Abstract :
In 1939, the Admiralty successfully used the industrial type BT9 high-voltage mercury-vapour thyratron as a modulator switch. As the development of high-power radar progressed, special valves were needed to meet conditions very different from those of industry. The requirement of high peak-current, with very rapid build-up, set new problems in cathode design. These were met by new cathodes, one indirectly and two directly heated, in which the surface had maximum availability for the emission of electrons and the reception of positive ions. The gauze cathode prevents flaking off of the emissive coating. Cathode life is limited by the effects of bombardment by positive ions, the greatest effect being produced during the ionization period, when the current reaches a considerable value before the anode-cathode voltage has fallen to its steady arc-drop value. The higher the voltage prior to conduction, the more severe the cathode bombardment and the shorter the life. The paper discusses ionization, de-ionization, the effects of cathode temperature and of vapour pressure, anode sputter, and other features experimentally investigated in the course of the development. A short account is given also of mercury-vapour thyratrons used as auxiliaries in radar service. The use of mercury vapour necessitates a time delay for cathode heating before current can be drawn, and also temperature control to keep the vapour pressure within prescribed limits. These are recognized disadvantages, but with their acceptance, the mercury-vapour thyratron assisted the development of high-power radar and its application at an early stage and throughout the war.
