ELECTRONIC STABILIZER OF PULSE VOLTAGE AT THE BETATRON INJECTOR

V. M. Razin

(Presented by the scientific seminar of the Physico-Technical Faculty)

The use of one of the methods for stabilizing betatron radiation [1] requires maintaining the constancy of the injection voltage with a high degree of accuracy. The required accuracy of voltage constancy can be obtained with the electronic stabilization circuit developed by us. The schematic diagram of the stabilizer is shown in Fig. 1.

Fig. 1

Negative voltage pulses from a divider, connected directly to the cathode of the betatron injector, are applied to the grid of tube \(L1\). The main purpose of the amplification stage on tube \(L1\) is to reverse the polarity of the pulse. Positive voltage pulses from the anode of tube \(L1\) are fed to the grid of the cathode follower on tube \(L2\). The low output resistance of the cathode follower makes it possible to charge capacitance \(C4\), through the diode rectifier on tube \(L3\), almost up to the amplitude voltage in a time equal to the pulse duration. This circumstance makes it possible to reduce the inertia of the circuit when

during rapid increases of the mains voltage and practically eliminates the possibility of breakdowns in the injector.

At the output of the diode rectifier there is a Π-shaped capacitive–resistive filter. The filter must reduce the voltage ripple to such a value that the voltage variations at the input of the next amplifier stage do not exceed the limits of the dynamic range. The resistor \(R7\) reduces the time constant of the filter, i.e., reduces the inertia of the circuit’s action. To ensure stable operation of the circuit, the value of the resistance \(R7\) must be many times smaller than the insulation leakage resistance.

The dc voltage from the filter output is applied to the grid of the dc-voltage amplifier on tube \(L4\). The voltage on the grid of this stage is compared with the reference voltage at the cathode. The value of the reference voltage from the stabilizer on tube \(L5\) is regulated by potentiometer \(R12\) and determines the value of the high pulsed voltage.

The voltage from the output of the amplifier on tube \(L4\) is applied to the grids of the parallel-connected tubes \(L6\) and \(L7\) of the power amplifier. The dc component of the positive voltage at the anode of tube \(L4\) is compensated by a negative voltage from a separate source. The power amplifier on tubes \(L6\) and \(L7\) controls the bias current of the saturable choke \(D_r\), through the series-connected ac windings of which the charging transformer of the injection circuit is supplied.

The direction of variation of the voltages and currents in the electronic stabilizer is not difficult to trace from the circuit in Fig. 1. Suppose that an increase in the mains voltage has occurred, as a result of which the injection voltage also tends to increase. An increase in the amplitude of the negative pulse at the input of circuit \(L1\) causes an increase in the positive dc voltage at the output of the diode rectifier on tube \(L3\). The anode current of tube \(L4\) then increases and the voltage at its anode drops. The voltage drop is transmitted through the direct galvanic connection to the grids of the power amplifier. The anode current of tubes \(L6\) and \(L7\) decreases, the bias of the saturable choke decreases, and the inductive resistance of the ac windings increases so much that the high pulsed voltage remains practically constant.

From consideration of the principle of operation of the circuit it is clear that this electronic stabilizer is a compensating system of automatic regulation. As is known, in such systems [2] there is also load stabilization, i.e., when the load resistance changes the voltage will also be stabilized.

Theoretical calculations show that, in our circuit, a voltage stabilization factor of the order of several hundreds and a load stabilization factor of the order of several tens of units can be obtained. In practice, the value of the stabilization factor will apparently be limited by the stability of the reference-voltage source. Stabilization with respect to changes in mains voltage was checked experimentally by varying the supply voltage of the saturable choke. The results of the investigations are presented in Fig. 2. The graphs in Fig. 2 show the dependence of the high pulsed voltage on the supply voltage of the saturable choke for three different positions of the slider of potentiometer \(R12\). As is evident from the graphs in Fig. 2, over a wide range of variation of the supply voltage of the saturable choke the pulsed voltage changes so little that experimental measurement of the stabilization factor presents serious difficulties.

The results of the experimental investigation of load stabilization are presented in Fig. 3. Here curve 1 shows the dependence of the high pulsed voltage on the load resistance in the absence of elec-

...ron stabilization; curve II gives the same dependence in the presence of stabilization. As is seen from Fig. 3, variation of the load resistance over wide limits does not cause any noticeable change in the pulsed high voltage.

Fig. 2

For an accurate determination of the stabilization coefficients in our circuit, a special measurement procedure had to be developed, making it possible to detect very small changes in the pulsed high voltage.

Fig. 3

In conclusion, let us note that the high periodic pulsed-voltage stabilization circuit developed by us may find application in other physical installations as well, and not only in the betatron.

The design and experimental investigation of the electronic stabilization circuit considered here were carried out by Yu. V. Grigor’ev and E. M. Belov under the author’s supervision.

REFERENCES

1. V. M. Razin. Stabilization of betatron radiation. Izvestiya TPI, vol. 87, 1957.
2. A. M. Bonch-Bruevich. The Use of Electron Tubes in Experimental Physics. GITTL, 1955.