Dual anode electron gun & PPM focusing for Ka-Band TWT

A low perveance (0.14µp) convergent electron gun with dual anodes has been designed using codes EGUN (2D), TRAK (2D), OPERA (3D) and OMNITRAK (3D) for Ka-Band helix (TWT) for space applications. Pierce type electron gun with an electrically isolated beam forming electrode (BFE), control anode (A1) and an ion barrier anode (A2) has been opted. An M-type dispenser cathode with low cathode emission loading (∼1.0 Amp/cm²), has been selected to ensure the desired long life of more than 10 years and high reliability. The application of +100 to + 250 volts at the ion barrier anode with respect to ground anode (A1) protects the cathode surface from the ion bombardment and hence enhances the life and reliability of the tube. Potential at A1 can be used to regulate the current over the life of the tube without changing the accelerating potential; this also contributes towards the life and reliability of the tube. In order to further enhance the life and reliability, non-evaporating type SAES ST-175 getter, has been incorporated in the gun assembly. Application of BFE negative bias of the order of 10 to 20 volts with respect to the cathode is used to achieve the desired beam transmission in the CW tube.