摘要:

An eddy‐current apparatus for measurement of the superconducting transition of high‐Tcfilms has been developed. It uses a LC resonant circuit driven by a voltage controlled oscillator. The system is ideally suited for nondestructive screening of many high‐Tcfilms. High‐Tcfilms deposited on both sides of a substrate can be investigated successfully.

ISSN:0034-6748    

DOI:10.1063/1.1143410

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Time‐dependent resistance of a high voltage, high current, closing switch was measured via the discharge of a low impedance transmission line. With this technique, only the current waveform of the switch, the initial open circuit voltage, and the transmission‐line characteristics need to be known to determine the switch resistance waveform. Under certain conditions a simple lossless transmission‐line formula is sufficient for the calculation of the switch resistance waveform. However, if the transmission‐line losses cannot be neglected a correction can be readily implemented through numerical signal processing. The bandwidth of this technique is limited to the bandwidth of the switch current record only. This is particularly useful in the measurement of the resistance of high voltage, high current switches where inductive pick‐up noise in the switch voltage waveform is large and where high voltage probes of sufficiently high bandwidth are not readily available. In this article, both simulated and experimental examples are given, together with the method of calculation of switch resistance which takes into account practical transmission line losses. It was found that best results are obtained if the magnitude of the switch resistance is higher than or of a similar order as the characteristic impedance of the transmission line.

ISSN:0034-6748    

DOI:10.1063/1.1143411

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The single‐frequency method of Hill and Coleman [Solid‐State Electron.23, 987 (1980)] for the evaluation of interface state density has been modified to provide more accurate information about the interface state density. Interface state density obtained by this method has been found to be in good agreement with the interface state density obtained from Nicollian–Goetzberger’s conductance method.

ISSN:0034-6748    

DOI:10.1063/1.1143412

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A high‐pressure dilatometer capable of measuring volume changes of 2×10−6cm3is described. Fractional volume changes of 1 part in 104can be measured over a temperature range of −30–90 °C, and a pressure range of 1 bar (0.1 MPa)–3 kbar (300 MPa). The dilatometer is constructed out of a commercial high‐pressure valve and a compact home‐built pressure sensor. The system was specially designed to study lipid‐water biological systems. Preliminary studies on the lipid dioleoylphosphatidylethanolamine (DOPE) revealed the existence of phases not previously reported for this lipid.

ISSN:0034-6748    

DOI:10.1063/1.1143413

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We discuss here the use of a mechanical suspension for the new generation of resonant gravitational wave antennas, cooled at temperatures below 0.1 K. The crucial problem of these detectors is the isolation from all external mechanical vibrations in the presence of the unavoidable solid thermal link with a dilution refrigerator. Measurements of the mechanical transfer function performed with two types of suspension‐thermal link configurations are reported. This study has led to the design of the suspension and thermal link of the first ultralow temperature antenna of the Rome group ‘‘Nautilus.’’

ISSN:0034-6748    

DOI:10.1063/1.1143414

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A microcomputer‐aided ultrasonic system that can be used to measure the vibratory displacements of an object is presented. A pair of low cost 40‐kHz ultrasonic transducers is used to transmit ultrasound toward an object and receive the ultrasound reflected from the object. The relative motion of the object modulates the phase angle difference between the transmitted and received ultrasound signals. A single‐chip microcomputer‐based phase detector was designed to record and analyze the phase shift information which is then sent to a PC‐AT microcomputer for processing. We have developed an ingenious method to reconstruct the relative motion of an object from the acquired data of the phase difference changes. A digital plotter based experiment was also designed for testing the performance of the whole system. The measured accuracy of the system in the reported experiments is within ±0.4 mm and the theoretical maximal measurable speed of the object is 89.6 cm/s. The main advantages of this ultrasonic vibration measurement system are high resolution, low cost, noncontact measurement and it is easily installed.

ISSN:0034-6748    

DOI:10.1063/1.1143415

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Optical probes are now commonly used in industrial conditions as well as in laboratory experiments. Although they have been primarily devoted to void fraction measurements, additional information could be extracted from the raw signals they deliver. For a stretched optical probe, it is shown that the modulus of the ensemble velocity of a bubble ‖V0‖ could be inferred from the rise timeTuassociated with the liquid/gas transition. These two parameters are correlated thanks to piercing experiments in which the interface curvatureRand the angle &bgr; between the probe and the normal to the interface are controlled. While the influence ofRis negligible, the rise time is very sensitive to &bgr;. A one‐to‐one relation betweenTuand ‖V0‖ is ensured for quasiperpendicular impactions which must be accordingly distinguished among the bubble signatures. The software which includes the adequate signal processing criteria is described. It gives access to void fraction and to the chord‐velocity joint distribution which provides a guide to determine the flow regime. The void fraction is shown to be weakly sensitive to the criteria. Besides, velocity and chords estimates are in rather good agreement with those deduced from direct visualization, both in a bubble and a slug flow. Such extensions of the measuring capabilities of optical probes are expected to be also applicable to other phase detection probes.

ISSN:0034-6748    

DOI:10.1063/1.1143416

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A normally on gating circuit with reduced gate‐associated artifacts and improved extinction has been developed for focusing electrode‐equipped photomultiplier tubes (PMTs) such as the Thorn EMI, Inc. model 9816. Unlike previous gating methods where the focusing electrode is pulsed negative with respect to the photocathode (PC) to gate the tube off, this method holds the focusing electrode at a constant potential of +80 V with respect to the PC and pulses the first dynode from +80 to −246 V. This technique effectively eliminates post‐gate signals induced by PC irradiation during the gating period, though at the cost of lower extinction efficiencies. By also pulsing the fifth dynode capacitatively with −326 V, attenuation is restored to greater than 107:1. The circuit assembly is compact enough to fit within a standard PMT housing and operates from the PMT dynode current. Turn‐off and turn‐on times of 55 and 40 ns, respectively, are obtained. The gate is thus suitable for exciting light pulse rejection in low level, submicrosecond time‐resolved fluorescence, and phosphorescence measurements.

ISSN:0034-6748    

DOI:10.1063/1.1143417

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A laboratory‐scale arc furnace encompassing a nonconsumable electrode and a cold hearth and operating at inert gas pressures of up to 20 bar is described. Using this equipment, weight losses due to volatilization when melting high vapor pressure metallic materials have been reduced to about 20% of those encountered with a conventional arc furnace operating at 1 bar. The arc furnace has been designed to be evacuable to high vacuum levels, as well as attaining high pressures, so that specimen purity can be retained by thoroughly evacuating and degassing the system prior to admitting inert gas and melting. Increases in gas pressure have been found to have a pronounced effect on the stability of the arc, especially at low arc currents, and to achieve minimum volatilization losses a balance is required between pressure and arc current for a particular sample.

ISSN:0034-6748    

DOI:10.1063/1.1143418

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A high speed data acquisition system which is closely coupled with a high speed digital processor is described. Data acquisition at a rate of 40 million 14 bit data values per second is possible simultaneously with data processing at a rate of 80 million floating point operations per second. This is achieved by coupling a commercially available VME format single board computer based on the Intel i860 microprocessor with a custom designed first‐in, first‐out memory circuit that transfers data at high speed to the processor board memory. Parallel processing to achieve increased computation speed is easily implemented because the data can be transferred simultaneously to multiple processor boards. Possible applications include high speed process control and real time data reduction. A specific example is described in which this hardware is used to implement a feedback control system for 18 parameters which uses 100 input signals and achieves a 100 &mgr;s cycle time.

ISSN:0034-6748    

DOI:10.1063/1.1143843

出版商:AIP    

年代:1992

数据来源: AIP