A BOUNDED WAVE TRANSMISSION
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Abstract
The important aspects in the design, construction and performance of a Nuclear Electromagnetic Pulse (NEMP) simulator are described with consideration of (a) reliably simulating the NEMP environment in a limited experimental volume without nuclear detonation and, (b) to measure accurately the time varying E, D-dot and B-dot field waveforms by suitable non-interfering sensors. Plane-wave simulation of the radiated electromagnetic fields is done through the Quasi-Transverse Electromagnetic (TEM) bound fields of the transmission line. An active volume of 300 mm (L) x 150 mm (W) x 50 mm (H) is chosen for electronic sub-systems 'and components' hardness/vulnerability assessment. The Bell Lab specified electromagnetic pulse waveform - the difference of two exponentials, is selected for synthesizing the circuit parameters of the transmission line guide. I. INTRODUCTIONThe threat perception arising from the electromagnetic environment generated in the event of a nuclear (NEMP)[1], has resulted in wide ranging debates that are evident from the voluminous publication of numerous scientific papers. It is of utmost importance to (a) reliably simulate such an environment in a limited experimental volume without nuclear detonation, (b) measure accurately the time varying E and B field waveforms by suitable non-interfering sensors, (c) assess vulnerability thresholds of electronic circuits and (d) evolve electromagnetic hardness assurance by shielding and transient surge-suppression techniques. This paper addresses the issues in (a) and (b) mentioned above. The need to develop an NEMP simulation facility was perceived with a view to evaluate the electronic sub-systmes' and components' vulnerability and to harden them to sustain the eventuality of an exo-atmospheric nuclear detonation. Though, adequate analytical tools are available and many more being developed for vulnerability predictions, validation through experimental simulations while complementing the analysis improves the confidence level in the predictions thus made. |
The Bounded Wave Transmission Line (BWTL) NEMP simulator developed in-house [2], consists - as the name suggests - a parallel plate aluminium structure tapered down at both ends to an aluminium ground plane thereby constituting a flat plate transmission line geometry. The simulating volume is 'bounded' within the parallel plate region. The electromagnetic fields in the dominant transverse electromagnetic (TEME) mode within the structure very closely simulate the free space EM wave propagation. The electric field remains perpendicular to the plates but fringes at the edges. The magnetic field in the uniform field region is parallel to the plates. The two field vectors are transverse to the direction of wave propagation. The dimensions of the structure are sufficient enough to accommodate and uniformly illuminate within the test volume a standard sized printed circuit board. Though the IEC specified double exponential curve fit for the early-tile field is the latest accepted norm [3], the more stringent original Bell Laboratories specified electromagnetic pulse waveform is selected for synthesizing the circuit parameters of the transmission line guide. An important off shoot of this simulator development was that it necessitated the development of the fast transient field sensors. E-field, D-dot and B-dot field sensors had to be developed to measure these field quantities within the simulator volume to accurately determine the field magnitudes and their corresponding wave shapes the spatial variations of the peak electric field strengths along and across the simulator axis are plotted. The facility has built-in provision to measure the terminated load current by means of a co-axial current shunt integrated along with the copper sulphate load. Though, no repetition rate requirement is stipulated to NEMP simulation, a 2 - 5 Hz repetition rate on the existing facility can be very easily achieved. It is not feasible to change the direction of field polarization in this facility. Though, the difficulty can be partially obviated by changing the orientations of the Device Under Test (DUT), the facility still lacks to take into account the ground reflections that are more likely in an actual event. Barring some of these limitations, a transmission line type simulator has the advantages of achieving peak field |
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