IMPROVEMENT OF THE ACCELERATION CHAMBER OF A COMPACT BETATRON

Yu. N. ZELENOV, I. P. OKS, V. S. PUSHIN, V. L. CHAKHLOV

(Presented by the scientific seminar of the PMB sector)

In the process of manufacturing sealed acceleration chambers for compact betatrons, significant difficulties arise, associated with the high precision required in assembling the injector electrodes and installing it inside the chamber volume.

In known designs of sealed chambers for compact betatrons [1], the comparatively low electrical strength of the injector assembly (not more than 30 kV) is determined mainly by the electrical strength of the stem. In these designs, the anode lead is made directly in the region of the injector branch pipe, and increasing the electrical strength is possible by increasing the overall dimensions of the injector stem. However, in compact betatrons, because of the limited space available for accommodating the injector stem, this way of increasing the electrical strength is unsuitable. In addition, the induction method of welding it to the bulb does not provide the required accuracy of injector installation and leads to irreversible oxidation of the tantalum electrodes.

Fig. 1. Design of the injector and getter assemblies: 1 — stem, 2 — chamber branch pipe, 3 — injector, 4 — spring contacts, 5 — anode crosspieces, 6 — getter lead, 7 — titanium strip.

To eliminate the shortcomings listed above, a new design of a sealed acceleration chamber for a betatron was developed, which made it possible, without increasing the dimensions of the stem, to raise the injection voltage and reduce the total number of metal-to-glass seals.

The general appearance of the chamber is shown in Figs. 1 and 2. It consists of an injector assembly, a getter assembly, and a bulb. The injector assembly contains stem 1, injector 3, and spring contacts 4 welded to anode crosspieces 5. At the point where the spring contacts touch the inner ...

to the ring of branch pipe 2 and, in the region of one of the getter leads 6, a layer of silver is fired onto the conductive coating, ensuring reliable electrical contact between the getter lead, the conductive coating, and the injector anode. Joining the anode leads and the conductive coating inside the chamber made it possible to eliminate from the region of the injector stem the anode lead, which is at zero potential, and thereby to simplify its design and increase its electric strength.

For installing the injector in the chamber and welding the stem to the bulb, a device was used whose schematic is shown in Fig. 3. The required position of the injector is ensured by a special template 7, unambiguously fixed relative to the mounting clamp 5. The injector stem 9 is clamped by the cathode leads in the traversing mechanism 10 and, with its aid, is set so that the anode box 8 coincides with the seat of the template. The template is then removed, and bulb 4 is secured in its place. For this purpose the mounting clamp is moved downward along guides 2 of the device and, after the bulb has been installed, is returned to its initial position. Thus, adjustment of the injector is performed not relative to the chamber branch pipe, but relative to the template of the mounting clamp, the position of the bulb in which corresponds to the position of the chamber in the interpole space of the betatron.

Fig. 2. Annealed chamber.

Welding of the stem to the branch pipe of the bulb is carried out without preliminary heating, using a hand torch. The temperature in the injector region during sealing, as measurements have shown, does not exceed 80°C, and this does not lead to oxidation of the electrode surfaces. The symmetrical designs of the injector stem and branch pipe, and the comparatively large distance between the seam and the bulb, do not lead during welding to the occurrence of dangerous stresses in the glass, which made it possible to eliminate subsequent annealing of the chamber.

To improve the quality and increase the productivity of sealing, it is possible to use a vertical sealing machine, which is especially important in serial manufacture of chambers.

In manufacturing a series of sealed-off chambers, special attention was paid to the identity of the electron-optical characteristics of the injector. Preliminary studies showed that an injector forming an electron beam with a divergence angle \(\gamma = 20 \div 30^\circ\) is less critical with respect to the accuracy of its installation in the chamber than an injector forming an almost parallel beam. The optimal mutual arrangement of the injector electrodes is shown in Fig. 4. To ensure the required geometry, the electrodes were assembled with the aid of special fixtures. Mechanical stresses in the cathode core were relieved by threefold annealing in vacuum at a temperature of \(2000^\circ\text{C}\). After each annealing, the position of the cathode core relative to the focusing electrode was corrected. As the emis-

Fig. 3. Schematic of the device for installing and welding the injector stem: 1 — base, 2 — guide post, 3 — clamp-displacement screw, 4 — cylinder, 5 — installation clamp, 6 — fixing stop, 7 — template, 8 — injector anode, 9 — stem, 10 — stem-displacement mechanism.

powder applied to the core by the cataphoresis method was used as the emission layer. To simplify manufacture and assembly, the anode was made without side walls.

In order to simplify the getter assembly, the lead of the conducting coating was combined with one of the leads of the straight-heated getter. Titanium foil measuring \(0.1 \times 2 \times 20\) mm was used as the gas absorber. However, from operating practice with sealed-off chambers it became clear that it is possible to use one-time getters of the “BaTi” type, used, for example, in cathode-ray tubes.

According to the technology described, 15 sealed-off chambers were manufactured. When they were tested on a production betatron of the PMB-6 type, in which the injection voltage does not exceed 30 kV, the \(\gamma\)-radiation intensity was 19–24 r/h, which is somewhat higher than the nominal rated value of the betatron. Bench tests showed an increase in the electric strength of the injector stem to 45–50 kV. It is known that, all other conditions being equal, increasing the injection voltage in betatrons leads to a proportional increase in the radiation intensity. Therefore, the use of the developed design of the accelerating chamber will make it possible to improve significantly the efficiency of production betatrons, while the relatively high stability of the results obtained for intensity, due mainly to the identity of the injector parameters, will simplify their adjustment. In addition, reducing by half the number of metal-to-glass seals sharply reduced rejects in the process of manufacturing the chambers and simplified a number of technological operations.

Fig. 4. Diagram of the Kerst injector: 1 — cathode traverse, 2 — anode, 3 — target, 4 — cathode, 5 — focusing electrode.

REFERENCES

1. M. A. Lebedinskii. Technology of Electrovacuum Production. Moscow, Gosenergoizdat, 1961.

2. L. M. Anan’ev, L. A. Levshuk, D. I. Proskurovskii, V. L. Chakhlov. Proceedings of the Fourth Interuniversity Conference on Electron Accelerators. Moscow, “Vysshaya shkola,” 1964.

3. L. M. Anan’ev, L. A. Vorob’ev, V. N. Gorbunov. Induction Electron Accelerators—Betatrons. Moscow, Atomizdat, 1961.