A Closed Time-Reversal Cavity for Electromagnetic Waves in Transmission Line Networks

A closed time-reversal (TR) cavity has been considered a purely theoretical concept whose experimental realization was assumed to be impossible. In this article, we demonstrate that it is, in fact, possible to realize a closed TR cavity using a network of transmission lines. The network is excited by either lumped voltage or current sources at arbitrary locations, and it is terminated on matched impedances. This system is an exact closed TR cavity in the sense that, in the backward-propagation stage, by back-injecting time-reversed measured responses (voltage or current) generated by a source at each line terminal, a time-reversed copy of the current and voltage distribution in the forward-propagation stage is obtained. We report for the first time an experimental realization of a TR cavity formed by a network of inhomogeneous transmission lines, in which the spatial and temporal signal distribution along the network is reproduced as in a sequence of the system state (i.e., voltages and or current along the lines) being played in reverse. We also discuss the interfering effect of the so-called diverging wave for observation points in the vicinity of the source. An active realization of a TR sink is proposed to effectively overcome the interfering effect caused by the diverging waves.

Mario Paolone, Reza Razzaghi, Marcos Rubinstein, Zhaoyang Wang

A closed time-reversal cavity has been considered a purely theoretical concept whose experimental realization was assumed to be impossible. In this paper, we demonstrate that it is, in fact, possible to realize a closed time-reversal cavity using a network of transmission lines. The network is excited by either lumped voltage or current sources at arbitrary locations, and it is terminated on matched impedances. This system is an exact closed time-reversal cavity in the sense that, in the backward-propagation stage, by back-injecting time-reversed measured responses (voltage or current) generated by a source at each line terminal, a time-reversed copy of the current and voltage distribution in the forward-propagation stage is obtained. We report for the first time an experimental realization of a time-reversal cavity formed by a network of inhomogeneous transmission lines, in which the spatial and temporal signal distribution along the network is reproduced as in a sequence of the system state (i.e., voltages and or current along the lines) being played in reverse. We also discuss the interfering effect of the so-called diverging wave for observation points in the vicinity of the source. An active realization of a time-reversal sink is proposed to effectively overcome the interfering effect caused by the diverging waves.

2020

Electromagnetic Time Reversal: Theory, Application to Fault Location in Power Networks and Experimental Validation

Zhaoyang Wang

The time-reversal invariance is readily described and universally embodied in most laws of nature, either in a strict or in a soft sense. Based on this underlying physical principle, time-reversal theory has been theoretically and experimentally studied in the field of acoustics first, and later introduced into electromagnetism, under the general denomination of Electromagnetic Time Reversal (EMTR). With the development of modern power system, especially represented by the technology of Smart Grids and Active Distribution Networks (ADNs), a reliable and efficient fault detection and location functionality is increasingly required to ensure the security and quality of supply. It has recently been shown that an EMTR-based fault location technique can provide superior performance with respect to existing techniques in terms of location accuracy and robustness against uncertainties. At the same time, several theoretical and technological challenges have to be addressed before such a technique can be used in real-world applications. The present work aims at addressing these challenges, which are summarized hereunder. Time reversal cavity applied to power networks. The concept of the time-reversal cavity was proposed by Cassereau and Fink for acoustic waves and later extended to electromagnetic waves. It is accepted that a time-reversal cavity is a theoretical concept and cannot be realized experimentally because it requires in principle an infinite number of transducers covering a closed surface around the medium to obtain information about all wavefronts propagating in all directions. Although this is true in a 3D situation, we believe that in the case of power networks in which waves are confined in a specific region of the space and propagate along the longitudinal direction of the lines (1D situation), such a time-reversal cavity is realizable. We propose to design, simulate, and experimentally validate such a cavity for the case of inhomogeneous power networks. Improving the performance of EMTR-based fault locating techniques. The currently-developed EMTR-based technique requires multiple simulations to identify the location of the fault. We will investigate possible ways of reducing the number of required simulation runs. Practical implementation of an EMTR-based fault locator. The EMTR-based method technologically requires the use of a hardware platform, such as a Field Programmable Gate Array (FPGA), capable of performing Electromagnetic Transient (EMT) simulations and post Digital Signal Processing (DSP). In order to address constraints of application in real-world power networks and robustness requirements, further improvement and optimization will be considered, namely, the determination of the minimum time-reversal window required to achieve a certain location accuracy, the development of a library of transmission lines and cables commonly used in power networks, analysis of the location error caused either by incomplete knowledge of the network, or other disturbances such as those originated by lightning, as well as limitations associated with the measurement. The final aim of the thesis is to further develop a fault location hardware platform integrating EMTR theory with numerous functionalities, like EMT simulations, DSP, interactive communication. Its performance will be verified by hardware-in-the-loop tests, and eventually by live tests in real power networks.

EPFL2020

Fluid-dynamic and thermo-mechanical analyses of the Monoblock Mitre Bend for the ITER electron cyclotron Upper launcher

René Chavan, Timothy Goodman, Avelino Mas Sanchez

The ITER Electron Cyclotron Heating (ECH) system will be used to counteract magneto-hydrodynamic plasma instabilities by aiming up to 20 MW of mm-wave power at 170 GHz. The primary vacuum boundary at the Electron Cyclotron Upper Launcher (EC UL) extends into the port cell region through eight beamlines, defining the so-called First Confinement System (FCS). Each beamline, designed for the transmission of 1.31 MW, is delimited by the closure plate at the port plug back-end and by a diamond window in the port cell. The FCS essentially consists of a Z-shaped set of straight corrugated waveguides (WG) connected by Mitre Bends (MB) with a nominal inner diameter of 50 mm. Due to the space restrictions, the eight MBs at the last FCS section are grouped into two monoblocks. Each Monoblock Mitre Bend (MBMB) consists of a body with corrugated feedthroughs defining a specific angle for each beamline and four mirrors attached by bolted connection to reflect the mm-wave power. The thermal expansion arising from the ohmic losses, the cooling pressure, the bolt pre-tension and the imposed displacements coming from the connection with the transmission lines are the primary design loads for the MBMBs. The fluid-dynamic analyses performed for both upper and lower MBMBs show that highest temperature takes place in the MB mirrors, reaching a maximum value of 203 degrees C at the beam center. The thermomechanical analyses demonstrate that the peak stress also occurs at this location with a maximum value of 324 MPa. These stresses are categorized and compared with the allowable limits defined in the ASME code, what probes that the current design of both upper and lower MBMBs are able to withstand the loads taking place during the mm-wave normal operation.

ELSEVIER SCIENCE SA2021

Electromagnetic Time Reversal: Theory, Application to Fault Location in Power Networks and Experimental Validation

Zhaoyang Wang

EPFL2020