摘要:

The variation in heat transfer between a fine wire and the surrounding fluid has traditionally been the most accepted method for measuring the detailed movements of a flow fluid. This paper reviews the present understanding of thermal techniques as used in research and recent advancements of the technique. While the basic research sensor is still a fine wire, modern electronics has greatly simplified data analysis as well as established constant temperature operation of the sensor as the control method of choice. Film sensors have extended the use of thermal sensors to new applications. Comparisons with laser Doppler velocimeters help to identify experiments where thermal sensors still represent the best choice. The ability of modern electronics to easily address the typical nonlinear response of thermal sensors has resulted in increased use for commercial applications. While in these applications fast response can be a factor (e.g., fuel flow control in engines), generally only average flows are needed. As a result, calibration shifts due to deposits on the sensor surface when exposed to contaminated fluids can generally be reduced to acceptable levels by using larger sensors.

ISSN:0034-6748    

DOI:10.1063/1.1145187

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The design and performance of an instrument to measure cosmic microwave background (CMB) anisotropy on angular scales from 10’to 30’is reported. The instrument utilizes a single‐mode waveguide bolometer cooled by an adiabatic demagnetization refrigerator. This is the first time a 100 mK bolometer has been used in a CMB experiment. Observations of the CMB were made using this radiometer from the South Pole during the 1991–1992 season and, with minor modifications, during the 1992–1993 season. The radiometer is used with a fully steerable altitude‐azimuth Cassegrain telescope built for this observation, also described here.

ISSN:0034-6748    

DOI:10.1063/1.1145188

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A novel method has been developed to stabilize the amplitude of a multiwavelength laser beam. The method is based on the use of an acousto‐optic tunable filter to diffracted light which has more than one wavelength. The number of the wavelengths in the diffracted light is controlled by the number of the rf signals which are applied into the filter. Because the intensity of each wavelength in the diffracted beam is dependent on the power of the corresponding applied rf signal, the total amplitude of the multiwavelength diffracted beam can be stabilized, i.e., adjusted at a constant amplitude and maintained at that level, by individually controlling the intensity of each corresponding wavelength by use of an rf driver with a feedback loop for each wavelength. In addition, the method also provides a unique means to amplitude‐modulate one (or more) wavelength(s) of a multiwavelength beam at a specific frequency while keeping the other wavelengths at a constant amplitude level.

ISSN:0034-6748    

DOI:10.1063/1.1145189

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The construction characteristics and the performances of an x‐ray diode working at 200 Hz are described. The temporal behavior of both the plasma cathode emission and the diode discharge has been studied, including the measurements of vacuum effects on the repetition rate and lifetime capability, preionization versus electrical features, and ionization density versus dosage. This simple and low‐cost x‐ray diode has been used to uniformly preionize a half‐liter XeCl laser, delivering an output laser energy of 800 mJ/shot at a 100 Hz repetition rate.

ISSN:0034-6748    

DOI:10.1063/1.1145190

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A pulsed power supply has been developed for exciting a copper vapor laser with a bore diameter of 80 mm and a discharge length of 2890 mm. The pulsed power supply has two thyratrons, an LC‐inversion circuit and a two‐staged magnetic pulse compression circuit. An efficiency of 82% has been obtained which is defined as the ratio of energy stored in the storage capacitor to the output energy of the magnetic pulse compression circuit, with a voltage greater than 40 kV, averaged output power of 24 kW, and voltage rise time of 70 ns. The pulsed power supply was successfully operated for more than 2000 h and an optical output power of 272 W was obtained.

ISSN:0034-6748    

DOI:10.1063/1.1145245

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A spectroscopic total integrated scattering instrument, which uses a focusing sphere and a broadband light source, has been constructed. It records total reflectance and transmittance spectroscopically, in the wavelength region 400–1000 nm. Diffuse reflectance and transmittance values below 10−4can be registered. These measurements require low scattering optical components and low noise electronics. Design details are given about the detection system, particularly the low noise preamplifier. The purpose of the instrument is to measure diffuse reflectance and transmittance spectra of interference coatings. Such spectra can give information about the amplitude of the roughness of the interfaces and reveal if the interfaces are topographically correlated or uncorrelated. Examples of spectra in both reflectance and transmittance mode are given.

ISSN:0034-6748    

DOI:10.1063/1.1145191

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We describe an apparatus for multiangle light scattering studies of samples that are under hydrostatic pressures. The mean hydrodynamic radius and mean radius of gyration can be determined using quasielastic and static light scattering at temperatures between 5 and 85 °C and pressures between 0.1 and 100 MPa.  

ISSN:0034-6748    

DOI:10.1063/1.1145192

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We have developed an x‐ray photoelectron spectrometer system which combines an adjustable grazing incidence angle source with reflected beam detection. When operated about the critical angle, this combination permits a variation of the x‐ray penetration depth which can be monitored by means of the reflectivity. At angles of incidence less than the critical angle, the sampling depth of the photoemission is diminished, but the photoemission from the surface is enhanced due to the constructive interference of the incident and reflected x‐ray beams. When used with Mg K&agr; radiation (E&ggr;=1253.6 eV), the spectrometry system obtains useful distributions of chemical species in surface layers of 10–40 A˚ thickness. We present data showing the depth dependence obtained with the spectrometer of different oxides in a sulfide‐treated, oxidized GaAs (100) surface.

ISSN:0034-6748    

DOI:10.1063/1.1145193

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We used a GaAs crystal operating in a negative electron affinity mode to produce an intense continuous electron beam by photoemission. The major drawback of photoemission from GaAs, i.e., rapid current decay, was overcome without continuously supplying cesium. After a little initial decay, the current remained constant at 1 mA over a few mm2, with no degradation. Seasoning of the vacuum chamber played a fundamental role in achieving this performance.

ISSN:0034-6748    

DOI:10.1063/1.1145194

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The design and characterization of a high sensitivity multichannel momentum dispersive electron momentum spectrometer is reported. Electron pair coincidences are detected over a ±30° range of azimuthal angle at the exit slit of a cylindrical mirror analyzer configured to measure symmetric (&thgr;=45°) noncoplanar (e,2e) scattering. A position sensitive microchannel plate/resistive anode encoder electron detector and a channeltron electron multiplier are employed in a novel multichannel coincidence detection system based upon detection of ‘‘pile‐up’’ of pairs of timing pulses from the detectors. Proof‐of‐concept results, exhibiting significantly higher coincidence count rates and statistical precision than have been previously observed in electron momentum spectroscopy, are presented for argon.

ISSN:0034-6748    

DOI:10.1063/1.1145195

出版商:AIP    

年代:1994

数据来源: AIP