The CV4018 thyratron is, as usual, a gas filled valve, in this case the gas at low pressure is xenon. This example is of tetrode construction and was designed for relay or grid-controlled rectifier applications. The valve is also marked 2D21W and it differs from the basic valve in being designed to withstand vibration as would be found in moving vehicles and aircraft.
When a certain critical anode current flows the ionisation of the gas begins an avalanche discharge that overrides the grid control and leads to saturation current flow. The anode current can then only be extinguished by reducing the anode volts below the ionisation threshold.
The gm value in a normal hard valve is replaced by the control ratio parameter. In this valve the ratio is 250. At saturation the valve voltage drop is 8 Volts.
The box anodes and thick mica sheets to reduce the effects of vibration.
The anode seen at the cathode end.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040. Type CV4018 was first introduced in 1952.