摘要:

We describe the design and construction of a new search for supernovae, based on the use of current computer and detector technology. This search uses a computer‐controlled telescope and charge coupled device (CCD) detector to collect images of hundreds of galaxies per night of observation, and a dedicated minicomputer to process these images in real time. The search system has a detection threshold of 17th visual magnitude, discovered its first supernova on 18 May 1986, and at this writing has found three additional supernovae. The system currently is being upgraded with an improved detector and a more powerful computer. At full speed and sensitivity, the system will be able to search 2500 galaxies per night with a limiting magnitude of 19 or fainter.

ISSN:0034-6748    

DOI:10.1063/1.1139773

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In this article we describe a SQUID‐based system used to measure the superconducting transition temperature in the range 4.2–300 K. The apparatus has been tested using a high‐critical‐temperature single crystal of YBa2Cu3O7−x(&bartil; 10−1mm3volume). The system is based on mutual inductance variations measurements, performed in a low‐frequency ac magnetic field of 2×10−5T. A feedback loop is closed on the input circuit in order to null out the current flowing in the pickup coil. This is achieved using an electronic scheme which does not involve any modification of the commercial SQUID electronics. Our experiment is performed using a copper wire pickup coil, but the scheme can also be used with a superconducting input circuit, thus allowing measurements in a dc regime. The obtained sensitivity is 5×10−5/&sqrt;Hz. This figure can be further improved by optimizing the circuit parameters.

ISSN:0034-6748    

DOI:10.1063/1.1139774

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new high‐stability scanning tunneling microscope (STM) has been developed. Its key design feature is the application of two, concentrically aligned, piezoelectric ceramic tubes, allowingx–y–zmotion of the tungsten tip as well as sample mounting to be cast into a single compact unit. The construction is small, very rigid, and temperature compensated, yielding reduced sensitivity to mechanical and acoustic vibrations and temperature variations. Other advantages of this ‘‘thimble‐size’’ STM include easy sample mounting and, at a later stage, operation under UHV conditions with access to other surface‐analysis (UHV) tools. At present, the microscope is operated in air, and its performance is tested by imaging a highly oriented pyrolytic graphite surface and a Si(111) surface on which a 100‐A˚‐thick Au layer has been deposited.

ISSN:0034-6748    

DOI:10.1063/1.1139775

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A cesium surface ionization source of the type and geometry customarily used in conjunction with sputter‐type negative heavy ion sources has been characterized. Measurements have been made of positive‐ion production and probabilities of ionization as functions of extraction voltage and cesium oven temperature for ionizer porosities of 0.7 and 0.8. The perveancePof the source, when operated in the space‐charge‐limited regime with the &rgr;=0.7&rgr;0ionizer, is found from experiment to be 7.61×10−4&mgr;P, while that for the &rgr;=0.8&rgr;0ionizer is 1.92×10−4&mgr;P. These values are lower by factors of 3.88 and 15.53, respectively, than those predicted by numerical solution to Poisson’s equation for full area emission from the source. Positive‐ion current versus ionizer temperature data are also presented along with mechanical design features of the source.

ISSN:0034-6748    

DOI:10.1063/1.1139776

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The basic parameters of the refocus geometry cesium sputter ion source for the production of negative ions from a low‐sputtering material (carbon) and a high‐sputtering material (copper) have been measured. The question of the influence of sample temperature on the negative‐ion generation process has been addressed. Negative‐ion yields from carbon samples are found to be weakly dependent on temperature over a range from 26° to 400 °C, while those from copper samples are found to be essentially independent of temperature over a range of 26° to 200 °C. The results of these investigations indicate that (1) sample temperature does not strongly influence negative formation in these sources, and (2) the maximum negative‐ion yields for a given sample are dependent on both cesium‐ion energy and positive‐ion current. In addition to these data, the negative‐ion extraction optics of the source are briefly discussed and cursory estimates of the probabilities for negative‐ion formation presented.

ISSN:0034-6748    

DOI:10.1063/1.1139777

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new method for the evaluation of the electron temperature from the voltage–current characteristic of a single Langmuir probe under the application of an ac signal was developed and tested. This method exploits harmonics in the current spectrum which arise from the nonlinearity of the characteristic; from the amplitude of these harmonics the electron temperature can be deduced. Langmuir probe measurements on the TEXTOR edge plasma show a good agreement of the new method with the conventional one. The advantages of this method over other methods are discussed. Proposals for further improvements are given.

ISSN:0034-6748    

DOI:10.1063/1.1139724

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Several sinusoidal and saddle coils have been used to measure the toroidal current centroid of discharges produced in a pulsed tokamak experiment. Careful analysis of the external coil measurements and elaborate calibrations made the measurements possible even for large displacements of the plasma current and the noncircular geometry of the diagnostic coils. The calibration techniques and the data analysis formula are described, and examples of the use of the external coils to measurem=1 kinks of unstable toroidal Z‐pinch discharges are presented.

ISSN:0034-6748    

DOI:10.1063/1.1139725

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A multichannel, two‐color, quadrature heterodyne interferometer is used to measure the line density in the DIII‐D tokamak. The unique feature of this real‐time vibration‐compensated interferometer is the combination of high speed (1 MHz), high resolution (2&pgr;/256), and wide range (±8193 fringes). Quadrature phase information from a CO2laser (10.6 &mgr;m) and a He–Ne laser (0.63 &mgr;m) are digitized with high‐speed (6 MHz) flash digitizers. Zero crossings of the signals are counted with digital circuitry yielding quarter fringe resolution with a 4‐MHz bandwidth. Further fringe resolution of 1/256 is provided at 350 kHz by a PROM which uses the digital signals as input to a look‐up table. Analog line density is presently available at 80 kHz with a system noise equivalent phase shift of ±2/256. Error monitoring is provided for low signal amplitude and exceeding the maximum fringe rate. In addition, a method to prevent coating of in‐vessel mirrors due to plasma and vessel wall cleaning discharges has been developed.

ISSN:0034-6748    

DOI:10.1063/1.1139726

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An algorithm has been developed for fast analysis of a sparse multichord interferometer array, yielding peak density, Gaussian width, radial offset, ellipticity, and instability amplitude and frequency from line‐integrated density measurements. A nonlinear least‐squares fit is performed over many short time intervals, with independent variables reduced to one dimension. Assumptions of plasma rigidity and slowly evolving rotation rate are necessary to trade temporal resolution (on instability timescales) for spatial resolution across the moving plasma column. Results from the Tara tandem mirror experiment are shown.

ISSN:0034-6748    

DOI:10.1063/1.1139727

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Distributed electron cyclotron resonance or DECR, a new plasma excitation technique well adapted to microwave multipolar plasmas, is described. The novel reactor concept is based upon the use of several microwave linear applicators working at the ECR mode along the multipolar confinement magnets. At a microwave frequency of 2.45 GHz, large volumes of highly homogeneous plasmas with an electron temperature of the order of 3 eV and ion densities better than 1011cm−3are currently being produced in DECR reactors.

ISSN:0034-6748    

DOI:10.1063/1.1139728

出版商:AIP    

年代:1988

数据来源: AIP