No description available.

This work was motivated by questions raised from multifrequency microwave interferometer measurements taken in a cylindrical plasma on the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) project. Standard data analysis based on a thin beam model neglecting refraction yields inconsistent electron densities and density profiles for different frequencies. This work focuses on the development of a model for the wave propagation through cylindrical plasmas when the plasma radius is on the order of the beam waist. For the calculations presented a Gaussian beam profile and plasma spatial profile were assumed. Both refraction by density gradients and finite beam sizes are found to play important roles and explain polychromatic differences in the electron densities and profiles. Calculations for the new model are compared to a thin beam model not accounting for refraction and experimental data from VASIMR.

published_or_final_version / abstract / toc / Mechanical Engineering / Doctoral / Doctor of Philosophy

Η παρούσα διπλωματική εργασία καταπιάνεται με τη μελέτη της συμπεριφοράς των φωτοβολταϊκών πλαισίων σε καταπόνηση με κρουστικές τάσεις κεραυνών. Πιο συγκεκριμένα, εφαρμόστηκαν σε ένα φωτοβολταϊκό πλαίσιο κρουστικές τάσεις, συμφωνά με τον κανονισμό IEC 61730-2,και μετρήθηκε το αν και κατά πόσο επηρεάστηκε η λειτουργία του πλαισίου. Το πρώτο κεφάλαιο παρέχει το θεωρητικό πλαίσιο της έρευνας. Αναλυτικότερα, παρέχεται η αρχή λειτουργίας του μονοκρυσταλλικού πλαισίου, και αναφέρονται τα τεχνικά χαρακτηριστικά αυτού που χρησιμοποιήσαμε στην παρούσα εργασία. Παρατίθεται η αρχή λειτουργίας της κρουστικής γεννήτριας που χρησιμοποιήθηκε στο πειραματικό σκέλος και δίνεται ο ορισμός της κρουστικής τάσης. Τέλος, δίνονται πληροφορίες ως προς τη μετρητική συσκευή που χρησιμοποιήθηκε για τη μελέτη της καλής λειτουργίας του πλαισίου. Το δεύτερο κεφάλαιο σχετίζεται με το πειραματικό κομμάτι της έρευνας. Πιο συγκεκριμένα, παρατίθενται οι κρουστικές τάσεις που έπληξαν το πλαίσιο και γίνεται εκτενής και σαφής αναφορά στον κανονισμό IEC 61730-2 που υπαγορεύει τους κανόνες διεξαγωγής του πειράματος. Τέλος, παρουσιάζονται οι ακριβείς συνθήκες διεξαγωγής της πειραματικής διαδικασίας ενώ περιέχονται φωτογραφίες που πάρθηκαν κατά τη διάρκεια υλοποίησης της έρευνας. Στο τρίτο κεφάλαιο καταγράφονται τα αποτελέσματα των μετρήσεων καλής λειτουργίας του φωτοβολταϊκού πλαισίου και παρουσιάζονται σε πίνακες και χαρακτηριστικές I-V. Αποτελέσματα που πάρθηκαν τόσο πριν όσο και μετά από κάθε καταπόνηση. Κλείνοντας, στο τέταρτο και τελευταίο κεφαλαίο, εξάγονται συμπεράσματα και γίνεται εκτενής ανάλυση των αποτελεσμάτων που καταγράφηκαν στο κεφάλαιο 3. Τα τελικά συμπεράσματα σχετίζονται με το αν επηρεάστηκε ή όχι η καλή λειτουργία του κυττάρου. Ακόμα, παρουσιάζονται κάποιες αδυναμίες της έρευνας μας καθώς και κάποια ερωτήματα-προτάσεις περεταίρω διευρένησης, που μας γεννήθηκαν κατά την ενασχόλησή μας με το θέμα. / This thesis deals with the study of the behavior of the modules when applying lightning impulse voltages. In particular, percussive tendencies were applied to a solar cell, according to the regulation IEC 61730-2, and it was measured whether and how the functioning of the framework was affected. The first chapter provides the theoretical framework of the research. More specific, it is given the operating principle of the single crystal framework and there are referred the technical features which were used in this work. It is presented the operating principle of the impulse generator used in the experimental arm and it is given the definition of “voltage impulse”. Finally, there are presented information on the measuring device used to the study of the proper functioning of the framework. The second chapter is related to the experimental part of the investigation. To be more specified, there are given the percussive tendencies that affected cell and a detailed and clear reference to IEC 61730-2 regulation that dictates the rules of our experiments. Finally, we present the precise conditions under which the experimental procedure was done and there are contained photographs which were took during the implementation of the survey. In the third chapter, they are displayed the results of the measurements proper operation of photovoltaic panel and they are presented in tables and characteristic IV. The results were obtained both before and after each exposure Finally, in the fourth and final chapter, conclusions are drawn and there is a detailed analysis of the results recorded in chapter 3. The final conclusions relating to whether or not the proper cell function was affected . In conclusion, they are presented some shortcomings in our research and some questions (further recommendations for research), there were entered to our minds during our involvement with the issue.

No description available.

Ultra-wideband (UWB) is a revolutionary radio communication system that utilizes a large portion of the frequency spectrum while maintaining low power levels and high data rates. UWB systems can be used both indoors and outdoors within the power-level masks regulated by the Federal Communications Commission, thus making the technology very versatile. One of the main advantages of UWB is its robustness to multi-path diversity. The technology has attracted the interests of research and industry alike, owing to the possibility of implementing low-power, low-complexity, and low-cost devices. A widely recognized method of transmitting UWB signals is the use of Impulse Radio technology to transmit information. Impulse Radio Ultra-Wideband (IR-UWB) uses repetitive pulses of very short duration, low duty cycle, and low power levels within FCC regulations. One implementation of IR-UWB pulses in non-coherent transmission is the use of Differential Code-Shifted Reference (DCSR) pulses. In this technique, one of the main challenges at the receiver is pulse-level synchronization that times the received pulses at the right moments for accurate pulse detection. This thesis will introduce two design proposals in attempt to achieve the pulse synchronization. The first proposal is based on a fast-switch-controlled integrator circuit, while the second focuses on the use of an active low pass filter and phase-locked loop circuits to achieve proper clock timing. Both proposals will be presented, together with schematics, computer-aided simulations, and lab tests results.

<p>This dissertation investigates the use of intraprocedure acoustic radiation force impulse (ARFI) imaging for visualization of radiofrequency ablation (RFA) lesions during cardiac transcatheter ablation (TCA) procedures. Tens of thousands of TCA procedures are performed annually to treat atrial fibrillation (AF) and other cardiac arrhythmias. Despite the use of sophisticated electroanatomical mapping (EAM) techniques to validate the modification of the electrical substrate, post-procedure arrhythmia recurrence is common due to incomplete lesion delivery and electrical conduction through lesion line discontinuities. The clinical demand for an imaging modality that can visually confirm the presence and completeness of RFA lesion lines motivated this research.</p><p>ARFI imaging is an ultrasound-based technique that transmits radiation force impulses to locally displace tissue and uses the tissue deformation response to generate images of relative tissue stiffness. RF-induced heating causes irreversible tissue necrosis and contractile protein denaturation that increases the stiffness of the ablated region. Preliminary in vitro and in vivo feasibility studies determined RF ablated myocardium appears stiffer in ARFI images.</p><p>This thesis describes results for ARFI imaging of RFA lesions for three research milestones: 1) an in vivo experimental verification model, 2) a clinically translative animal study, and 3) a preliminary clinical feasibility trial in human patients. In all studies, 2-D ARFI images were acquired in normal sinus rhythm and during diastole to maximize the stiffness contrast between the ablated and unablated myocardium and to minimize the bulk cardiac motion during the acquisition time.</p><p>The first in vivo experiment confirmed there was a significant decrease in the measured ARFI-induced displacement at ablation sites during and after focal RFA; the displacements in the lesion border zone and the detected lesion area stabilized over the first several minutes post-ablation. The implications of these results for ARFI imaging methods and the clinical relevance of the findings are discussed.</p><p>The second and third research chapters of this thesis describe the system integration and implementation of a multi-modality intracardiac ARFI imaging-EAM system for intraprocedure lesion evaluation. EAM was used to guide the 2-D ARFI imaging plane to targeted ablation sites in the canine right atrium (RA); the presence of EAM lesions markers and conduction disturbances in the local activation time (LAT) maps were used to find the sensitivity and specificity of predicting the presence of RFA lesion with ARFI imaging. The contrast and contrast-to-noise ratio between RFA lesion and unablated myocardium were calculated for ARFI and conventional ICE images. The opportunities and potential developments for clinical translation are discussed. </p><p>The last research chapter in this thesis describes a feasibility study of intracardiac ARFI imaging of RFA lesions in clinical patients. ARFI images of clinically relevant ablation sites were acquired, and this pilot study determined ARFI-induced displacements in human myocardium decreased at targeted ablation sites after RF-delivery. The challenges and successes of this pilot study are discussed.</p><p>This work provides evidence that intraprocedure ARFI imaging is a promising technology for the visualization of RFA lesions during cardiac TCA procedures. The clinical significance of this research is discussed, as well as challenges and considerations for future iterations of this technology aiming for clinical translation.</p> / Dissertation

This thesis reports on the investigations of resistive-capacitive (RC) loaded bow-tie antennas with special emphasis on impulse ground penetrating radar applications. Impulse radiation for ground penetrating radar is a challenging research topic because of the unique problem arising from impulse radiation: late-time ringing, which usually masks the important echo signals from the targets. While resistive loading is a common solution for eliminating late-time ringing, use of resistive loading typically sacrifices the radiation efficiency. In this thesis, a resistive-capacitive loading technique is investigated for a circular bow-tie antenna in the attempt to reduce/suppress the late-time ringing as well as to maintain a relatively high radiation efficiency. To implement the system, a microstrip differentiator, which converts a monopulse into a Gaussian-like monocycle to be used as input impulse, is presented. Further, specially designed coplanar waveguide/coplanar strip (CPW/CPS) baluns embedded with Chebyshev transformers of characteristic impedance up to 120 have been constructed and tested. To evaluate the system, instead of using the conventional peak voltage value of the radiated waveform, average radiated energy, average ringing energy, relative radiation efficiency and relative ringing efficiency are utilized and these metrics are easily established using low-cost low-sensitivity probes. Measurement results show that the RC-loading scheme is functioning as expected and the impulse system as a whole is capable of reducing the late-time ringing energy to 50% while maintaining average radiation energy as 83% when compared with capacitive loading cases.

When a microphone array is mounted on a mobile aerial platform, such as an unmanned aerial vehicle (UAV), most existing beamforming methods cannot be used to adequately identify continuous and impulsive ground. Here, numerical simulation results and laboratory experiments are presented that validate a proposed time-frequency beamforming method based on the Multiple Signal Classification (MUSIC) algorithm to detect these acoustic sources from a mobile aerial platform. In the numerical simulations three parameters were varied to test the proposed algorithm’s location estimation performance: 1) the acoustic excitation types; 2) the moving receiver’s simulated flight conditions; and 3) the number of acoustic sources. Also, a distance and angle error analysis was done to quantify the proposed algorithm’s source location estimation accuracy when considering microphone positioning uncertainty. For experimental validation, three laboratory experiments were conducted. Source location estimations were done for: a 600 Hz sine source, a banded white noise source between 700-800 Hz, and a composite source combined simultaneously with both the sine and banded white noise sources. The proposed algorithm accurately estimates the simulated monopole’s location coordinates no matter the excitation type or simulated trajectory. When considering simultaneously-excited, multiple monopoles at high altitudes, e.g. 50 m, the proposed algorithm had no error when estimating the source’s locations. Finally, a distance and angle error analysis exposed how relatively small microphone location error, e.g. 1 cm maximum error, can propagate into large averaged distance error of about 10 m in the far-field for all monopole excitation types. For all simulations, however, the averaged absolute angle error remained small, e.g. less than 4 degrees, even when considering a 5 cm maximum microphone location error. For the laboratory experiments, the sine source had averaged distance and absolute angle errors of 0.9 m and 14.07 degrees from the source’s true location, respectively. Similarly, the banded white noise source’s averaged distance and absolute angle errors were 1.9 m and 47.14 degrees; and lastly, the averaged distance and absolute angle errors of 0.78 m and 8.14 degrees resulted when both the sources were simultaneously excited.

This thesis reports on the investigations of resistive-capacitive (RC) loaded bow-tie antennas with special emphasis on impulse ground penetrating radar applications. Impulse radiation for ground penetrating radar is a challenging research topic because of the unique problem arising from impulse radiation: late-time ringing, which usually masks the important echo signals from the targets. While resistive loading is a common solution for eliminating late-time ringing, use of resistive loading typically sacrifices the radiation efficiency. In this thesis, a resistive-capacitive loading technique is investigated for a circular bow-tie antenna in the attempt to reduce/suppress the late-time ringing as well as to maintain a relatively high radiation efficiency. To implement the system, a microstrip differentiator, which converts a monopulse into a Gaussian-like monocycle to be used as input impulse, is presented. Further, specially designed coplanar waveguide/coplanar strip (CPW/CPS) baluns embedded with Chebyshev transformers of characteristic impedance up to 120 have been constructed and tested. To evaluate the system, instead of using the conventional peak voltage value of the radiated waveform, average radiated energy, average ringing energy, relative radiation efficiency and relative ringing efficiency are utilized and these metrics are easily established using low-cost low-sensitivity probes. Measurement results show that the RC-loading scheme is functioning as expected and the impulse system as a whole is capable of reducing the late-time ringing energy to 50% while maintaining average radiation energy as 83% when compared with capacitive loading cases.