Relativistic current collection by a cylindrical Langmuir probe

The current I to a cylindrical Langmuir probe with a bias Φp satisfying β≡eΦp/mec2∼O(1) is discussed. The probe is considered at rest in an unmagnetized plasma composed of electrons and ions with temperatureskTe∼kTi≪mec2. For small enough radius, the probe collects the relativistic orbital-motion-limited (OML) current I OML , which is shown to be larger than the non-relativistic result; the OML current is proportional to β1/2 and β3/2 in the limits β≪1 and β≫1, respectively. Unlike the non-relativistic case, the electron density can exceed the unperturbed density value. An asymptotic theory allowed to compute the maximum radius of the probe to collect OML current, the sheath radius for probe radius well below maximum and how the ratio I/I OML drops below unity when the maximum radius is exceeded. A numerical algorithm that solves the Vlasov-Poisson system was implemented and density and potential profiles presented. The results and their implications in a possible mission to Jupiter with electrodynamic bare tethers are discussed density value. An asymptotic theory allowed to compute the maximum radius of the probe to collect OML current, the sheath radius for probe radius well below maximum and how the ratio I/IOML drops below unity when the maximum radius is exceeded. A numerical algorithm that solves the Vlasov-Poisson system was implemented and density and potential profiles presented. The results and their implications in a possible mission to Jupiter with electrodynamic bare tethers are discussed.

Self-consistent numerical dispersion relation of the ablative Rayleigh-Taylor instability of double ablation fronts in inertial confinement fusion

The linear stability analysis of accelerated double ablation fronts is carried out numerically with a self-consistent approach. Accurate hydrodynamic profiles are taken into account in the theoretical model by means of a fitting parameters method using 1D simulation results. Numerical dispersión relation is compared to an analytical sharp boundary model [Yan˜ez et al., Phys. Plasmas 18, 052701 (2011)] showing an excellent agreement for the radiation dominated regime of very steep ablation fronts, and the stabilization due to smooth profiles. 2D simulations are presented to validate the numerical self-consistent theory.

Exploring polysilicon deposition conditions through a laboratory CVD prototype

Cost and energy consumption related to obtaining polysilicon impact significantly on the total photovoltaic module cost and its energy payback time. Process simplifications can be performed, leading to cost reductions. Nowadays, among several approaches currently pursued to produce the so called Solar Grade Silicon, the chemical route, named Siemens process, is the dominant one. At the Instituto de Energía Solar research on this topic is focused on the chemical route, in particular on the polysilicon deposition step by chemical vapor deposition (CVD) from Trichlorosilane through a laboratory prototype. Valuable information about the phenomena involved in the polysilicon deposition process and the operating conditions is obtained from our experiments. A particular feature of our system is the inclusion of a mass spectrometer. The present work comprises spectra characterization of the polysilicon deposition chemical reaction, temperature and inlet gas mixture composition influence on the deposition rate and analysis of polysilicon deposition conditions for the ?pop-corn' phenomenon to appear, based on experimental experience (Actas de la Special Issue: E-MRS 2012 Spring Meeting ? Symposium A

Supersonic regime of the Hall-magnetohydrodynamics resistive tearing instability

An earlier analysis of the Hall-magnetohydrodynamics (MHD) tearing instability [E. Ahedo and J. J. Ramos, Plasma Phys. Controlled Fusion 51, 055018 (2009)] is extended to cover the regime where the growth rate becomes comparable or exceeds the sound frequency. Like in the previous subsonic work, a resistive, two-fluid Hall-MHD model with massless electrons and zero-Larmor-radius ions is adopted and a linear stability analysis about a force-free equilibrium in slab geometry is carried out. A salient feature of this supersonic regime is that the mode eigenfunctions become intrinsically complex, but the growth rate remains purely real. Even more interestingly, the dispersion relation remains of the same form as in the subsonic regime for any value of the instability Mach number, provided only that the ion skin depth is sufficiently small for the mode ion inertial layer width to be smaller than the macroscopic lengths, a generous bound that scales like a positive power of the Lundquist number

Two-dimensional plasma expansion in a magnetic nozzle: Separation due to electron inertia

A previous axisymmetric model of the supersonic expansion of a collisionless, hot plasma in a divergent magnetic nozzle is extended here in order to include electron-inertia effects. Up to dominant order on all components of the electron velocity, electron momentum equations still reduce to three conservation laws. Electron inertia leads to outward electron separation from the magnetic streamtubes. The progressive plasma filling of the adjacent vacuum region is consistent with electron-inertia being part of finite electron Larmor radius effects, which increase downstream and eventually demagnetize the plasma. Current ambipolarity is not fulfilled and ion separation can be either outwards or inwards of magnetic streamtubes, depending on their magnetization. Electron separation penalizes slightly the plume efficiency and is larger for plasma beams injected with large pressure gradients. An alternative nonzero electron-inertia model [E. Hooper, J. Propul. Power 9, 757 (1993)] based on cold plasmas and current ambipolarity, which predicts inwards electron separation, is discussed critically. A possible competition of the gyroviscous force with electron-inertia effects is commented briefly.

La ciudad interior : infinitud y concavidad en la no-stop city (1970- 1971)

A finales de los 60 se había hecho evidente como la tecnificación del ambiente había permitido a algunas tipologías (supermercados, aparcamientos, fábricas) alcanzar profundidades construidas potencialmente ilimitadas e independizarse del afuera. La No-Stop City nace de una idea sencilla: extender esta tecnificación a la totalidad de lo construido para englobar, no sólo la práctica totalidad de funciones, sino, en última instancia, toda la ciudad. Esta operación tiene efectos paradójicos: a medida que la arquitectura crece, pierde la mayoría de características que la han definido tradicionalmente. Una disolución por hipertrofia que da lugar a un espacio homogéneo, cóncavo y potencialmente infinito. Pero, además de la pura factibilidad técnica, existen dos influencias clave y aparentemente contradictorias que explican esta apuesta por una ciudad interior e ilimitada: el marxismo y al Pop Art. El proyecto es, en muchos sentidos, un manifiesto construido que refleja la militancia de los miembros del grupo en el seno del marxismo italiano. Pero es también la plasmación del interés declarado del grupo por el Pop Art, la cultura popular y la sociedad de masas. La influencia cruzada de comunismo y consumismo explica esta “utopía cuantitativa” en la que se hacen coincidir la sociedad y la fábrica, la producción y el consumo. Una ciudad basada en la centralidad de los objetos de consumo y la subsiguiente pérdida de protagonismo de la arquitectura, en la que lo urbano, al tiempo que se extiende sin límites sobre el territorio, ignorando su exterioridad rural, disuelve el hogar como ámbito de privacidad, ignorando su interioridad doméstica. Un proyecto que, en la estela también de Marshall McLuhan, ilustra como pocos la conversión de lo urbano en una “condición” virtualmente omnipresente y que nos sigue interrogando con preguntas que son, por otra parte, eternas: ¿Qué es un edificio? ¿Qué es una ciudad?.

Damping models in the truncated derivative nonlinear Schrödinger equation

Four-dimensional flow in the phase space of three amplitudes of circularly polarized Alfven waves and one relative phase, resulting from a resonant three-wave truncation of the derivative nonlinear Schrödinger equation, has been analyzed; wave 1 is linearly unstable with growth rate , and waves 2 and 3 are stable with damping 2 and 3, respectively. The dependence of gross dynamical features on the damping model as characterized by the relation between damping and wave-vector ratios, 2 /3, k2 /k3, and the polarization of the waves, is discussed; two damping models, Landau k and resistive k2, are studied in depth. Very complex dynamics, such as multiple blue sky catastrophes and chaotic attractors arising from Feigenbaum sequences, and explosive bifurcations involving Intermittency-I chaos, are shown to be associated with the existence and loss of stability of certain fixed point P of the flow. Independently of the damping model, P may only exist as against flow contraction just requiring.In the case of right-hand RH polarization, point P may exist for all models other than Landau damping; for the resistive model, P may exist for RH polarization only if 2+3/2.

Structure of intermediate shocks in collisionsless anisotropic Hall-magnetohydrodynamics plasma models

The existence of discontinuities within the double-adiabatic Hall-magnetohydrodynamics (MHD) model is discussed. These solutions are transitional layers where some of the plasma properties change from one equilibrium state to another. Under the assumption of traveling wave solutions with velocity C and propagation angle θ with respect to the ambient magnetic field, the Hall-MHD model reduces to a dynamical system and the waves are heteroclinic orbits joining two different fixed points. The analysis of the fixed points rules out the existence of rotational discontinuities. Simple considerations about the Hamiltonian nature of the system show that, unlike dissipative models, the intermediate shock waves are organized in branches in parameter space, i.e., they occur if a given relationship between θ and C is satisfied. Electron-polarized (ion-polarized) shock waves exhibit, in addition to a reversal of the magnetic field component tangential to the shock front, a maximum (minimum) of the magnetic field amplitude. The jumps of the magnetic field and the relative specific volume between the downstream and the upstream states as a function of the plasma properties are presented. The organization in parameter space of localized structures including in the model the influence of finite Larmor radius is discussed

The Interior City Infinity and Concavity in the No-Stop City (1970-1971)

In the late 60s it had become clear how the environment technification had allowed some typologies (supermarkets, car parks, factories) to reach potentially unlimited built depths becoming, therefore, independent from the outside. The No-Stop City is born from a very simple idea: to extend this technification to the totality of built reality encompassing, not only almost all functions, but ultimately, the whole city. This operation has paradoxical effects: as architecture grows, it loses most of the features that have traditionally defined it. A dissolution by hypertrophy that gives rise to an homogeneous, concave and potentially infinite space. But beyond the pure technical feasibility, there are two key influences, seemingly contradictory, that explain this endeavor for an interior and endless city: Marxism and Pop Art. The project is, in many senses, a built manifesto reflecting the militancy of the group members within the Italian Marxism. But it is also the embodiment of the groups declared interest in Pop Art, popular culture and mass society. The cross-influence of communism and consumerism explains this "quantitative utopia" in which the society and the factory, the production and consumption, would match. A city based on the centrality of consumer products and the subsequent loss of prominence of architecture, in which the urban phenomenon, while spreading endlessly over territory, ignoring its rural exteriority, dissolves the home as a sphere of privacy, ignoring its domestic interiority. A project, also in the wake of Marshall McLuhan, that illustrates like few others the conversion of the urbane into a virtually omnipresent "condition" and that still interrogates us with questions that are, on the other hand, eternal: What is a building? What is a city?

Pneumatic Serendipity

El término inglés serendipity hace referencia al hallazgo de algo interesante, por casualidad, o a la habilidad de reconocer un descubrimiento –aunque este no tenga exactamente relación con lo que se está buscando. Durante los últimos cuatro años, Pneumatic Serendipity ha buscado de manera intencionada la coincidencia, el accidente, que permita establecer nuevas relaciones a través de la construcción de prototipos. A lo largo de diferentes talleres y seminarios, se han ido proponiendo experimentos que lleven a los estudiantes a reflexionar sobre la forma, estructura y construcción en el proyecto arquitectónico. Experimentos planificados por los tutores de los talleres –y que forman parte de una investigación más extensa–, pero espontáneos en la exploración y análisis por parte de los alumnos. Todos esos son aspectos que no solo resultan especialmente indisociables en las estructuras neumáticas sino que, a través de ellos, se llega a poner en cuestión la idea misma de habitar y percibir la arquitectura. La vuelta a las cuestiones que animaron el debate de la Arquitectura Radical de los años 60 y 70, en el momento de máxima apertura a esa clase de experimentaciones, ha sido fundamental para formar una conciencia común sobre las propuestas desarrolladas. Con ello se ha animado a la discusión y anclaje de los proyectos a temas de aquel contexto, reinterpretados en la lógica de la época del digital y de la comunidad 2.0 –la tecnología en relación al espesor o la ligereza, las revueltas sociales, la arquitectura-protesta y la arquitectura pop-up, la crisis energética y la energía en el proyecto de arquitectura.

Direct Vlasov simulations of electron-attracting cylindrical Langmuir probes in flowing plasmas

Current collection by positively polarized cylindrical Langmuir probes immersed in flowing plasmas is analyzed using a non-stationary direct Vlasov-Poisson code. A detailed description of plasma density spatial structure as a function of the probe-to-plasma relative velocity U is presented. Within the considered parametric domain, the well-known electron density maximum close to the probe is weakly affected by U. However, in the probe wake side, the electron density minimum becomes deeper as U increases and a rarified plasma region appears. Sheath radius is larger at the wake than at the front side. Electron and ion distribution functions show specific features that are the signature of probe motion. In particular, the ion distribution function at the probe front side exhibits a filament with positive radial velocity. It corresponds to a population of rammed ions that were reflected by the electric field close to the positively biased probe. Numerical simulations reveal that two populations of trapped electrons exist: one orbiting around the probe and the other with trajectories confined at the probe front side. The latter helps to neutralize the reflected ions, thus explaining a paradox in past probe theory.