278 research outputs found
Noise performance of frequency- and phase-locked CW magnetrons operated as current-controlled oscillators.
No full textLow-power continuous wave ``cooker'' magnetrons driven from industrial-quality switch-mode power supplies have been frequency locked by driving them as current-controlled oscillators in a phase-lock loop (PLL). The noise performance of these frequency-locked oscillators is reported as a function of heater power. The injection of -30- to -40-dB signals derived from the reference oscillator of the PLL into the magnetron's output waveguide while the anode current is controlled by the PLL is shown to phase lock the magnetron's output. Results for locking performance are presented.
Frequency and phase modulation performance of an injection-locked CW magnetron.
No full textIt is demonstrated that the output of a 2.45-GHz magnetron operated as a current-controlled oscillator through its pushing characteristic can lock to injection signals in times of the order of 100-500 ns depending on injection power, magnetron heater power, load impedance, and frequency offset of the injection frequency from the natural frequency of the magnetron. Accordingly, the magnetron can follow frequency and phase modulations of the injection signal, behaving as a narrow-band amplifier. The transmission of phase-shift-keyed data at 2 Mb/s has been achieved. Measurements of the frequency response and anode current after a switch of phase as a function of average anode current and heater power give new insight into the locking mechanisms and the noise characteristics of magnetrons
Microwave and RF Vacuum Electronic Power Sources
No full textDo you design and build vacuum electron devices, or work with the systems that use them? Quickly develop a solid understanding of how these devices work with this authoritative guide, written by an author with over fifty years of experience in the field. Rigorous in its approach, it focuses on the theory and design of commercially significant types of gridded, linear-beam, crossed-field and fast-wave tubes. Essential components such as waveguides, resonators, slow-wave structures, electron guns, beams, magnets and collectors are also covered, as well as the integration and reliable operation of devices in microwave and RF systems. Complex mathematical analysis is kept to a minimum, and Mathcad worksheets supporting the book online aid understanding of key concepts and connect the theory with practice. Including coverage of primary sources and current research trends, this is essential reading for researchers, practitioners and graduate students working on vacuum electron devices
Role of aggressivity on reactivity and craving before and after cue exposure in recently detoxified alcoholics: Results from an experimental study
No full textThe role of aggressivity and cue exposure in induction of craving were investigated in a clinical setting. Thirty abstinent alcoholic patients were divided into a low and a high aggressive group based on scores on the physical aggression subscale of the Buss-Durkee Hostility Inventory and exposed to alcohol cues. Craving was measured by means of the Alcohol Craving Questionnaire (ACQ) and Visual Analogue Scales (VAS). Important findings are: (1) main effects of aggressivity on `emotionality', `purposefulness' and `expectancy' of ACQ were very significant; (2) on `drinking intention' and `craving for alcohol' of VAS, aggressivity and cue exposure showed a significant interaction; (3) the main effect of cue exposure on heart rate also reached a significance level of 0.007. The results were discussed in the context of the Classical, Operant Conditioning Theory, the Cognitive Craving Theory of Tiffany, Gilbert's STAR Model, and the Self-Medication Hypothesis Copyright (C) 2001 S. Karger AG, Basel
Fabrication process changes for performance improvement of an RF MEMS resonator : conformable contact lithography, Moiré alignment, and chlorine dry etching
No full textThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.Includes bibliographical references (p. 103-106).This thesis presents fabrication process improvements for a RF MEMS resonator for the purpose of improving the quality factor (Q) and extending the frequency range. The process changes include the use of conformable contact lithography (CCL) and chlorine-based dry etching for improved fine-feature patterning and Moiré -based alignment techniques to allow for a non-self-aligned process. The resulting control over feature size and structure are expected to improve Q and enable higher frequency resonators. A CCL process utilizing moir6 alignment marks is described. An automated Moiré -based alignment system using Labview software is presented which demonstrates sub-100 nm alignment accuracy for a single alignment mark. A full-wafer alignment experiment is described that demonstrates average pattern placement errors of ... for the x- and y-directions respectively. The experimental limitations are analyzed and suggested improvements to the system are detailed. Chlorine dry etching experiments are conducted in order to produce a straight sidewall etch through the "stack" of resonator materials (chrome, aluminum nitride, and molybdenum). A combination of Cl₂/0₂, Cl₂/Ar, and CF₄0₂ plasmas at low pressure (2 mTorr), high microwave/source power (500W), and a moderate DC bias (-150V) demonstrates a straight sidewall angle (>80⁰ measured from horizontal) with no undercut for all layers of the stack. RF resonators fabricated with these process modifications are presented. An average overlay error of 55 nm (110 nm min-max) is recorded for 11 devices located closest to the line between the alignment marks in an aligned release of the resonators. The design modifications enabled by the new process are described and the prospect for higher-frequency devices and higher-Q device performance is discussed.by Mark Sakai.S.M
An apparatus for studying interactions between Rydberg atoms and metal surfaces
No full textA system suitable for studying interactions between ⁸⁷Rb Rydberg atoms and metal surfaces has been constructed. This thesis describes the design and construction of the apparatus, and some test results. Atoms in a vapor cell magneto-optical trap are transferred to a macroscopic Ioffe-Pritchard trap, where they will be RF evaporatively cooled and loaded into a magnetic microtrap (atom chip). Confinement of cold clouds at controllable distances (5–200 μm)} from a metal surface is possible. The effects of atom-surface interactions can be studied with Rydberg atom spectroscopy.
Some functionality of the apparatus has been demonstrated. Approximately 1.5×10⁷ atoms were loaded into a mirror MOT, and about 6×10⁶ atoms were optically pumped to the |F=2, m_F=2> hyperfine ground state and confined in a macroscopic Ioffe-Pritchard trap. The temperature of the cloud in the trap was 42 ± 5 μK, and the 1/e lifetime is 1–1.5 s. Forced RF evaporation has been used to measure the magnetic field at the trap minimum, but RF evaporative cooling has not yet been demonstrated
Extracellular Vesicles: Emerging Modulators of Cancer Drug Resistance
No full textDrug resistance still represents the main reason for therapy failure in cancer patients. In the last decade, extracellular vesicles (EVs), a heterogeneous group of particles implicated in cell-to-cell communication, have been shown to substantially contribute to this phenomenon. This review summarizes the molecular mechanisms underlying the EV-mediated development of chemoresistance, shedding light on the potential role of these vesicles as both diagnostic/prognostic markers and therapeutic targets.
Extracellular vesicles (EVs) have recently emerged as crucial modulators of cancer drug resistance. Indeed, it has been shown that they can directly sequester anti-tumor drugs, decreasing their effective concentration at target sites. Moreover, they facilitate the horizontal transfer of specific bioactive cargoes able to regulate proliferative, apoptotic, and stemness programs in recipient cells, potentially conferring a resistant phenotype to drug-sensitive cancer cells. Finally, EVs can mediate the communication between the tumor and both stromal and immune cells within the microenvironment, promoting treatment escape. In this context, clarifying the EV-driven resistance mechanisms might improve not only tumor diagnosis and prognosis but also therapeutic outcomes. Detailed cellular and molecular events occurring during the development of EV-mediated cancer drug resistance are described in this review article
Design of helix slow-wave structures for high efficiency TWTs.
No full textTWTs for space applications commonly have a helix pitch profile which incorporates a section with increased phase velocity followed by a negative phase velocity taper. A simple method is described for the initial design of a helix slow-wave structure of this kind to achieve high overall efficiency. It is shown that the use of a section with increased phase velocity increases the beam efficiency of a TWT while reducing the generation of second harmonic power. The technique is illustrated by its application to a 70 W Ku-band TWT and the performance is shown to be comparable with that of an existing TWT
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