SU1124819A1 - Method of absolute calibration of multiple superconducting measure of magnetic flux - Google Patents

Description

The invention relates to a technique of precise magnetic and electrical measurements using the phenomenon of superconductivity and quantum effects in superconductors, and can be applied for the absolute calibration of multi-valued superconducting measures of magnetic flux.

For magnetic and electrical measurements, magnetic flux measures are used, i.e. devices reproducing a given value of the magnetic flux coupled with the turns of the inductor. These measures do not have their own readout device, so their graduation usually reduces to the determination of their constants, which are either calculated by

4;: a or experimentally by comparing

00 with an exemplary measure on installations with a ballistic galvanometer.

The error of the calculated measures is determined by the error of measuring the geometrical dimensions of the inductors and is 0.001-0.01%; the error of the experimental methods for determining the constant measures of magnetic flux does not exceed 0.1%.

The field of practical application of non-superconducting measures of magnetic flux is limited by its use for the calibration of ballistic galvanometers and webermeters. I In a closed superconducting circuit, the magnetic flux coupled to this circuit is equal to an integer number of magnetic flux quanta Φ g 2.07 and does not change with time. This property makes it possible to increase the accuracy of measures of magnetic flux and expand the area of their practical use. A multi-valued superconducting magnetic flux measure, consisting of series-connected superconducting inductance coils and an electric current measuring instrument with zero internal resistance, can be scaled by the magnetic flux. The output value during calibration is the reading of the reading device of the electric current measuring instrument. Closest to the present invention is a method of calibrating a superconducting measure of magnetic flux, including changing the magnetic flux coupled to the turns of the measure, changing the electric current in the circuit of the measure, measuring the change in the magnetic flux by integrating the time voltage of the pin and measuring the difference counts before and after the specified change in current. The prior art calibration method is as follows. 1. A graduated measure is connected in parallel to a resistor bounded by a thermal switch, through which a constant electric current flows. 2. When the key is in the superconducting state, K is read out from the measure reading device. 3.Dp change magnetic flux coupled with. by turns of the inductor, the thermal switch is brought to the normal state for the time during which the instantaneous values of the voltage drop V (t) are measured and recorded on the resistor. 4. After the thermal key is transferred (at the time t) to the superconducting state, a counting device K is read out. The magnitude is proportional to the electric current and magnetic flux in the superconducting circuit of the measure being tested. The magnitude of this magnetic flux (m) is determined by the expression:. L (P (t:} I v (t) dt. (1) t, Calibration with the help of the non-stationary Josephson effect of the device that measures and records the voltage V (t), allows you to express the value of the magnetic flux in the form g uP (T ) (Z, CI 9e (t) dt, (2) where 9 & t (t) is the instantaneous value of the readings of the device; From its constant. 5. Graduation is completed after repeated repetition of steps 1-4 with varying time interval T and setting between the values of the measured magnetic flux and the readings of the K reading device. The disadvantages of this method are The layouts are the multilink of the verification circuit, through which the magnitude of the magnetic flux quantum is transmitted, and the errors associated with it, as well as the presence of additional errors related to the change in voltage V (t) and the time interval T, the purpose of the invention is to reduce absolute calibration errors of multi-valued superconducting magnetic flux measures. The goal is achieved by changing the magnetic flux by forming a rectangular voltage pulse in a Jo-Zefson junction, connected to the conclusions of the measure and the stepwise voltage-current characteristic induced by an external electromagnetic RF field, and at the same time automatically controlling the electric current in the circuit of the measure, the number of quanta by which the magnetic flux changed is measured by registering the increment of the oscillation phase of the RF field during the time period voltage pulse. It is known that the volt-ampere characteristic of the Josephson junction, placed in an external electromagnetic RF field, acquires a stepped character with parts of zero

5 differential resistance (stu

pen mi) at voltage x.,

where, 1,2, ... is an integer;

f is the oscillation frequency of the HF-floor,

This stepwise voltage-current characteristic of the Josephson junction is used to calibrate the measure.

If the electric current through the Josephson junction is close to zero, the operating point on the volt-ampere characteristic is at the step. By feeding the transition to the corresponding current jumps, you can abruptly shift the working point on the volt-ampere characteristic from step from step to step, and thus get Josephson junction is a rectangular voltage pulse with amplitude Vn and duration. If a load with a finite impedance is connected to the Josephson junction, then during the time interval T, the bias current must be adjusted so that the operating point is within the current step, and after a reverse transition to the step, the bias current must remain constant. If the Josephson junction is applied to a superconducting inductor, then the regulator current should linearly increase up to time t, up to time tg at speeds Up / bo, where L is the inductance value to: at.

| . .. The change in the magnetic flux in a measure over time, g –lg uP (T) (, J ((t) t, -tfCtj), (3)

whereeLp (t) is the oscillation phase of the high-frequency field.

Thus, the measurement of the number of quanta by which the magnetic flux changed over time is reduced in the proposed method to the measurement of the phase difference ((t -) - u7 (t), which can be measured (with an absolute error of ± If) by an electronic counter of oscillation periods.

One of the options for implementing the proposed calibration method is illustrated in the drawing.

A graduated magnetic flux measure consisting of a superconducting inductance coil 1 and an electric current meter 2.

196

zero internal resistance, connected to pins 3 of the Josephson junction. The Josephson junction has a capacitive coupling to an RF oscillator 5, designed to obtain a stepped current-voltage characteristic, and a galvanic coupling to an electric current regulator 6, the input of which is connected by an information transmission channel 7 to a control device 8. The generator 5 is connected to a counter 9 periods of oscillation by the RF cable 10. Counter 9 is also connected to the control unit B with information transmission channel 11, via which commands of beginning and end of counting are transmitted at times t and t, respectively.

The formation of a rectangular voltage pulse at the Josephson junction and a change in current at interval T is performed using a current controller 6, which receives control signals on channel 7 from control device 8 in accordance with the program previously entered into it. This program uses information about the value of the inductance of coil 1, the voltage-current characteristic of the Josephson junction 4, the number of the working stage n and the frequency of the RF generator 5.

In another embodiment of the proposed calibration method, the controller 6 can be controlled not by the previously entered program, but by using the measurement information of the device 2. It is also possible to use analog devices for generating a voltage pulse and adjusting the current.

As an example, let us consider the calibration error of the proposed method of superconducting magnetic flux measure, the coil of which has an inductance of 10 mH, and a superconducting quantum interference device whose recording device allows operation in digital-analog mode with a maximum count rate of 1-10, period is used as a measuring device over the input winding current is lxO, 1 µA. The maximum current rise rate for such a meter is mA / s. The voltage Vq should be no more than

 1 µV. Such a voltage can be obtained at step c at 241.8 MHz.

If the flow of the regulator during the flow change is programmed using information about the voltage and inductance Lg, then with an error of L of i1%, the operating point is shifted by the current-voltage characteristic to the moment tg at T 10 10 with no. will exceed +10 uA. It follows that with a current width of more than 30 µA, the operating point remains within the step during the interval T „

The measurement error of the phase difference tf (tj,) - cp (t (), if using the frequency meter 43-54 as a counter, is less than 1 - 10 at intervals T 5-, 1 s. The measurement error of the difference is determined by the value of the interval T, noise flux and zero-level stability of a superconducting quantum interference device. With a mean square value of the spectral density of a third-party flux S-0 FHz and a zero-level drift, it does not exceed 1 40

yuschem

h, this error

It can be reduced below the level, that error and will determine basically the calibration error by the proposed method.

The use of the proposed method of calibration, as can be seen from the above example, has the following advantages compared with the prototype:

a) the calibration error using relatively simple means can be less than 11СГ, while even using the most accurate digital recording voltmeter at the present time, the method chosen for the prototype will give an error of 1.10;

b.) the graduation error is not affected by the frequency instability of the generator of the high-frequency shift of the Josephson junction, which allows to significantly simplify the necessary equipment;

c) if a superconducting inductance measure is included in the magnetic flux measure, then the proposed method can be used to calibrate the direct current of digital quantum interference DC meters and control their linearity, which makes it possible to use them in DC and DC measures this voltage.

Compared to the method for determining the constant most accurate calculated measure of the magnetic flux currently used in the measurement technique of the national economy, the proposed method can provide an increase of accuracy by at least two orders of magnitude.

Claims (1)

METHOD OF ABSOLUTE GRADING OF A MULTI-VALUED SUPERCONDUCTIVE MEASURE. MAGNETIC FLOW, including changing the magnetic flux coupled to the turns of the measure by changing the electric current in the circuit of the measure, measuring the change of the magnetic flux by integrating the voltage across the terminals of the measure over time and registering the difference in counts before and after the specified current change, characterized in that, in order to reduce the calibration error, the magnetic flux is changed by simultaneously forming a rectangular voltage pulse at the terminals of the measure using a Josephson junction having an induced External Expansion RF field stages ~ chatuyu voltage-current characteristic, and automatic control of the electric current in the circuits measures, and measure the number of photons is measured at - nilsya magnetic flux registering ® phase increment RF field oscillations per / duration ymlulsa rectangular voltage. FROM