摘要:

The kinetic inductanceLkis associated with the inertial mass of the current carriers. For a long superconductor of lengthland very small cross‐sectional area &sgr;,Lk= (m*/nse*2) (l/&sgr;) and is the main contribution to the temperature‐dependent inductanceLT. We have measuredLTin superconducting tin wires and thin film meander lines by a technique which uses a counter to determine frequency changes of a 15 MHz tunnel‐diode oscillator whose tank circuit containsLT. The measured frequency changes are proportional toz= [&lgr;(t)/&lgr;(0)]BCSfor wires and thick films; for thin films the frequency changes are proportional toz2. This result agrees with calculations ofLTfrom the London theory and the values of &lgr;(0) agree well with the values expected for these samples. This technique allows measurement of the carrier concentration or penetration depth in thin films over the wholeH, Tplane of the superconducting state and can be independent of the penetration law assumed. Frequency shifts observed with thin film alumimum meander lines have been used to detect changes of temperature of 5∼10−7°K, changes of magnetic field of 10−5G, and changes of current associated with one quantum in a flux quantum magnetometer.

ISSN:0021-8979    

DOI:10.1063/1.1657905

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A high‐efficiency electrically and physically small superconducting antenna is reported. Antenna factors and matching circuit equivalents are developed. Theoretical predictions of efficiency andQincrease are derived on the basis of the superconducting surface resistance of lead. The antenna is a loop supsended through a ground plane and matched through a symmetrical coaxial cavity having a resonant frequency of 400 MHz. The loop is lead‐plated and has an open area of approximately 1 cm2. Coupling to the system is achieved through a movable electric‐field probe inserted into a slot in the wall of the matching cavity. A normal‐state electrically small antenna of this size has a theoretical efficiency of less than 1% due to the fact that the loop resistance is much larger than the equivalent radiation resistance. Upon cooling to 4.2°K, theQof the system increases to 20 000 from a room‐temperature value of 156, indicating limitation by the radiation resistance only. The power radiation efficiency increases by a factor of 500, as measured by a fixed external receiving system. The VSWR of the system is maintained near unity in order to avoid the complications of mismatching.

ISSN:0021-8979    

DOI:10.1063/1.1657906

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Slow‐wave propagation of electromagnetic waves in transmission lines formed of thin‐film superconductors has been studied theoretically and experimentally. Previous theoretical analyses have been extended to include nonlocal theories. Strong dependence of phase velocity is found on film thickness and interfilm spacing when these become less than a few penetration depths. Velocity is also modified by coherence length, mean free path, nature of reflection of electrons at the film surfaces, and by temperature and magnetic field. Experimental measurements were made to verify the dependence on thickness, spacing, and temperature by means of a resonance technique. Agreement with theory was excellent in the case of temperature. Data taken for varying thickness and spacing verified the general trend of theoretical predictions. They indicate a nonlocal behavior with some specular reflection, but scatter of the data taken for different films prevents precise comparison of theory and experiment. Estimates of bulk penetration depths were made for indium, &lgr;In= 648±130 Å. For tantalum a rough estimate could be made of &lgr;Ta= 580 Å. Data were consistent with the estimate of coherence length for indium of &xgr;0≈ 3000 Å. Velocity was found to be independent of frequency in the range 50–500 MHz, while losses increased as the square. Pulse measurements indicated that delays of several microseconds and storage of several thousand pulses on a single line are feasible.

ISSN:0021-8979    

DOI:10.1063/1.1657907

出版商:AIP    

年代:1969

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1657909

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Calorimetrically determined energy losses for a series of niobium and niobium‐25 at.% zirconium samples carrying an af transport current agreed with those calculated from hysteresis loops determined by dc magnetization measurements. The results of this study showed that the energy losses in ultra‐high‐purity annealed Nb are less than those for any of the other samples studied at magnetic fields less than 1500 G. The magnetic field dependence of the energy losses in all of the samples studied is given byEL=E12hm, whereELis the energy dissipated per unit surface area per cycle and is independent of frequency,E12is a constant which depends on the material and surface finish, andh=Hp/Hc1, whereHpis the peak ac field amplitude, andHc1is the lower critical magnetic field;m=n1whenh≤1, andm=n2whenh≥1. For both highly reversible and highly irreversible samples,n1≈ 3. For the least reversible samples,n2≈ 4, increasing to ∼8 for the most reversible samples studied.E12generally increased with decreasing reversibility and was strongly dependent on the surface finish of the sample, increasing as the surface roughness increased. The power lossPLper unit surface area is given byPL= (EL)f. This linear frequency dependence is consistent with a hysteretic loss mechanism as is the agreement between the calorimetrically determined losses and those calculated from the measured hysteresis loops.

ISSN:0021-8979    

DOI:10.1063/1.1657910

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A careful numerical analysis of a measured steady‐stateI‐V(current‐voltage) characteristic of a superconducting compound conductor with very fine strands is reported. The actual, nonlinear heat transfer from copper to the helium bath is considered. It is shown that in the current‐sharing state the flux flow resistance of the superconductor,Rf, increases from zero to values much greater than the copper resistance,RCu. ForRf∼RCu, the superconductor currentIsis a function of the fieldH, the voltageV, and the temperatureT. ForRf»RCu,Isis a function ofH, independent ofV, and a linear function ofT. The value of this function extrapolated to the bath temperatureTbis appreciably greater than the critical current. In the absence of flux jumps, the current‐sharing state should be terminated by the steady‐state conditiondI/dV= 0. The actual ``take off'' (sudden transition into the normal state) observed, however, took place below the mentioned steady‐state limit and before the transition from nucleate to film boiling. The new findings on the steady‐state performance of compound conductors with very thin strands make essential modifications of the usual theoretical approaches necessary. It is shown how the new approach can be applied to compound conductors with thick filaments.

ISSN:0021-8979    

DOI:10.1063/1.1657911

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Steady‐state flux jumping has been observed for a thin‐walled tubular sample of NbTi subjected to coaxial superimposed ac and dc magnetic fields. By steady state we mean flux jumping which occurs reproducibly each cycle. The magnitude &Dgr;hof the field shielded by a sample in the critical state decreases with increasing field rate. A rate‐dependent effective resistivity &rgr;effthat exists just prior to a given flux jump has been calculated from &Dgr;hand the field rate. It is found that &rgr;effincreases nonlinearly to a value of about 5×10−9&OHgr;·cm ath= 7×105Oe/sec in a background field of 10 kOe. Such a &rgr;effis characteristic of the flux‐creep state.

ISSN:0021-8979    

DOI:10.1063/1.1657912

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The problem of electrical and thermal stability of a superconductor when carrying a steady‐state transport current beyond the critical current has been analyzed for a bare superconductor. The following relation is derived &ngr; =I(I−1)/1−&thgr;I2, where &ngr; is the nondimensional voltage,Iis the current in units of the critical current,Ic, of the superconductor, and &thgr; is a parameter involving only the constants of the superconductor, and the heat transfer coefficient of the cooling medium. To have a reversible voltage current characteristic beyond the critical current, the parameter &thgr; should be «1. Factors affecting this parameter are analyzed. The influence of this parameter on the propagation of a resistive front is discussed. Furthermore, the influence of a normal metal in good electrical and thermal contact with the superconductor is also included.

ISSN:0021-8979    

DOI:10.1063/1.1657913

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A previously proposed model used to predict transient stabilization in high‐current superconductive solenoids is assessed in the light of results on actual test magnets made with a variety of fabrication parameters. The possibilities for the use of the model is discussed with reference to the missing elements and when considering special ``short‐sample'' measurements on thermal propogation characteristics and magnetization characteristics. Results of recovery‐current tests are presented.

ISSN:0021-8979    

DOI:10.1063/1.1657914

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Magnetization measurements were performed on variously prepared samples of multifilament (Ti&sngbnd;Nb) and Cu composites. All measurements were performed at 4.2°K, utilizing peak magnetic fields of 50 kG. The magnetization data were obtained in a manner similar to that described in earlier papers. The impetus for these particular measurements was the hypothesis of P. F. Smith in regard to the coupling of the small filaments of superconductors when they are embedded in a low‐resistivity matrix such as Cu. This coupling, of course, is a function of ∂Ha/∂t, but for very long lengths (e.g., magnets), or even short lengths in the case of well annealed Cu, the time constant for the decoupling is extremely long. The coupling, it was suggested by Smith, could be reduced by increasing the resistance of the matrix material, and then twisting the composite, the equivalent of ``transposition'' of the individual filaments in a two‐dimensional case. Various resistance layers are formed around each strand by the proper heat treatment of these multifilament composites. The filaments of the sample composites were found to be decoupled to varying degrees depending on applied magnetic field, sample length, and matrix resistivity.

ISSN:0021-8979    

DOI:10.1063/1.1657915

出版商:AIP    

年代:1969

数据来源: AIP