OXIDE–THORIUM CATHODES FOR BETATRON INJECTORS

A. G. Vlasov, B. V. Okulov, V. P. Ponomarev, A. I. Slupskaya

1. Choice of Cathode Type and Technology for Their Manufacture

Specific requirements are imposed on the cathodes of electron-accelerator injectors: sufficiently high emission density and stability over time, resistance to poisoning, i.e., the ability to operate at a comparatively low vacuum of \(10^{-5}\)—\(10^{-6}\) torr, resistance to ion bombardment, mechanical strength of the emitting layer and its adhesion to the substrate, and a sufficiently long service life.

A number of cathodes used in electron-vacuum devices satisfy these requirements: cathodes made of pure thoriated tungsten, oxide cathodes, oxide–thorium cathodes, and others.

Cathodes made of pure tungsten have one substantial drawback: a sufficient emission density of \(10\)—\(20\ \mathrm{a/cm^2}\) in a pulse is ensured only at high temperature, which considerably shortens their service life.

For a long time, thoriated-tungsten cathodes were used in sealed-off electron-vacuum devices. In [1] the use of thoriated tungsten for injector cathodes was described. But such cathodes have a number of disadvantages:

1. The thorium film on the tungsten surface is rapidly destroyed by ion bombardment at anode voltages of several kilovolts.

2. At temperatures above \(2000^\circ\mathrm{K}\), the cathode sharply loses emission, as shown in Fig. 1.

Thoriated-tungsten cathodes operate more stably after they have been carburized by heating in hydrocarbon vapors. In this case an atomic thorium film forms on tungsten carbide, with which its adhesion is considerably better than with pure tungsten. In addition, the reduction of thorium from its oxide within the tungsten wire proceeds better in the presence of carbon [2]. However, tungsten carbides are brittle, which leads to cracking of the cathode surface upon cooling and to rapid failure because of mechanical destruction.

Oxide cathodes have found the widest use; barium is used as the active substance for them. Such cathodes, while having major advantages in emission properties, nevertheless also have disadvantages: they require activation at high vacuum, and if the vacuum is impaired they irreversibly lose emission.

Therefore, in demountable vacuum systems, which are the systems of most betatrons, it was proposed to use an oxide cathode based on thorium oxide.

The operating temperature of an oxide-thorium cathode is somewhat lower than in the case of thoriated tungsten (Fig. 1). These cathodes have no blocking layer between the oxide layer and the core, which makes it possible to use them in pulsed operation when drawing large current densities, equal to \(10\)—\(20 \, a/cm^2\). The emission of oxide-thorium cathodes changes with a large time constant—several hundred hours.

Fig. 1. Dependence of emission density on cathode temperature.
Curve 1—thoriated tungsten cathode.
Curve 2—oxide-thorium cathode.
Curve 3—pure tungsten cathode.

To obtain high emission from an oxide-thorium cathode, it is necessary to activate it. To activate a cathode means to create an excess quantity of free thorium atoms on its emitting surface. Cathode activation proceeds best with current drawn at \(1750\)—\(1800^\circ C\). Activation at higher temperatures is undesirable, since when the temperature is lowered to the operating value the cathode may become deactivated [3].

Oxide-thorium cathodes operate most stably at a temperature of \(1650^\circ C\). After contact with the atmosphere, oxide-thorium cathodes rapidly restore their properties upon repeated activation, are readily degassed, and can operate for a long time in a poor vacuum of the order of \(10^{-4}\) torr without losing their emission properties.

2. Technology for Manufacturing Oxide-Thorium Cathodes

The oxide-thorium cathodes used in betatron injectors consist of a spiral, or several spirals connected in parallel, made of tungsten wire with an applied oxide layer.

The service life of such a cathode depends to a large extent on the durability of the core and the reserve of thorium oxide.

For oxide-thorium cathodes, as the core we use tungsten wire of grades VA-3 and VA-5 with a diameter of \(0.2 \, mm\).

Tungsten, unlike other metals, has the highest melting temperature, a low evaporation rate, and therefore a long service life.

Fig. 2. Injector with an oxide–thorium cathode for small-size betatrons at 6 MeV

Fig. 3. Cathode assembly of the injector of a high-current betatron at 25 MeV

Coils wound on a winding machine are cleaned by electrolytic etching in a 20% alkali solution, washed, dried, mounted, and, to remove residual deformations, annealed in a vacuum furnace at a pressure of \(10^{-2}—10^{-4}\) torr and a temperature of \(900—1000^\circ\mathrm{C}\) for 5 minutes.

The second critical operation in the fabrication of an oxide-thorium cathode is the application of the oxide.

Thorium oxide quickly flakes off from pure tungsten. Various methods are used to improve the adhesion of thorium oxide to the core material. We use a method of coating the core with a mixture of tungsten and molybdenum powders. A mixture of powders, \(50\% W + 50\% Mo\), is applied by cataphoresis to the cathode core.

When sintered in a vacuum furnace at a pressure of \(10^{-5}\) torr and a temperature of \(2200^\circ\mathrm{C}\), such a coating forms a spongy surface. The sponge makes it possible to create the required reserve of active substance, improves heat transfer from the core to the active substance, and increases the strength of adhesion of the thorium oxide to the core.

Thorium oxide is applied to the spongy surface of the core by cataphoresis or is rubbed in with a brush.

The cathode coated with thorium oxide is placed in a system for vacuum impregnation. The system is evacuated. Then, by a sharp increase in pressure to 2–3 atmospheres, the thorium oxide is driven into the pores of the sponge.

After impregnation, the cathode is once again coated with active mass and immersed for several seconds in a binder, which forms on the cathode surface a thin film that firmly binds the thorium oxide crystals together. A more complete cathode technology is described in [4].

3. Results of tests of oxide-thorium cathodes

We manufacture oxide-thorium cathodes for injectors of small-sized betatrons at 6 MeV (Fig. 2), for a high-current betatron at 25 MeV (Fig. 3), and for a high-current stereobetatron at 15 MeV (Fig. 4).

Fig. 4. Cathode assembly of the injector of a high-current 15 MeV stereobetatron

Cathodes for injectors of small betatrons are made in the form of a spiral 1 mm in diameter and 7 mm long. The cathodes operate for 300–350 hours with a constant emission of 1.5–2 A in the pulse.

The cathodes of a high-current 25-MeV betatron are made in the form of 16 parallel-connected spirals 1 mm in diameter and 11 mm long. The heating power of such cathodes is 200–250 W. The current from the cathode is 40 A; the service life is 350–400 hours. The cathodes of a high-current 15-MeV stereobetatron are 5 parallel-connected spirals 1 mm in diameter and 8 mm long. The heating power is 60 W; the current from the cathode is 5–10 A; the service life is 400–450 hours.

All these cathodes are relatively simple to manufacture, operate well in demountable systems, make it possible to obtain high current densities in pulsed operation without fear of vacuum degradation down to a pressure of \(10^{-4}\) torr, and have a service life of several hundred hours.

All this has enabled us to make extensive use of oxide-thorium cathodes for betatron injectors.

REFERENCES

1. A. G. Vlasov. Izv. TPI, 87, 297, 1957.
2. B. M. Tsarev. Calculation and Design of Electron Tubes. GEI, 1951.
3. Effective Thermocathodes. Collection of translations edited by A. G. Shulman. GEI, issue 268, 1958.
4. A. G. Vlasov, A. I. Slutskaya. Oxide-Thorium Cathodes of Accelerator Injectors. Report of the Research Institute of Nuclear Physics No. 3171, Tomsk, 1965.