The water tunnel tests demonstrated the existence of several modes of flow instabilities in a highly swirling flow, modes which were shown to affect the combustion process. Mean and turbulent characteristics of the internal and external swirling shear layers were measured and unstable flow modes were identified. Instability modes during combustion were visualized by phase locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. Closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx emissions. Microphone and OH emission detection sensors monitored the combustion process and provided input to the control system. An acoustic source modulated the airflow and thus affected the mixing process and the combustion. Effective suppression of the pressure oscillations and the concomitant reduction of NOx emissions were associated with a reduced coherence of the flow structures which excited the thermoacoustic instability.
Numerical studies of flow over a circular cylinder at ReD=3900
Authors: Kravchenko Arthur G. , Moin Parviz
Published: July 2002

Flow over a circular cylinder at Reynolds number 3900 is studied numerically using the technique of large eddy simulation. The computations are carried out with a high-order accurate numerical method based on B-splines and compared with previous upwind-biased and central finite-difference simulations and with the existing experimental data. In the very near wake, all three simulations are in agreement with each other. Farther downstream, the results of the B-spline computations are in better agreement with the hot-wire experiment of Ong and Wallace [Exp. Fluids 20, 441–453 (1996)] than those obtained in the finite-difference simulations. In particular, the power spectra of velocity fluctuations are in excellent agreement with the experimental data. The impact of numerical resolution on the shear layer transition is investigated.
Communication through plasma sheaths via Raman (three-wave) scattering process
Authors: Nazarenko S. V. , Newell A. C. , Zakharov V. E.
Published: July 2002

A mechanism of communication through plasma sheaths which surround vehicles traveling at hypersonic velocities is proposed. The idea is to place a source of an intensive, high frequency electromagnetic pump wave on the vehicle and to use the nonlinearity of the medium to create a backscattered Stokes wave through the interaction of the pump wave with the signal-carrying wave sent by a ground transmitter. Such an interaction will be mediated by the plasma oscillations generated in the process of the resonant absorption of the signal-carrying wave. The advantages of this mechanism for information transfer are that the high frequency pump wave only is present in a small region near the vehicle and that the signal wave can have relatively low frequency.
Simulations of magnetically insulated multistage ion diodes
Authors: Slutz S. A. , Poukey J. W. , Pointon T. D.
Published: July 2002

An analytic theory for magnetically insulated multistage acceleration of high intensity ion beams has been presented [J. Appl. Phys. 67, 6705 (1990)]. This theory predicts an operating behavior that is strongly dependent on the electron density profile. A numerical investigation, using both two-dimensional (2-D) and three-dimensional (3-D) particle-in-cell codes, of multistage diode operating behavior is presented in this paper. The 2-D results are consistent with the analytic results based on a very thin electron sheath. In contrast, the 3-D simulations are consistent with the analytic theory based on a thick electron sheath. The different results are due to the growth of electromagnetic instabilities in the 3-D simulations, which generate fluctuations that broaden the electron sheath. The 2-D simulations did not properly model these instabilities because they propagate in the direction that was ignored. In addition to these results, the 3-D code was used to study the generation of ion divergence due to the instability induced fluctuations. These simulations show a positive correlation between the ion current density (normalized for space-charge effects) and the growth of transverse ion velocities during acceleration. It is found that, at low beam current densities, ion divergence can be reduced significantly by postacceleration.
Bifurcation to a marfing divertor plasma governed by charge exchange
Authors: Ghendrih Ph.
Published: July 2002

A one-dimensional (1-D) model is used to investigate the effect of a high neutral density on the divertor plasma via charge exchange. A regime characterized by a strong plasma pressure decrease along the field line is obtained above a critical neutral density. The bifurcation is characterized by the location of the ionization front, which reaches 70% of the distance from the divertor plate to the X point at the threshold. At this position, the Mach number is large, which has a beneficial aspect on impurity control. The weak dependence of the bifurcation control parameter on the heat outflow from the core allows one to extrapolate this regime to the ignited plasma in the next step devices without requiring large modification of the divertor geometry with respect to that of present experiments.
Stability of magnetohydrodynamic equilibria with flow in the presence of a resistive wall
Authors: Bhattacharyya S. N.
Published: July 2002

The effect of a resistive wall on the stability of a magnetohydrodynamic equilibrium with flow is examined. Gyroscopically stabilized negative energy modes are shown to become unstable in the presence of a resistive wall. An estimate for the growth rate is obtained. The effect of this destabilization on the limits on the safety factor and the plasma beta for a rigidly rotating plasma column are studied.
Experimental observation of filamentation growth in laser-produced plasmas
Authors: Young P. E.
Published: July 2002

The filamentation instability has been observed and systematically studied in a low Z plasma under a variety of density gradients and electron temperatures. Growth lengths have been measured for several plasma conditions and as a function of the spatial size of the imposed intensity perturbation. The filamentation growth is compared to a model that assumes nonlocal electron energy transport and to ponderomotive theory.
Wavelet bicoherence: A new turbulence analysis tool
Authors: van Milligen B. Ph. , Sánchez E. , Estrada T. , Hidalgo C. , Brañas B. , Carreras B. , García L.
Published: July 2002

A recently introduced tool for the analysis of turbulence, wavelet bicoherence [van Milligen, Hidalgo, and Sánchez, Phys. Rev. Lett. 16, 395 (1995)], is investigated. It is capable of detecting phase coupling—nonlinear interactions of the lowest (quadratic) order—with time resolution. To demonstrate its potential, it is applied to numerical models of chaos and turbulence and to real measurements. It detected the coupling interaction between two coupled van der Pol oscillators. When applied to a model of drift wave turbulence relevant to plasma physics, it detected a highly localized coherent structure. Analyzing reflectometry measurements made in fusion plasmas, it detected temporal intermittency and a strong increase in nonlinear phase coupling coinciding with the L/H (low-to-high confinement mode) transition.
The interaction of researchers with teachers: What scientists can offer elementary and secondary schools
Authors: Holt Rush D.
Published: July 2002

Research scientists are engaged in a variety of programs to assist schools in teaching scientific principles by involving students and teachers in investigative processes. Scientists who want to assist in science education should understand the major reform efforts underway in the educational community. Current pedagogical practice has moved away from declarative science and into hands-on participation by students, in which they learn about science as a process. Scientists can help teachers ensure not only that curricular elements are technically accurate and current, but also that the elements represent the process of authentic science.
Fast magnetic reconnection and sudden enhancement of current sheets due to inward boundary flows
Authors: Wang Xiaogang , Ma Z. W. , Bhattacharjee A.
Published: July 2002

Magnetic reconnection is widely believed to be involved in dynamical phenomena such as solar flares or magnetospheric substorms. The Sweet–Parker model of magnetic reconnection in a Y-type geometry predicts a characteristic time scale proportional to S1/2 (where S is the Lundquist number), which is too slow to account for the observed time scales. The Petschek model, in contrast, predicts a time scale proportional to ln S in an X-point geometry. Numerical magnetohydrodynamic (MHD) simulations in the high-S regime generally validate the Sweet–Parker model, unless the resistivity is enhanced in the diffusion region to large values (such that typically S<103). It is demonstrated in this paper that nonlinear reconnection dynamics in a Harris sheet driven by inward boundary flows occurs on a nonlinear time scale that is proportional to S1/5 and thus has a weaker dependence on resistivity than the Sweet–Parker time scale. The current sheet amplitude at the separatrix (spanning Y points) grows algebraically in the linear regime but is suddenly enhanced after it makes a transition to the nonlinear regime. An analytical calculation is given for both the linear and the nonlinear regimes, and supported by two-dimensional resistive MHD simulations. The features of sudden current sheet enhancement and fast reconnection, controlled by boundary flows, are relevant to the phenomena of substorm onset or the impulsive phase of flares.
Stimulated Raman backscattering instability in short pulse laser interaction with helium gas
Authors: Malka V. , De Wispelaere E. , Marquès J. R. , Bonadio R. , Amiranoff F. , Blasco F. , Stenz C. , Mounaix Ph. , Grillon G. , Nibbering E.
Published: July 2002

Experimental and theoretical results on the stimulated Raman backscattering (SRS) reflectivity of a short laser pulse (120 fs) interaction with an optically ionized helium gas are presented. The reflectivity is measured as a function of the gas pressure from 1 to 100 Torr. A monodimensional (1-D) theoretical model, including the refraction induced during the ionization process, describes the dependence of the SRS reflectivity with the gas pressure and explains its maximum at around 35 Torr. In the very low pressure case (<15 Torr), the radial ponderomotive force expels the electrons out of the propagation region before the laser pulse reaches its peak intensity and significantly reduces the observed reflectivity. A 1-D hydrodynamic calculation, included in the model, describes this density depletion and a good agreement is obtained between theory and experiments in the whole range of pressures.
Analytical tokamak equilibrium for shaped plasmas
Authors: Zheng S. B. , Wootton A. J. , Solano Emilia R.
Published: July 2002

A general analytical solution of the Grad–Shafranov equation is presented. Specific functional forms of pressure and plasma current are used; the solution allows arbitrary plasma size, aspect ratio, elongation, triangularity, current, and poloidal beta, without imposing undue constraints amongst those variables.
One-dimensional intense laser pulse solitons in a plasma
Authors: Sudan R. N. , Dimant Y. S. , Shiryaev O. B.
Published: July 2002

A general analytical framework is developed for the nonlinear dispersion relations of a class of large amplitude one-dimensional isolated envelope solitons for modulated light pulse coupled to electron plasma waves, previously investigated numerically [Kozlov et al., Zh. Eksp. Teor. Fiz. 76, 148 (1979); Kaw et al., Phys. Rev. Lett. 68, 3172 (1992)]. The analytical treatment of weakly nonlinear solitons [Kuehl and Zhang, Phys. Rev. E 48, 1316 (1993)] is extended to the strongly nonlinear limit.
Equilibrium of highly asymmetric non-neutral plasmas
Authors: Fajans J. , Backhaus E. Yu. , McCarthy J. E.
Published: July 2002

Pure electron plasmas are usually confined within cylindrically symmetric Penning–Malmberg traps. When azimuthally asymmetric potentials are imposed on the trap walls, the plasmas deform into asymmetric shapes. Such deformed plasmas have been observed experimentally, and are long lived. This paper analyzes the equilibria of these plasmas. Wall potentials can be found which place many asymmetric, flat-top plasmas into exact equilibrium; virtually any flat-top plasma can be placed into approximate equilibrium.
Dissipation of weakly dispersive Alfvén waves
Authors: Champeaux S. , Passot T. , Sulem P. L.
Published: July 2002

The influence of the dispersion magnitude on the dissipation of weakly nonlinear Alfvén waves induced by transverse collapse is investigated using an envelope formalism. It is shown that when the dispersion is too small to enforce circular polarization, the dissipative structures are more uniformly distributed in space and the total dissipation is enhanced.
The diffusion region in collisionless magnetic reconnection
Authors: Hesse Michael , Schindler Karl , Birn Joachim , Kuznetsova Masha
Published: July 2002

The structure of the dissipation region in collisionless magnetic reconnection is investigated by means of kinetic particle-in-cell simulations and analytical theory. Analyses of simulations of reconnecting current sheets without guide magnetic field, which keep all parameters fixed with the exception of the electron mass, exhibit very similar large scale evolutions and time scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities and magnetic field structures. The electron-scale phenomena in the reconnection region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing electrons in a field reversal region. The dissipation is explained by the electric field generated by nongyrotropic electron pressure tensor effects. In the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of the electron mass. The similarities of the gradients as well as the behavior of the electron flow velocity can be derived from the electron trapping scale and the electron mass independence of the reconnection electric field. A further model which includes a significant guide magnetic field exhibits almost identical behavior. The explanation of this result lies in a Hall-type electric field which locally eliminates the magnetizing effect on the electrons of the guide magnetic field. The resulting electron dynamics is nearly identical to the one found in the model without guide magnetic field. This result strongly supports the hypothesis that the local physics in the dissipation region adjusts itself to the demands of the large-scale evolution. A further verification of this notion is provided by Hall-magnetohydrodynamic simulations which employ simple resistive dissipation models in otherwise similar large-scale models. These results also pertain to the inclusion of local reconnection physics in larger scale simulation models.
Disruption mitigation studies in DIII-D
Authors: Taylor P. L. , Kellman A. G. , Evans T. E. , Gray D. S. , Humphreys D. A. , Hyatt A. W. , Jernigan T. C. , Lee R. L. , Leuer J. A. , Luckhardt S. C. , Parks P. B. , Schaffer M. J. , Whyte D. G. , Zhang J.
Published: July 2002

Data on the discharge behavior, thermal loads, halo currents, and runaway electrons have been obtained in disruptions on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 2A 441 (1985)]. These experiments have also evaluated techniques to mitigate the disruptions while minimizing runaway electron production. Experiments injecting cryogenic impurity “killer” pellets of neon and argon and massive amounts of helium gas have successfully reduced these disruption effects. The halo current generation, scaling, and mitigation are understood and are in good agreement with predictions of a semianalytic model. Results from “killer” pellet injection have been used to benchmark theoretical models of the pellet ablation and energy loss. Runaway electrons are often generated by the pellets and new runaway generation mechanisms, modifications of the standard Dreicer process, have been found to explain the runaways. Experiments with the massive helium gas puff have also effectively mitigated disruptions without the formation of runaway electrons that can occur with “killer” pellets.
Observation of a highly directional γ-ray beam from ultrashort, ultraintense laser pulse interactions with solids
Authors: Norreys P. A. , Santala M. , Clark E. , Zepf M. , Watts I. , Beg F. N. , Krushelnick K. , Tatarakis M. , Dangor A. E. , Fang X. , Graham P. , McCanny T. , Singhal R. P. , Ledingham K. W. D. , Creswell A. , Sanderson D. C. W. , Magill J. , Machacek A. , Wark J. S. , Allott R. , Kennedy B. , Neely D.
Published: July 2002

Novel measurements of electromagnetic radiation above 10 MeV are presented for ultra intense laser pulse interactions with solids. A bright, highly directional source of γ rays was observed directly behind the target. The γ rays were produced by bremsstrahlung radiation from energetic electrons generated during the interaction. They were measured using the photoneutron reaction [63Cu(γ,n)62Cu] in copper. The resulting activity was measured by coincidence counting the positron annihilation γ rays which were produced from the decay of Cu62. New measurements of the bremsstrahlung radiation at 1019 W cm−2 are also presented.
Nonlinear relativistic gyrokinetic Vlasov-Maxwell equations
Authors: Brizard Alain J. , Chan Anthony A.
Published: July 2002

A set of self-consistent nonlinear gyrokinetic equations is derived for relativistic charged particles in a general nonuniform magnetized plasma. Full electromagnetic-field fluctuations are considered with spatial and temporal scales given by the low-frequency gyrokinetic ordering. Self-consistency is obtained by combining the nonlinear relativistic gyrokinetic Vlasov equation with the low-frequency Maxwell equations in which charge densities and current densities are expressed in terms of moments of the gyrokinetic Vlasov distribution. For these self-consistent gyrokinetic equations, a low-frequency energy conservation law is also derived.
Covariant formulation of the Fokker–Planck equation for moderately coupled relativistic magnetoplasma
Authors: Baral K. C. , Mohanty J. N.
Published: July 2002

The covariant approach of using the Fokker–Planck equation to treat large angle as well as small angle ion–electron collisions in a magnetoplasma is formulated. It includes third term velocity differential coefficients relevant for moderately coupled plasma valid for large Coulomb logarithm (2≲ln Λb⩽10). The kinetic equation is derived from first principle; the collision integral is formulated, including the appropriate Chapman–Enskog type of approximation in order to facilitate further evaluations of diffusion and transport coefficients.
Applicability of the ballooning transform to trapped ion modes
Authors: Falchetto G. L. , Vaclavik J. , Maccio M. , Brunner S. , Villard L.
Published: July 2002

In the scope of the study of low-frequency electrostatic microinstabilities in tokamak plasmas, attention has been focused on the effect of trapped ions. The ballooning transform has been applied to the gyrokinetic equation, for the case of a large aspect ratio plasma with circular magnetic surfaces. A new eigenvalue code has been developed to solve the resulting integral equation, for the case of adiabatic electrons and full ion dynamics, thus taking into account both circulating and trapped ions. The goal has been to assess the validity of the ballooning transform for trapped ion modes. A scan over the parameter kθρL has been carried out to determine a lower threshold for applicability of the ballooning representation. Illustrative results of trapped ion modes (TIM) are presented, together with the comparison with the ones obtained using a global gyrokinetic code, for low toroidal wave numbers, and a local kinetic dispersion relation.
Relativistic effects on magnetoresonance in free-electron lasers
Authors: Maraghechi B. , Farokhi B. , Mirzanejhad S.
Published: July 2002

Orbital motions of magnetized and relativistic electron beams in free-electron lasers (FEL) with electromagnetic and magnetostatic wiggler are considered. The orbit and dispersion curves of an electromagnetic wiggler demonstrate the magnetoresonance when the transverse motion of electrons in the wiggler field is treated nonrelativistically. Magnetoresonance is also shown in the nonrelativistic case of the magnetostatic wiggler. It is shown that relativistic effects, due to the transverse motion of electron in the wiggler field, destroy the magnetoresonance in an electromagnetic wiggler and displace it in an electrostatic wiggler. Magnetoresonance in the growth rate of FEL in the nonrelativistic case and its destruction in the relativistic case are shown.
Electron, photon, and ion beams from the relativistic interaction of Petawatt laser pulses with solid targets
Authors: Hatchett Stephen P. , Brown Curtis G. , Cowan Thomas E. , Henry Eugene A. , Johnson Joy S. , Key Michael H. , Koch Jeffrey A. , Langdon A. Bruce , Lasinski Barbara F. , Lee Richard W. , Mackinnon Andrew J. , Pennington Deanna M. , Perry Michael D. , Phillips Thomas W. , Roth Markus , Sangster T. Craig , Singh Mike S. , Snavely Richard A. , Stoyer Mark A. , Wilks Scott C. , Yasuike Kazuhito
Published: July 2002

In recent Petawatt laser experiments at Lawrence Livermore National Laboratory, several hundred joules of 1 μm laser light in 0.5–5.0-ps pulses with intensities up to 3×1020 W cm−2 were incident on solid targets and produced a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations have all been diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. About 40%–50% of the laser energy is converted to broadly beamed hot electrons. Their beam centroid direction varies from shot to shot, but the resulting bremsstrahlung beam has a consistent width. Extraordinarily luminous ion beams (primarily protons) almost precisely normal to the rear of various targets are seen—up to 3×1013 protons with kTion∼several MeV representing ∼6% of the laser energy. Ion energies up to at least 55 MeV are observed. The ions appear to originate from the rear target surfaces. The edge of the ion beam is very sharp, and collimation increases with ion energy. At the highest energies, a narrow feature appears in the ion spectra, and the apparent size of the emitting spot is smaller than the full back surface area. Any ion emission from the front of the targets is much less than from the rear and is not sharply beamed. The hot electrons generate a Debye sheath with electrostatic fields of order MV per micron, which apparently accelerate the ions.
Novel Landau level laser in the quantum Hall regime
Authors: Aoki H.
Published: July 2002

A novel laser is proposed for the two-dimensional electrons in semiconductor heterostructures in strong magnetic fields. The laser operates under the two-dimensional Landau quantization with population inversion in the Landau levels. The laser frequency, which is directly tunable by the magnitude of the external magnetic field (H), falls on the far infrared for H∼1 T.
Model for degradation of band-gap photoluminescence in GaAs
Authors: Guidotti Daniel , Hovel Harold J.
Published: July 2002

A recombination-enhanced defect reaction model is described. It is based on nonradiative recombination of photoexcited carriers at particular crystal defect sites near a semiconductor surface. The model is general and correctly predicts, with only one adjustable parameter, the photoluminescence degradation rate in GaAs, its power dependence, and its independence on lattice temperature.
Molecular determinants of terminal growth arrest induced in tumor cells by a chemotherapeutic agent
Authors: Chang Bey-Dih , Swift Mari E. , Shen Mei , Fang Jing , Broude Eugenia V. , Roninson Igor B.
Published: July 2002

Treatment with chemotherapy or radiation is not invariably
cytotoxic to all tumor cells. Some of the cells that survive treatment
recover and resume proliferation, whereas others undergo permanent
growth arrest. To understand the nature of treatment-induced terminal
growth arrest, colon carcinoma cells were exposed to doxorubicin,
and surviving cells were separated into proliferating and
growth-arrested populations. Only growth-arrested cells displayed
phenotypic markers of cell senescence and failed to form colonies. Gene
expression was compared between senescent and proliferating fractions
of drug-treated cells by using cDNA microarray hybridization and
reverse transcription–PCR. Drug-induced senescence was associated with
inhibition of genes involved in cell proliferation and with coinduction
of multiple intracellular and secreted growth inhibitors. Several tumor
suppressors and other genes that are down-regulated in carcinogenesis
were up-regulated in senescent tumor cells. Induction of most growth
inhibitors was delayed but not abolished in cells with homozygous
knockout of p53, in agreement with only limited p53 dependence of
drug-induced terminal growth arrest. On the other hand, senescent cells
overexpressed secreted proteins with antiapoptotic, mitogenic,
and angiogenic activities, suggesting that drug-induced senescence is
associated with paracrine tumor-promoting effects. About one-third of
the genes up-regulated in senescent cells and almost all of the
down-regulated genes showed decreased or delayed changes in
p21
Waf1/Cip1/Sdi1
-deficient cells, indicating that p21 is a
major mediator of the effects of p53 on gene expression. Elucidation of
molecular changes in tumor cells that undergo drug-induced senescence
suggests potential strategies for diagnostics and therapeutic
modulation of this antiproliferative response in cancer treatment.
Induction of a bystander mutagenic effect of alpha particles in mammalian cells
Authors: Zhou Hongning , Randers-Pehrson Gerhard , Waldren Charles A. , Vannais Diane , Hall Eric J. , Hei Tom K.
Published: July 2002

Ever since the discovery of X-rays was made by Röntgen more than a hundred years ago, it has always been accepted that the deleterious effects of ionizing radiation such as mutation and carcinogenesis are attributable mainly to direct damage to DNA. Although evidence based on microdosimetric estimation in support of a bystander effect appears to be consistent, direct proof of such extranuclear/extracellular effects are limited. Using a precision charged particle microbeam, we show here that irradiation of 20% of randomly selected A
L
cells with 20 alpha particles each results in a mutant fraction that is 3-fold higher than expected, assuming no bystander modulation effect. Furthermore, analysis by multiplex PCR shows that the types of mutants induced are significantly different from those of spontaneous origin. Pretreatment of cells with the radical scavenger DMSO had no effect on the mutagenic incidence. In contrast, cells pretreated with a 40 μM dose of lindane, which inhibits cell–cell communication, significantly decreased the mutant yield. The doses of DMSO and lindane used in these experiments are nontoxic and nonmutagenic. We further examined the mutagenic yield when 5–10% of randomly selected cells were irradiated with 20 alpha particles each. Results showed, likewise, a higher mutant yield than expected assuming no bystander effects. Our studies provide clear evidence that irradiated cells can induce a bystander mutagenic response in neighboring cells not directly traversed by alpha particles and that cell–cell communication process play a critical role in mediating the bystander phenomenon.
Suppression of chromosome segregation defects of
Escherichia coli
muk
mutants by mutations in topoisomerase I
Authors: Sawitzke James A. , Austin Stuart
Published: July 2002

Escherichia coli
muk mutants are temperature-sensitive and produce anucleate cells. A spontaneously occurring mutation was found in a
ΔmukB∷kan
mutant strain that suppressed the temperature-sensitive phenotype and mapped in or near
topA
, the gene that encodes topoisomerase I. Previously characterized
topA
mutations,
topA10
and
topA66
, were found to be general suppressors of
muk
mutants: they suppressed temperature sensitivity and anucleate cell production of cells containing null or point mutations in
mukB
and null mutations in
mukE
or
mukF
. The suppression correlated with excess negative supercoiling by DNA gyrase, and the gyrase inhibitor, coumermycin, reversed it. Defects in
topA
allow 99% of cell division events in
muk
null mutants to proceed without chromosome loss or loss of cell viability. This observation imposes important limitations on models for Muk activity and is consistent with a role for MukBEF in chromosome folding and DNA condensation.
Sequence of the 1918 pandemic influenza virus nonstructural gene (NS) segment and characterization of recombinant viruses bearing the 1918 NS genes
Authors: Basler Christopher F. , Reid Ann H. , Dybing Jody K. , Janczewski Thomas A. , Fanning Thomas G. , Zheng Hongyong , Salvatore Mirella , Perdue Michael L. , Swayne David E. , García-Sastre Adolfo , Palese Peter , Taubenberger Jeffery K.
Published: July 2002

The influenza A virus pandemic of 1918–1919 resulted in an
estimated 20–40 million deaths worldwide. The hemagglutinin and
neuraminidase sequences of the 1918 virus were previously determined.
We here report the sequence of the A/Brevig Mission/1/18 (H1N1)
virus nonstructural (NS) segment encoding two proteins, NS1 and nuclear
export protein. Phylogenetically, these genes appear to be close to the
common ancestor of subsequent human and classical swine strain NS
genes. Recently, the influenza A virus NS1 protein was shown to be a
type I IFN antagonist that plays an important role in viral
pathogenesis. By using the recently developed technique of generating
influenza A viruses entirely from cloned cDNAs, the hypothesis that the
1918 virus NS1 gene played a role in virulence was tested in a mouse
model. In a BSL3+ laboratory, viruses were generated that possessed
either the 1918 NS1 gene alone or the entire 1918 NS segment in a
background of influenza A/WSN/33 (H1N1), a mouse-adapted virus
derived from a human influenza strain first isolated in 1933. These
1918 NS viruses replicated well in tissue culture but were attenuated
in mice as compared with the isogenic control viruses. This attenuation
in mice may be related to the human origin of the 1918 NS1 gene. These
results suggest that interaction of the NS1 protein with host-cell
factors plays a significant role in viral pathogenesis.