Hamiltonian formalism in the problem on the modification of the current density profile during the development of tearing instability in a tokamak

The formation of a magnetic island as a result of tearing instability can be interpreted as the bifurcation of an axisymmetric equilibrium configuration at which nested magnetic surfaces are preserved. The modification of the current density profile due to such bifurcation is studied using the Hamiltonian formalism. In the case of a long narrow island, the gradient profile changes to a profile with an extremum on the axis of the magnetic island.     

Determination of the resonant harmonics of the error field from dynamic magnetic measurements in a tokamak

The possibility is discussed of determining the amplitude and phase of a static resonant error field in a tokamak by means of dynamic magnetic measurements. The method proposed assumes measuring the plasma response to a varying external helical magnetic field with a small (a few gauss) amplitude. The case is considered in which the plasma is probed by square pulses with a duration much longer than the time of the transition process. The plasma response is assumed to be linear, with a proportionality coefficient being dependent on the plasma state. The analysis is carried out in a standard cylindrical approximation. The model is based on Maxwell’s equations and Ohm’s law and is thus capable of accounting for the interaction of large-scale modes with the conducting wall of the vacuum chamber. The method can be applied to existing tokamaks.     

Analysis of quasistatic MHD perturbations and stray magnetic fields in a tokamak plasma

Mechanisms for the development of quasistatic MHD perturbations in a viscous rotating tokamak plasma are considered. The influence of stray magnetic fields on the stability of MHD modes in the plasma of the TFTR tokamak is analyzed.     

Empirical mode decomposition method for investigating the structure of large-scale MHD instabilities in a tokamak

A new approach to the processing of experimental data from MHD diagnostics is developed for the purpose of investigating the spatial structure of large-scale MHD instabilities in a tokamak. The empirical mode decomposition method is applied to expand a multimode MHD perturbation in individual modes, which are then identified by constructing a spatial analytic signal with the help of the Hilbert transform. The method can be used to analyze the structure of the tearing instability and of the resistive wall modes.     

Coupled rotational dynamics of two MHD modes in the T-10 tokamak

The coupled rotational dynamics of the m/n = 2/1 and 3/2 MHD modes in an ohmic discharge and the m/n = 2/1 and 4/1 modes in a regime with on-axis electron-cyclotron resonance heating were studied experimentally. The specific features of rotation of these modes in the presence of an error magnetic field, in particular the dependence of the instantaneous frequency Ω(t) of the MHD mode on its spatial orientation at the current instant, were revealed. In analyzing the results obtained, MHD modes were identified with tearing modes (magnetic islands) having a corresponding spatial structure. A possible mechanism for the mutual influence of magnetic islands resulting in their consistent rotation is discussed.     

Turbulent dynamics of helical perturbations in the edge plasma of the T-10 tokamak

Turbulent dynamics of the edge plasma in the T-10 tokamak is simulated numerically by solving nonlinear MHD equations in the framework of the four-field {?, n, p _e , p _i } reduced two-fluid Braginskii hydro-dynamics. It is shown that the transition from ohmic to electron-cyclotron heating is accompanied by a decrease in the amplitudes of turbulent fluctuations in plasma. This is caused by the enhancement of longitudinal dissipation due to the increase in the electron temperature. However, phase relations between potential fluctuations of different modes change in such a way that the Reynolds turbulent force increases, which leads to an increase in the poloidal velocity in the direction of ion diamagnetic drift. Since the poloidal and ion diamagnetic drift velocities enter into the equation of the radial force balance for ions with different signs, the radial electric field decreases. The simulation results agree qualitatively with the results of experiments in the T-10 tokamak. The dependence of the radial electric field on the plasma density, ion pressure, and neutral density is also calculated.     

The effect of emigration and immigration on the dynamics of a discrete-generation population

The recent paper of Sinha and Parthasarathy investigated the effect of modifying the Ricker and logistic population models to simulate the effects of immigration to, and emigration from, the population. Immigration of a fixed number of individuals was shown to reduce the probability of observing chaos in the Ricker model but not the logistic one. Here, isocline analysis is used to investigate why these effects occur. The stabilization effect for the Ricker equation occurs over a wide range of values of the immigration parameter. There are no values of the parameter, however, which increase the stability of the logistic equation substantially. In contrast density-dependent immigration is found to destabilize both the Ricker and logistic models. Density-dependent emigration serves to reduce the propensity of both models to exhibit chaos.     

The effect of podophyllotoxin on microtubule dynamics

We have investigated the effects of podophyllotoxin on the dynamic properties of microtubules assembled from pure tubulin dimer. Excess podophyllotoxin causes the complete disassembly of microtubules, through formation of a tubulin-GTP-podophyllotoxin ternary complex with a dissociation rate constant of 160 s-1 at 37 degrees C, similar to that found upon extensive isothermal dilution in this buffer system. Addition of substoichiometric concentrations of podophyllotoxin causes partial disassembly of microtubules through production of an equivalent amount of the ternary complex. Microtubule length measurements and incorporation of [3H]GTP-tubulin dimer show that podophyllotoxin can suppress the dynamic instability of tubulin dimer microtubules and that it acts substoichiometrically in so doing. We interpret the action of substoichiometric podophyllotoxin on microtubule ends in terms of effects on interconversion of growing and shrinking microtubules in a dynamic system in which tubulin-GTP-podophyllotoxin is kinetically analogous to tubulin-GTP in addition and to tubulin-GDP in dissociation. The ability to suppress dynamic instability may be one way in which drugs such as podophyllotoxin, acting at relatively low concentrations, are able to arrest cell growth and development in a selective way, without necessarily affecting the integrity of the major part of the cytoskeletal microtubule network.     

The effect of water on protein dynamics

Neutron diffraction and spectroscopy were applied to describe the hydration and dynamics of a soluble protein and a natural membrane from extreme halophilic Archaea. The quantitative dependence of protein motions on water activity was clearly illustrated, and it was established that a minimum hydration shell is required for the systems to access their functional resilience, i.e. a dynamics state that allows biological activity.     

The effect of ohmic return currents on biomagnetic fields

To illustrate the calculation methods used for biomagnetic fields we present a detailed calculation of the B-field from a spherical cell in an infinite ohmic bath. The calculation is done from three approaches and the results are used to clarify some misinterpretations that may seem to be biophysical problems but are, in fact, creatures of the formalism used.     

The effect of fertilization and irrigation on the vegetation dynamics of a pine-heath ecosystem

The year-to-year changes in the field-and bottom layers were studied by non-destructive recording of cover percentage on permanent plots in a young Scots pine (Pinus sylvestris) stand, subjected to experimental treatment. The treatments were: control (0), daily irrigation (I), fertilization once a year (F) and daily irrigation plus fertilization five days a week (IF). The cover of the total field layer of the 0-, I-, F-and IF-plots increased during the period of study from 51, 40, 47 and 50% in 1973 to 70, 79, 86 and 92% in 1980. The bulk of this increase consisted of Vaccinium vitis-idaea with the exception of the IF-plots, where the increase especially during the later part of the period of study was due to Chamaenerion angustifolium and and to some extent, to Rubus idaeus. The cover of the total bottom layer of the 0-, I-, F-and IF-plots changed from 63, 59, 54 and 41% in 1973 to 68, 90, 37 and 36% in 1980. The most drastic change took place in the IF-plots, where the composition changed from almost exclusively lichens to bryophytes. Pohlia nutans and Pleurozium schreberi were recorded in substantial amounts filling the gap left by the vanishing lichens. There were no conclusive changes on the I-plots as compared with the 0-plots. The cover estimates, however, were generally more substantial. The overall decline in the bottom layer on the F-plots was mainly due to a decrease in the cover of Pleurozium schreberi.The work was carried out within the Swedish Coniferous Forest Project and supported by the Swedish Natural Research Council, the Swedish National Environmental Protection Board, the Swedish Council of Forestry and Agricultural Research and the Wallenberg Foundation.Nomenclature follows Lid (1974) for vascular plants, Nyholm (1954–69) and Arnell (1956) for bryophytes, Dahl & Krog (1973) for lichens, except for Lecidia granulosa, and the collective group Cladonia silvatica, where Magnusson (1929, 1952) was followed.I wish to thank J. G. K. Flower-Ellis for taking an active part in the planning of this work; S. Bråkenhielm for help and co-operation in the field; H. Sjörs, C. O. Tamm and many colleagues in the Swedish Coniferous Forest Project, among them F. Andersson, A. Aronsson & B. Axelsson for encouragement and support.     

The effect of correlations on the population dynamics of lymphocytes

Recent studies of the population dynamics of a system of lymphocytes in an in vitro immune response have reported strong correlations in cell division times, both between parents and their progeny, and between those of sibling cells. The data also show a high level of correlation in the ultimate number of divisions achieved by cells within the same clone. Such correlations are often ignored in mathematical models of cell dynamics as they violate a standard assumption in the theory of branching processes, that of the statistical independence of cells. In this article we present a model in which these correlations can be incorporated, and have used this model to study the effect of these correlations on the population dynamics of a system of cells. We found that correlation in the division times between parents and their progeny can alter the mean population size of clones within the system, while all of the correlations can affect the variance in the sizes of different clones. The model was then applied to experimental data obtained from time-lapse video microscopy of a system of CpG stimulated B lymphocytes and it was found that inclusion of the correct correlation structure is necessary to accurately reproduce the observed population dynamics. We conclude that correlations in the dynamics of cells within an ensemble will affect the population dynamics of the system, and the effects will become more pronounced as the number of divisions increases.     

Numerical simulation of drift-resistive ballooning turbulence in the presence of the GA mode in the plasma edge tokamak

Drift-resistive ballooning turbulence is simulated numerically based on a quasi-three-dimensional computer code for solving nonlinear two-fluid MHD equations in the scrape-off layer plasma in a tokamak. It is shown that, when the toroidal geometry of the magnetic field is taken into account, additional (geodesic) flux terms associated with the first poloidal harmonic (∼sinθ) arise in the averaged equations for the momentum, density, and energy. Calculations show that the most important of these terms is the geodesic momentum flux (the Stringer-Windsor effect), which lowers the poloidal rotation velocity. It is also shown that accounting for the toroidal field geometry introduces experimentally observed, special low-frequency MHD harmonics—GA modes—in the Fourier spectra. GA modes are generated by the Reynolds turbulent force and also by the gradient of the poloidally nonuniform turbulent heat flux. Turbulent particle and heat fluxes are obtained as functions of the poloidal coordinate and are found to show that, in a tokamak, there is a “ballooning effect” associated with their maximum in the weak magnetic field region. The dependence of the density, temperature, and pressure on the poloidal coordinate is presented, as well as the dependence of turbulent fluxes on the toroidal magnetic field.     

不同类型棉田棉蚜种群动态研究

根据华北棉区8种不同时空类型棉蚜种群密度调查结果,分析和比较了不同播种时间、间套作及免耕措施对棉蚜种群密度的影响。     

The effect of oxycholesterols on thermo-induced membrane dynamics

The effect of temperature change(s) on the dynamics of giant unilamellar vesicles containing oxidized and non-oxidized cholesterol was investigated and characterized. We have demonstrated that (i) major cholesterol auto-oxidation products, 7β-hydroxycholesterol (7β) and 7-ketocholesterol (7keto), rendered vesicles more responsive to temperature changes; (ii) 7keto imparted greater thermo-induced membrane dynamics than 7β; (iii) 7β and 7keto vesicles synergistically were more thermo-responsive than the individual oxysterols; (iv) the thermo-responsiveness of 7keto-containing vesicles was equivalent to that of 25 hydroxycholesterol (25OH)-containing vesicles; and (v) we have characterized the observed membrane dynamics. The results provide a new plausible mechanism: oxidative-stressed membranes in conjunction with temperature change induce membrane dynamics. These findings improve the mechanisms reported previously that attributed the induced dynamics solely to membrane oxidation.