DEVELOPMENT OF A COMPACT HIGH-FREQUENCY STEREOBETATRON

V. I. Kudryavchenko, V. A. Moskalev

(Presented by the scientific seminar of the Research Institute of Nuclear Physics)

In certain fields of science and technology, stereobetatrons find broad application (1, 2, 3, 4). At the Research Institute of Nuclear Physics, Electronics, and Automation of the Tomsk Polytechnic Institute, a high-frequency compact stereobetatron is being developed with an accelerated-electron energy of 3 MeV and a gamma-radiation dose rate on the order of 10 R/min at a distance of 1 m from the target. The supply frequency of the exciting windings is 400 Hz.

In order to reduce losses in the steel and, consequently, to reduce the dimensions and weight, the magnetic circuit of the installation is made of strip electrical steel grade E310 with a thickness of 0.08 mm. The use of strip electrical steel for the manufacture of the magnetic circuit determined the design features and manufacturing technology of the electromagnet.

The Stereobetatron Electromagnet

The stereobetatron magnetic circuit consists of two yokes, two pairs of poles, and two “wafer” blocks. The electromagnet is excited by two pairs of magnetizing windings, which are arranged symmetrically on the pole tips and connected in parallel with one another. The general appearance of the electromagnet is shown in Fig. 1.

The yokes of the electromagnet consist of five ring-shaped sections forming a toroid, and in cross section they have a stepped shape approximating a circle. This form of magnetic circuit makes it possible to obtain a high coefficient of filling of the yoke with steel and ensures a uniform loading of the yoke steel by the magnetic flux, which improves the natural azimuthal phase nonuniformity.

To manufacture the electromagnet yokes from strip steel, a machine was used for winding the cores of powerful pulse transformers, developed at TPI [5]. The necessary working gap of the installation is ensured by cutting the toroid diametrically into two parts and gluing profiled pole pieces, made from strips of equal length of E310 strip steel 0.08 mm thick and assembled along an involute, to the surfaces formed in the cut. This method of manufacturing the poles provides the highest coefficient of filling of the poles with steel and reduces nonuniformities of the magnet-

magnetic field [6]. Central inserts—“packets” made of parallel-stacked plates—are inserted between the poles. Since the magnetic core of the stereobetatron is made of thin steel, special attention was paid to developing the manufacturing technology for the individual parts and assemblies, in order to obtain a sufficiently high coefficient of filling with steel.

Fig. 1.

The individual parts and assemblies are fastened together with bakelite varnish and epoxy resin, resulting in a monolithic and compact structure without tie studs or bolts. The use of thin steel for manufacturing the electromagnet makes it economical; losses in the steel are reduced by a factor of 2.

The windings of the electromagnet are powered from a converter at 400 Hz. A pulsed version of the power supply is possible, with the material of the magnetic core allowing pulses of milli- and microsecond duration to be used. The use of increased-frequency currents to power the stereobetatron makes it possible correspondingly to increase the average dose rate of the radiation in comparison with betatrons having an electromagnet made of ordinary electrical steel.

Magnetic Characteristics

In developing the electromagnet of the stereobetatron, the objective was to obtain a design that would provide good magnetic characteristics and would not require additional tuning and adjustment.

Figure 2 shows the nature of the variation of the curve of the magnetic-field fall-off index. The curve agrees well with the results obtained on a flat model.

The static nonuniformity of the magnetic field, measured at radii of 3.5 cm, 4.5 cm, and 6 cm, does not exceed 0.7% and does not require additional

corrections, since it will not cause significant deviations of the electron motion from the equilibrium orbit.

The azimuthal phase nonuniformity was measured in the planes \(z=0\) and \(z=7\) mm and does not exceed \(\pm 0.25\ \mu\text{s}\) for \(z=0\) and \(\pm 0.3\ \mu\text{s}\) for \(z=7\) mm. This comparatively high intrinsic phase uniformity is ensured by the stepped shape of the yoke and by evolvent poles.

Fig. 2.

The principal magnetic characteristics presented above give grounds to conclude that additional tuning is not required in this electromagnet design.

Electronic circuits and vacuum system

In the stereobetatron provision is made for the use of an injection voltage of up to 100 kV. Figure 3 shows the injection circuit. To obtain a voltage pulse of up to 100 kV and a duration of 2–4 \(\mu\text{s}\), the discharge of a double pulse-forming line through the primary winding of a pulse transformer and a hydrogen thyratron of type TGI1-400/16 is used. The rectifier unit is built according to a single-half-period circuit using V1-01/40 kenotrons. The injection circuit is triggered from the synchronization circuit (Fig. 4) and can be adjusted from 5 to 100 \(\mu\text{s}\). In the initial version it is planned to feed the electron guns from two analogous circuits; subsequently a transition is envisaged to feeding 2 injectors from one circuit.

The displacement of electrons from the equilibrium orbit onto the target is accomplished by the circuit shown in Fig. 5, and occurs in two stages:

1) from the radius of the equilibrium orbit \(r_0 = 4.5\) cm to a radius of 5.2 cm by means of central windings wound on the central inserts;

2) from a radius of 5.2 cm to a radius of 6.9 cm by means of an extra-orbital winding located on the poles.

The displacement circuit is common to the two accelerating gaps. As the switching device, a hydrogen thyratron TGI1-400/16 is used. The charging device is made with two parallel-connected V1 01/30 kenotrons. The switching device is triggered by the commutating ...

Fig. 3.

Visible labels in the schematic:

• to the electron gun
• \(C_2\)
• \(\sim 200\), 400 Hz
• \(L\)
• \(C\)
• TGI1-400/16
• \(R_{\text{ch}}\)
• from the synchronization circuit
• V1-0.1/40
• \(R_{\text{lim}}\)
• \(\sim 200\), 400 Hz

Fig. 4.

a device with a synchronization circuit that makes it possible to regulate the ignition time from 300 to 600 μsec.

The vacuum system of the stereobetatron consists of two porcelain chambers, which have branch pipes for evacuating the system, for housing the injectors, and for carrying out experiments. Evacuation of the system is carried out by an RVN-20 pump and titanium pumps. The inner surface of the chambers is coated with tin oxide, and, to bring out the layer, silver is fired into individual sections of the branch pipes.

Fig. 5.

The stereobetatron can be used for X-raying rapidly occurring physical and chemical processes, for radiographing moving parts and assemblies in order to study their mutual arrangement and detect defects, for obtaining two consecutive images at intervals of several tens of microseconds in order to study the dynamics of rapidly occurring processes, for stereodefectoscopy of special products, etc.

The stereobetatron described has been fabricated and is in the stage of preparation for start-up.

REFERENCES

1. V. A. Moskalev. ZhTF, 32, 2060—2061, 1956.
2. V. A. Moskalev, Izv. vuzov MVO SSSR, Fizika, No. 6, 1959.
3. V. A. Moskalev, Yu. M. Akimov, Izv. vuzov, SSSR MVO SSSR, Fizika, No. 5, 1959.
4. V. A. Moskalev et al. Izv. vuzov MVO SSSR, Fizika, No. 5, 1959.
5. N. I. Vorob'ev. Machine for winding magnetic cores of large dimensions, Proceedings of the Third Inter-University Conference, Tomsk, 1961.
6. A. M. Anan'ev, V. L. Chakhlov, Yu. B. Yarushkin. Advanced scientific, technical, and production experience. Issue 10, topic 14M-62-263/10, GOSINTI, 3—20. Moscow, 1962.