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  • 1
    Digitale Medien
    Digitale Medien
    A prospective trial of computed tomography in the staging of ovarian malignancy (1985)
    Oxford, UK : Blackwell Publishing Ltd
    BJOG 92 (1985), S. 0 
    Details
    ISSN: 1471-0528
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Medizin
    Notizen: Summary. Thirty patients, thought clinically to have ovarian tumours, were studied prospectively by pre-operative computed tomographic (CT) scans of the abdomen and pelvis. In six patients (20%) small metastases in mesentery, omentum and on subdiaphragmatic peritoneum were not detected by the scans. CT did not improve the accuracy of staging or assist the surgeons by drawing their attention to disease which they might otherwise have missed. Although CT gives an elegant demonstration of anatomy, it is not an alternative to extended laparotomy in patients with ovarian tumours.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1471-0528.1985.tb01116.x
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  • 2
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.860716
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  • 3
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.860360
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  • 4
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859905
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  • 5
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859367
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  • 6
    Digitale Medien
    Digitale Medien
    Relativistic and nonrelativistic plasma dispersion functions (1989)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 30 (1989), S. 2484-2487 
    Details
    ISSN: 1089-7658
    Quelle: AIP Digital Archive
    Thema: Mathematik , Physik
    Notizen: A number of new results involving the functions that determine the emission, dispersion, and absorption of waves in nonrelativistic and weakly relativistic plasmas are presented. These results comprise series, integrals, recursion relations, symmetry properties, interrelations, approximations, and connections with standard transcendental functions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.528528
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  • 7
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859223
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  • 8
    Digitale Medien
    Digitale Medien
    Conditions for the validity of unmagnetized-plasma theory in describing weakly magnetized plasmas (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 525-534 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: An investigation is made of the conditions under which the expression for the dielectric tensor from magnetized-plasma theory may be approximated by that from unmagnetized-plasma theory. Surprisingly, the conditions usually quoted are shown to be inaccurate chiefly because they make no reference to the form of the plasma distribution function. Corrected conditions depend strongly on the distribution function and their application is thus highly problem-dependent. For completely arbitrary distributions these conditions are generally stronger than the commonly quoted ones for weakly damped waves. By contrast, the conditions applicable to axisymmetric distributions can be weaker than some of those commonly quoted. A geometric interpretation of the revised conditions for axisymmetric distributions is given in terms of the resonance ellipses of cyclotron maser theory and is illustrated with reference to dispersion and instability of Langmuir and Bernstein waves in weakly magnetized plasmas. These results apply to all weakly magnetized plasmas that can be described by the linearized Vlasov equations and imply that some investigations which have used the commonly quoted conditions to justify use of unmagnetized-plasma theory may be in error on this point.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.866834
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  • 9
    Digitale Medien
    Digitale Medien
    Weakly relativistic dispersion of Bernstein waves (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 107-114 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Weakly relativistic effects on the dispersion of Bernstein waves are investigated for waves propagating nearly perpendicular to a uniform magnetic field in a Maxwellian plasma. Attention is focused on those large-wave-vector branches that are either weakly damped or join continuously onto weakly damped branches since these are the modes of most interest in applications. The transition between dispersion at perpendicular and oblique propagation is examined and major weakly relativistic effects on dispersion at perpendicular propagation are found to persist at other angles; these effects can dominate even in low-temperature plasmas. A number of simple analytic criteria are obtained which delimit the ranges of harmonic number and propagation angle within which various types of weakly damped Bernstein modes can exist.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.866557
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  • 10
    Digitale Medien
    Digitale Medien
    Relativistic plasma dispersion functions: Series, integrals, and approximations (1987)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 28 (1987), S. 1203-1205 
    Details
    ISSN: 1089-7658
    Quelle: AIP Digital Archive
    Thema: Mathematik , Physik
    Notizen: A number of results are presented involving the plasma dispersion functions appropriate to waves in weakly relativistic, magnetized, thermal plasmas. These results include generating functions, series, integral forms and interrelations, and several useful approximations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.527515
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