ISSN:0021-8979    

DOI:10.1063/1.1657891

出版商:AIP    

年代:1969

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1657893

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Heat transfer from a cyclindrical copper surface to liquid helium at 4°K has been measured. The sample consisted of a horizontally mounted 3 mm o.d., evacuated glass tube, 6.5 cm long, onto which was deposited a 2000 Å‐thick copper film. It was heated by passing an electrical current through the copper film, and the temperature was monitored by a carbon resistor attached to the inside of the glass tube with varnish. In initial experiments, large temperature fluctuations were observed in the nucleate boiling region. These temperature fluctuations disappeared when the sample was either close to the liquid helium surface or when vertical walls were placed around the sample. The large temperature fluctuations were probably due to interactions of the convective flow of liquid helium with the boundaries of the system (Dewar walls, liquid surface, etc.). Similar temperature fluctuations were observed with a stainless steel sample of approximately the same dimensions. The peak heat flux obtained in the nucleate boiling regime was 0.98 W/cm2for Cu and 0.66 W/cm2for stainless steel.

ISSN:0021-8979    

DOI:10.1063/1.1657894

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Superconductivity studies have been performed on someA‐15 pseudo‐binary systems to determine whether low‐temperature annealing, conducive to atomic ordering, will markedly raise the superconducting transition temperature. The following systems were examined for this behavior: Nb3Ge&sngbnd;Nb3Al, Nb3Sn&sngbnd;Nb3Al, and Nb3Ga&sngbnd;Nb3Al. Greater attention was devoted to the Nb&sngbnd;Al&sngbnd;Ge system in view of the recent observation of superconductivity above 20°K in this system. We have also observed transition temperatures approaching 20°K in nearly single phase samples of Nb&sngbnd;Al&sngbnd;Ge after low‐temperature heat treatment at 700° and 800°C. It is suggested that crystallographic long‐range ordering is responsible for the highTc's observed in this system. Attempts to confirm this hypothesis by determining the degree of long‐range order were unsuccessful. The otherA‐15 systems which we investigated also exhibited an increase ofTcwith low‐temperature heat treatment; however, none exceeded 18°K.

ISSN:0021-8979    

DOI:10.1063/1.1657895

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Many high‐field superconducting systems make use of copper as part of the conductor configuration. Copper is present primarily because it combines low resistance with moderately high strength. As copper is exposed to high transverse magnetic fields its resistance increases. This resistance increase, or magnetoresistance, has been experimentally measured for several different grades and tempers of commercially available copper at 4.2°K and in the field range from 0 to 100 000 G. Zero field‐resistance ratios varied from 60 to 1200 due to differing degrees of purity and amounts of cold work. The results of these measurements fit a normalized equation of the type which gives the fractional change in resistance as a function of transverse field and zero field‐resistance ratio. The mechanical properties for these samples have been measured at room temperature and 4.2°K and the yield stress correlated with the resistance properties.

ISSN:0021-8979    

DOI:10.1063/1.1657896

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A computer model is described which is capable of predicting the maximum current, the ``threshold current'', which can be carried without quenching a Nb&sngbnd;Ti copper composite superconductor undergoing flux jumping. The model utilizes standard equations governing the generation and flow of heat in a wire to predict its voltage‐versus‐time response as a function of current. Calibration of the model for any specific thermal environment is done by means of a simple test procedure involving measurement of the response following the triggering of a flux jump in a small noninductively wound solenoid of one to four layers. The combination of theoretical and experimental techniques indicates the value of adequate copper cross section and good thermal environment, and suggests the superiority of porous fabric or cadmium foil over more conventional interleaving materials, and of thin insulation over the alternatives of thick insulation or none at all.

ISSN:0021-8979    

DOI:10.1063/1.1657897

出版商:AIP    

年代:1969

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1657901

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A new process for fabricating niobium rf cavities for use in beam separators and linear accelerators is described. The best performance to date for a cavity made by this process was obtained with a TE011mode cavity, operating at 11.2 GHz at a peak magnetic field of ∼500 G, for whichQ≈2×1010. The principle advantages of the process described over other methods of fabricating niobium rf cavities are the following. First, a multiplicity of integrally connected cavities can be fabricated in a single joint free section. Second, the finished niobium cavities have a very high‐quality surface finish which is produced without the need of mechanical or chemical polishing the cavities after fabrication. Third, the niobium in the completed cavity is ultrapure and thus has an extremely low normal state resistance at liquid helium temperatures. Fourth, a high thermal conductivity between the niobium on the inside of the cavity and the surrounding liquid helium bath can be obtained when the niobium is supported with porous tungsten.

ISSN:0021-8979    

DOI:10.1063/1.1657902

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The resonant frequency of a superconducting cavity has been photodielectrically controlled by a feedback loop which contains an optical path. The cavity is an 870‐MHz lead‐plated quarter‐wave reentrant structure. A 19 000 &OHgr;·cm silicon wafer terminates the quarter‐wave stub. The loadedQis approximately 105indicating the losses in the semiconductor dominate the superconducting surface resistance. The highQprovides a large phase error between the input and output voltage when the driving oscillator drifts or is modulated away from the very stable resonant frequency of the cavity. The error is processed by a wide‐bandwidth phase detector that drives a gallium arsenide diode to follow the frequency excursion of the oscillator by photodielectric tuning. The 9000 Å light is the optical feedback signal which corrects the phase error. The system response typically provides a frequency deviation of ±50 kHz at a 0.2 MHz rate. Higher cavity frequency and purer semiconductor samples extend the deviation limits and shorter free‐carrier lifetimes increase the speed of response.

ISSN:0021-8979    

DOI:10.1063/1.1657903

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The excellent frequency stability and cryogenic environment of a superconducting resonant cavity provides a sensitive method for observing trap‐filling in CdS and similar materials. When used with thermally stimulated conductivity and dc photoconductivity, it is possible to solve for trap energy, capture cross section, density of trap states, and free‐carrier lifetime. The technique is that used by Arndt, Hartwig, and Stone to observe optically induced changes in the complex dielectric constant by inertia forces on free carriers in Si and other indirect‐gap semiconductors. Using TSC, pure CdS crystals showed very weak trapping effects and CdS: Al displayed electron traps at 0.52, 0.35, and a group at 0.265, 0.20, and 0.15 eV. A quenching, or hole trapping, level was seen at 1.6 eV below the conduction band. Deep electron trap densities were about 1015cm−3and the shallow set was about 1017cm−3each. Hole trap density was slightly in excess of 1018cm−3. In CdS: Al, the photodielectric frequency shift of an 840 MHz cavity was proportional to the integral of photon flux, indicating the effect was caused by polarization of electrons in traps. Photodielectric data yielded an effective trap energy for the three shallow levels at 0.176 eV and a total density of 3×1017cm−3. Density of trapped electrons is calculated from frequency change. Capture cross section for the shallow levels was about 10−14cm2. Additional data from dc photoconductivity provide free‐carrier lifetime and location of the electron Fermi level as a function of filled trap density. CdS:Al displays a ``tap'' effect which can erase the accumulated frequency change without warming.

ISSN:0021-8979    

DOI:10.1063/1.1657904

出版商:AIP    

年代:1969

数据来源: AIP