EFFICIENT CATHODES FOR A MICROTRON

A. P. Arutyunova, A. G. Vlasov, A. I. Slutskaya, M. T. Shivyrtalov

The specific nature of cathode operation in an injector for a microtron imposes stringent requirements on the parameters and design of the cathode assembly. The best capture of electrons into acceleration in the microtron resonator is possible from a small area, on the order of several square millimeters, located at a definite place in the resonator. On this basis, cathodes for providing a sufficient accelerated current in the microtron must meet the following requirements:

1. With a small emitting surface of \(4\ \mathrm{mm}^2\), they must provide a current on the order of \(2\text{–}3\ \mathrm{A}\). Therefore, the cathode emission density must be on the order of \(50\text{–}100\ \mathrm{A}/\mathrm{cm}^2\), with a service life of more than 100 hours.

2. The cathode must not be destroyed by ion bombardment, which in the high-frequency field of the microtron resonator is very intense, especially when the vacuum deteriorates.

3. It must have a smooth emitting surface to ensure reliable operation at a high electric-field strength, which in the microtron resonator reaches several hundred kilovolts per \(1\ \mathrm{cm}^2\).

4. It must have stable emission over time.

5. It must allow disassembly of the microtron without poisoning the emitting surface.

The above requirements can be met by cathodes made of various materials—tungsten, oxide-thorium, lanthanum hexaboride, etc. We have developed and tested cathodes made from the following emission materials: purely metallic (tungsten), semiconductor (oxide-thorium), and lanthanum hexaboride. The cathodes were made both directly heated and with indirect and electron heating. Cathodes of the first two types and the results of their tests are described in [2, 3].

Lanthanum hexaboride proved to be the most suitable material for microtron cathodes. The best results were obtained with a cathode design using electron heating.

In its properties, lanthanum hexaboride satisfies all the listed requirements imposed on microtron cathodes.

The thermionic properties of cathodes made of lanthanum hexaboride are evident from the following table [1].

The properties of lanthanum hexaboride are given in detail in [1] and may be summarized as follows: specific resistance \(17.4\ \mu\Omega/\mathrm{cm}\), velo-

the evaporation rate in vacuum at a temperature of \(1680^\circ\mathrm{C}\) is \(2.89 \cdot 10^{-8}\ \text{g/cm}^3\), at \(1790^\circ\mathrm{C}\)—\(2.78 \cdot 10^{-7}\ \text{g/cm}^3\), at \(1910^\circ\mathrm{C}\)—\(2.66 \cdot 10^{-6}\ \text{g/cm}^3\). The melting point is \(2210^\circ\mathrm{C}\), the boron content is 31.7%, and the density is \(7.7\ \text{g/cm}^3\). Cathodes made of lanthanum hexaboride can be manufactured as directly heated cathodes and with indirect heating. Cathodes with electron heating are economically

Table 1

advantageous and convenient. Their heating power is considerably lower than that of directly heated cathodes. The manufacture of cathodes from hot-pressed lanthanum hexaboride rods makes it possible to avoid complicated cathode-unit designs providing protection against ion bombardment.

Fig. 1. Cathode assembly of a microtron with a lanthanum-boride cathode in the form of a rod

On Fig. 1 a cathode with electron heating is shown in the form of a rod fastened in a tantalum boat and heated by electron bombardment with an incandescent tungsten coil. Such a cathode operated in a microtron for 50 hours with an emission of \(1\ \text{A}\). The service life of such a cathode depends on burn-through of the tantalum boat, the durability of the coil, and cathode sputtering of the emitting material itself, lanthanum hexaboride. In practice, as a result of tests and calculations

the optimum dimensions of the heater coil were selected (diameter 0.3 mm, pitch 0.2 mm, length 7 mm) and the distance from the post to the heater (1.5–2 mm). In manufacturing such cathodes it is necessary to take into account the chemical interaction between the tantalum boat, in which the lanthanum hexaboride tablet is fastened, and the post, as a result of which a boride of the substrate metal is formed. The tantalum substrate is chemically insulated from the post by means of molybdenum disilicide.

Fig. 2. Cathode assembly of a microtron with an end-face lanthanum-boride cathode

In addition, the tantalum boat is affected by a known property of tantalum—embrittlement near incandescent tungsten, especially when the vacuum deteriorates. Long-term operation of the microtron with such cathodes showed that there is no need to use cathodes with so large an emitting surface (1.8–7 mm²). Since it is known from the theory of microtron operation that not all particles emitted from the cathode surface enter acceleration, but only a certain fraction, the area can therefore be reduced to \(2 \times 2\ \text{mm}^2\), and the cathode can be made as an end-face type. The design of such a cathode is shown in Fig. 2. In this design, uniform temperature over the working emitting surface, constancy and uniformity of the emission current, mechanical stability, small overall dimensions, and a heating power of 20–30 W have been achieved. The advantage of such a design is the possibility of obtaining high current densities, up to 60–70 A/cm², from a small emitting area of 4–5 mm², and the possibility of varying the point at which the cathode is installed relative to the resonator slot.

But for an end-face cathode it is necessary to have cylindrical posts.

On a specially manufactured machine put into operation, shown in Fig. 3, cylindrical posts of lanthanum hexaboride with a diameter of 2 mm and a height of 8 mm are cut. After fabrication and etching, the post is pressed into a cup,

Fig. 3. Electric-spark machine for cutting cylindrical rods from lanthanum hexaboride

Fig. 4. General view of the test stand

mounted on a rigid tantalum bracket. The spiral is made of VA-3 tungsten wire with a diameter of \(0.4\ \mathrm{mm}\). The spiral has three turns with an outside diameter of \(6\ \mathrm{mm}\).

The microtron cathodes were tested on a special test stand, whose general appearance is shown in Fig. 4. The basic electrical circuit for the tests is shown in Fig. 5.

Fig. 5. Basic circuit for testing cathodes with electron heating

As a result of tests on the test stand and directly during operation in the microtron, it was established that, with a comparatively low heating power of \(20\text{--}30\ \mathrm{W}\), a current of up to \(3\ \mathrm{A}\) can be drawn from the end-face cylindrical cathode, whose emitting surface is \(4.5\text{--}5\ \mathrm{mm}^2\); this corresponds to a current density of \(60\text{--}70\ \mathrm{A/cm^2}\). The cathodes withstand several disassemblies of the system and, under very severe operating conditions during microtron tuning, operate for \(40\text{--}50\) hours. Under normal operating conditions, the service life of the end-face cathode reaches 100 hours or more.

REFERENCES

1. G. V. Samsonov, Yu. B. Paderno. Borides of Rare-Earth Metals. Publishing House of the Academy of Sciences of the Ukrainian SSR, Kiev, 1961.

2. A. P. Arutyunova, A. G. Vlasov, T. I. Danilina. Development of efficient cathodes for a microtron. Report at the VI Interuniversity Conference on Electron Accelerators. (In press).

3. A. P. Arutyunova, A. G. Vlasov, T. I. Danilina. Development of cathodes for a microtron. Report of the Research Institute of Nuclear Physics No. 3149, Tomsk, 1965.