SETUP FOR STUDYING THE MECHANISM OF ELECTRICAL BREAKDOWN IN ULTRAHIGH VACUUM

S. P. Bugaev, D. I. Proskurovskii, V. I. Eshkenazi

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

At present, studies of the mechanism of breakdown in high vacuum are being carried out in many countries. This interest in the electrical strength of vacuum is due to the fact that the insulating properties of vacuum are widely used in physical research and in technology. At present there are several theories of the mechanism of vacuum breakdown. The bulk of experiments confirming one or another theory of vacuum breakdown has been performed for gaps from 0.1 to 10 cm under dc voltage and pulsed voltage with \(\tau_{\text{imp}} > 1\ \mu\text{sec}\). But the processes initiating vacuum breakdown in small gaps take place in about \(1—10\ \text{nsec}\) [1]. Therefore the study of electrical breakdown of vacuum by high-voltage pulses of nanosecond duration should shed light on the breakdown mechanism.

A vacuum setup for such studies must satisfy a whole series of requirements. One of the main requirements is the use of oil-free pumping means for evacuating the gas, which excludes the use of rotary oil pumps at the forevacuum stage and oil-vapor pumps on the high-vacuum side. This necessity for using the newest oil-free pumping means is caused by the fact that films of oil vapor, with which the electrodes under study become coated, can substantially affect the breakdown mechanism [2]. The limiting vacuum attainable in the working volume must be better than \(1 \times 10^{-9}\) torr. This, in turn, requires baking the ultrahigh-vacuum part of the setup to \(450^\circ\) and higher, which makes it necessary to fabricate it from stainless steel [3, 4].

Fulfillment of the above requirements led to the following scheme of the vacuum setup (Fig. 1). A photograph of the setup is shown in Fig. 2.

The preliminary vacuum is obtained by means of zeolite pumps. Further pumping is carried out by means of ion-sorption pumps. The zeolite pumps are made from a stainless-steel tube of grade \(1\times18\text{N}9\text{T}\), \(50\ \text{mm}\) in diameter, with a wall \(1\ \text{mm}\) thick and \(300\ \text{mm}\) long, welded by argon-arc welding to a flange. A nickel mesh is placed inside the tube. Zeolite granules are poured between the nickel mesh cylinder and the stainless-steel wall. During baking to \(450^\circ\), the zeolite pumps \(Ц_1\) and \(Ц_2\) are cut off by valves \(B_1\) and \(B_2\) from the ultrahigh-vacuum part of the setup.

from one another, and the open traps \(P_1\) and \(P_2\) make it possible to remove adsorbed gases and water vapor. To obtain rarefaction, traps \(P_1\) and \(P_2\) are closed and the stainless-steel cylinder is cooled with liquid nitrogen. After 15 minutes, with the valves open, a pressure of \(10^{-3}\) torr is obtained in the system; it is measured with an LT-2 lamp.

Fig. 1. Vacuum diagram of the setup for studying the mechanism of electrical breakdown in ultrahigh vacuum

The next task is to start the titanium pump \(T_2\), for which a pressure of \(10^{-4}\) torr is required. This reduction of pressure is carried out by the evaporative titanium pump \(T_1\). With the aid of pump \(T_2\), a pressure of \(1 \times 10^{-7}\) torr is obtained in the system; it is measured with an LM-1 lamp. To the left of flanges \(\Phi_1\) is shown the high-vacuum part of the setup, the parts of which are made mainly of brass. To the right of flanges \(\Phi_1\) is located the ultrahigh-vacuum part of the setup; the parts of the latter are made of stainless steel and are heated in a furnace to a temperature of \(450^\circ\text{C}\). The power of the electric furnace is 5 kW.

Valve \(B_5\) has a clear bore of 25 mm. The seal is an annealed copper cone, which cuts into a stainless-steel knife edge. With the aid of valve \(B_5\), the ultrahigh-vacuum part is isolated from the high-vacuum part and is pumped by pump \(T_3\) down to a pressure of \(1 \times 10^{-9}\) torr.

All assemblies of the ultrahigh-vacuum system are connected through metal vacuum seals to a cube, three faces of which are blanked off, while to the remaining three are attached pump \(T_3\), the discharge chamber, the IM-12 ionization manometer, and valve \(B_5\).

The metal seals are rings of annealed M1 copper 1–1.5 mm thick, which are compressed between knife edges by six M10 bolts. The knife edges have an average diameter of 40 mm, a height of 0.9–1 mm, a width of 0.6–0.7 mm, and are polished to cleanliness class 8.

The IM-12 manometer is a triode ionization manometer with an axial collector, mounted in a glass bulb which is connected to the flange through a kovar transition.

Pump \(T_3\) consists of a cylindrical housing with a water-cooling jacket, to which flanges are welded on both sides. A metal-ceramic stem is attached to one flange, on which the pump electrodes are mounted; by the other flange the pump is attached to the cube. The titanium evaporator is located along the axis of the pump and is a cylindrical titanium rod on a molybdenum core. The evaporator diameter is 3.3 mm, height 50 mm. The evaporator is heated by electron bombardment from a directly heated tungsten cathode (Ø 0.3 mm, length 125 mm) of loop shape. The power expended on heating the evaporator is 40 W (voltage 500 V, current 80 mA). The cathode is kept under tension by means of a spring fastened in a special support. To protect the metal-ceramic stem from deposition, a screen is provided.

Fig. 2. General view of the setup for studying the mechanism of electrical breakdown of vacuum

The discharge chamber is a metal-glass coaxial structure with an internal diameter of 60 mm. The choice of the main diameters of the chamber was made so that the characteristic impedance of the latter was 75 ohms, which makes it possible to match the chamber with the supply cables [5, 6]. Four branch pipes ending in flanges are welded by argon-arc welding into the side surface of the outer cylinder. By one of the flanges the chamber is attached to the cube, while the others are closed off with flanges containing an optical window.

The optical windows, as well as the glass insulators of the chamber, are made with Kovar-glass seals. The Kovar is welded to stainless steel by argon-arc welding. Part of the outer housing of the chamber is replaced by four corrugations of a thin-walled (0.2 mm) stainless-steel bellows.

The presence of the bellows makes it possible to adjust the distance between the electrodes from 0 to 10 mm. The chamber withstands voltages up to 50 kV and transmits without distortion pulses with a rise time of 1 nsec. The chamber dimensions are \(230 \times 140 \times 140\) mm.

REFERENCES

1. G. M. Kassirov, B. M. Kovalchuk, ZhTF, 1964, 34, 484.
2. S. Dushman. Scientific Foundations of Vacuum Technique. IL, 1950.
3. Modern Vacuum Technique. Collection of articles edited by N. V. Cherepnin, IL.
4. E. Trendelenburg. Ultrahigh Vacuum, Mir Publishing House, 1966.
5. G. A. Vorob’ev, G. A. Mesyats. Technique for forming nanosecond pulses. Gosatomizdat, 1963.
6. L. A. Morugin, G. V. Glebovich. Nanosecond Pulse Technique. “Sovetskoe Radio,” 1964.