Turbulent Contributions to Ohm's Law in Axisymmetric Magnetized Plasmas

The effect of magnetic turbulence in shaping the current density in axisymmetric magnetized plasma is analyzed using a turbulent extension of Ohm's law derived from the self-consistent action-angle transport theory. Besides the well-known hyper-resistive (helicity-conserving) contribution, the generalized Ohm's law contains an anomalous resistivity term, and a turbulent bootstrap-like term proportional to the current density derivative. The numerical solution of the equation for equilibrium and turbulence profiles characteristic of conventional and advanced scenarios shows that, trough "turbulent bootstrap" effect and anomalous resistivity turbulence can generate power and parallel current which are a sizable portion (about 20-25%) of the corresponding effects associated with the neoclassical bootstrap effect. The degree of alignment of the turbulence peak and the pressure gradient plays an important role in defining the steady-state regime. In fully bootstrapped tokamak, the hyper-resistivity is essential in overcoming the intrinsic limitation of the hollow current profile.Comment: 19 pages, 6 figures, journal pape

Acid Precipitation and Catastrophes in Forest Dynamics: A Conceptual Framework

The focus of this paper will be on the intrinsic nonlinear nature of the vegetation response to acid precipitation, which plays a principal role in causing catastrophes in forest dynamics. We contemplate three possible mechanisms of forest disruption from acidic deposition: (i) through direct effects upon vegetation (such as physical damage to tissues), (ii) through increased soil acidity, which inter alia entails the release of toxic amounts of aluminum and manganese, (iii) through accumulation in the tree biomass of excessive amounts of nutrients, which may be harmful to the plants. We analyze the three corresponding modes of forest decline and demonstrate, within the framework of catastrophe theory, that the second and third mechanisms give rise to a so-called fold catastrophe

Comparing electroweak data with a decoupling model

Present data, both from direct Higgs search and from analysis of electroweak data, are starting to become rather restrictive on the possible values for the mass of the standard model Higgs. We discuss a new physics scenario based on a model with decoupling (both in a linear and in a non linear version) showing how it allows for an excellent fit to the present values of the $\epsilon$ parameters and how it widens the allowed ranges for the Higgs mass (thought as elementary in the linear version, or as composite in the non linear one).Comment: 10 pages, 3 Figures Late

Stability Analysis of Predator-Prey Models via the Liapunov Method

As is well known from the classical applications in the electrical and mechanical sciences, energy is a suitable Liapunov function; thus, by analogy, all energy functions proposed in ecology are potential Liapunov functions. In this paper, a generalized Lotka-Volterra model is considered and the stability properties of its non-trivial equilibrium are studied by means of an energy function first proposed by Volterra in the context of conservative ecosystems. The advantage of this Liapunov function with respect to the one that can be induced through linearization is also illustrated

Mean Value and Variability of Fish Catches in Fluctuating Environments

The mean value of the catch and its variability due to environmental fluctuations are analyzed for a very general stock-recruitment model. Particular attention is devoted to the comparison of two standard fishing strategies (constant effort and constant escapement) in terms of mean catch, variance in catches and maximum deviation of catch. It is demonstrated analytically that constant escapement policies should always give higher mean catch, but should also give higher catch variance and more extreme catches only under certain conditions of environmental variability

Catastrophes in Exploited Forests

(1) This paper shows why small variations of the human exploitation of a natural forest can give rise to dramatic changes in forest biomass. (2) Two simple mechanisms for catastrophes, already pointed out for other ecosystems, are briefly discussed at the beginning of the paper. The first occurs when depensation phenomena are present in the forest growth curve, while the second is due to the concavity of the harvesting function, which can be interpreted as the functional response of the forest exploiters. In both cases a small increase in the exploitation can lead to the collapse of the forest. (3) A more interesting mechanism for catastrophes is then presented. It is based on the dynamics of the soil nutrient and the fact that tree mortality may become very high when soil acidity exceeds a threshold. (4) In particular, it is shown that an increase of the exploitation may give rise to a catastrophic collapse of the forest if the exogenous nutrient inflow (i.e., acidic deposition) is sufficiently high. Moreover, such a catastrophe is irreversible, i.e., reforestation is not possible, if the nutrient inflow is too high