Proceedings of the Tomsk Polytechnic Institute,
awarded the Order of the October Revolution
and the Order of the Red Banner of Labor

Vol. 206　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1973

HIGH-FREQUENCY GENERATOR FOR CONDUCTING EXPERIMENTS TO STUDY THE EFFECT OF ADDITIONAL FOCUSING BY MICROWAVE FIELDS ON THE CAPTURE OF ELECTRONS INTO ACCELERATION IN A BETATRON

Yu. A. NEPOKRYTYKH, V. I. ZELENTSOV

(Presented by the scientific seminar of the sector of ultrahigh frequencies and accelerator theory,
Research Institute of Nuclear Physics, Electronics, and Automation)

Ultrahigh-frequency generators find very wide application in accelerator technology. They make it possible to obtain very high electromagnetic-field intensities, which are used for the purposes of acceleration, formation, and focusing of beams of charged particles. The generator described below was built for an experimental study of the processes involved in the influence of additional focusing of an electron beam by microwave fields during its capture into acceleration. A block diagram of the generator is shown in Fig. 1. The main element of the generator is a powerful magnetron operating in a pulsed mode. The most important and decisive advantage of the magnetron is that it has mechanical frequency tuning over a very wide range and permits frequency adjustment with high precision.

Fig. 1. Block diagram of the generator

For a generated microwave power-pulse duration of 3 μsec, the repetition frequency of the pulse trains reaches 400 Hz. The magnetron has forced water cooling. In view of the comparatively large filament current, the cathode leads overheat; they must therefore be blown by a fan. To create an operating magnetic field with intensity \(H = 2200 \pm 50\) oersted, an electromagnet of U-shaped profile was designed and built with three coils wound with a copper busbar \(2 \times 6\ \mathrm{mm}^2\) and connected in a matching arrangement. The total number of ampere-turns is \(IW = 13000\) A-turns. The generator is powered from a standard modulator assembled according to the circuit of a forming-line discharge through two series-connected thyratrons of type TGI1-400/16 and the primary winding of a pulse transformer.

The electrical circuit of the generator chains is shown in Fig. 2. A positive high-voltage pulse from the modulator is applied to the primary winding of the pulse transformer. From the secondary winding, a signal of negative polarity is applied to the magnetron cathode and excites oscillations in it. The leading edge of the pulse is \(\tau_{\mathrm{f}} < 0.2\ \mu\mathrm{sec}\). Capacitors \(C_{1,2,3}\) serve to equalize the potentials between the arms of the secondary windings. Power for the magnetron filament is supplied through contacts \(K_1\) and \(K_2\) of the water relay and fan, which close—

the circuit when water and air are supplied. The laboratory transformer serves for smooth current regulation, and instrument \(A_1\) for measuring it. The autotransformer is installed because the winding of the pulse transformer is not designed for large currents. When the filament circuit is switched on, the electromagnet winding is automatically energized, since contact \(K_3\) of relay \(P\) closes. The laboratory autotransformer and instrument \(A_2\) serve for regulating and measuring the magnetic-field intensity in the anode block of the magnetron. Contact \(K_4\) of relay \(P\) and contact \(K_5\) in the interlock of the generator cabinet doors are connected in series in the modulator power-supply circuit and prevent the arrival of high-voltage pulses in the absence of filament and magnetic

Fig. 2. Electrical circuit diagram of the generator circuits

field in the magnetron. The current flowing through the magnetron is monitored by a magnetic sensor that encloses the high-voltage insulator of the pulse transformer. The sensor is a ring 18 cm in diameter and \(15 \times 15\ \mathrm{mm}^2\) in cross section, made of permalloy tape \(0.05\ \mathrm{mm}\) thick. The winding consists of 180 turns of PELSHO-0.1. Retuning of the magnetron frequency is carried out remotely by means of selsyns \(C\)—\(c\) and a reducer \(P\)—\(p\). The microwave-power pulse from the magnetron (Fig. 1) passes through a transition waveguide and a ferrite isolator to the load. The isolator serves to suppress waves reflected by the load [1], and protects the magnetron from overloads during a short circuit. Testing, calibration, and measurement of the generator characteristics were carried out with the standard water-cooled load of an IM-4 power meter connected.

Figure 3 shows the dependence of the generated power on the voltage in the primary circuit of the anode-power transformer of the modulator forming line and on the magnetron filament current.

Comparison of the curves shows that the generated power depends strongly on the filament current. The generator provides high frequency stability. Figure 4 shows the change in the generator frequency as a function of the number of revolutions of the selsyn rotating the tuning mechanism. The frequency variation range is more than 7%. In designing resonant loads with high \(Q\) and using magnetrons with a fixed generation frequency, such high requirements are imposed on manufacturing accuracy that special devices for frequency trimming must be provided in them. When the load parameters change (for example, temperature), frequency trimming is required, and this presents considerable difficulties and inconveniences if the resonator is located in a vacuum space [2, 3]. The present generator makes it possible to design resonators with larger tolerances and without frequency-retuning devices.

Fig. 3. Current–voltage characteristics

Fig. 4. Range of variation of the generator frequency during tuning

Fig. 5. Dependence of the generated power on frequency

Fig. 6. Microwave-power pulse

Fig. 7. Pulse from the magnetic sensor

Because of the considerable power, small detunings of the load frequency have little effect on operation. Figure 5 shows the dependence of the generated power on frequency over the tuning range. The envelope of the microwave-power pulse is shown in Fig. 6, and the control pulse from the magnetic sensor is shown in Fig. 7. The generator makes it possible to obtain pulses with a duration of up to \(10\ \mu\text{s}\) at nominal power, with a drop of no more than 3%. To correct the pulse, \(RC\) circuits may be used (Fig. 8) and [4]. The parameters of the circuits are selected experimentally. According to the requirements for technical operation of the magnetron, the quartz-glass window through which the power is radiated must be air-cooled. In the present case this proved impossible, since during operation the load and the high-frequency line were evacuated. No cases of melting of the window were observed.

Fig. 8. Connection of the correction circuit

REFERENCES

1. I. V. Lebedev. Microwave Devices and Engineering, vol. 1, 2, 1964.
2. Technical report. Library of the Research Institute of Nuclear Physics and Electronics.
3. Technical report. Library of the Research Institute of Nuclear Physics and Electronics.
4. N. I. Vorob'ev et al. Electron Accelerators, 229, 1964.