INCREASING THE INTENSITY AND STABILITY OF BETATRON BREMSSTRAHLUNG BY SELECTING OPTIMAL INJECTION CONDITIONS

L. N. DROZDOV

In the practice of radiation flaw detection, electron accelerators—betatrons—are finding ever wider application. The effectiveness of the use of betatrons is determined in large measure by the intensity and stability of the radiation they generate.

A number of methods have been proposed for increasing the intensity [1, 2] and stability [3, 4] of betatron bremsstrahlung. However, the relative complexity of implementation and the low efficiency limit the application of these methods. The present communication gives the results of research aimed at increasing the intensity of the bremsstrahlung of the B-30 betatron designed by TPI by optimizing the shape of the injection pulse; and, in order to increase the stability of the intensity, a scheme is proposed for automatic tuning to the optimal injection phase in accordance with changes in the injection voltage. Control of the shape of the injection pulse according to the circuit shown in Fig. 1 is carried out directly from the betatron control panel by changing the voltage on the storage capacitors \(C_1, C_2\) (injection units \(I, II\)) and subsequently discharging them through thyratrons \(L_8, L_{10}\), with an adjustable time shift, into the primary winding of the high-voltage injection pulse transformer. In this case, a voltage pulse is produced in the secondary winding of the pulse transformer, the duration and shape of which can be smoothly varied over wide limits. The timing adjustment of the firing of thyratron \(L_{10}\) relative to the firing of thyratron \(L_8\) is set by phantastron \(L_6\) in the interval \(0 \div 20\ \mu\text{sec}\). The greatest intensity, exceeding the initial value by a factor of three, was obtained with the optimal shape of the injection pulse shown in Fig. 2, c. In contrast to the initial injection pulse (Fig. 2, a), which has a duration at the 0.5 level of \(2.5\ \mu\text{sec}\), the optimal pulse is characterized by a duration at the 0.5 level of \(3\ \mu\text{sec}\) and by the presence at the top of a plateau with a duration of \(1.25\ \mu\text{sec}\). A feature of the plateau is the smooth rise of the pulse top by 10% from the level 0.9 to 1.0 of the pulse amplitude.

The radiation intensity was estimated using a “Kaktus” milliroentgenometer and a “Dim-60” integrating dosimeter (lead phantom). An injection pulse of the shape shown in Fig. 2, c made it possible to obtain an intensity equal to \(250\ \text{R/min}\) at a distance of \(1\ \text{m}\) from the target.

Studies on optimizing the shape of the injection pulse were carried out by us on several betatrons manufactured at TPI. As a result of the tests, the intensity of betatrons operating in optimal regimes was increased on average by a factor of 2–3, and in some cases the injection pulse with the shape shown in ...

Fig. 1. Circuit for optimizing the phase and shape of the injection pulse.

Visible labels in the diagram include: “Injection block I,” “Injection block II,” “to injector,” and “injection pulse-shape simulator.”

Fig. 2, b. Operation of the injection-phase optimization channel is as follows. The injection voltage from the preceding cycle is taken from the divider (Fig. 1) and fed to a diode-capacitor integrating circuit; it is amplified by \(L_5\) (left half) and then goes to the input of the DCT. The time constant of the integrating circuit was chosen experimentally and is equal to 0.3 sec. At the output of the DCT there is a voltage that sets the bias current of the peaker through an additional winding. This current determines the optimum phase of electron capture into acceleration when the injection voltage and the gun current are changed. The initial setting of the optimum injection phase is carried out with potentiometer \(R_2\). By changing the resistance \(R_1\) in the bias circuit of the triode \(L_5\), its operating point is selected, ensuring the optimum capture phase within the operating range of the dependence of intensity on injection voltage. Adjustment of the transfer coefficient of the amplitude of the injection pulse into the peaker bias current is carried out with potentiometer \(R_3\). After the optimum power-supply and injection-phasing modes have been set, the tracking point corresponding to the maximum in the dependence of intensity on injection phase is finally corrected with potentiometer \(R_2\).

Fig. 2. Oscillograms of injection-voltage pulses (time marks: 1 μsec)

A feature of using such a circuit for optimizing the conditions of electron capture into acceleration is that, at normal gun current, as the injection voltage is increased the radiation intensity automatically increases, settling at the maximum value. Switching to manual control of the injection phase is carried out by means of switch VK-1 and is performed by changing the peaker bias current with potentiometers \(R_4\), \(R_5\) for fine and coarse adjustment. The degree of automatic optimization of the conditions of electron capture into acceleration was evaluated by measuring the integral radiation dose of the betatron in comparison with the manual injection-phase control mode. The measurement results showed that the integral dose when operating with the optimization circuit is 30% greater than the dose obtained when operating under manual control. The simplicity and good operating characteristics make it possible to recommend the developed circuits for optimizing the injection-pulse shape and capture conditions for increasing the radiation intensity of betatrons used in defectoscopic installations.

REFERENCES

1. I. P. Chuchalin. Study of the process of electron capture in a betatron, accelerated with various shapes of the injection-voltage pulse. Izv. TPI, vol. 87, 1957.

2. B. V. Gelperin, M. G. Merkur’ev, N. M. Sokolov. Experimental studies of the influence of the shape of the injection pulse and the stability of the power-supply elements of a 25-MeV betatron installation on radiation intensity. Proceedings of the 6th Interuniversity Conference on Electron Accelerators. Energiya Publishing House, 1968.

3. V. M. Razin. Stabilization of betatron radiation. Izv. TPI, vol. 87, 1957.

4. N. N. Balamatov, B. I. Goryachev. System of automatic optimization of the gamma-radiation intensity of a betatron. Proceedings of the 5th Interuniversity Conference on Electron Accelerators, Atomizdat, Moscow, 1966.