Mode conversation losses in overmolded millimeter wave transmission lines

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 106-109).Millimeter wave transmission lines are integral components for many important applications like nuclear fusion and NMR spectroscopy. In low loss corrugated transmission lines propagating the HE,1 mode with a high waveguide radius to wavelength ratio (a/X), the transmission line loss is predominantly a result of mode conversion in components such as miter bends. The theory for determining losses in miter bends though is only approximate, and is based instead on the problem of the loss across a diameter-length gap between two waveguide sections. Through simulation, we verified that the existing analytic theory of this gap loss is correct; however, our simulations could not verify the assumption that the miter bend loss is half the loss in the gap. We also considered the problem of higher order modes (HOMs) mixed with an HE11 input entering the miter bend. Using a numerical technique, we found that the loss through the miter bend is dependent on both the amplitude of the HOM content as well as its phase relative to the phase of the HE11 mode. While the overall loss averaged across all phases remains the same with increasing HOM content, the power that fails to traverse the gap tends to increase, and it is this power that appears as very high order modes that will cause heating around the miter bend. For the ITER transmission line, the loss based on gap theory is 0.027 dB and, using a coherent technique, we measured a loss of 0.05 + 0.02 dB with a vector network analyzer (VNA).(cont.) We also set out to measure the mode conversion caused by a miter bend by using a 3-axis scanner system to measure the field patterns within the ITER waveguide. Due to the presence of higher order modes output by the HE I launcher, definitive results on the mode conversion attributed to the miter bend could not be obtained. Using a phase retrieval code, we were able to calculate the mode purity of the launcher output and found it to be 98 + 0.5 %. Future work will concentrate on reducing this HOM content to enable measurements of the miter bend mode conversion.by David S. Tax.S.M

Experimental study of a high efficiency step-tunable MW gyrotron oscillator

Thesis (Ph. D.)--Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 191-205).The gyrotron is a source capable of producing megawatt power levels at millimeter-wave frequencies for many important applications, including electron cyclotron heating and current drive in magnetic fusion devices. It is important that the gyrotron operates with high efficiency and provides a high quality output beam to minimize system size, maximize reliability and avoid additional losses in external systems. This thesis presents the experimental study of such a gyrotron designed to operate at MW power levels and whose initial 110 GHz operation was expanded to include operation at 124.5 GHz. To this end, a new set of components, including a cavity, mode converter, and output window were designed for operation at both frequencies. The cavity was designed using the code MAGY and the Q factors of 830 for the TE22,6,1mode at 110 GHz and 1060 for the TE24,7,1 mode at 124.5 GHz would be suitable for CW operation in an industrial gyrotron. The mode converter consisting of a dimpled-wall launcher and 4 phasecorrecting mirrors could theoretically produce an output beam with 99 % Gaussian beam content at each frequency while a single-disc window was implemented with over 99.5 % power transmission at both frequencies. The achieved output power in experiment was 1.1 MW at 110 GHz and 850 kW at 124.5 GHz for the design parameters of 96 kV and 40 A. At 98 kV and 42 A, the gyrotron achieved 1.25 MW and 1 MW at 110 and 124.5 GHz, respectively. Mode competition is typically a major limitation in such gyrotrons, and stable single-mode operation was demonstrated at both frequencies. At 110 GHz, the output beam had 98.8 % Gaussian beam content, while at 124.5 GHz, the output beam quality was 94.4 %. Another experiment within this thesis demonstrated the implementation of a mode converter with smooth mirrors that would be less susceptible to machining and misalignment errors. A Gaussian beam content of 96 % was measured in that experiment. In addition, a thorough study of the gyrotron start-up scenario was performed, for which experimental work had been lacking in the literature. The start-up scenario is the sequence of modes that are excited during the rise of the voltage pulse and is essential for the gyrotron to operate in its most efficient regime known as the hard self-excitation regime. This gyrotron operates nominally in the TE22,6,1 mode near the 110 GHz cutoff frequency with an axial field profile that is approximately Gaussian at the steady-state peak voltage. In experiments performed in the smooth mirror mode converter configuration, lower frequency modes were observed at lower voltages as opposed to higher frequency modes as predicted by theory. Analysis of these modes showed that they are backward-wave modes far from their cutoff frequency which have higher order axial field profiles, i.e. TE21,6,3 and TE21,6,4 modes at frequencies of 108-109 GHz. The excitation of these modes was investigated and shown to be possible by using theory and single-mode simulations with the code MAGY. This discovery was important as these modes were not included in past code runs, and thus future improvements can be made to incorporate this effect.by David S. Tax.Ph.D

Explaining Predictive Uncertainty with Information Theoretic Shapley Values

Researchers in explainable artificial intelligence have developed numerous methods for helping users understand the predictions of complex supervised learning models. By contrast, explaining the $\textit{uncertainty}$ of model outputs has received relatively little attention. We adapt the popular Shapley value framework to explain various types of predictive uncertainty, quantifying each feature's contribution to the conditional entropy of individual model outputs. We consider games with modified characteristic functions and find deep connections between the resulting Shapley values and fundamental quantities from information theory and conditional independence testing. We outline inference procedures for finite sample error rate control with provable guarantees, and implement an efficient algorithm that performs well in a range of experiments on real and simulated data. Our method has applications to covariate shift detection, active learning, feature selection, and active feature-value acquisition

Molecular Foundations of Reproductive Lethality in Arabidopsis thaliana

The SeedGenes database (www.seedgenes.org) contains information on more than 400 genes required for embryo development in Arabidopsis. Many of these EMBRYO-DEFECTIVE (EMB) genes encode proteins with an essential function required throughout the life cycle. This raises a fundamental question. Why does elimination of an essential gene in Arabidopsis often result in embryo lethality rather than gametophyte lethality? In other words, how do mutant (emb) gametophytes survive and participate in fertilization when an essential cellular function is disrupted? Furthermore, why do some mutant embryos proceed further in development than others? To address these questions, we first established a curated dataset of genes required for gametophyte development in Arabidopsis based on information extracted from the literature. This provided a basis for comparison with EMB genes obtained from the SeedGenes dataset. We also identified genes that exhibited both embryo and gametophyte defects when disrupted by a loss-of-function mutation. We then evaluated the relationship between mutant phenotype, gene redundancy, mutant allele strength, gene expression pattern, protein function, and intracellular protein localization to determine what factors influence the phenotypes of lethal mutants in Arabidopsis. After removing cases where continued development potentially resulted from gene redundancy or residual function of a weak mutant allele, we identified numerous examples of viable mutant (emb) gametophytes that required further explanation. We propose that the presence of gene products derived from transcription in diploid (heterozygous) sporocytes often enables mutant gametophytes to survive the loss of an essential gene in Arabidopsis. Whether gene disruption results in embryo or gametophyte lethality therefore depends in part on the ability of residual, parental gene products to support gametophyte development. We also highlight here 70 preglobular embryo mutants with a zygotic pattern of inheritance, which provide valuable insights into the maternal-to-zygotic transition in Arabidopsis and the timing of paternal gene activation during embryo development

Integrating the Genetic and Physical Maps of Arabidopsis thaliana: Identification of Mapped Alleles of Cloned Essential (EMB) Genes

The classical genetic map of Arabidopsis includes more than 130 genes with an embryo-defective (emb) mutant phenotype. Many of these essential genes remain to be cloned. Hundreds of additional EMB genes have been cloned and catalogued (www.seedgenes.org) but not mapped. To facilitate EMB gene identification and assess the current level of saturation, we updated the classical map, compared the physical and genetic locations of mapped loci, and performed allelism tests between mapped (but not cloned) and cloned (but not mapped) emb mutants with similar chromosome locations. Two hundred pairwise combinations of genes located on chromosomes 1 and 5 were tested and more than 1100 total crosses were screened. Sixteen of 51 mapped emb mutants examined were found to be disrupted in a known EMB gene. Alleles of a wide range of published EMB genes (YDA, GLA1, TIL1, AtASP38, AtDEK1, EMB506, DG1, OEP80) were discovered. Two EMS mutants isolated 30 years ago, T-DNA mutants with complex insertion sites, and a mutant with an atypical, embryo-specific phenotype were resolved. The frequency of allelism encountered was consistent with past estimates of 500 to 1000 EMB loci. New EMB genes identified among mapped T-DNA insertion mutants included CHC1, which is required for chromatin remodeling, and SHS1/AtBT1, which encodes a plastidial nucleotide transporter similar to the maize Brittle1 protein required for normal endosperm development. Two classical genetic markers (PY, ALB1) were identified based on similar map locations of known genes required for thiamine (THIC) and chlorophyll (PDE166) biosynthesis. The alignment of genetic and physical maps presented here should facilitate the continued analysis of essential genes in Arabidopsis and further characterization of a broad spectrum of mutant phenotypes in a model plant

Clinical characteristics of women captured by extending the definition of severe postpartum haemorrhage with 'refractoriness to treatment': a cohort study

Background: The absence of a uniform and clinically relevant definition of severe postpartum haemorrhage hampers comparative studies and optimization of clinical management. The concept of persistent postpartum haemorrhage, based on refractoriness to initial first-line treatment, was proposed as an alternative to common definitions that are either based on estimations of blood loss or transfused units of packed red blood cells (RBC). We compared characteristics and outcomes of women with severe postpartum haemorrhage captured by these three types of definitions. Methods: In this large retrospective cohort study in 61 hospitals in the Netherlands we included 1391 consecutive women with postpartum haemorrhage who received either ≥4 units of RBC or a multicomponent transfusion. Clinical characteristics and outcomes of women with severe postpartum haemorrhage defined as persistent postpartum haemorrhage were compared to definitions based on estimated blood loss or transfused units of RBC within 24 h following birth. Adverse maternal outcome was a composite of maternal mortality, hysterectomy, arterial embolisation and intensive care unit admission. Results: One thousand two hundred sixty out of 1391 women (90.6%) with postpartum haemorrhage fulfilled the definition of persistent postpartum haemorrhage. The majority, 820/1260 (65.1%), fulfilled this definition within 1 h following birth, compared to 819/1391 (58.7%) applying the definition of ≥1 L blood loss and 37/845 (4.4%) applying the definition of ≥4 units of RBC. The definition persistent postpartum haemorrhage captured 430/471 adverse maternal outcomes (91.3%), compared to 471/471 (100%) for ≥1 L blood loss and 383/471 (81.3%) for ≥4 units of RBC. Persistent postpartum haemorrhage did not capture all adverse outcomes because of missing data on timing of initial, first-line treatment. Conclusion: The definition persistent postpartum haemo

Calculation and measurement of higher order mode losses in ITER ECH transmission lines

The ITER transmission lines (TLs) must be designed to deliver 20 MW from a 24 MW, 170 GHz gyrotron system. Miter bends are the main source of loss for these highly overmoded, corrugated, cylindrical waveguide TLs. Previous estimates have used only a pure HE[subscript 11] mode for the analysis of the loss due to a miter bend, however higher order modes (HOMs) must be considered for a practical analysis. For the linearly-polarized, Gaussian-like beam from a gyrotron, the LP[subscript mn] mode basis set should be used to describe the fields in the corrugated waveguide. The HOM content greatly affects the propagation of HE[subscript 11] content in a miter bend, with a large emphasis placed on the percentage of HOMs and the phase difference between HE[subscript 11] and each HOM. By considering LP modes, a complete basis set is used to investigate the HOM effects on HE11 loss in a miter bend. We also present a new conservation theorem relating the power centroid offset and propagation angle due to any two LP[subscript mn] modes propagating in the corrugated waveguide

Erratum: Experimental Results on a 1.5 MW, 110 GHz Gyrotron with a Smooth Mirror Mode Converter

We present an internal mode converter (IMC) design for a 1.5 MW, 110 GHz gyrotron operating in the TE 22,6 mode. The launcher, designed using the codes Surf3d and LOT, converts the cavity waveguide mode into a nearly pure Gaussian beam. The Gaussian beam output from the launcher is shaped by a series of 4 smooth, curved mirrors to provide a circular output beam with a flat phase front at the gyrotron window. By employing smooth mirrors rather than mirrors with phase correcting surfaces, such an IMC is less sensitive to alignment issues and can more reliably operate with high efficiency. The IMC performance was verified by both cold test and hot test experiments. Beam pattern measurements in each case were in good agreement with theoretical predictions. The output beam was of high quality with calculations showing that the Gaussian Beam content was 95.8 ?? 0.5% in both hot and cold test.close0