FABRICATION AND APPLICATION OF MAGNETICALLY SATURATED PERMALLOY SENSORS

B. N. Kalinin, V. N. Kuzmin, Yu. K. Petrov

Permalloy sensors with longitudinal saturation are widely used in measurements of constant, alternating, and pulsed magnetic fields in charged-particle accelerators and other devices.

A permalloy sensor consists of a strip or wire of permalloy with a signal winding consisting of 100–500 turns of fine wire. At the moment when the component of the magnetic field directed along the axis of the core passes through zero, the core is remagnetized, and a voltage pulse arises in the signal winding. The pulse parameters depend on many factors: the dimensions and material of the core, the methods of mechanical and thermal treatment, and also on the rate of change of the magnetic field at the moment the pulse occurs. The magnitude of the magnetic field being measured at the moment the pulse occurs is determined from the current in the measuring winding, which creates a field equal and opposite to the measured one.

At the Research Institute of Nuclear Physics at TPI, a design and fabrication technology were developed for permalloy sensors for measuring magnetic fields, which make it possible to obtain pulses with a duration of up to 2 μsec and an amplitude of 0.05–0.1 V at a magnetic-field variation rate of 10 T/sec. For the cores of the permalloy sensors, wire permalloy of grade 80NKhS with a diameter of 0.025–0.075 mm and a length of 2–5 cm was used. The permalloy was annealed in a hydrogen atmosphere at a temperature of 500–700°K and placed in a thin quartz tube with a diameter of 0.5–1 mm. A signal winding was wound onto the quartz tube using PEL wire of 0.02–0.03 mm diameter, with 100–500 turns. The sensor fabricated in this way was inserted into a glass tube with a diameter of 3–4 mm, and the latter into a special device for stretching the permalloy (Fig. 1) [1]. This device was placed in a solenoid powered by industrial-frequency current. By stretching the permalloy wire with a stress of \((4–5)\cdot10^8\ \text{N}/\text{m}^2\) and simultaneously twisting it, one ensures that the voltage pulse has a triangular shape and a minimum duration. This stressed state is then fixed by means of a clamping screw, and the glass tube is filled with epoxy resin. After curing for 24 hours, the sensor is ready.

A disadvantage of this fabrication method is that the pulse shape and amplitude, as well as the magnitude of the coercive force in a stressed sensor, depend to a considerable degree on the temperature of the sensor. This is due to the fact that the temperature coefficients of expansion of permalloy and epoxy resin are different. Therefore, a change in the temperature of the sensor leads to a change in the stress in the permalloy wire, which causes a change in the pulse parameters. In unstressed sensors, no dependence of the pulse parameters on temperature was observed.

Annealing the permalloy core in a quartz tube at a temperature of \(1300^\circ\mathrm{K}\), without subsequently creating stresses, makes it possible to obtain a bell-shaped pulse with a duration of about \(4 \div\) µsec.

It is known that, when the rate of increase of the magnetic field changes, the coercive force changes because of the presence of eddy currents in the core (the width of the hysteresis loop), and, consequently, the instant at which the pulse occurs changes relative to the passage of the magnetic field through zero. In some cases this phenomenon may lead to error in measurements.

The width of the hysteresis loop of the permalloy core was measured by the authors using two methods.

The essence of the first method [2] is that pulses from the signal winding of the sensor are fed to the vertical sweep of an oscilloscope, while the horizontal sweep is produced by means of the voltage taken from a resistor connected in series with a choke, in which a magnetic field of industrial frequency is produced (Fig. 2). As a result, an oscillogram of two pulses of opposite polarity is obtained on the screen, the distance between them being proportional to the width of the hysteresis loop. From the magnitude of the direct current through the measuring winding of the sensor, which shifts these pulses along the \(X\) axis by a distance equal to the distance between the pulses, the width of the hysteresis loop of the permalloy core can be determined.

Fig. 1. Device for drawing a permalloy core in a magnetic field: 1—holder, 2—permalloy core, 3—quartz tube, 4—signal winding, 5—glass tube, 6—clamping screws.

Measurement of the width of the hysteresis loop was also carried out according to the circuit shown in Fig. 3. The permalloy core under study “D,” with signal winding \(w_2\), is placed inside the excitation coil \(w_1\), which is supplied with a sinusoidal current at a frequency of \(500 \div 800\) Hz. The magnitude of the excitation field is determined from the known geometrical parameters of the coil and the current through it. A voltage proportional to the excitation current is fed to the vertical input of the oscilloscope, producing a continuous sinusoid on the screen. At the moment when the field in the core is equal to its coercive force, a voltage pulse arises in the signal winding and is fed to the brightness modulator of the oscilloscope. As a result, alternating bright and dark points appear on the sinusoid; the vertical distance between them is proportional to the width of the hysteresis loop. Knowing the magnitude of the excitation field, the width of the hysteresis loop can be determined from the oscillogram.

Fig. 2. Circuit for measuring and automatically correcting the width of the hysteresis loop of a permalloy core by the pulse method

For automatic correction of the change in the width of the hysteresis loop with changing rate of rise of the field, the authors proposed a correction circuit [2], shown in Fig. 2. The correction circuit consists of two windings: an inner and an outer one. These windings are connected to one another in such a way that the emfs induced in them by the alternating field are oppositely directed, and the outer winding must produce a larger emf than the inner one. The ends of the correcting winding are closed through a resistance \(R\). In this case, in the region of the permalloy core an advancing field is produced, linearly dependent on the rate of rise of the magnetic field. The width of the hysteresis loop depends nonlinearly on the rate of rise of the magnetic field; therefore, correction of the width

Fig. 3. Circuit for measuring the width of the hysteresis loop of a permalloy core by the method of modulation of the excitation field

the hysteresis loop can be carried out only within a certain range of rates of rise of the magnetic field.

To adjust the correction circuit, the sensor is placed in a field with a specified rate of field rise, and the resistance value is selected so that the pulses on the oscillogram coincide. The correctness of the adjustment is checked by changing the rate of field rise over the range of operating rates of rise; in this case the pulses must not diverge.

In one of the sensors made by the authors, without correction of the hysteresis-loop width when the rate of rise of the magnetic field was varied by \(\pm 5\%\), the measurement error was \(\pm 1 \cdot 10^{-5}\ \mathrm{T}\), while with correction this error was reduced to \(\pm 1 \cdot 10^{-7}\ \mathrm{T}\).

Permalloy sensors were used by the authors in phase meters—devices used in betatrons to determine the instant at which an alternating magnetic field passes through zero. The phase meter has two sensors, one of which is fixed, while the other is moved and placed at the point of measurement. Both pulses from the signal windings are fed to the vertical input of an oscilloscope. The phase shift between the magnetic fields at the points where the sensors are installed is determined from the magnitude of the direct bias current in the measuring winding of one of the sensors when the pulse on the oscillogram coincides. If the measuring winding of the second sensor is supplied with current and its value is kept constant throughout all measurements, then the instantaneous field values can be measured, with the level of the measured field being measured by the magnitude of the current in the measuring winding of the second sensor. The error in measuring the field by this method is \(\pm 5 \cdot 10^{-7}\ \mathrm{T}\).

In addition, permalloy sensors were used by us in a magnetometer for measuring instantaneous values of pulsed magnetic fields [3], in an instrument for measuring constant magnetic fields [4], and in a device for determining the median surface of the magnetic field of a synchrotron [5].

References

1. Yu. K. Petrov. Application of elastic deformations of cores in the fabrication of permalloy probes. Electron Accelerators. Proceedings of the IV Interuniversity Conference on Electron Accelerators, 259, 1964.

2. Yu. K. Petrov, B. N. Kalinin. Allowance for hysteresis in permalloy sensors of zero magnetic field. Electron Accelerators. Proceedings of the III Interuniversity Conference on Electron Accelerators, 213, 1963.

3. B. N. Kalinin. Measurement of the current ratio in the sensors of a permalloy magnetometer (this volume).

4. V. N. Eponechnikov, V. N. Kuzmin. Instrument for measuring weak constant magnetic fields. Electron Accelerators. Proceedings of the III Interuniversity Conference on Electron Accelerators, 218, 1963.

5. V. N. Kuzmin, Yu. K. Petrov. PTE, 1, 177, 1962.