Mechanical Engineering - Ph.D. / Sc.D.
http://hdl.handle.net/1721.1/7683
2014-09-03T04:45:42ZA "permanent" high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen
http://hdl.handle.net/1721.1/88893
A "permanent" high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen
Haid, Benjamin J. (Benjamin John Jerome), 1974-
This thesis explores a new design for a portable "permanent" superconducting magnet system. The design is an alternative to permanent low-temperature superconducting (LTS) magnet systems where the magnet is cooled by a bath of liquid helium. The new design involves a high-temperature superconducting (HTS) magnet that is cooled by a solid heat capacitor. An apparatus was constructed to demonstrate stable operation of a permanent magnet wound with Bi2223/Ag conductor while in thermal communication with a mass of solid nitrogen. The system includes a room-temperature bore and can func tion while it stands alone, detached from its cooling source, power supply, and vacuum pump. The magnet is operated in the 20-40 K temperature range. This apparatus is the first to demonstrate the operation of a superconducting magnet with a permis sible temperature variation exceeding a few degrees kelvin while a magnetic field is maintained for a useful duration. Models are developed to predict the experimental system's warming trend and magnetic field decay. The models are validated with a good agreement between simulations based on these models and experimental results. Potential performance advantages of a solid nitrogen cooled permanent HTS (SN2/HTS) magnet system over a liquid helium cooled LTS (LHe/LTS) system are explored for various applications. The SN2/HTS system design includes a second solid heat capacitor that cools a radiation shield. Recooling of the heat capacitors is performed with a detachable cryocooler. The SN2/HTS system offers both improved stability and improved portabilit over an LHe/LTS system design.; (cont.) Design codes are constructed to comiiie the SN2/HTS system design with a LHe/LTS design for two different applicatiohs. The first application is a general permanent superconducting magnet employing a room temperature bore. The second application is a superconducting mine countermeasures system (SCMCM) that is used to sweep passive magnetic influence mines. The codes predict the important system attributes, namely minimum volume and minimum weight, that should be expected fora given set of design requirements (i.e. field magnitude and bore size, or magnetic dipole moment) and a given set of conductor properties. Their results indicate that present HTS conductor critical current and index are not yet sufficient for producing SN2/HTS systems of a size that is comparable to that expected for a LHe/LTS system. However, the conductor properties of Bi2223/Ag have been consistently improving over the last decade, and new HTS conductors are expected to be developed in the near future. Therefore, the codes are used to determine the minimum HTS properties that are necessary for constructing a cryocooled SN2/HTS system with a size comparable to that expected for a LHe/LTS system.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.; Includes bibliographical references (p. 249-254).
2001-01-01T00:00:00ZHigh precision stereo profilometry
http://hdl.handle.net/1721.1/88892
High precision stereo profilometry
Aumond, Bernardo Dantas, 1972-
Metrological data from sample surfaces can be obtained by using a variety of profilome try methods. Atomic Force Microscopy (AFM), which relies on contact inter-atomic forces to extract topographical images of a sample, is one such method that can be used on a wide range of surface types, with possible nanometer resolution (both vertical andlateral). However, AFM images are commonly distorted by convolution, which reduces metrological accuracy. This type of distortion is more significant when the sample surface containshigh aspect ratio features such as lines, steps or sharp edges or when probe and sample share similar characteristic dimensions. Therefore, as the size of engineered features arepushed into the micrometer and sub-micrometer range by the development of new high precision fabrication techniques, convolution distortions embedded in the images becomeincreasingly more significant. Aiming at mitigating these distortions and recovering metrology sound ness, we introduce a novel image deconvolution scheme based on the principle of stereo imaging. Multiple images of a sample, taken at different angles, allow for separation ofcon volution artifacts from true topographic data. As a result, accurate samplereconstruction and probe shape estimation can be achieved simultaneously. Additionally, shadow zones, which are areas of the sample that cannot be reached by the AFM probe, are greatly re duced. Most importantly, this technique does not require a priori probe characterizationor any sort of shape assumption. It also reduces the need for slender or sharper probes,which, on one hand, induce less convolution distortion but, on the other hand, are more prone to wear and damage, thus decreasing the overall inspection system reliability.; (cont.) This research project includes a survey of current high precision metrology tools and an in-depthanalysis of the state of the art deconvolution techniques for probe based metrology instruments. Next, the stereo imaging algorithm is introduced, simulation results presented and anerror analysis is conducted. Finally, experimental validations of the technique are carried outfor an industrial inspection application where the characteristic dimensions of the samplesare in the nanometer range. The technique was found to be robust and insensitive to probe or shape geometries. Furthermore, the same framework was deemed to be applicable to other probe based imaging techniques such as mechanical stylus profilometers and scanning tunneling microscopy.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.; Includes bibliographical references (leaves 186-190).
2001-01-01T00:00:00ZDynamics and stability of gravity-capillary solitary waves
http://hdl.handle.net/1721.1/88871
Dynamics and stability of gravity-capillary solitary waves
Calvo, David C. (David Christopher)
Over the past several years, it has been recognized that a new class of solitary waves can propagate in nonlinear dispersive wave systems if the phase speed of linear waves attains a local extremum at some finite wavenumber. Near such a point, solitary waves in the form of small-amplitude wavepackets can be obtained for which the phase speed of the carrier oscillations matches the group speed of their envelope. Such an extremum is found in the analysis of water waves when the restoring forces of both gravity and surface tension are taken into account, and certain kinds of these gravity-capillary solitary waves have been observed in experiments. While past theoretical studies have focussed mainly on determining steady solitary wave profiles, very little work has been done on examining their stability properties which is the thrust of this thesis. Beginning in the weakly nonlinear regime, an asymptotic analysis of linear stability is presented and comparison is made with numerical computations. Contrary to predictions of the nonlinear Schrbdinger (NLS) equation, some free solitary wave types are found to be unstable owing to exponentially effects terms that lie beyond standard two-scale perturba- tion theory. Moreover, numerical simulations show that unstable gravity-capillary solitary waves may decompose into stable solitary waves that have soliton properties. Stability results are then extended to the fully nonlinear regime to treat both free and forced situa- tions using numerical techniques to solve the full hydrodynamic equations in steady form. A dramatic difference is found between the linear stability of free and forced waves in both weakly and fully nonlinear cases, and results obtained here are compared with laboratory experiments.; (cont.) The analysis followed in the free-surface problem is then generalized to examine the dynamics of gravity-capillary interfacial solitary waves in a layered two-fluid system. Here, the linear stability and limiting wave forms of free solitary waves are determined over a range of system parameters using the full hydrodynamic equations. Finally, a related problem of gravity-capillary envelope solitons is considered under the general situation of unequal phase and group speeds. By asymptotic and numerical techniques it is found that envelope solitons are generally nonlocal-tails are radiated owing to a resonance mechanism that is beyond the NLS equation.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.; Includes bibliographical references (leaves 137-143).
2001-01-01T00:00:00Z3D reconstruction from 2D images and applications to cell cytoskeleton
http://hdl.handle.net/1721.1/88870
3D reconstruction from 2D images and applications to cell cytoskeleton
Cheng, Yuan, 1971-
Approaches to achieve three dimensional (3D) reconstruction from 2D images can be grouped into two categories: computer-vision-based reconstruction and tomographic reconstruction. By exploring both the differences and connections between these two types of reconstruction, the thesis attempts to develop a new technique that can be applied to 3D reconstruction of biological structures. Specific attention is given to the reconstruction of the cell cytoskeleton from electron microscope images. The thesis is composed of two parts. The first part studies computer-vision-based reconstruction methods that extract 3D information from geometric relationship among images. First, a multiple-feature-based stereo reconstruction algorithm that recovers the 3D structure of an object from two images is presented. A volumetric reconstruction method is then developed by extending the algorithm to multiple images. The method integrates a sequence of 3D reconstruction from different stereo pairs. It achieves a globally optimized reconstruction by evaluating certainty values of each stereo reconstruction. This method is tuned and applied to 3D reconstruction of the cell cytoskeleton. Feasibility, reliability and flexibility of the method are explored.; (cont.) The second part of the thesis focuses on a special tomographic reconstruction, discrete tomography, where the object to be reconstructed is composed of a discrete set of materials each with uniform values. A Bayesian labeling process is proposed as a framework for discrete tomography. The process uses an expectation-maximization (EM) algorithm with which the reconstruction is obtained efficiently. Results demonstrate that the proposed algorithm achieves high reconstruction quality even with a small number of projections. An interesting relationship between discrete tomography and conventional tomography is also derived, showing that discrete tomography is a more generalized form of tomography and conventional tomography is only a special case of such generalization.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, February 2001.; Includes bibliographical references (leaves 121-129).
2001-01-01T00:00:00Z