The voltage-current relationship in a TWT electron gun is defined by the perveance, as follows:
Gun Perveance = Cathode Current / [(Cathode-to-Anode Voltage).
Selecting low voltage, high current operation means that the gun perveance must be high, and this presents several practical limitations.
Cathode current density and gun perveance considerations can be removed by designing the TWT for high voltage, low current (low perveance) operation.
An electron gun for a practical TWT will operate at an intermediate perveance, based on a tradeout study in which the factors shown in Table 2 are considered.
The electron gun must be designed for the desired perveance while drawing uniform current density from across the cathode surface and producing an electron beam that has the required diameter.