摘要:

A miniature single element effusion cell has been fabricated and tested that allows for the high-vacuum deposition of a variety of transition-metal and rare-earth elements. The cell is designed to operate under high-vacuum conditions,≈10−9 Torr,with low power demands,<200 W.The virtues of this evaporator are the simplicity of design and ease of fabrication, assembly, maintenance, and operation.

ISSN:0034-6748    

DOI:10.1063/1.1148262

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A new low-pressure flow reactor operated as a Knudsen cell and intended for chemical kinetic studies is described. The reactor is specifically designed to study the kinetics of heterogeneous reactions. Gas-phase species are detected either by mass-spectrometric sampling or byin situoptical techniques, e.g., laser-induced fluorescence, resonantly enhanced multiphoton ionization. A feature of the reactor is its modular design, allowing full interchangeability of several sample holders at minimal effort, allowing the measurement of uptake coefficients ranging from10−7to 1.0. Sample supports operating at low and high temperatures have been developed which cover the stated temperature range. Several experimental examples of the utility of the reactor are detailed. The reliability and error bars of the kinetic results due to the errors and uncertainties associated with the experimental procedures are discussed, in particular for fast heterogeneous processes. It is found that even in the molecular flow regime, for fast reaction, the effects of diffusion limitations within the cell must be taken into account. This fact has been shown here from an experimental point of view. In a companion article the phenomena are studied using Monte Carlo simulation of the gas dynamics under molecular flow conditions. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148263

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A Monte Carlo program has been developed to simulate the gas dynamics inside flow reactors (Knudsen cells) specially adapted to study heterogeneous reactions. Because the reactors are operated under molecular flow conditions, the Monte Carlo technique can be applied to calculate trajectories of individual molecules considering only gas-wall collisions. Ensembles of trajectories are then used to generate synthetic kinetic data as a function of specific reactor geometries (surface-to-volume ratio, injector position, reactive-surface diameter, etc.) and analyzed to yield rate constants. The calculated rate constants thus obtained are useful in assessing the systematic error associated with a rate-constant determination and in the designing of future geometries of Knudsen cells. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148264

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The development ofin vivoelectron paramagnetic resonance spectrometers has generated a need for a better method of inductive coupling between a microwave resonator and a transmission line. We describe here an approach, based on introducing an additional impedance transformer between a coupling loop and the transmission line. This additional transformer decreases (by a factor of 20) the resonance frequency shift that occurs during adjustment of the coupling. The calculated optimal length is slightly greater than a quarter of the wavelength of the electromagnetic waves. Experimental measurements of the return loss for the matching circuit has shown the validity of the calculations and the usefulness of the proposed theoretical model. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148265

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

It is well known that a swing can help infants fall asleep easily. In conditions of deep sleep, infants can grow faster. But how and when to swing (including swing speed and amplitude) for better growth is a serious question. As of now, of concern is that there is no objective quantitative research concerning this topic, although there exist some qualitative studies in the literature. This project is directed to designing a swing system that can simulate a natural or forced swing vibration with a quiet close-loop magnetic driving technique. Both the frequency and amplitude of the system can be changed independently in the range of 0.4–1 Hz and 0.36°–3.6°, respectively. Therefore, the infant’s quantitative physiological signals (e.g., electroencephalogram, energy consumption, sleep duration) can be measured to determine the best method of swinging. This system would help a lot of neonates, especially premature infants whose growth rate is much lower than it is in the mother’s body. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148266

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Interferometric gravitational wave detectors like VIRGO or LIGO will use mirrors suspended as pendulums, forming systems of optical cavities; this requires precise control of alignment and of the mirror pendular motions. We describe a system for the measurement of position and orientation of a mass with respect to a local reference frame. We use a single computer read out charge coupled device camera to determine all six degrees of freedom by observing reference marks on the mass, and, in the case of small mass misalignment, reflections of two auxiliary laser beams. The derived information is available in a shared memory segment on an industrial standard bus system (VME) crate for further digital processing, which will be necessary for alignment and damping of the pendular movements. The mass coordinates can be calculated at the camera frame rate of 60 Hz. In a table top experiment with fixed test mass, the short term rms measurement errors for the translational degrees of freedom were measured to be better than 0.5 &mgr;m; for two rotational degrees of freedom, the errors were better than 0.5 &mgr;rad, and for the third one about 15 &mgr;rad. The long term stability over several days is about an order of magnitude less; it is limited by drifts, to a large extent of thermal origin. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148267

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A new laser interferometer for geophysical purposes is being operated in an underground laboratory in central Italy. A simple electro-optical technique has been used to limit the costs. The main characteristics of this instrument are: high sensitivity(&Dgr;l/l≈10−12), fast sampling rate (up to 6 kHz), large dynamic range (unbounded in principle), the capability of following strain rates as high as7×10−5 s−1, and good reliability (two failures in nearly two years). ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148268

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

To extend the operating frequency band of earth-based interferometric gravitational wave (GW) detectors down to 1 Hz, an unconventional system is required that provides approximately a factor of one million vibration isolation for the horizontal and more for the vertical, starting at 1 Hz. To this end, a three-stage ultra-low-noise, high-performance active and passive isolation system is being designed and constructed in order to demonstrate that a high degree of reduction can be achieved for vibrations at the support points of a GW detector’s final pendulum system. The first stage of this system has been fully characterized and is the main subject of this article. It is an equilateral triangular platform, 1.1 m on a side, with a total mass of 460 kg, including the vacuum system that will contain the other two stages. Active isolation is achieved by six servocontrol loops, using signals from low-frequency displacement sensors to feed back to noncontacting force transducers. The first stage has been actively isolated from vibrations in all six degrees of freedom by at least a factor of 100 at 1 Hz and above. The system noise limitations at 2 Hz and above are roughly2×10−11and5×10−11 m/Hzfor vertical and horizontal translations, respectively. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148269

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A small, low-cost sensor for measuring orbital atomic oxygen fluxes and their corrosive effect on spacecraft materials has been developed. Its operation is based on the reaction between hyperthermal atomic oxygen and a thin silver film. The electrical conductivity of the silver oxides formed is negligible allowing the reaction rate to be monitored through resistance measurements of the film. Atomic oxygen effects on spacecraft coatings can be investigated by placing a sample above the silver film. The construction of the sensor emphasizes simple techniques and components to improve reliability and reduce costs, while its small size and mass will allow it to be used on a wide range of spacecraft, increasing flight opportunities. An instrument comprising 12 of the sensors has been tested in orbit on the Space Technology Research Vehicle (STRV-1a) microsatellite mission. Results from this experiment agree well with those from ground-based simulation facilities. The successful outcome of this demonstration flight shows that the sensor can operate successfully and usefully in space. Several improvements to the technique are suggested for follow-on flights. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148270

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We describe a microwave source that utilizes a plasma to bunch the electron beam. The electron beam interacts with the plasma via the wakefield produced by the head of the beam. The resulting plasma oscillations bunch the beam at a frequency proportional to the plasma frequency. Thus the device can be tuned by varying the plasma density. The theory of the device is described and numerical simulations of the interaction are presented. The theory and simulations were validated by a proof-of-principle experiment using a simple klystron configuration. ©1997 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1148271

出版商:AIP    

年代:1997

数据来源: AIP