摘要:

A simple, low‐cost pulse nuclear magnetic resonance (NMR) spectrometer is described. There are two key features of the instrument: (1) all radio frequency phase shifting is done digitally with a new‐generation frequency synthesizer and (2) the pulse programmer is based on a high‐speed 32‐bit parallel interface card in a desktop computer. The spectrometer is well suited for single‐frequency channel experiments such as nuclear quadrupole resonance and deuterium quadrupole echo NMR experiments. Because the rf phase shifting is done digitally, the spectrometer operates easily over a wide frequency range, here 10–300 MHz. The pulse programmer and other software functions are written in LabView, a graphical programming language.

ISSN:0034-6748    

DOI:10.1063/1.1144122

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A high‐speed data acquisition and control system for atom‐resolved tunneling spectroscopy based on an 80386 computer and a commercially available data acquisition card is descibed. An analog proportional‐integral feedback controller with sample and hold is used to control the scanning tunneling microscope. Constant tip‐sample separation during data collection is maintained by interrupting the feedback loop. Assembly language routines taking advantage of the direct memory access capabilities of the data acquisition card are used to minimize the data collection time. A current‐to‐voltage amplifier based on measuring the voltage drop across a resistor in series with the tunnel junction is used to amplify the tunnel current. This amplifier has a bandwidth of 250 kHz at a gain of 109V/A. The complete system is capable of obtaining an entire tunneling spectrum in 1.5 ms. The system can also simultaneously collect 128 spectroscopic images, a topographic image, and spatially resolved tunneling spectra. Similar techniques can also be used to obtain local barrier height measurements and barrier height images. Scanning tunneling microscopy and spectroscopy data for C60adsorbed on Au(111) are presented as examples of data collected using this system.

ISSN:0034-6748    

DOI:10.1063/1.1144123

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The development and application of a new dynamic measuring method of velocity variation of piezoelectric materials, especially of lead zirconate titanate (PZT) ceramics, with an alternating electric field is presented. This method has the following advantages over the conventional static method. (1) The velocity variation due to nonlinear effect is separable from the velocity variation due to temperature change. (2) The adiabatic nonlinear coefficients are obtainable. (3) In spite of the dynamic nature of the method, it fixes the sign of the nonlinear coefficient. (4) Separating the orders of the velocity variation as a function of the applied electric field, each order of coefficients is determinable.  

ISSN:0034-6748    

DOI:10.1063/1.1144124

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A large‐nanostructure probe with optically guided macroscopic scanning has been developed for high‐resolution imaging and characterization of nanostructures. The novel optical viewing system allows placement of the imaging tip to within 1 &mgr;m of a desired site on the sample during coarse positioning. Fine positioning and imaging are accomplished with nanometer‐scale resolution using a segmented‐tube piezoelectric scanner. High‐resolution images of identifiable quantum dots have been obtained to demonstrate the efficacy of the method.

ISSN:0034-6748    

DOI:10.1063/1.1144125

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A contamination‐free Langmuir probe of novel design is described. Surface contaminants, which lead to erroneous evaluation of plasma parameters by distortion of the probe’s current‐voltage characteristic, are removed by the indirect heating of the probe tip with separate heating elements running parallel to the probe wire in multibore alumina tubing. This configuration minimizes magnetic field perturbations, while maintaining the compact profile and construction ease of an unheated probe. The design and operating characteristics of such a probe are reported. Temperatures at the probe tip, as measured by a subminiature thermocouple, can exceed 500 °C. Experiments determining the effect of probe temperature,Tprobe, on the characteristic have been performed in aQ‐machine plasma (Ti≊Te≊0.2 eV). We find that forTprobe<250 °C, the measured electron temperature is up to several times too large, indicating the presence of a contamination layer. This contamination layer is removed forTprobe≥300 °C, and accurate electron temperatures are recovered.

ISSN:0034-6748    

DOI:10.1063/1.1144074

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The characteristics of a compact space and wavelength‐resolving spectrometer‐detector arrangement are described which is used to measure spectral line shapes simultaneously along several viewing chords. The system is based upon a freely programmable slow‐scan charge‐coupled device (CCD) camera gated by an image intensifier tube. Visible and near‐UV light emitted near the plasma boundary is collected from up to 12 viewing chords and transmitted to the spectrograph by quartz fibers. Spectra can be recorded with a temporal resolution of 20 ms by means of multiple pixel binning during CCD readout. Integration of the spectral line profiles yield particle fluxes of various low‐Zspecies for ohmically heated helium discharges. The calibration, sensitivity, and signal‐to‐noise ratio of the system are reconstructed considering each individual element, allowing an easy prediction of the performance of similar applications.

ISSN:0034-6748    

DOI:10.1063/1.1144075

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Tokamaks require toroidal loop voltage for breakdown of the neutral gas, current rise, and the flat top phase. The temporal profile of the loop voltage established by the change of flux linked by the ohmic transformer has to be a noncosine waveform. In this paper a multistage capacitor bank is described which was used to energize the ohmic transformer in tokamak ADITYA with a major radius of 0.75 m, minor radius of 0.25 m, and a toroidal field of 1.5 T at the plasma center. A combination of capacitors charged to different voltages are switched in at appropriate times, to realize an experimental demand for initial high loop voltage followed by a lower sustaining loop voltage. Theoretical prediction for the duration of the secondary loop voltage as a function of circuit parameters, for a fast bank operation of 6 kV, slow bank, 4–4.5 kV, and slow bank, 2–2.5 kV yieldt0=1.25 mS,t1=4.95 mS, andt2=24.1 mS. These values are in close agreement to the measured values oft0=1.39 mS,t1=5.7 mS, andt2=23.7 mS. The trigger delays to the various capacitor bank sections are parameter dependent. To avoid repetitive adjustments in the delays, a novel scheme for consistent triggering is also highlighted.  

ISSN:0034-6748    

DOI:10.1063/1.1144076

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The experimental setup has been designed that allows measurements of the minority‐carrier lifetime versus drive current in InGaAsP/InP LEDs. The small‐signal technique has been used to obtain the lifetime data and a special averaging system has been built for measurements at very low injection levels. The measurement system is controlled by an IBM personal computer and supporting software packages have also been designed.

ISSN:0034-6748    

DOI:10.1063/1.1144077

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This article describes an economical short‐pulse ultrasound system with a goniometer stage, with which both compression and shear wave reverberations in thin ceramic or other material samples may be separately time resolved. Arrival times of individual reverberations are compared to estimates made by computer simulation of the wave system by means of a parameter fitting routine. The system output gives compression and shear velocities based on several reverberations in the test material. Wide frequency bandwidths are achieved by using acoustically thick PZT transducers in time intervals less than one half period of their fundamental compression wave frequency.

ISSN:0034-6748    

DOI:10.1063/1.1144078

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new method of determination of ultrasound velocity and attenuation in single crystals by means of dynamic x‐ray diffraction is substantiated theoretically and experimentally. The diffractional response in a crystal undergoing stationary or pulsed acoustic excitation is considered. The frequency dependence of the diffraction intensity yields information on ultrasound velocity. Time scanning of the diffraction response under pulsed excitation allows one to determine the attenuation coefficient. Enhancement of the ultrasound‐induced x‐ray diffraction contrast is achieved for high‐frequency acoustic waves, when x‐ray extinction length exceeds the ultrasonic wavelength. The sensitivity and accuracy limitations of the method due to diffraction spreading of the acoustic beam, to the finite thickness of the sample and its inhomogeneity, and to lateral dissipation of the acoustic energy are evaluated. The pertinent experimental data were obtained for silicon and quartz single crystals. The advantages of the proposed method versus the conventional acoustic technique are analyzed.  

ISSN:0034-6748    

DOI:10.1063/1.1144079

出版商:AIP    

年代:1993

数据来源: AIP