Phasemeter for a Betatron

A. P. Grigor'ev, V. E. Ogarkov, O. A. Shevchenko

(Presented by the scientific seminar of the Scientific Research Institute of Electrical Engineering)

At present there have been certain achievements in the field of measuring phase inhomogeneities of a magnetic field. However, as indicated in [1], the existing measurement methods do not fully meet the increased requirements and need improvement. In particular, in connection with the wide introduction of betatrons into industry and medicine and the organization of their serial production, it is necessary to increase the productivity of tuning operations, including work associated with measuring the phase inhomogeneity of the magnetic field. Experience in measuring the phase structure of the magnetic field of a betatron shows that the measurement process requires large expenditures of labor and time, and the measuring operations are performed by highly qualified operators. In this connection it is advisable to measure phase inhomogeneity by converting the time interval (or phase), set by zero-field sensors (for example, peak transformers), into a voltage without the use of an oscilloscope or other means of visual indication. Similar phasemeters have been described in the literature. One of their shortcomings is the ambiguity of the sign of the measured phase shift [2, 3], which substantially complicates, and sometimes even excludes, the possibility of automating the measurement process.

It is known that, when the process of measuring and analyzing the phase inhomogeneity of a magnetic field is automated, labor productivity can be increased almost 100-fold [1], while measurement errors are substantially reduced (misses, inaccuracy in setting the field sensor by azimuth, etc.). The circumstances set forth above demonstrate the importance of developing measuring instruments and means and, in particular, phasemeters for betatrons.

The authors have proposed a principle for constructing a phasemeter [4] that makes it possible simultaneously to determine both the magnitude and the sign of the phase shift between magnetic fluxes at different azimuths (i.e., the lag or lead of one flux relative to another).

Below is presented a variant of the implementation of the proposed principle, developed and tested at the Scientific Research Institute of Electrical Engineering. Using the schematic diagram (Fig. 1) and the voltage diagram (Fig. 2), we shall briefly consider the operation of the circuit. Pulses of negative polarity from the reference and measuring sensors (r.s., m.s.) are amplified by the triodes \( \frac{1}{2}L_1 \) and \( \frac{1}{2}L_4 \). The r.s. is installed immovably, while the m.s. is moved in azimuth in the magnetic field of the betatron. After amplification, the pulse from the r.s. is applied to trigger the monostable multivibrator \(L_2\). The duration of the positive ...

Fig. 1. Schematic diagram

Fig. 2. Voltage diagrams

The pulse $\tau_{\text{w.m}}$ from the one-shot multivibrator is chosen to be deliberately greater than the maximum possible phase shift of the magnetic fluxes for this type of betatron. In the circuit under consideration, the value $\tau_{\text{w.m}} = 50\ \mu\text{s}$ was adopted.

The pulse from the measuring pickup is amplified ($\frac{1}{2}L_4$) and, through phase inverter $\frac{1}{2}L_4$ and diode $D_6$, is fed to the input of the balanced trigger $L_3$. The return of the trigger to its initial state is accomplished by a negative pulse obtained as a result of differentiating the trailing edge of the pulse from the one-shot multivibrator, which, through cathode follower $\frac{1}{2}L_1$ and diode $D_2$, is fed to the other input of the trigger. The duration of the pulse from the trigger, $\tau_{\text{tr}}$, may be either less than the duration $\tau_{\text{w.m}}$, when the pulse from the i.p. lags behind the pulse from the k.p., or greater than $\tau_{\text{w.m}}$, if the pulse from the i.p. leads the pulse from the k.p. By carrying out the operation of summing the resulting opposite-polarity pulses (or subtraction, in the case of same-polarity pulses), we obtain, respectively, a positive or negative pulse whose duration is proportional to the phase shift, while its polarity indicates the order in which the pulses from the i.p. and k.p. arrive.

In the proposed circuit, unipolar (positive) pulses from the monostable multivibrator and the trigger are used; subtraction is carried out in the balanced stage Л5, in whose arms the measuring instrument is connected.

Let us indicate the purpose of several elements of the circuit. Resistor \(R_{35}\) is used to set the zero with the instrument switched on and with no signals at the input, as well as when tubes are replaced. Resistor \(R_{34}\) serves to adjust the sensitivity of the instrument. The amplitude of the pulse from the monostable multivibrator is set by resistor \(R_{10}\). Resistor \(R_6\) is used in the process of adjusting the monostable multivibrator. The measurement result is read from a pointer instrument (a microammeter) with zero at the middle of the scale; in this case, the magnitude of the pointer deflection from zero is proportional to the interval between the pulses from the c.d. and i.d., and the direction of its deflection indicates the sign of the phase shift. To extend the measurement range, the instrument may have a switch.

It is not difficult to see that, when the proposed circuit is used, the process of measuring the phase inhomogeneity of the magnetic field can be readily automated; for this purpose, with the aid of an electric drive, uniform rotation must be imparted to the measuring disk with the i.d. mounted on it [3], and a self-recording instrument must be connected to the low-ohmic output of the balanced stage.

The advantages of the described circuit include higher labor productivity due to a reduction in the number of operations during measurements, greater measurement accuracy (in amplitude measurements using an oscilloscope, an error of \(\approx 10\%\) occurs [5]), the possibility of automating the measurement processes, and the absence of the need to calibrate the sensors.

The disadvantages of the circuit, revealed during adjustment and testing of the circuit, include certain difficulties in obtaining equivalent scale values for positive and negative shifts, which arise because of the nonidentity of the shapes of the pulses being added; however, they can be eliminated by careful adjustment or by using additional stages in the circuit for normalizing the pulses (upper limiters).

At present, tests of a modified transistor circuit, based on the same principle of operation, are being completed.

REFERENCES

1. L. M. Anan’ev, S. F. Vasilevskii. Analysis of the errors and performance of a compensation phase magnetometer. Izvestiya TPI, vol. 180, 1971.

2. A. M. Merson. Radio-Measurement Engineering. Moscow, “Energiya,” 1967.

3. L. M. Anan’ev et al. Induction Electron Accelerator—Betatron. Moscow, Gosatomizdat, 1961.

4. A. P. Trigor’ev, V. E. Ogarkov. Phasemeter for measuring the phase inhomogeneity of a magnetic field. Author’s certificate No. 312399, dated May 28, 1970.

5. M. I. Gryaznov et al. Measurement of Pulse Voltages. “Sovetskoe radio,” 1969.