摘要:

A new type of low‐temperature heat switch is described. The switch uses unforced convection of helium gas within a closed loop. We report measurements of the performance of a simple version of the heat switch.

ISSN:0034-6748    

DOI:10.1063/1.1138884

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Using a SQUID‐based second‐order gradiometer, we have measured the thermal magnetic noise generated by a thick aluminum plate and an eddy current shielded room used for biomagnetic measurements. The measured noise agrees with the theoretical prediction for the thermal noise fields generated by infinite conducting slabs. The noise is substantial near the walls of the shielded room, but is predicted to be less than 0.02 fT/&sqrt;Hz at 1 Hz at the center of the room. This is too small to influence the biomagnetic measurements usually taken at this location.

ISSN:0034-6748    

DOI:10.1063/1.1138885

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Diode temperature sensors are capable of being used at the accuracy level of a few hundredths of a kelvin. However, in order to achieve this performance, proper measurement techniques must be used. Poorly shielded or improperly grounded measurement systems can introduce ac noise which will create an apparent shift in the dc voltage reading across a diode sensor. This results in a temperature measurement error which may approach several tenths of a kelvin. The presence of the ac noise in question is not obvious during normal usage and several quick tests are outlined to verify whether or not a noise problem exists. Experimental data and derivations from theoreticalp‐njunction characteristics are given which correlate the ac noise level with possible voltage/temperature measurement errors. These results can be used in estimating the accuracy and performance of a temperature measurement system. Several of the more common problems which introduce noise into diode circuitry are described.

ISSN:0034-6748    

DOI:10.1063/1.1138886

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

An instrument is described for laboratory measurements of the fluctuating static pressure in the turbulent boundary layer above progressive water waves. It consists of a disk‐shaped sensing head properly designed to minimize the dynamic pressure variation to an acceptable level, a commercially available piezocrystal transducer housed inside a casing, and a forward‐bent connecting tube. Pressure fluctuations sampled by the disk are converted into an electrical signal by the piezocrystal transducer. Through low‐pass filtering, only the frequency range of interest is retained. The instrument was tested successfully for frequency response, dynamic and mechanical noise sensitivity, and response to spurious pressure fluctuations (produced when operating in a Eulerian wave‐following mode) inside a cylindrical chamber and in a wind–wave facility, and some sample results along with the calibration procedures and data analysis are presented.

ISSN:0034-6748    

DOI:10.1063/1.1138887

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A new instrument has been developed to map the photovoltaic response of a semiconductor to very penetrating sub‐band‐gap radiation (for silicon, &lgr;=1.3 &mgr;m). The minority carrier density being nearly uniform throughout the thickness of the crystal, the photovoltaic response is dominated by the carrier diffusion length and thus reflects the quality of the material. A computer‐controlled scan of the semiconductor presents a map of the gross quality of the specimen. The actual probe is a small transparent electrode (water) illuminated by an optical fiber carrying 1.3‐&mgr;m light from an LED.

ISSN:0034-6748    

DOI:10.1063/1.1138888

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A quench detector is a device that interrupts the flow of current through a superconductor in the event the superconductor reverts to the normal, resistive state. This new design has adjustable filtering and sensitivity. The input is well isolated from the output, eliminating any possible ground loop through the detector. It also has excellent noise immunity. A detector has operated with no false trips for more than two years, detecting hundreds of quenches.

ISSN:0034-6748    

DOI:10.1063/1.1138889

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A systematic approach to passive shimming is proposed, and has been tested experimentally using a simultaneous gradient cancellation technique. In finding coil adjustments to correct gradients, the iterative application of the pseudoinverse (or generalized inverse) technique is used and a solution provided at each step. Each solution consists of a set of optimum values of adjustments. If the degree of freedom for adjustments is larger than the number of gradients to be corrected, the solution is the minimum norm solution. Through computer simulations, the algorithm is found to converge to a magnetic field homogeneous enough to allow nuclear‐magnetic‐resonance (NMR) imaging in only a few steps even in the presence of internal imperfections and misalignments as large as several 10 mm and an external asymmetric field affecting materials as near as a few meters. High homogeneity as well as fast convergence to a uniform magnetic field has been confirmed experimentally with the KAIS 1.5‐kG magnet. By use of the proposed algorithm, we have experimentally obtained a field homogeneity of a 1.5‐kG resistive magnet twice as homogeneous as that of the specified value with only a few iterations. Although it has been tested in the available low magnetic field, there are no restrictions in applying the technique to various magnets including superconductive magnets widely used in NMR imaging (in this case the algorithm is applied at the assembly stage).

ISSN:0034-6748    

DOI:10.1063/1.1138890

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A microprocessor‐based pulse programmer for pulsed nuclear‐magnetic resonance (NMR) based on the Intel 8086 has been developed. The pulse sequences are specified using a programming language developed specifically for this purpose. Pulse and delay timing is generated by a single timebase operating at 20 MHz, controlled by a state memory. The pulse programmer offers a high degree of flexibility at a moderate cost.

ISSN:0034-6748    

DOI:10.1063/1.1138891

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

This paper describes the design and operation of a time‐of‐flight mass spectrometer that uses fast, heavy ions from the spontaneous fission of Cf‐252 to desorb species from the surfaces of solids. The technique provides for a simultaneous, multimass, isotope specific, elemental, and molecular surface characterization without complications due to surface charging.

ISSN:0034-6748    

DOI:10.1063/1.1138892

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A microprocessor‐based interface is described which interfaces a wide variety of digital instruments to the IEEE‐488 Bus in a very simple manner. Intended for general laboratory use the interface is simple to program and features a modular design which facilitates expansion and modification.

ISSN:0034-6748    

DOI:10.1063/1.1138893

出版商:AIP    

年代:1986

数据来源: AIP