Ironless Cryogenic Current Comparators for AC and DC Applications

The ac and dc errors of cryogenic current comparators with toroidal-shield configurations are reported. Based on these experiments and on theoretical considerations, geometric parameters of the superconducting shields could be determined, which govern the errors. These results may be used for the construction of cryogenic current comparators with predictably low errors. It seems to be possible to decrease the dc error far below our present resolution limit of 3 X 10-11.     

Self-Calibrating Cryo Current Comparators for AC Applications

The outstanding dc properties of cryo current comparators make it possible to build ac comparators practically free of error. Special coaxial windings allow the capacitive error of any 1/N comparator to be compensated for by means of two separated self-calibration measurements for the primary and the secondary winding. It is shown that errors can be kept as low as 1 X 10-8 at 10 kHz.     

High-Precision DC-Current Source

For applications in the field of high-precision electrical measurements, specially on superconducting circuits, a dc-current source has been developed, which supplies two dc currents with selectable ratios in the range between 10 mA and 5 A. The performance of the instrument relies on the combination of conventional dc-current comparators with low ripple sampling detectors and ironless cryogenic current comparators with a SQUID as zero detector. Thus the current ratio is stabilized to 1 X 10-9 of its value.     

Field attenuation as the underlying principle of cryo-current comparators 2. Ring cavity elements

In Part 1 the connexion between the error of a cryo-current comparator and the magnetic field attenuation in the gap region of its superconducting shield has been treated for the special but very common case of co-axial gap regions. This paper extends the error analysis on gaps forming a ring cavity. Such gap elements occur in toroidal cryo-current comparators as a link between two adjacent co-axial cavities. The results for these comparators are: the only field component which may penetrate the gap without attenuation is the one containing the useful signal about the current linkage of the comparator. All other field components contribute to the error of the comparator. They are attenuated according to their degree of symmetry. As the exact way the error functions is known it will be possible to construct cryo-current comparators with a predictable upper limit to their dc error.     

Superconductive dc transmission lines: design study study and cost estimates

A conservative design study of a dc superconductive power line based on present day technology is presented. Power is transmitted by opposing currents in coaxial conductors, formed from helically wound tapes of composite, with a potential difference between them. Conductor temperature stability and fault current conditions are shown to be the factors which dominate the conductor cost; both are analysed. The capital cost of a cable is expressed as a function of a cable dimension, the basic cable parameters, and the material properties. This cost may be minimized with respect to cable radius. The resultant optimum magnetic fields vary little with power rating and are quite low at about 1–2 × 10⁵ A m^?1. The capital cost of such cables, exclusive of terminal costs, is less than the cost of conventional dc cables at power levels above about 1 GVA. Although extensive development would be necessary, the basic principles needed to build a dc superconductive cable are known.     

Precision Tests of a Quantum Hall Effect Device DC Equivalent Circuit Using Double-Series and Triple-Series Connections

Precision tests verify the dc equivalent circuit used by Ricketts and Kemeny to describe a quantum Hall effect device in terms of electrical circuit elements. The tests employ the use of cryogenic current comparators and the double-series and triple-series connection techniques of Delahaye. Verification of the dc equivalent circuit in double-series and triple-series connections is a necessary step in developing the ac quantum Hall effect as an intrinsic standard of resistance.     

A Low-Temperature Direct-Current Comparator Bridge

The application of superconducting direct-current comparators to the measurement of resistance ratios is described. One comparator consists of a binary set of ratios between 1:1 and 160:1 providing for self-calibration by a buildup procedure. A second comparator exhibiting discrete ratios of 1:1, 10:1, and 100:1 is also described. Ratio uncertainty of less than 1 part in 109 is achieved by enclosing the ratio windings in overlapping toroidal superconducting shields. Superconducting quantum interference devices (SQUID's) serve as flux sensors for the comparators. One of these current comparators is used to calibrate a 100-?:1-? resistive divider, which at a current of 10 mA exhibits a self-heating error of 0.0023 ppm.     

Flicker (1/f) noise in tunnel junction dc SQUIDS

We have measured the spectral density of the 1/f voltage noise in current-biased resistively shunted Josephson tunnel junctions and dc SQUIDs. A theory in which fluctuations in the temperature give rise to fluctuations in the critical current and hence in the voltage predicts the magnitude of the noise quite accurately for junctions with areas of about 2 × 10⁴ µm², but significantly overestimates the noise for junctions with areas of about 6 µm². DC SQUIDs fabricated from these two types of junctions exhibit substantially more 1/f voltage noise than would be predicted from a model in which the noise arises from critical current fluctuations in the junctions. This result was confirmed by an experiment involving two different bias current and flux modulation schemes, which demonstrated that the predominant 1/f voltage noise arises not from critical current fluctuations, but from some unknown source that can be regarded as an apparent 1/f flux noise. Measurements on five different configurations of dc SQUIDs fabricated with thin-film tunnel junctions and with widely varying areas, inductances, and junction capacitances show that the spectral density of the 1/f equivalent flux noise is roughly constant, within a factor of three of (10^?10/f)? ₀ ² Hz^?1. It is emphasized that 1/f flux noise may not be the predominant source of 1/f noise in SQUIDS fabricated with other technologies.     

Noise-Optimized Silicon Radiometers

This paper describes a new, experimentally verified, noise analysis and the design considerations of the dynamic characteristics of silicon radiometers. Transimpedance gain, loop gain, and voltage gain were optimized versus frequency for photodiode current meters measuring ac and dc optical radiation. Silicon radiometers with improved dynamic characteristics were built and tested. The frequency-dependent photocurrent gains were measured. The noise floor was optimized in an ac measurement mode using photodiodes of different shunt resistance and operational amplifiers with low 1/f voltage and current noise. In the dark (without any signal), the noise floor of the optimized silicon radiometers was dominated by the Johnson noise of the source resistance. The Johnson noise was decreased and equalized to the amplified 1/f input noise at a 9 Hz chopping frequency and 30 s integration time constant, resulting in an equivalent root-mean-square (rms) photocurrent noise of 8 × 10⁻¹⁷ A. The lowest noise floor of 5 × 10⁻¹⁷ A, equal to a noise equivalent power (NEP) of 1.4 × 10⁻¹⁶ W at the 730 nm peak responsivity, was obtained at a 100 s integration time constant. The radiometers, optimized for ac measurements, were tested in a dc measurement mode as well. Performances in ac and dc measurement modes were compared. In the ac mode, a ten times shorter (40 s) overall measurement time was needed than in the dc mode (400 s) to obtain the same 10⁻¹⁶ A noise floor.     

Characterization of the Input Noise Sources of a dc SQUID

From the theory of the intrinsic noise in a dc SQUID by Tesche and Clarke, we derive the expressions of the current and voltage input noise spectral densities in a dc SQUID current amplifier operated in a flux locked mode. The expected current and voltage noises are compared, at audio frequencies, with the experimental results obtained with a low noise dc SQUID in which the input load (resonant and not) and the operating temperature (1–4 K) are changed. In order to evaluate the input voltage noise, which is directly related to the current noise around the SQUID loop and is usually neglected, we have used as the input circuit a LC resonator with a very high quality factor (10⁶). Both the voltage and current input noises exceed the expected values by the same factor of about 8. This means that the modulus the optimum source impedance of the SQUID amplifier is still in agreement with the value expected from the theory, which is approximately given by the product of the input coil inductance and the angular frequency. To explain the excess noise results, we propose a model in which the voltage and current input noises are due to a thermal magnetic noise source which is present near the SQUID.     

Cryogenic performance of coaxial,multi-pin and coaxial-multi-pin complex bakable feedthroughs

Five BNC-type coaxial (BNC-receptacle, BNC-receptacle with floating shield, BNC-receptacle with floating and reverse sleeve, BNC-JJ, and BNC-JJ with floating shield), four N-type coaxial (N-receptacle, N-receptacle with floating shield, N-JJ, and N-JJ with floating shield), 22 pin, and 14 pin-BNC complex vacuum feedthroughs have been constructed. They are bakable (600 K) and can be used below helium lambda temperature (2.17 K), and remain leak-tight (? 10^?18 kNms^?1). Moreover, there is no leak at a pressure difference of 1.5 × 10³ kNm^?2 of helium at room temperature. BNC-JJ has a VSWR ? 1.35 from dc to 4.0 GHz. N-JJ has a VSWR ? 1.25 from dc to 6.5 GHz. 22 pin and 14 pin-BNC xcomplex vacuum feedthroughs can be quickly connected by Burndy connectors.     

Evidence for unpinned magnetic flux vortices above theI ctransition in the 2223-phase high-T csuperconductor

A similarity in the dc voltage-current (V-I) curves for both direct and alternating transport currents is used to propose that unpinned flux vortices are generated above theI _ctransition for dc transport currents, when Abrikosov flux vortices begin to penetrate the superconductor. Two methods can be used to give a dc voltage drop for an ac transport current: (1) if there is a slight dc offset voltage in the ac current which favors vortex loop collapse as in a traditional dcI _ctest, or (2) if an asymmetric transverse magnetic field is present which favors vortex loop collapse for current in one direction over the reverse direction.     

Electroluminescence Behavior of ZnS:Mn2+ Crystals

The electroluminescence spectra of ZnS and ZnS:Mn²⁺ crystals were studied under excitation in ac and dc electric fields from 10³ to 10⁴ V/cm. The relative intensities of the 465-, 510-, and 530-nm emissions in the ZnS crystals are found to depend on excitation conditions. The frequency-dependent brightness–voltage behavior of the ZnS:Mn²⁺ crystals is interpreted in terms of prebreakdown electroluminescence.     

Evidence for unpinned magnetic flux vortices above theIctransition in the 2223-phase high-Tcsuperconductor

A similarity in the dc voltage-current (V-I) curves for both direct and alternating transport currents is used to propose that unpinned flux vortices are generated above theI _ctransition for dc transport currents, when Abrikosov flux vortices begin to penetrate the superconductor. Two methods can be used to give a dc voltage drop for an ac transport current: (1) if there is a slight dc offset voltage in the ac current which favors vortex loop collapse as in a traditional dcI _ctest, or (2) if an asymmetric transverse magnetic field is present which favors vortex loop collapse for current in one direction over the reverse direction.     

Low frequency ac conduction and dielectric relaxation in poly(N-methyl pyrrole)

The ac conductivity and dielectric constant of poly(N-methyl pyrrole) thin films have been investigated in the temperature range 77–350 K and in the frequency range 10 ²-10² Hz. The well defined loss peaks have been observed in the temperature region where measured ac conductivity approaches dc conductivity. These loss peaks are associated with the hopping of the charge carriers. The frequency and temperature dependence of ac conductivity have been qualitatively explained by considering the contribution from two mechanisms; one giving a linear dependence of conductivity on frequency and other having distribution of relaxation times giving rise to broad dielectric loss peak. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

First dc Measurements of Electrons on Liquid Helium: the Helium-FET

We present the first dc-measurements on a 2-dimensional (2D) electron system floating above a liquid ⁴ He-film which covers a structured metal surface. With our arrangement of a source-, gate-, and drain-electrode a 2-dimensional charge transport is realized in analogy to a field-effect-transistor. The electrons which are moving along the x-direction due to different dc potentials are directly measured. This dc current, of the order of pA, is strongly dependent on the applied split-gate voltage. So the electrons were laterally confined to a narrow channel between the two gate electrodes. The effective width of the channel is reduced by the gate potential, so that a quasi-1D configuration can be realized. The measured electron current through the split-gate is analyzed and discussed on grounds of reduced dimensionality and 1D electron transport behaviour.     

Application of a standard two-stage transformer for checking purposes

Conclusions A standard two-stage current transformer at a frequency of 50 Hz when operating with domestic differential-null comparison instrument is not less accurate than three-winding magnetic current comparators and is superior to them in its speed of measurement and portability. For a simple and economic modernization of the standard measuring facilities employed in the USSR to check commercial-frequency current transformers the three-winding magnetic current comparators and uncompensated transformers should be replaced by standard two-stage current transformers.Translated from Izmeritel'naya Tekhnika, No. 5, pp. 65–67, May, 1980.     

The Precise Measurement of Current Ratios

The development of the current comparator, a three-winding current ratio transformer, is reviewed and its characteristics as an alternating current ratio standard are analyzed. Particular attention is given to the use of magnetic shielding and its effect on the accuracy and usefulness of the device. Some error characteristics of three types of audio-frequency current comparators are given and possible applications discussed. These include the calibration of current transformers and impedance comparisons. The adaptation of this device to dc operation is made possible by modulation techniques. A 20 000-ampere self-balancing direct current comparator, designed specifically for the calibration in situ of transductors or direct current transformers, is described. The application of this comparator to the calibration of shunts at high currents also is discussed and some results presented.     

A comparative study of CrNx coatings Synthesized by dc and pulsed dc magnetron sputtering

Chromium nitride (CrN_x) coatings were prepared by reactively sputtering chromium metal target with various nitrogen flow rate percentages (f_N2) using a closed field unbalanced magnetron sputtering system operated in dc and middle frequency pulsed condition (100 kHz and 50% duty cycle). In this study, plasma examination proved that a large amount of ions with a wide range of ion energies (up to 65 eV and mainly from 10-30 eV region) was identified in the pulsed plasma compared to the low ion flux and energy (0-10 eV) in a dc discharged plasma. The results showed that the phase structure of CrN_x coatings was changed from nitrogen doped Cr(N) to pure β-Cr₂N, and to a mixture of β-Cr₂N and c-CrN and then to pure c-CrN phases with an increase in the f_N2 in both dc and pulsed conditions. However, the pulsed CrN_x coatings exhibit lower N concentrations than dc CrN_x coatings prepared under the same f_N2, which leads to the existing of β-Cr₂N phase within a wide range of f_N2 (30-50%). In comparison with the typical large columnar structure in the dc sputtered coatings, the pulsed CrN_x coatings exhibit dramatic microstructure improvements which benefited from the improved plasma density and ion bombardment from the pulsed plasma, where the super dense and nearly equi-axial structures were observed in a wide range of f_N2. The microstructure improvements contributed to the enhancements in the hardness and wear resistance of pulsed CrN_x coatings. In the pulsed CrN_x coatings, the hardness values were above 30 GPa when the f_N2 is in the range of 30-40%, which is related to the formation of the β-Cr₂N phase. With the formation of a mixture of β-Cr₂N and c-CrN phases in the coatings deposited with 40-50% f_N2, a low COF of 0.36 and wear rate of 1.66 × 10^− 6 mm³ N^− 1 m^− 1 can be achieved.