Current collection by an active spherical electrode in an unmagnetized plasma

A theoretical model for the steady-state response of anodic contactors that emit a plasma current Ii and collect electrons from a collisionless, unmagnetized plasma is presented. The use of a (kinetic) monoenergetic population for the attracted species, well known in passive probe theory, gives both accuracy and tractability to the theory. The monoenergetic population is proved to behave like an isentropic fluid with radial plus centripetal motion, allowing direct comparisons with ad hoc fluid models. Also, a modification of the original monoenergetic equations permits analysis of contactors operating in orbit-limited conditions. Besides that, the theory predicts that, only for plasma emissions above certain threshold current a presheath/double layer/core structure for the potential is formed (the core mode), while for emissions below that threshold, a plasma contactor behaves exactly as a positive-ion emitter with a presheath/sheath structure (the no-core mode). Ion emitters are studied as a particular case. Emphasis is placed on obtaining dimensionless charts and approximate asymptotic laws of the current-voltage characteristic

Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries

Full Stokes filter-polarimeters are key instruments for investigating the rapid evolution of magnetic structures on the solar surface. To this end, the image quality is routinely improved using a-posteriori image reconstruction methods. We analyze the robustness of circular polarization asymmetries to phase-diversity image reconstruction techniques. We use snapshots of magneto-hydrodynamical simulations carried out with different initial conditions to synthesize spectra of the magnetically sensitive Fe I line at 5250.2 A. We degrade the synthetic profiles spatially and spectrally to simulate observations with the IMaX full Stokes filter-polarimeter. We also simulate the focused/defocused pairs of images used by the phase-diversity algorithm for reconstruction and the polarimetric modulation scheme. We assume that standard optimization methods are able to infer the projection of the wavefront on the Zernike polynomials with 10% precision. We also consider the less favorable case of 25% precision. We obtain reconstructed monochromatic modulated images that are later demodulated and compared with the original maps. Although asymmetries are often difficult to define in the quiet Sun due to the complexity of the Stokes V profiles, we show how asymmetries are degraded with spatial and spectral smearing. The results indicate that, although image reconstruction techniques reduce the spatial smearing, they can modify the asymmetries of the profiles, mainly caused by the appearance of spatially-correlated noise.Comment: 10 pages, accepted for publication in A&

A robust optimization hybrid algorithm for solving the direct kinematics of the general Gough-Stewart platform

El problema de cinemática directa para los robots paralelos se puede enunciar como sigue: dado un conjunto de valores de las variables articulares, se deben encontrar los valores correspondientes en las variables cartesianas, es decir, la posición y orientación del órgano terminal. En muchas ocasiones, el problema de cinemática directa requiere la resolución de un sistema de ecuaciones no-lineales. Los métodos más eficientes para resolver problemas de este tipo suponen convexidad de una función de costo cuyo mínimo es la solución del sistema. La capacidad de tales métodos de optimización para encontrar una solución adecuada depende fuertemente del punto inicial. Un problema bien conocido es la selección de tal punto inicial, el cual requiere información a priori sobre una vecindad convexa donde se encuentra la solución. Este artículo propone un método eficiente para seleccionar y generar el punto inicial basado en aprendizaje probabilístico. El método evita eficientemente los mínimos locales, sin necesidad de intervención humana o información a priori, lo cual lo hace más robusto si se compara con el método Dogleg u otro método de minimización local basado en gradiente. Con el propósito de mostrar el desempeño del método híbrido, se presentan experimentos y su discusión correspondiente. La propuesta se puede extender a otras estructuras con cadenas cinemáticas cerradas, o a la solución en general de sistemas de ecuaciones no-lineales, y por supuesto, para problemas de optimización no-lineales.The direct kinematics problem for parallel robots can be stated as follows: given values of the joint variables, the corresponding Cartesian variable values, the pose of the end-effector, must be found. Most of the times the direct kinematics problem involves the solution of a system of non-linear equations. The most efficient methods to solve such kind of equations assume convexity in a cost function which minimum is the solution of the non-linear system. In consequence, the capacity of such methods depends on the knowledge about an starting point which neighboring region is convex, hence the method can find the global minimum. This article propose a method based on probabilistic learning about an adequate starting point for the Dogleg method which assumes local convexity of the function. The proposed method efficiently avoids the local minima, without need of human intervention or apriori knowledge, thus it shows a more robust performance than the simple Dogleg method or other gradient based methods. To demonstrate the performance of the proposed hybrid method, numerical experiments and the respective discussion are presented. The proposal can be extended to other structures of closed-kinematics chains, to the general solution of systems of non-linear equations, and to the minimization of non-linear functions.Peer Reviewe

A Mathematical Model to Study the Meningococcal Meningitis

AbstractThe main goal of this work is to introduce a novel mathematical model to study the spreading of meningococcal meningitis. Specifically, it is a discrete mathematical model based on cellular automata where the population is divided in five classes: sus- ceptible, asymptomatic infected, infected with symptoms, carriers, recovered and died. It catches the individual characteristics of people in order to give a prediction of both the individual behavior, and whole evolution of population

Integral field spectroscopy of type-II QSOs at z=0.3-0.4

We present and analyse integral-field observations of six type-II QSOs with z=0.3-0.4, selected from the Sloan Digital Sky Survey (SDSS). Two of our sample are found to be surrounded by a nebula of warm ionized gas, with the largest nebula extending across 8" (40 kpc). Some regions of the extended nebulae show kinematics that are consistent with gravitational motion, while other regions show relatively perturbed kinematics: velocity shifts and line widths too large to be readily explained by gravitational motion. We propose that a ~20 kpc x20 kpc outflow is present in one of the galaxies. Possible mechanisms for triggering the outflow are discussed. In this object, we also find evidence for ionization both by shocks and the radiation field of the AGN.Comment: 6 pages, 5 figures, accepted by MNRAS Letter