摘要:

A superconducting quadrupole doublet for beam focusing has been operated with a 30‐kG field at the center of the 30‐cm‐long straight section of the magnets and a 3‐kG/cm field gradient. The magnets used twisted multifilament Nb&sngbnd;Ti superconductor imbedded in a Cu matrix of 0.050‐in. diameter and operate at currents up to 500 A. Each magnet is equipped with a persistent mode superconducting switch. The room‐temperature beam aperture of the magnet‐Dewar system is 15 cm. The focal length of the magnet pair is about 1 m for 500‐MeV pions. The magnets are mounted in a Dewar with thermally compensated supports to minimize motion upon cooling and a soft iron shield reduces external fields to less than 60 G.

ISSN:0021-8979    

DOI:10.1063/1.1659656

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Two superconducting 20‐kG dipole magnets have been designed and constructed at the National Accelerator Laboratory. They are of a type called ``superferric'' since they consist of an iron yoke and superconducting coil. A 76‐cm‐long model providing a 4×10‐cm aperture operated successfully at 20 kG with a conductor current density of 36 kA/cm2and a stored energy of 8 kJ.1A 3‐m‐long prototype requiring 17 kA/cm2to reach 20 kG in a 4×13‐cm aperture will be tested soon. The iron yokes of these magnets are refrigerated to 4.2 K. Niobium‐titanium multifilament conductor is used. The coils are short free and the magnets can be charged to full field in 5 min or less. A 60‐cm‐long, 4‐kG/cm iron‐free quadrupole with a 15‐cm bore is being constructed. The field is shaped by coil sectors of 18° and 36°. A conductor current density of 21 kA/cm2is required to achieve a gradient of 4 kG/cm. The stored energy will be about 300 kJ at this gradient.

ISSN:0021-8979    

DOI:10.1063/1.1659658

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Superconducting magnets with long charge times require a power supply that will hold a constant voltage across a magnet during the charge, hold, and discharge periods. Conventional supplies regulate poorly during hold and lose control completely during discharge of the magnet. A simple circuit has been designed to connect between a magnet and the remote sense input of a conventional power supply which permits control and regulation at all times. During the testing of multisection superconducting magnets, it is desirable to know the order and approximate time interval in which each section undergoes a transition to the normal conducting state. A circuit has been designed which detects and displays this information.

ISSN:0021-8979    

DOI:10.1063/1.1659660

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The temperature‐independent residual surface resistanceRresof high‐Q‐superconducting resonators is up till now not well understood. Experiments in lead‐plated copper cavities and massive niobium cavities (&ngr;≃GHz) have shownRres∝&ohgr;1.8±0.2, which excludes normal conducting regions as a cause for the rf losses. An explanation is presented which can describe the observed dependencies on frequency and on surface treatment: The losses are caused by the generation of phonons in fissured surfaces by locally excited rf fields. These losses are temperature independent, giveRres∝&ohgr;2and can also explain the observed mode dependence in lead‐plated copper cavities.

ISSN:0021-8979    

DOI:10.1063/1.1659661

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

A critical power dissipation resulting from an oscillating magnetic field,Hpcos&ohgr;t, can produce a magnetic breakdown field,Hp′ <Hc, the critical field of the superconductor. The analysis shows, for example, why the breakdown field of a superconducting microwave cavity can be well belowHcin some cases, and indicates what the functional dependence of the cavityQmay be for values ofHpnearHp′. The effective resistivity of a single isolated oscillating fluxoid, as well as that of a stationary normal region, is also derived for both type I and type II superconductors.

ISSN:0021-8979    

DOI:10.1063/1.1659662

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

We report the effect of adding up to 23% tin to the indium film of a thin‐film/tantalum‐oxide/bulktantalum transmission line. Addition of tin reduces the velocity and increases the delay for fixed length by 1.4% for each percent of tin. Agreement with the predictions of Pippard's nonlocal theory when meanfree path is reduced is excellent. Pulse attenuation and shape degradation are not increased by addition of tin. Attenuation as low as 10 dB per microsecond of delay was observed at 1.25 K. The added tin reduces by 50% the sensitivity of velocity to temperature nearTc, and improves the reproducibility of velocity from line to line. Critical temperature is increased to 6 K for 23% tin, in good agreement with previous measurements.

ISSN:0021-8979    

DOI:10.1063/1.1659663

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Magnetization and susceptibility data are presented for niobium rod samples in the temperature range of 1.2°–8.9°K. The niobium samples tested had a range of tantalum content from 700 to 4000 ppm. The samples were annealed and degassed in high vacuum at temperatures between 2100° and 2300°C. Values of &kgr;1, &kgr;2,Hc1,Hc2, andHcare presented as a function of tantalum impurity, and these values are compared with theoretical predictions.

ISSN:0021-8979    

DOI:10.1063/1.1659534

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Microwave superconducting mixers operating at 10–13 GHz utilizing Josephson junctions have been investigated analytically and experimentally. Measurements on mixer sensitivity were carried out at 13 GHz using waveguide‐mounted experimental point‐contact Josephson junctions at 4.2 K. Results showed sensitivities of −90 dBm/MHz (10−18W/Hz) which included mismatch losses encountered in the waveguide structure. The Josephson junction mixing mechanism was analyzed on the basis of small‐signal modulation of the staircase like dcI‐Vcharacteristics. It was shown that a small‐signal microwave excitation results in an ac lateral shift of the regions between steps, such that: (a) Optimum Josephson mixing occurs halfway between steps. (b) Mixing occurs between all steps with maximum conversion efficiency between the zero and first step. (c) Predicted low conversion loss and possibly internal gain for a matched mixer. The analytical model was simulated by a digital computer and the behavior of the Josephson mixer predicted for various conditions of excitation. The model chosen for the analysis and the computer simulation is based on the simplest assumptions concerning bulk‐formed Josephson junctions. In the laboratory an external local oscillator set 60 MHz below the signal frequency was used to induce the well‐known step structure in the dcI‐Vcharacteristic of the point‐contact Josephson junction. Mixing of theLoand signal was found to occur between constant voltage steps in agreement with both analysis and computer simulation. The correctness of the model is demonstrated by the agreement between the theory and experiment.

ISSN:0021-8979    

DOI:10.1063/1.1659535

出版商:AIP    

年代:1971

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1659538

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

It is well established that critical current densities in hard superconductors depend on pinning of the fluxoid lattice by microstructural inhomogeneities against electromagnetic forces. However, the connection between the measured critical current densities which are continuum quantities and the interactions between the pinning objects and the fluxoid lattice which arevirtualpoint interactions has presented a rather complex problem that is the focus of this review. An experimental approach to diagnosis of the mechanisms and a rather general phenomenological theory are described and illustrated by a summary of some studies of hard superconductors containing high lattice‐dislocation densities that provide a high density of rather weak pinning objects. Conclusions of general applicability are summarized.

ISSN:0021-8979    

DOI:10.1063/1.1659539

出版商:AIP    

年代:1971

数据来源: AIP