Roberto A. Clemente

On Current Drive in Field-Reversed Configurations

The problem of current drive in field-reversed configurations, by applying two transverse magnetic fields rotating in opposite sense, is analyzed within the framework of two-fluid collisional equations. It is shown that it is possible to generate opposite torques on electron and ions, while the net externally applied torque is vanishing. In this scenario, wall interactions, collisions with neutrals and diffusion effects are not required in order to maintain the steady state. It is also shown that, when electrons and ions are almost in phase with the two rotating fields, the injected power is mainly dissipated by collisions associated with the azimuthal motion of both species, which should permit the achievement of significant plasma current drive efficiency in field-reversed configurations.

A Simple Model for Solidification of Undercooled Metallic Samples

A simple model for reproducing temperature recalescence behaviour in spherical undercooled liquid metallic samples, undergoing crystallization transformations, is presented. The model is applied to constant heat extraction rate, uniform but time dependent temperature distribution inside the sample (even after the start of crystallization), a classical temperature dependent rate of nucleation (including contributions from different specific heats for different phases and also a catalytic factor to model the possibility of heterogeneous distributed impurities) and the solidified grain interface velocity is taken proportional to the temperature undercooling. Different assumptions are considered for the sample transformed fraction as function of the extended volume of nuclei, like the classical Kolmogoroff, Johnson–Mehl, Avrami one (corresponding to random distribution of nuclei), the Austin–Rickett one (corresponding to some kind of clusterized distribution) and also an empirical one corresponding to some ordering in the distribution of nuclei. As an example of application, a published experimental temperature curve for a zirconium sample in the electromagnetic containerless facility TEMPUS, during the 2nd International Microgravity Laboratory Mission in 1994, is modeled. Some thermo-physical parameters of interest for Zr are discussed.

Non-Linear Unidimensional Debye Screening in Plasmas

An exact analytical solution for T e = T i and an approximate solution for T e ≠ T i have been obtained for the unidimensional non-linear Debye potential. The approximate expression is a solution of the Poisson equation obtained by expanding up to third order the Boltzmanns factors. The analysis shows that the effective Debye screening length can be quite different from the usual Debye length, when the potential to thermal energy ratio of the particles is not much smaller than unity.

Minima of dissipated power in magnetic levitation

An approximate model for calculating the average lifting force and absorbed power, for a magnetically levitated metallic sphere, is presented. It is shown that by imposing suitable boundary conditions, expressed in terms of matching conditions for the magnetic field, it is possible to express the average absorbed power in terms of appropriate physical parameters. For a given conductive sphere, three cases of interest are analyzed, namely: (i) fixed frequency and resistivity; (ii) fixed external currents and resistivity; (iii) fixed external currents and frequency. The analysis displays the relevant parametric dependence and proves the existence of minima for the absorbed power in parameter space. The results are of potential interest for improving the temperature control of the metallic sample, which is of fundamental importance both in levitation melting and undercooling processes.

On axisymmetric double adiabatic MHD equilibria with plasma flow

The stationary equilibrium of an axisymmetric plasma characterized by toroidal and poloidal flows is considered within the framework of ideal double adiabatic magnetohydrodynamic equations. The problem is reduced to a nonlinear partial differential equation for the poloidal magnetic flux function, containing six surface functions, plus a nonlinear algebraic Bernoulli equation defining the plasma density. Ellipticity conditions and bifurcations of its solutions are discussed in the limit of small beta, appropriated for tokamak-like equilibria. Possible connections with the L-H transition are suggested.

Spherically symmetric stationary MHD equilibria with azimuthal rotation

An equation for stationary MHD equilibrium with azimuthal rotation in spherical coordinates is derived. Following a procedure introduced by Maschke and Perrin, by supposing that the plasma temperature is a surface quantity, we describe a two-parameter family of axisymmetric toroidal configurations. The equilibrium equation is analytically solved for given hypotheses in such a way as to investigate the effect of plasma rotation on the magnetic field structure.

Rotating magnetic field current drive in a hollow plasma column with a steady toroidal field

The effect of a steady azimuthal magnetic field on rotating magnetic field current drive is studied. The configuration considered consists of an infinitely long plasma column with a finite radius conductor, which carries a steady longitudinal current, running along its axis. The ions are assumed to be fixed and the electrons are described using an Ohm’s law that contains the Hall term. A fully two-dimensional computer code is developed to solve the resulting time-dependent equations. For some values of the steady azimuthal field, two steady-state solutions with different efficiencies are found.

MINIMUM TEMPERATURE IN ELECTROMAGNETIC LEVITATION MELTING UNDER TERRESTRIAL GRAVITY

Recently, the existence of minima for the dissipated power in electromagnetic levitation melting has been reported. Such results will be used in order to estimate the temperature of the surface of the sample, when the only allowed mechanism for energy transport from the levitated drop is radiation cooling. It is also possible to estimate the difference in temperature, between the bulk and the surface of the specimen, when the principal mechanism for energy transport inside the liquid metal is thermal conductivity. The results suggest that even in experiments under terrestrial gravity, it should be possible to reach large undercooling for some metals of a high melting point, high total hemispherical emissivity, relatively low resistivity and density, without any need for a cooling gas atmosphere.

Anisotropic ideal magnetohydrodynamic cylindrical equilibria with incompressible adiabatic flow

The problem of ideal anisotropic magnetohydrodynamic cylindrical equilibria with incompressible adiabatic flow is treated. Two examples of exact analytical solutions with different cross shapes of the plasma column are shown. As a bypass result, the model is consistent with the difference between parallel and perpendicular plasma pressures sustained by the flow itself.

The effect of ion motion on rotating magnetic field current drive

The effect of ion motion on rotating magnetic field (RMF) current drive in field reversed configurations is studied using a fully two-dimensional numerical code that solves the two fluid equations with massless electrons and constant uniform temperature. The ion momentum equation includes viscosity and collisions with neutrals, which remain fixed. The electrons are described with an Ohms law that includes the Hall and pressure gradient terms. For full penetration of the RMF, ion spin-up due to collisions with electrons reduces the current drive efficiency and a large fraction of neutrals is needed to prevent the ions from acquiring high azimuthal velocities. For conditions that would result in incomplete penetration with the fixed ion model, ion rotation and variable density can facilitate the penetration of the RMF, thus increasing the efficiency. The rotation modifies the density profile and can trigger rotational instabilities.