HIGH-VOLTAGE CIRCUITS FOR POWERING INJECTOR DEVICES OF HIGH-CURRENT BETATRONS

Yu. M. Skvortsov

(Submitted by the Scientific Research Institute of Nuclear Physics, Electronics, and Automation)

To increase the effectiveness of the use of betatrons by increasing the number of accelerated electrons in each pulse, the most realistic and advantageous path is to raise the injection voltage while simultaneously increasing the aperture of the accelerator gap [1, 2]. At present, the advisability of using high-voltage injection to increase the radiation intensity of cyclic accelerators has been confirmed experimentally [3, 4], and the requirements imposed on this injection have been stated with sufficient clarity in [5, 6]. It was precisely the satisfaction of these requirements [7] that made it possible, in betatrons with high-voltage injection, to obtain a substantial increase in the number of accelerated electrons. The possibility of a proportional change in the number captured and the preservation, to the end of the acceleration cycle, of the number of electrons brought in is determined to a considerable extent by the electrical parameters of the pulse generator of the injection circuit and by the characteristics of the interpolar space of the betatron. From this point of view, in order to obtain the conditions for “optimal” capture* of the injected electrons, a number of requirements are imposed on high-voltage pulse-voltage generators, in particular.

In connection with the need, at the moment of injection, to satisfy the condition of equality between the forces of the electrical interaction of the electron beam and the forces of the magnetic field in the interpolar space, requirements are imposed on the voltage pulse generated to feed the high-voltage injector both in terms of power, which corresponds to the required injection current, and in terms of shape. Obtaining the necessary correspondence between the amplitudes of the voltage and current of the pulse applied to the injector determines the power source and the type of switch in the pulse-generator circuit. At the same time, generation of a pulse of specified shape depends on the design features and electrical parameters of the storage element of the high-voltage modulator.

Since, according to the studies of a number of authors [9, 10], no limit is established between the magnitude of the captured charge and the injection voltage for the initial value of the energy of the particles introduced into acceleration—

* “Optimal” capture corresponds to the introduction of such a number of electrons as is approximately equal in magnitude to the equilibrium charge capable of being held by the magnetic-field forces in the interpolar gap of the betatron under the given conditions.

electrons; therefore, the actually permissible voltage amplitude is associated with ensuring the necessary electrical strength of the injector and of the electromagnet parts in the air gap where this injector is located.

For injection in a high-current betatron at 25 MeV, we selected an injection energy approximately 10 times greater than that previously used in ordinary betatrons of comparable energy, i.e., \(E_{\mathrm{i}} = 400 + E_0\) keV. At the specified voltage, the dimensions of the injection system as a whole do not exceed reasonable limits. Such an increase in injection energy, in accordance with theoretical predictions, gives an increase in radiation output by more than an order of magnitude, all other conditions being equal. For a 15 MeV betatron, the injection energy was chosen equal to \(E_{\mathrm{i}} = 200 + E_0\) keV; here and above \(E_0 = 511\) keV is the electron rest energy.

Fig. 1

Increasing the injection voltage to hundreds of kilovolts requires the use of an inflector in the injection channel. According to the existing relationship between the voltages on the electron gun and on the electrostatic-type inflector, and taking into account its geometric characteristics for the selected range of injection voltages \((U_{\mathrm{i}} = 200\text{--}400\ \text{kV})\), a voltage of \(U_{\mathrm{vu}} = 30\text{--}60\ \text{kV}\), respectively, was required on the deflecting plate of the inflector.

Since, with high-voltage injection, the power of the pulse-voltage generator becomes significant, for the injection systems of the first high-current betatrons at 15–20 MeV the magnitude of the required current was calculated by the empirical formula:

\[ I_{\mathrm{ig}} = (6 \div 8)\cdot 10^{-6}\frac{E_{\mathrm{i}}}{R_0} \left[ \left(\frac{E_{\mathrm{i}}}{E_0}\right)^2 - 1 \right] S_{\mathrm{i}}. \]

Here \(R_0\) and \(S_{\mathrm{i}}\) are the radius of the equilibrium orbit and the region of focusing forces in the interpole space; \((6\text{--}8)\cdot 10^{-6}\) is a coefficient accounting for the change in the required generator current depending on the type of injector device and the power of the circuit.

The modulators for high-voltage pulse generators constructed by the author cover the entire range:

a) by type of storage element,
b) by type of switch.

Fig. 2

Fig. 3

For a high-current betatron of 25 MeV, operating at the industrial frequency, a circuit using a high-power electronic tube was envisaged (Fig. 1), with a lumped capacitance with partial discharge to be used as the storage element. However, the small value

Table 1

* Pulse repetition frequency 0.25 Hz.
** Of the correction line.

the pulse power permitted by modern electronic modulator tubes, and the complexity of operation associated with the use of separate rectifiers for supplying the grids, as well as the need for liquid cooling of their electrodes, make their use impossible at the present stage in powerful generators of high-voltage injection systems.

The use as modulators of ionic devices of the air-controlled spark-gap type or a hydrogen thyratron (Fig. 2) made it possible to fully satisfy the requirements imposed on a high-voltage injection system. Switching in the modulator of pulse currents with amplitudes of up to 1000 A at a voltage of 60 kV made it possible to provide, with a generator for voltages up to 400 kV, the currents required in the load—the high-voltage electron gun of the end type [11]. In addition, by changing the characteristic impedance of the line through reduction of its longitudinal inductance, it is possible to carry out the necessary correction of the pulse shape. This correction is intended to increase the stability of capture, which is reflected in the average intensity of the bremsstrahlung radiation of a high-current betatron.

Fig. 4. Oscillograms of pulses at different values of $L_k$ of a section of artificial line

The positive properties of a capacitive storage unit and a double shaping line were realized in the proposed and implemented pulse modulator (Fig. 3) with a combined storage unit [2]. The original connection of a section of artificial line, open at the end, makes it possible, by changing its characteristic impedance, to carry out pulse correction within known limits (Fig. 4).

Below, the table gives the principal characteristics of high-voltage modulators and pulse transformers [13] manufactured under the author’s direction for high-current betatrons with energies of 15–25 MeV.

In conclusion, we note that the use of the manufactured high-voltage pulse-voltage generators in the injection system of high-current betatrons [14, 15], which have an increased region of focusing forces in the interpolar space of the electromagnet, made it possible to increase the accelerated charge by more than 200–500 times in comparison with ordinary betatrons of the same energy.

The author takes this opportunity to express his sincere gratitude to Professor V. A. Moskalev for formulating the problem and for valuable advice.

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