Microwave device investigations Semiannual progress report, 1 Oct. 1969 - 1 Apr. 1970

Beam-plasma interactions, cyclotron harmonic instability study, and millimeter and submillimeter wave detection by paramagnetic material

Utilizing tunable signal interference control topologies with electromechanical resonators

Exploiting knowledge gained from previous investigations of channelized and trans-versal filters, signal interference filters use transmission line differences to generate transmission zeros through phase-shifted combinations of signals at the output of a device. The transmission lines used in these circuits are straightforward to design, but are limited to high-frequency signals (on the order of a few gigahertz) due to the necessity for spatial compactness and low loss. More recent studies have used electromechanical resonators to achieve phase shifting and quality factor improvements at slightly-lower frequencies. These concepts may prove useful if extended to micro- and nanoscale resonators. To explore signal interference topologies outside of purely-electrical, high-frequency filtering domains, a generic system model is proposed herein, which is based upon high quality factor resonant elements and continuously-tunable amplitude and phase components. The mathematical models developed in this work are generalized to apply the concept of signal interference to a variety of linear resonant systems. With this approach, frequency response behaviors can be quickly modified from amplification to cancellation through appropriate tuning of the phase and gain components. The analytical models are simulated and implemented in electromechanical circuitry as a first step towards system integration. The prototypical circuits qualitatively match the desired frequency response and tuning behaviors, proving the use of the mathematical models in the design of linear resonant signal interference systems

Core Losses and Torque Ripple in IPM Machines: Dedicated Modeling and Design Trade Off

The proper combination of stator and rotor slot numbers is pursued in the design of interior permanent-magnet (IPM) motors with wide constant-power speed range. At high speed, in the flux-weakening region, the arising of stator and rotor iron losses due to magnetomotive-force (MMF) spatial harmonics limits the IPM motor performance. Torque ripple is another problem for this kind of machines, both at low and high speed. The numbers of stator slots and rotor equivalent slots have a major impact on both the loss and ripple aspects. A simplified model is proposed here in order to evaluate both problems with a general approach and point out the possible design tradeoff. With respect to previous models in the literature, both stator and rotor losses are included, and a more comprehensive approach is followed in the description of the rotor MMF harmonics. The model's effectiveness is tested through finite element analysis simulations and some experimental results. The proposed approach is useful for the selection of the IPM machine structure according to the specific requirements of the applicatio

Development and evaluation of a fault-tolerant multiprocessor (FTMP) computer. Volume 4: FTMP executive summary

The FTMP architecture is a high reliability computer concept modeled after a homogeneous multiprocessor architecture. Elements of the FTMP are operated in tight synchronism with one another and hardware fault-detection and fault-masking is provided which is transparent to the software. Operating system design and user software design is thus greatly simplified. Performance of the FTMP is also comparable to that of a simplex equivalent due to the efficiency of fault handling hardware. The FTMP project constructed an engineering module of the FTMP, programmed the machine and extensively tested the architecture through fault injection and other stress testing. This testing confirmed the soundness of the FTMP concepts

Doctor of Philosophy

dissertationPhotonic integration circuits (PICs) have received overwhelming attention in the past few decades due to various advantages over electronic circuits including absence of Joule effect and huge bandwidth. The most significant problem obstructing their commercial application is the integration density, which is largely determined by a signal wavelength that is in the order of microns. In this dissertation, we are focused on enhancing the integration density of PICs to warrant their practical applications. In general, we believe there are three ways to boost the integration density. The first is to downscale the dimension of individual integrated optical component. As an example, we have experimentally demonstrated an integrated optical diode with footprint 3 Ã- 3 m2, an integrated polarization beamsplitter with footprint 2.4 Ã- 2.4 m2, and a waveguide bend with effective bend radius as small as 0.65 m. All these devices offer the smallest footprint when compared to their alternatives. A second option to increase integration density is to combine the function of multiple devices into a single compact device. To illustrate the point, we have experimentally shown an integrated mode-converting polarization beamsplitter, and a free-space to waveguide coupler and polarization beamsplitter. Two distinct functionalities are offered in one single device without significantly sacrificing the footprint. A third option for enhancing integration density is to decrease the spacing between the individual devices. For this case, we have experimentally demonstrated an integrated cloak for nonresonant (waveguide) and resonant (microring-resonator) devices. Neighboring devices are totally invisible to each other even if they are separated as small as /2 apart. Inverse design algorithm is employed in demonstrating all of our devices. The basic premise is that, via nanofabrication, we can locally engineer the refractive index to achieve unique functionalities that are otherwise impossible. A nonlinear optimization algorithm is used to find the best permittivity distribution and a focused ion beam is used to define the fine nanostructures. Our future work lies in demonstrating active nanophotonic devices with compact footprint and high efficiency. Broadband and efficient silicon modulators, and all-optical and high-efficiency switches are envisioned with our design algorithm

AVERAGE-VALUE MODELING OF HYSTERESIS CURRENT CONTROL IN POWER ELECTRONICS

Hysteresis current control has been widely used in power electronics with the advantages of fast dynamic response under parameter, line and load variation and ensured stability. However, a main disadvantage of hysteresis current control is the uncertain and varying switching frequency which makes it difficult to form an average-value model. The changing switching frequency and unspecified switching duty cycle make conventional average-value models based on PWM control difficult to apply directly to converters that are controlled by hysteresis current control. In this work, a new method for average-value modeling of hysteresis current control in boost converters, three-phase inverters, and brushless dc motor drives is proposed. It incorporates a slew-rate limitation on the inductor current that occurs naturally in the circuit during large system transients. This new method is compared with existing methods in terms of simulation run time and rms error. The performance is evaluated based on a variety of scenarios, and the simulation results are compared with the results of detailed models. The simulation results show that the proposed model represents the detailed model well and is faster and more accurate than existing methods. The slew-rate limitation model of hysteresis current control accurately captures the salient detail of converter performance while maintaining the computational efficiency of average-value models. Validations in hardware are also presented

Doctor of Philosophy

dissertationDriven by a myriad of potential applications such as communications, medical imaging, security, spectroscopy, and so on, terahertz (THz) technology has emerged as a rapidly growing technological field during the last three decades. However, since conventional materials typically used in microwave and optical frequencies are lossy or do not effectively respond at these frequencies, it is essential to find or develop novel materials that are suitable for device applications in the THz range. Therefore, there is wide interest in the community in employing novel naturally-occurring materials, such as 2D materials, as well as in designing artificial metamaterial structures for THz applications. Here, we combined both of these approaches so to develop reconfigurable THz devices capable of providing amplitude modulation, phase modulation, and resonance frequency tuning. First, graphene is employed as the reconfigurable element in metamaterial phase modulators. For this purpose, we propose the use of unit cells with deep-subwavelength dimensions, which can have multiple advantaged for beam shaping applications. The analyzed metamaterials have one of the smallest unit cell to wavelength ratios reported or proposed todate at THz frequencies. By systematic analysis of the geometrical tradeoffs in these devices it is found that there is an optimal unit cell dimension, corresponding roughly to ~λ/20, which can deliver the best performance. In addition to this, we explored other applications of graphene in metamaterial devices, including amplitude modulation and resonance-shifting. These studies motivated us to analyze what is the most suitable role of graphene from a THz device perspective: is graphene a good plasmonic material? Or it is better suited as a reconfigurable material providing tunability to otherwise passive metallic structures? Our studies show that the Drude scattering time in graphene is an important parameter in this regard. In order to attain strong plasmonic resonances graphene samples with τ >> 1ps are required, which is challenging in large area CVD samples. But graphene is just one example of a wider class of 2D materials. In this work we also studied for the first time the application of 2D materials beyond graphene as reconfigurable elements in THz devices. For this purpose, Molybdenum Disulfide (MoS2) was employed as the reconfigurable element in cross-slot metamaterial amplitude modulators. Our results evidence that smaller insertion loss is possible when employing 2D materials with a bandgap, such as MoS2, rather than a zero-gap material such as graphene. Furthermore, because of a stronger optical absorption active control of the metamaterial properties is possible by altering the intensity of an optical pump. We later investigate and discuss transparent conductive oxides (TCOs), which constitute an interesting choice for developing visible-transparent THz-functional metamaterial devices for THz applications. These materials show a metallic THz response and thus can substitute the metal patterns in metamaterial devices. In our particular studies we analyzed samples consisting of: (i) two-dimensional electron gases at the interface between polar/nonpolar complex oxides having record-high electron density, and (ii) thin-films of La-doped BaSnO3 having record-high conductivity in a TCO. These materials exhibit a flat THz conductivity across a broad terahertz frequency window. As a result of their metal-like broadband THz response, we demonstrate a visible-transparent THz-functional electromagnetic structure consisting of a wire-grid polarizer

Synthetic mammalian signaling circuits for robust cell population control

In multicellular organisms, cells actively sense, respond to, and control their own population density. Synthetic mammalian quorum sensing circuits could provide insight into principles of population control and improve cell therapies. However, a key challenge is avoiding their inherent sensitivity to “cheater” mutations that evade control. Here, we repurposed the plant hormone auxin to enable orthogonal mammalian cell-cell communication and quorum sensing. Further, we show that a “paradoxical” circuit design, in which auxin stimulates and inhibits net cell growth at different concentrations, achieves population control that is robust to cheater mutations, controlling growth for 43 days of continuous culture. By contrast, a non-paradoxical control circuit limited growth but was susceptible to mutations. These results establish a foundation for future cell therapies that can respond to and control their own population sizes