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  • 1
    Electronic Resource
    Electronic Resource
    Interaction of cavitation bubbles with a free surface (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 8225-8237 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The motion of single- and two-cavitation bubbles generated by laser beams directly beneath a free surface is studied experimentally, using high-speed photography, and theoretically using the highly accurate boundary integral method. Favorable comparisons of bubble shape history and centroid motion are observed while the numerical calculations provide information on the pressure field surrounding the bubbles. A range of responses, including the null impulse state, is obtained for the two bubbles depending on the bubble size ratio and the interbubble and bubble-free surface distances, although in all cases reported in this article, the bubble nearest the free surface yields a high-speed liquid jet directed away from the free surface. It is also found that when the free-surface–bubble interaction is strong, a fast free-surface spike is formed for both the single- and two-bubble cases. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1368163
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  • 2
    Electronic Resource
    Electronic Resource
    Stochastic growth of localized plasma waves : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2394-2400 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Localized bursty plasma waves are detected by spacecraft in many space plasmas. The large spatiotemporal scales involved imply that beam and other instabilities relax to marginal stability and that mean wave energies are low. Stochastic wave growth occurs when ambient fluctuations perturb the system, causing fluctuations about marginal stability. This yields regions where growth is enhanced and others where damping is increased; bursts are associated with enhanced growth and can occur even when the mean growth rate is negative. In stochastic growth, energy loss from the source is suppressed relative to secular growth, preserving it far longer than otherwise possible. Linear stochastic growth can operate at wave levels below thresholds of nonlinear wave-clumping mechanisms such as strong-turbulence modulational instability and is not subject to their coherence and wavelength limits. These mechanisms can be distinguished by statistics of the fields, whose strengths are lognormally distributed if stochastically growing and power-law distributed in strong turbulence. Recent applications of stochastic growth theory (SGT) are described, involving bursty plasma waves and unstable particle distributions in type III solar radio sources, the Earth's foreshock, magnetosheath, and polar cap regions. It is shown that when combined with wave–wave processes, SGT also accounts for associated radio emissions. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1345505
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  • 3
    Electronic Resource
    Electronic Resource
    Analytic treatment of weak-turbulence Langmuir wave electrostatic decay (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 428-440 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The three-wave decay of beam-driven Langmuir waves into ion-sound waves, and backscattered Langmuir waves is analyzed. Realistic approximations are used for the spectra of Langmuir and ion-sound waves to enable calculations of growth rates of ion-sound and backscattered Langmuir waves in terms of a single well behaved integral. This integral is evaluated numerically, and approximated analytically and the results are compared with previous estimates. The case of intense beam-driven Langmuir waves decaying amid a weak thermal background of pre-existing Langmuir and ion-sound waves is examined in detail and the analytical approximation in this case is found to reproduce the numerical results accurately. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1339839
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  • 4
    Electronic Resource
    Electronic Resource
    Properties of transit-time interactions in magnetized plasmas: Analytic and numerical results (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2751-2763 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The recently developed perturbation theory of transit-time interactions between particles and coherent wave packets in magnetized plasmas is applied to particular field structures. Limits of validity are determined by comparison with test-particle simulations, showing that the theory is accurate everywhere except near certain well-determined resonances, for wave fields exceeding a characteristic threshold, and for particles below a particular velocity. The properties of transit-time interactions in magnetized plasmas are investigated in detail to determine their dependence on the fields and parameters of the particle motion. Resonant particle scattering is found to occur at low particle velocities when the frequency of the coherent wave packet is an integer multiple of the gyrofrequency. Two different types of resonant transit-time dissipation are also observed: one arises from transient cyclotron acceleration in the localized wave packet, the other from beating between the gyration of the particles and the oscillation of the wave packet field. Both effects involve an interplay between the field geometry and resonant oscillations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860716
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  • 5
    Electronic Resource
    Electronic Resource
    Driven random-phase three-wave interactions: Cycles, bursts, and stochasticity (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 3524-3532 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Steadily driven, undriven, and stochastically driven three-wave decay processes involving groups of random-phase waves are investigated analytically and numerically. Steadily driven systems in which one product wave is suppressed exhibit neutrally stable Lotka–Volterra cycles, as for the true two-component case, whereas three-component systems are stable below a critical driver strength and unstable beyond that point. Initially unstable, but undriven, systems produce bursts of product waves, after which the parent waves fall to a final level that is an exponentially decreasing function of their initial level. Three-component systems where the product waves have near-equal dissipation rates are an exception to the latter behavior; in such systems the final parent-wave level is almost independent of the initial one. Stochastic driving gives rise to bursts of product waves in a cycle of fluctuating period, whereas a low-level noise source tends to stabilize the system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860360
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  • 6
    Electronic Resource
    Electronic Resource
    Transit-time damping and the arrest of wave collapse (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 545-554 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Power dissipation by transit-time damping is investigated analytically and numerically using a perturbation expansion and a test-particle code, respectively. Excellent agreement between the two methods is found for both one-dimensional and multidimensional systems. It is shown that the local power dissipation can take on positive or negative values depending on position, implying that particles not only carry off energy from localized fields, but redistribute it within them. The results are applied to estimate the arrest scales of the collapsing wave packets found in strongly turbulent plasmas. Arrest scales in the ranges (14–23)λD and (16–26)λD are found for two- and three-dimensional wave collapse, respectively. These estimates are consistent with results from particle-in-cell simulations, which yielded arrest scales of ∼14λD in 2D and ∼20λD in 3D, and with experimental results that implied arrest at scales of (17–30)λD in 3D. The previously problematical outcome that 3-D collapse is arrested at a longer scale than in 2D, despite its stronger nature, results because the larger fraction of high-velocity particles in the 3-D plasma velocity distribution leads to stronger transit-time damping than in the corresponding 2-D system. It is argued that transit-time dissipation will arrest the collapse of particularly intense wave packets at even longer scales due to the increase in the local Debye length caused by localized heating near the center of these wave packets and due to the formation of high-velocity tails by transit-time acceleration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859905
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  • 7
    Electronic Resource
    Electronic Resource
    Two-component model of strong Langmuir turbulence: Scalings, spectra, and statistics of Langmuir waves (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2999-3016 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-component model of strong Langmuir turbulence is developed, in which intense coherent Langmuir wave packets nucleate from and collapse amid a sea of low-level background waves. Power balance between these two components determines the overall scalings of energy density and power dissipation in the turbulence, and of the rate of formation, number density, volume fraction, and characteristic nucleation time of collapsing wave packets. Recent insights into the structure and evolution of collapsing wave packets are employed to estimate the spectra and field statistics of the turbulence. Extensive calculations using the Zakharov equations in two and three dimensions demonstrate that the predictions of the model are in excellent agreement with numerical results for scalings, spectra, and the distribution of fields in the turbulence in isotropic systems; strong support is thus found for the nucleation model. The scaling behavior proves to be insensitive to the form of the damping of the waves at large wave numbers. Wave collapse is approximately inertial between the nucleation and dissipation scales, yielding power-law energy spectra and field distributions in this range. The existence of a fixed arrest scale manifests itself in exponentially decreasing energy and dissipation spectra at high wave numbers and exponentially decreasing field distributions at high field strengths. It is suggested that such an exponential decrease may explain the field distributions seen in recent beam–plasma experiments. Generalizations to turbulence driven anisotropically by beams or governed by equations other than the Zakharov equations are outlined. It is shown that a previously unrecognized scaling observed in beam-driven systems is correctly predicted by the generalized model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859367
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  • 8
    Electronic Resource
    Electronic Resource
    Strong Langmuir turbulence generated by electron beams: Electric-field distributions and electron scattering (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 3120-3133 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Strong-turbulence theory is used to predict the statistics of intense Langmuir fields generated when an electron beam is injected into a plasma, and the theory of transit-time interactions is then used to calculate the beam scattering caused by the turbulent waves. The theory predicts that the distribution of field strengths will have a Gaussian tail corresponding to fields in nonlinearly collapsing coherent wave packets that are near the arrest of their collapse by damping. The functional form of the tail of the field distribution is determined by the statistical distribution of wave packets at the time of their formation, whereas its exponent depends on the arrest scale. Comparison of numerical calculations of this exponent with experimental measurements confirms the Gaussian form of the tail and implies that collapse is arrested at a scale of (16±5)λD, where the peak electrostatic energy density is of the same order as the thermal energy density, in good agreement with independent particle-in-cell calculations. Transit-time calculations of beam scattering in strong turbulence yield rms energy changes in good agreement with experimental values and mean energy changes that are well within the experimental limits. These results support the validity of the recently developed scaling theory of strong turbulence, and the predicted form of transit-time interactions with coherent wave packets.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859223
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  • 9
    Electronic Resource
    Electronic Resource
    Transit-time interactions in magnetized plasmas (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1045-1056 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The theory of transit-time interactions between particles and coherent wave packets is generalized to include the effects of ambient and induced magnetic fields. Using a Born approximation, the wave–particle energy transfer is calculated analytically to second order in the perturbing fields, which is sufficient to treat both particle scattering and wave damping. The results possess the correct unmagnetized limit, and can be approximated in a simple form in the small gyroradius regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860951
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  • 10
    Electronic Resource
    Electronic Resource
    Simulation of the collapse and dissipation of Langmuir wave packets (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2600-2622 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The collapse of isolated Langmuir wave packets is studied numerically in two dimensions using both particle-in-cell (PIC) simulations and by integrating the Zakharov partial differential equations (PDE's). The initial state consists of a localized Langmuir wave packet in an ion background that either is uniform or has a profile representative of the density wells in which wave packets form during strong plasma turbulence. Collapse thresholds are determined numerically and compared to analytical estimates. A model in which Langmuir damping is significantly stronger than Landau damping is constructed which, when included in the PDE simulations, yields good agreement with the collapse dynamics observed in PIC simulations for wave packets with initial wave energy densities small compared to the thermal level. For more intense initial Langmuir fields, collapse is arrested in PIC simulations at lower field strengths than in PDE simulations. Neither nonlinear saturation of the density perturbation nor fluid electron nonlinearities can account for the difference between simulation methods in this regime. However, at these wave levels inhomogeneous electron heating and coherent jets of transit-time accelerated electrons in phase space are observed, resulting in further enhancement of wave damping and the consequent reduction of fields in the PIC simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859385
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