Alfvén wave far field from steady-current tethers

We analyzed he Alfvén waves ignaturele ft behindi n the ionospherbe y orbiting insulatedt etherso peratinga s thrusterso r generatorws ith steadyc urrentsU. sing a recentd escription of tetherr adiationt o determineth e far field, we showt hat, at their leadinge dges,t heA lfv6n wingsh avea n Airy functionc rosswises tructureT. he field amplitudef alls off weakly as the inversec uber oot of the distancea longt he wavef ront.T he fronti tself, whichc arriesa negligible fractiono f the powerr adiateda sA lfven waves,b roadensp roportionallyto the cuber oot of that distanceW. e showt hatc ollisionadl ecayb ecomesim portanat t about1 03k m alongt he front.F or tethersl ongert han about2 km, the top andb ottoms tructures("wings")a re fully disjoint

Air Entrainment Processes in a Circular Plunging Jet: Void-Fraction and Acoustic Measurements

Circular plunging jets were studied by both void fraction and acoustic techniques. There were two aims: to measure the structure of the jet flow and its regimes as a function of jet speed and free-jet length; and to develop and validate the acoustic measurement technique in the developing flow. Void fractions and bubble count rates were measured in the developing shear layer of a large-size plunging jet (d_1 = 25 mm). The data compared well with a solution of an advective diffusion equation and showed an increased air entrainment rate with increasing free-jet length for x_1 /d_1 =/ < 12. The acoustic data were processed by a novel technique to extract both bubble count and bubble size data. Three plunging jet flow regimes were noted. Near inception, acoustic pulses are isolated and indicate individual bubble entrainment as observable visually. Above a characteristic jet velocity, the number of the bubble pulses increases sharply although bubbles are still produced intermittently. At higher velocities, bubble production becomes quasi-continuous. The study suggests that an acoustic technique calibrated through detailed laboratory measurements can provide useful, absolute data in high-void fraction flows. The robust acoustic sensor can then be used in hostile industrial or environmental flows where more delicate instruments are impractical

On the use of electron-multiplying CCDs for astronomical spectroscopy

Conventional CCD detectors have two major disadvantages: they are slow to read out and they suffer from read noise. These problems combine to make high-speed spectroscopy of faint targets the most demanding of astronomical observations. It is possible to overcome these weaknesses by using electron-multiplying CCDs (EMCCDs). EMCCDs are conventional frame-transfer CCDs, but with an extended serial register containing high-voltage electrodes. An avalanche of secondary electrons is produced as the photon-generated electrons are clocked through this register, resulting in signal amplification that renders the read noise negligible. Using a combination of laboratory measurements with the QUCAM2 EMCCD camera and Monte Carlo modelling, we show that it is possible to significantly increase the signal-to-noise ratio of an observation by using an EMCCD, but only if it is optimised and utilised correctly. We also show that even greater gains are possible through the use of photon counting. We present a recipe for astronomers to follow when setting up a typical EMCCD observation which ensures that maximum signal-to-noise ratio is obtained. We also discuss the benefits that EMCCDs would bring if used with the next generation of extremely large telescopes. Although we mainly consider the spectroscopic use of EMCCDs, our conclusions are equally applicable to imaging.Comment: 18 figures, 3 tables, 18 page

PZT Sensor Arrays for Integrated Damage Monitoring in Concrete Structures

The broad objective of the work reported here is to provide a fundamental basis for the use of Lead Zirconate Titanate (PZT) patches in damage detection of concrete structures. Damage initiation in concrete structures starts with distributed microcracks, which eventually localize to form cracks. By the time surface manifestation in the form of visible cracking appears there may be significant degradation of the capacity of the structure. Early detection of damage, before visible signs appear on the surface of the structure is essential to initiate early intervention, which can effectively increase the service life of structures. Development of monitoring methodologies involves understanding the underlying phenomena and providing a physical basis for interpreting the observed changes in the parameters which are sensed. PZT is a piezoelectric material, which has a coupled constitutive relationship. In the case of the PZT patches bonded to a concrete structure, any sensing strategy requires developing an understanding of the coupled electromechanical (EM) response of the PZT-concrete system. The challenges associated with the use of PZT patches for damage monitoring in a concrete substrate include providing the following: a clear understanding of the fundamental response of the PZT patch when bonded to a concrete substrate; interpretation of the coupled response of the PZT patch under load induced damage; and development of an efficient, continuous monitoring methodology to sense a large area of the concrete substrate. Due to a lack of a fundamental basis, the use of PZT patches in concrete structures often involves inferring the measured response using model-based procedures. The work outlined in this thesis addresses the key issue of developing the theoretical basis and providing an experimental validation for PZT-based damage monitoring methodology for concrete structures. A fundamental understanding of response of the PZT patch when bonded to concrete substrate is developed. The outcome of the work is an integrated local and distributed sensing methodology for concrete structures by combining the electromechanical impedance and stress wave propagation methods using an array of bonded PZT patches. The work presented in this thesis is focused on using PZT patches bonded to a concrete substrate. A fundamental understanding of the coupled electromechanical behaviour of a PZT patch under an applied electrical excitation in an electrical impedance (EI) measurement, is developed. The influence of the substrate size and its material properties on the frequency dependent EI response of a PZT patch is investigated using concrete substrates of different sizes. The dynamic response of a PZT patch is shown to consist of resonance modes of the PZT patch with superimposed structural response. The resonance behaviour of the PZT patch is shown to be influenced by the material properties of the substrate. The size dependence in the EI response of a PZT patch bonded to a concrete substrate is produced by the dynamic behaviour of the structure. The size of the local zone of the concrete material substrate in the vicinity of the bonded PZT patch, which influences the frequency dependent EI response of the PZT patch is identified. For each resonant mode, a local zone of influence, which is free from the influence of boundary is identified. The dynamic response of the PZT resonant mode is influenced by the elastic material properties and damping within the zone of influence. The structural effects of the concrete substrate produced by the finite size of the specimen are separated from the material effects produced by the material properties and the material damping in the coupled EM response of the bonded PZT patch. The influence of size of the concrete substrate on the coupled impedance response of the PZT is identified with peaks of structural resonance, which are superimposed on the resonant peaks of the bonded PZT patch The EI response of the PZT patch when bonded to concrete for detecting load-induced damage from distributed microcrack to localized cracks within the zone of influence of the PZT patch is investigated. Using an approach which combines an understanding of the coupled EM constitutive behaviour of PZT with experimental validation, a methodology is developed to decouple the effects of stress and damage in the substrate on the coupled EM response of a PZT patch. The features in the EI signature of a bonded PZT patch associated with stress and damage are identified. An increasing level of distributed damage in the concrete substrate produces a decrease in the magnitude and the frequency of the resonant peak of the bonded PZT patch. The substrate stress produces a counter acting effect in the EI spectrum of the bonded PZT patch. A measurement procedure for the use of bonded PZT patches for continuous monitoring of stress-induced damage in the form of distributed microcracks in a structure under loading is developed. An integrated methodology for damage monitoring in concrete structures is developed by combining the EI method for local sensing and the stress wave propagation-based method in a distributed sensing mode. An array of surface mounted PZT sensors are deployed on a concrete beam. The EI measurements from individual PZT sensors are used for detecting damage within the local zone of influence. PZT sensor pairs are used as actuators and sensors for distributed monitoring using stress wave propagation. A stress-induced crack is introduced in a controlled manner. It is detected very accurately from the full-field displacement measurement obtained using digital image correlation. The crack opening profile in concrete produced by the fracture is established from the surface displacement measurements. From the measurements of bonded PZTs, the localized crack is detected in the zone of influence by EI. The change in compliance of the material medium due to a localized crack is small and it is reflected in the smaller change in the measured EI when compared to distributed damage. Stress wave based measurements sensitively detect crack openings on the order of 10m. The material discontinuity produced by a closed crack, after removal of the stress is also detected. A damage matrix is developed for stress wave based method which is independent of transmission path to assess the severity of damage produced by the crack in a concrete structure

Plasma Dynamics

Contains reports on two research projects.U. S. Energy Research and Development Administration (Contract E(l1-1)-3070)National Science Foundation (Grant ENG75-06242

Investigation of photoelectrons from molecules in a strong field

When intense and few-cycle laser pulses interact with an atomic/molecular beam highorder Above Threshold Ionisation (HATI) can take place. The resulting electrons can be re-scattered from the parent atoms/molecules and gain kinetic energy. The HATI electrons contain information about the atomic/molecular structure thus providing a method to probe atomic and molecular dynamics with sub-fs temporal and sub-angström spatial resolution. In this thesis the development of a Velocity Map Imaging apparatus capable of measuring the two-dimensional (2D) projection of the velocity distribution of electrons with energies up to 400 eV is described. This device was implemented with a molecular beam apparatus to study the electron re-scattering process that occurs when atoms/molecules are subjected to strong laser fields. Time-of-fight measurements were carried out to find the molecular beam. To perform the experiments a method to generate intense and few-cycle pulses based on hollow fibre pulse compression has been implemented. Pulses of 14 fs with energies of 500 μJ have been measured in a differentially pumped fibre set-up, with input pulses of 42 fs and 700 μJ using a home-made Frequency-Resolved Optical Gating device. The performance of the VMI apparatus was investigated by first studying the ATI rings formed by low energy electrons. Then, a study of the high energy electrons was carried out in different gases and re-scattered electrons with energies up to 100 eV were measured. The photoelectron spectra recorded with linearly polarised laser exhibit a plateau with a cut-off at 10 UP that is a characteristic of the re-scattering process. The observation of rescattered electrons was confirmed by two techniques: comparison of the data obtained with vertical polarisation (re-scattering) and circular polarisation (no re-scattering) and analysis of the structure in the angular distribution obtained in Xenon

The flight of the GAPS prototype experiment

The General AntiParticle Spectrometer experiment (GAPS) is foreseen to carry out a dark matter search using low-energy cosmic ray antideuterons at stratospheric altitudes with a novel detection approach. A prototype flight from Taiki, Japan was carried out in June 2012 to prove the performance of the GAPS instrument subsystems (Lithium-drifted Silicon tracker and time-of-flight) and the thermal cooling concept as well as to measure background levels. The flight was a success and the stable flight operation of the GAPS detector concept was proven. During the flight about 106 charged particle triggers were recorded, extensive X-ray calibrations of the individual tracker modules were performed by using an onboard X-ray tube, and the background level of atmospheric and cosmic X-rays was measured. The behavior of the tracker performance as a function of temperature was investigated. The tracks of charged particle events were reconstructed and used to study the tracking resolution, the detection efficiency of the tracker, and coherent X-ray backgrounds. A timing calibration of the time-of-flight subsystem was performed to measure the particle velocity. The flux as a function of flight altitude and as a function of velocity was extracted taking into account systematic instrumental effects. The developed analysis techniques will form the basis for future flights