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The photograph shows the cathode temperature being measured during the exhaust schedule of the M-O Valve Companys E2986 hot-cathode modulator thyratron. This valve has power ratings of 200MW (peak) and 150kW (mean). It is thought to be the largest of its type available in the world at present. A metal envelope construction is used to enable all the the electrodes to be easily water-cooled. By eliminating grid emission and gas density changes due to hot electrodes, this also enables a very high power to be achieved in a relatively small envelope. This valve is filled not with hydrogen, but with deuterium, whose greater dielectric strength enables an anode voltage of 40kV to be achieved. While the valve is in use the deuterium gas filling is continuously replenished by means of a titanium deuteride reservoir, The heater of this is controlled by a barretter and thermistor to counteract the effect of changes in the supply voltage or ambient temperature on the gas pressure. An impregnated tungsten cathode is used in association with a secondary emitter to give a maximum peak current of 10,000A with a heater power of 1.8kW.
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