In order to obtain long life from an inert gas-filled thyratron it is essential to take precautions to ensure that:
- The cathode shall not be destroyed by bombardment.
- The gas shall not be driven into the electrode structure or 'cleaned-up'.
The two effects are inter-related in that failure to observe either condition will lead to the other fault.
However, for design purposes condition (1) is met by observing the limiting anode current and filament voltage ratings quoted for the valve.
The gas clean-up effect operates whenever a high voltage exists between two valve electrodes in the presence of an active, or residual ion population.
This occurs if, when the valve is conducting a substantial forward current, this current is stopped in a few micro-seconds and it is immediately followed by a rapid rise of inverse voltage across the valve.
Under these conditions residual positive ions are accelerated by the high voltage across the valve and become imbedded in the anode. This effect is expressed quantitatively as:
The product of rate of anode current decay and the rate of initial inverse anode voltage rise.
The rate of anode current decay is expressed in amperes per microsecond and is taken over the last 10 microseconds of current conduction. The average rate of inverse voltage rise is expressed in volts/microsecond and is taken over the first 200V.
This product is known as the valve commutation factor and when a maximum value is quoted, care must be taken to ensure that it is not exceeded.
If the commutation factor is too high it may be lowered by the use of snubbing circuits. These usually take the form of simple resistance-capacitance or inductance-capacitance networks placed in the thyratron anode-cathode circuit. Such circuits are effectively low-pass filters and they prevent rapid voltage or current changes from being impressed on the valve.