A High Reliability, Fail Safe, Thyratron Crowbar System For The Protection Of High Power Microwave Tubes In Television Transmitters

Abstract

EEV is a world leader in the production of Inductive Output Tubes (IOT's), which are now beginning to replace conventional klystrons in television transmitters. Unlike conventional TV klystrons, the I O T requires a crowbar to protect its carbon control grid and its cathode in the event of a fault. A bought-in IOT test equipment at EEV originally used a spark gap crowbar, but this was replaced by a thyratron crowbar when an IOT under test was destroyed because the spark gap failed to fire. The thyratron choice for this socket, which ages and tests 40kW IOT's at 28kVd.c., was a single gap glass thyratron type CX1722A/3 . This was installed in November 1991 and is still working satisfactorily after 10,000 filament and 4,200 high voltage hours and 2,600 crowbar events. Commercial transmitters require 24 hour a day, seven day a week operation. Transmitter reliability is essential and fault conditions must be dealt with by a fail-safe crowbar system. Accordingly, EEV has developed a two gap ceramic thyratron, CX2708, for use in commercial equipments. This 3" diameter thyratron is rated for 40kV d.c. hold-off, and for a fault level of 6 coulombs. All CX2708 thyratrons are tested for 24 hour hold-off at 40Kv d.c. at the maximum specified heater and reservoir voltages, and are also tested discharging 16pF from 40kV through a resistance of 23R followed by 6.4C of power supply follow-on at the minimum specified heater and reservoir voltage. These exacting tests were essential to meet the severe reliability reqirements, and are being performed on all production CX2708 thyratrons to ensure continued reliability and quality. Measurements have been made of the capabilities of the CX2708 with regard to capacitor discharge and follow-on current level and duration. These measurements show that the requirements of commercial equipments are well within the capabilities of the CX2708. A complete crowbar unit (type MA2447A) has also been developed for use with the CX2708. This unit incorporates automatic fault current triggering of the crowbar when a preset current level is exceeded. Interlocking of the grid 1 current to indicate that the crowbar is ready to fire and external triggering via optic links are additional important features. Two methods of setting the triggering level are discussed in detail with particular reference to noise immunity. Under typical operating conditions the unit will protect a 0.05mm diameter wire (47 SWG). Protection is fast with a firing delay of only 1lOns under fault conditions and 50011s via the external trigger. Under fault conditions at the full 40kV the "let through" to the fault is 2.3mC and only 0.023A2s. Many CX2708 thyratrons are now operating satisfactorily in transmitters throughout the world. The target spurious firing rate from the thyratron is less than one per year to ensure first class reliability. A d.c. hold-off test at 40kV at EEV has reached 3,500 hours without a single spurious firing and is ongoing. A further j~mportant consideration for the user is the thyratron warm-up time. Current production CX2708's have a warm-up time of 10 minutes, but a new design has now matched the IOT warm-up time of 5 minutes.