摘要:

A method to determine the variation of thermal conductivity as a function of temperature in silicon is described. The method is based on the application of a photothermal displacement technique in the temperature range 30–300 K. The results obtained on samples with different types and dopant concentrations are shown to be in good agreement with those reported in the literature.

ISSN:0034-6748    

DOI:10.1063/1.1143966

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An interferometer is presented which is capable of measuring the free‐surface velocities and displacements of both specular and diffuse surfaces. The setup utilizes a previously used principle of producing a virtual image of one mirror at the same distance from the photodiode as the second mirror of the interferometer, albeit with considerable simplification. It is shown that use of a He–Ne laser of only 5‐mW power can produce high contrast displacement fringes from surfaces of materials with nonuniform microstructure, including composites. Substrates of carbon–carbon composites and polycrystalline alumina with nonuniform microstructure on the scale of 5–10 &mgr;m, and with peak velocities up to 150 m/s were considered. An experimental strategy which allows one to covert the optical setup to either a velocity or a displacement interferometer is also discussed. It is further shown that use of a fast photodiode and a high‐speed digitizer with a 5‐ps rise time provides a time resolution of 0.2 ns for recording the displacement fringes, and allows measuring free surface velocities up to 800 m/s. This is demonstrated by measuring such transient surface velocities with rise times of 1 ns on a specular Si surface. In all the experiments reported here, the surface velocities were produced by the reflection of a stress wave, which in turn was generated on the back surface of the substrate, using a Nd:YAG laser pulse.

ISSN:0034-6748    

DOI:10.1063/1.1144452

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We have developed a triple‐pendulum vibration isolation system for suspended mirrors used in a laser interferometer for gravitational wave detection. Owing to the passive damping method using high‐performance permanent magnets, the system has a very simple, compact structure. We measured the isolation ratio for vibration below 40 Hz and confirmed that the observed and calculated values are in good agreement. The stability of the system has been demonstrated by operating a Michelson laser interferometer incorporated with a mirror mounted on the vibration isolation system.

ISSN:0034-6748    

DOI:10.1063/1.1143967

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Using a block of anisotropic material from the point of view of thermal expansion, six bars are cut in different spatial directions. A double‐exposure photograph of speckle is taken of the six bars, one before and one after the temperature is increased. Only one double exposure is required for the six bars altogether. Illuminating two points of the double specklegram of each bar with a narrow laser beam and measuring the angle between the diffraction maxima, the longitudinal expansion is obtained for each bar and the thermal expansion tensor of the material is calculated.

ISSN:0034-6748    

DOI:10.1063/1.1143917

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A nanometer displacement probe is created by using a knife edge, a two‐part‐divided photodiode, and high quality lenses. The laser beam, of wavelength 780 nm, is focused on the sample surface, where the image formed of the beam is read by the photodiode through the same optical path. The displacement signal is found to originate from the diffracted image change at the surface. A dynamic range of 2 &mgr;m and a minimal resolution of 1 nm for a lens off=3.6 mm are obtained. The difference current, when normalized by the summation current, becomes a function of displacement and is independent of the sample surface reflectivity. As an application, a piezoceramic piece having a mean roughness of 1.03 &mgr;m was used. The frequency characteristics measured by this mechanical displacement method and the conventional electric oscillation measurement method were compared and found to be in good agreement.

ISSN:0034-6748    

DOI:10.1063/1.1143918

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Capacitive transducers have proven to be very effective sensors of small displacements, because of inherent stability and noninvasive high resolution. The most versatile ones have been those of a differential type, in which two elements are altered in opposite directions in response to change of the system parameter being monitored. Oftentimes, this differential pair has been incorporated into a bridge circuit, which is a useful means for employing synchronous detection to improve signal to noise ratios. Unlike previous differential capacitive dilatometers which used only two active capacitors, the present sensor is a full‐bridge type, which is well suited to measuring low‐level thermal expansions. This analog sensor is capable of 0.1 &mgr;m resolution anywhere within a range of several centimeters, with a linearity of 0.1%. Its user friendly output can be put on a strip chart recorder or directed to a computer for sophisticated data analysis.

ISSN:0034-6748    

DOI:10.1063/1.1143919

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Differential capacitive sensors are common to pressure gauges. Previous sensors of the bridge type have utilized only two variable (active) components, resulting in a half‐bridge electrical equivalent circuit. There are numerous advantages of a symmetric full bridge, using four active components. The symmetry improves linearity and increases signal to noise ratio, especially when supported by electronics based on synchronous detection. Another advantage of symmetry is invariance to scaling, through immunity to stray capacitance. This feature is especially important in relationship to the current trend toward micro‐electro‐mechanical systems. A crude, inexpensive, macroscopic prototype of the present design was built with attention to high sensitivity. Constructed with a thin aluminized Mylar diaphragm, this device can readily resolve pressure changes smaller than 10−2Pa.

ISSN:0034-6748    

DOI:10.1063/1.1144451

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In magnetic fusion toroidal confinement devices, the confining magnetic fields may have errors which produce magnetic islands. The standard method of investigating magnetic field errors involves measuring the maximum radial spatial extent of a given magnetic island at a fixed toroidal location and calling this measurement the island width. However, because of magnetic flux conservation inside an island, the spatial width of the island is not toroidally invariant. Magnetic error fields and their associated islands have been investigated in the Compact Auburn Torsatron through the measurement of the local rotational transform about the fixed points of the island. This method of magnetic field error investigation is valid at any poloidal or toroidal angle because the local rotational transform is spatially invariant. The technique of measuring the local rotational transform is described and the results are compared to those obtained using standard methods of measuring island widths.

ISSN:0034-6748    

DOI:10.1063/1.1143920

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A sensitive interferometer based on a Michelson scheme was constructed and applied to low‐density pulsed‐plasma measurements. Using a moderate power He–Ne laser and common photodetectors, a sensitivity as high as 1/1000 FWHM of an interference fringe was reached with a minimum line‐integrated plasma density as low as ⟨NeL⟩min=8×1013cm−2. This sensitivity was obtained in a real plasma experiment with space and time resolution of 1–2 mm and 120 ns, respectively. The time resolution for this technique is restricted mainly by the oscilloscope finite input capacitance and could be made higher but with a loss of sensitivity. In order to obtain such high a sensitivity, we used loads for the photocurrent registration higher than the 50‐&OHgr; matched load. The initial phase is not assumed to be optimized and is recorded separately during the operation of the interferometer. Suppression of electrical noise and of instabilities associated with the laser power requires a supplementary detector for background subtraction to improve the signal‐to‐noise ratio. A description of the high sensitivity interferometer with the passive beam power stabilization, analysis of the data treatment procedure, as well as measurements for an electron density distribution, are presented.  

ISSN:0034-6748    

DOI:10.1063/1.1143921

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Results of an experimental study of a relativistic plasma generated by means of the gyromagnetic autoresonance in the plasma synchrotron accumulator called GYRAC‐D are presented. The use of a new heating scheme in the regime of a train of pulses with the further accumulation of the generated plasma in a mirror magnetic configuration and improvement of vacuum conditions owing to a system of external injection make it possible to obtain a plasma with the following parameters:Te≊200 keV,ne≊1×109cm−3, and lifetime approximately 1 s. No major obstacles to further increase the density are observed.

ISSN:0034-6748    

DOI:10.1063/1.1143922

出版商:AIP    

年代:1993

数据来源: AIP