GB2311617A - Cryogenic current comparator - Google Patents

Description

CRYOGENIC CURRENT COMPARATOR The present invention relates to a cryogenic current comparator.

A cryogenic current comparator is a device to balance two DC or low frequency AC currents to great accuracy.

At DC it is possible to compare, control and maintain two currents to an accuracy of 10-10. Such devices are used for metrology purposes to make highly accurate comparisons of resistors and to obtain an absolute value of resistance from the von Klitzing effect. The work has been reported by I K Harvey in "A precise low power DC ratio transformer" Rev Sci Inst, Vol. 43,, pp. 16261629 (1972); and by Jeckelmann B, Inglis AD, and Jeanneret B in "Material Device and step independence of the Quantised Hall Resistance" IEEE Transactions on Instrumentation and Measurements, Vol. 44, No. 2, 1995, pp. 265 to 272.

The use of a current comparator in the prior art is easily disturbed by outside interference and noise.

This is particularly problematic because a device known as a SQUID (Superconducting Quantum Interference Device) is used to measure and control any imbalance of the two currents. SQUIDS are notoriously sensitive to very low levels of RF interference over a very broad frequency range. To try to reduce this to the minimum, it is normal to fit filters at the input to the device but this is found to be very difficult due to the nature of the system.

A diagram of a typical cryogenic current comparator bridge circuit is shown in Figure 1. There are two current sources (power supplies) PS1 and PS2, usually battery powered to ensure complete isolation. They are designed to sweep the current from zero to a test value, typically 5 milliamps, in preferably both positive and negative senses. The current passes through the two resistors R1 and R2, whose values are typically 1O0Q, giving a voltage across the resistors of 0.5 volts. The difference is read by a highly sensitive amplifier Al and usually recorded by a computer (not shown) which controls the operation of the bridge by driving PS1 and PS2 and analyzing the results. All these parts of the bridge are at ambient temperatures.

The currents after passing through the resistors pass through the two superconductive coils C1 and C2 of the cryogenic current comparator, which is normally at 4.2K absolute temperature and installed in a liquid helium cryostat. Any error in the ratio of the two currents gives rise to a stray magnetic field which is detected by the input coil of the SQUID amplifier. The SQUID amplifier applies a correction signal to current source PS1 and under feedback control can maintain the ratio of the currents to an accuracy of 10-10.

The ratios of turns on the two coils defines the ratio of the two currents. These currents are identical to those passing through the resistors, provided that the two current sources PS1 and PS2 are truly isolated from each other and the whole circuit is only grounded at one point, usually near the input to the voltage amplifier Al.

In this example, two coils of 400 turns each are used in the cryogenic current comparator.

In order that the cryogenic current comparator can work effectively it must be screened from outside magnetic fields and the coils themselves are enclosed in a complex shielding system, more details of which can be found in the first reference.

To allow different currents to be compared, a number of different coil ratios is provided so that a typical cryogenic current comparator will have 10 or more coils of different turn values.

The problems mainly arise because coils of superconductive wire have internal self capacity and will resonate with a high Q, often over 100,000. In addition, connection to the world outside of the cryostat by wiring to the power supplies injects both RF interference and white noise from the power supplies.

The resonant nature of the coils amplifies the noise making the SQUID and cryogenic current comparator unusable in the worst case or very noisy at best. To overcome this, sophisticated filtering of the power supply outputs at room temperature, together with the use of feed-through capacities on the lines to the cryogenic current comparator are used but this is often insufficient. Further improvement comes from the use of a faraday cage or screened room around the entire experimental apparatus. This is an expensive procedure.

This invention seeks to provide an improved design which makes a cryogenic current comparator easier to use and far more stable.

According to an aspect of the present invention, there is provided a cryogenic current comparator comprising first and second current sources, first and second superconductive coils coupled to the first and second current sources respectively, and first and second resistances coupled in parallel to the first and second coils respectively.

Each resistor and coil combination is preferably chosen so as to act substantially as a Q killer, thereby to ensure that the circuit produces substantially no resonant or other non-linear behaviour.

Each coil and resistance combination preferably has a cut off frequency of substantially between 100 Hz and 10kHz.

Each resistor preferably has a value of lfl.

The comparator preferably includes a capacitive feedthrough at room temperature prior to the input to the superconductive coils.

The present invention uses the unique property of superconductive wire to provide screening and removal of resonant and non-linear effects while retaining the DC performance of the cryogenic current comparator.

According to another aspect of the present invention, there is provided a magnetic shield for a cryogenic current comparator including an inner shield for enclosing a plurality of superconductive coils disposed around an axis, and an outer shield completely enclosing the inner shield so as to enclose the coils and also for enclosing completely a comparator SQUID.

A high permeability magnetic enclosure is preferably provided between the inner and outer shields.

The inner shield is preferably not continuous around the axis.

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an example of prior art cryogenic current comparator; Figure 2 is a block diagram of an embodiment of part of a cryogenic current comparator; and Figure 3 is a schematic diagram of an embodiment of magnetic shield for the current comparator of Figure 2.

Referring to Figure 2, there is shown an embodiment of current source and coil/resistance arrangement to replace the equivalent arrangements of the current comparator shown in Figure 1. It will be apparent that two such circuits would be provided, in a manner similar to the prior art example of Figure 1. An amplifier Al and SQUID are also provided.

In the embodiment shown, current is fed down as normal after passing through a simple capacitive feed-through at room temperature, to the input to the superconductive coil of the cryogenic current comparator. Each lead of the cryogenic current comparator has two connections.

One is the current feed, the second is to a resistor of typical value 1 ohm as shown in Figure 2.

Two separate contacts to the superconductive coil must be made since there is always contact resistance between the normal conductors and the superconductor. These contacts are one for the coil and one for the resistor.

In other words, the coil and resistor are not coupled to the conductors at either ends thereof by common contacts but have their own separate contacts. This feature, it has been found, considerably improves on prior art deficiencies.

The value of the resistor is chosen so that the time constant and frequency response of the superconductive coil and resistor combination is appropriate; typically a cut off frequency of 100Hz to 10kHz is appropriate.

Resistor values of 1Q are preferred. The effect of the resistor is to act as a Q killer and to ensure the circuit produces no resonant or other non-linear behavior. Since the coil is superconductive, once the current has been injected into the superconductor, all the current flows through the coil and no current will flow through the resistor at DC. The accuracy of the current ratios can thus be maintained and relied upon.

If there was a common connection between the coil, the resistor and the current input, the contact resistance to the superconductor would result in series resistance to the coil and a leakage would flow through the resistor, destroying the accuracy of the cryogenic current comparator and the bridge.

The above-described system has been able to operate with very simple filtering without a screened room.

Comparisons of two resistors to better than 10-8 and to the von Klitzing standard to an accuracy of 10-9 have been made successfully.

A further improvement relates to the design and magnetic shielding of the whole device. Since the control of the two currents depends on measuring the stray flux from the two coils, it is important that the pick-up coil does not see any effects from the outside environment or nearby sources of magnetic fields such as the superconductive magnet used for the quantum hall device.

In the preferred design there are twelve or more superconductive coils 1 wound on a single bobbin made of insulating material, shown in cross-section in Figure 3.

These coils are arranged in a superconductive shield 2 made of lead, the design of which is described in the article by Harvey but which is not continuous around the axis. The field that appears at the axis of the coil is only dependent upon the number of turns and current flowing in the coil and is independent of the exact location of the coil inside the shield.

This field is detected by the pick-up coil 3 which is connected to the SQUID 4. In order to ensure that the SQUID and pick-up coils do not see external magnetic fields and interference, there is an outer shield 5 which is continuous around the axis and totally encloses the coils, the inner shield and the SQUID.

To ensure that the outer shield does not reduce the sensitivity of the device by acting as a shorted turn around the inner shield, there is fitted a high permeability magnetic enclosure 6 between the superconductive shields 2 and 5. This is a closed cylinder with holes for the input wires 7 and the connections to the pick up coil. The base is demountable as is the base of the outer shield 5 to allow access to the inner parts for service or repair.

The magnetic enclosure provides a low reluctance path for the flux that escapes from the inner shield due to differences in the currents being measured. This ensures that the sensitivity of the current comparator is maintained at the highest level. By use of this design, it is possible to make a cryogenic current comparator with a sensitivity of 10-10 Ampere turns in a compact housing of 50mm diameter. This has the advantage that it can be installed in a storage vessel for liquid helium and so minimise consumption of the expensive liquid helium.

Claims (11)

1. A cryogenic current comparator comprising first and second current sources, first and second superconductive coils coupled to the first and second current sources respectively, and first and second resistances coupled in parallel to the first and second coils respectively.

2. A cryogenic current comparator according to claim 1, wherein each resistor and coil combination is chosen so as to act substantially as a-Q killer.

3. A cryogenic current comparator according to claim 1 or 2, wherein each coil and resistance combination has a cut off frequency of substantially between 100 Hz and 10 kHz.

4. A cryogenic current comparator according to claim 1, 2 or 3, wherein each resistor has a value of 1=;.

5. A cryogenic current comparator according to any preceding claim, including a capacitive feed-through at room temperature prior to the input to the superconductive coils.

6. A cryogenic comparator according to any preceding claim, wherein for each coil and resistor, there is provided an electrical connection from the current source to the coil and a separate electrical connection to the resistor.

7. A magnetic shield assembly for a cryogenic current comparator including an inner shield for enclosing a plurality of superconductive coils disposed around an axis, and an outer shield completely enclosing the inner shield so as to enclose the coils and also for enclosing completely a comparator SQUID.

8. A magnetic shield assembly according to claim 7, including a high permeability magnetic enclosure disposed between the inner and outer shields.

9. A magnetic shield assembly according to claim 7 or 8, wherein the inner shield is not continuous around the axis.

10. A cryogenic comparator substantially as described herein with reference to, and as illustrated in, accompanying Figure 2.

11. A magnetic shield assembly substantially as described herein with reference to, and as illustrated in, accompanying Figure 3.