bluuebunny/crossref_metadata_2025 huggingface dataset:
Conditional subgrid force and dissipation in locally isotropic and rapidly strained turbulence
Authors: Meneveau Charles , Katz Joseph
Published: July 2002

Modeling of the small scales of turbulence for large eddy simulation (LES) requires a sound understanding of the relationships between large and small scales of motion. Conditional averaging allows us to isolate the effects of specific large-scale flow features in a statistically meaningful fashion. We study how regions of large-scale straining, rotation, and energy cascade rate affect SGS quantities of direct dynamical relevance for LES, namely the subgrid-scale (SGS) force (the divergence of the SGS stress tensor), and the SGS dissipation rate. Conditional averages are measured from experimental data in two flows: (i) the far field of a turbulent jet, and (ii) initially near-isotropic turbulence undergoing rapid axisymmetric expansion. These data are representative of near-equilibrium, locally isotropic turbulence, and of a nonisotropic, nonequilibrium turbulent flow, respectively. The separation between large and small scales is performed with a spatially compact box filter. Results for the locally isotropic data show that the SGS force surrounding points of large strain-rate magnitude is nearly radial. Due to the divergence-free condition of the velocity field, such SGS force can only affect the resolved pressure field. Being directed outwards, the SGS force decreases the resolved pressure in regions of high strain-rate magnitude. Similar results are obtained in regions of large resolved dissipation, defined as the energy flux into the smallest resolved band of scales. No effect of the resolved vorticity is found. In the rapidly distorted flow, the SGS force in regions of large positive resolved dissipation is found to affect both the resolved pressure (again decreasing it) and the resolved velocity (the SGS motions oppose the mean deformation). Regions of large negative resolved dissipation (backscatter) exhibit the opposite effect, acting in such a way as to favor the mean deformation. The mixed model is shown to provide better predictions of conditional SGS dissipation than the Smagorinksy or the similarity models alone. However, observed discrepancies in the conditional SGS force imply the need for further model improvements.
Crenulative turbulence in a converging nonhomogeneous material
Authors: Romero Casildo A. , Harlow Francis H. , Rauenzahn Rick M.
Published: July 2002

Crenulative turbulence is a nonlinear extension of the Bell–Plesset instability, usually observed in a converging system in which there is a nonhomogeneous response of stress to strain and/or strain rate. In general, crenulation occurs in any circumstance in which the mean flow streamlines converge the material more strongly than the compressibility can accommodate. Elements of the material slip past each other, resulting in local fluctuations in velocity from that of the mean flow, producing a type of turbulence that is more kinematic than inertial. For a homogeneous material, crenulation occurs at the atomic or molecular scale. With nonhomogeneous stress response at larger scales, the crenulative process can also occur at those larger scales. The results are manifested by a decrease in the rate of dissipation to heat, and by the configurationally irreversible mixing of nonhomogeneities across any mean-flow-transported interface. A mathematical description of the crenulative process is obtained by means of Reynolds decomposition of the appropriate variables, and the derivation of transport equations for the second-order moments that arise in the mean-flow momentum and energy equations. The theory is illustrated by application to the spherical convergence of an incompressible fluid with nonhomogeneous distribution of kinematic viscosity.
Coulomb-driven flow of a dielectric liquid subject to charge injection by a sharp electrode
Authors: Higuera F. J. , Vázquez P. A.
Published: July 2002

Injection of charge by a sharp electrode into a surrounding dielectric liquid leads to Coulomb forces that set the liquid into motion. An analysis is presented of this motion in a small region around the edge of the electrode, which determines the injected current as a function of the far electric potential seen by this region. By using an injection law appropriate for nonpolar liquids, the analysis predicts an electric current that increases first exponentially and then as the power 73 of the harmonic part of the electric potential, sometimes with a range of multiplicity in between.
Bursts in inclined layer convection
Authors: Busse F. H. , Clever R. M.
Published: July 2002

A new instability of longitudinal rolls in an inclined fluid layer heated from below is analyzed in the case of the Prandtl number P=0.71. The instability assumes the form of subharmonic undulations and evolves into a spatially chaotic pattern when the angle of inclination is of the order of 20°. The chaotic state rapidly decays and longitudinal rolls recover until the next burst of chaotic convection occurs. The theoretical findings closely correspond to recent experimental observations by Daniels et al. [Phys. Rev. Lett. (to be published)].
Bifurcations and modulational interaction in negative compressibility turbulence
Authors: Charlton L. A. , Carreras B. A. , Lynch V. E. , Sidikman K. L. , Diamond P. H.
Published: July 2002

The results of detailed numerical studies of phenomena in negative compressibility turbulence with sheared perpendicular (i.e., poloidal) flow are presented. The turbulence model is based on the parallel ion flow gradient instability, a representative paradigm for ion drift waves. Studies of coupled turbulence and mean flow evolution indicate the existence of two distinct nonlinear states. In the first state, saturation occurs via nonlinear transfer to damped high-k modes and sheared flow is heavily damped. In the second state, the turbulence level is controlled by the self-consistently generated sheared flow. Transition between these states is determined by the competition between instability growth and damping of rotation. The dynamics of the observed transition is well described and consistent with a simple set of coupled envelope equations. Modulational interaction between small scale turbulence and large scale m≠0 shear flows is observed, as well.
Ideal stability of a toroidal confinement system without a toroidal magnetic field*
Authors: Steinhauer Loren C. , Ishida A. , Kanno R.
Published: July 2002

New results show that a toroidal plasma can be ideally stable to gross modes without a toroidal magnetic field. Previous ideal-magnetohydrodynamic (MHD) studies for such systems [commonly called field-reversed configurations (FRC)] have consistently predicted instability to the tilting mode (lowest-order kink mode). However, a new range of equilibria not previously considered are found, which are stable to tilting in ideal-MHD theory. The equilibrium properties that promote stability are hollow current profile, and racetrack separatrix shape. Stable equilibria may not be possible in a θ-pinch system, but could be achieved with a properly designed vertical field coil set. The stability of FRC’s in past θ-pinch experiments arises partly from nonideal effects, but benefits considerably from hollow current profile and racetrack separatrix shape.
Simulation of charged-particle beam transport in a gas using a hybrid particle-fluid plasma model
Authors: Welch D. R. , Olson C. L. , Sanford T. W. L.
Published: July 2002

The simulation of charged-particle beam transport in a ∼1 Torr gas requires accurate plasma-electron modeling. A simple resistive model, which assumes local energy deposition and a thermal plasma-electron distribution, is inadequate. A hybrid model has been implemented into the particle-in-cell simulation code, iprop (The iprop Three-Dimensional Beam Propagation Code, AMRC-R-966, available from D. Welch, Mission Research Corporation, 1720 Randolph Road SE, Albuquerque, NM 87106, September 1987), in which plasma electrons are divided into high-energy macroparticle and thermal-fluid components. This model, which includes ‘‘knock-on’’ bound-electron collision and runaway sources for high-energy electrons, is then used in the simulation of relativistic electron-beam and ion-beam experiments. Results are found to be in agreement with HERMES III [Performance of the HERMES III Gamma Ray Simulator, in Digest of Technical Papers, 7th IEEE Pulsed Power Conference, Monterey, CA, 11 June 1989 (Institute of Electrical and Electronic Engineers, New York, 1989), pp. 26–31] and GAMBLE II [Phys. Rev. Lett. 70, 2573 (1993)] experimental observables.
Frequency up-conversion and time-dependent tunneling of electromagnetic radiation in step-ionized plasmas*)
Authors: Rappaport H. L. , Striffler C. D.
Published: July 2002

The frequency spectra is evaluated and the energy balance investigated for the transmitted and reflected transient electromagnetic waves that are generated when a monochromatic source drives a finite width plasma in which a step increase in density occurs. The source wave undergoes up-conversion to a frequency which depends on the initial and final plasma frequencies. When the final density is many times the plasma critical density transmission resonances result in multiple peaks in the frequency spectra. A tunneling effect is described in which a burst of energy is transmitted from the plasma immediately following the density transition.
Modulational interaction between drift waves and trapped ion convective cells: A paradigm for the self-consistent interaction of large-scale sheared flows with small-scale fluctuations
Authors: Lebedev V. B. , Diamond P. H. , Shapiro V. D. , Soloviev G. I.
Published: July 2002

The linear and nonlinear dynamics of modulational interaction between small-scale drift waves and large-scale trapped ion convective cells are investigated. This example is a paradigm of the more general problem of describing the self-consistent interaction of small-scale fluctuations with mean sheared flows. The growth rate of modulational instability is determined by spectral properties of drift waves and can exceed the linear growth rate of the trapped ion mode. An anisotropic spectrum of drift waves is always modulationally unstable. The spatial orientation of the convective cell pattern and structure (i.e., shear strength) is determined by drift wave spectrum anisotropy and propagation direction. In the presence of a sheared magnetic field, which pins small-scale drift waves to mode rational surfaces, the modulational growth rate becomes intrinsically anisotropic, on account of the modified radial structure of drift waves.
Application of the kinetic transport theory to a unified treatment of longitudinal perturbations in neutral and charged gases
Authors: Sukhorukov A. I. , Stubbe P.
Published: July 2002

A uniform treatment of longitudinal waves or wave-like perturbations in neutral gases and plasmas is presented, using the method of kinetic transport theory in conjunction with a ten-moment collision model. The perturbations are considered to be generated by an oscillating boundary, and their spatial evolution for given frequency is investigated. Special emphasis is placed on the damping of the perturbations, as apparent from their spatial decay. The close intrinsic relation between longitudinal perturbations in plasmas and neutral gases is revealed by systematic variation of the collision to wave frequency ratio and of the charge number. The strong damping of neutral sound found in the small collision frequency limit is seen to be a continuous extension of Landau damping, either of ion-acoustic or Langmuir waves, when starting at the full charge number and reducing the latter to zero. Application of the present theory to experimental neutral sound data leads to almost quantitative agreement, from the near-collisionless to the collision-dominated limit.
Characteristic speeds in high β isotropic/anisotropic plasmas
Authors: Karimabadi H. , Krauss-Varban D. , Omidi N.
Published: July 2002

Given the importance of linear mode properties (e.g., characteristic speeds) in identification/classification of discontinuities, a detailed comparison between the mode properties in fluid theory and kinetic theory in high β plasmas is carried out. It is found that conventional fluid theories of linear modes in both isotropic and anisotropic plasmas do not yield the correct mode properties, even in the long-wavelength limit. In particular, fluid phase velocities are very sensitive to the model and parameters (polytropic indices) employed. Because of this, fluid theory loses its predictive power. In linear kinetic theory, modes cannot be ordered according to their phase velocities. For instance, at small and moderate propagation angles, the slow/sound (S/SO) mode can have the fastest phase velocity. In such cases, a (quasiparallel) fast shock would be associated with the S/SO mode rather than the usual fast/magnetosonic (F/MS) mode. This has important implications for fast shocks. Since it is the F/MS rather than S/SO mode that connects to the whistler branch, low Mach number quasiparallel shocks associated with S/SO would not be expected to have a phase standing whistler wave train upstream, and their thickness is determined by dissipation rather than dispersion. The consequences of the kinetic mode properties are demonstrated via hybrid simulations (fluid electron, kinetic ions) using the quasiparallel shock as an example.
Laboratory observation of the dust-acoustic wave mode
Authors: Barkan A. , Merlino R. L. , D’Angelo N.
Published: July 2002

A laboratory observation of the dust-acoustic instability is reported. The results are compared with available theories.
Reduction of bootstrap current in the Modular Helias-like Heliac stellarator
Authors: Garabedian P. R. , Gardner H. J.
Published: July 2002

A geometric estimate for the size of the bootstrap current has been implemented in a plasma equilibrium code and applied to the Modular Helias-like Heliac (MHH) stellarator [Nucl. Fusion 34, 881 (1994)]. The predicted change in the rotational transform of the MHH due to the bootstrap current has been brought down to an acceptable level, and this has resulted in one of the smallest stellarator reactor candidates to be discovered so far.
Modeling the excitation of global Alfvén modes by an external antenna in the Joint European Torus (JET)
Authors: Huysmans G. T. A. , Kerner W. , Borba D. , Holties H. A. , Goedbloed J. P.
Published: July 2002

The active excitation of global Alfvén modes using the saddle coils in the Joint European Torus (JET) [Plasma Physics and Controlled Nuclear Fusion Research 1984, Proceedings of the 10th International Conference, London (International Atomic Energy Agency, Vienna, 1985), Vol. 1, p. 11] as the external antenna, will provide information on the damping of global modes without the need to drive the modes unstable. For the modeling of the Alfvén mode excitation, the toroidal resistive magnetohydrodynamics (MHD) code CASTOR (Complex Alfvén Spectrum in TORoidal geometry) [18th EPS Conference On Controlled Fusion and Plasma Physics, Berlin, 1991, edited by P. Bachmann and D. C. Robinson (The European Physical Society, Petit-Lancy, 1991), Vol. 15, Part IV, p. 89] has been extended to calculate the response to an external antenna. The excitation of a high-performance, high beta JET discharge is studied numerically. In particular, the influence of a finite pressure is investigated. Weakly damped low-n global modes do exist in the gaps in the continuous spectrum at high beta. A pressure-driven global mode is found due to the interaction of Alfvén and slow modes. Its frequency scales solely with the plasma temperature, not like a pure Alfvén mode with a density and magnetic field.
Transverse equilibrium and stability of the primary beam in the plasma wake-field accelerator
Authors: Krall J. , Joyce G.
Published: July 2002

The primary electron beam in the plasma wake-field accelerator is studied, using multidimensional particle simulation, for cases in which a shaped electron beam is used, with length L≳λp, where λp is the plasma wavelength. Excellent agreement is found between radial equilibrium conditions and simulation results. Wake amplitudes are significantly reduced when the beam is initialized in a nonequilibrium state. The electron-hose and the transverse two-stream instabilities are observed to cause transverse deflections of the beam, with the transverse two-stream instability having a lower growth rate. Operation in the electron-hose regime can be avoided by reducing the beam density.
Vlasov simulation of modulational instability and Langmuir collapse
Authors: Wang J. G. , Payne G. L. , DuBois D. F. , Rose H. A.
Published: July 2002

Vlasov simulations are developed to study the one-dimensional modulational instability and Langmuir collapse in a plasma driven by an external field. Both electrons and ions in the plasma are described by the Vlasov equations. The external driving field energy density is ∼10−3 times the initial electron thermal energy density. The plasma has a mass ratio of mi/me=1836, and a temperature ratio of Ti/Te=0.1 or 1.0, where mi, me, Ti, and Te are the ion mass, electron mass, ion temperature, and electron temperature. The modulational instability is saturated by the formation of Langmuir cavitons which later collapse and burn out due to the strong wave–particle interactions. The particle heating during a single Langmuir collapse is small under this weak driving field. The accelerated electron and ion jet-like streams in phase space as well as the nucleation of cavitons are observed during the evolution. Zakharov equations with the same parameters and similar initial conditions are also simulated, and the results are compared with those of the Vlasov simulations. The two models agree with each other quantitatively up to the first collapse following the saturation of exponential growth; dynamic differences appear after the first collapse.
Core temperature fluctuations and related heat transport in the Texas Experimental Tokamak-Upgrade
Authors: Cima G. , Bravenec R. V. , Wootton A. J. , Rempel T. D. , Gandy R. F. , Watts C. , Kwon M.
Published: July 2002

The mechanism(s) responsible for anomalous heat transport in the tokamak plasma core has remained elusive to experimental verification. In this paper the hypothesis that high-frequency electrostatic turbulence can account for the measured electron heat transport in Ohmically heated tokamak discharges of the Texas Experimental Tokamak-Upgrade (TEXT-U) [Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier, Amsterdam, 1989), Vol. 1, p. 342] is tested. To accomplish this, core temperature fluctuations have been determined from the measured correlation between two electron cyclotron radiation signals detected by a multichannel high-frequency-resolution heterodyne radiometer. It is found that long wavelength modes (poloidal wave number ≲1 cm−1) are present, with an electron temperature fluctuation amplitude comparable to the density fluctuation amplitude. However, these modes cannot account for observed transport. An extrapolation of the observed turbulent temperature spectrum to the shorter wavelengths measured by far infrared scattering shows that shorter-wavelength electrostatic turbulence could still be responsible for transport.
Saturation of backward stimulated Raman scattering and enhancement of laser light scattering in plasmas
Authors: Kolber T. , Rozmus W. , Tikhonchuk V. T.
Published: July 2002

Theoretical and numerical calculations are performed using the system of Zakharov and electromagnetic wave equations, to describe the nonlinear behavior of stimulated Raman scattering (SRS) in a finite homogeneous plasma slab. The enhancement of secondary scattering processes due to the nonlinear SRS saturation is investigated. The parametric decay of the resonantly driven Langmuir wave provides a mechanism which saturates SRS and greatly broadens the Langmuir and ion acoustic wave spectra. These enhanced electrostatic fluctuations scatter the incident electromagnetic radiation. Scaling laws for enhanced Brillouin, forward Raman, and anti-Stokes forward and backward Raman scattering as well as criteria for their strong enhancement are given. The frequency spectra of enhanced Brillouin scattering shows red- and blue-shifted components, with different amplitudes depending on the plasma density and laser intensity. The numerical results have been compared with experimental data providing new or alternative explanations for the observations.
Fluctuation-induced flux at the plasma edge in toroidal devices
Authors: Carreras B. A. , Hidalgo C. , Sánchez E. , Pedrosa M. A. , Balbín R. , García-Cortés I. , van Milligen B. , Newman D. E. , Lynch V. E.
Published: July 2002

Fluctuation-induced fluxes have a bursty character. As a consequence, a significant part of the total particle flux is carried out by sporadic, large transport bursts. The local flux distribution function is consistent with a near-Gaussian character of the fluctuations. The radial dependence of the statistical properties of plasma fluctuations and induced fluxes have been investigated in the plasma boundary region of the TJ-I tokamak [I. García-Cortés et al., Phys. Fluids B 4, 4007 (1992)] and the TJ-IU torsatron [E. Ascasibar et al., in Plasma Physics and Controlled Fusion Research, Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville (International Atomic Energy Agency, Vienna, in press)]. There is a striking similarity between the statistical properties of turbulent transport in both devices.
Observation of beam-enhanced sheath instability in a double plasma device
Authors: Sarma A. , Bailung H. , Chutia Joyanti
Published: July 2002

The effects of ion beams on sheath properties are experimentally investigated in a double plasma device. The dispersion relations of the ion beam plasmas are measured by interferometer method. The low-frequency instability due to sheath around the negatively biased grid is found to be controlled by two parameters, namely the grid biasing voltage and source anode biasing voltage. The instability is caused by the resonant coupling of the three ion beams that arise due to asymmetry of the sheath potential. The sheath structure follows the Child–Langmuir law and the frequency of the instability is also found to be inversely proportional to the sheath thickness. Therefore, the transit time model is considered to explain the observed phenomena. The coupling between the beam and the oscillating component of the ions through the sheath enhances the instability growth which occurs mainly in the presheath region. The excitation of the instability occurs within certain range of velocity ratio of different beam modes.
Threshold for neoclassical magnetic islands in a low collision frequency tokamak
Authors: Wilson H. R. , Connor J. W. , Hastie R. J. , Hegna C. C.
Published: July 2002

A kinetic theory for magnetic islands in a low collision frequency tokamak plasma is presented. Self-consistent equations for the islands’ width, w, and propagation frequency, ω, are derived. These include contributions from the perturbed bootstrap current and the toroidally enhanced ion polarization drift. The bootstrap current is independent of the island propagation frequency and provides a drive for the island in tokamak plasmas when the pressure decreases with an increasing safety factor. The polarization drift is frequency dependent, and therefore its effect on the island stability cannot be deduced unless ω is known. This frequency is determined by the dominant dissipation mechanism, which for low effective collision frequency, νeff=ν/ε<ω, is governed by the electrons close to the trapped/passing boundary. The islands are found to propagate in the electron diamagnetic direction in which case the polarization drift is stabilizing and results in a threshold width for island growth, which is of the order of the ion banana width. At larger island widths the polarization current term becomes small and the island evolution is determined by the bootstrap current drive and Δ′ alone, where Δ′ is a measure of the magnetic free energy.
Langmuir field structures favored in wave collapse
Authors: Robinson P. A. , Wouters M. J. , Broderick N. G. R.
Published: July 2002

Study of Langmuir collapse thresholds shows that they have little polarization dependence and that moving packets have the lowest thresholds in the undamped case. However, incorporation of damping into the density response inhibits collapse of packets moving at more than a small fraction of the sound speed. Investigation of energy transfer to packets localized in density wells—the nucleation process—shows that at most a few trapped states can exist and that energy transfer is most effective when there is a single barely-trapped state. Coupled with an argument that closely packed wave packets have lower collapse thresholds, this argument yields an estimate of the number density of localized nucleating states in a turbulent plasma. It also leads to a simple and direct semiquantitative estimate of the collapse threshold. All these results are in accord with previous numerical simulations incorporating ion-sound damping, which show a preponderance of slow-moving or stationary packets with little or no intrinsic polarization dependence of thresholds. Likewise, the number densities obtained are in good agreement with simulation values, and the simple estimate of the threshold is semiquantitatively correct. The extent of the agreement supports the nucleation scenario with close-packed nucleation sites in the turbulent state.
Plasma diagnostics in the Tokamak Fusion Test Reactor using emission of electron cyclotron radiation at arbitrary frequencies
Authors: Fidone I. , Giruzzi G. , Taylor G.
Published: July 2002

Emission of cyclotron radiation at arbitrary wave frequency for diagnostic purposes is discussed. It is shown that the radiation spectrum at arbitrary frequencies is more informative than the first few harmonics and it is suited for diagnosis of superthermal electrons without any ‘‘ad hoc’’ value of the wall reflection coefficient. Thermal radiation from the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)] is investigated and it is shown that the bulk and the tail of the electron momentum distribution during strong neutral beam injection is a Maxwellian with a single temperature in all ranges of electron energies.
Magnetohydrodynamic kink stability of high pressure (εβpol≫1) toroidal plasmas
Authors: Kleva Robert G.
Published: July 2002

High pressure toroidal equilibria with εβpol≫1 are shown to be violently unstable to magnetohydrodynamic (MHD) kink modes. Here, ε is the inverse aspect ratio of a torus with a square conducting wall and βpol is the ratio of the plasma pressure to the poloidal magnetic field pressure. A broad spectrum of toroidal mode numbers is unstable at large εβpol. These modes grow rapidly, with a very short exponential growth time of the order of the Alfvén time. The growth rate of the kink mode is independent of the magnitude of the resistivity of the plasma.
Ballooning instability precursors to high β disruptions on the Tokamak Fusion Test Reactor
Authors: Fredrickson E. D. , McGuire K. M. , Chang Z. Y. , Janos A. , Manickam J. , Taylor G. , Mirnov S. , Semenov I. , Kislov D. , Martynov D.
Published: July 2002

Toroidally localized ballooning modes have been found as precursors to high β disruptions in many regimes on the Tokamak Fusion Test Reactor (TFTR) [D. Meade et al., Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, pp. 9–24]. Lower frequency, global magnetohydrodynamic (MHD) activity, typically an ideal n=1 kink mode, causes the toroidal localization. Larger-amplitude n=1 modes result in stronger toroidal localization of the ballooning modes. The modes are typically localized to a region spanning about 90°–120° in the toroidal direction.
Acceleration in femtosecond laser-produced plasmas
Authors: Sauerbrey R.
Published: July 2002

The influence of the acceleration of a femtosecond laser-generated plasma on the reflected spectrum of the plasma-producing pulse is analyzed quantitatively, and compared to experimental results. It is shown that the spectral positions of the reflected laser light are complicated functions of the temporally varying optical properties of the plasma and the hydrodynamic motion. The linewidths, however, depend only on the acceleration of the plasma mirror and the chirp of the laser pulse and can consequently be used to measure the acceleration in a laser-produced plasma. Plasma accelerations on the order of 1018 m s−2≊1017 g directed both away from the solid target at intensities of I≤1017 W cm−2 and into the target for I≥3×1017 W cm−2 are obtained from an analysis of the experiments. The results demonstrate that during the short subpicosecond laser pulse the plasma motion is actually dominated by acceleration rather than by a constant expansion velocity. The measured accelerations are among the highest accelerations that have been generated in the laboratory for macroscopic objects.
Tearing mode with strong flow shear in the viscosity-dominated limit
Authors: Shen C. , Liu Z. X.
Published: July 2002

Using a standard boundary layer approach, the tearing mode with shear flow comparable with shear magnetic field and fluid viscosity much larger than resistivity has been studied analytically. The results show that, the growth rate in this case scales as ν2/3ν, where νν is the normalized viscosity, in agreement with the numerical results of Einaudi and Rubini [Phys. Fluids B 1, 2224 (1989)]. It is found analytically that large viscosity may have a destabilizing effect on the instability.
Electron capture from one- and two-electron atoms by fast positrons in dense plasmas
Authors: Jung Young-Dae
Published: July 2002

Electron capture processes by positrons from one- and two-electron atoms in dense plasmas are investigated in accordance with the classical Bohr–Lindhard model using the modified hyperbolic-orbit trajectory method. The interaction potential in dense plasmas has been approximated by the Debye–Hückel model. The modified hyperbolic-orbit trajectory for the positron path is obtained by the combination of the hyperbolic-orbit and straight-line trajectories with the modified impact parameter. The maximum value of the electron capture probability per electron for two-electron (He-like) atoms is smaller than that for one-electron (H-like) atoms. The maximum position of the electron capture probability is slightly shifted to the target nucleus with an increase of the target charge. The plasma-screening effect on the electron capture probability for He-like atoms is more effective than that for H-like atoms. However, the capture probabilities obtained by the straight-line trajectory for H-like and He-like atoms are identical.
New one-dimensional model equations of magnetohydrodynamic turbulence
Authors: Yanase Shinichiro
Published: July 2002

New one-dimensional model equations of magnetohydrodynamic turbulence depending on the position x and the time t are proposed considering the x component of the velocity u(x,t) and the y and z components of the magnetic field by(x,t) and bz(x,t). It is found that the model equations conserve the total energy and the quantity similar to the cross helicity, and that they have a similarity solution, u(x,t)∼x/t and by(x,t), bz(x,t)∼C/t in the inviscid limit. A shock type solution is obtained where both the velocity and the magnetic fields have a jump in a narrow viscous or diffusive region. Numerical simulations are also carried out and the results show that the k−2 energy spectrum develops both for the velocity and the magnetic fields.
Theory of the spatiotemporal dynamics of transport bifurcations
Authors: Lebedev V. B. , Diamond P. H.
Published: July 2002

The development and time evolution of a transport barrier in a magnetically confined plasma with nonmonotonic, nonlinear dependence of the anomalous flux on mean gradients is analyzed in the context of a one-field model. Upon consideration of both the spatial inhomogeneity and the gradient nonlinearity of the transport coefficient, it is shown that the transition develops as a bifurcation front with radially propagating discontinuity in the local gradient. The spatial location of the transport barrier as a function of input flux is calculated. The analysis indicates that for powers slightly above threshold, the barrier location xb(t)∼[Dnt(P−Pc)/Pc]1/2, where Pc is the local transition power threshold and Dn is the neoclassical diffusivity. This result suggests a simple explanation for the high disruptivity observed in reversed shear plasmas. The basic conclusions of this theory are insensitive to the details of the local transport model.
Enhanced scattering in magnetized laser-produced plasma
Authors: Mourenas D.
Published: July 2002

A simple model of Bernstein mode parametric excitation near the electron plasma frequency in a subcritical laser-produced plasma is presented. It is argued that the instability threshold of such modes might become much lower than the usual thresholds for both convective and absolute Raman scattering, in particular, near the so-called “ωqn” resonances. This would occur in high-temperature plasmas (2–5 keV) and over very small (about ten microns long) regions of flat magnetic field and plasma density, corresponding to hot spot locations. A special emphasis is placed on a simplified formulation of the growth rates and thresholds for parameters domains wherein Bernstein mode parametric excitation might be of importance, to allow applications in any experimental situation. Many features of the resulting backscatter compare favorably with experimental observations.
A formula for reconstructing fully relativistic electron distributions from incoherent Thomson scattering data
Authors: Naito O. , Yoshida H. , Hatae T. , Nagashima A.
Published: July 2002

A simple formula for reconstructing fully relativistic electron momentum distribution functions from incoherent Thomson scattering data is presented. Although the formula assumes the isotropy in momentum space, it may be useful for obtaining detailed information on the electron distribution function in fusion plasmas.
Coalescence of magnetic islands including anomalous resistivity
Authors: Schumacher J. , Kliem B.
Published: July 2002

Two-dimensional compressible magnetohydrodynamic simulations of the coalescence instability in a low-beta plasma are presented in which anomalous resistivity is permitted to occur. The instability is considered in a uniform current sheet configuration, initially perturbed by an infinite chain of spots of anomalous resistivity. The two phases of the instability known from simulations based on the Fadeev equilibrium—ideal and resistive—can clearly be distinguished also in this configuration. It is found that the conversion of magnetic energy into kinetic energy dominates Ohmic heating. The main energy release occurs within a few Alfvén transit times. The scaling of several key parameters in the current layer formed between the coalescing islands is compared to the scalings obtained with uniform resistivity. The numerical results indicate that the peak reconnection rate decreases toward large Lundquist numbers (S≫104) as S−α with α=15–13 and that a transition to slow reconnection (α∼12) may occur at S≳107, where S is based on the background resistivity.
Ion temperature and plasma flows in improved confinement mode in the H-1 heliac
Authors: Shats M. G. , Rudakov D. L. , Boswell R. W. , Borg G. G.
Published: July 2002

Low and improved confinement modes in the H-1 heliac [M. G. Shats et al., Phys. Rev. Lett. 77, 4190 (1996)] are studied experimentally in rf-sustained (<100 kW, 7 MHz) argon discharges at low magnetic fields (<0.15 T). Surprisingly high ion temperature, measured using a retarding field energy analyzer, is found which increases across the transition to improved confinement mode from 40 to 80 eV, while the electron density increases by about 50%. Both toroidal and poloidal plasma flow velocities do not change across the transition. The increase in a radial electric field in high mode is balanced on average by a corresponding increase in the ion pressure gradient.
The two-dimensional structure of radiative divertor plasmas in the DIII-D tokamak
Authors: Fenstermacher M. E. , Allen S. L. , Brooks N. H. , Buchenauer D. A. , Carlstrom T. N. , Cuthbertson J. W. , Doyle E. J. , Evans T. E. , Garbet P.-M. , Harvey R. W. , Hill D. N. , Hyatt A. W. , Isler R. C. , Jackson G. , James R. A. , Jong R. , Klepper C. C. , Lasnier C. J. , Leonard A. W. , Mahdavi M. A. , Maingi R. , Meyer W. H. , Moyer R. A. , Nilson D. G. , Petrie T. W. , Porter G. D. , Rhodes T. L. , Schaffer M. J. , Stambaugh R. D. , Thomas D. M. , Tugarinov S. , Wade M. R. , Watkins J. G. , West W. P. , Whyte D. G. , Wood R. D.
Published: July 2002

Recent measurements of the two-dimensional (2-D) spatial profiles of divertor plasma density, temperature, and emissivity in the DIII-D tokamak [J. Luxon et al., in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1987), p. 159] under highly radiating conditions are presented. Data are obtained using a divertor Thomson scattering system and other diagnostics optimized for measuring the high electron densities and low temperatures in these detached divertor plasmas (ne⩽1021 m−3, 0.5 eV⩽Te). D2 gas injection in the divertor increases the plasma radiation and lowers Te to less than 2 eV in most of the divertor volume. Modeling shows that this temperature is low enough to allow ion–neutral collisions, charge exchange, and volume recombination to play significant roles in reducing the plasma pressure along the magnetic separatrix by a factor of 3–5, consistent with the measurements. Absolutely calibrated vacuum ultraviolet spectroscopy and 2-D images of impurity emission show that carbon radiation near the X-point, and deuterium radiation near the target plates contribute to the reduction in Te. Uniformity of radiated power (Prad) (within a factor of 2) along the outer divertor leg, with peak heat flux on the divertor target reduced fourfold, was obtained. A comparison with 2-D fluid simulations shows good agreement when physical sputtering and an ad hoc chemical sputtering source (0.5%) from the private flux region surface are used.
Generalized ballooning and sheath instabilities in the scrape-off layer of divertor tokamaks
Authors: Myra J. R. , D’Ippolito D. A. , Goedbloed J. P.
Published: July 2002

The stability of the scrape-off layer to high toroidal mode number ballooning-type instabilities is considered. The equilibrium includes a simple model of the X-point geometry, and parallel (as well as cross-field) equilibrium variations of temperature, density, and potential. The latter are computed numerically from the Braginskii form for Ohm’s law. The stability analysis includes the effects of curvature, resistivity, parallel variation of the E×B drift frequency, and sheath boundary conditions at the divertor plate. Importantly, the equilibrium model assures consistency among the possible instability drives; i.e., the pressure weighting of the curvature, the plasma potential (E×B drift), and the conditions at the divertor plate are coupled by the equilibrium model. Numerical solutions indicate two modes: (i) the curvature-driven mode with growth rate enhanced by the sheaths; and (ii) the E×B shear mode driven by equilibrium variations in the region between the X point and the plate. The latter mode is shown to be partly driven by the X-point geometry. The effect of finite Larmor radius, resistivity, and electron inertia on these modes is investigated.
Direct observation of potential profiles with a 200 keV heavy ion beam probe on the Compact Helical System
Authors: Fujisawa A. , Iguchi H. , Lee S. , Crowley T. P. , Hamada Y. , Kubo S. , Idei H. , Sanuki H. , Itoh K. , Minami T. , Tanaka K. , Nishimura S. , Ida K. , Hidekuma S. , Kojima M. , Takahashi C. , Okamura S. , Matsuoka K.
Published: July 2002

In this paper we present space potential profiles directly observed in a toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings, 12th International Conference on Plasma Physics and Controlled Nuclear Fusion, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411], using a 200 keV heavy ion beam probe. The potential profiles exhibit widely varied characteristics, including positive and negative polarities for electron cyclotron and neutral beam-heated plasmas, respectively. The behavior of high-energy particles in the CHS plasmas are deduced from loss cone diagrams evaluated from the observed potential profiles.
Studies of transport scaling and reduction under feedback
Authors: Chiu J. S. , Sen A. K.
Published: July 2002

A feedback control system using an ion beam as a remote suppressor has been previously shown to be very effective in suppressing plasma instabilities in the Columbia Linear Machine [G. A. Navratil et al., Plasma Phys. 24, 184 (1982)]. The first experimental measurements for the effect of this feedback system on anomalous particle transport, as determined from the cross-correlation of density and potential fluctuations is presented. It is shown that feedback reduces transport due to a rotational E×B mode by up to a factor of 3 in this experiment. Also, it was found that particle transport scales linearly with fluctuation amplitude and feedback control does not alter this scaling. Last, the experimentally observed scaling of particle transport does not agree with any theoretical predictions.
Ion wave response to intense laser beams in underdense plasmas
Authors: Elisseev V. V. , Ourdev I. , Rozmus W. , Tikhonchuk V. T. , Capjack C. E. , Young P. E.
Published: July 2002

Numerical and theoretical studies of laser beam interaction with underdense plasmas involving ion wave instabilities are presented. The theoretical model that is used involves realistic distribution of laser intensity in a focal spot and a non-paraxial electromagnetic wave equation coupled to the ion acoustic wave equation in a two-dimensional geometry. Included among the important results is a weak correlation between backscattered stimulated Brillouin scattering (SBS) reflectivity and filamentation or self-focusing instabilities. The transmitted light demonstrates angular spreading and frequency shifts consistent with near-forward SBS. The role of filamentation and self-focusing on the transmitted light is also discussed.
Turbulent heat and particle flux response to electric field shear
Authors: Ware A. S. , Terry P. W. , Carreras B. A. , Diamond P. H.
Published: July 2002

Turbulent heat and particle fluxes are shown to have different responses to E×B shear flow in edge plasmas. Recent work has shown that E×B shear can affect the cross phase between electrostatic potential and pressure fluctuations as well as fluctuation levels, thus affecting the heat and particle fluxes. Resistive pressure gradient driven turbulence (RPGDT) is considered including a collisional electron temperature equation. While the particle flux is sensitive to moderate amounts of shear in the radial electric field, the heat flux is insensitive to moderate shear. In the collisional regime, the difference in the sensitivity of the heat and particle fluxes to E×B shear is due to the large parallel heat conductivity. The large parallel heat conductivity dominates the temperature response to electrostatic perturbations. In lower collisionality regimes, the difference in the dissipation rates between density and temperature is negligible, but other effects can still produce different responses in the heat and particle fluxes.
Growth rates of the ablative Rayleigh–Taylor instability in inertial confinement fusion
Authors: Betti R. , Goncharov V. N. , McCrory R. L. , Verdon C. P.
Published: July 2002

A simple procedure is developed to determine the Froude number Fr, the effective power index for thermal conduction ν, the ablation-front thickness L0, the ablation velocity Va, and the acceleration g of laser-accelerated ablation fronts. These parameters are determined by fitting the density and pressure profiles obtained from one-dimensional numerical simulations with the analytic isobaric profiles of Kull and Anisimov [Phys. Fluids 29, 2067 (1986)]. These quantities are then used to calculate the growth rate of the ablative Rayleigh–Taylor instability using the theory developed by Goncharov et al. [Phys. Plasmas 3, 4665 (1996)]. The complicated expression of the growth rate (valid for arbitrary Froude numbers) derived by Goncharov et al. is simplified by using reasonably accurate fitting formulas.
The klystron: A microwave source of surprising range and endurance
Authors: Caryotakis George
Published: July 2002

This year marks the 60th anniversary of the birth of the klystron at Stanford University. The tube was the first practical source of microwaves and its invention initiated a search for increasingly more powerful sources, which continues to this day. This paper reviews the scientific uses of the klystron and outlines its operating principles. The history of the device is traced from its scientific beginnings to its role in WWII and the Cold War, its subsequent decline in use for military systems, and to its current resurgence as the key component in a major accelerator project. Finally, the paper describes the development of a modular klystron, which may someday power future accelerators at millimeter wavelengths.
On the theory of internal kink oscillations
Authors: Breizman B. N. , Candy J. , Porcelli F. , Berk H. L.
Published: July 2002

In this paper a time evolution equation for internal kink oscillations is derived. It is valid for both stable and unstable plasma regimes, and incorporates the response of an energetic particle population. A linear analysis reveals a parallel between (i) the time evolution of the spatial derivative of the internal kink radial displacement and (ii) the time evolution of the perturbed particle distribution function in the field of an electrostatic wave (Landau problem). It is shown that diamagnetic drift effects make the asymptotic decay of internal kink perturbations in a stable plasma algebraic rather than exponential. However, under certain conditions the stable root of the dispersion relation can dominate the response of the on-axis displacement for a significant period of time. The form of the evolution equation naturally allows one to include a nonlinear, fully toroidal treatment of energetic particles into the theory of internal kink oscillations.
Nonlocal electron transport in laser heated plasmas
Authors: Brantov A. V. , Bychenkov V. Yu. , Tikhonchuk V. T. , Rozmus W.
Published: July 2002

Nonlocal theory of an electron transport in laser-produced plasmas with the large ion charge and arbitrary ratio of the characteristic spatial scale length to the electron mean free path has been developed for small potential perturbations. Closure relations have been derived from the solution to the electron Fokker–Planck equation which includes inverse bremsstrahlung heating and ponderomotive effects. All electron transport coefficients and their dependence on the laser intensity have been found. An expression for the electron heat flux includes laser field and plasma flow contributions. Identification of these different sources is necessary for the unique definition of the thermal transport coefficient which is independent of the particular application. A complete derivation of the potential part of the ponderomotive force in the presence of inverse bremsstrahlung heating has been presented.