Design and development of electron gun for space helix travelling wave tube

The design and development of an electron gun for space travelling wave tube (TWT) needs special considerations in order to achieve high reliability, high efficiency, high linearity and long life. A Pierce type convergent gun has been designed and developed for a C-band 60 W (CW) space helix TWT. The anode is isolated with the body (Integral Pole Piece Barrel assembly with the helix SWS) and it is kept at slightly higher potential (+100 to +250 volts) with respect to the body. In practice, the cathode is supplied with negative potential (around -3.0 kV) with respect to the body which is at the ground potential. Therefore, the positive potential on the anode with respect to the body not only controls the beam current over the life time of the tube but also acts as an ion barrier and helps to increase the life and reliability of the cathode. The high beam perveance (/spl ap/0.5 /spl mu/P) gun is designed for beam voltage 3.0 kV and beam current 75 mA. The high perveance gun is selected for achieving high gain with small size and reduced mass of the space tube. An M-type dispenser cathode is used with a loading of nearly 1 Amp/cm/sup 2/ for long life. The convergence factor is selected less than 6:1 for high beam linearity and the beam fill-factor is selected 0.5 for optimum beam transmission.