High-efficiency 20 GHz traveling wave tube development for space communications

A 75 watt CW high efficiency helix TWT operating at 20 GHz was developed for satellite communication systems. The purpose was to extend the performance capabilities of helix TWTs by using recent technology developments. The TWT described is a unique design because high overall efficiency is obtained with a low perveance beam. In the past, low perveance designs resulted in low beam efficiencies. However, due to recent breakthoughs in diamond rod technology and in collector electrode materials, high efficiencies can now be achieved with low perveance beams. The advantage of a low perveance beam is a reduction in space charge within the beam which translates to more efficient collector operation. In addition, this design incorporates textured graphite electrodes which further enhance collector operation by suppressing backstreaming secondaries. The diamond supported helix circuit features low RF losses, high interaction impedance, good thermal handling capability and has been designed to compensate for the low perveance beam. One more discussed tube feature is the use of a velocity taper in the output helix that achieves low signal distortion while maintaining high efficiency

Donnan equilibrium and the osmotic pressure of charged colloidal lattices

We consider a system composed of a monodisperse charge-stabilized colloidal suspension in the presence of monovalent salt, separated from the pure electrolyte by a semipermeable membrane, which allows the crossing of solvent, counterions, and salt particles, but prevents the passage of polyions. The colloidal suspension, that is in a crystalline phase, is considered using a spherical Wigner-Seitz cell. After the Donnan equilibrium is achieved, there will be a difference in pressure between the two sides of the membrane. Using the functional density theory, we obtained the expression for the osmotic pressure as a function of the concentration of added salt, the colloidal volume fraction, and the size and charge of the colloidal particles. The results are compared with the experimental measurements for ordered polystyrene lattices of two different particle sizes over a range of ionic strengths and colloidal volume fractions.Comment: 8 pages, 4 Postscript figures, uses multicol.sty, to be published in European Physical Journal

Where the linearized Poisson-Boltzmann cell model fails: (II) the planar case as a prototype study

The classical problem of two uniformly charged infinite planes in electrochemical equilibrium with an infinite monovalent salt reservoir is solved exactly at the mean-field nonlinear Poisson-Boltzmann (PB) level, including an explicit expression of the associated nonlinear electrostatic contribution to the semi-grand-canonical potential. A linearization of the nonlinear functional is presented that leads to Debye-H\"uckel-like equations agreeing asymptotically with the nonlinear PB results in the weak-coupling (high-temperature) and counterionic ideal-gas limits. This linearization scheme yields artifacts in the low-temperature, large-separation or high-surface charge limits. In particular, the osmotic-pressure difference between the interplane region and the salt reservoir becomes negative in the above limits, in disagreement with the exact (at mean-field level) nonlinear PB solution. By using explicitly gauge-invariant forms of the electrostatic potential we show that these artifacts -- although thermodynamically consistent with quadratic expansions of the nonlinear functional -- can be traced back to the non-fulfillment of the underlying assumptions of the linearization. Explicit comparison between the analytical expressions of the exact nonlinear solution and the corresponding linearized equations allows us to show that the linearized results are asymptotically exact in the weak-coupling and counterionic ideal-gas limits, but always fail otherwise, predicting negative osmotic-pressure differences.Comment: 24 pages, 3 PostScript figures, submitted to J. Chem. Phy

Liquid-state theory of charged colloids

A simple theory of the fluid state of a charged colloidal suspension is proposed. The full free energy of a polyelectrolyte solution is calculated. It is found that the counterions condense onto the polyions forming clusters composed of one polyion and n counterions. The distribution of cluster sizes is determined explicitly. In agreement with the current experimental and Monte Carlo results, no liquid-gas phase separation was encountered.Comment: 4 pages, 2 Postscript figures, uses multicol.sty; changed conten

The one-component plasma: a conceptual approach

The one-component plasma (OCP) represents the simplest statistical mechanical model of a Coulomb system. For this reason, it has been extensively studied over the last forty years. The advent of the integral equations has resulted in a dramatic improvement in our ability to carry out numerical calculations, but came at the expense of a physical insight gained in a simpler analytic theory. In this paper we present an extension of the Debye-Hueckel (DH) theory to the OCP. The theory allows for analytic calculations of all the thermodynamic functions, as well as the structure factor. The theory explicitly satisfies the Stillinger-Lovett and, for small couplings, the compressibility sum rules, implying its internal self consistency.Comment: 28 pages, 5 Postscript figures, uses elsart.sty, accepted for publication in Physica

Absolute Calibration of the Auger Fluorescence Detectors

Absolute calibration of the Pierre Auger Observatory fluorescence detectors uses a light source at the telescope aperture. The technique accounts for the ombined effects of all detector components in a single measurement. The calibrated 2.5 m diameter light source fills the aperture, providing uniform illumination to each pixel. The known flux from the light source and the response of the acquisition system give the required calibration for each pixel. In the lab, light source uniformity is studied using CCD images and the intensity is measured relative to NIST-calibrated photodiodes. Overall uncertainties are presently 12%, and are dominated by systematics.Comment: 4 pages, 3 figure. Submitted to the 29th ICRC, Pune, Indi

Constraint-induced movement therapy: reviews of clinical trials

INTRODUÇÃO: A Terapia por Contensão Induzida (TCI) é uma terapêutica que visa à recuperação da função do membro superior (MS) parético de pacientes com sequelas de um acidente vascular cerebral por meio de treinamento intensivo, uso de uma restrição no MS não-parético durante 90% do dia e pelo pacote de transferência. OBJETIVO: O objetivo deste trabalho é realizar uma revisão entre os estudos que compararem a TCI com outras terapêuticas. METODOLOGIA: Foi realizada uma pesquisa nas bases de dados MEDLINE, SciELOo e ISI Web of Science com o descritor constraint induced movement therapy durante o mês de junho de 2008. RESULTADOS: Foram encontrados seis artigos onde a TCI foi comparada com os métodos Bobath e Kabat. Discussão: Dentre os artigos encontrados, percebemos que há uma variação entre as amostras de cada estudo, assim como o método de avaliação entre as terapêuticas. CONCLUSÃO: A TCI mostrou-se superior em relação às outras terapêuticas.INTRODUCTION: The Constraint-induced Movement Therapy (CIMT) is a therapeutic which main goal is the functional recuperation of paretic upper extremity (UE) of stroke survivors by an intensive treatment, wear of restriction in non-paretic UE during 90% of activity day and by for the transfer package. OBJECTIVE: The main of this paper is to do a review between studies which compare the CIMT between other therapeutics. METHODS: Had been made an search between MEDLINE, SciELO and ISI Web of Science with the principal word constraint induced movement therapy during the month of June of the 2008. RESULTS: We founded six articles where the CIMT was compared with the therapeutics Bobath and Kabat. Discussion: Inside the articles founded, we observed a variation between the population in each study, as by the outcomes to evaluate the therapeutics. CONCLUSION: The CIMT showed itself superior by the other therapeutics.Universidade São MarcosUniversidade Federal de São Paulo (UNIFESP) Departamento de Neurologia/NeurocirurgiaUNIFESP, Depto. de Neurologia/NeurocirurgiaSciEL