<p>Nonlinear transmission lines (NLTLs) are of great interest for high power microwave (HPM) generation because they can sharpen pulses to create an electromagnetic shockwave to produce oscillations from 100 MHz to low GHz. NLTLs provide frequency agility, compactness, durability and reliability, providing a solid-state radiofrequency (RF) source for producing HPM. The essential component of NLTLs is the nonlinear material, typically a dielectric that varies with voltage or a magnetic material whose permeability varies with current, incorporated in the transmission line in various topologies. This thesis presents an alternative approach involving designing composites comprised of nonlinear dielectric inclusions (barium strontium titanate (BST)) and/or nonlinear inductive inclusions (nickel zinc ferrites (NZF)) in a polymer base host material, analogous to electromagnetic interference designs that incorporate stainless steel inclusions of various shapes in a plastic to tune the composite’s electromagnetic properties at GHz. Appropriately designing NLTL composites requires predicting these effective properties both in linear (for a fixed and low voltage and current) and nonlinear regions (permittivity and permeability become voltage dependent and current dependent, respectively) prior to designing HPM systems comprised of them. As a first step, this thesis evaluates and benchmarks composites models in the commercial software CST Microwave Studios (CST MWS) to various effective medium theories (EMTs) to predict the permittivity and permeability of composites of BST and/or NZF inclusions in the linear regime, compared with experimental measurements. The manufacturing and measurement of the nonlinear composites will be briefly discussed with an analysis of the homogeneity of a composite sample using 3D X-ray scan. Long-term application of these approaches to predicting the effective nonlinear composite permittivity and permeability and future work will be discussed.</p>

<p>Nonlinear transmission lines (NLTLs) provide a solid state alternative to conventional vacuum based high power microwave (HPM) sources. The three most common NLTL implementations are the lumped element, split ring resonator (SRR), and the nonlinear bulk material based NLTLs. The nonlinear bulk material implementation provides the highest power output of the three configurations, though they are limited to pulse voltages less than 50 kV; higher voltages are possible when an additional insulator is used, typically SF₆ or dielectric oil, between the nonlinear material and the outer conductor. The additional insulator poses a risk of leaking if structural integrity of the outer conductor is compromised. The desire to provide a fieldable NLTL based HPM system makes the possibility of a leak problematic. The work reported here develops a composite based NLTL system that can withstand voltages higher than 50 kV and not pose a risk of catastrophic failure due to a leak while also decreasing the size and weight of the device and increasing the output power.</p> <p>Composites with barium strontium titanate (BST) or nickel zinc ferrite (NZF) spherical inclusions mixed in a silicone matrix were manufactured at volume fractions ranging from 5% to 25%. The dielectric and magnetic parameters were measured from 1-4 GHz using a coaxial airline. The relative permittivity increased from 2.74±0.01 for the polydimethylsiloxane (PDMS) host material to 7.45±0.33 after combining PDMS with a 25% volume fraction of BST inclusions. The relative permittivity of BST and NZF composites was relatively constant across all measured frequencies. The relative permeability of the composites increased from 1.001±0.001 for PDMS to 1.43±0.04 for a 25% NZF composite at 1 GHz. The relative permeability of the 25% NZF composite decreased from 1.43±0.05 at 1 GHz to 1.17±0.01 at 4 GHz. The NZF samples also exhibited low dielectric and magnetic loss tangents from 0.005±0.01 to 0.091±0.015 and 0.037±0.001 to 0.20±0.038, respectively, for all volume fractions, although the dielectric loss tangent did increase with volume fraction. For BST composites, all volume fraction changes of at least 5% yielded statistically significant changes in permittivity; no changes in BST volume fraction yielded statistically significant changes in permeability. For NZF composites, the change in permittivity was statistically significant when the volume fraction varied by more than 5% and the change in permeability was statistically significant for variations in volume fraction greater than 10%. The DC electrical breakdown strength of NZF composites decreased exponentially with increasing volume fraction of NZF, while BST composites exhibited no statistically significant variation with volume fraction. </p> <p>For composites containing both BST and NZF, increasing the volume fraction of either inclusion increased the permittivity with a stronger dependence on BST volume fraction. Increasing NZF volume fraction increased the magnetic permeability, while changing BST volume fraction had no effect on the composite permeability. The DC dielectric breakdown voltage decreased exponentially with increased NZF volume fraction. Adding as little as 5% BST to an NZF composite more than doubled the breakdown threshold compared to a composite containing NZF alone. For example, adding 10% BST to a 15% NZF composite increased the breakdown strength by over 800%. The combination of tunability of permittivity and permeability by managing BST and NZF volume fractions with the increased dielectric breakdown strength by introducing BST make this a promising approach for designing high power nonlinear transmission lines with input pulses of hundreds of kilovolts.</p> <p>Coaxial nonlinear transmission lines are produced using composites with NZF inclusions and BST inclusions and driven by a Blumlein pulse generator with a 10 ns pulse duration and 1.5 ns risetime. Applying a 30 kV pulse using the Blumlein pulse generator resulted in frequencies ranging from 1.1 to 1.3 GHz with an output power over 20 kW from the nonlinear transmission line. The output frequencies increased with increasing volume fraction of BST, but the high power oscillations characteristic of an NLTL did not occur. Simulations using LT Spice demonstrated that an NLTL driven with a Blumlein modulator did not induce high power oscillations while driving the same NLTL with a pulse forming network did. </p> <p>Finally, a composite-based NLTL could be driven directly by a high voltage power supply without a power modulator to produce oscillations both during and after the formed pulse upon reaching a critical threshold. The output frequency of the NLTLs is 1 GHz after the pulse and ranged from 950 MHz to 2.2 GHz during the pulse. These results demonstrate that the NLTL may be used as both a pulse forming line and high power microwave source, providing a novel way to reduce device size and weight, while the use of composites could provide additional flexibility in pulse output tuning. </p>

Society’s dependence on electronic and electrical systems has increased rapidly over the past few decades, and people are relying more and more on these gadgets in their daily life because of the efficiency in operation which these systems can offer. This has revolutionized many areas of electrical and electronics engineering including power sector, telecommunication sector, transportation and many other allied areas. With progress in time, the sophistication in the systems also increased. Also as the systems size reduced from micro level to nano level, the compactness of the systems increased. This paved the way for development in the digital electronics leading to new and efficient IC 0s that came into existence. Power sector also faced a resurge in its technology. Most of the analog meters are now replaced by digital meters. The increased sophistication and compactness in the digital system technology made it susceptible to electromagnetic interference especially from High Power Electromagnetic Sources. Communication, data processing, sensors, and similar electronic devices are vital parts of the modern technological environment. Damage or failures in these devices could lead to technical or financial disasters as well as injuries or the loss of life. Electromagnetic Interference (EMI) can be explained as any malicious generation of electromagnetic energy introducing noise or signals into electric and electronic systems, thus disrupting, confusing or damaging these systems. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. These effects can range from a simple degradation of data to a total loss of data. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit. The sources of electromagnetic interference can be either unintentional or intentional. The sources producing electromagnetic interference can be of different power levels, different frequency of operation and of different field strength. One such classification of these sources are the High Power Electromagnetic Sources (HPEM) High Power Electromagnetic environment refers to sources producing very high peak electromagnetic fields at very high power levels. These power levels coupled with the extremely high magnitude of the fields are sufficient to cause disastrous effects on the electrical and electronic systems. There has been a lot of developments in the field of the source technology of HPEM sources so that they are now one of the strongest sources of electromagnetic interference. High Power Electromagnetic environment refers to the sources producing very high peak electromagnetic fields at very high power levels. These power levels coupled with the extremely high magnitude of the fields are sufficient to cause disastrous effects on the electrical and electronic systems. HPEM environments are categorized based on the source characteristics such as the peak electric field, often called threat level, frequency coverage or bandwidth, average power density and energy content. The sources of electromagnetic interference can be either unintentional or intentional. Some examples of unintentional sources are the increased use of electromagnetic spectrum which generates disturbance to various systems operating in that frequency band, poor design of systems without taking care of other systems present nearby as well as lightning. Intentional sources are High altitude Electromagnetic Pulse (HEMP) or Nuclear Electromagnetic Pulse (NEMP) due to nuclear detonations, Ultra Wide Band (UWB) field from Impulse Radiating Antennas (IRA), Nar-row band fields like those coming from High Power Microwaves (HPM), High Intensity Radio Frequency (HIRF) sources. Of these the lightning is natural and all other sources are man-made. The significant progress in the Intentional High-Power Electromagnetic (HPEM) sources and antenna technologies and the easy access to simple HPEM systems for anyone entail the need to determine the susceptibility of electronic equipment as well as coupling of these fields with systems such as cables (buried as well as aerial), airborne vehicle etc. to these types of threats. Buried cables are widely used in the communication and power sectors due to their efficient functioning in urban cities and towns. These cables are more prone to electromagnetic interferences from HPEM sources. The buried communication cables or even the buried data cables are connected to sensitive equipments and hence even a slight rise in the voltage or the current at the terminals of the equipments can become a serious problem for the smooth operation of the system. In the first part of the thesis the effect of the electromagnetic field due to these sources on the cables laid underground has been studied. The second part of this thesis deals with the study of the interaction of the EM field from the above mentioned HPEM sources with an airborne vehicle. Airborne vehicle and its payload are extremely expensive so that any destruction to these as a result of the voltages and currents induced on the vehicle on account of the incoming HPEM fields can be quite undesirable. The incoming electromagnetic fields will illuminate the vehicle along its axis which results in the induction of currents and voltages. These currents and voltages will get coupled to the internal control circuits that are extremely sensitive. If the induced voltage/ current magnitude happen to be above the damage threshold level of these circuits then it will result in either a malfunction of the circuit or a permanent damage of it, with both of them being detrimental to the success of the mission. This will even result in the abortion of the mission or possible degradation of the vehicle performance. Hence it is worthwhile to see what will be the influence of an incoming HPEM electromagnetic field on the airborne vehicle with and without the presence of an exhaust plume. In this work, the HPEM sources considered are NEMP, IRA and HPM. The electromagnetic fields produced by the EMP can induce large voltage and current transients in electrical and electronic circuits which can lead to a possible malfunction or permanent damage of the systems. The electric field at the earth 0s surface can be modelled as a double exponential pulse as per the IEC standard 61000-2-9. The NEMP field incident on the earth’s surface is considered as that coming from a source at a distance far away from the earth’s surface; hence a plane wave approximation has been used. Impulse radiating antennas are the ones that are used as the major source of ultra wide band radiation. These are highly powerful antennas that use a pulsed power source as the input and this power source is conditioned to get an extremely sharp rise time pulse. These antennas are very high power antennas that are capable of producing a significant electromagnetic field. Impulse radiating antenna is a paraboloidal reflector and hence is an aperture antenna. Initially the radiated field due to this aperture needs to be found out at any observation point from the antenna. In this thesis, the aperture distribution method is used to accurately determine the field due to the aperture. In this method the field reflected from the surface of the reflector is first found on an imaginary plane through the focal point of the reflector that is normal to the axis of the reflector, by using the principles of geometrical optics, which then is extended to the observation point. The IRA considered for the present work is the one of the most powerful IRA as per the published literature available in the open domain. This has an input voltage of 1.025 MV. The far field electric field measured at the boresight (at r =85 m) being equal to 62 kV/m, and the uncorrected pulse rise time (10%-90%) is 180 ps for this IRA. HPM sources are usually electromagnetic radiators having a reflector with a horn antenna kept at their focal point for excitation. HPM sources generally operate in single mode or at tens or hundreds of Hz repetition rates. Many HPM radiators are developed in the world each with their own peculiar geometry and power levels. In the present thesis, a single waveguide (WR-975) fed HPM antenna assembly has been studied. The chosen waveguide has a cut-o_ frequency of 1 GHz and a power level of 10 GW. The wavelength associated with the waveguide is 0.3 m. The field pattern shows a definite peak in its response when the frequency is 1 GHz, the cut-off frequency of the waveguide. The electric field coming out of the HPEM sources travel through the medium that is either air alone or a combination of air and soil respectively depending upon whether the circuit on which the coupling is analysed is an airborne vehicle or an underground cable. The media plays a major role in the coupling, as the field magnitude is influenced by the characteristic properties of the media. As height increases the magnitude of the electric field decreases for all types of sources and also the time before which the field waveform starts is increased. The electric field in the soil is decided by the soil properties such as its conductivity and permittivity. The soil is modelled in frequency domain and the high frequency behaviour of soils is considered with its conductivity and permittivity taken as functions of frequency, as the incident field has high frequency components. A soil medium can be electromagnetically viewed as a four component dielectric mixture consisting of soil particles, air voids, bound water, and free water. When electric field is incident on the soil, it gets polarized. This is as a result of a wide variety of processes, including polarization of electrons in the orbits around atoms, distortion of molecules, reorientation of water molecules, accumulation of charge at interfaces, and electrochemical reactions. Whatever is the HPEM source, an increase in the soil conductivity results in an increased attenuation of the field. Also there is a significant loss of high frequency components in the GHz range in the field due to the selective absorption by the soil. This effect causes the percentage attenuation to be maximum for HPM and minimum for NEMP and IRA lying in between these two extremities. Increase in permittivity of the soil causes attenuation of the electric field for all HPEM sources. This is due to the relaxation mechanisms in the soil due to atomic- or molecular-scale resonances. The coupling of the electromagnetic fields due to HPEM sources is considered in the first phase. Two cables are considered (i) buried shielded and (ii) buried shielded twisted pair cables. The results are arrived at using the Enhanced Transmission Line model. The induced current is more for a shielded cable than a twisted pair cable of the same configuration. The induced current magnitude depends upon the type of the HPEM source, the depth of burial of the cable and the point on the cable where the current/ voltage is computed. Current is maximum at the centre of the cable for a matched termination and the voltage is the minimum at this point. The ratio of the induced current in the inner conductor with respect to the shield current of a shielded cable is the least for an HPM, and maximum for NEMP. This is due to the fact that higher frequencies are absorbed more by the shield of the cable. This affects HPM induced current the maximum and NEMP the least because of the presence of the lower frequency components in NEMP. Induced current in the twisted pair cable depends upon the number of pairs of the cable and the pitching of the cable. The electromagnetic field from the HPEM sources propagates with less attenuation in air due to the lower resistance this medium offers for electromagnetic wave propagation. Hence any system in air, be it electrical or electronic, will be under the strong illumination by these electromagnetic fields. As the second part of this thesis, the influence of the electromagnetic fields from all the three HPEM sources on an airborne vehicle in flight is analysed. For this part of study, the Electromagnetic (EM) fields radiated by all the three sources at different heights from the earth 0s surface have been computed. The coupling study has been done for the case of a vehicle with plume as well as without plume. For the second case, the electromagnetic modelling of the plume has been done taking into consideration its conductivity, which in turn depends on the different ionic species present in the plume. The species of the exhaust plume depends upon the chemical reactions taking place in the combustion chamber of the nozzle of the vehicle. The presence of the alkali metals as impurity in the airborne vehicle propellant will generate considerable ion particles such as Na+, Cl in addition to e- in the plume mixture during combustion which makes the plume electrically conducting. But it does not influence the pressure, temperature and velocity of the plume. After the nozzle throat, the exhaust plume regains the supersonic speed, so the flow of the exhaust plume is assumed as compressible flow in the second region. The electrons have high collision frequency, high number density, high plasma frequency and lower molecular mass and hence the highly mobile electrons dominate the heavy ion particle in the computation of the electrical conductivity of the plume. The plume conductivity decreases marginally from the axis till a distance equal to the nozzle radius but the peak value increases sharply towards the exit plane edge of the nozzle radius. The induced current is computed using Method of Moments. The induced current depends upon the type of interference source, its characteristics, whether the plume is present or not and the type of the plume. The HPM induces maximum current in the vehicle because of the fact that the plume has a tendency to become more conductive at these frequencies. The induced currents due to the EM fields from IRA and NEMP comes after the HPM. The presence of the plume enhances the magnitude of the induced current. If the plume is homogeneous then the current induced in it is more.