ON THE OPERATION OF TR-85/15 THYRATRONS IN PULSED MODE

V. M. KUZNETSOV, I. P. CHUCHALIN

At present, designers of electrophysical installations are increasingly confronted with the problem of switching currents of several thousand amperes for durations of up to tenths of a second and more.

The electrovacuum devices manufactured by industry (thyratrons, ignitrons, etc.) cannot switch current pulses of such long duration. In fact, the most powerful pulsed hydrogen thyratrons, for example TGI1-400/16 [1], are rated for a maximum current of 400 A and a voltage of 16 kV at a maximum current duration of 5 μsec.

The most powerful generator tubes of the GMI-90 type pass current pulses of 20 A at a voltage of 36 kV and a pulse duration of 2.5 μsec [2].

Ionic devices with a liquid cathode—ignitrons—have great capabilities. The most powerful of them, for example IVU-100/15 and IVS-200/15 [3], at voltages up to 15 kV can pass pulsed currents from 300 to 1600 A. But their disadvantage is that they cannot operate at long current durations, since in this case interruption of the anode current occurs as a result of narrowing of the interelectrode space [1, 4, 5].

In addition, ignitrons operate with insufficient reliability because of frequent malfunctions of the ignitors by means of which the arc is initiated.

In our opinion, the most suitable devices for switching powerful current pulses with durations of up to tenths of a second are thyratrons with mercury filling.

Taking into account the advantages of thyratrons over ignitrons in the duration of the current passed, we carried out studies of the operation in pulsed mode of the most powerful mercury-filled thyratrons of type TR1-85/15, intended for operation in rectifier devices.

The TR1-85/15 thyratron has the following rated specifications: amplitude of direct and reverse voltage \(U_a = 15\) kV, amplitude of anode current \(I_{am} = 300\) A, average anode current not more than 85 A [2].

The studies showed that such a thyratron can be considerably overloaded in current in pulsed modes, while keeping the average current within the rated limits, i.e., not more than 85 A.

The limit to increasing the maximum current through a thyratron is determined by the following factors: a) the cathode emission current; b) the dynamic stresses arising during the passage of a current pulse; c) the durability of the cathode in pulsed operation.

The area of the oxide cathode in the TR1-85/15 thyratron is approximately 1400 cm². In modern electron-vacuum devices the maximum pulsed emission density from oxide cathodes reaches \(10 \div 50\ \mathrm{A/cm^2}\) [6, 7]. On this basis, the maximum current from the cathode of the TR1-85/15 thyratron may reach several tens of kiloamperes. The thyratrons were investigated in the circuit shown in Fig. 1. A capacitor bank \(C\), previously charged from a rectifier, was discharged into the inductance \(L\) through the thyratron under test \(T_p\). To reduce the power consumed, the capacitors were recharged through another thyratron \(T_n\), connected in inverse-parallel with the main thyratron.

Fig. 1.

The maximum current was recorded by means of a standard active shunt and a previously calibrated oscillograph. The dependence of the anode current on the anode voltage was linear, i.e., saturation of the device did not occur.

The voltage drop in the arc was measured on the thyratron \(T'_p\), which was open at all times and passed a small current from an additional voltage source. The voltage drop in the arc of the conducting valve \(\Delta U\) did not show a noticeable increase in any of the experiments.

At first the experiments were carried out at an anode voltage \(U_a = 3000\ \mathrm{V}\). The duration of the current pulse through one thyratron was \(\tau = 0.084\ \mathrm{s}\). In this regime the thyratron was tested up to a maximum current \(I_{am} = 3000\ \mathrm{A}\) at a pulse repetition rate of 2 times per second.

Up to a current amplitude of 1700 A no special changes in the operation of the thyratron were observed. At currents above 1700 A the thyratron flashed throughout the entire bulb. This was apparently caused by diffusion of the plasma into the interelectrode region and by ionization of the entire gas filling. At an anode voltage \(U_a = 3000\ \mathrm{V}\) and a maximum current \(I_{am} = 1700\ \mathrm{A}\), the thyratrons operated continuously for more than 50 hours without deterioration of their characteristics. The overload capability of the thyratrons was then checked at an anode voltage of 15 kV and a current-pulse duration of 3.5 μs. Up to an anode-current value of 1550 amperes the thyratrons operated without noticeable changes. At higher anode-current values, up to 5100 A, the thyratron again began to flash throughout the entire bulb. However, even when switching a current of 5100 A, the characteristics of the thyratron did not deteriorate.

More prolonged tests of the thyratrons—over a period of 90 hours—were carried out at an anode voltage of 15 kV and a current of 1200 amperes.

The mechanical strength of the thyratrons under the indicated overloads proved sufficient. The experiments also showed that, for stable

during thyratron operation it is necessary to monitor the temperature in the grid region. Overheating of the grid causes spontaneous ignition of the thyratron. Therefore, when operating thyratrons under overload conditions, it is necessary to keep careful watch over the cooling system.

The possibility of operating TR-85/15 thyratrons under current-overload conditions is confirmed by experience with their operation in several installations of the Institute of Nuclear Physics at the Tomsk Polytechnic Institute.

LITERATURE

1. G. A. Vorontsov. Pulse Thyratrons, Sovetskoe Radio Publishing House, 1958.
2. Reference Book. Electric Vacuum Devices, GEI, 1956.
3. A. A. Sakovich, T. A. Suetin. Investigation of sealed IVS-100/15 ignitrons, Elektrichestvo, No. 1, 1959.
4. B. M. Shlyaposhnikov. Ignitron Rectifiers, Transzheldorizdat, 1947.
5. V. L. Granovskii, T. A. Suetin. Generation of powerful electrical oscillations in a low-pressure discharge, ZhTF, issue 9, 1946.
6. V. B. Marchenko. Modern Cathodes, GEI, 1958.
7. M. L. Gurevich, M. D. Gurevich. Electric Vacuum Devices, Military Publishing House of the Ministry of Defense of the USSR, 1955.