MEASUREMENT OF THE PHASE INHOMOGENEITY OF A MAGNETIC FIELD

A. P. GRIGOR’EV, I. I. KRIMKER, V. E. OGARKOV

(Presented by the scientific seminar of the Research Institute of Electron Introscopy)

Theoretical and experimental studies of the compensation method and device hitherto used for measuring the phase inhomogeneity of the magnetic fields of betatrons have shown [1] that the accuracy and productivity of the measurement process do not meet the increased requirements of betatron engineering; it is therefore advisable to develop new, more accurate measurement methods that make it possible to automate the measurement processes.

These recommendations were implemented in the devices we proposed [2, 3], which make it possible simultaneously to measure the magnitude and sign of the phase shift between magnetic fluxes at different azimuths and to automatically record the measurement results on the tape of a recording instrument.

Below is a description of a new version of the device intended for measuring phase inhomogeneity, having a number of advantages in comparison with the known one [3].

It is seen from Fig. 1 that, in terms of its construction principle, the meter is a typical two-channel phase meter in which the phase shift is converted into a time interval, with subsequent measurement of the average current proportional to this interval [4, 5].

A substantial simplification of known phase-meter circuits and the possibility of simultaneously measuring the magnitude and sign of the phase shift between magnetic fluxes without any additional comparison and switching operations proved possible thanks to a very valuable property of a symmetrical trigger with separate starting through diode gates: the dependence of the polarity of the output pulses of the stage on the order (sequence) of the incoming starting pulses. This is explained by Fig. 2, which presents voltage diagrams for the cases of lagging (2, a, b) and leading (2, c, d) of the pulses from the measuring sensor (movable) relative to the pulses from the reference sensor; for both cases, variants of the trigger states 0 and 1 are given, since when the circuit is switched on their appearance is equally probable.

In the instrument circuit, transistors T₁, T₆ contain limiter amplifiers from below, which is necessary for obtaining short pulses from sufficiently wide sensor pulses ($\tau_d = 10 \div 30$ μsec); T₂, T₅ are amplifiers, T₃, T₄ are the trigger, and T₇, T₈ are the output balanced stage.

Fig. 1. Schematic circuit diagram.

Fig. 2. Voltage diagrams.

The use of the peaks of the pulses from the sensors to trigger the flip-flop makes it possible, with its aid, to form pulses with a minimum duration of up to 0.5 μsec. Experimental investigation has shown that this approach is the most expedient compared, for example, with the use of upper limiting followed by differentiation and triggering of the flip-flop by pulses corresponding to the initial portions of the fronts of the pulses from the sensors, since in the latter case a certain constant component (systematic error), caused by the different durations of the pulses from the reference and measuring sensors,

\[ \Delta \tau = \frac{|\tau_{\text{и.д}}-\tau_{\text{к.д}}|}{2}. \tag{1} \]

is included in the instrument reading.

It can be determined directly from the readings of the instrument at the moment when the sensors are at the same azimuth, and is taken into account in the measurements as follows: if \(\tau_{\text{к.д}}>\tau_{\text{и.д}}\), then, when the pulse from the measuring sensor lags, the value \(\Delta \tau\) must be added to the instrument readings, and when it leads, subtracted; and, correspondingly, the reverse is done for the case \(\tau_{\text{к.д}}<\tau_{\text{и.д}}\).

The same effect as when lower limiting is used is obtained in the case where the pulse from the sensors is first differentiated and then limited and amplified.

To automate the measurement process, a recorder is connected at the output of the balanced stage instead of the pointer instrument (M265M), and the measuring disk is given uniform rotation by means of a drive.

Thus, it is evident that, by using the proposed meter, the accuracy of measurements can be substantially increased, since most of the errors of the compensation method listed in [1] are either substantially reduced in magnitude or eliminated altogether. In particular, errors due to inaccurate setting of the measuring disk are eliminated; errors due to EMF induced in the sensors and in the supply currents, as well as errors in the calibration of the sensors, the need for which is practically eliminated; replacement of the oscilloscope used in the compensation method by a direct-reading instrument also makes it possible to reduce the measurement error by almost an order of magnitude.

When calibrating the meter with a generator of pulses calibrated by duration, it was found that the measurement error is 0.2–0.3 μs on a 20 μs scale, i.e., 1–1.5% (not including methodological errors). Testing of experimental prototypes of the instrument showed that it is easily adjusted and calibrated (the instrument scale is linear), and its operation on various types of betatrons demonstrated high reliability and ease of use. The instrument scale is calibrated in μs; however, if necessary it can be calibrated in units of magnetic-field induction, in which case the scale remains linear owing to the smallness of the measured phase shifts ($<1^\circ$).

The dimensions of the instrument are $170 \times 160 \times 130$, its weight is 0.75 kg, it is powered from the industrial mains, and the power consumption is 1 W.

REFERENCES

1. L. M. Anan’ev, S. F. Vasilevskii. Analysis of the error and performance of a compensating phase magnetometer. Izvestiya TPI, vol. 180. Tomsk, Tomsk State University Press, 1970.

2. A. P. Grigor’ev, V. E. Ogarkov. A phase meter for measuring the phase inhomogeneity of a magnetic field. Inventor’s certificate No. 312399 of May 28, 1970.

3. A. P. Grigor’ev, V. E. Ogarkov. Development of control circuits and measuring equipment for accelerators. Abstracts of a report at the All-Union Conference “Development and Practical Application of Electron Accelerators.” Tomsk, Tomsk State University Press, 1972.

4. R. A. Valitov et al. Radio-Engineering Measurements. Moscow, “Sovetskoe radio,” 1970.

5. Phase meter F2-4. Description and operating instructions. Kiev, 1969.